EXTRACTION OF PHENOLICS FROM GRAPE SOLIDS DURING FERMENTATION

Pulp - HCTA (mainly in juice), Benzoic phenolic acids and Flavans (in cellular debris);

Seeds - Gallic acid, Catechins, Proanthocyanidins.

At crushing HCTA held in vacuoles of big cellules of pulp, pass in the liquid phase, while compounds held in vacuoles of cellules of skins, seeds and fibro-vascular tissues and those bonded to the polymeric structure remain in the solid phase.

Before fermentation, as a consequence of the contact between must and grape solids, some phenolics go from the solid into the liquid phase. Nevertheless most of phenolics is extracted in presence of alcohol during fermentation such as all grape phenolics are extracted better in presence of alcohol.

The extraction of phenolics can be described as a diffusion process depending from the composition of the liquid phase and from the sites of solids in which these compounds are held. The ethanol can have also a role of breaking hydrogen bonds between phenolics and polymeric structures of solids.

In practice the mechanism of extraction of phenolics from grape solids during fermentation, appears more complex because of the interference of additional factors such as PPO activity as well as some products of yeast metabolism (i.e. acetaldehyde), initial amount of SO₂ and temperature of must.

All above factors have a great influence or the reactions leading to polymerisation reactions and as a consequence on chemical and organoleptical characteristics of wine.

If SO₂ is absent or present in small amount, PPO oxidise polyphenols to o-quinones which begin polymerisation reactions as well as do carbocations produced by reaction between acetaldehyde and anthocyanins or between acetaldehyde and flavans.

As a consequence of these additional processes which modify the natural diffusion one, polyphenols of fermented must can have a different structure than those of grapes. This is true particularly for polymeric phenolics.

The formation of polymers can cause the loss of a part of phenolics by precipitation with proteic and polisaccharidic materials of must. In addition yeasts absorb mainly anthocyanins and flavans at the end of fermentation.

At present it does not appear possible to propose some tests to acquire informations about the amount and the nature of wine phenolics from the knowledge of grape phenolics, because of the above interfering factors which largely condition the extraction and activate polymerisation reactions.

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