21 Biochemistry of Fruits 509color to the fruits, but also are important nutritional
ingredients in the human diet. Beta-carotene is con-
verted to vitamin A in the human body and thus
serves as a precursor to vitamin A. Carotenoids are
strong antioxidants. Lycopene is observed to pro-
vide protection from cardiovascular diseases and
cancer (Giovanucci 1999). Lutein, a xanthophyll,
has been proposed to play a protective role in the
retina, maintaining vision.
ANTHOCYANINBIOSYNTHESIS
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 ac-
cumulate in the vacuole of the cell and are often
abundant in the cells closer to the surface of the fruit.
Anthocyanin biosynthesis starts with the conden-
sation of three molecules of malonyl-CoA with
p-coumaroyl-CoA to form tetrahydroxychalcone,
mediated by the enzyme chalcone synthase (Fig.
21.8). Tetrahydroxychalcone has the basic flavonoid
structure C 6 -C 3 -C 6 , with two phenyl groups separat-
ed by a three-carbon link. Chalcone isomerase
enables the ring closure of chalcone, leading to the
formation of the flavanone naringenin, which pos-
sesses a flavonoid structure having two phenyl
groups linked together by a heterocyclic ring (Fig.
21.9). 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 dihydro-
kaempferol, dihydromyricetin, and dihydroquerce-
tin, which differ in their number of hydroxyl moieties.
Dihydroflavonol reductase converts the dihydroflav-
onols into the colorless anthocyanidin compounds
leucocyanidin, leucopelargonidin, and leucodelphi-
nidin. Removal of hydrogens and the induction of
unsaturation of the C ring at C2 and C3, mediated by
anthocyanin synthase, result in the formation of
cyanidin, pelargonidin, and delphinidin, the colored
compounds (Fig. 21.9). 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. Pelar-
gonidins give orange, pink, and red color; cyanidins
provide magenta and crimson coloration; and del-
phinidins provide the purple, mauve, and blue color
characteristics to several fruits. The color character-
istics of fruits may result from a combination of sev-
eral forms of anthocyanins existing together, as well as
the conditions of pH and ions present in the vacuole.
Anthocyanin pigments cause the diverse colora-
tion of grape cultivars, resulting in skin colors vary-
ing from translucent to red and black. All the forms
of anthocyanins including 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 or 5 position 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 towards the end
of ripening and is highly influenced by sugar levels,
light, temperature, ethylene, and increased metab-
olite translocation from leaves to fruits. All these
factors positively influence the anthocyanin levels.
Most of the anthocyanin accumulation may be limit-
ed to epidermal cell layers and a few of the subepi-
dermal 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
1500 and 3000 mg/kg fresh weight. In some high-
anthocyanin-containing varieties such as Vincent,
Lomanto, and Colobel, the anthocyanin levels can
exceed 9000 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 the early stage
of development, which is termed astringency; it is
characteristic to fruits such as banana, kiwi, and
grape. The astringency is due to the presence of tan-
nins 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.ESTERVOLATILEBIOSYNTHESISThe sweet aroma characteristic to several ripe fruits
is due to the evolution of several types of volatile com-
ponents that include monoterpenes, esters, organic