Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

448 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
HMG-CoA reductase (HMGR). MVA undergoes phosphorylation, and produces IPP, a
five-carbon condensational unit of many terpenes. DMAPP is an isomer of IPP, which is
generated in the presence of the enzyme IPP isomerase. These components undergo condensation
and produce geranyl (C 10 ), farnesyl (C 15 ), and geranylgeranyl (C 20 ) pyrophosphate
compounds, which form monoterpenes, sesquiterpenes, and diterpenes. Monoterpenes and
their derivatives (aldehydes, esters, and alcohols) are major volatile compounds in fruits
(Paliyath et al., 1997; Paliyath and Murr, 2006). The DXP pathway starts with the condensation
of pyruvate and glyceraldehyde-3-P, which generates DXP. Further, a series of
reactions produce IPP and DMAPP simultaneously in this pathway (Rohmer et al., 1993).
21.4.3 Biosynthesis of ester volatiles
Every fruit has its own characteristic aroma, which is due to the presence of unique monoterpenes,
esters, organic acids, aldehydes, and ketones. During lipid catabolism, several alcohols
and acids are generated that combine with fatty acyl CoA to form ester volatiles.
Fruits, including banana, apples, and strawberry, are enriched in ester volatiles. With the
advancement of ripening, production of volatile components increases. The key enzyme in
the synthesis of ester volatiles is alcohol acyl CoA transferase (AAT) that transfers an acyl
moiety of the acyl-CoA to a corresponding alcohol (Paliyath and Murr, 2006).
In fruits such as apple, tomato and melon, the aroma formation depends on ethylene
production (Bauchot et al., 1998; Fan et al., 1998; Griffiths et al., 1999). It has been reported
that repression of ACC synthase and ACC oxidase reduces aroma components in tomato
and melon (El-Sharkawy et al., 2005; Zhu et al., 2005). Lipoxygenase activity resulting in
the production of hexanal and hexanol is also affected by ethylene biosynthesis in tomato
and apple fruits. It is also noted that the use of AVG, an inhibitor of ethylene synthesis, and
diazocyclopentadiene, an ethylene action inhibitor, also reduces the biosynthesis of aroma
components in apples (Fan et al., 1998). The application of 1-MCP, an ethylene action
inhibitor, also decreases aroma volatile production in apples (Fan et al., 1999).
The activity of AAT enzyme was lower in transgenic apples with suppressed ethylene
biosynthesis. Furthermore, the exposure of transgenic fruits to 80 μL/L ethylene increased
AAT activity to the same level observed in nontransformed apples (Defilippi et al., 2005),
suggesting that biosynthesis of flavor volatiles depends on ethylene production. However,
it is an open question how ethylene affects the activities of volatile components.
21.4.4 Biosynthesis of anthocyanins
Anthocyanins give red, purple, and blue colors to many fruits, vegetables, cereal, and flowers.
The stability of anthocyanins depends on the pH and the chelation of metal ions.
They are stable under acidic conditions and are rapidly broken down under neutral conditions.
Anthocyanins are glycosylated anthocyanidins, and sugars are mostly attached to
the 3-hydroxyl position of the anthocyanidins (sometimes to the 5 or 7 position). There are
several hundred types of anthocyanins depending on the number of hydroxyl groups, their
methylation patterns, and the nature and number of sugars that are attached the molecule.
In higher plants, six anthocyanidins are common that include cyanidin (Cy), delphinidin
(Dp), peonidin (Pn), petunidin (Pt), malvidin (Mv), and pelargonidin (Pg).