Heat treatment and fruit ripening

Postharvest heat treatments lead to an alteration of gene expression and fruit ripening can sometimes be either delayed or disrupted. The extent of the alternation of fruit ripening is a function of the exposure temperature and duration and how quickly the commodity is cooled following the heat treatment. The most commonly measured components of fruit ripening affected by heat treatments include fruit softening, membrane and flavor changes, respiration rate, ethylene production, and volatile production. Cell wall degrading enzymes and ethylene production are frequently the most disrupted and are sometimes not produced or their appearance is delayed following heating. Other processes associated with ripening are not altered to the same extent or soon recover. Fruit sensitivity to heat treatments is modified by preharvest weather conditions, cultivar, rate of heating, and subsequent storage conditions. The amount of sensitivity or tolerance to heat stress of a commodity is related to the level of heat protective proteins at harvest and the postharvest production of heat shock proteins. Two types of heat responses are seen. The first is a normal cellular response (< 42<degrees>C) that can lead to reduced chilling sensitivity, delayed or slowed ripening and a modification of quality. The second occurs near the threshold for damage > 45 degreesC and is modified by the pre-stress environmental conditions, the cellular response to stress and cellular recovery. Loss of membrane integrity appears to be an effect and not a cause of injury. The site of the injury lesion is still unknown and could be associated with transcription, translation and cellular recovery capacity after an injury threshold has been exceeded. (C) 2000 Elsevier Science B.V. All rights reserved.