postharvest physiology of fresh cut tomato slices
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Postharvest physiology of fresh cut tomato slicesTRANSCRIPT
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POSTHARVEST PHYSIOLOGY OF FRESH-CUT TOMATO SLICES
A thesis submitted to The University of Queensland, Australia in fulfillment of the requirements for the degree of
Doctor of Philosophy in Horticultural Science
By
Darwin H. Pangaribuan
Ir., Bogor Agricultural University, Indonesia PG Dipl., International Institute for Geo-Information Science
and Earth Observation, Enschede, The Netherlands M.Sc., Wageningen University, The Netherlands
School of Agronomy and Horticulture
2005
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DECLARATION OF ORIGINALITY
The work presented in this thesis is to the best of the author’s
knowledge and belief, original and the author’s own work, except as acknowledged in the text and that the material
has not been submitted, either in whole or in part, for a degree at this or any other university.
Darwin Pangaribuan
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LIST OF PRESENTATIONS
5th Australian Horticultural Society Meeting, September 2002, University of Sydney, Australia.
1. Effect of Temperature and Maturity Stage (Poster)
2. The Physiology of Fresh-cut Tomato Slices (Poster) 21st ASEAN/3rd APEC Seminar on Postharvest Technology, August 2003, Bali, Indonesia.
1. The Effect of Fruit Maturity and Storage Temperature on
Postharvest Physiology of Fresh-Cut Tomato Slices (Oral)
2. Biochemical Changes in Tomato Slices as Affected by Fruit
Maturity and Storage Temperature (Poster)
3. Effect of Slicing on Ethylene Production and Respiration Rate of
Tomato Slices (Poster)
Australasian Postharvest Horticulture Conference, October 2003, Brisbane, Australia.
1. 1-Methylcyclopropene Delays Softening in Tomato Slices (Oral)
2. Effect of an Ethylene Absorbent on Quality of Tomato Slices
(Poster)
3. Exposure of Tomato Fruit to 1-MCP Improves Quality of Stored
Slices (Poster)
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List of Publications
Available at:
http://unila.academia.edu/DarwinPangaribuan
https://www.researchgate.net/profile/Darwin_Pangaribuan
No Journal Publications 1 Pangaribuan, D.H., and D. Irving 2011. Effect of maturity and 1 MCP on
quality of tomato slices. Jurnal Agronomi Indonesia Vol. 39(2):92-96
2 Pangaribuan, D.H, D. Irving and Tim O’Hare. 2010. Effect of Maturity and 1-MCP on Physiology of Tomato Slices. Acta Horticulturae No 880:361-366
3 Pangaribuan, D.H and D. Irving. 2010. The Effect of 1-MCP in Maintaining the Quality of Tomato Slices. Jurnal Teknologi dan Industri Pangan, Vol XXI (1): 80-86
4 Pangaribuan, D.H. 2009. The Effect of Ethylene in Maintaining Quality of Tomato Slices. Jurnal Teknologi dan Industri Pangan, Vol XX(1) : 50-55
5 Pangaribuan, D.H. and D. Irving. 2008. Effect of Fruit Maturity and Storage Temperature on the Quality and Storage Life of Tomato Slices. Jurnal Agrista. Vol 12 (1):51 – 61
6 Pangaribuan, D.H. and D. Irving. 2006. The Physiology and Nutrition of Tomato Slices as Affected by Fruit Maturity and Storage Temperature. Jurnal Agrista, Vol 10 (3):142 – 151.
7 Pangaribuan, D.H. and D. Irving. 2006. The Effect of Heat Treatments on the Postharvest Quality of Tomato Slices. Jurnal Agrotropika, Vol 11 (2): 74 – 82.
8 Pangaribuan, D.H. 2006. Ethylene Production and respiration rate in fruit and sliced tomatoes. Jurnal Agrotropika, Vol 11 (1): 15 - 22.
9 Pangaribuan, D.H. 2005. The Physiological Characters of Tomato from Different Portions of Fruit. Jurnal Agrotropika, Vol 10 (2): 101 – 106.
10 Pangaribuan, D.H., D. Irving. 2004. The Involvement of Ethylene in Softening of Tomato Slices. Thai Journal of Agricultural Sciences, Vol.37
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Proceeding Publications
11 Pangaribuan, D. 2011. Effect of Storage Temperature and Ethylene Absorbent on the Quality of Cherry Tomatoes. Proceeding on Seminar Nasional Sains dan Teknologi IV. Lembaga Penelitian Universitas Lampung. Pages 247-256.
12 Pangaribuan, D. 2010. The effect of chemical treatment on the quality of tomato slices. Proceeding on International Seminar on Horticulture for Food Security. ISHFS. Juni 2010.
13 Irving, D. E. and Pangaribuan, D. 2009. The Effects of Cultivar and Storage Temperature on Postharvest Characteristics on Tomato Fruits. Proceeding International Seminar on Sustainable Biomass Production and Utlization: Challenges and Opportunities August 3-4, 2009
14 Pangaribuan, D; D.E. Irving; T,J, O’Hare. 2003. 1-MCP delays ripening in tomato slices. Proceeding of Australasian Postharvest Horticulture Conference, pp. 169 – 171.
15 Pangaribuan, D; D.E. Irving; T,J, O’Hare. 2003. Effect of ethylene absorbent on quality of tomato slices. Proceeding of Australasian Postharvest Horticulture Conference, pp. 252 – 253.
16 Pangaribuan, D; D.E. Irving; T,J, O’Hare. 2003. Exposure of tomato fruit to 1-MCP improves quality of stored slices. Proceeding of Australasian Postharvest Horticulture Conference, pp. 254 – 255.
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ACKNOWLEDGMENTS
I sincerely thank my principal advisor, Dr Donald Irving, for his
valuable guidance, his understanding of an international student to
whom English is a secondary language, and for valuable criticism and
editing during writing each chapter of the thesis. His kindness has
contributed significantly to my learning in the preparation of posters
and seminars, and in the completion of this thesis.
I wish to thank to my co-advisors, Dr Gavin Porter, for his support at
the early stage of the study and Dr Tim O’Hare (Queensland
Department of Primary Industries and Fisheries) and Assoc Prof. Alan
Wearing, for their advice during my study. My gratitude is expressed
especially to Victor Robertson, for help in running laboratory
equipment; Katherine Raymont for help in running the HPLC and
spectrophotometer; Dr Andrew Macnish who helped me with the GC
and in the preparation and quantification of 1-MCP; Alan Lisle for
statistical advice; Leigh Baker (Queensland Department of Primary
Industries and Fisheries) for help running the autotitrater; Darren
Zielke (Syngenta Seeds Pty. Ltd) for helping me to obtain tomato
materials; Julie Hilton (Student Support Advisor) who proof-read part
of my thesis.
Pursuing Ph.D studies in Australia would not have been possible
without a scholarship from the Australian Agency for International
Development (AusAID). Thanks also to the Rector of the University of
Lampung, Indonesia for his permission to pursue postgraduate study.
The completion of this thesis is also supported by my family,
especially my loving wife Dame Trully Gultom, who worked hard to
provide financial assistance, and my son, Daniel Gramy Pratama.
Darwin Pangaribuan [email protected]
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ABSTRACT
Fresh-cut products are becoming increasingly popular as an option for
processing fruit and vegetable commodities. Rapid deterioration
during storage of tomato slices is the main problem with fresh-cut
tomato. Slicing disrupts the plant tissue so the products become
more perishable, which leads to a relatively short storage life, tissue
softening, and results in tomato slices with poor quality. The
scientific basis for maintaining quality of tomato slices during storage,
and postharvest handling techniques to extend storage life, is the
focus of this thesis.
The major research objectives of this study focused on the
physiological (ethylene and respiration), quality (firmness, colour,
soluble solids, titratable acidity, and electrolyte leakage) and
nutritional (ascorbic acid and lycopene) changes that occur in fresh-
cut tomato slices from cv. ‘Revolution’ during storage. The specific
objectives of the research were:
1. To determine the effects of slicing on the postharvest physiology
of tomato slices
2. To study the quality changes in tomato slices taken from fruit at
different stages of maturity and stored at different storage
temperatures
3. To characterise the involvement of ethylene in the loss of slice
quality
4. To determine the efficacy of 1-MCP in maintaining quality of
tomato slices
5. To determine the effect of fruit maturity and 1-MCP on the
quality of tomato slices
6. To evaluate the effect of applying a brief heat shock to intact
tomatoes on the quality of slices.
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Study of the physiology of fresh-cut of tomato slices was started with
comparisons of ethylene production and respiration between intact
tomatoes and sliced tomatoes (arranged stacked or scattered in
storage containers). Ethylene production and respiration initially
increased in response to slicing. The rate of ethylene production and
respiration by tomato slices was higher than in intact fruit. Slices
arranged in stacks had lower rates of ethylene production and
respiration compared with slices that were scattered. To reduce
ethylene production and respiration rates by tomato slices, regrouping
slices into their original shape is desirable during storage.
Tomato fruits at different stages of maturity have different
physiological and metabolic activities when stored at different
temperature regimes. Slices taken from fruit at four stages of
maturity, characterized by colour as ‘turning’, ‘pink’, ‘light-red’, and
‘red’, were evaluated for quality when stored at 0, 5 or 10 °C. The
slices taken from the ‘turning’ stage of maturity were firmer and had
longer storage life compared with those slices taken from the ‘red’
maturity tomatoes. Tomato slices stored at 0 °C were firmer and had
longer storage life compared with those slices stored at 10 °C. Storage
life of tomato slices could be maintained for 12 days at 0 °C, 10 days
at 5 °C, or 8 days at 10 °C. Tomato slices obtained from the ‘pink’ and
‘light-red’ stages of maturity would be acceptable for marketing.
Experiments were conducted to investigate whether ethylene
absorbents and ethylene influence the quality of tomato slices.
Ethylene absorbent resulted in reduced ethylene, less CO2
accumulation, and firmer slices. In contrast, ethylene applied 2 days
after slicing stimulated the rate of ethylene production, CO2
production, and produced softer slices during storage. These
experiments show that endogenous ethylene produced by slicing of
intact tomatoes or application of exogenous ethylene to slices in
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containers had the undesirable effects of inducing softening during
storage.
Changes in firmness are ethylene-mediated in tomatoes and can be
prevented by exposure of fruit to 1-methylcyclopropene (1-MCP).
When intact tomatoes at the ‘pink’ maturity stage were treated with
0.1, 1.0 or 10.0 µL L-1 1-MCP (20 °C, 12 h), 1-MCP reduced both
ethylene production and respiration rate, delayed softening of the
pericarp, and inhibited loss in titratable acidity in slices when
compared with slices from fruit not treated with 1-MCP. The storage
life of tomato slices was extended by application of 1-MCP to intact
tomatoes, but application 1-MCP to slices was of no benefit. The most
effective concentration of 1-MCP for inhibiting the ethylene-induced
softening of tomato slices was 1 µL L-1.
A study was carried out to determine whether 1-MCP would be more
effective if applied at an early maturity stage or a late maturity stage.
1-MCP (1 µL L–1 at 20 °C) was applied directly to intact tomatoes at
‘turning’ and ‘pink’ (early maturity) and ‘light-red’ (late maturity)
stages. The efficacy of 1-MCP was affected by fruit maturity, as later
maturity fruit were usually less responsive to 1-MCP. This study has
shown that application of 1-MCP to intact tomato retarded the
progress of ripening and reduced the rate of loss in slice quality if
applied at the early stages of maturity (‘turning’ and ‘pink’ stages).
The effect of heat treatment on tomato slice quality was determined
when intact ‘pink’ maturity stage tomato fruit were dipped in water at
38 °C, 42 °C or 46 °C for 1 hour or treated with hot air at 38 °C for 24
h, 36 h, or 48 h. Dipping intact tomatoes in hot water or treating
intact tomatoes with hot air prior to slicing did not extend the storage
life of tomato slices.
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This thesis showed that pre-slicing treatments such as selection of
slice portion and arrangement, appropriate fruit maturity and storage
temperature, and application of the ethylene inhibitors 1-MCP, as well
as post-slicing treatments such as ethylene reduction strategies, can
minimise the negative effects of wounding. The information obtained
from this study will provide valuable information for the development
of tomato slice production and marketing.
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TABLE OF CONTENT
1. INTRODUCTION .................................................................... 13
1.1 FRESH-CUT PRODUCTS ................................................................ 13 1.2 PROBLEMS WITH FRESH-CUT PRODUCTS ......................................... 15
2. LITERATURE REVIEW ........................................................... 17
2.1 TOMATO BIOLOGY ...................................................................... 17 2.1.1 Introduction............................................................................ 17 2.1.2 Fruit structure ........................................................................ 17 2.1.3 Fruit composition ................................................................... 19
2.2 FRUIT QUALITY .......................................................................... 20 2.2.1 Introduction............................................................................ 20 2.2.2 Colour .................................................................................... 21 2.2.3 Texture ................................................................................... 23 2.2.4 Sugars ................................................................................... 24 2.2.5 Titratable acidity ................................................................... 25 2.2.6 Flavour ................................................................................... 26 2.2.7 Nutritional value .................................................................... 26 2.2.8 Summary ............................................................................... 28
2.3 FACTORS AFFECTING TOMATO QUALITY ........................................... 28 2.3.1 Introduction............................................................................ 28 2.3.2 Cultural practices .................................................................. 29 2.3.3 Cultivars ................................................................................ 29 2.3.4 Maturity ................................................................................. 31 2.3.5 Low temperature .................................................................... 33 2.3.6 Relative humidity ................................................................... 34 2.3.7 Chilling injury ........................................................................ 35 2.3.8 Summary ............................................................................... 38
2.4 PHYSIOLOGICAL CHANGES DURING FRUIT RIPENING ........................... 38 2.4.1 Introduction............................................................................ 38 2.4.2 Ethylene biosynthesis and production .................................. 39 2.4.3 Respiration ............................................................................ 44 2.4.4 Loss of chlorophyll and synthesis of lycopene ...................... 45 2.4.5 Fruit softening........................................................................ 46 2.4.6 Degradation of starch, and sugar changes ........................... 47 2.4.7 Changes in cellular membranes ............................................ 48 2.4.8 Summary ............................................................................... 49
2.5 CONTROL OF RIPENING ................................................................ 50 2.5.1 Introduction............................................................................ 50 2.5.2 Effect of ethylene on quality attributes .................................. 50 2.5.3 Control of ethylene action ...................................................... 51 2.5.4 1-methylcyclopropene ............................................................ 53 2.5.5 Heat treatment ....................................................................... 56 2.5.6 Summary ............................................................................... 60
2.6 PHYSIOLOGICAL CHANGES AFTER SLICING ....................................... 60 2.6.1 Introduction............................................................................ 60 2.6.2 Wound-ethylene synthesis .................................................... 61
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2.6.3 Elevated respiration............................................................... 63 2.6.4 Membrane deterioration......................................................... 65 2.6.5 Wound healing ....................................................................... 66 2.6.6 Water loss .............................................................................. 66 2.6.7 Susceptibility to micro-organisms .......................................... 67 2.6.8 Loss of firmness ..................................................................... 68 2.6.9 Flavour changes .................................................................... 69 2.6.10 Nutritional changes.............................................................. 69 2.6.11 Summary ............................................................................. 71
2.7 GENERAL SUMMARY ................................................................... 72 2.7.1 Summary ............................................................................... 72 2.7.2 Objectives .............................................................................. 73
3. MATERIALS AND METHODS.................................................. 74
3.1 OVERVIEW ................................................................................ 74 3.2 TOMATO FRUIT .......................................................................... 75 3.3 ASSESSMENTS MADE BEFORE EXPERIMENTATION ............................. 75 3.4 HANDLING OF SLICES .................................................................. 76 3.5 EXPERIMENTAL MEASUREMENTS ................................................... 77
3.5.1 Ethylene and CO2 evolution ................................................... 77 3.5.2 Slice firmness ........................................................................ 79 3.5.3 Soluble solids (SS), titratable acidity (TA) and ratio SS/TA ... 79 3.5.4 Juice colour ............................................................................ 80 3.5.5 Electrolyte leakage ................................................................ 81 3.5.6 Ascorbic acid.......................................................................... 81 3.5.7 Lycopene ................................................................................ 83
3.6 STATISTICS AND DATA ANALYSES ................................................... 84
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1. INTRODUCTION
1.1 Fresh-cut products
Fresh-cut products have been defined as those products arising from
procedures such as washing, sorting, trimming, peeling, slicing, or
chopping, that do not affect the ‘fresh-like’ quality of the fruit or
vegetable (Burns, 1995; Cantwell and Suslow, 2002). A more
encompassing definition for fresh-cut fruit or vegetables was proposed
by Salunkhe et al. (1991b), as “a fresh fruit or vegetable that has gone
through a process to increase its functionality without greatly
changing its fresh-like properties”. Reyes (1996) and Ahvenainen
(2000) defined the requirements of fresh-cut as (1) delivering to the
consumer a fresh-like product and at the same time ensuring food
safety and maintaining sound nutritional and sensory quality, and (2)
ensuring that the product should have sufficient shelf-life to make
distribution feasible within the region of consumption.
