Short communication
Effects of various heat treatments on the induction of cold
tolerance and on the postharvest qualities of ‘Star Ruby’
grapefruit
Ron Porat *, David Pavoncello, Jacob Peretz, Shimshon Ben-Yehoshua,
Susan Lurie
Department of Posthar6est Science of Fresh Produce,Agricultural Research Organization,The Volcani Center,PO Box6,
Bet Dagan50250,Israel
Received 9 August 1999; accepted 28 October 1999
Abstract
We examined the effects of various heat treatments on the induction of cold tolerance in red grapefruit (Citrus paradisicv. Star Ruby) and evaluated their effects on various other postharvest quality parameters such as decay development, weight loss, peel color, and juice total soluble solids (TSS) and acid content. Various heat treatments, including prestorage conditioning regimes of 3 days at 21°C and 7 days at 16°C, a hot water dip (HWD) at 53°C for 2 min, a hot water brushing (HWB) treatment at 60°C for 30 s, and curing (3 days at 36°C) achieved 70 – 90% reduction in the chilling injury (CI) index, as compared with control untreated fruit, following 6 weeks of cold storage at 2°C and an additional week at 20°C. Of the various treatments that induced cold tolerance, only HWB significantly reduced postharvest decay development. Conditioning and curing, which required longer exposure periods of 3 – 7 days at relatively high temperatures, significantly increased fruit weight loss, enhanced peel color alteration, and increased the juice TSS/acid ratios. On the other hand, the short postharvest heat treatments, including the HWD at 53°C for 2 min and the HWB at 60°C for 30 s, did not affect fruit weight loss, color, or TSS and acidity levels. Overall, it is concluded that short postharvest heat treatments, including either HWD or HWB are preferable, since they effectively induce tolerance to cold temperatures in ‘Star Ruby’ grapefruit without impairing any other postharvest qualities. Of these treatments, the new HWB is faster and could be used to clean and disinfect the fruit, and simultaneously to enhance its CI tolerance. © 2000 Elsevier Science B.V. All rights reserved.
Keywords:Chilling injury; Citrus; Grapefruit; Hot water; Postharvest; Star Ruby
www.elsevier.com/locate/postharvbio
* Corresponding author. Tel.: +972-3-968-3624; fax: + 972-3-968-3622.
E-mail address:[email protected] (R. Porat)
1. Introduction
Citrus fruits are subtropical and are known to be susceptible to chilling injury (CI) development during cold storage (Chalutz et al., 1985; Paull, 1990; Kader and Arpaia, 1992). This sensitivity to low temperatures has serious economic implica-tions, since cold storage still provides an impor-tant quarantine treatment required to export citrus fruit to fly-free zones in many countries (US Department of Agriculture. Animal and Plant Protection Services, 1976).
Several postharvest heat treatments have been reported to induce fruit tolerance to cold tempera-tures and to reduce the development of CI symp-toms during cold storage and cold quarantine treatments (Hatton, 1990; Wang, 1993; Lurie, 1998a,b; Schirra and Ben-Yehoshua, 1999). In citrus, these heat treatments include prestorage conditioning for either 3 days at 21°C or 7 days at 16°C (Hatton and Cubbedge, 1982, 1983; Mc-Donald, 1986), hot water dips (HWD) at 53°C for 2 – 3 min (Wild and Hood, 1989; Rodov et al., 1995; Schirra and D’hallewin, 1997; Schirra et al., 1997), and curing for 3 days at 36°C under water-saturated conditions (Ben-Yehoshua et al., 1989; Rodov et al., 1995; Mulas et al., 1996). In addi-tion to these established heat treatments, a new hot water brushing (HWB) treatment, involving the application of hot water to fruit moving along a set of brush rollers, which has recently been developed to clean and disinfect fruit and vegeta-bles (Fallik et al., 1999), was also found to induce tolerance of low temperatures in ‘Star Ruby’ grapefruit (Porat et al., unpublished).
