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Preharvest chilling reduces low temperature breakdown incidence of kiwifruit
Postharvest Biology and Technology 38 (2005) 169–174
Preharvest chilling reduces low temperature breakdown incidence of kiwifruit Evangellos Sfakiotakis a , Georgios Chlioumis a , Dimitrios Gerasopoulos b,∗ b
a Aristotle University of Thessaloniki, Department of Horticulture, 54 124 Thessaloniki, Greece Aristotle University of Thessaloniki, Department of Food Science and Technology, 54 124 Thessaloniki, Greece
Received 4 February 2005; accepted 15 June 2005
Abstract To study the effects of preharvest chilling on low temperature breakdown incidence (LTB) of kiwifruit (Actinidia deliciosa [A. Chev.] C.F. Liang and A.R. Ferguson), a mist evaporative cooling system was established in the orchard and the accumulation of temperatures below 10 ◦ C in hours was recorded during fruit maturation. The fruit were harvested at three dates, and soluble solids content (SSC) and firmness were measured at harvest, while LTB incidence was determined following 24-week storage at −0.5 ◦ C. Fruit harvested immature had a high LTB incidence, while late harvested fruit had low disorder incidence. Misted fruit of mid harvest and control fruit of the late harvest accumulated 180 h below 10 ◦ C preharvest, and had significantly reduced LTB, while misted fruit of late harvest did not show any LTB incidence. The use of chilling time, rather than maturation time, as a basis of data analysis of harvest maturity indices (SSC and firmness) and LTB might allow the construction of charts for precise determination of harvest date and prediction of postharvest kiwifruit quality free of LTB. © 2005 Elsevier B.V. All rights reserved. Keywords: Actinidia deliciosa [A. Chev.] C.F. Liang and A.R. Ferguson; Storage; Firmness; Soluble solids content
1. Introduction Kiwifruit are normally harvested late October in Greece when the soluble solids content (SSC) reaches the minimum value of 6.2–6.5%, though for long-term storage, it is recommended that the fruit should have a SSC level of 7–9% when harvested.
∗ Corresponding author. Tel.: +30 310 998792; fax: +30 310 998791. E-mail address: [email protected] (D. Gerasopoulos).
0925-5214/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.postharvbio.2005.06.010
Growers, however, are not willing to delay harvest to avoid the risk of early frosts during November or even late October. Kiwifruit subjected to prolonged storage often develop the physiological disorder low temperature breakdown (LTB), a disorder that causes severe losses to kiwifruit after storage (Lallu, 1997). Lallu and Webb (1997) found that the incidence of LTB was higher in precooled than in passively cooled fruit. LTB incidence (occurrence and severity) of kiwifruit is dependent on the time to reach 0 ◦ C after precooling, and storage duration at temperatures below 2.5 ◦ C (Lallu, 1997).
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Cell wall bulk porosity of LTB-affected fruit decreased with increasing severity of the symptoms while the amount of cell wall material was 30% higher in grainy than in unaffected tissues (Bauchot et al., 1999). Lower LTB incidence has been associated with greater fruit Ca content (Gerasopoulos and Drogoudi, 2005), this sustaining greater fruit cell wall integrity and membrane permeability (Ferguson, 1984; Conway, 1987). The susceptibility of kiwifruit to LTB has been suggested to be affected by seasonal factors (Arpaia et al., 1985), and Harman (1981) reported that maturity stage and harvest greatly influence LTB of kiwifruit during storage. Thus, LTB which is considered a chilling injury (Lallu, 1997), appears also to be related to factors affecting membrane function at low storage temperatures, and susceptible therefore to acclimation by moderate to low preharvest orchard chilling temperatures occurring late during fruit maturation. Postharvest responses of fruit and vegetables to chilling stress are often greatly influenced by preharvest field temperature (Wang, 1982). A mist evaporation system has been employed in the past to enhance cool weather below 10 ◦ C (Merrit et al., 1961) to control scald, a disorder that is also considered to be a result of chilling injury; about 120 h below 10 ◦ C before harvest were required to reduce scald from 100% to producing nearly scald-free ‘Granny Smith’ apples (Thomai et al., 1998). Scald reduction was also correlated with low preharvest temperatures in ‘Starking Delicious’ (Blanpied et al., 1991) and in ‘Cortland’ or ‘Delicious’ apples (Barden and Bramlage, 1994). LTB is a serious problem in stored kiwifruit and to develop strategies for protection it is necessary to define its relationship with preharvest field temperature. The aim of the present work was to study the relationship of mist-induced preharvest low temperatures during fruit maturation on LTB of kiwifruit after long-term cold storage.