Fresh-cut products were previously termed ‘minimally processed’, and
included handling, preparation, packaging and distribution of
agricultural commodities in a fresh-like state (Shewfelt, 1987). The
term evolved into ‘lightly processed’ products (Abe and Watada, 1991;
Huxsoll and Bolin, 1989), to the currently accepted term of ‘fresh-cut’
(Watada et al., 1996). In this study the term ‘fresh-cut’ will be used
throughout, as used in papers on kiwifruit (Agar et al., 1999), pear
(Gorny et al., 2000), and tomato (Hong and Gross, 2000; Hong et al.,
2000).
Production of fresh-cut vegetables and fruits has increased
dramatically in the last decade both as a result of changes in
lifestyles, and in response to consumer demand for convenient, ready-
to-use and ready-to-eat products with fresh-like quality (Ahvenainen,
1996; Garrett, 2002). The International Fresh-Cut Produce
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Association (IFPA) estimated the market value of fresh-cut produce in
the USA to reach approximately $10-12 billion in sales in 2004 (IFPA,
2004).
Fresh-cut products are value-added products that provide an
additional outlet to consumers and could increase the consumption of
fresh produce due to its convenience and attractive appearance and
flavour. For the food service industry, fresh-cut products are
convenient because labour for preparation and special systems to
handle waste is not required. In addition, specific forms of fresh-cuts
can be delivered to the consumer at short notice and there is no
additional preparation for their use in restaurants, fast food outlets
and retail markets (Cantwell and Suslow, 2002; Watada et al., 1996).
Maintenance of quality is a challenge to the rapidly expanding fresh-
cut sector. Maintaining quality of fresh-cut vegetables products has
been possible in carrot (Barry-Ryan and O'Beirne, 1998; Bolin and
Huxsoll, 1991), lettuce (Varoquaux et al., 1996), mushrooms (Brennan
et al., 2000) and pak-choi (O'Hare et al., 1995). Research on apples
(Buta et al., 1999), kiwifruit (Varoquaux et al., 1990), and oranges
(Pretel et al., 1998) has also been reported.
Tomato is a vegetable fruit and little attention has been paid to fresh-
cut tomato slices. Tomatoes have great potential for fresh-cut
processing (Anonymous, 2002a). As tomato slices are a fresh
commodity, there is a need for ready-to-eat tomato slices that have an
acceptable condition for the food service industries, for use on
sandwiches, burgers and in restaurants.
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1.2 Problems with fresh-cut products
Fresh-cut products remain biologically and physiologically active, as
the tissues are living and respiring (Reyes, 1996). The main problems
with fresh-cut products are two-fold. Firstly, fresh-cut products are
highly perishable because mechanical processes such as cutting,
slicing, shredding, and trimming disrupt cellular structures (Rolle and
Chism, 1987; Watada et al., 1996). Secondly, mechanical wounding
results in increased production of ethylene, weight loss, and
respiration rates (Watada et al., 1996). In addition, the cellular
breakdown leads to undesirable enzymatic reactions and tissue
softening, leakage of ions and other cellular components, and
consequently storage life is often reduced (Burns, 1995; Luna-
Guzman et al., 1999). If all these changes cannot be properly
controlled, they can lead to rapid senescence and deterioration of the
product.
It is known that fresh-cut fruit are more difficult to produce than
fresh- cut vegetables. Part of the reason for this difference is that
many fruits have different maturity stages and must be ripened before
they are processed (Cantwell and Suslow, 2002). Commercially,
tomato is a vegetable type of product, but botanically tomato is a
fleshy fruit that undergoes ripening during development. Ripening in
tomatoes is distinct and dramatic (Madhavi and Salunkhe, 1998).
Fresh-cut tomato slices have a relatively short storage life (Artes et al.,
1999; Hong and Gross, 1998), due to the inherent perishability of
tomato and to the cellular damage caused by the slicing process
which leads to tissue softening and associated loss of tissue integrity.
These factors can result in tomato slices with poor quality. Study of
the physiology, biochemistry and nutritional changes of fresh-cut
tomato slices is needed to ensure high quality of sliced tomato
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products during storage, and to provide a scientific basis for
developing effective handling procedures for such a delicate product.
This thesis is divided into 10 Chapters. In Chapter 2, the literature
review will discuss the postharvest physiology of intact and sliced
tomatoes, and give emphasis to the differences that slicing imposes.
The objectives of the study are detailed at the end of literature review.
Chapter 3 covers the general methods applied in all experiments to
prevent repetition in the subsequent chapters. Chapters 4 through 9
present results of the experiments. The final chapter (Chapter 10)
contains the overall discussion and conclusions.
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2. LITERATURE REVIEW
2.1 Tomato Biology
2.1.1 Introduction
The tomato (Lycopersicon esculentum L.) belongs to the family
Solanaceae and is an important horticultural crop. The plant is
indigenous to South America and was domesticated and cultivated in
Mexico but was introduced to Europe and North America in the early
19th century. The popularity of tomato led to extensive breeding
programs to produce cultivars suitable for fresh and processed
consumption (Tigchelaar, 1986; Varga and Bruinsma, 1986). The
tomato is now one of the most widely grown and consumed vegetables
in the world (Rubatzky and Yamaguchi, 1997).
In Australia, tomatoes are gaining in popularity, as annual
consumption per capita has risen from 15 kg to 17 kg during the five
year period 1990 to 1995 (Ross, 1998). Queensland is a leading state
for vegetable production and mainly produces fresh market tomatoes
with only a small percentage of the total crop grown for processing.
The main Queensland growing areas are Bowen, Bundaberg, and
southeast Queensland (Redlands, Lockyer Valley and the Granite Belt)
(Fullelove et al., 1998).
2.1.2 Fruit structure
Tomatoes follow the pattern of growth of parthenocarpic fruit i.e. the
growth of a fruit without embryo development (Varga and Bruinsma,
1986). Botanically, the tomato is a fleshy fruit, specifically a berry,
since the seeds are formed within a fleshy mesocarp (Davies and
Hobson, 1981). Generally, tomato fruit consists of flesh
(skin/epidermis and pericarp) as well as pulp (placenta and locular
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contents) (Ho and Hewitt, 1986). Figure 2.1 depicts the transverse
section of a mature tomato fruit showing the various structures and
regions.
Figure 2.1 The transverse section of a mature tomato fruit (Brecht, 1987)
The skin. The skin or peel of the pericarp consists of the epidermal
layer and 3-5 layers of thick-walled collenchymous tissue (Davies and
Hobson, 1981; Varga and Bruinsma, 1986). The outer epidermis cells
do not have any stomata (Varga and Bruinsma, 1986).
The pericarp. The pericarp consists of an outer pericarp, radial
pericarp and inner pericarp/columella (Ho and Hewitt, 1986). As the
fruit matures, the pericarp continues to enlarge by cell division and
cell enlargement, then the pericarp cells become large with many
intercellular spaces (Davies and Hobson, 1981; Varga and Bruinsma,
1986). The structure of the radial pericarp and inner pericarp is
similar to that of the outer pericarp. The columella has larger air
spaces (Ho and Hewitt, 1986), and contains many starch grains until
the fruit begin to colour (Varga and Bruinsma, 1986). Salunkhe et al.
(1974) indicated that outer and inner pericarp regions play an
important role in the quality of the tomato because of the highest
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contents of dry matter, insoluble solids, and reducing sugars are
found there.
The locules. The locular section contains the seeds that are
surrounded by a gel of parenchyma cells (Madhavi and Salunkhe,
1998). Brecht (1987) reported that the locular or juicy tissue of the
tomato was derived from the placenta of the fruit, overgrowing the
seeds and filling the loculus. However, these are not fused with either
the seeds or the pericarp. Sawhney and Dabbs (1978) found that
increasing locule number does not necessarily result in an increase in
seed number. They also concluded that both locule number and seed
number may independently affect final fruit size.
Shape and Size. The ultimate fruit shape and size are determined by
the rate and duration of cell enlargement (Ho, 1992), cultivar (Bertin
et al., 1998), and environmental and nutritional conditions (Barrett et
al., 1998). Tomato shape varies greatly with cultivar, and can be
elongated or pear like, oblate, or spherical (Barrett et al., 1998).
2.1.3 Fruit composition Tomato fruit consists of 93 - 95% of water (Davies and Hobson, 1981;
Wills, 1987). Generally, 5 - 7.5% of tomato content is dry matter
(Davies and Hobson, 1981) with approximately 1% in cuticles and
seeds, and 4 to 6% in soluble solids (Petro-Turza, 1986). The main
reducing sugars are glucose and fructose with the fructose content
being slightly higher (Baldwin et al., 1991a; Baldwin et al., 1991b;
Stevens et al., 1977b; Young et al., 1993). Sucrose has also been
found in tomatoes, but at much lower concentrations (Davies and
Hobson, 1981; Picha, 1986). Citric acid is the predominant organic
acid in tomatoes but malic acid is also present as a major constituent,
and together they account for 12% of the dry matter (Knee and Finger,
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1992; Stevens, 1985). Figure 2.2 shows the dry matter composition of
the tomato.
Figure 2.2 Composition of dry matter in tomato fruits (Stevens, 1985)
2.2 Fruit quality
2.2.1 Introduction It is generally accepted that consumer satisfaction is associated with
product quality. Quality can be defined as "the sum total of those
attributes which combine to make fruits and vegetables acceptable,
desirable, and nutritionally valuable as human food" (Salunkhe et al.,
1991a). Many preharvest and postharvest factors determine tomato
quality. Quality components that are important for tomatoes are both
external (colour and texture) and internal (flavour and nutritive value)
(Table 2.1). These quality parameters are related to fruit composition
at harvest and they change during postharvest handling.
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Table 2.1 Quality components of tomato
Main factors Components Appearance (visual)
Size: dimensions, weight, volume Shape Colour Defects: physical, physiological, pathological
Texture (feel) Firmness, softness Flavour (taste, smell)
Sweetness Sourness (acidity) Aroma (volatile compounds) Off-flavours and off-doors
Nutritive value Carbohydrates Proteins Lipids Vitamins
Adapted from Salunkhe et al. (1991a).
The following section of this literature review will discuss colour,
firmness, flavour, and nutritional value.
2.2.2 Colour Colour has a strong influence on consumer perceptions (D'Souza et
al., 1992; Francis, 1980) and is an important quality attribute in the
tomato industry (Gould, 1992; Stevens and Rick, 1986). Tijskens and
Evelo (1994) stated that colour is often used as an external index for
overall quality. Consumers prefer tomatoes that are harvested ‘red
ripe’ (Kader et al., 1977; Watada and Aulenbach, 1979).
Shewfelt et al. (1988) proposed that measurement of colour is closely
related to visual perception in tomatoes. Fruit surface colour can be
evaluated by a tristimulus colorimeter, which is known as the Hunter
method. This instrument provides 3 values for each colour
measurement. These values are derived from 3 scales defined as L*,
a*, b* (CIELAB) (McGuire, 1992). "L*" represents brightness/lightness
on a scale 0 - 100, with 0 being perfect black and 100 being perfect
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white. On the horizontal axis, the "a*" scale progresses from green
(negative values) to red (positive values). On the vertical axis, the “b*”
scale covers the range from blue to yellow. Positive “b*” indicates
yellow and negative “b*” indicates blue. As tomatoes turn from green
to red, changes in the colour space are characterised by lower L
readings, a change from –a* to +a* readings and decreased +b*
readings (Shewfelt et al., 1987).
Little (1975) and McGuire (1992) suggested that a more appropriate
measure of colour can be obtained from the calculation of hue angle
(h°) (Figure 2.3). The hue is the actual colour (for example, red,
yellow, blue, etc.), which is effective for visualising the colour
appearance of food products (Little, 1975; McGuire, 1992). A hue of
180° represents pure green and a hue of 0° pure red (Shewfelt et al.,
1988) and is calculated from the arctangent of b*/a*, designated hº.
Arctangent, however, assumes positive values in the first and third
quadrants and negative values in the second and fourth quadrants.
For a useful interpretation, h° should be a positive value between 0º
and 360º.
Figure 2.3. Hue angle, a* and b* colour (McGuire, 1992)
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Hue angle provides the best objective means of ripeness classification.
Shewfelt et al., (1992) proposed ranges as follows: for ‘mature-green’
(hº > 114º), ‘breaker’ (101º < hº < 114º), ‘turning’ (85º < hº < 101º),
‘pink’ (64º < hº < 85º), ‘light red’ (36º < hº < 64º) and ‘red’ (hº< 36º)
classes using average hue at the circumference. Hue angle at the
blossom end was 2-12° lower than at the circumference due to
initiation of colour development at the blossom end (Shewfelt et al.,
1992).
2.2.3 Texture
The texture of fruit is another major quality factor. It is commonly
agreed that consumer preference is for firm fruit that do not lose juice
during eating or slicing (Frenkel and Jen, 1989). The textural quality
of tomatoes is greatly influenced by flesh firmness, the fruits internal
structure (ratio between amount of pericarp and locular tissue), and
skin toughness (cuticle thickness) (Dorais et al., 2001; Frenkel and
Jen, 1989; Kader et al., 1978a). Moreover, the proportion of these
components affects the overall firmness of the fruit (Frenkel and Jen,
1989). This was shown by Barrett et al. (1998) who proposed that
thicker pericarp walls, more pericarp tissue, and fewer locules
ensured the structural integrity of tomato fruit. Moreover, Wann
(1996) explained that fruit firmness can be controlled by the integrity
of cell wall tissues, the elasticity of pericarp tissues, and the
enzymatic activity involved in fruit softening during the process of
ripening. Fruit softening is discussed further in section 2.4.5.
The texture of the tomato can be measured as firmness. In measuring
tomato slice firmness, Wu and Abbott (2002) suggested that a 4 mm
cylindrical probe provided more consistent firmness measurements
than a 6.4 mm spherical probe at harvest, and penetration depth of 3
mm provided more consistent results than 1 mm. Hall (1987)
24
measured the firmness of outer, radial, and inner pericarp tissues
using a 4.9 mm cylindrical probe on 12.5 mm thick tomato slices, and
stated that the outer pericarp was generally firmer than the columella.
