influence of high temperature stress on postharvest quality of processing and non-processing tomato...

INFLUENCE OF HIGH TEMPERATURE STRESS ON POSTHARVEST QUALITY OF PROCESSING AND

NON-PROCESSING TOMATO CULTIVARS

MAJEED MOHAMMED', L.A. WILSON and P.I. GOMES

Department of Crop Science Faculty of Agriculture,

The University of the West Indies St. Augustine, Trinidad, W.I.

Accepted for Publication January 30, 1995

ABSTRACT

Tomato fruits from a processing cultivar, 'Dorado ' and a non-processing cultivar, 'Star Pak ', were subjected to three prestorage heat treatments over a 5-hperiod under unshaded (34f2C), shaded (29f2C) andpre-cooled (20f2C) conditions followed by storage for I8 and 36 days at 20C. Terminal symptoms of heat injury and disease infection were evaluated in relation to changes in physical, physiological and chemical characteristics of the fruits during storage. Unshaded fruits demonstrated effects of heat injury resulting in reduced bioelectrical resistance, increased electrolyte leakage, increased total acidity and decreased total soluble solids during storage at 20C. Pre-cooling and shading proved to be effective methods to remove or prevent accumulation of field-heat and minimize heat injury. Pre-cooling was most effective, accounting for 28% and 14% less infected fruits for 'Dorado ' and 'Star Pak', respectively, compared with unshaded fruits, afer 36 days storage at 20C.

INTRODUCTION

Under normal harvesting procedures in the Caribbean, tomato fruit are allowed to remain in field containers up to 4-6 h with no protection from the sun. Moreover, after assembly of containers at field collection points and during transport to the packinghouse, the tight packing of field containers

'To whom correspondence should be addressed.

Journal of Food Quality 19 (1996) 41-55. All Rights Reserved. "Copyright 1996 by Food & Nutrition Press, Inc., Trumbull, Connecticut. 41

42 M. MOHAMMED, L.A. WILSON and P.I. GOMES

restricts air movement and respiratory heat loss. Also, containers of fruits sometimes remain unstacked for several hours before unloading or transfer to ripening and storage rooms. In a recent survey conducted by Mohammed (1992), tomato fruit temperatures above 40C were recorded at several stages in the handling system including harvesting and postharvest transportation, storage and sales display. Losses up to 21%, due to heat injury, were reported.

Tomatoes are sensitive to heat stress. Exposure of fruits to temperatures above 30C suppresses many of the parameters of normal fruit ripening including color development, softening, respiration rate and ethylene production (Buescher 1979; Chachin et al. 1980; Hicks el al. 1983). It is also well known that exposure of fruit to temperature extremes approaching 40C can induce metabolic disorders and facilitate fungal and bacterial invasion (Rickard and Coursey 1979). Although symptoms of heat injury and disease incidence are easily observed at the end of storage, the incipient incidence of these disorders is often not recognized in time to effect corrective treatment.

Chen el al. (1982) and Inaba and Crandall (1988) found that estimates of electrolyte leakage were good indicators of incipient heat injury in tomatoes. No attempt was made, however, to correlate those estimates with other parameters of normal fruit ripening which are known to be affected by heat injury.

In the experiments reported here, tomato fruits from one processing and one non-processing cultivar were subjected to three postharvest heat treatments, symptoms of heat injury and disease infection were recorded and these symptoms were compared with measurements of physical, physiological and chemical characteristics which might indicate incipient metabolic disorders. The significance of the data for understanding the genesis of heat injury symptoms and the management of the postharvest handling system to avoid heat injury are discussed.

MATERIALS AND METHODS

One processing tomato cultivar, 'Dorado', and one non-processing, 'Star Pak' , with good postharvest quality characteristics but susceptible to heat injury, were field grown at the University Field Station, Valsayn, Trinidad. Mature- green fruits were hand harvested, washed, and sorted for uniformity and freedom from defects. Fruit maturity was determined in the field on the basis of the size of the fruit, its position on the plant and the smoothness of its shoulder as well as by observing locule development in a representative sample of fruit cut mature-green as described by Kader et al. (1977) and Dodds and Ludford (1990). Fruits were harvested between 8:OO-8:30 with prevailing air temperatures of 34+2C and 60-70% R.H. External and internal fruit temperatures of both cultivars were determined on a few representative samples

HIGH TEMPERATURE STRESS ON TOMATOES 43

with a Fisher Scientific Digital Hygrometer-thermometer (Model EAI, Spring- field, NJ).