In the present study, we examined the effects of various postharvest heat treatments on the induc-tion of low temperature tolerance in ‘Star Ruby’ grapefruit, and their effects on various other postharvest quality parameters. Our major objec-tive was to evaluate whether any of the other heat treatments could substitute for the prolonged con-ditioning treatment required before cold steriliza-tion. ‘Star Ruby’ is one of the most important pigmented grapefruit varieties, and is widely used in local and export commerce. Nevertheless, few reports are available on its postharvest handling and cold tolerance characteristics.
2. Materials and methods
2.1. Plant material and storage conditions
Red grapefruit (C.paradisicv. Star Ruby) were harvested randomly from a local orchard and were used 1 day after harvest. For storage experi-ments, fruit were kept for 6 weeks at 2°C and then transferred for another week to shelf-life condi-tions at 20°C. The relative humidity was 90% in both storage rooms. Each treatment included four boxes, each containing 30 fruits. The experiment was repeated three times during the 1998/1999 harvest season at November 17, February 2 and March 22. The relative effects of the various heat treatments on the reduction of CI and on fruit quality were similar in all experiments. However, the absolute amount of CI and decay differed during the harvest season. All results presented in this study are from the second experiment, carried out at the middle of the harvest season.
2.2. Posthar6est heat treatments
Temperature conditioning for either 3 days at 21°C or 7 days at 16°C was conducted by keeping the fruit in separate storage rooms at the above temperatures for the required periods. After-wards, the fruit were transferred to cold storage at 2°C.
HWD treatments at 53°C for either 2 or 3 min were performed by putting the fruit in plastic containers with holes and dipping them into a 250-l thermostatic bath as described elsewhere (Rodov et al., 1995).
HWB treatments at 55 and 60°C were applied as a rinse onto fruit moving along a set of brush rollers as described by Fallik et al. (1999). The fruits were exposed to the respective HWB tem-peratures for 30 s.
Curing was conducted by placing the fruit in sealed polyethylene packages in cartons and keep-ing them at 36°C for 72 h, as described previously (Ben-Yehoshua et al., 1987).
2.3. E6aluation of CI
After 6 weeks of storage at 2°C and an addi-tional week at 20°C, fruits were sorted into four categories according to their CI severity: none (score 0, no pitting), slight (score 1, a few scat-tered pits), moderate (score 2, pitting covering up to 30% of the fruit surface), and severe (score 3, extensive pitting covering \30% of the fruit
sur-face). The CI index was determined for each treatment by multiplying the number of fruit in each category by their score, and then dividing this sum by the total number of fruit assessed.
2.4. E6aluation of decay
Evaluation of decay incidence was performed after 6 weeks of storage at 2°C and an additional week of shelf life at 20°C. The total number of fruit manifesting decay symptoms (mainly green mold) was determined in each treatment and ex-pressed as the decay percentage.
2.5. Chemical and color determination
The total soluble solids (TSS) content in the juice was determined with a refractometer, and the acidity percentage was measured by titration with 0.1 N NaOH to pH 8.3, the results being expressed as citric acid. Peel color was determined by measuring the hue angle with a Minolta Chromo Meter, model CR-200 (McGuire, 1992).
2.6. Statistical analysis
Results were analyzed with the SigmaStat statistical software (Jandel Scientific Software, San Rafael, CA). Student – Newman – Keuls one-way analysis of variance (ANOVA) tests on ranks were performed, and the results reported are sig-nificant at PB0.05.
3. Results
3.1. Effects of 6arious heat treatments on CI
incidence
All of the postharvest heat treatments examined in this study reduced CI incidence (Fig. 1). Among the various heat treatments tested, five treatments including prestorage conditioning for 3 days at 21°C and 7 days at 16°C, HWD at 53°C for 2 min, HWB at 60°C for 30 s, and curing (3 days at 36°C) were most effective in reducing the development of CI symptoms, and were not sig-nificantly different from each other. These treat-ments reduced the CI index of the fruit to 0.12 – 0.34 as compared with 1.07 in control non-treated fruit (Fig. 1). In our examinations, no visible CI symptoms were detected immediately after the 17 day quarantine treatment at 2°C (data not shown).