within the orchard of the Aristotle University of Thessaloniki Farm, Greece. 2.2. Treatments An over-tree mist system was installed for three groups of three vines per treatment (untreated vines were used as controls). Five nozzles were placed for each tree, misting water for 30 s at a frequency of 10 min. Temperature was monitored every 30 min by using a Squirrel Digital Meter/Logger (Grand Instrument, Cambridge, UK) with three sensors (thermistors), one placed under the peel of the fruit under mist, another under the peel of the untreated fruit and a third inside the canopy of an untreated tree. The cumulative preharvest hours below 10 ◦ C were obtained daily from temperature recording starting on 9 September (Merrit et al., 1961). Fruit were harvested at three harvest dates when the control fruit had accumulated 0 (early: 9 October), 90 (mid: 9 November) and 180 h (late: 23 November) below 10 ◦ C. Ninety fruit per group of vines (replications) of control and misted vines were harvested on each date and of those fruit, 75 were placed directly at −0.5 ◦ C and 90% relative humidity storage for 24 weeks, and 15 were used for firmness and SSC determination. 2.3. Fruit analysis Flesh firmness was measured using a Chatillon penetrometer (7.9 mm tip) on opposite sides, after fruit peel removal. SSC was determined using a digital refractometer (Atago model PR-1, Tokyo). Following storage, three replications, 75 fruit per replication for each mist and harvest date treatments were transferred at 20 ◦ C for 1 day, then cut into two halves and disordered fruit were counted based on typical LTB symptoms (water-soaking and grainy outer pericarp appearance). 2.4. Statistical analysis
2. Materials and methods 2.1. Plant material Kiwifruit vines (Actinidia deliciosa [A. Chev.] C.F. Liang and A.R. Ferguson, cv. Hayward), were selected for vegetative and crop load uniformity at random
Statistical analysis was based on a complete randomized design, using three replications for each treatment (control and mist). Statistical analyses were performed using a multi-factor ANOVA, based upon the replicate fruit, using the Statsoft statistical package SPSS (version 9.0, Chicago, USA). Percentage data were arcsine transformed, prior to analyses. LSDs for
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the comparisons between treatments during storage were also calculated. Treatment means were separated using LSDs at a P-value of 0.05. Regression analysis was performed using Sigmaplot package (Systat Software, Richmond, CA, USA).
3. Results Kiwifruit vines were exposed to lower temperatures than the ones normally occurring in the orchard environment by using an evaporative cooling method with an over-tree mist system. Fig. 1 shows the daily temperature change in the canopy environment of control vines as well as of control or misted fruit during a day of average temperature changes (15 October). The temperature difference between canopy environment and control fruit was 1–4 ◦ C, similar to the differences between control and misted fruit. The accumulation of hours below 10 ◦ C for that day was 7 and 11 h for the control and the misted fruit, respectively (Fig. 1). Control (unmisted) fruit harvested at the early harvest period had no accumulated hours below 10 ◦ C (Fig. 1, Table 1), SSC of 5.3% and firmness of 96 N (Table 1). Misted fruit of the early harvest date, although having accumulated 30 h below 10 ◦ C (Fig. 1) did not differ from control fruit in either SSC or firmness (Table 1). Control fruit developed postharvest an
Fig. 1. Accumulated chilling hours below 10 ◦ C of control fruit and fruit subjected to overhead misting during fruit maturation (main figure). Insert shows the daily temperature changes of control fruit and fruit subjected to overhead misting during an average day (15 October).
LTB incidence of 100%, while fruit of the same harvest period under mist developed 60% LTB (Table 1). Fruit harvested at the mid period (32 days later) that received no mist accumulated 90 h below 10 ◦ C (Fig. 1), showed higher SSC (6.9%) and firmness (84 N) (Table 1). However, misted fruit of the mid harvest period accumulated double the time below 10 ◦ C (Fig. 1) and had similar SSC but were softer by 12 N, compared to the control (Table 1).
