Effects of intermittent warming and temperature conditioning on the postharvest quality of 'Oroblanco' citrus fruit following long-term cold storage

'Oroblanco' is an early-maturing pummelo-grapefruit hybrid (Citrus grandis x C paradisi). The fruit of this cultivar are usually picked in October and are marketed while their peel color is still green. However, during long-term storage, the fruit turns yellow, and loses much of their commercial value. In a previous study, we found that application of gibberellic acid and low storage temperatures of 2 degreesC (35.6 degreesF) markedly reduced the rate of degreening. However, 'Oroblanco' fruit are sensitive to chilling injuries, and thus could not be stored at 2 degreesC for long periods. In the present study, we examined the possible application of intermittent warming (IW) and temperature conditioning (TC) treatments, in order to retain the green fruit color during long-term cold storage but without enhancing the development of chilling injuries. It was found, that following storage at 2 degreesC, either with or without IW and TC, the fruit retained green color up to 16 weeks, whereas at 11 degreesC (51.8 degreesF) fruit turned yellow after 8 weeks. However, untreated fruit held continuously at 2 degreesC developed 40, 51, and 68% chilling injuries after 8, 12, and 16 weeks, respectively. IW (storage at cycles of 3 weeks at 2 degreesC + 1 week at 11 degreesC) reduced the amount of chilling injuries to only 5, 7 and 11 % after the same periods of time, respectively. TC [a pre-storage treatment for 7 days at 16 degreesC (60.8 OF) before continuous storage at 2 degreesC] effectively reduced the development of chilling injuries to only 5% after 8 weeks of storage, but was ineffective in reducing chilling damage after longer storage periods. Because chilling damaged fruit is prone to decay, the IW and TC treatments also reduced the incidence of decay development during storage. The IW and TC treatments did not affect juice total soluble solids and acid percentages, but did affect fruit taste and the amounts of off-flavor volatiles emitted from the juice. Taste panels indicated that the taste score of untreated control fruit stored at 11 degreesC gradually decreased during long-term storage, and that this decrease was more severe in chilling damaged fruit stored continuously at 2 degreesC. The taste of IW-treated fruit remained acceptable even after 16 weeks of storage, and TC-treated fruit remained acceptable for up to 12 weeks. Fruit taste scores were inversely correlated with the concentrations of ethanol and acetaldehyde detected in the juice headspace.