3.2. Effects of 6arious heat treatments on decay
de6elopment
Among the various heat treatments tested, only the HWB treatments at either 55 or 60°C signifi-cantly reduced postharvest decay development as compared with control untreated fruit; they
Fig. 2. Effects of various heat treatments on postharvest decay development in ‘Star Ruby’ grapefruit. Decay was evaluated after 6 weeks of storage at 2°C and an additional week of shelf-life conditions at 20°C. Data are means of four replica-tions per treatment, each containing 30 fruits. Columns marked by different letters are significantly different atPB 0.05 according to a Student – Newman – Keuls one-way ANOVA test on ranks.
7 days at 16°C and HWD, also reduced decay development somewhat, but their effect was not significant (Fig. 2). Following all treatments, the total amount of decay increased throughout the harvest season (data not shown).
3.3. Effects of 6arious heat treatments on fruit weight loss
Prestorage conditioning treatments for 3 days at 21°C and 7 days at 16°C, and curing for 3 days at 36°C, significantly increased fruit weight loss, to 4.0, 3.7 and 4.5%, respectively, as compared with only 2.5% in control untreated fruit (Table 1). The HWD and HWB treatments did not affect fruit weight loss (Table 1).
3.4. Effects of 6arious heat treatments on peel
color
The peel color of untreated ‘Star Ruby’ grape-fruit is normally reddish – yellow. Prestorage con-ditioning treatments for 3 days at 21°C and 7 days at 16°C, and curing for 3 days at 36°C significantly enhanced the fruit peel color change towards red, and after 6 weeks of storage at 2°C and an additional week at 20°C the fruit hue angle was 72.9, 72.8 and 66.8°, respectively, as compared with 78.7° in control untreated fruit (Table 1). The HWD and HWB treatments had no significant effect on fruit peel color (Table 1). duced it to only 7.4 and 7.5%, respectively, as
compared with 22.2% in the control fruit (Fig. 2). On the other hand, the curing treatment given for 3 days at 36°C under water-saturated conditions increased decay development to 40.2% (Fig. 2). Other heat treatments, including conditioning for
Table 1
Effects of various heat treatments on the postharvest quality of ‘Star Ruby’ grapefruita
Color (h°) TSS/acid ratio
Acid (%)
Treatment W. loss (%) TSS (%)
2.55 c 9.87 a 1.11 a 8.89 c 78.7 a
Control
Conditioning (21°C, 3 days) 3.97 ab 10.32 a 1.02 ab 10.12 b 72.9 b Conditioning (16°C, 7 days) 3.70 b 10.00 a 1.01 ab 9.89 b 72.8 b 78.0 a 8.78 c
1.14 a
HWD (53°C, 2 min) 2.22 c 10.01 a
2.28 c 9.66 a 1.11 a 8.70 c 79.0 a
HWD (53°C, 3 min)
2.52 c 9.89 a
HWB (55°C, 30 s) 1.17 a 8.45 c 79.7 a
1.15 a
HWB (60°C, 30 s) 2.33 c 9.82 a 8.54 c 78.4 a
4.53 a
Curing (36°C, 3 days) 10.07 a 0.90 b 11.19 a 66.8 c
3.5. Effects of 6arious heat treatments on juice TSS and acid content
None of the postharvest heat treatments had any significant effect on the juice TSS content (Table 1). On the other hand, the curing treat-ment significantly reduced the acid content in the juice to only 0.90% as compared with 1.11% in control untreated fruit (Table 1). Conditioning for 3 days at 21°C and 7 days at 16°C also reduced the juice acid content to 1.02 and 1.01%, respec-tively, but this effect was not significant (Table 1). Overall, conditioning for 3 days at 21°C and 7 days at 16°C, and curing resulted in significant increases of the juice TSS/acid ratio to 10.12, 9.89 and 11.19, respectively, as compared with 8.89 in the juice of control non-treated fruit (Table 1). The HWD and HWB treatments did not affect the juice TSS/acid ratio.
