thermal pasteurization for minimally processed grape...
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THERMAL PASTEURIZATION FOR MINIMALLY PROCESSED GRAPE JUICE:
MICROBIAL STABILITY DURING SHELF LIFE AND SENSORY EVALUATION
A Project Paper
Presented to the Faculty of the Graduate School
of Cornell University
In Partial Fulfillment of the Requirements for the Degree of
Master of Professional Studies in Agriculture and Life Sciences
Field of Food Science and Technology
by
Michelle Maldonado
May 2015
© 2015 Michelle Maldonado
ABSTRACT
Minimally processed juices are preferred by consumers because they perceive
them to be closer to the organoleptic and nutritional characteristics of fresh juices. The
study was conducted to investigate the effect of mild thermal pasteurization on the quality
and shelf life of grape juice and to conduct sensory evaluation to compare minimally
thermal processed with UV treated grape juice. Two varieties of white grapes (Niagara
and Riesling) and one variety of red grapes (Concord) were heated to 71°C for 6 seconds
and UV treated (at 14 mJ/cm2). For the shelf life study, pH, °Brix, total plate and mold
and yeast counts were conducted biweekly. The findings of the study indicate that the
shelf-life of refrigerated, minimally thermal processed grape juice ranged from six weeks
(Concord) to eleven weeks (Niagara and Riesling). The sensory evaluation of the grape
juices demonstrated that participants were able to differentiate between the UV treated
and thermally pasteurized juice in all the varieties studied.
Keywords: Grape, Juice, Sensory Evaluation, Minimally processed juice, shelf life
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BIOGRAPHICAL SKETCH
Michelle Maldonado was born in Pomona, California in 1988. She attended
University of San Francisco de Quito in Ecuador to earn her bachelor’s degree in Food
Engineering. She worked with her classmate in developing a passion fruit carbonated
beverage for their final project. She graduated with Magna cum laude in 2013. After
graduation, she worked in a juice production company where she collaborated in
implementing good manufacturing practices in the company and worked in new product
development. She later worked in the Ecuadorian Service for Standardization by
developing national standards for the food and beverage sector.
In 2014, she received a Universities of Excellence - Ecuadorian Government
Scholarship allowing her to pursue a Masters in Professional Studies degree in Food
Science and Technology at Cornell University. She worked under the guidance of Dr.
Olga Padilla-Zakour. During her time at Cornell, she worked as a graduate teaching
assistant for FDSC 4100: Sensory Evaluation of Food. She also was a member of the
Ocean Spray product development team that won first place in the competition. She
plans to return to Ecuador to work for the government.
iv
This project is dedicated to my parents Miguel and Beatriz who with their
unconditional love and support have encouraged me every step of the way
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ACKNOWLEDGMENTS
I would like to express my gratitude to my faculty advisor Dr. Olga Padilla-Zakour
for her support during my stay in Cornell and the writing of this project. She is very
knowledgeable about her field of study and she cares deeply for all the students under
her guidance. I could not imagine having a better advisor. I would like to thank Jessie
Usaga for being so patient when you were teaching me about analytical methods I
needed to use for my project. My sincere thanks also goes to Herb Cooley and Tom
Gibson for their help in the pilot plant and John Churey for counting the plates when I
could not travel to Geneva. My sincere thanks goes to my friend Vanessa Moncayo, I
could not have imagined a better person to have worked side by side in this project. I
thank my fellow lab mates Marcela Patiño, Marcela Villareal and Elizabeth Buerman for
making the short time I spent in the lab very memorable.
This work could have not been completed without the encouragement of my family
members. Thanks mom, dad and my sister Stephanie for supporting my decision to go
into grad school.
In addition, I would like to thank my friends I met in Ithaca and back home with a
special mention to my friends at INEN for wishing me luck when I summited my
application for the scholarship and for asking me to keep them updated during every step
of the process. I am very grateful to all of you for becoming an important part of my life.
Financial support for this project was provided by the Secretaria de Educacion
Superior, Ciencia, Technologia e Inovacion. Thank you for awarding me with the
scholarship.
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TABLE OF CONTENTS
Biographical Sketch iii
Dedication iv
Acknowledgements v
Table of Contents vi
List of Figures vii
List of Tables viii
Chapter 1: Introduction 1
Chapter 2: Materials and Methods 12
Chapter 3: Results and Discussion 16
Chapter 4: Conclusions and Recommendations for Future Work 26
References 27
Appendix 31
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LIST OF FIGURES
Figure 1.1. Total US retail sales and forecast of juice, juice drinks and smoothies at
current prices 2009-19 .................................................................................................... 1
Figure 3.1. Changes in total plate counts in thermally processed grape juice (71°C for 6
s) over storage period (at 7°C) ...................................................................................... 20
Figure 3.2: Changes in yeast and mold counts in thermally processed grape juice (71°C
for 6 s) over storage period (at 7°C) .............................................................................. 21
Figure A.1. Sample test ballot for grape juice sensory triangle test ............................... 37
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LIST OF TABLES
Table 3.1. Physicochemical characteristics of Concord, Niagara and Riesling grape on
day 0 ............................................................................................................................. 18
Table 3.2. Chemical changes during storage (at 7°C) of thermally processed (71°C for 6
s) grape juice until end of shelf life ................................................................................ 23
Table 3.3. Results from triangle test for UV treated (at 14 mJ/cm2) and thermally
pasteurized (71°C for 6 s) juice for the three different grape varieties .......................... 24
Table A.1. Total Plate Count results for thermally processed Concord grape juice ....... 31
Table A.2. Mold and Yeast Count results for thermally processed Concord grape juice ...