Sixty fruits of each cultivar were subjected to 3 prestorage heat treatments. The first treatment consisted of fruits spread in a single layer on an open cardboard carton and exposed to the sun over a 4-h period where air temperatures averaged 34 f2C. The second treatment consisted of fruits subjected to shaded conditions under a galvanized iron-covered shed, (average air temperatures reached 29f2C). The third treatment consisted of fruits pre-cooled in a storage room at 1OC for five h until average fruit temperature was 20f2C. Each treatment was monitored over a 5-hour period and at the end of each hour, the external (fruit surface) and internal temperatures were recorded with a Fisher Scientific Digital Hygrometer-thermometer.

At the end of each hour a random sample of ten fruits of each cultivar was selected from each treatment and examined for heat injury, bioelectrical resistancy, and electrolyte leakage. Heat injury was rated from 1 - 5 with 1 = no injury; 2 = slight; 3 = moderate; 4 = extensive; 5 = severe. The methodology for bioelectrical resistance and electrolyte leakage was similar to that described by Lougheed et al. (1983) and King and Ludford (1983).

After five hours, unshaded, shaded and pre-cooled fruits of both cultivars were dipped in 300 ppm chlorine solution (25-27C) for 3 min, to control surface pathogens. Fruits were spread as a single layer on absorbent paper and left for 15 min under an oscillating fan until surface moisture had evaporated and then stored at 20C, 60-70% r.h. in separate cardboard cartons in a Gallenkarnp incubator (Citenco Ltd., England). Fruits of each cultivar were evaluated after 18 and 36 days for heat injury, bioelectrical resistance, electrolyte leakage, percentage infected or decayed fruits, pH, total titratable acidity and total soluble solids as described previously by Kader er aL(1977). Color was measured (mean of 8 circumferential measurements using a pink standard: L = 69.1, a = +23.4, b = +9.3), using a Gardner XL colorimeter (Gardner Pacific Scientific Co., Silver Springs, MD 20910). Readings for hue (tan-' bla), value (L) and chroma (aZ + b2) 1/2 were converted to a tomato color index as determined by Hobson (1987) using the equation, 2000 + L d a w ( H o b s o n et al. 1983). Firmness was determined on the opposite sides of each fruit at four locations with a Chatillon penetrometer ((Model DDP-5, J. Chatillon and Sons, Kew Gardens, NY) equipped with a 0.64 cm V-shaped chisel hammerhead and a gauge to record the force (9 cm-*) required for penetration (Miller et al. 1983).

The experiment consisted of three replicates with each replicate containing ten fruits. Data were analyzed as a completely randomized design with a factorial arrangement of variables, and significance tested by the F-test and Duncan's multiple range test where applicable after transformation for ranking (Steel and Torrie 1960). The experiment was repeated once.


RESULTS

Fruit and Pulp Temperatures

There was no significant difference between external or internal fruit temperatures for processing or non-processing tomato cultivars, thus the data in Fig. 1 are the means of both cultivars taken hourly. Unlike fruits from the pre-cooled and shaded treatments, unshaded fruits showed elevated and increasing temperatures as time progressed (Fig. 1).

Also, temperatures of pre-cooled fruits declined more rapidly (P S0.05) than that of shaded fruits (Fig. 1).

Y g

3 5 1 2 4

Duntion (hours)

FIG. 1. AVERAGE FRUIT TEMPERATURES OF TOMATOES SUBJECTED TO UNSHADED, SHADED AND PRECOOLED TREATMENTS AFTER 1-5 H


be-storage Effects of Heat Injury, Electrolyte Leakage and Bioelectrical Resistance

Visible evidence of heat injury as yellowish-white patches on the side of the fruit was not obvious until the fifth hour of exposure (Fig. 2). For both cultivars, 'Dorado' and 'Star Pak', unshaded fruits had higher heat injury ratings than shaded or pre-cooled fruits (Fig. 2). Heat injury ratings for shaded and pre-cooled fruits were not significantly different (Fig. 2).

