0.28
FIGURE 2.13 Correlation between volatile acidity production and the maximum cell population in high‐sugar botrytized musts.
FIGURE 2.14 Effect of the yeast‐assimilable nitrogen content in must (with or without ammonium supplements) on the production of volatile acidity (initial sugar content: 350 g/l).
In wines made from botrytized grapes, certain substances in the must inhibit yeast growth (Volume 2, Section 3.7.2) and increase the production of acetic acid and glycerol during fermentation. Botrytis cinerea secretes these “botryticine” substances (Ribéreau‐Gayon et al., 1952, 1979). Fractional precipitation with ethanol partially purifies these compounds from must and culture media of B. cinerea. These heat‐stable glycoproteins have molecular weights between 10,000 and 50,000. They are composed of a peptide (10%) and a carbohydrate part containing mostly mannose and galactose as well as some rhamnose and glucose (Dubourdieu, 1982). When added to healthy grape must, these compounds provoke an increase in glyceropyruvic fermentation and a more‐or‐less significant excretion of acetic acid at the end of fermentation (Figure 2.15). The mode of action of these glycoproteins on yeasts has not yet been identified. The physiological state of yeast populations at the time of inoculation seems to play an important role in the fermentation of botrytized grape must. Commercial dry yeast preparations are much more sensitive to alcoholic fermentation inhibitors than yeast starters obtained by preculture in healthy grape must.
FIGURE 2.15 Effect of an alcohol‐induced precipitate of a botrytized grape must on glycerol and acetic acid formation during the alcoholic fermentation of healthy grape must (Dubourdieu, 1982). 1, Evolution of acetic acid concentration in the control must; 2, evolution of acetic acid concentration in the must supplemented with the freeze‐dried precipitate; 1′, evolution of glycerol concentration in the control must; 2′, evolution of glycerol concentration in the must supplemented with the freeze‐dried precipitate.
Other winemaking factors favor the production of acetic acid by S. cerevisiae: anaerobic conditions, very low pH (<3.1) or very high pH (>4), certain amino acid or vitamin deficiencies in the must, and excessively high temperature (25–30°C) during the yeast growth phase. In red winemaking, temperature is the most important factor, especially when the must has a high sugar concentration. In hot climates, the grapes should be cooled when filling the tanks. The temperature should not exceed 20°C at the beginning of fermentation. The same procedure should be followed during thermovinification immediately following the heating of the grapes.
In dry white and rosé winemaking, excessive must clarification can also lead to the excessive production of volatile acidity by yeast. This phenomenon can be particularly pronounced with certain yeast strains. Therefore, must turbidity should be adjusted to the lowest possible level that enables a complete and rapid fermentation (Sections 3.7.3 and 13.5.3). The input of lipids made available to yeasts via solid sediments (grape solids), in particular long‐chain unsaturated fatty acids (C18:1 and C18:2), greatly influences acetic acid production during white and rosé winemaking.
The experiment in Figure 2.16 illustrates the important role of lipids in acetic acid metabolism (Delfini and Cervetti, 1992; Alexandre et al., 1994). The volatile acidity of three wines obtained from the same Sauvignon Blanc must was compared. After filtration but before yeast inoculation, must turbidity was adjusted to 250 Nephelometric Turbidity Units (NTU) by three different methods: reincorporating fresh grape solids (control), adding cellulose powder, and supplementing with a lipid extract (using methanol–chloroform) that contained the same quantity of grape solids adsorbed on the cellulose powder. The volatile acidities of the control wine and the wine that was supplemented with a lipid extract of grape solids before fermentation are identical and perfectly normal. Although the fermentation was normal, the volatile acidity of the wine made from the must supplemented with cellulose (therefore devoid of lipids) was practically twice as high (Lavigne, 1996). Supplementing the medium with lipids appears to favor the penetration of amino acids into the cell, which limits the formation of acetic acid.
FIGURE 2.16 Effect of the lipid fraction of grape solids on acetic acid production by yeasts during alcoholic fermentation (Lavigne, 1996).
During the alcoholic fermentation of red or slightly clarified white wines, yeasts do not continuously produce acetic acid. The yeast metabolizes a large portion of the acetic acid secreted in must during the fermentation of the first 50–100 g of sugar. It can also assimilate acetic acid added to must at the beginning of alcoholic fermentation. The assimilation mechanisms are not yet clear. Acetic acid appears to be reduced to acetaldehyde, which favors alcoholic fermentation to the detriment of glyceropyruvic fermentation. In fact, the addition of acetic acid to a must lowers glycerol production but increases the formation of acetoin and 2,3‐butanediol. Yeasts seem to use the acetic acid formed at the beginning of alcoholic fermentation (or added to must) via acetyl‐CoA in their lipid synthesis pathways.
Certain winemaking conditions produce abnormally high amounts of acetic acid. Since this acid is not used during the second half of the fermentation, it accumulates until the end of fermentation. When refermenting a tainted wine, yeasts can lower volatile acidity by metabolizing acetic acid. The wine is incorporated into freshly crushed grapes at a proportion of no more than 20–30%. The wine should be sulfited or filtered before incorporation to eliminate bacteria. The volatile acidity of this mixture should not exceed 0.6 g/l expressed as H2SO4. The volatile acidity of this newly made wine rarely exceeds 0.3 g/l expressed as H2SO4. The concentration of ethyl acetate decreases simultaneously.
2.3.5 Other Secondary Products of the Fermentation of Sugars
Lactic acid is another secondary product of fermentation. It is also derived from pyruvic acid, which is directly reduced by yeast L(+)‐ and D(−)‐lactate dehydrogenases. Under anaerobic conditions (the case in alcoholic fermentation), the yeast synthesizes predominantly D(−)‐lactate dehydrogenase. Yeasts form 200–300 mg of D(−)‐lactic acid per liter and only a few dozen milligrams of L(+)‐lactic acid. The latter is formed essentially at the start of fermentation. By determining the D(−)‐lactic acid concentration in a wine, it can be ascertained whether the origin of acetic acid is yeast or lactic acid bacteria (Section 14.2.3). Wines that have undergone malolactic fermentation can contain several grams per liter of exclusively L(+)‐lactic acid. On the other hand, the lactic acid fermentation of sugars (lactic spoilage) forms D(−)‐lactic acid. When D(−)‐lactic
