Water temperature for hydrocooling field cucumbers in
relation to chilling injury during storage
Jennifer R. DeEll
a,*, Cle´ment Vigneault
a, Ste´phanie Lemerre
baAgriculture and Agri-Food Canada,Horticultural Research and De
6elopment Centre,430Boul.Gouin,Saint-Jean-sur-Richelieu, Que´bec,Canada J3B3E6
bEtablissement National d’Enseignement Supe´rieur Agronomique de Dijon,21Boul.Oli
6ier de Serre,21800Que´tigny,France
Received 26 May 1999; received in revised form 9 August 1999; accepted 10 August 1999
Abstract
The objective of this study was to test the hypothesis that water temperatures less than the lowest recommended storage temperature (10°C) for cucumbers could be used for hydrocooling without inducing chilling injury or negatively affecting storage life. Field cucumbers were hydrocooled with water at 1.5, 3.5, 6, 8 or 10.5°C until the internal cucumber temperature reached 12°C, or hydrocooled with water at 1.5°C until the internal cucumber temperature reached 1.7, 8 or 12°C. Cucumber temperature at harvest was:20°C and the storage temperature was 12°C. Little or no visual symptoms of chilling injury were observed after 10 – 12 days of storage. However, chlorophyll fluorescence measurements indicated some chilling stress at the membrane level in cucumbers hydrocooled with water at temperatures below 6°C and in cucumbers hydrocooled with water at 1.5°C until the internal product temperature was 1.7°C, as indicated by lowerFv/Fm values. Approximately one third of the cucumbers from all hydrocooling
treatments developed rot. There were no significant differences in % marketable cucumbers or in % mass loss after 10 or 12 days of storage. These results suggest that cucumbers could be hydrocooled using water at temperatures below the recommended storage temperature of 10°C. However, it is not recommended to use water below 6°C or to cool the cucumbers below this temperature, due to increased risk of chilling injury as indicated by the chlorophyll fluorescence measurements. © 2000 Elsevier Science B.V. All rights reserved.
Keywords:Cucumis sati6usL.; Chlorophyll fluorescence
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1. Introduction
Cucumber (Cucumis sati6us L.) is a chilling sensitive commodity and thus should not be stored long-term at temperatures below 7 – 10°C
(Hardenburg et al., 1986; Lidster et al., 1988; Snowdon, 1991). Chilling injury may develop if cucumbers are stored at lower temperatures, as characterized by surface pitting and dark watery patches. This injury is generally followed by an increased tendency to decay, particularly when the temperature is raised. Time is a critical factor with chilling injury, and brief exposures (52 days) to * Corresponding author. Fax: +1-450-3467740.
E-mail address:[email protected] (J.R. DeEll)
temperatures below 10°C may cause no damage. The longer the period of exposure and the lower the temperature, the greater the damage associ-ated with chilling injury. However, visual symp-toms may not develop until the produce is returned to higher temperatures.
The recommended precooling methods for cu-cumbers are room cooling, air and forced-air evaporative cooling (Kasmire and Thompson, 1992). Nevertheless, hydrocooling may be used in certain situations (Wilson et al., 1995). It is gener-ally accepted that the temperature of the water used for hydrocooling is the same as the recom-mended storage temperature for the produce be-ing cooled. Therefore, a water temperature near 10°C is generally used for cucumbers. Lower wa-ter temperature would result in faswa-ter cooling, however, due to the chilling sensitive nature of cucumber it is often assumed that this would result in visual damage.
The objective of this study was to test the hypothesis that water temperatures less than the lowest recommended storage temperature (10°C) for cucumbers could be used for hydrocooling field cucumbers without inducing chilling injury or negatively affecting storage life. Chlorophyll fluorescence was used to assess the effects of chilling prior to the development of visual symp-toms, since it has been shown to be responsive to chilling stress in cucumber (van Kooten et al., 1992; Tijskens et al., 1994).
2. Materials and methods
2.1. Plant material
Freshly harvested field cucumbers (cv. ‘Speed-way’) were obtained from two local producers in Que´bec, during July and August 1998. Mar-ketable cucumbers of similar size, color, and ma-turity were selected for the experiments.