...................................................................................................................................... 32
Table A.3. Total Plate Count results for thermally processed Niagara grape juice ........ 33
Table A.4. Mold and Yeast Count results for thermally processed Niagara grape juice ....
...................................................................................................................................... 34
Table A.5. Total Plate Count results for thermally processed Riesling grape juice ....... 35
Table A.6. Mold and Yeast Count results for thermally processed Riesling grape juice ....
...................................................................................................................................... 36
1
CHAPTER 1
INTRODUCTION
Fruit Juice Market
Sales of fruit juice (juice, juice drinks and smoothies) accounted for $16 billion in
2014, a number that has remained relatively unchanged since 2009. Issues such as,
health concerns and competition from other drink categories (flavored water, ready-to-
drink tea and coffee) are the causes for lack of growth in this market (Mintel 2014).
Figure 1.1. Total US retail sales and forecast of juice, juice drinks and smoothies at
current prices, 2009-19
Source: Based on Information Resources INC, Infoscan reviews, US census bureau, Economic census,
USDA economic research/Mintel
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One hundred percent juice accounts for approximately half the sales (49.6%), with
the category juice drinks coming in second place (45.5%). Even though 100% juice takes
first place in the category, the segment is negatively perceived by customers as having a
high calorie and sugar content. Recommendations to boost market growth is to
emphasize the nutritional benefits (vitamins and minerals) they deliver and to develop
new flavor combinations (Mintel 2014).
Grape Juice Industry
During 2014, 7.9 million tons of grapes were grown in the United States, most of
the grapes were produced in the states of California, New York and Washington. Of those
grapes, only 6.8% (approximately 550 tons) were destined to grape juice production. The
major producers of domestic grape used for juice were the states of Washington, New
York, Pennsylvania and Michigan (NASS 2015).
The most common grape varieties grown are Concord grapes and Niagara grapes.
Concord grapes represent most of the grapes harvested with more than 505,000 tons
processed, followed by more than 70,000 tons of processed Niagara grapes (NASS
2015).
The New York State Department of Agriculture and Markets estimates that the
crop value for grapes was $52.3 million in 2012. Most of the grapes grown in the state
are destined for grape juice production (62%), followed by wine production (36%) and
fresh market (2%). The majority of grapes are produced in the Lake Erie area, the Finger
Lakes, the Hudson Valley and the eastern end of Long Island. (New York State, 2015).
3
Grape Varieties
Concord
This Native American (Vitis labrusca) grape variety is known as being the most
commonly used for grape juice processing. Typically Concords are used for the
traditional purple grape products and in New York State Concord accounts for 88 percent
of the juice grape utilization in the most recent 5 years (MKF 2005). Consumers can
recognize Concord for its balance of sweetness, acidity and astringent characteristics
even if the juice is highly diluted and sweetened (Bates 2001).
Riesling
Riesling is a variety of Vitis vinifera (wine grapes) originating in Germany no later
than 1350. Riesling vines grow small berries and compact and round leaves. Riesling
grapes have a low pH (2.9-3.2) and they are harvested late in the season so they have
the opportunity to produce high sugar levels to balance the acidity. This variety is cold
resistant and is grown in continental climates in Germany, Austria, New York, Washington
and Oregon (Sechrist 2012).
Niagara
Niagara grapes are members of the Vitis labrusca (native to United States) grapes.
This variety was first created when Concord grapes were cross-bread with white Cassady
grape in 1868 in Niagara County, New York. Niagara grapes are the second most
4
cultivated variety and they are used principally in the production of white grape juices
(MFK 2005).
Juice Safety
In 2001, the United States Food and Drug administration (FDA) issued a new
regulation “Part 120: Hazard Analysis and Critical Control Point (HACCP) Systems;
Procedures for the Safe and Sanitary Processing and Importing of Juice” (21CFR Part
120) due to the number of outbreaks of Salmonella spp., Escherichia coli O157:H7 and
Cryptosporidium parvum associated with the consumption of fresh apple and orange
juices. The regulation states that all processors shall develop a HACCP plan to determine
which biological, chemical and physical hazards more likely to occur in fruit juices. This
regulation requires processors to implement measures that will produce at a minimum a
5 log reduction for the most heat resistant microorganism of public health significance
under normal to moderate abuse storage conditions (FDA 2014).