-____.-

Dorado (Processing C Y . ) Star pak (wn-processing

Tomato Cultivars c v . )

FIG. 2. INCIDENCE OF HEAT INJURY ON PROCESSING AND NON-PROCESSING TOMATO CULTIVARS SUBJECTED TO UNSHADED, SHADED AND PRE-COOLED

TREATMENTS AFTER 5 H

Electrolyte leakage of 'Star Pak' and "Dorado', was higher (P<0.05) for unshaded fruits than pre-cooled fruits at the end of each consecutive hour (Table 1). 'Star Pak' exhibited more electrolyte leakage than 'Dorado' only for unshaded samples (Table 1). Unshaded 'Star Pak' and 'Dorado' tomatoes showed significant (P 50.05) increases in electrolyte leakage from the third and fourth hour onwards (Table 1). Similar increases in electrolyte leakage occurred for shaded 'Star Pak' tomatoes between the fourth and fifth hours (Table 1).

46 M. MOHAMMED. L.A. WILSON and P.I. GOMES

TABLE 1 . PERCENTAGE ELECTROLYTE LEAKAGE OF PROCESSING AND NON-PROCESSING

TOMATO CULTIVARS SUBJECTED TO UNSHADED, SHADED AND PRE-COOLED TREATMENTS AFTER 1-5 H

Electrolyte leakage ( % I

Time (Hour) Dorado (processing cv.) Star Pak (non-processing cv.)

Unshaded Shaded Pre-cooled unshaded Shaded Pre-cooled

1 14.1 cz 1 2 . 6 ab 11.4 a 15.9 c 12.2 ab 11.0 a

2 14.0 c 12.4 ab 11.5 ab 16.6 c 12.9 b 11.7 ab

3 14.2 c 1 2 . 7 ab 11.4 a 19.2 d 12.9 b 1 1 . 8 ab

4 17.3 d 12.9 ab 11.9 ab 22.0 e 13.2 b 11.9 ab

5 22.6 e 13.1 b 11.9 ab 25.3 f 15.2 c 12.2 ab

Mean separation by Duncan's multiple range test, P = 0.05

Bioelectrical resistance measurements of 'Dorado' showed no significant differences between unshaded, shaded or pre-cooled treatments until the fifth hour (Table 2). However, 'Star Pak', had higher (P10.05) bioelectrical resistance for pre-cooled tomatoes compared with unshaded fruits after the first hour and this continued for each consecutive hour thereafter (Table 2). Shaded fruits were also significantly (PlO.05) higher than unshaded fruits for 'Star Pak' after the fourth and fifth hours.

Linear regression models for the unshaded fruit treatment indicated that there were significant relationships between fruit temperature and both electrolyte leakage and bioelectrical resistance for both cultivars (Fig. 3). The threshold fruit temperature that initiated changes in membrane permeability leading to increasing electrolyte leakage and decreasing bioelectrical resistance differed according to exposure time to unshaded conditions and cultivar. For example, in Fig. 3, the mean threshold external and internal fruit temperatures at which significant (P 50.05) increases in electrolyte leakage were first recorded were 38.3C for 'Dorado' (9 = 0.87, s.e.= k1.46) and 37.0C for 'Star Pak' (3 = 0.98, see. = +0.57). These physical and physiological events coincided with fruit exposure times of 2.25 h and lh, respectively, when extrapolated against data presented in Fig. 1. Similarly, at mean threshold external and internal fruit temperatures of 40.3C for 'Dorado' (3 = 0.81, s.e. = +0.93) and s.e. = +0.55), significant decreases (PS0.05) in hioelectrical resistance occurred after 3.25 h and 2.25h, respectively (Fig. 3).


TABLE 2. BIOELECTRICAL RESISTANCE OF PROCESSING AND NON-PROCESSING TOMATO

CULTIVARS SUBJECTED TO UNSHADED, SHADED AND PRE-COOLED TREATMENTS AFTER 1-5 H

Bioelectrical resistance (kilohms) _ _ ~ ~~ __

T i m e (Hour) Dorado (processing cv.) Star Pak (non-processing c v . )

Unshaded Shaded Pre-cooled Unshaded Shaded Pre-cooled

1 15.2abz 17.333 79.2b 14.labcd 10.0def 79.0f

2 14.6ab 77.0b 79.0b 12.6abc 15.9cdef 10.4ef

3 14.0ab 76.913 79.210 12.0abc 75.lbcdef 70.ldef

4 74.lab 76.4b 70.9b 11.lab 15.3cdef 17.0def

5 10.4a 16.0b 10.0b 70.5a 74.0bcdef 77.2def

Mean separation by Duncan's multiple range test, P = 0 . 0 5

3.25 H

. . I I

BE$.-STAR PAK (B]

