Polyphenols - Fruit Ripening: Physiology, Signalling and Genomics - page 165

have been reported in grapes and berries
including blackcurrant, redcurrant, lingon-
berry, strawberry (Ehala et al. , 2005),
bilberry and blueberry (Moze et al. , 2011).
Major representatives are resveratrol,
piceatannol and their 3- O - E - D -glucosides
(R=glucoside), piceid and astringin (Fig.
9.2e), found in the trans and cis forms.
A variety of oligostilbenes are also found
in grape, including (+)- H -viniferin, a
resveratrol cylic dehydrodimer (Fig. 9.2f),
other dimers derived from it, and several
trimers (e.g. D -viniferin), and tetramers
such as E -viniferin and hopeaphenols
(Takaya et al. , 2002; Amira-Guebailia et al. ,
2006). Other minor polyphenol classes,
such as isofl avones, coumestans and
lignans, are attracting much interest be-
cause of their phyto-oestrogenic properties
(Kuhnle et al. , 2007). They occur mostly in
foods as glycosides but can be hydrolysed
by intestinal enzymes. Their major dietary
sources are legumes (soybean and chickpea
for isofl avones, legume sprouts for
coumestans) and cereals for lignans, but
low amounts have also been found in fruits
that contribute about 30% of the total
phytoestrogen intake in western diets
(Carmichael et al. , 2011). Major fruit
sources of isofl avones and lignans are
citrus fruits, although higher con-
centrations of lignans are found in apricot,
watermelon, blackcurrant and gooseberry
(a few 100 μg per 100 g DW; Kuhnle et al. ,
2007). Citrus fruits also contribute small
amounts of coumestrol (15 μg per 100 g
DW; Kuhnle et al. , 2007), accounting for
about half the dietary intake (Carmichael et
al. , 2011).

Anthocyanins are usually represented
(and analysed) under their fl avylium cation
form, whose colour shifts from red to
purple as the number of B-ring substituents
increases. However, this form is prevalent
only in very acidic conditions because
hydration and deprotonation reactions
convert fl avylium ions to colourless
hemiketal forms and blue quinoidal bases
as the pH is increased. Pigment stabili-
zation is provided by regulation of the pH
of intracellular compartments such as the
vacuole and by self-association of antho-
cyanins, association with other molecules
(co-pigmentation), or complexation with
metal ions, as reviewed recently (Yoshida
et al. , 2012). Co-pigmentation, involving
hydrophobic π-π interactions of the
anthocyanin fl avylium with other planar
structures (Goto and Kondo, 1991), results
in colour enhancement and a slight
bathochromic shift from red to purple. In
particular, fl avonols, which by themselves
are light-yellow pigments, are known to act
as co-pigments and enhance anthocyanin
colour.
Some of the anthocyanin-derived pig-
ments listed under 'Flavonoid reaction
products' also show particular colour
properties. In particular, among pigments
detected in fruits, pyranoanthocyanins are
orange (Fulcrand et al. , 1996), while
products resulting from condensation of
anthocyanins with aldehydes are purple,
presumably because of intra-molecular co-
pigmentation (Escribano-Bailon et al. ,
1996; Dueñas et al. , 2006).

9.3.2 Taste properties

Some polyphenols contribute to taste
properties, especially bitterness and
astringency. It should be emphasized that
bitterness is a taste, mediated by interaction
with taste receptors, while astringency is a
tactile perception, usually ascribed to
interaction with (and eventually pre-
cipitation of) salivary proteins.
Numerous phenolic compounds such as
oleoropein, the bitter principle of olive,
and most phenolic acids exhibit bitterness.

9.3 Polyphenols and Fruit Quality

9.3.1 Colour properties

Polyphenols comprise a variety of pigments:
red and blue anthocyanins, yellow fl avonols
and products formed by oxidative reactions
that are responsible for the brown colour of
seed coats, as explained previously under
'Flavonoid reaction products'.