Antioxidants and Bioactive Compounds in Fruits - Fruit Ripening: Physiology, Signalling and Genomics - page 102

Although in vivo data are rather limited,
there is accumulative in vitro evidence
showing that phenolic compounds affect
many cellular processes. The antioxidant
activity of phenolics is attributable to the
electron delocalization over the aromatic
ring and their high redox potential, which
allows them to act as reducing agents,
hydrogen donors and singlet oxygen
quenchers (Serrano et al. , 2011). The main
categories of phenolic compounds are
phenolic acids and fl avonoids. This
classifi cation is made based on the
structure of their basic skeleton. Other
compounds that belong to this category are
lignans, stilbenes, tannins, coumarins and
lignins.

catechin and epicatechin are fl avanols
(Manach et al. , 2004). The genus Citrus is
characterized by the accumulation of
fl avanone glycosides. Orange juice is a
source of the fl avanone glycoside
hesperidin (Tripoli et al. , 2007), whilst the
fl avanols catechin and epicatechin are
common in the peel and seeds of grape and
apple (Rice-Evans et al. , 1997). Isofl avones
are not commonly found in fruits.
Proanthocyanidins are oligomeric
fl avonoids, usually dimers or oligomers of
the fl avanols catechin and epicatechin.
Anthocyanidins are pigments giving several
fruits their characteristic red, blue or purple
colours (e.g. berries, grape, cherry, pome-
granate, plum, apple), although under some
conditions they remain uncoloured. In
addition, anthocyanidins have great anti-
oxidant potency, with the distribution of
hydroxyls in the molecule affecting the
antioxidant capacity of the different
anthocyanidins; the differences between
them result from the OH, H and OCH 3
substitutions on the phenolic ring. Among
the 23 anthocyanidins that have been
described, the most common ones found in
fruits are cyanidin, delphynidin, pelar-
gonidin, peonidin, petunidin and malvidin.
Anthocyanins are glycosides containing
a sugar moiety and an anthocyanidin unit.
More than 635 different anthocyanins exist
and their structural differences are related
to the number of hydroxyl groups in the
anthocyanidin skeleton, and the position
and the number of bonded sugar residues,
as well as by the aliphatic or aromatic
carboxylates bonded to them (Castañeda-
Ovando et al. , 2009). The content and
diversity of anthocyanins in fruits is
greatly affected by genetic factors, environ-
mental conditions, agricultural practices,
harvest maturity, storage conditions, post-
harvest treatments and processing.

Phenolic acids
Phenolic acids (C6-C1) are categorized
as: (i) benzoic acid derivatives (syringic
acid, gallic acid, vanillic acid,
p -hidroxybenzoic); (ii) cinnamic acid
derivatives ( p -coumaric acid, caffeic acid,
ferulic acid, sinapic acid, chlorogenic
acid); and (iii) hydroxyphenylacetic acids
(4-hydroxyphenylacetic acid, dihydroxy-
phenylacetic acid). Such derivatives differ
in the degree of hydroxylation, methoxyl-
ation and alkylation of the aromatic ring. A
structure-antioxidant activity relationship
has been found for phenolic acids
(Exarchou and Gerothanasis, 2006). The
most common phenolic acids in fruits are
caffeic and gallic acid, and thus the total
phenol content is usually determined in
terms of caffeic or gallic acid equivalents.

Flavonoids
Flavonoids have a structure of two
aromatic rings, which are associated
together by a three-carbon oxygenated
heterocycle (C6-C3-C6), and include
fl avonols, fl avones, isofl avones, fl avanols,
fl avanones, proanthocyanidins and antho-
cyanidins.
Rutin, luteolin and apigenin are the
most common fl avones, quercetin and
kampferol are typical fl avonols,

Other phenolic compounds
Fruit also contain polyphenolic com-
pounds, derived from the polymerization
of simple phenolics. The most well-known
group of these compounds is tannins. and