E L - STAR PAz((c)

\ 2.25 H

I L.S.D.=0.05 A: 'Dorado': B: 'Star Pak': C: 'Dorado': D: 'Star Pak':

y - -37.04 + 1.39X r2 - 0.87. s.e. - ~ 1 . 4 6 y = -40.73 + 1.58X r2 - 0.98, s.e. - k0.57 y - 94.53 - 0.68% r2 -0.81, s.e. - kO.93 y = 92.67 - 0.54X r2 - 0.86. 5.e. = kO.55

0135 36 37 38 39 40 41 41 43 44 4 5 , ~ TEMPERATURE (C)

FIG. 3. THE EFFECT OF FRUIT TEMPERATURE ON ELECTROLYTE LEAKAGE AND BIOELECTRICAL RESISTANCE OF UNSHADED 'DORADO' AND 'STAR PAK' TOMATOES


Post-storage Effects of Heat Injury, Electrolyte Leakage and Bioelectrical Resistance

'Dorado' tomatoes that were originally unshaded and subsequently transferred to 20C for 18 days showed more (P 50.05) visible symptoms of heat injury than shaded or pre-cooled fruits (Fig. 4). A similar relationship was observed between shaded and pre-cooled treatments (Fig. 4). These differences were inconsistent with changes in bioelectrical resistance measurements but consistent with declining electrolyte leakage values recorded between unshaded, shaded and pre-cooled fruits over the same time (Fig. 4).

After 18 days at 20C 'Star Pak' tomato fruit had higher (PSO.05) heat injury ratings in fruits that were originally unshaded compared with those shaded or pre-cooled fruits, respectively (Fig. 4). Shaded fruits showed more heat injury than pre-cooled fruits and had higher bioelectrical resistance (P < 0.05) values than unshaded fruits. For the same treatment an inverse relationship was obtained between bioelectrical resistance and electrolyte leakage (Fig. 4).

After 36 days at 20C, heat injury in shaded fruits of both cultivars was significantly lower (P10.05) than in unshaded fruits. Both unshaded and shaded fruits had more (P50.05) heat injury damage than pre-cooled fruits (Fig. 4). Although electrolyte leakage values showed a similar trend, those for bioelectrical resistance of shaded and pre-cooled fruits were higher (P 10.05) than for unshaded fruits (Fig. 4). Symptoms of heat injury became more obvious as fruit ripening progressed. Towards the latter part of the second storage period fruits developed blister-like areas, later forming large, slightly depressed, grayish-white areas with a dry, paper-like surface.

Firmness

Generally 'Dorado' was firmer than 'Star Pak' during storage periods at 20C (Table 3). Whereas for 'Dorado' after 18 days at 20C, unshaded samples were firmer (PSO.05) than shaded or pre-cooled fruit, for 'Star Pak' shaded fruits were firmer than unshaded or pre-cooled fruits. However, after 36 days at 20C, neither cultivar showed differences between treatments (Table 3).

Color Index

After 18 and 36 days at 20C pre-cooled fruit from both cultivars ripened more uniformly than shaded or unshaded fruits as indicated by superior color development (P<0.05), (Table 3). Unshaded fruits of the cultivar 'Dorado' ripened more slowly and had poorer color development than shaded and


5 4.5 1

3 s

.5 2.5

2

I .s I

0.5 0

;? 3

= .e

Dorndu Simr P o t [processing cv) _(iion.proccssinu cv?

70 L s W . 0 5 ~ T

LSoo.05 1 60

so I a

S n r Pal. Dvrnllo

LSD0.05T 1

3 6 Storage period ( d a y s )

Unshaded Rl Shaded El Pr-e-cooled

FIG. 4. HEAT INJURY, BIOELECTRICAL RESISTANCE AND ELECTROLYTE LEAKAGE OF TOMATO CULTIVARS PREVIOUSLY SUBJECTED TO UNSHADED,

FOR 18 AND 36 DAYS, RESPECTIVELY SHADED AND PRE-COOLED TREATMENTS AND SUBSEQUENTLY STORED AT 20C


TABLE 3. FIRMNESS, COLOR INDEX AND PERCENTAGE INFECTED FRUITS OF PROCESING

AND NON-PROCESSING TOMATO CULTIVARS PREVIOUSLY SUBJECTED TO UNSHADED. SHADED AND PRE-COOLED TREATMENTS FOLLOWED BY STORAGE