Table 1 Accumulated time below 10 ◦ C and changes in SSC and firmness of ‘Hayward’ kiwifruit harvested from control or misted vines at three different harvest dates Treatment
Harvest (date)
Accumulated (h)
SSC (%)
Firmness (N)
LTB (%)
Control Mist Control Mist Control Mist
Early (9 October)
0 30 90 180 180 360
5.3 c 5.5 c 6.9 b 7.1 b 10.7 a 10.9 a
96 a 94 a 84 b 72 c 65 c 53 e
99 a 60 b 21 c 13 d 11 d 0e
Firmness (N)
LTB (%)
0.8635 0.0026 0.0259
0.0139 0.0007 0.2152
Mid (9 November) Late (23 November)
P-values SSC (%) Mist Harvest date Mist × harvest date
0.8586 0.0002 0.9738
LTB was assessed following storage at −0.5 ◦ C for 24 weeks. P-values for the effects of mist applications and harvest date treatment are shown. Values in the same column followed by the same letter are not significantly different by Duncan’s multiple range test (a = 0.05). Probability values less than 0.05 are considered significant.
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Although control fruit of the late harvest accumulated the same amount of chilling as the misted fruit of the mid harvest (Fig. 1), these fruit had considerably higher SSC and tended to be softer than control fruit of the mid harvest (Table 1). Misted fruit of the late harvest accumulated double the amount of chilling compared to that of control fruit of the mid harvest or the misted fruit of the third harvest (Fig. 1). Misted fruit of the late harvest had similar SSC but were softer compared to the control (Table 1). Misted fruit of the late harvest, however, accumulated the highest amount of chilling (360 h, Fig. 1) and developed no LTB (Table 1). The results of the mist treatment are more clearly demonstrated by presenting the fruit maturation indices and LTB on the basis of accumulated time below 10 ◦ C, rather than the conventionally used maturation time: SSC development of control and misted fruit were completely separated in accumulated time below 10 ◦ C. Control fruit showed a faster increase in SSC than mist-acclimated fruit both following an exponential increase of R2 = 0.99 and P < 0.0501 and <0.0062, respectively (Fig. 2A). Unlike SSC increase, firmness loss was similar in control or misted fruit, regardless of the time basis used. Firmness loss decreased linearly with accumulated time below 10 ◦ C (Fig. 2B). LTB development when correlated with chilling time accumulated below 10 ◦ C showed a decrease following an exponential decay equation (Fig. 2C) of R2 = 0.967 (P < 0.0001) for both the control and the misted fruit. The preharvest accumulated chilling of 90 h below 10 ◦ C significantly decreased LTB incidence to below 20%. Plotting LTB development with chilling hours accumulated below 10 ◦ C and SSC (Fig. 3A) or firmness (Fig. 3B) allowed us to construct LTB prediction model charts.
4. Discussion Harvest maturity had a significant effect on postharvest LTB incidence. Early harvested fruit was found to be more susceptible to LTB than the mid or late harvested fruit. This was attributed to a lack of a cold acclimation period. SSC, the main maturity index of kiwifruit, increases during fruit maturation (Reid, 1977; Harman, 1981), and did so in both control and misted fruit. Although cooler weather has been
Fig. 2. Correlation of kiwifruit SSC (A), firmness (B) and LTB (C) with cumulative time below 10 ◦ C, in fruit harvested from control and misted vines at three different dates then held at −0.5 ◦ C for 24 weeks. Each data point is the mean of 15 fruit for SSC and firmness and 75 fruit for LTB.
reported to induce increases in SSC (Seager et al., 1991), SSC values of mist treated and control fruit were not statistically different at any of the three successive harvest periods (Table 1). Firmness, an index used mainly to determine postharvest storage quality or loss, decreases during fruit maturation (Harman, 1981), and this decrease was more pronounced in misted fruit (Table 1). LTB incidence was reduced with harvest delay (Fig. 2C), similarly to observations of Harman (1981) that early harvested fruit shows postharvest internal breakdown corresponding to LTB, also described by Lallu (1997). However, fruit harvested with more than the established 6.2% SSC (mid-
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Fig. 3. Correlation of kiwifruit LTB with cumulative time below 10 ◦ C and SSC (A) or firmness (B), in fruit harvested from control and misted vines at three different dates then held at −0.5 ◦ C for 24 weeks.