Quality of Juices
A food quality program is designed to ensure the suitability of a product during all
stages of handling, processing, preparation, packaging, storage and distribution for an
intended application. In order to meet product specifications, processing should be
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carried out in the correct manner using fruit of an optimum level of maturity, and that the
product should be stored under suitable conditions to limit effects of degradation during
shelf-life. There are some common parameters used to measure the quality of juices
(Taylor 2005).
Soluble solids
The soluble solid content relates primarily to sugars and fruit acids present in the
juice because they are the main contributors; pectins, glycosidic materials and the salts
of metals (sodium, potassium, magnesium, calcium etc.), are also present in some juices
but their quantity is very small. The amount of soluble solids is measured in Brix value
by using an optical refractometer. Refractometer readings can be affected by the
presence of other dissolved solids like fruit acids when there are appreciable levels of
acid in the juice, for example, in lemon and lime juices (Taylor 2005).
Titratable acidity
The acid content of a juice is determined using a pH meter by direct titration against
standardized alkali solution (e.g. 0.1 M sodium hydroxide) to an end-point at pH 8.1. As
a general rule, the acidity of juices will decrease with increasing maturity of the fruit
source, or with increasing levels of sugars in the resulting juice. Hence the ratio of soluble
solids (e.g. Brix values) to acidity is an important value in the assessment of juice quality.
The Brix/acid ratio is frequently used to establish standard sensory, or taste, qualities for
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incoming juice supplies and to minimize the effect of seasonal variation. The higher the
Brix value in relation to the acid content of the juice, the higher the ratio and the ‘sweeter’
the taste (Taylor 2005).
Color measurements
When using the tristimulus method, the absorbance or reflectance of a product is
measured at a range of wavelengths in the red, green and blue areas of the spectrum.
This three-dimensional space can be represented by the L, a, b system, used by Hunter
Lab. L represents the degree of whiteness or blackness (from 0 to 100). The chromatic
portion of the color space is based on rectangular Cartesian coordinates (a, b) with red
represented by +a, green represented by –a, yellow represented by +b, and blue
represented by –b (Lawless 2010).
Microbiological
Because of their low pH, fruit juices will present less than ideal conditions for
pathogenic bacteria species, and these therefore are generally of no major concern for
the juice producer who is operating under good manufacturing practices. There are acid
tolerant bacteria, however, whose presence can give rise to off-flavors, and this effect
can be encountered with citrus juices (Taylor 2005). Yeasts are the most significant group
of micro-organisms associated with spoilage of soft drinks and fruit juices. Spoilage will
be seen as the growth and production of metabolic byproducts, for example, CO2, acid,
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and tainting compounds. As noted above, most spoilage is therefore by yeasts and mold
species, with yeasts most important, and some spoilage is by acid tolerant bacteria
(Hocking & Jensen, 2001; Jay & Anderson, 2001).
Juice Processing
Thermal Treatment Processing
Normally for self-stable juices, pasteurization is achieved by hot-fill and hold or in
bottle pasteurization. The hot fill and hold process is used with high acid juices that must
pass through a heat exchanger to elevate the temperature to 88 to 95 ºC, then the
container is immediately filled with the juice and set aside to wait at least 3 minutes before
cooling (McLellan & Padilla-Zakour 2005).
For refrigerated juices, the time-temperature combination of 70 to 90 ºC for a few
seconds is used to destroy pathogenic bacteria that may be present in the juice. Even
though the shelf-life is shorter, many consumers prefer refrigerated juices because of the
retention of nutrients and organoleptic properties in contrast to shelf-stable juice
(McLellan & Padilla-Zakour 2005).
Non-thermal Treatment Processing
UV irradiation is a FDA approved method that is used to eliminate pathogens from
fruit juice products. UV light works by damaging the DNA of bacterial cells, protozoa and
viruses so they are not capable of reproduction and therefore they die. UV irradiation,
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however, does not have the same effect on molds and yeasts (Tandon et. al 2003). The
advantages of using UV irradiation is that UV treatment has no effect on the organoleptic
properties of the juice, the product can be marketed towards consumers who want less
processed products, and the equipment has a much lower cost compared to the
equipment used for pasteurization (McLellan & Padilla-Zakour 2005).
Sensory Evaluation
Sensory evaluation is defined as a method used to quantify, analyze and interpret
responses to a product perceived though the senses. There are three basic types of tests
used in sensory analysis, discriminatory, descriptive and affective tests. Discriminatory
tests are used to answer if there is a difference between two products that can be
perceived by participants. A significant difference between the products would be
declared if the number of correct choices was above the level expected by chance
(Lawless 2010).
There are a large variety of tests based on this principle that can be used: Paired
comparison, triangle, duo–trio, n-alternative forced choice, A-not-A, ABX discrimination,
etc. Although all these tests are used to investigate whether a difference can be
perceived, each test varies their ability to do so in an accurate and efficient manner. For
example, in triangle and duo-trio tests, panelists are not given the nature of the attribute
they are supposed to concentrate on, they just have to identity which sample is different.