AT 20C FOR 18 AND 36 DAYS, RESPECTIVELY

Tomato cultivars storage

20 c Parameters period at Dorado (processing cv.1 Star Pak lnon-processing c v . )

Unshaded Shaded Pre-cooled Unshaded Shaded Pre-cooled

Firmness (9) 18 days 131.1~" 127.6bc 126.0b 72.0a 76.lb 71.8a

36 days 80.2a 8l.Oa 80.4a 69.2a 71.0a 69. Oa

Color index 18 days 41.la 44.lb 56.6d 45.9b 43.6ab 5 9 . 2 4

36 days 42.3a 4 6 . 4 ~ 59.9e 51.1~ 42.la 61.4e

Infected fruits ( $ 1 18 days 45.9de 33.9bc 20.la 5 2 . 4 ~ 37.2ab 32.6a

36 days 50.8e 40.0cd 22.8a 48.9~ 45.1bc 34.9a

Mean separacion by DUncan's multiple range cest. P = 0.05

pre-cooled treatments (Table 3). For both cultivars, color development was more uniform and advanced for pre-cooled treatments with increasing storage duration, at 20C. A similar observation was obtained for shaded 'Dorado' as well (Table 3).

Percentage Infected Fruits

There were fewer infected fruit in pre-cooled 'Dorado', after 18 and 36 days than in shaded or unshaded fruits (Table 3). In the shaded treatment 'Star Pak' fruit, had more decay than 'Dorado' fruit. However, after 18 and 36 days at 20C, shaded or pre-cooled 'Dorado' fruits were less ( P I 0.05) infected than unshaded fruits (Table 3).

Chemical Analyses

After 18 and 36 days at 20C, fruits of both cultivars originally unshaded and shaded showed lower pH values pre-cooled fruit treatments (Table 4).


TABLE 4.

TOMATO CULTIVARS PREVIOUSLY SUBJECTED TO UNSHADED, SHADED AND PRE-COOLED TREATMENTS FOLLOWED BY STORAGE AT 2OC FOR 18 AND 16

DAYS, RESPECTNELY

CHEMICAL QUALITY ATTRIBUTES OF PROCESSING AND NON-PROCESSING

Tomato cultivars Chemical Storage quality attribute 20 c

period ac Dorado lprocessing cv.1 Star Pak (non-processing cv. )

Vnshaded Shaded Pre-cooled unshaded Shaded Pre-cooled

pn 18 days 4.15az 4 .15a 4.34bc 4.06a 4.10a 4.41b

36 days 4.18ab 4.18ab 4.42~ 4.14a 4.16a 4.39b

Titlacable acidity (mg/lOog fresh wt.) 18 days 0.48~ O.4lbc 0.34ab 0.30~ 0.26bc 0.21ab

36 days 0.4Oabc 0.37ab 0.31a 0.26bc 0.25bc 0.19a

Total soluble solids 18 days 4.31a 4.42b 4.71d 4.Ola 4.13b 5.24e

36 days 4.60~ 4.71d 5.16e 4.22~ 4.438 5.22e

2 Mean separation by mncan's multiple range test. P = 0.05

Total titratable acidity for unshaded and shaded fruits of both cultivars was significantly (P10.05) higher than pre-cooled fruit after 18 days (Table 4). With the exception of pre-cooled 'Star Pak' fruits, total soluble solids increased as storage time doubled for all treatments (Table 4). There was also a progressive increase in total soluble solids for both cultivars between unshaded, shaded and pre-cooled fruits at each storage interval (Table 4).

DISCUSSION

It is important to understand the effects of short periods of exposure to high temperatures on the postharvest quality characteristics of tomatoes, as high temperatures occur during harvesting in the tropics. The incidence of heat injury in unshaded versus pre-cooled fruits (Fig. 2) reported here was consistent with findings of Inaba and Crandall (1988). Accordingly it is concluded that under tropical conditions, critically high temperatures may persist in tomato fruits causing heat injury if appropriate postharvest treatments, such as pre- cooling, are not implemented.


The inverse relationship between electrolyte leakage and bioelectrical resistance is a more reliable indicator of changes in membrane integrity due to heat stress than subjective evaluations of visual heat injury symptoms (Tables 1 Fig. 3.).