harvest SSC of 6.9%, Table 1), still showed considerable LTB (Table 1). Correlation of SSC with chilling hours below 10 ◦ C revealed a slower increase of SSC in mist-acclimated than control fruit (Fig. 2A). While chilling hours were increased by treatment, there was no effect on SSC when maturation time was used as a basis (Table 1). Chilling hours, when used as a time basis, introduces a second time dimension (the first is maturation time), which is actually related to acclimation per se. Based on a chilling or acclimation time-dimension, a slower rate of SSC increase was associated with better fruit acclimation and less LTB incidence. On the contrary, correlation of firmness with chilling hours below 10 ◦ C did not reveal any treatment effect on firmness since both the control and misted fruit appeared to have similar rates of decrease when either the acclimation (Fig. 2B) or maturation (Table 1) time-dimension was used. A correlation of accumulated hours below 10 ◦ C preharvest with LTB was most pronounced. Preharvest accumulated chilling for 90 h below 10 ◦ C significantly reduced LTB to 20%. A similar response to chilling (<10 ◦ C) temperature induced by the mist technique was found in the reduction of scald incidence of apples (Blanpied et al., 1991; Barden and Bramlage, 1994; Thomai et al., 1998). Apples exposed to temperatures of less than 10 ◦ C for more than 100 h before harvest show an increase in the degree of unsaturation of fatty acids compared with that in control fruit (Thomai et al., 1998). A similar increase in the unsatu-
rated/saturated fatty acid ratio in kiwifruit membranes, linked with an increase in membrane permeability during fruit maturation and storage, has also been reported (Abdala et al., 1996). The results of this experiment suggest that factors other than ripening are involved in LTB development. It seems that exposure of fruit to preharvest temperatures of less than 10 ◦ C induces an acclimation of fruit and the susceptibility to LTB during storage decreases. Thus, low preharvest temperatures that are responsible in promoting fruit membrane acclimation to low storage temperatures might be the major factor of LTB development. A second factor, among others related to LTB development, is probably fruit calcium levels, since calcium has been reported to maintain membrane function and better fruit cell wall integrity (Ferguson, 1984; Conway, 1987). Low levels of fruit calcium at early harvest maturities (Gerasopoulos, unpublished data) are often associated with higher levels of LTB (Harman, 1981), while substantially less LTB incidence was recorded in fruit that received preharvest CaCl2 sprays (Gerasopoulos and Drogoudi, 2005). Acclimation by preharvest chilling, suggested as the main factor regulating postharvest LTB incidence, is proposed as an additional maturity index for fruit quality after storage along with SSC and firmness, and can be used as a means to construct LTB incidence model charts (Fig. 3) that might allow an optimisation of harvest date, following further research to establish more detailed data.
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References Abdala, A., Gerasopoulos, D., Stavroulakis, G., 1996. Effects of harvest maturity and storage on ripening and membrane permeability of Hayward kiwifruit. Adv. Hort. Sci. 10, 3–7. Arpaia, M.L., Mitchell, F.G., Kader, A.A., Mayer, G., 1985. Effects of 2% O2 and varying concentrations of CO2 with or without C2 H4 on the storage performance of kiwifruit. J. Am. Soc. Hort. Sci. 110, 200–203. Barden, C.L., Bramlage, W.J., 1994. Separating the effects of low temperature, ripening and lighton loss of scald susceptibility in apples before harvest. J. Am. Soc. Hort. Sci. 118, 807– 813. Bauchot, A.D., Hallett, I.C., Redgwell, R.J., Lallu, N., 1999. Cell wall properties of kiwifruit affected by low temperature breakdown. Postharvest Biol. Technol. 16, 245–255. Blanpied, G.D., Bramlage, W.J., Chu, C.L., Ingle, M., Kushad, M.M., Lau, O.L., Lidster, P.D., 1991. A survey of the relationship among accumulated orchard hours below 108 ◦ C, fruit maturity, and the incidences of storage scald on ‘Starkrimson Delicious’ apples. Can. J. Plant Sci. 71, 605–608. Conway, W.S., 1987. The effects of postharvest infiltration of calcium, magnesium, or strodium on decay, firmness, respiration, and ethylene production in apples. J. Am. Soc. Hort. Sci. 112, 300–303.