One disadvantage of the triangle and duo-trio test is that their low statistical power can
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result in not detecting sensory differences that can potentially be detected by consumers
and lead to the rejection of the reformulated product (Ishii 2014).
Discrimination tests are usually used by product developers when they reformulate
a product, when they change ingredients they do not want consumers to perceive a
difference. Discrimination tests are also used to identify if a change in a process changes
the sensory characteristics of the product. Difference tests are very popular due to the
simplicity of data analysis. Statistical tables derived from the binomial distribution give
the minimum number of correct responses needed to conclude statistical significance as
a function of the number of participants, so results can be reported quickly (Lawless
2010).
There have been a number of studies done to compare if UV treatment has an
effect on the sensory parameters of treated fruit juices. Tandon et al. (2002) investigated
the storage quality of hot-filled (at 63°C), flash pasteurized (at 71°C x 6 s) and UV
irradiated (at 14000 uW) apple cider. Participants carried out ranking tests on the three
treatments and rated the samples on a 7-point preference scale with 1 being “dislike
extremely” and 7 being “like extremely”. The results indicated no statistical difference
between UV treatment and thermal pasteurization between the preference ratings and
the average ranking scores in the beginning of the study.
Donahue et al. (2004) investigated if UV inactivation of Escherichia coli O157:H7
had an effect on flavor in apple cider. The apple cider used in the study was irradiated at
8.77 mJ/cm2 power intensity. A triangle test was used for the sensory evaluation of the
cider. Participants were given treated and untreated cider and asked to identify the
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different sample. Results from the participants indicated that a statistical significant
difference could not be found between the sensory qualities between the juices.
Another study conducted in 2010 by Caminiti et al. measured sensory
characteristics of apple juice treated with UV light at energy dosages ranging of 5.31,
10.62, 26.55 and 53.10 J/cm2. Panelists were asked to evaluate color, odor, sweetness,
acidity, flavor and overall acceptability on a 9-point preference scale on the UV treated
samples and an untreated control. Statistical analysis revealed that there was no
significant difference (p≥0.05) between the samples that were treated with 5.31 and 10.62
J/cm2 and the control. Panelists reported lower hedonic scores for juices that were treated
with dosages greater or equal to 26.55 J/cm2 indicating that those dosages caused
adverse changes in odor, flavor and color. In all these studies, low dosages of UV
treatment did not affect the sensory qualities of apple cider.
Pala & Toklucu (2013) have also done research on sensory properties of UV
processed orange juice. Panelists were asked to rate untreated, UV (48.12 kJ/L) and
heat treated (90°C for 2 min) orange juice on flavor and aroma and overall acceptability
using a 7-point hedonic scale. Participants were also given two sets of oranges juices for
a triangle test; one that contained untreated and UV treated and another one that
contained UV treated and heat treated. Hedonic results indicated that UV treated orange
juice samples were more liked than heat treated (P <0.05). Results from the triangle test
indicated that there was no statistical significant difference between the control and the
UV pasteurized orange juice (P <0.01). However, the panelists were able to detect a
11
difference between the UV treated and the thermally processed orange juice that was
statistically significant (P <0.01).
Another study on fruit juices by Guevara et al (2012) evaluated the sensory quality
of guava and passion fruit nectars treated by ultra violet radiation. Guava nectar was
treated at 14.55 and 23.62 kJ/L and passion fruit nectar was treated at 6.19 and 11.03
kJ/L. Panelists participated in triangle tests, each set contained treated and untreated
samples, and they had to identify which sample was different. In all trials, panelists were
able to detect statistically significant differences between the treated and untreated
samples. Panelists expressed that they were able to identify the different sample
because the UV treated samples differed from the control in color, aroma and taste.
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CHAPTER 2
MATERIALS AND METHODS
Grape Juice
Two varieties of white grapes (Niagara and Riesling) and one variety of red grapes
(Concord) obtained by a grape yard located in Geneva, NY were used. Niagara and
Riesling varieties were stored at 0°C until used and thawed at 7°C for seven days.
Concord grapes were stored at 7°C before processing. Grapes were pressed in batches
using a custom made hydraulic press rack and frame. The Riesling juice had 17.8° Brix
and 3.47 pH value, the Niagara juice had 16.2 °Brix and 3.47 pH value, and the Concord
juice was characterized by a 13.7 °Brix and 3.25 pH value. 125mL Nalgene™ PET Sterile
Square Media Bottles with HDPE Closure (Thermo Scientific, Lima, OH) were used to
pack the juice samples.