A higher (P 50.05) percentage electrolyte leakage occurred in unshaded 'Star Pak' than 'Dorado' tomatoes after each consecutive hour and increased progressively as the duration of exposure increased (Tables 1 and 2). Yakir et al. (1984) obtained similar responses to high temperature stress from several processing lines of tomatoes and attributed this to the sensitivity of specific pigments and the concentration of carotenoids among cultivars.

Other studies (Chen et al. 1982; Inaba and Crandall 1988) have reported electrolyte leakage as a suitable indicator of high temperature injury although no simultaneous comparisons were taken for other objective methods such as bioelectrical resistance.

The absence of objective methods to detect heat injury in tomatoes before visible symptoms develop would lead to passage of fruits with initial damage to later stages in the marketing system, possibly resulting in postharvest losses. Evidence of this was seen in this investigation as fruits from both cultivars that were pre-cooled showed less visible symptoms of heat injury compared with shaded and unshaded fruits when subsequently stored at 20C for 18 or 36 days, respectively (Fig. 4). However, values of physiological characteristics of electrolyte leakage and bioelectrical resistance, indicative of possible heat injury could be detected in these fruits as early as 2.25 h following exposure to temperatures in excess of 38C prior to storage.

For the processing cultivar 'Star Pak', after 18 and 36 days, respectively at 20C, distinct relationships between heat injury, electrolyte leakage and bioelectrical resistance were obtained (Fig. 4). Electrolyte leakage accelerated by the initiation of heat stress and fruit ripening during storage (Wang and Adams 1980; King and Ludford 1983) accounted for the changes in membrane permeability observed.

Previous studies (Ogura et al. 1976) were in partial agreement with the data presented in Table 3 to the effect that exposure of tomatoes to high temperatures (34 f2C) for short periods inhibited the development of red color and softening after fruits were transferred to lower temperatures. After 18 days, unshaded 'Dorado' fruits exhibited greater (P (0.05) suppression in fruit softening than pre-cooled fruits. Upon prolonged storage, that is, up to 36 days, this trend did not persist (Table 3). Ogura et al. (1976), argued that the suppression of pectinesterase and polygalacturonase activity was brought about by brief exposure of fruits to temperatures above 33C. In this study, unshaded fruits were exposed to external temperatures of up to 34f2C, with fruit temperatures increasing to 46.2C, compared with pre-cooled fruits where temperatures after 5 h averaged 2Ok2C (Fig. 1). These results contradict those


reported by Buescher (1979) where opposite effects for fruit firmness were obtained.

However, Buescher (1979) used fruits at the breaker stage while this study and that of Ogura ef al. (1976) used mature-green fruits. Perhaps the capacity to synthesize pectolytic enzymes was reduced in mature-green fruits when briefly exposed to temperatures above 33C in contrast to those held at the same temperature but at a more advanced stage of maturity in experiments conducted by Buescher (1979). This interpretation was in line with the data obtained in this study for the longer storage period where there were no differences in firmness after 36 days at 20C (Table 3). Apparently, the suppression of pectolytic enzymes was reversible, since increased fruit ripening occurred with longer duration at 20C. The data presented in Table 3 were in agreement with this deduction, with the overall effect of color development being more pronounced with the non-processing compared with the processing one.

Rickard and Coursey (1979) indicated that exposure of produce to temperature extremes approaching 40C can induce metabolic disorders and facilitate pathological invasion by fungi and bacteria causing diseases, increased losses and reduced shelf-life of produce similar to losses we experienced and presented in Table 3. Pre-cooling had its greatest effect on postharvest disease control for the processing cultivar (Table 3). The benefits of pre-cooling in extending shelf life as well as reducing pathological decay supported previous results reported by Mitchell et al. (1972), Kasmire (1976), Thompson ef al. (1960) and Sommer (1982).

The results of this investigation also suggested that a delay or failure to remove field heat immmediately after harvest in order to lower fruit temperatures could result in changes in chemical composition of fruits upon ripening. Thus exposure of fruits of both cultivars to elevated temperatures (unshaded treatments) resulted in thermal stress in the form of a 'heat-shock' (Dunlap et al. 1991) that affected quality determining characteristics of increased acidity and decreased total soluble solids (Table 4). This stress was retained in fruits such that the chemical changes described above were not altered to the levels reported for pre-cooled or even shaded fruits, when subsequently transferred to storage at 20C. Thus, the data in this investigation confirmed that temperature management before and during storage was paramount in modern systems for postharvest handling and marketing of tomatoes.

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