Ferguson, I.B., 1984. Calcium in plant senescence and fruit ripening. Plant Cell Environ. 7, 477–489. Gerasopoulos, D., Drogoudi, D.P., 2005. Summer pruning and preharvest calcium chloride sprays affect storability and low temperature breakdown incidence in kiwifruit. Postharvest Biol. Technol. 36, 303–308. Harman, J.E., 1981. Kiwifruit maturity. Orchardist N.Z. 54, 126–130. Lallu, N., 1997. Low temperature breakdown in kiwifruit. Acta Hort. 444, 579–585. Lallu, N., Webb, D.J., 1997. Physiological and economic analysis of precooling kiwifruit. Acta Hort. 444, 691–697. Merrit, R.H., Stiles, W.C., Havens, A.V., Mitterling, L., 1961. Effects of preharvest air temperature on storage scald of ‘Stayman’ apples. Proc. Am. Soc. Hort. Sci. 78, 24–34. Reid, M., 1977. Maturation and storage of kiwifruit. Proceedings of Kiwifruit Seminar, Tauranga, September 1977. Min. Agric. Fish. N.Z., 3–52. Seager, N., Hewett, E., Warrington, I., 1991. How temperature affects Brix increase. N.Z. Kiwifr., 17. Thomai, T., Sfakiotakis, E., Diamantidis, G., Vasilakakis, M., 1998. Effects of low preharvest temperature on scald susceptibility and biochemical changes in ‘Granny Smith’ apple peel. Sci. Hort. 76, 1–15. Wang, C.Y., 1982. Physiological and biochemical responses of plants to chilling stress. HortScience 17, 173–186.
Preharvest chilling reduces low temperature breakdown incidence of kiwifruit Evangellos Sfakiotakis a , Georgios Chlioumis a , Dimitrios Gerasopoulos b,∗ b
a Aristotle University of Thessaloniki, Department of Horticulture, 54 124 Thessaloniki, Greece Aristotle University of Thessaloniki, Department of Food Science and Technology, 54 124 Thessaloniki, Greece
Received 4 February 2005; accepted 15 June 2005
Abstract To study the effects of preharvest chilling on low temperature breakdown incidence (LTB) of kiwifruit (Actinidia deliciosa [A. Chev.] C.F. Liang and A.R. Ferguson), a mist evaporative cooling system was established in the orchard and the accumulation of temperatures below 10 ◦ C in hours was recorded during fruit maturation. The fruit were harvested at three dates, and soluble solids content (SSC) and firmness were measured at harvest, while LTB incidence was determined following 24-week storage at −0.5 ◦ C. Fruit harvested immature had a high LTB incidence, while late harvested fruit had low disorder incidence. Misted fruit of mid harvest and control fruit of the late harvest accumulated 180 h below 10 ◦ C preharvest, and had significantly reduced LTB, while misted fruit of late harvest did not show any LTB incidence. The use of chilling time, rather than maturation time, as a basis of data analysis of harvest maturity indices (SSC and firmness) and LTB might allow the construction of charts for precise determination of harvest date and prediction of postharvest kiwifruit quality free of LTB. © 2005 Elsevier B.V. All rights reserved. Keywords: Actinidia deliciosa [A. Chev.] C.F. Liang and A.R. Ferguson; Storage; Firmness; Soluble solids content
1. Introduction Kiwifruit are normally harvested late October in Greece when the soluble solids content (SSC) reaches the minimum value of 6.2–6.5%, though for long-term storage, it is recommended that the fruit should have a SSC level of 7–9% when harvested.
∗ Corresponding author. Tel.: +30 310 998792; fax: +30 310 998791. E-mail address: [email protected] (D. Gerasopoulos).
0925-5214/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.postharvbio.2005.06.010
Growers, however, are not willing to delay harvest to avoid the risk of early frosts during November or even late October. Kiwifruit subjected to prolonged storage often develop the physiological disorder low temperature breakdown (LTB), a disorder that causes severe losses to kiwifruit after storage (Lallu, 1997). Lallu and Webb (1997) found that the incidence of LTB was higher in precooled than in passively cooled fruit. LTB incidence (occurrence and severity) of kiwifruit is dependent on the time to reach 0 ◦ C after precooling, and storage duration at temperatures below 2.5 ◦ C (Lallu, 1997).
