Biochemistry of Fruits - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 21

et al., 2000) fruits. ACC synthase is the rate-limiting enzyme of the ethylene biosynthetic
pathway and requires pyridoxal-5-phosphate as a cofactor. ACC synthase is inhibited by
pyridoxal phosphate inhibitors such as aminoethoxyvinylglycine and aminooxyacetic acid.
Field application of aminoethoxyvinylglycine as a growth regulator (retain, valent chemi-
cals) is a practical method of delaying the ripening in fruits such as apples, peaches, and
pears. As well, controlled atmosphere storage at very low oxygen levels (1-3%) is a com-
mon practice in commercial operations for long-term storage of fruits such as apples to
reduce the production of ethylene, since oxygen is required for the conversion of ACC to
ethylene.
After the initiation of ripening or harvest, several biochemical changes occur in fruits
and vegetables. As some of these changes such as the development of color, flavor, and
sweet taste are desirable for fruits—any sort of quality changes are ideally not desired
in vegetables. Thus, strategies for the preservation of shelf life and quality in fruits and
vegetables could be entirely different. It is important to know the biochemical differences
between fruits and vegetables and several biochemical pathways that operate in these tissues
to develop ideal conditions of storage for the preservation of shelf life and quality.
3.2 Biochemical composition of fruits
Fruits contain a large percentage of water that can often exceed 95% by fresh weight.
During ripening, activation of several metabolic pathways often leads to drastic changes in
the biochemical composition of fruits. Fruits such as banana store starch during development
and hydrolyze the starch to sugars during ripening that also results in fruit softening. Most
fruits are capable of photosynthesis, store starch, and convert them to sugars during ripening.
Fruits such as apple, tomato, and grape have a high percentage of organic acids, which
decreases during ripening. Fruits contain large amounts of fibrous materials such as cellulose
and pectin. The degradation of these polymers into smaller water-soluble units during
ripening leads to fruit softening as exemplified by the breakdown of pectin in tomato and
cellulose in avocado. Secondary plant products are major compositional ingredients in fruits.
Anthocyanins are the major color components in grapes, blueberries, apples, and plums;
carotenoids, specifically lycopene and carotene, are the major components that impart color
in tomatoes. Aroma is derived from several types of compounds that include monoterpenes
(as in lime, orange), ester volatiles (ethyl, methyl butyrate in apple, isoamyl acetate in
banana), simple organic acids such as citric and malic acids (citrus fruits, apple), and small
chain aldehydes such as hexenal and hexanal (cucumber). Fruits are also rich in vitamin
C. Lipid content is quite low in fruits, the exceptions being avocado and olives, in which
triacylglycerols (oils) form the major storage components. The amounts of proteins are
usually low in most fruits.
3.2.1 Carbohydrates, storage, and structural components
As the name implies, carbohydrates are organic compounds containing carbon, hydrogen,
and oxygen. Basically, all carbohydrates are derived by the photosynthetic reduction of CO 2 ,
and the hexoses (glucose, fructose) and pentoses (ribose, ribulose) that are intermediates
in the pathway are further converted to several sugar monomers. Polymerization of several