Enzymes in potable alcohol and wine production - Enzymes in Food Technology - page 202

HG

RG II RG I

RG-I

Fig. 9.3 Simplified scheme representing pectin.

Rhamnogalacturonan II (RG-II) has a very complex structure which cannot be hydrolyzed
by enzymes.

There are several schools of thought on how these three constituents found in the cell
walls are organized. It has, however, been shown that they are combined by covalent
acidic links to form pectin (Fig. 9.3), with cross links between different pectin chains
(ionic, electrostatic and diester borate bridges).

9.2.3.2 Pectin physical properties; negative effects in winemaking

Pectin's gel forming and water retention properties prevent the diffusion of pheno-
lic compounds and aromas into the must during the pre-fermentation and fermentation
stages.
The high viscosity of pectin solubilized after crushing, hinders juice extraction, clarifica-
tion and filtration.

9.2.4 Polyphenols

9.2.4.1 Types of phenolic compounds found in grape

Phenolic compounds are responsible for major organoleptic characteristics in wines, par-
ticularly colour and structure in red wines. However, wine polyphenol composition is still
poorly understood; most studies have focused on classes of molecules that can be sepa-
rated and assayed with high performance liquid chromatography (HPLC) and have neglected
polymers that are not as easily determined. Furthermore, polyphenols are highly reactive
compounds and good substrates for various enzymes, including polyphenoloxidases, peroxi-
dases, glycosidases and esterases. They undergo numerous enzymatic and chemical reactions
during storage and processing, and the structures of the resulting products are still poorly
understood. 15
Chemical reactions of polyphenols are particularly important in wine because they are
responsible for the colour and taste changes that occur during aging. Grape polyphenols,
namely, anthocyanins, flavonols, hydroxycinnamic acids and flavanols, including catechins
and proanthocyanidins, represented only approximately one-half of the polyphenol content of
a 2-year-old red wine polyphenol extract. The other one-half consisted of unknown phenolic
species derived from grape polyphenol reactions during winemaking and aging. 5 Genuine
anthocyanins determined with HPLC contributed 50% of the red colour intensity measured
spectrophotometrically after dilution with 1% HCl; therefore, the other one-half could be
attributed to anthocyanin-derived pigments.