The desirable attributes associated with high-quality fruit develop during ripening. Nonclimacteric fruits develop these attributes while still attached to the plant and are characterized by low levels of carbon dioxide (C02) and ethylene (C2H4) evolution. Little further ripening occurs in nonclimacteric fruit after harvest. Nonclimacteric fruits should be harvested as close to the flavor peak as possible because flavor will not develop after detachment.

Climacteric fruits are characterized by a rapid respiratory burst of greatly increased C02 and C2H4 at the initiation of ripening. The climacteric rise, triggered by C2H4, can occur on or off the plant. The fruit will generate its own C2H4 from aminocyclopropane carboxylic acid (ACC) by ACC oxidase, and ACC is generated from S-adenosyl methionine (SAM) by ACC synthase (6). If the fruit is harvested in a physiologically mature state before the increased generation of ACC and is kept in an environment free of C2H4, it can be maintained in a mature, unripe state during the transportation and storage. If climacteric fruits are harvested too early, they will not develop full flavor potential during storage. If harvested too late, they will become unacceptably soft before purchase or consumption. The marketplace offers greater incentives to the grower who harvests too early than to the one who harvests too late. In crops such as peaches and tomatoes where this window between too early and too late is narrow, premium quality product is difficult to obtain. Ripening can be artificially triggered by using an external source of C2H4. This process permits the shipment of green bananas from the tropics to geographically distant markets where ripening is triggered close to the consumer, long-distance truck shipment of green tomatoes for ripening close to the distribution point, and controlled-atmosphere storage of apples before ripening to permit year-round consumption.

Desirable changes associated with ripening include changes in appearance, flavor, texture, and nutritive value. Color changes are the result of the degradation of chlorophyll (green), which unmasks xanthophylls and carotenes (yellow and orange), and the biosynthesis of anthocyanins (red, blue, or purple) and the carotenoid lycopene (red). The development of sweetness in fruits is the result of a disappearance of sour (acids) and bitter (including tannins) compounds and the accumulation of sweet (sugars) compounds. Flavor is the combination of taste (sweet, sour, bitter) and aroma that results from the accumulation of volatile compounds during ripening, producing characteristic flavors associated with specific fruits. Softening during ripening is the result of degradation of cell walls, primarily pectins, by a series of hydrolase enzymes (5). Vitamins (particularly A and C) accumulate during ripening, thereby enhancing the nutritional quality of the product.

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