Physicochemical measurements
Chemical and physical measurements were made on the grape juice samples in
triplicate. pH, titratable acidity (TA), soluble solids (°Brix), turbidity, absorption coefficient
and color were measured. pH was measured using an Accument Basic AB15 pH meter
(Fisher Scientific, Hampton, New Hampshire). Titratable acidity was measured using G20
Compact Titrator Mettler Toledo, 5 mL of grape juice were taken and diluted with distilled
water until a total volume of 40 mL was reached. The results were indicated as (w/v)
malic acid percentage. Total soluble solids of the samples were measured using an Auto
13
ABBE Refractometer Leica 10504 (Leica Inc., Buffalo, NY) and reported as °Brix.
Turbidity was measured using a Hach 2100P turbidimeter 4500-00 (Hach Co., Loveland,
CO) and expressed as Nephelometric Turbidity Units (NTU). Color was measured using
a Hunter UltraScan VIS spectrophotometer (Hunter Lab Assoc., Reston, VA) results were
expressed as L, a and b values. The absorption coefficient (α) was calculated following
the protocol described by Koutchma et al. (2004) by measuring the sample absorption at
254 nm using a UV-1800 spectrophotometer (Shimadzu Scientific Instruments, Columbia,
MD). Samples were subjected to a 10-fold dilution in distilled water and placed into
demountable fused quartz cuvettes of 0.1, 0.2, 0.5 and 1.0 mm path length (NSG
Precision Cells, INC., Farmingdale, NY).
Thermal Processing
For the thermal pasteurization treatment, a continuous tubular pasteurizer
(UHT/HTST unit, Micro Thermics, Raleigh, NC) was used. The grape juices were heated
to 71°C for 6 seconds, cooled to 20-25°C and manually packed. Juice bottles were
refrigerated at 7°C and three samples of each juice were taken biweekly for conducting
the shelf life study.
Sensory Evaluation
A sensory study was carried out one day after the production of the juice. A triangle
discrimination test was conducted in the New York State Agricultural Experimental Station
(Geneva, NY). The aim of the study was to determine if consumers were able to perceive
differences between the thermal and UV-treated grape juices. Forty volunteers including
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college students, faculty members and staff from the Experimental Station participated in
the study. Participants were at least 18 years old. Each participant was presented three
different sets of juices, each one of a different grape variety. Samples were kept at 7°C
and served in plastic cups labeled with randomly generated three digit codes. Panelists
were asked to indicate the different sample.
Shelf Life Study
Microbiological counts including total plate count and molds and yeast count were
made every fourteen days. Plate count Agar (PCA) was used to determine total aerobic
microbes, and acidified (3.5 pH) Potato Dextrose Agar (PDA) was used to determine the
presence of yeast and mold. Both media were supplied by Difco, Becton Dickinson
(Sparks, MD). Three bottles of each juice were taken for microbiological analysis. One
mL of sample was subjected to serial dilutions in 1% sterile peptone water and then
placed into Petri dishes. Agar was poured and mixed thoroughly, a duplicate of each
dilution was plated. Petri dishes were incubated for 48 h at 30°C. Colonies were reported
as log10 CFU/mL.
Statistical Analysis
Three independent analytical replicates were used for each physicochemical
measurement and microbiological analysis. Results were reported in mean ± standard
deviation. Results were subjected to analysis of variance (ANOVA) and Tukey’s
significant difference test was used for the comparison of means using JMP 11.0 version
15
statistical software (SAS Institute Inc., Cary, NC). Differences were considered significant
at a P value < 0.05.
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CHAPTER 3
RESULTS AND DISCUSSION
Quality of Raw Grape Juices used in the Study
The PCA counts for the raw Concord, Niagara and Riesling grape juice produced
in the pilot plant were 239883 CFU/mL (5.4 log10), 53703 CFU/mL (4.7 log10) and 54954
CFU/mL (4.7 log10) respectively. For PDA counts, the raw Concord juice had 281838
CFU/mL (5.5 log10), the raw Niagara juice had 14454 CFU/mL (4.2 log10) and raw Riesling
11220 CFU/mL (4.1 log10). Fruit juice produced by undamaged fruit are reported to have
yeast counts between 1,000 to 100,000 CFU/mL (Vasavada and Heperkan 2002).
Splittstoesser and Mattick (1981) cited that high yeast counts between 100,000 to
1,000,000 CFU/mL are common in unpasteurized juice extracted from both handpicked
and mechanically harvested fruit. The higher PCA and PDA counts in the raw Concord
juice than the raw Niagara and Riesling juices could have been caused by the one-week
storage period of the grapes in refrigeration temperatures (7°C).
Physicochemical Characteristics of Grape Juices
Juice was obtained from three different grape varieties (Concord, Niagara and
Riesling). Table 3.1 indicates the pH, soluble solids (°Brix), Titratable acidity (TA),
turbidity, color components and absorbance coefficient values from these varieties.
Juices from the three varieties were not significantly different in titratable acidity and
17
absorbance coefficient. White grape varieties (Niagara and Riesling) were significantly
different from Concord grapes in pH, soluble solids, turbidity and Hunter a values.