Most of the softening of the tissues had occurred by 4 days after
incipient colour development, and tissue-softening rates of the
different tissues were cultivar dependent.
2.2.4 Sugars
Quality of tomatoes is mainly determined by the amount of soluble
sugars. The soluble solids of tomatoes are predominantly sugars
which constitutes about 55-60% of the dry matter (Fig. 2.2)(Hobson
and Grierson, 1993; Stevens, 1985). These sugars, in turn, are
important contributors to flavour. The free sugars are mainly
D-glucose and D-fructose which are present in approximately equal
amounts (Salunkhe et al., 1974). Fructose contributes more to
sweetness than glucose (Salunkhe et al., 1974; Stevens et al., 1977a).
Davies and Kempton (1975) and DeBruyn et al. (1971) indicated that
soluble solids are higher in the pericarp than the locular tissue.
Lower and Thompson (1966) found soluble solids content to be about
equal between the locular and pericarp portions, but Brecht et al.
(1976) showed that the locular tissue contained more soluble solids
than the pericarp. According to Brecht et al. (1976) the soluble solids
of the whole fruit may be higher, equal to, or less than that of the
pericarp or locular gel, depending on the variety.
Sugar content in tomatoes can be estimated by total soluble solids
and is measured using a refractometer and expressed as % soluble
solids or °Brix. Dry matter content is positively related to the total
sugar content of the fruit or to the ratio of soluble to total solids
(Davies and Hobson, 1981). Most tomato varieties vary in soluble
solids from between 3.2 and 7.0% in ripe tomato fruit (Islam and
25
Khan, 2000; Islam et al., 1996; Jones, 1999; Saltveit and Sharaf,
1992). Refractometers are used to measure the soluble solid contents.
2.2.5 Titratable acidity Titratable acidity is a measure of acid content, more specifically, the
number of protons that can be neutralised. Numerous studies have
indicated that factors such as cultivar (Lower and Thompson, 1966),
and stage of ripeness (Iwahori and Lyons, 1970) influence titratable
acidity values in tomato fruit. In ripe fruit, 40 - 90% of the total acids
are in the form of citric acid (Baldwin et al., 1991a; Baldwin et al.,
1991b). The acidity in ripe tomatoes varies widely between 0.25 and
1.1% citric acid on a fresh weight basis (Baldwin et al., 1991a;
Baldwin et al., 1991b; Young et al., 1993). The acid concentration is
important because it contributes to the flavour or taste of tomato
(Barringer, 2004).
Malic acid is found predominantly in ‘immature-green’ fruit and
decreases during maturation, while citric acid increases (Baldwin et
al., 1991a). Tomato fruit are most acidic when ‘mature-green’ or at
the early ‘breaker’ stage, and then acidity declines during ripening
(Baldwin et al., 1991a; Baldwin et al., 1991b; Knee and Finger, 1992;
Richardson and Hobson, 1987).
Kader et al. (1978b) reported that high quality fruit are characterised
by more than 0.32% titratable acidity (TA) and 3% soluble solids (SS)
and by a SS/TA ratio greater than 10. According to Hobson and
Grierson (1993) the ratio of SS/TA plays a major role in determining
the taste of a tomato, with high sugars and high acids being favoured.
26
2.2.6 Flavour
Flavour is an important quality in tomatoes. The flavour of tomatoes
is a complex sensation based on the taste and aroma of several
volatile compounds (Shewfelt, 1993; Stevens et al., 1977a). Flavour
quality of tomatoes is affected by the concentrations of sugars
(sweetness), organic acids (titratable acidity), phenolic compounds
(astringency), and odor-active volatiles (aroma) (Kader, 2002c).
Tomato flavour is also determined by harvest maturity. Maul et al.
(1998) found higher volatile levels and better sensory scores when
tomatoes were harvested at more mature stages. Kader et al. (1977)
also reported that tomatoes have more 'tomato-like' flavour when
harvested more mature. According to Stevens et al. (1977b) a large
locular portion and high concentration of acids characterise flavourful
cultivars.
Dorais et al. (2001) stated that tomato flavour is related to the balance
between sugars and organic acids (sugars/acids ratio) in the fruit, and
total sugar or acid content. The ratio between sugar (soluble solids)
and acid (titratable acidity) of the tomato fruit is a significant factor in
tomato flavour (Jones, 1999) and frequently used to rate the taste of
tomatoes (Barringer, 2004). However Stevens et al. (1977a)
recommend against this because with the same ratio, tomatoes with a
higher concentration of both sugars and acids taste better than those
with low concentrations of both. Loss in flavour during and after
processing of tomato is especially of concern in fresh-cuts, but limited
research has been done in this area.
2.2.7 Nutritional value Tomatoes are important for human health and well-being.
Nutritionally, tomatoes are rich sources of vitamins, especially
ascorbic acid and β-carotene, and antioxidants such as pro-vitamin A
27
(lycopene) (Rubatzky and Yamaguchi, 1997). The stability of nutritive
value during storage is an important factor to be considered for fresh-
cut tomato slices.
Tomato fruit contains important natural antioxidant substances such
as vitamin C (ascorbic acid) and lycopene. Generally, ascorbic acid
concentration in plant material decreases with maturity, but total
ascorbic acid per plant or fruit increases (Klein, 1987). Moreover, the
ascorbic acid content of raw tomato fruit differs according to cultivar
type. Brecht et al. (1976) showed that the ascorbic acid content of
eight cultivars of ripened tomatoes was 12.5 to 22.5 mg 100 g-1, while
Watada et al. (1976) reported that in 11 cultivars of ripe tomatoes, the
range was 13.7 to 31.8 mg 100 g-1.
There are two main carotenoids in tomato: lycopene (ψ,ψ-carotene)
which is the major carotenoid, and β-carotene (Arias et al., 2000a;
Gould, 1992). As tomatoes develop from ‘immature-green’ to ripe, the
increase in carotenoid content is related to the increase in lycopene
content within the plastids (Fraser et al., 1994; Thompson et al.,
2000). Lycopene, the main red pigment, constitutes 50-83% of the
total pigment content in the ripe tomato fruit (Abushita et al., 1997;
Davies and Hobson, 1981; Fraser et al., 1994). With the onset of
ripening, the lycopene content increases (Davies and Hobson, 1981).
Lycopene concentrations were reported to increase from 0.25 mg kg–1
in green tomatoes to values greater than 40 mg kg–1 in fully ripe fruits
(Thompson et al., 2000). Davies and Hobson (1981) found that
lycopene contents may range between 55 and 80 mg kg–1. Sharma
and Rick (1996) showed that the tomato skin contains about five
times more lycopene (540 mg kg–1; fresh weight basis) than the whole
tomato pulp (110 mg kg–1). Lycopene is also more concentrated in the
outer pericarp than in the locular tissue (McGlasson, 1993). Lycopene
concentration in tomatoes can be determined accurately in the
laboratory by spectophotometric measurements of tissue extracts
28
obtained with acetone (Mencarelli and Saltveit, 1988; Shi and Maguer,
2000).
The second main carotenoid is β-carotene that is about 3 to 7% of the
total carotenoid content (Gould, 1992). According to Fraser et al.
(1994) the maximum level of β-carotene occurs at the breaker or light
red stages, and is more concentrated in the locular tissue than in the
outer pericarp (McGlasson, 1993). Dorais et al. (2001) stated that
pigment concentration in the fruit depends on the cultivar and
growing conditions.
2.2.8 Summary Tomato fruit quality components for both intact tomatoes and tomato
slices include colour, firmness, soluble solids, flavour and nutritional
value, in addition to size, shape and freedom from defects. These
quality attributes cannot be improved after harvest but must be
maintained during storage of tomatoes or tomato slices. Future
research should seek to maintain optimal levels of quality in intact
fruits, as quality of fresh-cut tomato slices is influenced by the quality
of corresponding intact product.
2.3 Factors affecting tomato quality
2.3.1 Introduction
The composition and quality of tomato fruit as well as the
physiological, biochemical and storage stability of fresh-cut products
is influenced by preharvest factors such as cultural practices,
cultivars, and harvest maturity. Storage temperature and relative
humidity are the critical postharvest factors in achieving maximum
storage life of products. This section will focus on the effect of
29
cultural practices, cultivars, harvest maturity, storage temperature,
relative humidity, and chilling injury on the quality of intact and
fresh-cut tomatoes.
2.3.2 Cultural practices Among agronomic factors affecting tomato composition and flavour
are water availability, soil fertility and potassium (Stevens, 1985).
Stevens (1985) indicated that careful water management could result
in an increase in fruit solids. In addition, he argued that there is a
positive relationship between nitrogen availability and soluble solids
content. Addition of potassium fertilisers increased the acid content
of fruits (Stevens, 1985). Only one publication exists on the
agronomic factors for fresh-cut tomato slices. Hong et al. (2000)
conducted experiments to compare changes in the quality of slices of
red tomato fruit during storage at 5 °C. Plants were grown using
black polyethylene or hairy vetch mulches. They reported that
tomatoes from plants grown using hairy vetch mulch may be more
suitable for fresh-cut slices than those grown using black polyethylene
mulch on the basis of quality parameters including firmness, soluble
solids, and titratable acidity (Hong et al., 2000). This indicates that
further information on agronomic factors or cultural practices are
needed to produce high quality fresh-cut tomato slices.
2.3.3 Cultivars
Fresh-cut products are more perishable than the intact fresh product
(Kader, 2002c; Watada et al., 1996), therefore it is imperative to grow
varieties with enhanced shelf life characteristics. Genetic factors have
a direct influence on the quality of tomato fruit. As a consequence,
cultivar selection is of primary importance in determining the final
quality of a tomato fruit. Cultivars vary greatly in selectable attributes
such as colour, flavour, firmness, pest and disease resistance, and
30
skin colour (Madhavi and Salunkhe, 1998; Romig, 1995). Sams
(1999) stated that fruit quality traits may be modified by
environmental factors, but that the genetic background of the plant is
the major factor controlling quality.
There are differences in quality between varieties with extended shelf
life, such as those that ripen slowly and conventional ripening
cultivars. Generally, long shelf life cultivars have less flavour than
traditional cultivars (Jones, 1986). The long shelf life cultivar
‘Daniella’ has been introduced in Australia recently (Fullelove et al.,
1998), and while this was initially welcomed by the processing
industry (Ho, 1999), consumers have complained that the fruit was
not firm or juicy when cut into slices.
Ideally, tomatoes intended for fresh-cut production require similar
cultural practices to tomatoes intended for fresh market production
and need no unique harvesting or postharvest requirements
(Ahvenainen, 1996). The main criteria in assessing the suitability of
tomato cultivars for fresh-cut processing are as follows:
1. Multi-locular structure (Fullelove et al., 1998; Ross, 1998).
2. Low sensitivity to physiological disorders and microbial diseases
(Varoquaux and Mazollier, 2002).
3. Resistance to disease and resistance to mechanical injury
(Varoquaux and Mazollier, 2002).
4. Resistance to elevated carbon dioxide concentration and/or low
oxygen concentration (Varoquaux and Mazollier, 2002).
5. Reduced activity of endogenous fruit softening enzymes, which
contribute to degradative processes (Brecht, 1995; Romig, 1995).
6. Reduced chilling sensitivity that will allow more flexibility in
temperature management and result in better storage life and
quality (Brecht, 1995).
31
2.3.4 Maturity
It is essential to distinguish between the ‘horticultural maturity’ i.e.
the stage that represents optimal eating quality and ‘physiological
maturity’ i.e. the stage of full maturation that allows storage and
subsequent good eating quality (Watada et al., 1984). Tomatoes are
harvested at physiological maturity (Watada et al., 1984) and allowed
to ripen off the plant. Tomatoes are harvested at various stages of
maturity from ‘immature-green’ to the full-red stage (Table 2.2).
Table 2.2 Maturity and ripeness classes for fresh-market tomatoes. Class USDA
classification Description
Immature Mature-green A Mature-green B Mature-green C Breaker Turning Pink Light-red Red Full-red
- 1 1 1 2 3 4 5 6 -
Seed cut by a sharp knife on slicing the fruit; no jellylike material in any of the locules; fruit is more than 10 days from breaker stage Seed fully developed and not cut on slicing fruit; jellylike material in at least one locule; fruit is 6 to 10 days from breaker stage; minimum harvest maturity Jellylike material well developed in locules but fruit still completely green; fruit is 2 to 5 days from breaker stage Internal red coloration at the blossom end, but no external colour change; fruit is 1 to 2 days from breaker stage First external pink or yellow colour at the blossom end More than 10% but not more than 30% of the surface in the aggregate; shows a definite change in colour from green to tannish-yellow, pink, red, or a combination thereof More than 30% but no more than 60% of the surface, in the aggregate, shows pink or red colour More than 60% of the surface, in the aggregate, shows pinkish-red or red, but less than 90% of the surface shows red colour More than 90% of the surface, in the aggregate, shows red colour Fruit has developed full final red colour; fruit is more aromatic and softer than at the red stage
Source: Cantwell and Kasmire (2002).
32
It is essential to use mature fruit with an acceptable eating quality for
processing. Over-mature fruit will deteriorate rapidly. Mallik and
Bhattacharya (1996) found that the shelf life of tomatoes picked at the
‘mature-green’ stage was longer than in those picked at the breaker
stage, however they also found that quality after storage was poor.
Several researchers (Kader et al., 1978b; Kader et al., 1977; Shewfelt,
1990a) have found that fruit harvested at earlier stages of maturity
and room-ripened are less sweet, more sour, less tomato-like, and had
more off-flavour than those harvested at the full-red colour stage. In
contrast, tomato fruit harvested at the turning-red stage were sweeter,
less sour and more tomato-like, with less off-flavour than earlier-
harvested fruit (Kader et al., 1977).
There is a general belief that tomatoes ripened on the vine are of
better quality than tomatoes ripened off the vine (Bisogni et al., 1976;
Kavanagh et al., 1986; Picha, 1986; Stevens, 1986). Bisogni et al.
(1976) pointed out that the ratio of soluble solids to titratable acidity
is higher in tomato fruits ripened on the plant compared to room-
ripened fruit. Moreover, Watada and Aulenbach (1979) found that
vine-ripe fruit were considered sweeter than fruit harvested at
‘mature-green’ and ‘breaker’ stages by sensory panels despite there
being no significant differences in soluble solids content or dry matter.
Arias et al. (2000b) showed that the tomatoes ripened on the vine were
more red and darker than the off-vine-ripened tomatoes. The a*/b*
ratio, the lycopene content, soluble solids, total solids, and firmness
were also higher for the vine-ripened tomatoes (Arias et al., 2000b).
Selecting the optimum harvest maturity of the individual variety for
fresh-cut production is another critical factor that affects eating
quality and storage life (Gorny et al., 1998; Salunkhe and Desai, 1988;
Shewfelt, 1990b; Watada and Qi, 1999b). Artes et al. (1999) reported
that the shelf life of tomato slices could be maintained for 10 days at
33
2 °C for the slow ripening tomato cv. ‘Durinta’ with a long storage life
when harvested at the pink stage of maturity (h° values between 65
and 75°). Mencarelli and Saltveit (1988) reported that slices of the
‘mature-green’ tomato cv. ‘Castlemart’ ripened in the same pattern
when compared with whole ‘mature-green’ fruit. It was concluded
that tomato fruit slices could be ripened to an acceptable level of
quality.