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Cell wall bulk porosity of LTB-affected fruit decreased with increasing severity of the symptoms while the amount of cell wall material was 30% higher in grainy than in unaffected tissues (Bauchot et al., 1999). Lower LTB incidence has been associated with greater fruit Ca content (Gerasopoulos and Drogoudi, 2005), this sustaining greater fruit cell wall integrity and membrane permeability (Ferguson, 1984; Conway, 1987). The susceptibility of kiwifruit to LTB has been suggested to be affected by seasonal factors (Arpaia et al., 1985), and Harman (1981) reported that maturity stage and harvest greatly influence LTB of kiwifruit during storage. Thus, LTB which is considered a chilling injury (Lallu, 1997), appears also to be related to factors affecting membrane function at low storage temperatures, and susceptible therefore to acclimation by moderate to low preharvest orchard chilling temperatures occurring late during fruit maturation. Postharvest responses of fruit and vegetables to chilling stress are often greatly influenced by preharvest field temperature (Wang, 1982). A mist evaporation system has been employed in the past to enhance cool weather below 10 ◦ C (Merrit et al., 1961) to control scald, a disorder that is also considered to be a result of chilling injury; about 120 h below 10 ◦ C before harvest were required to reduce scald from 100% to producing nearly scald-free ‘Granny Smith’ apples (Thomai et al., 1998). Scald reduction was also correlated with low preharvest temperatures in ‘Starking Delicious’ (Blanpied et al., 1991) and in ‘Cortland’ or ‘Delicious’ apples (Barden and Bramlage, 1994). LTB is a serious problem in stored kiwifruit and to develop strategies for protection it is necessary to define its relationship with preharvest field temperature. The aim of the present work was to study the relationship of mist-induced preharvest low temperatures during fruit maturation on LTB of kiwifruit after long-term cold storage.
within the orchard of the Aristotle University of Thessaloniki Farm, Greece. 2.2. Treatments An over-tree mist system was installed for three groups of three vines per treatment (untreated vines were used as controls). Five nozzles were placed for each tree, misting water for 30 s at a frequency of 10 min. Temperature was monitored every 30 min by using a Squirrel Digital Meter/Logger (Grand Instrument, Cambridge, UK) with three sensors (thermistors), one placed under the peel of the fruit under mist, another under the peel of the untreated fruit and a third inside the canopy of an untreated tree. The cumulative preharvest hours below 10 ◦ C were obtained daily from temperature recording starting on 9 September (Merrit et al., 1961). Fruit were harvested at three harvest dates when the control fruit had accumulated 0 (early: 9 October), 90 (mid: 9 November) and 180 h (late: 23 November) below 10 ◦ C. Ninety fruit per group of vines (replications) of control and misted vines were harvested on each date and of those fruit, 75 were placed directly at −0.5 ◦ C and 90% relative humidity storage for 24 weeks, and 15 were used for firmness and SSC determination. 2.3. Fruit analysis Flesh firmness was measured using a Chatillon penetrometer (7.9 mm tip) on opposite sides, after fruit peel removal. SSC was determined using a digital refractometer (Atago model PR-1, Tokyo). Following storage, three replications, 75 fruit per replication for each mist and harvest date treatments were transferred at 20 ◦ C for 1 day, then cut into two halves and disordered fruit were counted based on typical LTB symptoms (water-soaking and grainy outer pericarp appearance). 2.4. Statistical analysis
2. Materials and methods 2.1. Plant material Kiwifruit vines (Actinidia deliciosa [A. Chev.] C.F. Liang and A.R. Ferguson, cv. Hayward), were selected for vegetative and crop load uniformity at random
Statistical analysis was based on a complete randomized design, using three replications for each treatment (control and mist). Statistical analyses were performed using a multi-factor ANOVA, based upon the replicate fruit, using the Statsoft statistical package SPSS (version 9.0, Chicago, USA). Percentage data were arcsine transformed, prior to analyses. LSDs for
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the comparisons between treatments during storage were also calculated. Treatment means were separated using LSDs at a P-value of 0.05. Regression analysis was performed using Sigmaplot package (Systat Software, Richmond, CA, USA).