4. Discussion
Postharvest heat treatments have been used for many years to control fungal diseases and for insect disinfestation (Couey, 1989; Barkai-Golan and Phillips, 1991; Lurie, 1998a,b). In addition, heat treatments also enhance cold tolerance and can be commercially used to reduce CI damage (Hatton, 1990; Wang, 1993; Lurie, 1998a,b).
In the present study, we found that various postharvest heat treatments, including prestorage conditioning for 3 days at 21°C and 7 days at 16°C, HWD at 53°C for 2 min, HWB at 60°C for 30 s, and curing at 36°C for 3 days, reduced CI development in ‘Star Ruby’ grapefruit by about 70 – 90% as compared with control non-treated fruit (Fig. 1). No statistical differences were ob-served among the effectiveness of the above treat-ments, meaning that each of them could be used to reduce CI development (Fig. 1). Similar find-ings regarding the reduction of CI development after cold storage were observed also when the fruit were waxed and treated with fungicides after the heat treatments, as in commercial packing-houses (data not shown).
Our findings regarding the effects of the various postharvest heat treatments in reducing CI in
‘Star Ruby’ grapefruit are consistent with those of previous studies of the effects of each of these treatments separately on other citrus cultivars (Hatton and Cubbedge, 1982, 1983; McDonald, 1986; Wild and Hood, 1989; Rodov et al., 1995; Mulas et al., 1996; Schirra and D’hallewin, 1997; Schirra et al., 1997). However, as far as we know, the present study is the only one that compared the effects of all of these conditioning, HWD, HWB, and curing heat treatments together in the same citrus cultivar; the results show that they were all effective in reducing CI development (Fig. 1).
Although the various conditioning, HWD, HWB, and curing treatments were similarly effec-tive in reducing CI development, they had differ-ent effects on other postharvest quality parameters. The short postharvest heat treat-ments, including HWD for 2 min and HWB for 30 s, did not affect internal and external fruit quality parameters, whereas the longer treat-ments, including conditioning for 3 or 7 days and curing for 3 days, significantly increased fruit weight loss, enhanced changes in fruit peel color, and increased the juice TSS/acid ratios (Table 1). In general, these physiological changes that are caused by the long conditioning and curing heat treatments, especially the increase in fruit weight loss and the decrease in juice acid content, are undesirable and impair fruit quality. On the other hand, the enhancement of fruit pigmentation by the long heat treatments may be beneficial.
more stem-end rots and less green mold decay (data not shown). Probably, the curing treatment was excessive and resulted in heat damage that made the fruit more accessible to pathogens.
In conclusion, we recommend the use of short postharvest heat treatments, including either HWD or HWB, to reduce CI in ‘Star Ruby’ grapefruit, since they do not impair any of the other fruit postharvest quality parameters, as the longer conditioning and curing treatments do. Another advantage of the short HWD and HWB treatments is that they are simple to apply, since they can be incorporated into the packinghouse sorting line and do not require any special han-dling, whereas conditioning requires the use of different storage rooms and curing requires heat-ing and special wrappheat-ing. Nevertheless, it should be noted that hot water treatments at high tem-peratures may cause heat damage to sensitive citrus cultivars (Mulas et al., 1997; Schirra and D’hallewin, 1997). In ‘Star Ruby’ grapefruit, we did not observe any damaging effects with any of the hot water treatments examined in this study.
Among the short HWD and HWB treatments, HWB appears to be preferable since it also signifi-cantly reduced postharvest decay. Other advan-tages of the new HWB technique are that it also cleans the fruit and improves its general appear-ance, and requires a much shorter exposure time (10 – 30 s) than conventional hot water dip treat-ments, which usually require several minutes.
Acknowledgements
This research was partially funded by the Eu-ropean Union FAIR CT-4096. The current paper is a contribution from the Agricultural Research Organization, the Volcani Center, Bet Dagan, Israel, No. 415-99, 1999 series.
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