2.3.5 Low temperature In harvested products, temperature has a tremendous effect on the
rate of metabolic processes such as respiration. Brecht (1995) found
that metabolic reactions in fruit and vegetables are reduced 2 - 3
times for each 10 °C reduction in temperature. However, the Q10 (the
ratio of the respiration rates for a 10 °C interval) of various fresh-cut
products varies. Zucchini, tomatoes and kiwifruit have a Q10 of about
3.5, while bell peppers, muskmelon have values of about 8.3 (Watada
et al., 1996). Therefore it is crucial to reduce the temperature in order
to prolong shelf life (Paull, 1999).
Low temperatures are effective in minimising a number of adverse
effects of wounding (Watada et al., 1996), inhibiting respiration,
reducing water loss from plant tissue (Shewfelt, 1986), reducing
overall metabolic activity, inhibiting microbial growth (Brackett, 1987),
and reducing changes in texture, nutrition, aroma and flavour (Paull,
1999).
The optimum storage temperature for storage of tomatoes varies
according to the cultivar, and ripeness of the fruit (Agar et al., 1994;
Mallik and Bhattacharya, 1996). Jones (1999) proposed the optimum
storage temperature for ‘mature-green’ fruit as 12.7 to 15.5 °C for
several days without significant quality loss. At these temperatures,
chilling damage does not occur, but colour development is slow. An
34
optimum storage temperature for ripe fruit to prevent significant
quality loss is 7.2 to 10 °C with a relative humidity of 85 to 96%
(Jones, 1999). This implies that red fruit can tolerate storage at lower
temperatures than ‘mature-green’ fruit, as fruit at the advanced stage
(breaker, or turning stage) are less sensitive to chilling injury than
‘mature-green’ fruits (Madhavi and Salunkhe, 1998).
High quality can be assured by maintaining fresh-cut commodities at
lower temperatures, than those recommended for intact fruit and
vegetables (Senesi et al., 2000; Watada and Qi, 1999b). However,
optimal temperatures for fresh-cut product needs to be chosen
adequately in order to avoid damage caused by chilling injury.
Generally, the recommended storage temperature for fresh-cut
vegetables is in the range 0-8 °C (Ahvenainen, 1996; Varoquaux and
Wiley, 1994). Temperature of 0 °C is in most cases preferable,
however it is not economically achievable. Temperatures between 5
and 10 °C are more commonly found in practice (Verlinden and
Nicolai, 2000). Artes et al. (1999) observed significant differences in
quality attributes of fresh-cut tomato on the basis of temperatures.
They found that when compared to 10 °C, tomato slices kept at 2 °C
had better visual quality. Moreover, Gil et al. (2002) considered 5 °C
is the optimum storage temperature to prevent chilling injury and
promote maximum shelf life. In most experiments on tomato slices,
Hong and Gross (1998; 2000; 2001) used 5 °C for storage of fresh-cut
tomato slices.
2.3.6 Relative humidity The relative humidity during storage is another decisive environmental
factor as it directly affects water loss. Water loss causes weight loss
resulting in dehydration and deterioration of tomato fruit as well as
reducing their commercial value (Kays, 1991; Wills et al., 1998).
35
Water (weight) loss from a fruit primarily depends on the water vapour
pressure deficit between the fruit and the storage atmosphere and the
magnitude of resistance to water vapour movement between the fruit
and the air (Rajapakse, 2001; Wills et al., 1998).
Fresh-cut products have exposed internal tissues as well as a large
surface area without any skin. Thus, this product is highly
susceptible to water loss, particularly at higher temperatures where
the vapour pressure deficit is large (Watada et al., 1996; Watada and
Qi, 1999a). However, Watada and Qi (1999a) mentioned that low
relative humidity is not a problem in fresh-cut products, since they
are packaged in film bags or containers over-wrapped with film.
2.3.7 Chilling injury The quality and postharvest storage life of tomatoes can be limited by
chilling injury. Chilling injury refers to an irreversible physiological
disorder observed in plant tissue that results from the exposure of
chilling-sensitive plants or fruits to temperatures below some critical
threshold (Lyons and Breidenbach, 1987). Tomatoes, depending on
maturity, are highly sensitive to chilling injury at temperatures below
12 to 13 °C (Cote et al., 1993; Hobson, 1987). Izumi and Watada
(1995) suggested that for storage of some chilling sensitive
commodities, including tomatoes, 5 °C is preferred to 0 °C.
Raison and Orr (1990) indicated that chilling injury develops in two
stages, referred to as the primary and secondary events. The primary
events are initiated when the produce is stored below the critical
temperatures that cause metabolic dysfunction leading to internal
damage in the cells. The secondary events are a consequence of the
primary events and lead to cell death and visible symptoms (Raison
and Orr, 1990). Based on the observations of chilling-induced lipid
36
degradation in cucumber fruit, Parkin and Kuo (1989) showed that
lipid peroxidation may be associated with chilling injury.
A theory of the nature of chilling injury in plants was explained by
Lyons (1973) (Fig. 2.4). Chilling injury is considered to be a
consequence of bulk lipid-phase membrane transitions occurring at a
critical temperature that leads to a complete loss of permeability
control. Finally, an irreversible metabolic imbalance arises leading to
physiological dysfunction and tissue death (Lyons, 1973).
Figure 2.4 Schematic pathway of the events leading to chilling injury in sensitive plant tissue (Lyons, 1973).
Chilling causes cell membrane damage and increases solute or
electrolyte leakage (King and Ludford, 1983). The electrolyte leakage
is generally considered an indirect measure of plant cell membrane
damage (King and Ludford, 1983; Wang, 1989). The extent of chilling
injury can therefore be determined by measuring the electrical
conductivity of solutes that have leaked from tissues (Autio and
Bramlage, 1986; Murata and Tatsumi, 1979).
37
The disorders induced by chilling temperature usually become
apparent only upon returning the fruit to ambient temperature. The
main symptoms are uneven or partial ripening, fruit softening,
enhanced susceptibility to postharvest fungal pathogens, water
soaked areas and surface pitting (Hobson, 1987; King and Ludford,
1983). In tomatoes, Hobson (1987) used the extent of visible damage,
i.e. water-soaked areas and surface pitting, as an indicator of the
degree of chilling injury. Hong and Gross (2000) found that the
formation of water-soaked areas often occurs while tomato slices are
held in cold storage, and before removal of fruit to non-chilling
temperatures.
One distinct type of tomato chilling injury is the loss of aroma
compounds. Kader et al. (1978b) showed that chilling tomato fruit to
5 °C for one week with subsequent ripening at 20 °C reduced fruit
flavour. In other studies, Maul et al. (2000) demonstrated that
tomatoes stored at 2, 5, 10 or 12.5 °C had less aroma and ripe-
flavours as well as more off-flavours compared with fruit stored at
20 °C. Lurie and Klein (1992b) showed that chill-injured tomatoes
ripened abnormally or they lacked the ability to fully ripen. This was
associated with injury to membranes (Saltveit, 1989).
The most effective method for preventing chilling injury symptoms in
tomatoes is by avoiding exposure of fruit to temperatures below the
critical temperature (Salunkhe et al., 1991a). Madhavi and Salunkhe
(1998) suggested that holding tomatoes below 10 °C for more than 24
hours should be avoided because chilling injury might seriously affect
the market quality of the fruit. Hong and Gross (2000) demonstrated
that the accumulation of ethylene around tomato slices at the ‘light-
red’ maturity stage in containers inhibited the development of chilling
injury.
38
2.3.8 Summary The effects of preharvest (cultural practices and harvest maturity) and
postharvest (storage temperature, humidity, and chilling injury)
conditions have major impacts on the quality of intact tomatoes and
tomato slices. Selection of appropriate tomato cultivars for slicing is
an important breeding objective. Postharvest stress, including chilling
injury, often leads to products deterioration. Therefore by controlling
factors that have a deteriorative effect on the quality of tomato slices
during storage, it is possible to attain good quality tomato slices with
sufficient storage life. Further research should focus in manipulating
the preharvest and postharvest factors for optimal quality of tomato
slices.
2.4 Physiological changes during fruit ripening
2.4.1 Introduction
Ripening is the terminal phase in the development of fleshy fruits and
a process of highly coordinated synthetic and degradative reactions
(Kays, 1991; Rhodes, 1980). During senescence the balance in
dynamic processes shifts with the total degradative reactions
becoming greater than the total synthetic reactions (Watada et al.,
1990). During the ripening of the tomato, from the ‘mature-green’ to
the ‘fully-ripe’ state, the colour, flavour, aroma, texture of tomato fruit
changes dramatically (Grierson and Kader, 1986; Kinet and Peet,
1997), but ripening also initiates fruit senescence and deterioration
(Romani, 1984).
Ripening in tomatoes starts in the interior fruit tissue with gel
formation in the locule. It proceeds to the placenta tissue, columella,
radial pericarp, and eventually the outer pericarp (Brecht, 1987).
Davies and Hobson (1981) stated that tomatoes typically ripen from
39
the "inside-out" and internal colour development and tissue softening
precedes changes in external colour and firmness. Ripening also
progresses from the central columella region down to the blossom end
and then up to the stem-end. Figure 2.5 depicts changes in
metabolism and composition during tomato ripening.
Figure 2.5 Changes in metabolism and composition during ripening (Grierson and Kader, 1986).
The main physiological changes in tomato relate to ethylene
biosynthesis, respiration rates, colour change, fruit softening,
degradation of starch and changes in sugar composition, and tissue
permeability.
2.4.2 Ethylene biosynthesis and production Ethylene is a plant hormone that regulates many aspects of plant
growth, development and senescence (Abeles et al., 1992a; Yang and
Hoffman, 1984). Ethylene is synthesized in, and evolved from, cells of
all climacteric fruits during their growth and development (Abeles et
al., 1992a). It is commonly said that ethylene is “the ripening
40
hormone” (McKeon et al., 1995). It therefore plays an important
regulatory role in the postharvest physiology of horticultural
commodities. The efficiency of postharvest technology systems can be
improved by the ability to control ethylene synthesis and/or responses
to suit specific practical needs (Yang, 1985; Yang and Hoffman, 1984).
Ethylene is formed from methionine by the following steps:
S-adenosylmethionine is synthesised by the action of SAM synthase
on methionine (Fig. 2.6). ACC synthase then converts SAM to
1-aminocyclopropane-1-carboxylic acid (ACC), which is, in turn,
converted enzymatically to ethylene by the action of ACC oxidase
(Yang, 1987; Yang and Hoffman, 1984).
Enzymes catalyse each step in the ethylene biosynthesis pathway.
Two enzymes that are unique to this pathway are ACC synthase and
ACC oxidase (Fig. 2.6) (Imaseki, 1991; Yang and Hoffman, 1984).
Measurements by Su et al. (1984) indicate that ACC synthase activity
increases quite early during ripening of tomatoes, and Liu et al. (1985)
found that ACC oxidase activity is enhanced by exogenous ethylene in
preclimacteric tomatoes.
ACC synthase is the main control site for ethylene biosynthesis. ACC
synthase plays a major role during autocatalysis (positive feedback)
where ACC synthase is stimulated, as well as during auto-inhibition
(negative feedback) where ACC synthase is inhibited (Yang and
Hoffman, 1984). Cameron et al. (1979) have shown that the
application of ACC to various plant organs, including roots, stems,
leaves, inflorescences, and fruit, results in a marked increase in
ethylene production. ACC synthase is strongly inhibited by inhibitors
of pyridoxal phosphate-dependent enzymes such as AVG and AOA
(Yang, 1985).
41
Figure 2.6 Pathway of ethylene biosynthesis (Reid, 2002)
Ethylene moves by diffusion from its site of synthesis (Davies, 1995;
Reid, 1995). The stem-scar of tomatoes serves as the main avenue for
ethylene and CO2 exchange (Burg and Burg, 1965; De Vries et al.,
1995b). This was supported by De Vries et al. (1995a) who showed
that more than 90% of the total ethylene is released through the stem-
scar region after removal of the calyx.
Ethylene synthesis is stimulated in most tissues in response to stress.
In particular, it is synthesised in tissues undergoing senescence or
ripening (Davies, 1995; Picton et al., 1995; Reid, 1995). In addition,
various environmental stresses such as mechanical wounding
(bruising and cutting) (Abeles et al., 1992a) and chilling temperatures
(Wang, 1993) stimulate ethylene formation.
Methionine
ATP
PPi + Pi
SAM synthase
S-adenosylmethionine (SAM)
ACC synthase
1-aminocyclopropane-
O2
CO2 + HCN
ACC oxidase
Ethylene
42
Theologis et al. (1992) considered the function of ethylene as a
“coordinator” of ripening. They established that ethylene regulates
fruit ripening by coordinating the expression of many genes involved
in metabolic processes, such as increasing fruit respiration rates,
chlorophyll degradation, synthesis of carotenoids, conversion of starch
into sugars, and the activity of several enzymes involved in the
degradation of cell walls. Ethylene also stimulates its own production.
In studies with transgenic and mutant tomato lines with inhibited
ethylene biosynthesis or perception, Giovannoni (2001) showed that
the process of climacteric ripening represents a combination of
ethylene regulation and developmental control. He suggested that
both ethylene and additional developmental processes regulate fruit
ripening.
A simple scheme that depicts features of the mechanism of ethylene
action in plants is shown at Figure 2.7. It is believed that ethyene
binds to a molecule, the “receptor” at the cell membrane. At the
binding site an activated complex is formed which stimulates release
of a “second message” that migrates to the cell nucleus and in turn
causes synthesis of mRNA (messenger RNA). The new mRNA
molecules direct the synthesis of polypeptides that form enzymes that
cause ethylene-induced actions, including fruit ripening (Alexander
and Grierson, 2002). The physiological effects of ethylene can be
blocked by ethylene binding inhibitors such as 1-MCP, and this will
be discussed in section 2.6.4.
Sisler and Goren (1981) demonstrated that ethylene binds to a protein
receptor and that such binding is a prerequisite for the manifestation
of hormonal effects. Leshem (1992) stated that the total number of
ethylene binding sites gradually increases with time and peaks when
tissue is still relatively young and declines afterwards, whereas
maximal ethylene production is considerably later (Leshem, 1992).
43
Figure 2.7 Mechanism of ethylene action (Reid and Wu, 1991)
In climacteric fruit, ethylene production is generally very low until the
commencement of ripening. Internal ethylene concentrations during
fruit growth, up to the start of the climacteric respiration rate, are less
than 1µL L-1 (McMurchie et al., 1972). Tomato produces moderate
amounts of ethylene on a fresh weight basis, around 1.0 – 10.0
µL kg1 h-1 (Kader, 2002b) . Brecht (1987) reported that in ‘mature-
green’ fruit the ethylene production rate is 0.08 - 1.5 nL g-1 h-1.
McMurchie et al. (1972) referred to this basic level as ‘system 1’
production. System 1 is the basal low rate of ethylene production
present in preclimacteric fruits or before the onset of ripening
(McMurchie et al., 1972).
Ethylene synthesis begins to increase at the onset of ripening. This
takes place before any external colour change at the blossom-end of
green fruit becomes noticeable and precedes the synthesis of enzymes
44
such as polygalacturonase (Grierson and Tucker, 1983; Su et al.,
1984). According to Chalmers and Rowan (1971) the climacteric peak
in ethylene evolution occurs between the ‘mature-green’ and ‘pre-
breaker’ stages. Although, Lyons and Pratt (1964) reported that the
highest level of ethylene production in tomato fruit occurs at the ‘pink’
stage of ripening.
