Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

44 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
have a stabilizing role in the photosynthetic reactions. By virtue of their structure, they
can accept and stabilize excess energy absorbed by the light-harvesting complex. During
the early stages of fruit development, the carotenoids have primarily photosynthetic
function. As the fruit ripens, the composition of carotenoids changes to reveal the colored
xanthophylls pigments. In tomato, lycopene is the major carotenoid pigment that accumulates
during ripening. Lycopene is an intermediate of the carotene biosynthetic pathway. In
young fruits, lycopene formed by the condensation of two geranylgeranyl pyrophosphate
(C20) moieties, mediated by the enzyme phytoene synthase, is converted to β-carotene
by the action of the enzyme sesquiterpene cyclase. However, as ripening proceeds, the
levels and activity of sesquiterpene cyclase are reduced, leading to the accumulation of
lycopene in the stroma. This leads to the development of red color in ripe tomato fruits.
In yellow tomatoes, the carotene biosynthesis is not inhibited, and as the fruit ripens, the
chlorophyll pigments are degraded exposing the yellow carotenoids. Carotenoids are also
major components that contribute to the color of melons. β-Carotene is the major pigment
in melons with an orange flesh. In addition, the contribution to color is also provided by
α-carotene, δ-carotene, phytofluene, phytoene, lutein, and violaxanthin. In red-fleshed melons,
lycopene is the major ingredient, whereas in yellow-fleshed melons, xanthophylls and
β-carotene predominate. Carotenoids provide not only a variety of color to the fruits, but
are also important nutritional ingredients in human diet. β-Carotene is converted to vitamin
A in the human body and thus serves as a precursor to vitamin A. Carotenoids are strong
antioxidants. Lycopene is observed to provide protection from cardiovascular diseases and
cancer (Giovanucci, 1999). Lutein, a xanthophyll, has been proposed to play a protective
role in the retina, maintaining the vision.
3.4.2 Anthocyanin biosynthesis
The development of color is a characteristic feature of the ripening process, and in several
fruits, the color components are anthocyanins biosynthesized from metabolic precursors.
The anthocyanins accumulate in the vacuole of the cell and are often abundant in the cells
closer to the surface of the fruit. Anthocyanin biosynthesis starts by the condensation of
three molecules of malonyl CoA with p-coumaroyl CoA to form tetrahydroxychalcone,
mediated by the enzyme chalcone synthase (Fig. 3.8). Tetrahydroxychalcone has the basic
flavonoid structure C6–C3–C6, with two phenyl groups separated by a three-carbon link.
Chalcone isomerase enables the ring closure of chalcone leading to the formation of the
flavanone naringenin that possesses a flavonoid structure having two phenyl groups linked
together by a heterocyclic ring (Fig. 3.9). The phenyl groups are designated as A and B,
and the heterocyclic ring is designated as ring C. Subsequent conversions of naringenin
by flavonol hydroxylases result in the formation of dihydrokaempferol, dihydromyricetin,
and dihydroquercetin, which differ in their number of hydroxyl moieties. Dihydroflavonol
reductase converts the dihydroflavonols into the colorless anthocyanidin compounds leucocyanidin,
leucopelargonidin, and leucodelphinidin. Removal of hydrogens and the induction
of unsaturation of the C ring at C2 and C3, mediated by anthocyanin synthase, results in
the formation of cyanidin, pelargonidin, and delphinidin, the colored compounds (Fig. 3.9).
Glycosylation, methylation, coumaroylation, and a variety of other additions of the anthocyanidins
result in color stabilization of the diverse types of anthocyanins seen in fruits.
Pelargonidins give orange, pink, and red color, cyanidins provide magenta and crimson