9A Anthocyanins and Anthocyanin-Derived Compounds 447
Studies in model solution containing an anthocyanin and flavanol oligomers (up
to tetramers) at pH 3 carried out at 50◦C demonstrated that temperature is another
factor that affects the progress of directcondensation reactions (Malien-Aubert
et al. 2002). At acidic pH and high temperature, the anthocyanin is in equilibrium
with the colorless chalcone. Although breakage of the flavanol C-C bond occurred
under these conditions, the formation of thechalcone impeded the synthesis of F-A
products and only A-F adducts were formed (Malien-Aubert et al. 2002).
9A.2.3 Anthocyanin-Flavanol and Anthocyanin-Anthocyanin
Aldehyde-Mediated Condensation Reactions
Precursors.Acetaldehyde is a yeast metabolite present in wine as a result of
the alcoholic fermentation. Acetaldehyde is also gradually generated during wine
aging from the oxidation of ethanol in the presence of polyphenols (Wildenradt
and Singleton 1974). Other aldehydes such as formaldehyde, propionaldehyde, iso-
valeraldehyde, benzaldehyde, isobutyraldehyde and 2-methylbutyraldehyde are also
minor constituent of wines, but important components of spirits used for the fortifi-
cation of Port wines (Pissarra et al. 2005b). Furfural, 5-hydroxymethylfurfural and
vanillin are compounds resulting from oak toasting, could also be found in wines
submitted to barrel aging. Other compoundsbearing an aldehyde moiety, such as
glyoxylic acid (COOH-CHO), can be generated from the oxidation of tartaric acid
by traces of iron or copper ions (Oszmianski et al. 1996; Clark and Scollary 2002;
Es-Safi et al. 2003).
Mechanism of reaction.The mechanism for aldehyde-mediated condensation
reactions was first proposed by Timberlake and Bridle (1976). The aldehyde, in
the form of carbocation, reacts with the flavanol (or tannin) at position C-6 or
C-8 of the phloroglucinol ring (Chapter 9B). After dehydratation, this flavanol-
aldehyde adduct gives rise to a new carbocation that attacks the anthocyanin.
The resulting compound is stabilized by deprotonation forminga quinoidal base
of violet color. The proposed structure involves the flavanol and the anthocyanin
linked by an ethyl bridge -(CH-CH 3 ) (Fig. 9A.3d). The flavanol is attached to
ethyl bridge by its C-8 or C-6 positions and the anthocyanin by its C-8 position
resulting in the C8-(CH-CH 3 )-C8 and in the C6-(CH-CH 3 )-C8 isomers. In accord-
ing to this, model solutions containing malvidin-3-glucoside and either (+)-catechin
or (–)-epicatechin in the presence of acetaldehyde generally give rise to two major
products (Roggero et al. 1987; Bakker et al. 1993; Rivas-Gonzalo et al. 1995). Due
to the existence of an asymmetric carbon in the ethyl bridge, two diastereoisomers
differing on the stereochemistry (RorS) of the methine carbon are also formed for
each of the regioisomers (Rivas-Gonzalo et al. 1995; Escribano-Bail ́on et al. 1996).
Lee et al. (2004) have recently reported theNMR data of ethyl-linked malvidin-3-
glucoside-(epi)catechin pigments.
Ethyl-linked products usually undergopolycondensation; the initial products
evolve to new pigments with high degree of polymerization that finally precipitate