478 N. Terrier et al.
9B.3.2 Enzymatic Processes
9B.3.2.1 Enzymatic Oxidation
The major grape enzymes involved in flavonoid degradation is polyphenoloxidase
(PPO) that catalyseso-hydroxylation of monophenols (Fig. 9B.4(1)) and oxidation
ofo-diphenols to the correspondingo-quinones (Fig. 9B.4(2)). Its action is partic-
ularly important in white wine technology as extensive decompartmentation of the
enzyme and its substrates (phenolic compounds and oxygen) takes place at pressing.
Catechins and flavonol aglycones are rather poor substrates for grape PPO compared
to caffeoyltartaric acid. Glycosylated flavonoids, epicatechin gallate and proantho-
cyanidins cannot be oxidized directly by grape PPO, presumably due to steric hin-
drance. However, they can react with the enzymically generated caffeoyltartaric acid
quinone through coupled oxidation (Fig. 9B.3) and nucleophilic addition reactions
(Fig. 9B.4) (Cheynier and Van Hulst 1988; Cheynier et al. 1988, 1989a, 1995;
Cheynier and Ricardo Da Silva 1991), as described in Chapter II.2.3 for antho-
cyanins. In addition, disproportionation of ano-quinone and ano-diphenol can
yield two semiquinone radicals (Fig. 9B.5) which can then undergo radical cou-
pling (Fig. 9B.6). Enzymatic oxidation can also be catalysed by other enzymes such
as laccase arising fromBotrytis cinerea (grey mold or noble rot) and peroxidases
which accept a wider range of substrates.
9B.3.2.2 Enzymatic Hydrolysis
Other enzymes degrading flavonoids include various hydrolases, originating from
microorganisms or from pectolytic preparations added during wine-making. Some
yeast strains show glycosidase activities, including -glucosidase which is active
on flavono and anthocyanin 3-glucosides. Tannase activity (tannin acyl hydrolase,
EC 3.1.1.20) catalysing hydrolysis of galloyl esters has been observed in numerous
fungi, includingB. cinerea, and in lactic acid bacteria (Matthews et al. 2006). The
use of exogenous enzymes is becoming more and more popular to help clarification,
increase press yield, extraction, and release of aroma compounds from their gluco-
sidic precursors. These enzyme preparations are pectinases and -glucanases. They
may show side activities such as cinnamate esterase, tannase and -glucosidase.
Hydrolysis of flavonol glycosides has been reported to result in haze development
as the insoluble flavonol aglycones precipitated out (Somers and Ziemelis 1985).
9B.3.3 Chemical Reactions
As stated above, flavonoids react as nucleophiles through their C8 and C6 posi-
tions. Acid-catalysed cleavage of the interflavanic linkages of proanthocyanidins
(cf. Fig. 9B.3) also takes place spontaneously in wine, yielding an intermediate elec-
trophile. A third group of reactions involves theo-diphenolic B-ring which can be
oxidized to electrophilic quinones. Several mechanisms arise from these reactivities.