Changes in Nutritional Quality of Fruits and Vegetables During Storage - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 444

fruit with optimal quality features. A number of biochemical changes, such as the biosyn-
thesis of anthocyanins, degradation of chlorophyll, enhanced activity of cell wall-degrading
enzymes, evolution of aroma compounds, and an increase in respiration and energy produc-
tion, occur during the progression of ripening (Paliyath and Murr, 2006). Ripening initiates
the catabolic breakdown of starch releasing sugars, and organic acids are converted back to
sugars through a process called gluconeogenesis.
Fruit softening is an integral part of the ripening, which is associated with textural
changes. It involves the alteration in structure and composition of cell wall, degradation of
cellulose and pectin components, and breakdown of starch. Depending on the fruit, the nature
of softening may vary from one to another. Fruits, including cherries, grapes and banana,
undergo extensive fruit softening, whereas extensive softening is not a part of fruit ripening
in apple and citrus (Seymour et al., 1993). The cell wall is mainly made of cellulose, pectin,
and hemicelluloses. Several enzymes, such as cellulase or

-galactosidase,
pectin methylesterase, and polygalactouronase, are involved in degradation of cell wall
components. It has been found that polygalactouronase activity is lower in cherries than
other fruits. Its activity increases during maturation and storage of cherries (Barrett and
Gonzalez, 1994). Excessive ripening of the fruit can be reduced by application of calcium.
It binds and cross-links with free carboxyl groups of polygalacturonic acid in pectin, which
enhances firmness of the fruits (Paliyath and Murr, 2006).
During senescence, phospholipid content of the cell membrane declines with an asso-
ciated increase in the levels of neutral lipids such as diacylglycerols, free fatty acids, and
fatty aldehydes. Moreover, the levels of sterols also increase during senescence. Hence,
there is an increase in the ratio of sterol/phospholipids. These changes decrease membrane
fluidity ultimately resulting in the loss of cellular compartmentalization, and subsequently
leading to senescence (Paliyath and Droillard, 1992). Membrane lipid degradation is ini-
tiated by the enzyme phospholipase D (PLD), which liberates phospholipid head groups
(choline, ethanolamine, inositol, etc.), and is followed by the activity of enzymes such as
phosphatidate phosphatase, lipolytic acyl hydrolase, and lipoxygenases. In this process,
peroxidized fatty acids produce compounds like hexanal and hexenal, which are important
fruit volatiles. The short-chain fatty acids undergo

β -1,4-glucanase, β

-oxidation and form short-chain fatty
acyl CoAs and fatty alcohols. These products are esterified enzymatically by alcohol—acyl
coA acyl transferase, giving rise to the volatile flavor components in fruits (Paliyath and
Droillard, 1992).

β

21.3 Role of ethylene in ripening

The process of ripening is initiated by the plant hormone ethylene. On the basis of ethy-
lene production and response to externally applied ethylene, fruits can be classified into
climacteric and nonclimacteric. Climacteric fruits display a burst in ethylene production
and respiration during ripening. In these fruits, synthesis of ethylene is autocatalytic, which
can reach tissue internal levels of 30-500 L/L or more. However, nonclimacteric fruits,

such as cherries, strawberries, and grapes, neither require a high level of ethylene during the
initiation of ripening nor do they produce ethylene autocatalytically as in climacteric fruits.
The rate of ethylene biosynthesis is also influenced by several external factors that mainly
include storage temperature and the levels of O 2 and CO 2 during postharvest storage. It has

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