9B Flavanols, Flavonols and Dihydroflavonols 483
Evidence in Wine
A number of flavanol-methylmethine-anthocyanins have been detected in wine (see
Chapter 9A for extensive list) as well as methylmethine-linked flavanol dimers
(Cheynier et al. 1997a; Saucier et al. 1997b). More recently, a method to determine
methyl-methine linkages has been proposed (Drinkine et al. 2007a), showing that
they represented less than 4% of bonds between flavanol units in wine although
this proportion increased with aging (Drinkine et al. 2007b). The other aldehyde
derivatives have not yet been detected in wine.
Factors Affecting the Reaction
The rate of condensation reactions dependson the concentration of the precursors
involved, and especially on that of the aldehyde, and on its protonation rate, which
is primarily determined by pH. On the other hand, acidic conditions also favour
cleavage of the products. The methylmethine bridges are particularly susceptible
to acid-catalysed cleavage which leads to rearrangement and further polymerisa-
tion (Es-Safi et al. 1999a; Escribano-Bailon et al. 2001). This also yields vinylfla-
vanol structures which add to anthocyanins to form flavanyl-pyranoanthocyanins
(Cheynier et al. 1999a; Mateus et al. 2002) and flavanylvinylpyranoanthocyanins
(portisins) (Mateus et al. 2003) as described in Chapter 9A.
Acetaldehyde is generated during alcoholic fermentation but its concentration
can be much increased as a result of oxygen exposure in the wine-making or aging
process. Metal ions such as iron (Oszmianski et al. 1996) or copper (Clark and
Scollary 2002) much enhance browning of catechin in wine-like medium contain-
ing tartaric acid due to the formation of xanthylium pigments (Es-Safi et al. 1999b).
Metals were first postulated to catalyse oxidation of tartaric acid to glyoxylic acid
but it was latter demonstrated that copper(II) also accelerated the bridging of two
(+)-catechin units by glyoxylic acid (Clark et al. 2003). Ascorbic acid, after an initial
antioxidant phase, also increased formation of xanthylium salts, as it crossed-over
to pro-oxidant (Bradshaw et al. 2003). Catechin reaction was faster with glyoxylic
acid than with acetaldehyde and even faster when both aldehydes were present
(Drinkine et al. 2005). Under the conditions used in this experiment, methylmethine
and carboxymethine bridged dimers as well as mixed polymers were observed but
no xanthylium salt could be detected, either because of the rather short incubation
time used or because no metal catalyst was present. However, products arising from
glyoxylic acid condensation have never beendetected in wine, possibly because its
formation rate from tartaric acid is the limiting factor.
9B.3.3.3 Reactions Involving Quinones
Precursors
These reactions involve, on one hand, the nucleophilic A-rings of flavonoids such
as flavanols, and on the other hand, electrophilic quinones arising from enzymatic
or chemical oxidation of phenolic compounds.