2009_Book_FoodChemistry

18.1 Fruits 827
Reduction of the benzoic acid carboxyl groups
yields the corresponding aldehydes and alcohols,
as for instance vanillin and vanillyl alcohol (XVII
and XVIII respectively in Formula 18.18) from
3-methoxy-4-hydroxybenzoic acid and coniferyl
alcohol from ferulic acid.
Lignin is formed by the dehydrogenative polymerization
of coniferyl, sinapyl, and p-coumaryl
alcohol, which is catalyzed by a peroxidase and
requires H 2 O 2 . A section of the structure of lignin
formed by the polymerization of coniferyl alcohol
is shown in Fig. 18.5. Lignins strengthen the
walls of plant cells. They play a role as fiber in
foods (cf. 15.2.4.2).
(18.18)
The glucosides of cis-o-coumaric acid are the precursors
of coumarins. Disintegration of plant tissue
releases the free acids from the glucosides.
The acids then close spontaneously to the ring
forms (R: OH and OCH 3 in various positions):
18.1.2.5.2 Flavan-3-ols (Catechins),
Flavan-3,4-diols,
and Proanthocyanidins
(Condensed Tanning Agents)
These colorless compounds are the following:
[R, R 1 = H: a) catechin, b) epicatechin; R = H,
R 1 = OH: gallocatechin, epigallocatechin;
R = OH: flavan-3,4-diols]:
(18.19)
(18.20)
Fig. 18.5. Section of the structure of a lignin (according
to Kindl and Wöber, 1975)
Table 18.20 shows the occurrence of catechin and
epicatechin in fruit.
In flavonoid biosynthesis, flavan-3,4-diols can
be converted to flavan-3-ols (cf. 18.1.2.5.7).
The intermediate is assumed to be a carbocation
which is reduced to flavan-3-ol (Formula 18.21).
When the reducing agent, e. g., NADPH, is
limited, the cation can react with flavan-3-ol to