Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

BIOCHEMISTRY OF FRUITS 47
coloration, and delphinidins provide the purple, mauve, and blue color characteristic to
several fruits. The color characteristics of fruits may result from a combination of several
forms of anthocyanins existing together, as well as the conditions of pH and ions present in
the vacuole.
Anthocyanin pigments cause the diverse coloration of grape cultivars resulting in skin
colors varying from translucent, red, and black. All the forms of anthocyanins, along with
those with modifications of the hydroxyl groups, are routinely present in the red and dark
varieties of grapes. A glucose moiety is attached at the 3 and 5 positions or at both in
most grape anthocyanins. The glycosylation pattern can vary between the European (Vitis
vinifera) and North American (Vitis labrusca) grape varieties. Anthocyanin accumulation
occurs toward the end of ripening, and is highly influenced by sugar levels, light, temperature,
ethylene, and increased metabolite translocation from leaves to fruits. All these
factors positively influence the anthocyanin levels. Most of the anthocyanin accumulation
may be limited to epidermal cell layers and a few of the subepidermal cells. In certain
high-anthocyanin-containing varieties, even the interior cells of the fruit may possess high
levels of anthocyanins. In the red wine varieties such as merlot, pinot noir, and cabernet
sauvignon, anthocyanin content may vary between 1,500 and 3,000 mg/kg fresh weight.
In some high-anthocyanin-containing varieties such as Vincent, Lomanto, and Colobel, the
anthocyanin levels can exceed 9,000 mg/kg fresh weight. Anthocyanins are very strong
antioxidants and are known to provide protection from the development of cardiovascular
diseases and cancer.
Many fruits have a tart taste during early stage of development, which is termed as
astringency, and is characteristic to fruits such as banana, kiwi, and grape. The astringency
is due to the presence of tannins and several other phenolic components in fruits. Tannins are
polymers of flavonoids such as catechin and epicatechin, phenolic acids (caffeoyl tartaric
acid, coumaroyl tartaric acid, etc.). The contents of tannins decrease during ripening, making
the fruit palatable.
3.4.3 Ester volatile biosynthesis
The sweet aroma characteristic to several ripe fruits are due to the evolution of several
types of volatile components that include monoterpenes, esters, organic acids, aldehydes,
ketones, alkanes, etc. Some of these ingredients specifically provide the aroma characteristic
to fruits and are referred to as character impact compounds. For instance, the banana flavor
is predominantly from isoamyl acetate, apple flavor from ethyl-2-methyl butyrate, and the
flavor of lime is primarily due to the monoterpene limonene. As the name implies, ester
volatiles are formed from an alcohol and an organic acid through the formation of an ester
linkage. The alcohols and acids are, in general, products of lipid catabolism. Several volatiles
are esterified with ethanol giving rise to ethyl derivatives of aliphatic acids (ethyl acetate,
ethyl butyrate, etc.).
The ester volatiles are formed by the activity of the enzyme acyl CoA: alcohol acyltransferase
or generally called as alcoholacyltransferase. In apple fruits, the major aroma
components are ester volatiles (Paliyath et al., 1997). The alcohol can vary from ethanol,
propanol, butanol, pentanol, hexanol, etc. The organic acid moiety containing the CoA
group can vary in chain length from C2 (acetyl) to C12 (dodecanoyl). Alcoholacyltransferase
activity has been identified in several fruits that include banana, strawberry, melon,