Table 3.1. Physicochemical characteristics of Concord, Niagara and Riesling grape juices1 on day 0
Variety pH Soluble
solids (°Brix)
TA (% (w/v)
malic acid)
Turbidity
(NTU)
Color components Absorbance
Coefficient
(Abs 430
nm)
L a B
Concord 3.26 ±
0.06a 13.69 ± 0.60a 0.64 ± 0.27
607.33 ±
75.48a
28.13 ±
0.30b
-0.03 ±
0.01b
0.15 ± 0.07
a
0.175 ±
0.002
Niagara 3.47 ±
0.07b 16.17 ± 1.16b 0.37 ± 0.004
428.00 ±
18.73b
28.44 ±
0.25b
1.32 ±
0.02a
2.44 ±
0.26b
0.152 ±
0.012
Riesling 3.47 ±
0.01b 17.81 ± 0.08 b 0.44 ± 0.04
330.33 ±
27.01b
29.64 ±
0.21a
1.44 ±
0.09a
4.78 ±
0.28c
0.158 ±
0.010
1 Mean ± SD, n=3. Numbers followed by different letters are significantly different (p ≤ 0.05) within columns
18
19
Effect of Minimal Thermal Processing on Stability during Shelf-life Study of Grape
Juice
Comparing the initial PCA counts, the thermal treatment achieved a reduction in
Concord, Niagara and Rieslings of 3.3 to 3.6 log10; 2.2 to 2.8 log10 and 2.0 to 3.0 log10,
respectively. With thermal processing, large reductions in PDA counts were also
observed, ranging from 3.6 to 4.3 log10 in Concord grape juice; 3.8 to 4.4 log10 in Niagara
grape juice and 5.3 to 5.6 log10 in Riesling grape juice.
The results from the aerobic counts (PCA) and mold and yeast counts (PDA) from
the grape juice samples during storage at 7°C are presented in Figure 3.1 and Figure 3.2
respectively. In all three varieties, the shelf life study ended because of visible mold
growth on the surface of the juice sample. Because the sample had visible mold growth,
it was not tested and noted as “spoiled”, assuming that the microbiological counts were
higher than 106 CFU/mL. The shelf life of the thermally pasteurized Concord juice was 4
weeks (28 days) and the shelf life of both Niagara and Riesling grape juices was 11 weeks
(77 days). Although, the shelf-life of the Concord grape juice was much shorter, it was
still in the shelf life range for mild thermally processed juices reported by Esteve and
Frígola (2007). The shorter shelf for the Concord juice could have resulted from the
higher initial microbiological counts from the raw juice. Also there could have been a
possible contamination from the packaging materials, as the samples that were labeled
as “spoiled” had visible mold growth on the cap and on the neck of the bottle.
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* Shelf life study was terminated due to visible mold growth on the surface of the sample
Figure 3.1. Changes in total plate counts in thermally processed grape juice (71°C for 6
s) over storage period (at 7°C)
21
* Shelf life study was terminated due to visible mold growth on the surface of the sample
Figure 3.2. Changes in yeast and mold counts in thermally processed grape juice (71°C
for 6 s) over storage period (at 7°C)
22
For chemical analysis, soluble solids and pH were measured during the shelf life
study as indicated in table 3.2. In both Concord and Niagara there was an increase in pH
and in all grape varieties there was an increase in soluble solids throughout the storage
period. These results corroborate with Siricururatana and coworkers (2013) who
indicated that the changes in pH and brix in Concord and Niagara grape juices were
caused by the metabolic activity of the yeast present. Yeasts cause the increase in
soluble solids and pH by converting polysaccharides present in the juice into soluble
sugars (Bal 2014).
Table 3.2. Chemical changes during storage (at 7°C) of thermally processed (71°C for 6 s) grape juice until end of shelf
life1,2
1 Mean ± SD, n=3. Numbers followed by different letters are significantly different (p ≤ 0.05) within columns
2 --- indicates samples where not taken
pH °Brix
Weeks Concord Niagara Riesling Concord Niagara Riesling
0 3.3 ± 0.1 3.5 ± 0.0a 3.5 ± 0 13.7 ± 0.6a 17.8 ± 0.1a 18.0 ± 0.3abc
2 3.4 ± 0 --- --- 15.8 ± 0.1b --- ---
4 3.4 ± 0.1 3.7 ± 0.0c 3.6 ± 0 15.7 ± 0.0b 17.3 ± 0.6ab 17.7 ± 0.3ab
6 3.7 ± 0.0c 3.6 ± 0.1 17.9 ± 0.1b 18.0 ± 0.2abc
8 3.6 ± 0.0bc 3.6 ± 0.1 17.6 ± 0.3ab 17.5 ± 0.4a
10 3.5 ± 0.0ab 3.6 ± 0 18.1 ± 0.1b 18.5 ± 0.0c
11 3.6 ± 0.0bc 3.5 ± 0 17.9 ± 0.1b 18.3 ± 0.0bc
23
24
Sensory Analysis – Discrimination Test
This sensory study investigated if there is a statistically significant difference
between the sensory aspects of UV treated and thermally pasteurized grape juice. Table
3.3 lists the results of the triangle test performed by the untrained panelists comparing
the UV-treated (at 14 mJ/cm2) juice and thermally pasteurized (at 71°C for 6 s) juice.