2.2. Hydrocooling and storage
Five groups of 15 cucumbers from each pro-ducer were selected. One thermocouple (type T) was placed into the center of each of five
cucum-bers from every group. Each lot of 15 cucumcucum-bers was then hydrocooled using a water temperature of 1.5, 3.5, 6, 8 or 10.5°C until the temperature among the five instrumented cucumbers averaged 12°C. Water temperatures during hydrocooling were maintained using a temperature control bath (model 1187, VWR Canlab, Ville Mont-Royal, Que´bec). The five instrumented cucumbers were discarded after cooling and the remaining ten were used for chlorophyll fluorescence measure-ments and quality evaluations during 10 days of storage at 12°C and 95% RH. This experiment was repeated on three successive days for each producer, to give three replications.
In a second experiment, groups of 15 cucum-bers from only one producer were hydrocooled with water at 1.5°C. Five of the cucumbers in each group were instrumented as described for the first experiment, and cooling continued until the temperature among the cucumbers averaged 1.7, 8 or 12°C. The cucumbers from this experiment were stored at 12°C and 95% RH for 12 days. This experiment was also repeated on three suc-cessive days, to give three replications.
2.3. Chlorophyll fluorescence measurements
Chlorophyll fluorescence measurements were taken at 12°C, using a modulated fluorometer (OS-500, Opti-Sciences Ltd., Tyngsboro, MA). The first measurements were taken for all cucum-bers within 30 min after hydrocooling. Additional measurements were taken after 1, 2, 4, 6, 8 and 10 days of storage in the first experiment, and after 3, 6, 9 and 12 days of storage in the second experiment. The dark-adapted parameters F0
(minimum fluorescence), Fm (maximum
fluores-cence), and Fv/Fm (Fv=Fm−F0) were evaluated
(DeEll et al., 1999) on two opposite sides near the center of each cucumber. The first side of the cucumber measured was the lightest side, followed by the darker opposite side.
No specific dark-adaptation period was neces-sary, as the cucumbers were stored in complete darkness. The Fv/Fm test (method 1 on the
operator to see well enough to handle the cucum-bers and to operate the instrument. Modulation intensity (660 nm) was set at 80 on the fluorome-ter, with the saturation intensity (35-W halogen lamp) at 255 for 2.0 s.
2.4. Quality e6aluations
The color of each cucumber was measured on the day of the hydrocooling treatments and every second day thereafter. Measurements for the sec-ond experiment were performed every 3 days. The color attributesaandbof two opposite sides near the center of each cucumber were measured using a chroma meter (model CR-200; Minolta, Osaka, Japan). The first side of the cucumber measured was the lightest side, followed by the darker oppo-site side. Chroma and hue angle were then calcu-lated as (McGuire, 1992):
chroma=(a2 blue to yellow axis.
All cucumbers were evaluated for chilling in-jury, decay development, marketability and mass loss at the end of each storage period. The inci-dences of chilling injury and decay were expressed as the percentage of cucumbers with symptoms, regardless of severity. Marketability was ex-pressed as the percentage of cucumbers that were marketable (e.g. no chilling injury and free of disease).
2.5. Statistical analyses
Both experiments were executed as randomized complete block designs, with days (repetitions) being the blocking factor. In the first experiment, the temperature of the water used for cooling a given lot from each producer was randomized within each day. In the second experiment, the internal target temperature was randomized. Since all of the cucumbers were initially at room tem-perature (:20°C), and since a significant amount of time was needed to cool the various groups, the groups that were cooled last were exposed to
room temperature for several hours more than those cooled first. The statistical analyses were therefore done using the time at room tempera-ture as a covariable. The Least Squares Means option of SAS was used to compare adjusted means using the paired T-test.