The rise in ethylene production is an autocatalytic process (i.e.
ethylene stimulates its own synthesis) which coincides with a rise in
climacteric respiration (Jeffery et al., 1984). Campbell et al. (1990)
reported that ethylene production reaches a peak of 12 - 15 nL g-1 h-1
on a fresh weight basis. The rate of ethylene production during
maturation and ripening correlates well with endogenous levels of
ACC, ACC synthase and ACC oxidase (Hoffman and Yang, 1980).
Hobson and Grierson (1993) introduced the concept of ‘system 2
evolution’ to represents the high rate of autocatalytic ethylene
production accompanying ripening in climacteric fruit (Oetiker and
Yang, 1995).
2.4.3 Respiration Fruit have been classified as climacteric or non-climacteric based on
their respiratory and ethylene production patterns during ripening
and their response to exogenous ethylene (Biale and Young, 1981;
Kader, 2002b). The tomato has been classified as a climacteric fruit
(Kinet and Peet, 1997). Early in development, the respiration rate is
high and decreases to a pre-climacteric minimum during maturation.
At the onset of ripening, respiration increases to a maximum, the
climacteric peak, before it subsequently declines slowly (Biale and
Young, 1981). This respiratory peak is preceded by or associated with
a rise in ethylene production (Kinet and Peet, 1997; Wills et al., 1998).
At the climacteric peak, respiration rate, measured as CO2 production,
45
is approximately double the pre-climacteric minimum and ranges
from 26 to 50 µL CO2 g-1 h-1 on a fresh weight basis (Andrews, 1995;
Campbell et al., 1990). At the pink-red stage, the climacteric process
of respiration reaches the maximum level (Salunkhe and Desai, 1984).
According to Meir et al. (1992) and Maharaj et al. (1999) respiration
rate is one of the most important indicators of senescence in tomato
fruit.
2.4.4 Loss of chlorophyll and synthesis of lycopene
The principal pigments that are responsible for the colour of tomatoes
are chlorophyll and carotenoids, especially lycopene (Arias et al.,
2000a; Hobson and Davies, 1971). Chlorophyll is the major pigment
in the early stages of tomato fruit development that imparts the green
colour. As fruit mature and ripen, the chlorophyll content decreases
because chloroplasts are converted to chromoplasts and additional
carotenoids are synthesised (D'Souza et al., 1992; Hobson and Davies,
1971). According to Fraser et al. (1994) and Mencarelli and Saltveit
(1988) the chlorophyll content is reduced by 90% by the time
tomatoes are red-ripe.
The intensity of the bright red colour of tomatoes is mainly due to the
presence of lycopene (Johjima and Matsuzoe, 1995; Stevens and Rick,
1986). Lycopene increases as tomato mature (Shi and Maguer, 2000).
Dumas et al. (2002) mentioned that at lycopene concentrations (on a
fresh weight basis) between 32 and 43 mg kg–1, fruit colour turns from
orange to red. Moreover, they state that lycopene accounts for 90% of
total carotene at red colouration (Dumas et al., 2002).
After harvest, colour development of tomatoes during ripening is
influenced by many factors including temperature (Grierson and
Kader, 1986). Shewfelt et al. (1988), Tijskens and Evelo (1994), and
46
Dumas et al. (2002) proposed a relationship between temperature and
the presence of lycopene. They indicated that at normal temperatures
(12 to 25 °C) chlorophyll degrades, while lycopene, and to a minor
extent, β-carotene is formed, resulting in red-coloured tomatoes. At
extreme temperatures such as temperatures below 12 °C, chlorophyll
does not degrade and lycopene does not accumulate. Prolonged
chilling also leads to loss in the ability to ripen even at normal
ripening temperatures, resulting in yellowish-green tomatoes. At high
temperatures, above 30 °C, chlorophyll degrades and β-carotene
accumulates, but the synthesis of lycopene is inhibited, resulting in
orange-yellow fruit.
2.4.5 Fruit softening Softening is generally associated with the ripening of fleshy fruits.
Flesh softening during fruit ripening is dependent on changes in the
chemical structure of the cell wall components (Kojima et al., 1991;
Tucker, 1993), protein, and the 3 carbohydrate fractions of pectin,
cellulose, and hemicellulose (John and Dey, 1986). Fruit softening is
characterised by increases in soluble pectins (John and Dey, 1986).
Pectin is a major component of the middle lamella, which binds
adjacent cells. A textural change as a result of solubilization of pectin
during ripening has been demonstrated in tomatoes (Gross and
Wallner, 1979; Hobson and Davies, 1971).
Fruit softening is stimulated by enzymatic and non-enzymatic
processes during ripening (Gross, 1990). The production of enzymes
involved in cell wall degradation is greatly accelerated during tomato
ripening (Poovaiah et al., 1988). Enzymes capable of degrading pectin
in fruits include pectinmethylesterase and polygalacturonase (Gross,
1990; John and Dey, 1986; Rigney and Wills, 1981).
47
Pectinmethylesterase catalyses the hydrolysis of methyl esters in
pectin molecules (Kays, 1991). Pectinmethylesterase activity in
tomato occurs throughout fruit development and ripening (Tucker et
al., 1982). Hobson and Grierson (1993) indicated that
pectinmethylesterase may be implicated in softening initiation. They
also stated that pectinmethylesterase and polygalacturonase occupy
different sites in the cell wall and middle lamella.
Polygalacturonase plays a role in the dissolution of the middle lamella
during ripening (John and Dey, 1986). Tucker and Grierson (1982)
reported that polygalacturonase activity was absent in ‘green’ tomato
fruit, but was associated more with softening in the later stages of
ripening (Gross, 1990). Tieman and Handa (1989) demonstrated that
polygalacturonase first appeared in the columella region (Figure 2.1)
followed by sequential appearance in the exopericarp and
endopericarp, respectively. These results suggest a regional
degradation of pectic substances in the fruits by polygalacturonase
(Kojima et al., 1991). However, the precise role of polygalacturonase
in softening of tomatoes is not yet clear. Smith et al. (1988) found
that if transgenic tomato plants were produced in which the level of
polygalacturonase synthesis in the fruit was substantially reduced
(about 90%), fruit softening was not significantly different from normal
non-transformed fruit.
2.4.6 Degradation of starch, and sugar changes Sugars originate from photosynthetic assimilates. Tomato fruit
accumulates carbohydrate prior to the onset of ripening in the form of
starch (Tucker, 1993). Madhavi and Salunkhe (1998) have noted that
tomato fruits accumulated low levels of starch in the immature stages.
Starch accumulation continues up to the ‘mature-green’ stage and
then rapidly decreases as ripening begin (Yu et al., 1967). Hobson
and Davies (1971) found that starch constitutes 0.10 - 0.15% in ripe
48
tomato fruit on a dry weight basis, and was hydrolysed during
ripening.
The breakdown of starch to glucose, fructose or sucrose is associated
with activities of α- and β-amylases and starch phosphorylase (Presis
and Levi, 1980; Steup, 1988). Generally, the sugar content of tomato
fruit is a function of the stage of maturity (Salunkhe et al., 1974).
According to Richardson and Hobson (1987) and Baldwin et al.
(1991a) sugar content increases progressively from the ‘mature-green’
or ‘breaker’ stages to reach the maximum at ‘turning’ or ‘red-ripe’
stages, but decreases once the fruit has begun to colour. Sugars in
the locular gel and the outer pericarp also increase during
development and maturation (Winsor et al., 1962a; Winsor et al.,
1962b).
2.4.7 Changes in cellular membranes Among the mechanisms associated with tomato fruit ripening,
changes in membrane structure play an important role. The cell
membrane system (i.e. plasma membrane, endoplasmic reticulum,
vacuolar membrane etc.) acts as selectively permeable barriers to the
movement of compounds within and between cells. Membrane
structure consists of fluid bilayers containing phospholipids and
proteins (Marangoni et al., 1996; Paliyath and Droillard, 1992;
Stanley, 1991). Generally, there is a decline in membrane
phospholipid content during ripening of tomato fruit (Bergevin et al.,
1993; Nguyen and Mazliak, 1990; Whitaker, 1991). Gucluu et al.
(1989) reported a correlation between changes in lipid composition,
particularly in the sterol-phospholipids ratio, and increased
membrane permeability.
Senescence is characterised by degradation of cell membranes and a
loss of membrane integrity and function, which in turn leads to loss of
49
tissue structure, alterations in cellular metabolism and ultimately
accelerated death (Nooden and Leopold, 1988; Paliyath and Droillard,
1992). One significant change in the membranes occurring with
senescence is the change in fluidity. Thompson et al. (1982) showed
that with the development of senescence, fluidity decreases and the
membranes becomes more rigid. This change can alter the activity of
enzymes that are associated with the membrane, and the function of
receptors on the membrane (Kays, 1991). Another physiological
change is increased membrane permeability, expressed as increasing
leakage of ions (Cote et al., 1993; Palma et al., 1995). With the
assumption that leakage is a result of an increase in membrane
permeability, solute leakage has been used as an indicator of tissue
senescence (Brady et al., 1970). This suggests that decreased fluidity
of membranes is translated into leakage of ions and therefore reduced
functionality of the membranes (Marangoni et al., 1996). Maintaining
cell membrane integrity and functionality is therefore considered as
the physiological basis for the preservation of fresh produce (Lee et al.,
1995).
2.4.8 Summary The ripening of climacteric fruits such as the tomato is stimulated by
ethylene. Ripening has been viewed as a genetically programmed
event involving the regulated expression of specific genes (Grierson,
1987). Dramatic physiological and biochemical changes occur in
tomato fruit during ripening, and those events are associated with
changes in ethylene production, respiratory and enzyme activity,
including cell wall and membrane-associated proteins. The capability
to control tomato ripening by modulating ethylene responses could
extend the storage life of tomatoes, and therefore ethylene production
response to tissue wounding will be reviewed in the next section.
50
2.5 Control of ripening
2.5.1 Introduction
Ethylene is involved in the regulation of many physiological responses.
Ethylene is often used in fruit and vegetable production to achieve
uniform ripening or to initiate ripening (Watada, 1986). The following
review elaborates on ethylene physiology, the role of the anti-ethylene
gas 1-MCP, and physical (heat) treatments for controlling tomato and
tomato slice ripening and quality.
2.5.2 Effect of ethylene on quality attributes The plant hormone ethylene is a gas that is involved in fruit ripening.
Ethylene is effective at parts-per-million (µL L–1) to parts-per-billion
(nL L–1) concentrations (Saltveit, 1999). In banana, Inaba and
Nakamura (1986) maintained that the effect of ethylene treatment
depends on the sensitivity of the fruit, which increases with age. Most
fruit, including tomatoes, become increasingly sensitive to ethylene
with time after anthesis (McGlasson et al., 1978).
The response of plants to ethylene can be beneficial or detrimental.
One of the potentially beneficial responses is induced ripening
(Salunkhe et al., 1991a). Lyons and Pratt (1964) showed exposure to
ethylene accelerates the natural ripening of unripe tomatoes with an
accompanying climacteric rise in CO2 and C2H4 production. The
development of ‘ripe’ colour with ethylene treatment was also observed
in other fruits such as in banana (Liu, 1976) and muskmelon (Bianco
and Pratt, 1977). Detrimental effects can be seen in the work of Risse
and Harton (1982) who showed that exposing watermelons to 5, 30, or
60 µL L–1 C2H4 reduced firmness and rind thickness, accelerated
deterioration, and reduced acceptability after 3 days at 18 °C.
Moreover, cucumbers treated with C2H4 developed unacceptable
51
textural attributes (Poenicke et al., 1977). These confirm that
ethylene has a detrimental effect on the texture of fruits and
vegetables by promoting unwanted softening (Saltveit, 1999).
Biochemical changes in tomato fruits have been divided into three
different classes by Jeffery et al. (1984):
1. Changes in metabolism of starch, sugars and organic acids:
independent of ethylene;
2. Loss of chlorophyll: enhanced by ethylene but does take place in its
absence;
3. Formation of lycopene, and increase in polygalacturonase and
invertase activities: dependent on ethylene for initiation and
continuation of the response.
2.5.3 Control of ethylene action Ethylene action can be manipulated using inhibitors of ethylene and
by removal of ethylene.
Inhibitors of ethylene. The most frequently used inhibitors of ethylene
production in horticulture are AVG (aminoethoxyvinylglycine) and
AOA (aminooxyacetic acid). Both are effective in inhibiting the activity
of ACC synthase (Owens et al., 1971; Yang and Hoffman, 1984).
Baker et al. (1978) found that AVG inhibited ethylene production more
completely in ‘green’ than in ‘pink’ tomato fruit tissues. For example,
ethylene production was inhibited by 69% in ‘mature-green’ tomato
slices, but only by 11 and 13% in climacteric (‘pink’) and post-
climacteric (‘red’) fruit, respectively. Baker et al. (1978) suggested that
this difference in sensitivity to AVG might be explained by increased
competition for methionine by ethylene formation in the riper tissue.
Yang and Hoffman (1984) suggested that because AVG and AOA do
not inhibit the conversion of ACC to ethylene, the level of ACC already
present in the tissues limits their effectiveness.
52
The ethylene inhibitor that has been reported to bind to ethylene
receptors is silver nitrate (AgNO3). However, a problem with using Ag+
is toxicity (Abeles et al., 1992). Another potentially safer inhibitor of
ethylene perception is 1-MCP, which is described below.
Removal of ethylene. The simplest strategy to protect sensitive
produce from exogenous ethylene is to remove ethylene gas from the
environment. Ethylene can be removed from storage areas by
constant ventilation with fresh air (Reid, 2002). Chemically, ethylene
can be removed with compounds that trap or convert it to inactive
products. One such absorbent compound is potassium permanganate
(KMnO4), sold commercially as Purafil® (Reid, 2002).
Potassium permanganate has the ability to oxidise ethylene to form
CO2 and H2O. In this oxidation process, the colour changes from
purple to brown as MnO4- is reduced to MnO2 (Reid, 2002). The use of
KMnO4 absorbents in polyethylene bags has extended the storage life
of bananas (Scott et al., 1970) and avocados (Hatton and Reeder,
1972). However, although the use of KMnO4 is convenient, there are
toxicity problems associated with its safe handling and disposal
(Abeles et al., 1992a).
Ethylene removal will delay fruit softening. Abe and Watada (1991)
maintained that removal of C2H4 from the storage environment of
lightly processed fruit and vegetables can retard tissue softening. Abe
and Watada (1991) used an ethylene absorbent (charcoal with
palladium chloride) to prevent the accumulation of ethylene and found
the absorbent was effective in reducing the rate of softening in peeled
and sliced kiwifruit, and sectioned bananas. Studies by Risse and
Miller (1983) showed that removal of ethylene from the storage
atmosphere increased the number of days for tomatoes to reach full
red colour and they were firmer than tomatoes stored without
ethylene removal. Knee et al. (1985) showed that the use of potassium
53
permanganate can result in reduced softening and /or rotting in
apples, avocado, banana, kiwifruit, lemon and mango.
2.5.4 1-methylcyclopropene The plant growth regulator 1-methylcyclopropene (1-MCP) has been
reported to have inhibitory effects on ethylene action in various
horticultural products such as banana (Jiang et al., 1999a; Macnish
et al., 1998), apple (Fan et al., 1999), broccoli (Fan and Mattheis,
2000b) and lettuce (Fan and Mattheis, 2000a). Application of 1-MCP
has been reported also to fresh-cut products such as apple (Jiang and
Joyce, 2002) and pineapple (Budu and Joyce, 2003).
