Food Chemistry

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.

(18.18)

The glucosides of cis-o-coumaric acid are the pre-
cursors of coumarins. Disintegration of plant tis-
sue releases the free acids from the glucosides.
The acids then close spontaneously to the ring
forms (R: OH and OCH 3 in various positions):

(18.19)

Fig. 18.5.Section of the structure of a lignin (according
toKindlandWöber, 1975)

Lignin is formed by the dehydrogenative poly-

merization 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 alco-

hol 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).

18.1.2.5.2 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]:

(18.20)

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