Color

The major pigment changes that occur during ripening of fruit are losses of chlorophyll and either the unmasking of previously synthesized, or the synthesis of, pigments such as carotenoids and anthocyan-ins. Although chlorophyll loss and pigment synthesis are coordinated during ripening, the two events are not directly related or interdependent. In temperate tree fruit, the predominant red color pigments are anthocyanins. The timing, rate, and extent of color change vary greatly by fruit type and cultivar and can be affected by both preharvest and postharvest factors.

Chlorophylls are sequestered in the chloroplasts as two predominant forms, chlorophyll a and chlorophyll b. Chlorophylls are insoluble in water. Although the mechanism of chlorophyll degradation is not fully understood and may involve both enzymatic and chemical reactions, chlorophyllase activity is thought to play a major role. Most temperate tree fruit lose chlorophyll during ripening, although some apple and pear cultivars remain green. It can be difficult to separate the initial decline of chlorophyll, on a surface area basis, resulting from fruit expansion from the subsequent yellowing that results from chlorophyll breakdown. Chlorophyll loss can be accelerated by increased ethylene production by the fruit, and yellowing is typically associated with softening and reduced market appeal. However, change of the background color is used as a harvest guide for bicolored cultivars such as 'Gala', 'Braeburn', and 'Fuji'. Exposure to ethylene after harvest can result in rapid loss of chlorophylls.

Carotenoids are a large group of water-insoluble pigments associated with chlorophyll in the chloroplast. Carotenoids are terpene compounds derived from acetyl-CoA via the mevalonic acid pathway. During ripening, carotenoid production increases, producing yellow to red pigments, as the chloroplasts are transformed to chromo-plasts. The carotenoids found in developing apple fruit include -carotene, lutein, violaxanthin, neoxanthin, and cryptoxanthin. Concentrations of lutein and violaxanthin increase, while levels of -carotene decrease, during ripening. Relatively less is known about ripening-associated changes of carotenoids in other temperate tree fruit. Studies in peaches show that only traces of carotenoids are present in ripe fruit.

Anthocyanins are flavonoid compounds and are synthesized from phenylalanine. Anthocyanins are water soluble and accumulate in the vacuoles, producing pink, red, purple, and blue colors of fruit. A typical anthocyanin is cyanidin-3-galactoside, which is largely responsible for the color of apples and pears. Anthocyanins are distributed throughout the fruit, as in sweet cherry cultivars, or restricted to epidermal and subepidermal tissues, as in apples, pears, plums, and nectarines. Anthocyanin concentrations are affected by cultivar, and because of marketing pressures, extensive selection of early red color strains of apple cultivars has occurred. Preharvest factors that influence anthocyanin production include light quality and quantity and temperature. In apples, preharvest sprays of ethylene-producing compounds are sometimes used commercially to stimulate color development. Color changes during ripening of apple depend mainly on simultaneous disappearance of chlorophyll a and b. Anthocyanin concentrations increase little after harvest, even in the presence of ethylene, with apparent changes in appearance, such as redder fruit, being due to degradation of chlorophyll.

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