3. Results Kiwifruit vines were exposed to lower temperatures than the ones normally occurring in the orchard environment by using an evaporative cooling method with an over-tree mist system. Fig. 1 shows the daily temperature change in the canopy environment of control vines as well as of control or misted fruit during a day of average temperature changes (15 October). The temperature difference between canopy environment and control fruit was 1–4 ◦ C, similar to the differences between control and misted fruit. The accumulation of hours below 10 ◦ C for that day was 7 and 11 h for the control and the misted fruit, respectively (Fig. 1). Control (unmisted) fruit harvested at the early harvest period had no accumulated hours below 10 ◦ C (Fig. 1, Table 1), SSC of 5.3% and firmness of 96 N (Table 1). Misted fruit of the early harvest date, although having accumulated 30 h below 10 ◦ C (Fig. 1) did not differ from control fruit in either SSC or firmness (Table 1). Control fruit developed postharvest an
Fig. 1. Accumulated chilling hours below 10 ◦ C of control fruit and fruit subjected to overhead misting during fruit maturation (main figure). Insert shows the daily temperature changes of control fruit and fruit subjected to overhead misting during an average day (15 October).
LTB incidence of 100%, while fruit of the same harvest period under mist developed 60% LTB (Table 1). Fruit harvested at the mid period (32 days later) that received no mist accumulated 90 h below 10 ◦ C (Fig. 1), showed higher SSC (6.9%) and firmness (84 N) (Table 1). However, misted fruit of the mid harvest period accumulated double the time below 10 ◦ C (Fig. 1) and had similar SSC but were softer by 12 N, compared to the control (Table 1).
Table 1 Accumulated time below 10 ◦ C and changes in SSC and firmness of ‘Hayward’ kiwifruit harvested from control or misted vines at three different harvest dates Treatment
Harvest (date)
Accumulated (h)
SSC (%)
Firmness (N)
LTB (%)
Control Mist Control Mist Control Mist
Early (9 October)
0 30 90 180 180 360
5.3 c 5.5 c 6.9 b 7.1 b 10.7 a 10.9 a
96 a 94 a 84 b 72 c 65 c 53 e
99 a 60 b 21 c 13 d 11 d 0e
Firmness (N)
LTB (%)
0.8635 0.0026 0.0259
0.0139 0.0007 0.2152
Mid (9 November) Late (23 November)
P-values SSC (%) Mist Harvest date Mist × harvest date
0.8586 0.0002 0.9738
LTB was assessed following storage at −0.5 ◦ C for 24 weeks. P-values for the effects of mist applications and harvest date treatment are shown. Values in the same column followed by the same letter are not significantly different by Duncan’s multiple range test (a = 0.05). Probability values less than 0.05 are considered significant.
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Although control fruit of the late harvest accumulated the same amount of chilling as the misted fruit of the mid harvest (Fig. 1), these fruit had considerably higher SSC and tended to be softer than control fruit of the mid harvest (Table 1). Misted fruit of the late harvest accumulated double the amount of chilling compared to that of control fruit of the mid harvest or the misted fruit of the third harvest (Fig. 1). Misted fruit of the late harvest had similar SSC but were softer compared to the control (Table 1). Misted fruit of the late harvest, however, accumulated the highest amount of chilling (360 h, Fig. 1) and developed no LTB (Table 1). The results of the mist treatment are more clearly demonstrated by presenting the fruit maturation indices and LTB on the basis of accumulated time below 10 ◦ C, rather than the conventionally used maturation time: SSC development of control and misted fruit were completely separated in accumulated time below 10 ◦ C. Control fruit showed a faster increase in SSC than mist-acclimated fruit both following an exponential increase of R2 = 0.99 and P < 0.0501 and <0.0062, respectively (Fig. 2A). Unlike SSC increase, firmness loss was similar in control or misted fruit, regardless of the time basis used. Firmness loss decreased linearly with accumulated time below 10 ◦ C (Fig. 2B). LTB development when correlated with chilling time accumulated below 10 ◦ C showed a decrease following an exponential decay equation (Fig. 2C) of R2 = 0.967 (P < 0.0001) for both the control and the misted fruit. The preharvest accumulated chilling of 90 h below 10 ◦ C significantly decreased LTB incidence to below 20%. Plotting LTB development with chilling hours accumulated below 10 ◦ C and SSC (Fig. 3A) or firmness (Fig. 3B) allowed us to construct LTB prediction model charts.
