Christopher B. Watkins
Postharvest fruit physiology describes the interaction of the physiological and biochemical events associated with ripening and senescence. Fruit continue to function metabolically after harvest, but in the absence of carbohydrates, nutrients, and water supplied by the tree. In some cases, the fruit are eaten immediately. However, handling, storage, and transport technologies are usually used to maintain the rate of fruit ripening and therefore quality. The postharvest period hence involves the appropriate management of stress to minimize metabolic rates and/or enhancement of injurious metabolic processes.
The botanical definition of a fruit is "a seed receptacle developed from an ovary," but a range of fruit types exist (Kays, 1997). The fleshy part of the apple and pear develops from the accessory tissue of the floral structure, while the drupe fruit, characterized by the peach and apricot, develops from the mesocarp. While it is not surprising that differences in metabolism occur among fruit types, the central metabolic pathways of glycolysis, the tricarboxylic acid cycle, and the mitochondrial electron transport chain are common to all fruit. Fruit ripening is characterized by many events, the most important of which for perception of fruit quality in the marketplace, are changes of texture, color, and flavor. Other ripening-associated events include seed maturation, fruit abscission, changes in respiration rate, ethylene production, alterations in tissue permeability and protein contents, and development of surface waxes. Ripening changes involve a series of coordinated but loosely connected biochemical pathways. Many of these require anabolic processes that need energy and carbon skeleton building blocks supplied by respiration.
This chapter is restricted to the consideration of physiology and biochemistry of fruit ripening as it pertains to respiration and ethylene production, texture, color, and flavor because of the importance of these factors in affecting consumer acceptability of temperate tree fruit. Excellent overviews of postharvest physiology are available in books by Kays (1997), Knee (2002), and Seymour, Taylor, and Tucker (1993).
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