Winemaking, white acidity

Dry white table wines differ from red table wines in characteristics other than color. They contain lower concentrations of the puckery and astringent tannins. They usually have a higher acid content which makes them somewhat lighter in body and fresher on the palate. Dry white wines also are usually simpler in their aroma in contrast to the complicated aroma and bouquet that can develop in a red table wine. While it is possible to make white table wines from red-skinned varieties, for the home winemaker this task is very difficult because it is almost impossible to remove all traces of the pigment-containing skins from the juice quickly enough to prevent some reddish or amber color from developing. 

In California, winemakers do not want white wine to undergo the bacterial malo-lactic fermentation that changes malic acid to lactic acid. The loss of acidity, increased pH, and freshness is considered to be detrimental. On the other hand, malo-lactic fermentation of red wines before secondary fermentation is common. This is because red wines undergo malo-lactic fermentation more easily than do whites and they generally are aged longer before use. The producers do not want a malo-lactic fermentation in the bottle. 

As noted earlier, white wine grape varieties generally are crushed at a lower °Brix range than red varieties. Winemakers prefer to ensure a higher total acid. 

In white winemaking, Saccharomyces cerevisiae must develop at low temperature, which reduces membrane fluidity. To maintain adequate fluidity in their membranes, yeasts increase the proportion of UFA in the phospholipids (Thurston et al. 1981 Torrija et al. 2003). Phospholipids with unsaturated fatty acids have a lower melting point and more flexibility than phospholipids with saturated acyl chains (Rodriguez et al. 2007). Such adaptation involves inducing the fatty acid desaturase OLEl which incorporate unsaturated bonds at defined positions in fatty acids (Nakagawa et al. 2002). 

Presence of medium chain fatty acids The presence of MCFA can decrease yeast viability and even stop alcoholic fermentation. This problem is more prevalent in white winemaking because fermentation is usually carried out at low temperatures and without any aeration. Yeast hulls have been very useful for avoiding this problem. Yeast hulls adsorb MCFA from the media and provide sterols and UFA to the yeasts. Yeast hulls can be used as preventives (20 g/hl) or as curatives (40-50 g/hl) of stuck and sluggish fermentations. 

The yeast strain also plays an essential role in determining the volatile phenol concentration in white wines. For many years now in the brewing industry (Goodey and Tubb, 1982), yeast strains have been selected for their low production of vinyl-phenols, as malt has a high phenol acid content. These are called Pof- (phenol off-flavor) strains. The selection of winemaking yeast has... 

In dry white and rose 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 which permits a complete and rapid fermentation (Chapter 13). Solids sedimentation (must lees) furnishes long-chain unsaturated fatty acids (C18 l, C18 2). Yeast lipidic alimentation greatly... 

In white winemaking, suspended solids activate fermentation (Section 3.7.3). Certain constituents, probably sterols and fatty acids, are involved in this phenomenon (Ribereau-Gayon et al., 1975b). Although these substances are not very soluble, yeasts are capable of using them to improve fermentation kinetics. They probably act in conjunction with other factors, such as oxygenation and possibly nitrogen additions. Yeast hulls have a similar effect independent of their ability to eliminate inhibition (Section 3.6.2). 

Must lees particles and even glucidic macromolecules, making up part of the colloidal turbidity of musts such as yeast hulls, can adsorb short-chain fatty acids (C8 and CIO) (Section 3.6.2) (Ollivier et al., 1987). In consequence, the level of must clarification should be controlled for each type of white winemaking by measuring must cloudiness or turbidity, expressed in NTU... 

An effective protection can be obtained in red wines sensitive to oxidasic casse as well as white musts during winemaking. At present, however, the use of ascorbic acid is not widespread in winemaking and not authorized in France probably because the required concentrations are too high to protect musts against oxidations and because sulfur dioxide is more effective. 

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