Wine Chemistry and Biochemistry

558 C. Santos-Buelga and V. de Freitas

In fact, the cellular structure of fruits softens during ripening as a result of the

enzymatic depolymerization of structural carbohydrates in cell walls (pectins, hemi-

cellulose, cellulose) releasing soluble fragments (Fry 1995; Huber 1983; Brummell

and Harpster 2001; Prasanna et al. 2007). The increase in soluble pectin frag-

ments could inhibit salivary protein-tannin interactions in the mouth leading to

a decreased astringent response. Some other mechanisms have been proposed to

explain this phenomenon. The decrease of astringency could also result from the

changes of polyphenolic composition during ripening (Haslam 1998b; Goldstein

and Swain 1963). Some authors have shown that the concentration of flavan-3-ols

and namely procyanidins in grapes decrease during maturity (de Freitas et al. 2001;

Czochanska et al. 1979; Kennedy et al. 2000). It has also been demonstrated that

during grape ripening, tannins bind to the internal face of tonoplasts, and also to the

cell wall polysaccharides through osidic bonds (Amrani Joutei et al. 1994). Other

findings showed that acetaldehyde formed insitu during ripening of some fruits,

such as persimmon, induces the polymerization (insolubilization) of water-soluble

proanthocyanidins, decreasing astringency (Tanaka et al. 1994).

Regarding the influence of polysaccharides, two mechanisms have been proposed

to explain their influence on protein-tannin interactions in solution (Haslam 1998b;

Luck et al. 1994) (Fig. 9D.13):

(i) Polyelectrolyte polysaccharides could form ternary complexes with the protein-

tannin aggregates, enhancing its solubility in aqueous medium

(ii) Polysaccharides could encapsulate tannins interfering with their ability to bind

and precipitate proteins

Tannin-Carbohydrate

Soluble complex

Protein

Protein/Tannin

Insoluble complex

Protein-Tannin-Carbohydrate

Soluble complex

i)

ii)

Fig. 9D.13Mechanism of carbohydrate inhibition of protein tannin interactions (Mateus

et al. 2004) (adapted from Mateus et al. 2004a)