3. Results and discussion
Cucumber temperature at harvest was :20°C. The time taken to cool the cucumbers to 12°C using water at 1.5, 3.5, 6, 8 or 10.5°C was 21.5, 24, 27, 37 and 50 min, respectively. This demon-strates the rate advantage in using colder water. In the second experiment, hydrocooling was done using water at 1.5°C until the cucumbers reached 1.7, 7.5 or 12.4°C. The time taken to reach these temperatures was 61, 26 and 15 min, respectively. Little or no chilling injury was observed after 10 – 12 days of storage at 12°C in the hydrocooled cucumbers (Table 1). The small amount of injury observed in cucumbers hydrocooled with water at 1.5, 3.5, or 6°C until the cucumber temperature reached 12°C was not significantly different from the absence of chilling injury in cucumbers hydro-cooled with warmer water (experiment 1). In addi-tion, no chilling injury was observed when cucumbers were hydrocooled with water at 1.5°C until the cucumbers reached lower temperatures, 1.7 or 8°C (experiment 2).
Approximately one third of the cucumbers from all hydrocooling treatments developed rot after 10 or 12 days of storage at 12°C, and there were no significant differences in % marketable cucumbers (Table 1). Similar cucumbers that were not hydrocooled also showed the same level of disease (34.4% incidence) after 10 or 12 days at 12°C, indicating that the high incidence of rot after storage was not due to the hydrocooling treatment. Weather conditions before and during harvesting were very conducive to disease, with higher precipitation and temperatures than nor-mal, (Gae´tan Bourgeois, AAFC-HRDC, personal communication) and thus were likely the reason for the high incidence of rot during storage.
Table 1
Incidence of chilling injury and rot, marketable cucumbers, and mass loss After 10 days of storage at 12°C, after hydrocooling with water at 1.5, 3.5, 6, 8 or 10.5°C until the cucumber temperature reached 12°C (experiment 1); and after 12 days of storage at 12°C, after hydrocooling with water at 1.5°C until the cucumber temperature reached 1.7, 8 and 12°C (experiment 2).
Rot (%)
Temperature (°C) Chilling Injury (%) Marketable (%) Mass Loss (%)
Experiment1water
1.5 2.7 35.1 64.1 5.09
31.4
3.5 1.5 56.3 5.24
36.8 51.8
0.1 4.84
6
0
8 33.2 56.5 4.98
33.5 54.7
10.5 0 4.93
NS NS
NS NS
Significancea
Experiment2cucumber
1.7 0 32.3 42.6 6.80
33.0 37.2
0 6.95
8
0
12 31.4 63.6 6.63
Significancea – NS NS NS
aNS, not significant.
had similar mass loss after 10 days of storage (4.9 – 5.2%), while cucumbers hydrocooled with water at 1.5°C until the cucumbers reached 1.7, 8 or 12°C had similar mass loss after 12 days of storage (6.6 – 6.9%). Generally, up to 7% mass loss is acceptable before cucumbers become unmar-ketable (Kays, 1997).
Although few visual symptoms of chilling in-jury developed, chlorophyll fluorescence measure-ments indicated some chilling stress in cucumbers hydrocooled with water below 6°C (Fig. 1). Cu-cumbers hydrocooled with water at 1.5 or 3.5°C had lower Fv/Fm values than cucumbers
hydro-cooled with warmer water, indicating that the cold temperatures induced changes in the thy-lakoid membranes resulting in reduced exciton transfer efficiency of photosystem II (DeEll et al., 1999). Although the interaction of hydrocooling temperature and storage duration was not signifi-cant, some interesting trends were observed. Fv/Fm of cucumbers hydrocooled with water at
10.5°C averaged between 0.78 and 0.79 immedi-ately after cooling and during storage (Fig. 1). Immediately after hydrocooling with water at 8 or 6°C, Fv/Fm was below 0.78 and below 0.77,
re-spectively, but then increased to similar levels of cucumbers hydrocooled with 10.5°C water after 2 and 4 days of storage, respectively. On the other hand, Fv/Fmof cucumbers hydrocooled with
wa-ter at 1.5 or 3.5°C was below 0.76 immediately after cooling, and although it increased to be-tween 0.76 and 0.77 after 1 day in storage, it never reached the levels of the cucumbers hydrocooled with warmer water.
Fig. 1. Chlorophyll fluorescence (Fv/Fm) of cucumbers after
Fig. 2. Chlorophyll fluorescence (Fv/Fm) of cucumbers after
hydrocooling with water at 1.5°C until the cucumber tempera-ture reached 1.7, 8 and 12°C, and during 12 days of storage at 12°C. The main effects of cucumber temperature and storage duration are significant atPB0.001.