4. Discussion Harvest maturity had a significant effect on postharvest LTB incidence. Early harvested fruit was found to be more susceptible to LTB than the mid or late harvested fruit. This was attributed to a lack of a cold acclimation period. SSC, the main maturity index of kiwifruit, increases during fruit maturation (Reid, 1977; Harman, 1981), and did so in both control and misted fruit. Although cooler weather has been
Fig. 2. Correlation of kiwifruit SSC (A), firmness (B) and LTB (C) with cumulative time below 10 ◦ C, in fruit harvested from control and misted vines at three different dates then held at −0.5 ◦ C for 24 weeks. Each data point is the mean of 15 fruit for SSC and firmness and 75 fruit for LTB.
reported to induce increases in SSC (Seager et al., 1991), SSC values of mist treated and control fruit were not statistically different at any of the three successive harvest periods (Table 1). Firmness, an index used mainly to determine postharvest storage quality or loss, decreases during fruit maturation (Harman, 1981), and this decrease was more pronounced in misted fruit (Table 1). LTB incidence was reduced with harvest delay (Fig. 2C), similarly to observations of Harman (1981) that early harvested fruit shows postharvest internal breakdown corresponding to LTB, also described by Lallu (1997). However, fruit harvested with more than the established 6.2% SSC (mid-
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Fig. 3. Correlation of kiwifruit LTB with cumulative time below 10 ◦ C and SSC (A) or firmness (B), in fruit harvested from control and misted vines at three different dates then held at −0.5 ◦ C for 24 weeks.
harvest SSC of 6.9%, Table 1), still showed considerable LTB (Table 1). Correlation of SSC with chilling hours below 10 ◦ C revealed a slower increase of SSC in mist-acclimated than control fruit (Fig. 2A). While chilling hours were increased by treatment, there was no effect on SSC when maturation time was used as a basis (Table 1). Chilling hours, when used as a time basis, introduces a second time dimension (the first is maturation time), which is actually related to acclimation per se. Based on a chilling or acclimation time-dimension, a slower rate of SSC increase was associated with better fruit acclimation and less LTB incidence. On the contrary, correlation of firmness with chilling hours below 10 ◦ C did not reveal any treatment effect on firmness since both the control and misted fruit appeared to have similar rates of decrease when either the acclimation (Fig. 2B) or maturation (Table 1) time-dimension was used. A correlation of accumulated hours below 10 ◦ C preharvest with LTB was most pronounced. Preharvest accumulated chilling for 90 h below 10 ◦ C significantly reduced LTB to 20%. A similar response to chilling (<10 ◦ C) temperature induced by the mist technique was found in the reduction of scald incidence of apples (Blanpied et al., 1991; Barden and Bramlage, 1994; Thomai et al., 1998). Apples exposed to temperatures of less than 10 ◦ C for more than 100 h before harvest show an increase in the degree of unsaturation of fatty acids compared with that in control fruit (Thomai et al., 1998). A similar increase in the unsatu-
rated/saturated fatty acid ratio in kiwifruit membranes, linked with an increase in membrane permeability during fruit maturation and storage, has also been reported (Abdala et al., 1996). The results of this experiment suggest that factors other than ripening are involved in LTB development. It seems that exposure of fruit to preharvest temperatures of less than 10 ◦ C induces an acclimation of fruit and the susceptibility to LTB during storage decreases. Thus, low preharvest temperatures that are responsible in promoting fruit membrane acclimation to low storage temperatures might be the major factor of LTB development. A second factor, among others related to LTB development, is probably fruit calcium levels, since calcium has been reported to maintain membrane function and better fruit cell wall integrity (Ferguson, 1984; Conway, 1987). Low levels of fruit calcium at early harvest maturities (Gerasopoulos, unpublished data) are often associated with higher levels of LTB (Harman, 1981), while substantially less LTB incidence was recorded in fruit that received preharvest CaCl2 sprays (Gerasopoulos and Drogoudi, 2005). Acclimation by preharvest chilling, suggested as the main factor regulating postharvest LTB incidence, is proposed as an additional maturity index for fruit quality after storage along with SSC and firmness, and can be used as a means to construct LTB incidence model charts (Fig. 3) that might allow an optimisation of harvest date, following further research to establish more detailed data.
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