Food Biochemistry and Food Processing (2 edition)

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27 Biochemistry of Fruits 549

lycopene formed by the condensation of two geranylgeranyl py-

rophosphate (C20) moieties mediated by the enzyme phytoene

synthase is converted to beta-carotene by the action of the en-

zyme sesquiterpene cyclase. However, as ripening proceeds, the

levels and activity of sesquiterpene cyclase are reduced lead-

ing to the accumulation of lycopene in the stroma. This leads

to the development of red colour in ripe tomato fruits. In yel-

low tomatoes, the carotene biosynthesis is not inhibited, and as

the fruit ripens, the chlorophyll pigments are degraded expos-

ing the yellow carotenoids. Carotenoids are also major compo-

nents that contribute to the colour of melons. Beta-carotene is

the major pigment in melons with an orange flesh. In addition,

the contribution to colour is also provided by alpha-carotene,

delta-carotene, phytofluene, phytoene, lutein and violaxanthin.

In red-fleshed melons, lycopene is the major ingredient, whereas

in yellow-fleshed melons, xanthophylls and beta-carotene pre-

dominate. Carotenoids provide not only a variety of colour to the

fruits but also important nutritional ingredients in human diet.

Beta-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 and prevention of age-related

macular degeneration.

Anthocyanin Biosynthesis

The development of colour is a characteristic feature of the

ripening process, and in several fruits, the colour components

are anthocyanins biosynthesised 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. Antho-

cyanin biosynthesis starts by the condensation of three molecules

of malonyl CoA withp-coumaroyl CoA to form tetrahydroxy-

chalcone, mediated by the enzyme chalcone synthase (Fig. 27.7).

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 lead-

ing to the formation of the flavanone, naringenin that possesses

a flavonoid structure having two phenyl groups linked together

by a heterocyclic ring. The phenyl groups are designated as A

and B and the heterocyclic ring is designated as ring C. Subse-

quent conversions of naringenin by flavonol hydroxylases result

in the formation of dihydrokaempferol, dihydromyricetin and

dihydroquercetin, which differ in their number of hydroxyl moi-

eties. Dihydroflavonol reductase converts the dihydroflavonols

into the colourless 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 coloured compounds

(Figs. 27.7 and 27.8). Glycosylation, methylation, coumaroyla-

tion and a variety of other additions of the anthocyanidins result

in colour stabilisation of the diverse types of anthocyanins seen

in fruits. Pelargonidins give orange, pink and red colour, cyani-

dins provide magenta and crimson colouration, and delphinidins

provide the purple, mauve and blue colour characteristic to sev-

eral fruits. The colour 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 colouration of grape

cultivars resulting in skin colours 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 towards the end of ripening,

and is highly influenced by sugar levels, light, temperature, ethy-

lene 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 epider-

mal cell layers and a few of the sub-epidermal 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 lev-

els 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 early stage of develop-

ment, which is termed as astringency, and is characteristic to

fruits such as banana, kiwi, grape and so on. The astringency is

due to the presence of tannins and several other phenolic com-

ponents 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.

Ester Volatile Biosynthesis

The sweet aroma characteristic to several ripe fruits are due to

the evolution of several types of volatile components that in-

clude monoterpenes, esters, organic acids, aldehydes, ketones,

alkanes and so on. Some of these ingredients specifically provide

the aroma characteristic to fruits and are referred to as character

impact compounds. For instance, the banana flavour is predom-

inantly from isoamyl acetate, apple flavour from ethyl-2 methyl

butyrate, and the flavour of lime is primarily due to the monoter-

pene 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 alco-

holacyltransferase (AAT). In apple fruits, the major aroma com-

ponents are ester volatiles (Paliyath et al. 1997). The alcohol

can vary from ethanol, propanol, butanol, pentanol, hexanol and