Neither hydrocooling treatment nor storage du-ration affected the color of the cucumbers (data not presented). However, the chroma and hue angle of the two cucumber sides were significantly different (Table 2). The lighter side of the cucum-ber had greater chroma and less hue angle, com-pared to the opposite cucumber side. Chlorophyll fluorescence also reflected differences between cu-cumber sides, as Fv/Fm was higher on the lighter
side of the cucumbers than on the opposite side (Table 2). However, there were no significant correlations between Fv/Fm and chroma (r2=
0.143) or hue angle (r2
= −0.132).
Eaks and Morris (1957) showed that cucumbers have a time – temperature relationship to chilling injury. Cucumbers held at 0°C for 2 days showed very slight chilling injury development (pitting) after an additional 2 days at 25°C, while cucum-bers held at 5°C began to exhibit chilling injury symptoms after 8 days, plus an additional 2 days at 25°C. Therefore, it is not surprising that hydro-cooling with water at temperatures from 1.5 to 10.5°C for 50 min or less did not result in high incidence of chilling injury in this study.
The visual symptoms of chilling injury, such as pitting or incipient decay, are secondary effects enhanced by higher temperatures, whereas the actual or primary damage is membrane alter-ations (Morris, 1982). Measurements of elec-trolyte leakage of cucumber peel tissue indicate that irreversible injury requires at least 7 days of continuous chilling at 4°C (Hariyadi and Parkin, 1991). van Kooten et al. (1992) found that mem-brane leakage in cucumber correlates well with Fv/Fm. In this study, Fv/Fm reflected changes in
the thylakoid membranes immediately after hy-drocooling (Figs. 1 and 2), even though the time – temperature combination was not enough to induce high incidence of the secondary visual symptoms of chilling injury during storage at 12°C (Table 1). The cucumbers were not held for additional days at 25°C because of the high inci-dence of rot development in all treatments and therefore, we cannot say whether or not more chilling injury would have developed in subse-quent higher temperatures.
Chlorophyll fluorescence measurements also in-dicated some chilling stress in cucumbers hydro-cooled with water at 1.5°C until the product temperature was 1.7°C (Fig. 2). Fv/Fm of these
cucumbers was below 0.74 immediately after cool-ing and only increased to :0.76 during storage. Although Fv/Fm was :0.76 immediately after
cooling for cucumbers hydrocooled with water at 1.5°C until the internal product temperature was 6 or 12 °C, it increased subsequently and averaged between 0.78 and 0.79 during storage, similar to the levels of cucumbers hydrocooled with water ]6°C (Fig. 1).
Table 2
Chroma, hue angle and chlorophyll fluorescence (Fv/Fm) of
lighter and opposite cucumber sides, during10 days of storage at 12°C, after hydrocooling with water at 1.5, 3.5, 6, 8 or 10.5°C until the cucumber temperature reached 12°C
Chroma
Cucumber side Hue angle Fv/Fm
123.1
31.2 0.785
Lighter side
11.4
Opposite side 132.51 0.775
*** ***
Significance ***
4. Conclusion
These results suggest that cucumbers could be hydrocooled using water at temperatures below the recommended storage temperature of 10°C. However, when water below 6°C was used and/or cucumbers were cooled to 1.7°C there was suffi-cient chilling stress to affect the exciton transfer efficiency of photosystem II and thus affect chlorophyll fluorescence. This indicates that some chilling stress at the membrane level was present in the cucumbers, although it was not enough to result in visual symptoms during storage at 12°C. Therefore, it is not recommended that water tem-peratures below 6°C be used for the hydrocooling of cucumbers or that the product is cooled down to 1.7°C, due to increased risk of chilling injury development. However, there is no need for the latter situation since the recommended storage temperature for cucumber is much warmer than this (10°C).
Acknowledgements
The authors wish to thank Yvan Garie´py, Bernard Goyette, and Dominique Roussel for their technical assistance, and Dr Shahrokh Kha-nizadeh for his statistical assistance.
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