WINETECH Technical Yearbook 2021

During alcoholic or malolactic fermen­ tation, the formation of vinylphenols from coumaric and ferulic acids and their reduct ion to ethylphenols by Brettanomyces is the cause of characteristic olfactory defects. In the presence of anthocyanin molecules, these vinylphenols react to give pyranoanthocyanins. Fur­ thermore, vinylphenols do not accumulate during ageing provided there is a sufficient quantity of monomeric anthocyanins. B. Evolution of anthocyanins and tannins 1. Diffusion of anthocyanins and tannins during fermentation In conventional red vinification, the extract ion of compounds from the skins and seeds occur during alcoholic fermentation, specifically at the maceration stage. The anthocyanins start to diffuse as soon as the grapes are crushed and their concentration peaks after four to five days of fermentation. The extraction of flavonols, monomeric flavanols (catechin and epicatechin), and low molecular mass oligomers from the skins, occur in parallel to extraction of the anthocyanins. At this stage, the coloured and colourless forms are in equilibrium depending on the pH and addition reactions with sulphites take place. When their concentration is

sufficient, a fraction of the anthocyanins interact in the phenomenon of self- association. With the extraction of other colourless phenolic compounds and when their concentrations are suitable, the anthocyanins take part in co-pigmentation phenomena. The diffusion of skin proanthocyanidins and procyanidins from seeds, is slower and is favoured by the increase in ethanol. However, the highest molecular procyanidins from seeds and those with the highest relative galloylation rate, remain in the pomace. This diffusion continues gradually, depending on the technical means used, until the separation of the solid phase when the wine is racked and the press wine is obtained. 2. Influence of the metabolism of fermentation microorganisms During alcoholic fermentation, yeasts produce various carbonyl compounds which can react with anthocyanins and lead to the formation of vitisins. In addition to their resistance to bleaching by SO 2 , these vitisins are red-orange pigments of interest, because they are coloured irrespective of the pH, while anthocyanins in the coloured form represent only 5 to 25% of the anthocyanins at wine pH. Acetaldehyde is also competitively involved

in condensation reactions that lead to the formation of anthocyanin or tannin compounds linked by an ethyl bridge. When this polymer includes anthocyanins, its colour is purple and it is not bleached by sulphites. Once an anthocyanin molecule is incorporated in the polymerisation with flavanols, it is less reactive towards acetaldehyde and the polymerisation kinetics are slowed down, leading to the formation of polymeric pigments that remain more soluble. In pigments formed through the acetaldehyde pathway, the constituent anthocyanin molecules take part in intramolecular co-pigmentation phenomena, giving red wine its typical purple-red colour at the end of fermentation. 3. Involvement of anthocyanase activity Yeasts may have anthocyanase activity, an enzyme responsible for the release of glucose molecules from monomeric anthocyanins, and can in this way be responsible for the formation of less stable aglycone anthocyanins, leading to a loss of colour. In general, the winemaking strains of Saccharomyces cerevisiae are low producers of anthocyanases. In contrast, non- Saccharomyces grape flora are potentially a more important source of anthocyanase activity: the genera Candida , Hanseniaspora and Pichia show the most

activity. In extreme cases, colour loss can reach 30 to 70%. 4. Adsorption of phenolic compounds on the cell walls of fermenting yeasts and lees Yeast cell walls have the ability to adsorb phenolic compounds during fermentation. The profile of the anthocyanins and their derivatives are modified in different ways depending on the yeast strain. Overall, the anthocyanins bound to the yeast cell walls represent 2 to 6% of the total anthocyanin loss with the removal of lees, even though release can be seen after long ageing on lees. Tannins also interact with the cell walls of dead yeast cells, particularly mannoproteins, but most of the polyphenols pass through the cell wall to interact with the intracellular constituents. During ageing and storage, as the content of monomeric anthocyanins involved in various reaction mechanisms is reduced, the formation of yellow to brown pigments resulting from the oxidation reactions of flavanols and proanthocyanidins will become predominant, while the proportion of purple-red pigments is reduced at the same time. If anthocyanins are absent or present in small quantities (as with white and rosé wine) the acid-catalysed cleavage leads,

WINETECH TECHNICAL YEARBOOK 2021 | 108