1-MCP is a gas that has been used on many horticultural products to
delay ripening and senescence (Watkins and Miller, 2003). 1-MCP is
an effective inhibitor of ethylene action at low concentrations (Sisler
and Serek, 1997). Figure 2.8 shows the chemical structure of 1-MCP.
Since 1-MCP is a non-toxic gas and leaves low measurable residues in
food commodities (Watkins, 2002), it can be used as a tool to study its
utility for extending the storage life and quality of plant products
(Blankenship and Dole, 2003).
Figure 2.8 Chemical structure of 1-MCP (Prange and DeLong, 2003)
Generally, 1-MCP has been applied at temperatures ranging from 20 –
25 °C (Blankenship and Dole, 2003; Watkins, 2002). Ku and Wills
(1999) showed that the application of 1-MCP produced better results
54
at 20 °C than at 5 °C. Similarly, 20 °C was best for application of 1-
MCP to broccoli (Able et al., 2002). Blankenship and Dole (2003)
hypothesised that lower temperature might lower the affinity of the
binding site for 1-MCP.
Most literature shows that treatment duration considered sufficient to
achieve a full response to 1-MCP is 12 to 24 hours (Blankenship and
Dole, 2003; Watkins, 2002). Higher concentrations of 1-MCP are
required for shorter treatment times (Ku and Wills, 1999; Wills and
Ku, 2002).
1-MCP is thought to act by binding “for a long time” to the ethylene-
receptor (Sisler and Serek, 1997) so that ethylene cannot elicit
subsequent signal transduction and translation (Sisler and Serek,
1997). The recent theory that 1-MCP may act to block ethylene action
was modelled by Binder and Bleecker (2003). They proposed that 1-
MCP suppresses the ethylene response pathway by permanently
binding to a sufficient number of ethylene receptors (ETR1, ETR2,
EIN4, ERS1 and/or ERS2) that keeps CTR1 in its active (inhibiting)
state (Fig. 2.9).
Figure 2.9 Model of mode of action of 1-MCP proposed by Binder and Bleecker (2003)
Generally, the ripening processes of most climacteric fruit, such as
softening and loss of titratable acidity, are usually delayed or inhibited
by 1-MCP application. 1-MCP delayed softening in intact and cut
apples (Jiang and Joyce, 2002). Hofman et al. (2001) showed that 1-
55
MCP delayed softening in avocado by 4.4 days, custard apple by 3.4
days, and mango by 5.1 days. Titratable acidity loss was inhibited in
tomatoes (Wills and Ku, 2002) and carrots (Fan and Mattheis, 2000a)
by 1-MCP. Application of 1-MCP produced higher soluble solids in
apples (Fan and Mattheis, 1999a), papaya (Hofman et al., 2001) and
pineapples (Selvarajah et al., 2001).
Rohwer and Gladon (2001) demonstrated that tomatoes treated with
1-MCP at the ‘breaker’ and ‘turning’ stages for 8 hours at 20 °C did
not develop any red colour over the 10-day holding period. They
reported that the most desirable delay in ripening was obtained by
treating fruits at the ‘pink’ stage with 0.058 µL L-1 (ca. 3 day delay) or
fruits in the ‘light-red’ stage with 580 nL L-1 1-MCP (ca. 4 day delay).
Mir et al. (1999) applied 1-MCP to tomatoes at 22 °C and reported an
extension of shelf life at all stages of maturity, due to reductions in the
rate of red colour development and loss in firmness.
Postharvest application of 1-MCP is an efficient method for delaying
the ripening of ‘green’ tomatoes and delaying the senescence of ripe
tomatoes. Wills and Ku (2002) reported that exposure of ‘green’
tomatoes to 5 µL L–1 1-MCP for 1 hour resulted in a 70% increase in
time to ripen. They also reported that exposure of ripe tomatoes to 20
µL L–1 for 2 hours resulted in 25% increase in postharvest life, based
on fruit appearance. Furthermore, Moretti et al. (2001) reported that
tomato fruit treated with 1-MCP at 1 µL L–1 for 22 hours at 22 °C were
about 88% firmer than the control fruit, and had a 38% lower a*/b*
ratio (more green colour), than control fruit at the end 17 days
storage.
Little literature exists on the use of 1-MCP on fresh-cut fruit products.
Fresh-cut apples treated with 1-MCP at concentrations of 1 or 10 µL
L–1 for 6 h at 20 °C showed reduced respiration and ethylene
56
production rates and delayed softening (better firmness maintenance)
and colour changes (Jiang and Joyce, 2002). These authors
mentioned that compared with untreated tissue, quality was better
maintained at 4 °C in apple slices treated with 1-MCP before or after
the fruit were processed. In fresh-cut pineapple fruit, Budu and Joyce
(2003) showed that the application of 1-MCP at higher concentrations
(up to 5 µL L–1) reduced the respiration rate and maintained the colour
of the cut pineapple pieces.
Limited information also exists on the efficacy of 1-MCP on fresh-cut
vegetable products. Ku and Wills (1999) showed that broccoli florets
held at low temperature (5 °C) after treatment with 1-MCP from 0.02
to 50 µL L–1 at either 20 or 5 °C had a longer storage life than control
florets. Their data showed that low concentrations of 1-MCP strongly
inhibited the loss of green colour of broccoli florets. The storage life of
shredded lettuce was extended by application of 1-MCP (Wills et al.,
2002), with the optimal treatment being 0.1 µL L–1 for 1 h at 5 °C
which resulted in an extension in storage life by 50% over untreated
lettuce.
2.5.5 Heat treatment
There has been increasing interest in the postharvest heat treatment
(thermotherapy) of vegetables and fruit to control insects pests,
prevent fungal decay, and to modify the ripening of commodities
(Lurie, 1998; McDonald et al., 1999). This is primarily because heat
treatment substitutes as a non-damaging physical treatment. It is a
non-carcinogenic, non-polluting, non-damaging treatment for
prevention of chilling injury and maintenance of fruit and vegetable
quality (Holton, 1990).
57
Several researchers, including Key et al. (1981) and Lurie (1998),
argue that exposure of plant tissues to thermal stress results in the
rapid induction of a small set of specific proteins called heat shock
proteins (HSPs). These proteins are produced within 30 minutes of
exposure to temperatures in the range 34 to 42 °C (Kanabus et al.,
1984). Saltveit (2000) reported that a brief heat shock interferes with
‘normal’ protein synthesis by preferentially inducing the synthesis of a
unique set of stress proteins.
Several authors have obtained prolonged postharvest life using heat
treatments. These include Teitel (1989) with melons, Barrancos et al.
(2003) with apples, and Miller et al. (1991) with mangoes. By
contrast, other authors such as Kerbel et al. (1987) and Chun et al.
(1988) on avocados and grapefruits respectively, have reported
negative effects on fruit quality after the application of heat
treatments. Generally, the main problem in using heat treatment is
the increased weight loss arising from the use of relatively high
temperatures, and the damage related to over-heating, such as
wrinkling or pitting of the fruit skin (Klein and Lurie, 1991; Lurie,
1998).
Methods for heat treatment of harvested fresh fruit and vegetables
include hot water, vapour heat and hot air (Lurie, 1998). Depending
upon exposure duration, if the temperature is too high (≥ 45 °C) or if
the treatment too prolonged, it can be lethal for the fruit (Paull, 1990;
Paull and Chen, 2000). Brief hot water treatments heat only the
surface cells of the product, whereas heated air treatments applied for
several hours increase the pulp temperature significantly (Salunkhe et
al., 1991a).
Tomatoes have been reported to tolerate different exposures to heat
without injury (Lurie and Klein, 1992b). Lurie and Klein (1991) found
that ‘mature-green’ tomatoes held 3 days at 36-40 °C before chilling at
58
2 °C did not develop chilling injury (whereas unheated tomatoes did)
and quality attributes were maintained for 4 weeks.
High temperatures are known to inhibit ethylene production in
tomatoes. Hot air treatment of 35 - 40 °C inhibits ethylene synthesis
within hours (Biggs et al., 1988). These authors correlated this
inhibition with a rapid decline in 1-aminocyclopropane-1-carboxylic
acid synthase activity (Figure 2.6). Yu et al. (1980) showed that heat
treatment blocks, at least transiently, the conversion of ACC to
ethylene. Atta-Aly (1992a) found that during incubation of tomato at
temperatures between 20 and 30 °C, inhibition of ethylene
biosynthesis was attributed to the reduction in ACC synthesis,
whereas at 35 °C, both ACC synthesis and its conversion to ethylene
were inhibited. Some researchers (Klein and Lurie, 1990; Paull and
Chen, 1990) reported a rapid loss (75%) of ACC oxidase in papaya and
other fruits exposed for short periods to temperature greater than
40 °C.
Fruit firmness is also affected by heat treatment. Biggs et al. (1988)
showed that tomatoes softened more slowly when held continuously at
temperatures between 30 °C and 40 °C than at 20 °C. Similarly,
Hinton and Pressey (1980) demonstrated that in the range 35 - 60 °C,
87% loss of activity of purified glucanase from tomato fruit can occur.
This was confirmed by Pressey (1983) who found that 50% glucanase
activity is lost by exposure to 50 °C for 5 minute. Kim et al. (1993a)
showed that heat treatment of whole apples improved apple slice
firmness. In a related experiment, Kim et al. (1994) demonstrated that
slices prepared from heat-treated apples showed increased firmness
during storage for up to 7 days for ‘Golden Delicious’ (firmness 34%
higher than on day zero of storage) and up to 14 days for ‘Delicious’
apple (48% higher firmness than at the beginning of storage).
59
Heating enhances colour development. Lurie and Klein (1992b)
showed that heated (38 °C for 3 days) tomatoes were redder, but not
softer, than non-heated fruit after storage at 12 °C and shelf life at
20 °C. Lycopene synthesis, which is responsible for the development
of red colour, was inhibited in fruits held at temperatures above 30 °C
(Chang et al., 1977). They also showed that when returned to 20 °C,
the inhibition of ripening was reversed and the fruits ripened
normally, although more slowly than fruits kept continually at 20 °C.
Generally, there are variable results on the effect of heat treatments
on soluble solids. This has been shown by several researchers (Lurie
and Klein, 1991; Lurie and Sabehat, 1997; McDonald et al., 1999)
who demonstrated that postharvest heat treatments, either water or
hot air (38 - 48 °C for 1 hour to 3 days), have no effect on tomato
soluble solids or titratable acidity. In addition, Lurie and Klein
(1992b) found that titratable acidity was the same between heated and
non-heated tomatoes, but in this work soluble solids contents
remained higher in heated fruit. They also showed that heating
induced higher sugar/acid ratio by 10 – 30%, which makes it
attractive for consumers. Paull and Chen (2000) maintained that the
variable effects on soluble solids and titratable acidity depends upon
the temperature used and duration.
Most research on the effects of hot-air treatments on tomatoes has
focussed on intact tomatoes at ‘mature-green’ maturity rather than on
‘pink’ or ‘red’ maturity (Klein and Lurie, 1991; Lurie and Klein, 1992b;
Saltveit and Cabrera, 1987). Postharvest heat treatments applied to
fresh-cut commodities have therefore targeted mainly improvement of
post-processing quality rather than shelf life extension such as in
fresh-cut apples by Kim et al. (1994) and Barrancos et al. (2003), in
lettuce by Loaiza-Velarde and Saltveit (2001), in fresh-cut celery
petioles by Loaiza-Velarde et al. (2003), and in fresh-cut cantaloupe by
60
Luna-Guzman et al. (1999). Less information is available on the
effects of high temperatures on tomato slices.
2.5.6 Summary
Control of ripening of tomatoes through the management of ethylene
physiology and its exposure to tomatoes is a vital component in the
postharvest physiology of tomato slices. In order to retard or prevent
quality loss in fresh-cut tomato slices, various possible external
treatments have been described. The inhibitor of endogenous
ethylene action (1-MCP) could be used as a tool to unravel the effects
of ethylene on postharvest quality of tomato slices. Heat treatment is
a promising alternative to chemical treatments, however several
aspects of the application of this technique to fresh-cut tomato slices
requires further research.
2.6 Physiological changes after slicing
2.6.1 Introduction
The physiological changes in fresh-cut products are concerned with
wounded tissue (Brecht, 1995) as the tissue integrity of these
products has been altered during slicing (Rolle and Chism, 1987;
Shewfelt, 1986). Any opening of tissue generally leads to metabolic
changes, so wounding elicits changes in fresh-cut products (Fig. 2.10).
Wounding stress results in metabolic activation including increased
respiration and ethylene production, membrane deterioration and in
some cases, induction of wound-healing processes and enhanced
water loss. Other consequences of wounding include flavour changes
and softening (Brecht, 1995; Saltveit, 1997). Wounding also provides
a suitable media for the growth of micro-organisms with visible
61
disease symptoms while the increased respiratory activity contributes
to a loss of nutritional value (Reyes and Gould, 1996).
Figure 2.10 The interrelationship among the many effects of slicing on the physiological process in fresh-cut products (Saltveit, 1997).
2.6.2 Wound-ethylene synthesis Ethylene production is stimulated when plant tissues are subjected to
stress, such as from wounding. This ethylene has been named wound
ethylene or ‘stress ethylene’ (McGlasson and Pratt, 1964; Yang and
Hoffman, 1984). The stimulation of wound ethylene production can
occur over a short time frame, often within 1 hour of wounding, with
peak rates usually within 6 to 12 hours (Abeles et al., 1992a; Yang
and Hoffman, 1984; Yang and Pratt, 1978). Wounding increases the
activity of 1-aminocyclopropane-1-carboxylic acid (ACC) synthase and
results in the increased production of ACC, which can be oxidised to
ethylene (Boller and Kende, 1980; Yu and Yang, 1980). Mattoo and
Anderson (1984) and Abeles et al. (1992a) stated that membrane-
associated ACC may interact with membrane-bound ethylene-forming
enzymes (ACC oxidase). During this interaction, ACC is produced in
Slicing
Respiration
Reduced carbohydrates
& flavour changes
Ethylene
Ripening
Softening
Membrane deterioration
Loss of normal cell
function
Nutritional changes Water loss
Wound healing
Suberin
Pathological invasion
62
close proximity to the ACC oxidase that then uses the ACC to produce
ethylene.
As a consequence of cutting a tomato, large increases in ethylene
production have been observed by several researchers (Artes et al.,
1999; Brecht, 1995; Lee et al., 1970; Mencarelli et al., 1989).
Mencarelli et al. (1989) reported that slicing ‘breaker’ maturity tomato
fruit increased ethylene production 3-4 fold and increased the rate of
ripening compared with intact fruit. Slicing also caused increased
ethylene production in tomato disks (Lee et al., 1970). After 15 - 20
minutes of cutting the tomato into small disks, ethylene production
was approximately 20-fold higher than for the intact fruit (Lee et al.,
1970). Disks taken from different portions of the fruit have different
levels of ethylene production (Brecht, 1995). Within an individual
‘mature-green’ tomato fruit, tissue excised from the distal (blossom)
end entered the climacteric phase before the tissue taken from the
equator or proximal (stem-end) regions. Excised tissue from all
regions produced much more ethylene than intact fruit during the
climacteric phase (Brecht, 1995). Artes et al. (1999) showed that
ethylene production by tomato slices was higher than for whole fruit
at 10 °C from the first hour after slicing and up to 7 days in storage.
From the above studies, it has been shown that the level of ethylene
from tomato slices increases in proportion to the amount of wounding.
