Flavor is a function of two primary attributes, taste and odor. Whereas taste is related to perception of sweetness, sourness, bitterness, and saltiness by the taste buds in the mouth, odor depends on the contributions of specific odor volatiles perceived by the olfactory receptors in the nose. The flavor, and therefore consumer acceptance, of fruit is a complex interaction between the concentrations of sugars, organic acids, phenolics in some fruit, and volatile compounds.

Sugar concentrations increase during fruit ripening and are major determinants of sweetness. In nonclimacteric cherries, which cannot ripen after harvest, sugar concentration is solely a function of translocation of carbohydrates into the fruit before harvest. Therefore, fruit quality is greatly affected if harvest occurs before adequate sugar accumulation has occurred. In climacteric fruit, sugar increases also occur because of carbohydrate translocation, but hydrolysis of stored carbohydrates, especially starch in apples and pears, can be a major contributor to increased sugar concentrations. These fruit can reach acceptable sugar levels and flavor during ripening off the tree.

Acidity levels are important factors in the flavor of many temperate tree fruit. The major organic acids vary by fruit type, for example, malic acid being present in apples and cherries, malic and quinic acids in pears, and malic and citric acids in peaches and nectarines. The concentrations of these acids typically decline during ripening and are utilized as respiratory substrates and as carbon skeletons for synthesis of new compounds. Organic acid concentrations are greatly in excess of those required for energy during ripening, but they may decline markedly during ripening.

Astringency in fruit is determined by concentrations of phenolic compounds. These are usually derived from phenylalanine via cin-namic and coumaric acids. Astringency can be a characteristic of certain apple cultivars. In peaches, research indicates that low-quality fruit have higher concentrations of phenolics such as chlorogenic acid and catechin than high-quality fruit, but no changes are detected during ripening. The overall taste of a fruit can be affected by the balances among sugars, acids, and phenolics, rather than the concentrations of each one alone.

The aroma of each fruit results from distinct quantitative and qualitative differences in compositions of volatile compounds produced during ripening. The major classes of flavor compounds are aldehydes, esters, ketones, terpenoids, and sulfur-containing forms. Their respective biosynthetic pathways are diverse, including those involved in fatty acid, amino acid, phenolic, and terpenoid metabolism. Increases in volatile production are often, but not always, associated with ethylene production. While large numbers of individual volatiles have been identified in fruit, relatively few make up the characteristic aroma perceived by the consumer. In apple fruit, for example, over 200 volatiles have been identified, but ethyl 2-methylbutyrate is responsible for much of the characteristic apple odor. Important "character" volatiles may occur in very low concentrations. In some cases, a single or few volatiles that make the "character" aroma have been identified, while in others, aroma is made up of a complex mixture of compounds that cannot be reproduced.

The ripening of fruit is a complex phenomenon that varies greatly among and within fruit types. Understanding the physiology of these fruit is critical to application of handling and storage protocols that will result in acceptable quality in the marketplace. Knowledge of res piratory and ethylene responses, for example, allows harvest decisions and application of correct storage temperatures that will minimize unwanted ripening changes. Although the successes of horticultural industries around the world are evidence of tremendous progress, serious issues exist with fruit quality in the marketplace. Unfortunately, appearance of the fruit is one of the last characteristics to be lost; a fruit may appear attractive but be soft and flavorless. Moreover, some of the methods used to prolong storage life, especially low-oxygen storage, appear to affect detrimentally recovery of flavor volatiles.



Kays, S. J. (1997). Postharvest physiology of perishable plant products. Athens, GA: Exon Press.

Knee, M., ed. (2002). Fruit quality and its biological basis. Sheffield, UK: Sheffield Academic Press.

Seymour, G. B., J. E. Taylor, and G. A. Tucker, eds. (1993). Biochemistry of fruit ripening. London, UK: Chapman and Hall.

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