Table 3.3. Results from triangle test for UV treated (at 14 mJ/cm2) and thermally
pasteurized (71°C for 6 s) juice for the three different grape varieties.
Grape Variety Panelists
(N)
Correct Judgments
reported
Correct Judgments required for
significance (p < 0.05)
Concord
Niagara
Riesling
40
40
40
24*
23*
24*
19
19
19
* indicates statistical difference between treatments at p = 0.05
The untrained panelists were instructed to taste a set of three juice samples and
to identify the odd sample. The minimum number of correct responses to establish
significance at p = 0.05 with 40 panelists is 19 (Lawless and Heymann 2010). Therefore,
with all three grape varieties (Concord, Niagara and Riesling) participants were able to
perceive a statistically significant difference between the UV treated and thermally
pasteurized juice.
25
Most studies that perform sensory evaluation of UV treated juice compare the
samples with untreated juice to see if the process has any effect on the sensory
characteristics of the juices (Donahue et al., 2004; Caminiti et al., 2010; Guevara et al.,
2012). Tandon and coworkers (2002), found there was no statistical significance
difference between thermally processed and UV treated apple cider. However, these
findings are consistent with those reported by Pala & Toklucu (2013) with panelists being
able to differentiate UV treated and the thermally processed orange juice. The difference
in the findings of the studies conducted could be due to the different composition of the
fruit juices used and the UV dose applied.
26
CHAPTER 4
CONCLUSIONS AND RECOMMENDATION FOR FUTURE WORK
The thermal treatment applied (71°C for 6 seconds) to the three varieties of grape
juices produced 2.0 to 3.6 log10 reductions in total aerobic microbe counts and 3.6 to 5.6
log10 reductions in mold and yeast counts. The findings of the study indicate that the
shelf-life of refrigerated, minimally thermal processed grape juice ranged from four weeks
(Concord) to eleven weeks (Niagara and Riesling). The difference could be due to the
varieties used, the initial microbiological counts or contamination of the packaging
material. The shelf life of the three varieties didn’t end due to high microbiological counts
but due to visible mold growth in the samples. In future research, in addition to
microbiological analysis it would be useful to conduct sensory analysis and
physicochemical measurements during the shelf life study to detect if there is a decline in
quality before the end of the shelf-life.
The sensory evaluation of the grape juices demonstrated that participants were
able to differentiate between the UV treated and thermal pasteurized juice in all the
varieties studied. There should be further investigation in order to determine which
sensory characteristics (color, aroma or flavor) of the juices the consumers were able to
perceive as different by using descriptive analysis. Also, preference testing should be
done to determine if there is a difference in consumer preference between thermal
processed grape juice and UV treated grape juice.
27
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Caminiti, I.M. Palgan, I. Muñoz, A. Noci, F. Whyte, P. Morgan, D.J. Cronin, D.A.
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Donahue, D., Canitez, N., & Bushway, A. (2004). UV inactivation of E. Coli
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31
APPENDIX
A. DATA FROM THE SHELF LIFE STUDY
Concord Grape Juice
Table A.1. Total Plate Count results for thermally processed Concord grape juice (71°C
for 6 s) over storage period (at 7°C)
