Antioxidants and Bioactive Compounds in Fruits - Fruit Ripening: Physiology, Signalling and Genomics - page 103

Tannins are segregated into hydrolysable
tannins, with a base unit of a gallic acid
moiety, and condensed tannins, with a
fl avone base unit. Tannins are found
mainly in pomegranate, persimmon,
berries and nuts, and are major con-
tributors to the bitter, astringent taste com-
ponent of fruits. Lignans are polyphenolic
substances derived from phenylalanine via
dimerization of substituted cinnamic
alcohols, such as cinnamic acid. Apricots,
strawberries and acai fruits are rich in
lignans. An analytical description of the
properties of major fruit polyphenols, as
well as biosynthetic and regulatory pro-
cesses that affect their composition, are
presented by Ageorges et al. , Chapter 9,
this volume.

7.2.3 Vitamin C

Ascorbic acid (AsA) is usually mentioned
as being synonymous with vitamin C;
however, vitamin C is considered the sum
of AsA and its fi rst oxidation product,
dehydroascorbic acid (DHA). Ascorbic acid
can even be oxidized during eating while
the food is being chewed. However, it is
important to consider that the fi rst
breakdown product of AsA, DHA, still has
vitamin C activity, and all activity is lost if
oxidation proceeds beyond this stage
(Salunkhe et al. , 1991). Ascorbic acid is a
water-soluble carbohydrate-derived com-
pound showing antioxidant and acidic
properties due to the presence of a
2,3-enediol moiety. The effi cacy of AsA in
disease prevention has been associated
with its capacity to neutralize ROS. AsA is
highly susceptible to oxidation, either
directly or through the enzyme ascorbate
oxidase, catalysing the oxidation of AsA to
DHA, with the concomitant reduction of
molecular oxygen to water (Sanmartin et
al. , 2007). Ascorbic acid concentration is
highly dependent on the individual
commodity considered (Lee and Kader,
2000). Persimmon, strawberry, kiwifruit,
and citrus fruit can be considered excellent
sources of AsA (reviewed by Davey et al. ,
2000; Vicente et al. , 2009). Wide variations
in its content within cultivars of the same
species also exist. For instance, AsA
content in fuzzy kiwifruit ( Actinidia
deliciosa ) varies from 29 to 80 mg 100 g -1
of fresh weight (Nishiyama et al. , 2004)
and from 14 to 103 mg 100 g -1 of fresh
weight in berry fruits (Pantelidis et al. ,
2007). The retention of AsA is also
markedly affected by storage and pro-
cessing, with high temperatures having the
highest impact (Lee and Kader, 2000).
Complex control networks affected by
multiple factors regulate ascorbate bio-
synthesis and metabolism in higher plants.
Perturbation of the ascorbate redox state by
either increasing or decreasing the reduced
ascorbate status alters the signalling path-
way and consequently the plant responses
to various stimuli and stresses. The

7.2.2 Carotenoids

Carotenoids are liposoluble pigments,
responsible for the yellow, orange and red
colour of several fruits, divided into
carotenes (e.g. D -carotene, E -carotene and
lycopene) and oxygenated derivatives
known as xanthophylls (e.g. lutein, crypto-
xanthin, violaxanthin and zeaxanthin).
Some carotenoids (like D -carotene,
E -carotene and cryptoxanthin) have pro-
vitamin A activity (Kopsell and Kopsell,
2006; Meléndez-Martínez et al. , 2007).
Carotenoids are usually present at low
concentrations and their levels vary
signifi cantly among species. Fruit are
generally not such good sources of
carotenoids as vegetable crops, although
there are a few notable exceptions such as
apricot, mango, citrus, papaya and
watermelon (Vicente et al. , 2009). To date,
more than 600 different carotenoids have
been identifi ed, but few are common; for
example, 25 carotenoids have been
identifi ed in loquat cultivars (de Faria et
al. , 2009). E -Carotene is the most studied
carotenoid, and lycopene is common in
tomato and watermelon. As lycopene is
regarded as a powerful natural antioxidant,
high-lycopene tomato cultivars are being
promoted in the market (Ilahy et al. , 2011).