Wine Chemistry and Biochemistry

9B Flavanols, Flavonols and Dihydroflavonols 465

9B.1.2 Flavanols

9B.1.2.1 Structure and Localisation

Grape flavanols, more accurately called flavan 3-ols as they are hydroxylated in the

3 position, are found as monomers but also as oligomers and polymers.

The major flavan 3-ol monomers in grape are (+)-catechin and its isomer,

(–)-epicatechin, and, to a lesser extent, the gallic ester of (–)-epicatechin, (–)-

epicatechin 3-gallate (Su and Singleton 1969) (Fig. 9B.2). Gallocatechin (Piretti

et al. 1976; Czochanska et al. 1979b) has also been reported inVitis viniferaand

catechin 3-gallate (Lee and Jaworski 1987) and gallocatechin 3-gallate (Lee and

Jaworski 1990) have been detected in some non-Viniferavarieties.

Flavanol oligomers and polymers are also called condensed tannins or proan-

thocyanidins. The term tannin refers to their capacity to interact or react with

proteins and precipitate them out. When heated under acidic conditions, these

molecules release red anthocyanidin pigments, hence the term proanthocyanidins.

The term leucoanthocyanidin, also referring to this particular property, is sometimes

encountered in the literature. However, this should be restricted to another group of

compounds, flavan 3,4-diols, which are intermediates in the biosynthetic pathway

leading to flavanols and anthocyanins (Stafford and Lester 1984; Nakajima et al.

2001; Abrahams et al. 2003) but have never been isolated from grapes, presumably

due to their instability.

In B-type proanthocyanidins (Fig. 9B.2), the flavanol constitutive units are linked
by C4-C8 and/or C4-C6 bonds, opening thepossibility for branched structures.

HOO

OH

OH

HO O

OH

OH

OH

OH

R 2

R 1

R 2

R 1

HO O

OH

R

OH

OH

8

6

4

OH

OH

OH

O

(+)-catechin: R=H, R 1 =OH, R 2 =H

(–)-epicatechin: R=H, R 1 =H,R 2 =OH

(–)-epicatechin 3-gallate: R=H, R 1 =H, R 2 =O-G

(+)-gallocatechin: R=OH, R 1 =OH, R 2 =H

(–)-epigallocatechin: R=OH, R 1 =H, R 2 =OH

(+)-gallocatechin 3-gallate: R=OH, R 1 =H, R 2 =O-G

(–)-epigallocatechin 3-gallate: R=OH, R 1 =O-G, R 2 = H

G =

HO

OH

O

OH

OH

HO OH

O

OH

HO

R 2

R 1

R 2

R 1 HOO

OH

OH

O O

OH

OH

OH

OH

R 2

R 1

R 2

R 1

4

4

4

8

8

7

2

6

C4–C8 bond

B-type procyanidins

A-type procyanidin

C4–C6 bond

R 2

R 1

Fig. 9B.2Structures of flavanol monomers and dimers