Ethylene resulting from wounding induces ripening process and
advances the onset of climacteric ethylene production (Saltveit, 1997).
Elevated ethylene after slicing in other fruits has been reported.
Varoquaux et al. (1990) reported that ethylene production from
kiwifruit slices decreased for 2 hour at 20 °C, then 2 - 4 hours later,
increased sharply, peaking at 7 times the rate of intact fruit, and
decreasing slightly or remaining constant after about 10 hours.
Watada et al. (1990) also showed ethylene production rate 16-fold
higher in sliced kiwifruit than in intact fruit. The continual increase
63
in ethylene production resulting from the slicing process was probably
due to the stimulation by endogenous ethylene. It was concluded that
ethylene production rates were proportional to the injured surface
area, and hence to the intensity of the stress (Watada et al., 1990).
McGlasson and Pratt (1964) showed a ten-fold increase in ethylene
production by cantaloupe flesh tissue slices compared with intact
fruit. Slices from nearly mature fruits showed a climacteric pattern,
and respiratory increase could be induced in slices treated with
ethylene.
2.6.3 Elevated respiration As a consequence of wounding in plant tissue respiration rate is
stimulated (Kays, 1991), but the initiation of this response is delayed
compared to that found for wound-induced ethylene (Brecht, 1995).
Asahi (1978) proposed that the increase in respiration is due to
enhanced aerobic mitochondrial respiration. Toivonen and DeEll
(2002) explained that increases in respiration is partially due to
removal of physical barriers (i.e. periderm or cuticle) to gas exchange
in the tissues. Another explanation for the rise in respiration was
stated by Laties et al. (1972). He demonstrated that in cut potatoes,
the rise in respiration after cutting or wounding is at least partially a
result of α-oxidation of long-chain fatty acids.
Increased respiration results in physiological changes. Saltveit (1997)
stated that enhanced respiration, coupled with decreased gas
diffusion due to liquid on the surface, can elevate carbon dioxide and
deplete oxygen to levels that stimulate anaerobic respiration. For
instance, shredded carrots are more susceptible to developing
anaerobic metabolism than whole carrots (Rolle and Chism, 1987).
Elevated respiration also causes a rapid decrease in stored reserves
(Saltveit, 1997). Laties (1978) demonstrated that starch breakdown is
64
enhanced, and both the tricarboxylic acid cycle and electron transport
chain are activated.
Increases in respiration as a consequence of cutting may be quite
substantial. Varoquaux and Wiley (1994) reported that the
respiration of fresh slices is, in most cases, 3 to 7 times that of the
intact organ, e.g. 4 to 7 times for grated carrots (Varoquaux and Wiley,
1994) and double for sliced and peeled ripe kiwifruit compared to
whole fruit (Brecht, 1995). Sliced strawberries and pears consumed
O2 at higher rates than whole fruit throughout 7 days of storage at
2.5 °C, and after transfer to 20 °C for 1 day (Rosen and Kader, 1989).
Short shelf life of the product is associated with higher respiration
rates (Kader, 1987; Rolle and Chism, 1987). Therefore, Watkins
(2002) suggested that keeping respiration rates at minimal levels is
desirable for maintaining quality.
Fresh-cut products generally have higher respiration rates than
corresponding intact products (Watada et al., 1996). Slicing of
‘mature-green’ tomatoes results in increased respiration by up to 40%
when stored at 8 °C, compared to the intact product (Mencarelli et al.,
1989). Watada et al. (1996) reported that respiration rates of fresh-
cut fruit increased with temperature. In the 0 to 10 °C storage
temperature range, the Q10 for tomato slices (7.1) was higher than
that for the whole product (2.9). Moreover, the Q10 was lower in the
10 - 20 °C temperature range (3.5) than in the 0 - 10 °C range (7.1).
Artes et al. (1999) reported that the respiration rates of fresh-cut
tomatoes increased significantly after 2 days at 10 °C. Cantwell
(1997) stated that higher respiration rates indicate a more active
metabolism and usually a faster deterioration rate. Higher respiration
rate can also result in more rapid loss of acids, sugars and other
components that determine flavour quality (Cantwell, 1997).
65
2.6.4 Membrane deterioration
Tomato tissue destined for fresh consumption begins to irreversibly
deteriorate following slicing. The process of cutting tissue into slices
damages the cells and cell membranes at the cutting surfaces
(Saltveit, 1997). Figure 2.11 depicts the events of membrane
deterioration in plant tissue.
Figure 2.11 Mechanism for membrane deterioration in plant tissue due to senescence or postharvest stress (Marangoni et al., 1996)
Wounding of plant tissue during the preparation of fresh-cut products
may cause membrane lipid degradation (Deschene et al., 1991; Rolle
and Chism, 1987). The ethylene produced by membrane systems may
play a role in this process by increasing the permeability of
membranes and reducing phospholipid biosynthesis (Watada et al.,
1990). Sheng et al. (2000) demonstrated in tomatoes that ethylene
production is a consequence of the metabolism of free fatty acids by
lipoxygenase, suggesting that the wound-induced membrane
66
breakdown may be directly associated with wound-induced ethylene
production.
Membrane deterioration results in plant tissue becoming vulnerable to
discolouration (Brecht, 1995; Watada and Qi, 1999b),
decompartmentation of cellular structure and organisation and loss of
normal cellular function (Brecht, 1995; Rolle and Chism, 1987). Rolle
and Chism (1987) stated that to maintain the quality of fresh-cut
products, membrane integrity must be maintained and the onset of
senescence must be delayed.
2.6.5 Wound healing Plant tissue can sometimes naturally seal the site of injury, such as in
potato and sweet potato (Brecht, 1995). The phrase wound healing is
used in some instances to refer to formation of suberin, callus, and
lignin production and deposition of cell walls at the wound site to form
a wound periderm (Burton, 1982; Kays, 1991). Suberisation of the
cell layers occurs in many tissues such as in cucumber pericarp and
in potato (Walter et al., 1990). To date, there is no information about
this process in tomato slices.
2.6.6 Water loss
Epidermis or skin is a very important barrier to water loss that would
otherwise lead to loss of turgor and desiccation. Fresh-cut fruit can
have large surfaces without any skin. In addition, processing leads to
an increase in the surface area/volume ratio. Therefore, fresh-cut
products tend to be more vulnerable to water loss (Brecht, 1995;
Garcia and Barrett, 2002).
67
Burton (1982) reported the differences in rate of water loss between
intact and wounded plant surfaces as varying 5 - 10-fold for organs
with lightly suberised surfaces such as carrots and parsnips, to
10 - 100-fold for organs with cuticularised surfaces such as spinach
leaf, bean pods, and cucumber fruit, and to as much as 500-fold for
heavily suberised potato tubers (Burton, 1982). Reduction of water
loss can be achieved by appropriate handling techniques including
lowering temperature and/or increasing the vapour pressure of the
surrounding air using packaging (Garcia and Barrett, 2002).
2.6.7 Susceptibility to micro-organisms Fresh-cut products provide an ideal media for the growth of micro-
organisms, so sanitation is essential to keep the microbial population
to a minimum (Watada and Qi, 1999a). Bolin et al. (1977) showed
that storage life is shorter with higher initial microbial loads. The
primary pathogen microorganisms living on fresh-cut products are
mesophilic microflora, lactic acid bacteria, fecal coliforms, yeasts and
molds, and pectinolytic microflora (Nguyen-the and Carline, 1994).
The sources of contamination in these products include the raw
material, the plant workers, the slicer, and the processing room
(Heard, 2002; Verlinden and Nicolai, 2000).
To reduce the contamination by microorganisms in fresh-cut
products, the whole and fresh-cut produce are generally washed by
50-200 µL L–1 chlorine solution (Watada and Qi, 1999a). After
pathogens have infected the products however, chlorination is not
very effective (Sawyer, 1978). If washing is conducted properly,
microbial populations can be lowered. In most studies on fresh-cut
tomato slices (Artes et al., 1999; Gil et al., 2002) the researchers used
sodium hypochlorite at 100 µL L–1. Although chlorination has no
residual effect (Sawyer, 1978), fresh produce exposed to pathogens
after treatment remains susceptible to re-infection (Hong and Gross,
68
1998). Another method to prevent microorganisms proliferation is low
temperature storage (Heard, 2002). However, storage of produce at
low temperatures does not eradicate microorganisms, it generally
reduces the growth rates of spoilage organisms and food-borne
pathogens (Heard, 2002; Verlinden and Nicolai, 2000).
Surface sterilisation of tomato fruit for sanitation purposes with
sodium hypochlorite has been found to alter firmness of tomato slices.
Hong and Gross (1998) showed that pericarp firmness of tomato slices
from fruit that had been treated with 1.05% sodium hypochlorite
(10500 µL L–1) for 60 seconds was less than one-half the firmness of
water-treated controls and lower than fruit pericarps from a 0.26 %
(2600 µL L–1) treatment. Artes et al. (1999) observed Rhizopus spp
colonies after 6 days storage of tomato slices at 10 °C. Hong et al.
(2000) observed the fungus Rhizopus stolonifer in slices from ‘red’
tomatoes grown using black polyethylene mulch with nofungicide
treatment after 12 days storage at 5 °C, and no fungi were detected in
tomatoes grown using black polyethylene with weekly fungicide
application. Blue mold rot, caused by Penicillium species, was
observed by Hong and Gross (2001) on fresh-cut tomato slices at the
end of 20 days storage at 5 °C. These results suggest that prevention
of microbial growth is an essential component in the storage of tomato
slices.
2.6.8 Loss of firmness In general, slicing fruit tissue results in loss of firmness (Varoquaux
and Wiley, 1994). The changes in texture of sliced fruits and
vegetables are either directly or indirectly affected by ethylene (Kader,
1985). Rosen and Kader (1989) have shown that wound-induced
ethylene production is associated with an increased rate of softening
in pear and strawberry slices.
69
The loss of firmness after slicing has been reported for apple slices
(Ponting et al., 1972), kiwifruit (Varoquaux et al., 1990) and kiwifruit
packed with banana sections (Watada et al., 1990). Varoquaux et al.
(1990) reported that kiwifruit slices lose 50% of their initial firmness
in less than 2 days at 2 °C. It was stated by Gross (1990) that
textural breakdown of kiwifruit slices during storage was due to
enzymatic hydrolysis of cell wall components. These results were
confirmed by Watada et al. (1990) who noted that the average firmness
of 1 cm-thick kiwifruit slices decreased by about 25% after 24 h and
by 40% after 48 h at 20 °C. They also reported that exposure of slices
to 2 or 20 µL L–1 ethylene accelerated the loss of firmness.
2.6.9 Flavour changes Flavour changes or flavour loss during and after processing are
especially of concern in fresh-cut products. Biochemical changes due
to wounding can affect flavour quality (John and Baldwin, 2002).
Fresh produce that exhibits high respiration due to wounding may
catabolize sugars or acids as a carbon source during storage (John
and Baldwin, 2002; Saltveit, 1997). According to Huxsoll et al. (1989),
flavour change may result from the loss of flavour compounds or from
the accumulation of compounds that produce off-flavours.
2.6.10 Nutritional changes Fresh-cut processing influences nutritional quality of produce (Klein,
1987). Mozafar (1994) and Lee and Kader (2000) maintained that if
vegetables are severely cut or shredded, as in the case of cabbage,
lettuce, carrots, and other vegetables sold as salad mixes, loss in
vitamin C or decrease in ascorbic acid content occurs. Cellular
disruption in fresh-cut vegetables and fruit increases enzymatic
activity by allowing substrate and enzymes to come in contact, and
results in rapid breakdown of vitamin C (Klein, 1987). Therefore,
70
Klein (1987) proposed that as long as cellular integrity remains,
vitamin content is not markedly changed. Mechanism of ascorbic acid
degradation is depicted in Figure 2.12.
Figure 2.12 Degradation of ascorbic acid (Klein, 1987)
The nutritional quality of fresh-cut products, including fresh-cut
tomato slices, changes as fruit maturity progresses. Kader et al.
(1977) reported that tomato fruit harvested green and ripened at 20 °C
to table-ripeness contained less ascorbic acid than those harvested at
the table-ripe stage. Betancourt et al. (1977) also demonstrated that
tomato fruit analyzed at the ‘breaker’ stage contained only 69% of its
potential ascorbic acid concentration if ripened on the vine compared
to table ripe-tomatoes. These results indicate the significant role of
maturity on the nutritional concentrations in products that are
subsequently processed. In kiwifruit, Agar et al. (1999) reported that
kiwifruit slices stored at 5 and 10 °C exhibited a gradual decrease in
ascorbic acid and an increase in L-dehydroascorbic acid. The total
71
vitamin C was 8%, 13%, or 21% lower than initial values in slices kept
for 6 days at 0 °C, 5 °C, or 10 °C, respectively.
Current information on lycopene bioavailability and stability of
lycopene in fresh-cut products is very limited. Barringer (2004)
suggested that the main cause of lycopene degradation is oxidation,
including oxidation during processing, but lycopene may also be
destroyed in processed tomato products by heating (Shi and Maguer,
2000). Fish and Davis (2003) evaluated the rate of deterioration of
lycopene in cut watermelon tissue during frozen storage. They found
that a small percentage of lycopene (4 - 6%), degraded during an
initial freeze-thaw, and then a loss of 30 - 40% lycopene occurred over
12 months storage at –20 °C and a loss of 5 - 10% over the same
period at –80 °C. Lycopene was slightly more stable in pureed
compared with diced watermelon tisue stored at –20 °C. Tavares and
Rodriguez-Amaya (1994) reported that the lycopene content in
concentrated tomato products is generally lower than expected,
because of losses during tomato processing.
2.6.11 Summary
Physiological changes in fresh-cut products are related to wounding of
tissue. The slicing of tomatoes therefore causes reactions that lead to
quality deterioration. It is obvious that slicing results in a significant
effect on physiological activities including flavour changes, softening,
nutritional changes, and pathological attack. Control of the wound
response is the key to providing a fresh-cut product with good quality
(Cantwell, 1997). It has been shown that low temperature storage is
an important component for reducing the wounding responses in
fresh-cut products. It is also evident from this review that research on
tomato slices needs to concentrate on the physiological and
biochemical changes during storage and how these affect the quality
72
of tomato slice products. There is therefore, a challenge to control all
of these deteriorative factors.
2.7 General Summary
2.7.1 Summary Fresh-cut products are new forms of processed fruit and vegetable
commodities. Tomato, because of its widespread utility, is an obvious
target for fresh-cut production. At the present time however, little
postharvest information is available about the physiological and
biochemical changes that may occur in fresh-cut tomato slices during
storage. The quality factors that are important for this product are
texture, flavour and nutritional value (Section 2.3).
The biochemical and physiological changes during ripening of intact
tomatoes and of tomato slices (Section 2.4) and the physiological
consequences of slicing tomatoes (Section 2.6) have been highlighted
in this review. Fresh-cut products may be subjected to a variety of
postharvest handling techniques to retain quality and extend storage-
life as described in Section 2.5. Deterioration of tomato slices during
storage could also be controlled by external modifications, including
physiological manipulation (harvest maturity) and environmental
manipulation (temperature) as well as by manipulation of ethylene
action using an ethylene inhibitor (1-MCP).
There are a number of key research areas that could potentially assist
in the development of fresh-cut tomato slices with high sensory
quality, long storage life, and satisfactory nutritional value. These key
research areas have been developed into the objectives of this thesis.
The major research objectives of this study will therefore focus on the
physiological (ethylene and respiration), biochemical (firmness, juice
73
colour, soluble solids, titratable acidity, and electrolyte leakage), and
nutritional (ascorbic acid and lycopene) changes that occur in fresh-
cut tomato slices during storage.