Week Replicates Dilution
Count
1
Count
2 Average Log10
Total
log10
CFU/mL
0
1 0 63 89 76 1.8808 1.88
2 0 57 79 68 1.8325 1.83
3 0 58 75 66.5 1.8228 1.82
2
1 0 58 38 48 1.6812 1.68
2 0 57 47 52 1.716 1.72
3 0 55 31 43 1.6335 1.63
4
1 1 5 9 7 0.8451 1.85
2 1 1 5 3 0.4771 1.48
3 1 12 5 8.5 0.9294 1.93
32
Table A.2 Mold and Yeast Counts for thermally processed Concord grape juice (71°C for
6 s) over storage period (at 7°C)
Week Replicates Dilution Count
1
Count
2 Average Log10
Total
log10
CFU/mL
0
1 0 0 0 0 --------- 0
2 0 0 0 0 ---------- 0
3 0 0 0 0 ---------- 0
2
1 0 0 0 0 ---------- 0
2 0 0 0 0 ---------- 0
3 0 0 0 0 ---------- 0
4
1 0 0 0 0 ---------- 0
2 0 0 0 0 ---------- 0
3 0 0 0 0 ---------- 0
33
Niagara Grape Juice
Table A.3. Total Plate Count for thermally processed Niagara grape juice (71°C for 6 s)
over storage period (at 7°C)
Week Replicates Dilution Count 1 Count 2 Average Log10
Total
log10
CFU/mL
0
1 1 7 16 11.5 1.0607 2.06
2 1 15 18 16.5 1.2175 2.22
3 1 22 24 23 1.3617 2.36
2
1 2 8 3 5.5 0.7404 2.74
2 2 1 2 1.5 0.1761 2.18
3 2 3 1 2 0.301 2.3
4
1 2 6 14 10 1 3
2 2 11 13 12 1.0792 3.08
3 2 14 16 15 1.1761 3.18
6
1 2 16 10 13 1.1139 3.11
2 2 9 17 13 1.1139 3.11
3 2 50 82 66 1.8195 3.82
8
1 1 56 15 35.5 1.5502 2.55
2 1 47 6 26.5 1.4232 2.42
3 1 7 6 6.5 0.8129 1.81
10
1 2 86 24 55 1.7404 3.74
2 2 1 2 1.5 0.1761 2.18
3 2 86 1 43.5 1.6385 3.64
11
1 2 1 2 1.5 0.1761 2.18
2 2 1 1 1 0 2
3 2 1 1 1 0 2
34
Table A.4. Mold and Yeast Counts for thermally processed Niagara grape juice (71°C for
6 s) over storage period (at 7°C)
Week Replicates Dilution Count
1
Count
2 Average Log10
Total
log10
CFU/mL
0
1 0 0 0 0 ---------- 0
2 0 0 0 0 ---------- 0
3 0 0 0 0 ---------- 0
2
1 0 0 0 0 ---------- 0
2 0 0 0 0 ---------- 0
3 0 0 0 0 ---------- 0
4
1 0 0 0 0 ---------- 0
2 0 0 0 0 ---------- 0
3 0 0 0 0 ---------- 0
6
1 1 9 14 11.5 1.0607 2.06
2 1 3 5 4 0.6021 1.6
3 1 2 11 6.5 0.8129 1.81
8
1 2 56 32 44 1.6435 3.64
2 1 30 7 18.5 1.2672 2.27
3 1 25 6 15.5 1.1903 2.19
10
1 2 28 14 21 1.3222 3.32
2 0 1 1 1 0 0
3 1 69 42 55.5 1.7443 2.74
11
1 0 1 2 1.5 0.1761 0.18
2 0 1 1 1 0 0
3 0 1 1 1 0 0
35
Riesling grape juice
Table A.5. Total Plate Count for thermally processed Riesling grape juice (71°C for 6 s)
over storage period (at 7°C)
Week Replicates Dilution Count 1 Count 2 Average Log10
Total
log10
CFU/mL
0
1 2 7 2 4.5 0.6532 2.65
2 1 9 6 7.5 0.8751 1.88
3 1 13 6 9.5 0.9777 1.98
2
1 2 2 3 2.5 0.3979 2.4
2 2 3 2 2.5 0.3979 2.4
3 2 6 1 3.5 0.5441 2.54
4
1 2 5 2 3.5 0.5441 2.54
2 2 28 19 23.5 1.3711 3.37
3 2 24 35 29.5 1.4698 3.47
6
1 1 23 90 56.5 1.752 2.75
2 2 11 10 10.5 1.0212 3.02
3 2 8 10 9 0.9542 2.95
8
1 1 19 28 23.5 1.3711 2.37
2 2 9 5 7 0.8451 2.85
3 2 2 6 4 0.6021 2.6
10
1 1 16 12 14 1.1461 2.15
2 1 22 15 18.5 1.2672 2.27
3 2 4 9 6.5 0.8129 2.81
11
1 1 19 12 15.5 1.1903 2.19
2 1 5 8 6.5 0.8129 1.81
3 1 13 17 15 1.1761 2.18
36
Table A.6. Mold and Yeast Count for thermally processed Riesling grape juice (71°C for
6 s) over storage period (at 7°C)
Week Replicates Dilution Count
1
Count
2 Average Log10
Total
log10
CFU/mL
0
1 0 0 0 0 ---------- 0
2 0 0 0 0 ---------- 0
3 0 0 0 0 ---------- 0
2
1 0 0 0 0 ---------- 0
2 0 0 0 0 ---------- 0
3 0 0 0 0 ---------- 0
4
1 0 0 0 0 ---------- 0
2 0 0 0 0 ---------- 0
3 0 0 0 0 ---------- 0
6
1 0 42 59 50.5 1.7033 1.7
2 0 83 66 74.5 1.8722 1.87
3 0 51 43 47 1.6721 1.67
8
1 0 1 1 1 0 0
2 2 17 52 34.5 1.5378 3.54
3 0 1 6 3.5 0.5441 0.54
10
1 1 1 1 1 0 1
2 0 1 1 1 0 0
3 0 1 1 1 0 0
11
1 0 10 5 7.5 0.8751 0.88
2 0 6 1 3.5 0.5441 0.54
3 0 1 1 1 0 0
37
Figure A.1. Sample test ballot for grape juice sensory triangle test