Polyphenols - Fruit Ripening: Physiology, Signalling and Genomics - page 160

9.2 Structures and Occurrence of Major
Fruit Polyphenols

delphinidin series. Further diversity arises
from the nature of the sugar (mono-
saccharide, often glucose but also galactose,
xylose, rhamnose and arabinose, or di-
saccharide, such as rutinose, sambubiose
and sophorose) and from additional sub-
stituents (glycosylation in the 5-position,
acylation of the sugar with aliphatic (e.g.
acetic, malic and oxalic) or hydroxy-
cinnamic ( p -coumaric, caffeic) acids).
Anthocyanin profi les are extremely variable
and are characteristic of each plant species
(Mazza and Miniati, 1993; Wu and Prior,
2005), with only two anthocyanins in
apples and peaches, and up to 31 in
'Concord' grape. Quantitatively, large
varietal differences are also observed: for
example, anthocyanin content in grape
berries ranges from zero in white cultivars
to 5 g kg -1 in the teinturier cultivar 'Alicante
Bouschet' (Mattivi et al. , 2006).

Polyphenols are found in most fruit
species. Fruit polyphenols are represented
primarily by fl avonoids, including some
widespread families such as anthocyanins,
fl avan-3-ols and fl avonols, and rather un-
common groups such as fl avanones and
dihydrochalcones. Major non-fl avonoid
polyphenols in fruits are ellagitannins and
ellagic acid conjugates. Other families, such
as stilbenoids and lignans, often mentioned
for their potential health effects, are also
encountered in fruits, in minor quantities.
Information available in the literature on
the occurrence and quantities of phenolic
compounds in common foods has recently
been compiled in the Phenol-Explorer
database (Neveu et al. , 2010). Specifi c
information on fruit fl avonoids can also be
found in the USDA fl avonoid database
( http://www.ars.usda.gov/ba/bhnrc/ndl).

Flavonols
Flavonols (Fig. 9.1b) are widespread in
fruits and are structurally diverse, differing
by the B-ring substitution pattern, and the
nature and position of sugar substituents,
which can also be acylated. According to
the USDA database, fl avonols are present
only in small quantities in fruits, but
concentrations in the range 10-50 mg per
100 g are found in some berries.

9.2.1 Flavonoids

Flavonoids share a common structure (C6-
C3-C6) consisting of two aromatic rings (A
and B) and an oxygen heterocycle (C).
They are subdivided into several sub-
classes that differ by the oxidation state of
the heterocycle and position of the B-ring,
as illustrated in Fig. 9.1(a-h).

Flavan-3-ols
Flavan-3-ols (Fig. 9.1c) comprise mono-
mers, oligomers and polymers, called
proanthocyanidins (PAs, syn. condensed
tannins). The presence of asymmetric
carbons in the saturated C-ring gives rise to
several isomers (2,3- trans (2R,3R; 2S,3R)
and 2,3- cis (2R,3S; 2S,3S), designated by
the prefi x epi-). In PAs, fl avan-3-ol units
are linked by C4-C6 or C4-C8 linkages in
the B-type series, plus C7-O-C2 or C5-O-
C2 bonds in the A-type series. PAs are
present in most fruits, except citrus,
cantaloupe, watermelon and tomato (Gu
et al. , 2003; Hellstrom et al. , 2009). Pro-
cyanidins, consisting of catechin and
epicatechin units, are the most widespread,

Anthocyanins

Anthocyanins are the pigments of most red,
blue and black fruits and are rather
common. In a recent screening, they were
detected in 14 fruits out of 25 (Wu and
Prior, 2005). Fruit anthocyanins are based
on six aglycones (anthocyanidins) that
differ by the number of hydroxyl groups on
their B-ring and their methylation pattern
(Fig. 9.1a) and are glycosylated in the
3-position. Pelargonidin is often missing,
while cyanidin derivatives are ubiquitous
and delphinidin derivatives are found in
some species (e.g. grape, blueberry and
cranberry). Methylation is commonly
encountered in both the cyanidin and