2.7.2 Objectives
The specific objectives of the research are:
1 To determine the effects of slicing on the postharvest physiology
of tomato slices (Chapter 4)
2 To study the quality changes in tomato slices taken from fruit at
different stages of maturity and stored at different storage
temperatures (Chapter 5)
3 To characterise the involvement of ethylene in the loss of slice
quality (Chapter 6)
4 To determine the efficacy of 1-MCP in maintaining quality of
tomato slices (Chapter 7)
5 To determine the effect of fruit maturity and 1-MCP on the
quality of tomato slices (Chapter 8)
6 To evaluate the effect of applying a brief heat shock to intact
tomatoes on the quality of slices (Chapter 9).
74
3. MATERIALS AND METHODS
3.1 Overview The general overview of the method during minimal processing and
assessments is depicted in Figure 3.1.
Figure 3.1 Generalised flow-chart for the preparation and measurements made
Store at controlled temperature
Selection of unblemished fruit, with uniform size, firmness and colour
Whole fruit washed with 100 ppm NaOCl solution for 1 min at 10 °C
Drained and kept overnight at 10 °C
Sliced (7 mm-thick) and drained at 10 °C
Allocate slices to treatments in ventilated plastic containers
Harvest tomato and transport within air-conditioned car
Firmness measurements
Homogenise for soluble solids, titratable acidity
and juice hue angle
Electrolyte leakage, ascorbic acid and
lycopene
Ethylene and respiration measurements
75
3.2 Tomato fruit
Tomatoes used were the cultivar ‘Revolution’, obtained from local
growers at Gatton supplied by Syngenta Seeds Pty. Ltd. This
indeterminate variety is a processing tomato newly selected for slicing.
It is multilocular and not juicy when sliced (Anonymous, 2002b).
Immediately after harvest, fruits were transported to the Postharvest
Laboratory, University of Queensland, Gatton, in an air-conditioned
car within 2 hours of harvest. Then fruit were carefully sorted for
absence of visual defects, as well as sorted based on the size and stage
of maturity. To minimise the diseases, whole fruit washed with 100
ppm NaOCl solution for 1 min, drained and kept overnight at 10 °C.
3.3 Assessments made before experimentation
Before starting each experiment, fruit fresh weight (g) was measured
using a balance (Sartorius, Germany), and fruit diameter (mm)
determined with a circumference meter. Firmness and colour were
evaluated in order to have uniformly mature material. Whole fruit
firmness was measured on each fruit using the Instron materials
tester (Autograph, Shimadzu AGS-H 500N) equipped with flat plates
and a probe. This equipment measured firmness based on the
resistance of the fruit to deformation. The maximum force required to
deform the fruit surface 5 mm by a 10-mm diameter spherical probe
with a head speed of 10 mm/min was determined, with the fruit lying
transversally and the plate positioned on the fruit equatorial zone
(Artes et al., 1999). Results were expressed in Newtons.
Surface colour of each fruit was measured using a Minolta CR-200
chromameter (Minolta Camera Co. Ltd. Osaka, Japan). To reduce
variability, six measurements of surface colour were made at the
equatorial, and the blossom-end and stem-end of the fruit.
76
3.4 Handling of slices Slices (7 mm thick) for all experiments were obtained after tomatoes
were sliced parallel to the equator region fruit with a commercial
slicing machine (Fasline®, model 919/927, Carol Stream Illinios, Plate
3.1). The tomato slicer and all equipment required for the handling
and preparation of tissue slices were sanitised before each experiment
by rinsing with 70% (v/v) ethanol.
Plate 3.1 The commercial slicing machine (Fasline®, model 919/927,
Carol Stream Illinios) used in experiments.
All slicing process and all operations associated with preparation and
handling of the tomato slices were conducted in a fresh-cut room at
10 °C to minimise contamination. In addition, in order to minimise
any contamination of the slices, sterile gloves were used in handling
tomato slices. After slicing, care was taken to prevent further damage
77
to slices as they were put into glass jars or into the plastic containers
for storage and quality assessments.
Each plastic container (high density polyethylene with length: 16.5
cm, width: 10.5 cm, depth: 6.5 cm) was capped with a lid perforated
with 2 holes (10 mm-diameters). The holes were packed with clean
cotton wool to assist in maintaining sterility, and to enable adequate
ventilation of the atmosphere inside (Wu and Abbott, 2002). Each
containers containing two layers of absorbent paper on the bottom to
prevent juice accumulation (Gil et al., 2002).
3.5 Experimental measurements
3.5.1 Ethylene and CO2 evolution
A static system using 1 litre glass jars was used to measure ethylene
and CO2 production (Kader, 2002a). Tomato slice samples were
weighed and the volume of the jars measured by water displacement.
The headspace was calculated as the difference between the volume of
the jar and the fresh weight of fruit. Background ethylene was
measured from an empty jar that was sealed along with the sample
jars. All the jars were flushed with fresh air for 5 minutes before lids
were closed and sealed. Ethylene and CO2 evolution was determined
from the headspace after incubation for 2 hours at storage
temperature or at treatment temperature. Headspace gas samples
were taken with a 1 mL disposable syringe through the gas sampling
port in the lids. Syringes were flushed at least 5 times with air
between sampling from different treatments. Then the syringes were
flushed 5 times with air from inside the glass jars to stir the internal
atmosphere before a sample was withdrawn. Syringes were regularly
disposed of to avoid potential contamination.
78
For ethylene determination, samples were injected into a gas
chromatograph (Shimadzu model GC-8A) fitted with a flame ionisation
detector. Temperatures of the injector port, column and detector were
120, 90, and 120 °C, respectively. The 900 mm-long and 5 mm
internal-diameter glass column was packed with activated alumina
mesh size 80/100. The Shimadzu CRGA Chromatopac integrator
output was calibrated using an ethylene standard gas (0.09 ± 0.02 µL
L–1, BOC Gases β-grade) and the balance gas was nitrogen. The
carrier gas (1 kg cm-2 pressure) was high purity nitrogen (BOC Gases).
Oxygen (0.3 kg cm-2) was supplied as medical grade air, and hydrogen
(0.45 kg cm-2) was high purity grade, both from BOC Gases.
Ethylene production rate on a fresh weight basis was calculated as
follows:
Ethylene production (nmol g -1 h-1) =
∆ C2H4 × wt(g) fresh
vol(L) space head×
t(h)1
× Ctemp273K
273K°+
× 22.41000
where:
∆ C2H4 = ethylene concentration in sample – background (µL L–1)
t = incubation time
For CO2 analyses, headspace samples were injected into a gas
chromatograph (Shimadzu model GC-8A) fitted with a thermal
conductivity detector. Temperatures of the injector port, column and
detector were 30, 35, and 30 °C, respectively. The 1.5 m-long and 1.8
mm-internal diameter glass column was packed with activated
alumina mesh size 80/100. The gas chromatograph signal was
recorded using a Shimadzu CRGA Chromatopac integrator calibrated
with a CO2 standard of 0.575% (v/v) in nitrogen (BOC Gases β-grade).
The carrier gas (1 kg cm-2 pressure) was high purity helium (BOC
Gases).
79
The rate of carbon dioxide production was used to indicate the
respiration rate. The respiration rate on a fresh weight basis was
calculated using the following equation:
Respiration rate (µmol g-1 h -1) =
∆ CO2 × 104 × wt(g) fresh
vol(L) space head × t(h)1 ×
Ctemp273K273K
°+ ×
22.41
where:
∆ CO2 = CO2 in sample – CO2 in background (% v/v).
t = incubation time
3.5.2 Slice firmness
Slices were brought from the storage room, then columella and
pericarp firmness was measured immediately before the sample
temperature could change. Columella firmness (Chapter 4 only) was
assessed at the centre of columella, whereas pericarp firmness was
assessed at the outer pericarp at two opposite locations using a
materials tester (Autograph, Shimadzu AGS-H 500N). The firmness
measurements were undertaken by placing each slice on a flat plate
held perpendicular to the probe. Firmness (N) was determined by
measuring the force required for a 4 mm-diameter cylindrical probe to
penetrate the cut surface 3 mm at a speed 1 mm/sec, following Wu
and Abbott (2002).
3.5.3 Soluble solids (SS), titratable acidity (TA) and ratio SS/TA
Juice was extracted from slices by homogenising them at high speed
for 1 minute in a food blender. The homogenate was filtered through
two layers of cheesecloth to obtain the clear filtrate. Soluble solids
content of the resulting clear juice (about 5 g) was determined at
80
20 °C using an Atago Digital Refractometer (PR-101, Fuji, Japan), in
units of °Brix. The refractometer was initially zeroed using distilled
water, and the prism was wiped with clean tissue paper and then
rinsed with distilled water after each measurement.
Titratable acidity was measured on 10 g of juice diluted with 50 ml of
distilled water. The diluted juice was stirred (IKAMAG, Janke &
Kunkel, Australia) then titrated with 0.1 N NaOH to an end point of
pH 8.2. An automatic titrator (Metrohm, Swiss) equipped with a 632
pH meter and 765 Dosimat Autoburette was used to measure
titratable acidity. Titratable acidity (TA) was expressed as percentage
(w/w) citric acid. The percent total titratable acidity as citric acid was
calculated by the following equation (Roberts et al. (2002) and Baker
(Queensland of Department of Primary Industry and Fisheries,
personal communication).
% citric acid = juice ofg 10
1000.064NaOH) of (normality 0.1(mL) vol.NaOH ×××
Where 0.064 = milliequivalent factor for citric acid
The ratio soluble solids to titratable acidity was calculated as
acidity titratable content solids soluble
3.5.4 Juice colour
Juice colour was measured using juice extracted as in section 3.5.3
using the CIELAB L*, a*, and b* values obtained with a Minolta CR-
200 (Minolta Camera Co. Ltd. Osaka, Japan) tristimulus colormeter.
Colour, as hue angle (hº), was measured by aiming the sensor through
the base of a glass jar containing 40 ml of juice (Artes et al., 1999).
81
Hue angle was calculated on the basis of the following equations
(Arias et al., 2000a; Lancaster et al., 1997):
h° = arctangent (b*/a*), when a* > 0 and b* ≥ 0
or
h° = 180° + arctangent (b*/a*), when a* < 0 and b* > 0;
3.5.5 Electrolyte leakage Electrolyte leakage rate was determined from conductivity
measurements according to the procedure of King and Ludford (1983)
and Hong et al. (2000). Four sections of pericarp discs (about 6 mm
diameter) were excised from each slice and combined. The discs were
weighed (about 2 g) and washed three times in distilled water and
placed in beakers containing 30 ml 0.4 M mannitol solution. In an
initial experiment it was found that conductivity of the bathing
solution did not increase appreciably with incubation for up to 6 h.
The discs were held at 30 °C for 6 hours and then 20 mL of solution
was taken and initial electrical conductivity readings were taken using
a conductance meter (Activon Model 301, Conductivity Meter,
Australia). The beakers were swirled for 10 s before the electrolyte
measurements were taken. The discs and bathing solutions were then
frozen at –20 °C for 24 hours and then thawed. Final conductivity
readings were taken and electrolyte leakage (%) was calculated
(McCollum and McDonald, 1991):
Electrolyte leakage (%) = 100 tyconductivi Finalty conductivi Initial
×
3.5.6 Ascorbic acid Ascorbic acid concentrations were determined using High Performance
Liquid Chromatography (HPLC, LC-10 AD Liquid Chromatograph,
Shimadzu, Japan) according to Rizzolo et al. (1984). Approximately 20
82
g of slices stored at -80°C were thawed and homogenised with a
homogenizer (Janke & Kunkel IKA, Ultra Tunax T25) at 13 500 rpm
for 2 minutes at room temperature. Ten gram of homogenate was
weighed and mixed with 25 ml of 6% (w/v) metaphosphoric acid. The
solution was centrifuged (P Selecta, Centronic, Spain) for 20 min at 6
000 rpm (2500 x g). The extract was transferred into a 100 ml
volumetric flask after filtration through Whatman No. 1 filter paper.
The residue from the filtration was extracted with metaphosphoric
acid once more and the second extract was combined with the first.
The mixture was filtered through Whatman No. 1 filter paper and the
filtrate was diluted to 100 ml with 6% (w/v) metaphosphoric acid. An
aliquot of the acid extract was then filtered through a Millipore filter
(Millex HA) prior to injection of 10 µL into the HPLC (SIL-10AXL,
Shimadzu, Japan). The ascorbic acid was separated on a column of
Luna 5µ C18 (Phenomenex, USA) with length 150 mm x diameter 4.6
mm, equipped with a guard column C18 5µ. The mobile phase was
0.2 N orthophosphoric acid, the flow rate was 0.8 mL/min, and the
detection wavelength was 254 nm. L (+) ascorbic acid (0.01 mg/mL)
(Merck) was used as the external standard for quantification. To
determine recovery of the procedure, a known amount of pure
ascorbic acid standard (i.e. 2 x the amount found normally) was added
to disc samples and then the extraction and chromatographic
procedures were applied to samples with and without standard
ascorbic acid, in duplicate. The recovery was 96 %, indicating
complete extraction. Ascorbic acid concentration on a fresh weight
basis was calculated:
Ascorbic acid (mg 100 g-1) =
sampleg g
standard of Area sample of Area 100×
83
3.5.7 Lycopene
Lycopene analyses were performed according to Beerh and Siddappa
(1959), Adsule and Dan (1979), and Hakim et al. (2000). Pigments
were measured by homogenizing ca. 3 g of frozen pericarp tissue with
a homogenizer (Janke & Kunkel IKA, Ultra Tunax, T25) at 13 500 rpm
in 10 ml of acetone in a centrifuge tube at room temperature. The
tubes used were covered with aluminium foil to prevent light-induced
lycopene oxidation. The tubes were shaken on a shaker (B.Braun,
Melsungen AG, Germany) for 15 min at 150 cycles/min and then
centrifuged (P Selecta, Centronic, Spain) at 6000 rpm (2500 x g) for 10
minutes at room temperature. The supernatant was decanted and
adjusted to 15 ml with acetone. Lycopene concentrations were
determined from the absorbance at 503 nm in an acetone extract
using a spectrophotometer (Pharmacia LKB, Ultrospec III, Japan).
This wavelength is best suited for tomato lycopene because the
influence of carotenoids is negligible (Beerh and Siddappa, 1959).
Lycopene content was calculated using the molecular extinction
coefficient of 17.2 x 104 mol cm-1 (Beerh and Siddappa, 1959;
Mencarelli and Saltveit, 1988) and was expressed on a fresh weight
basis:
Lycopene (mg kg-1) =
mL10L 1
moleg
cmΜ1017.2A
34503 ××
××9.536
1g mg 103
×tissuekg
mL 10.0×
tissuekg 0.0312 A ×
= 503
tissueg 31.2A 503 ×=
84
3.6 Statistics and data analyses Treatment means and standard errors of means were calculated using
Microsoft® Excel (Version 2002 Microsoft Inc.). Figures were drawn
using Sigmaplot® (Version 8). Analyses of variance were performed
using Minitab version 13.2 (2002) for Windows using the General
Linear Model. All measurements had equal sample size (balanced
data) and the least significant difference (LSD) procedure at P = 0.05
was used to test for differences between treatment means. Only
significant differences are discussed, unless otherwise stated.
Residual analyses of the data were performed to check that data
satisfied the assumptions of ANOVA. Analyses are presented in full in
the Appendix.