Tannins, winemakers

The fermentation usually is complete within 30 or 40 days. During this period additional anthocyanins are extracted from pomace and the color stabilizes due to polymerization between the anthocyanins and tannins. Winemakers call this process pomace "maturation". Moreover, due to the increasing alcohol concentration, many other metabolites are extracted from the pomace. In addition, yeast metabolites contribute to the aromatic complexity of the wine. 

Even when no additive is used in winemaking, the necessity for small-lot trial before production scale operation is apparent. Because a high percentage of wine is consumed only after chilling, and because chilling may accelerate the precipitation of potassium acid tartrate, ill-defined colloids, anthocyanin-tannin polymers, proteins, etc., simple cold stabilization by refrigeration in the winery may irreversibly alter the product and its eventually-perceived quality level. It often happens, especially in heavy-bodied red varietal wines, that a dark, amorphous precipitate may form in the bottle over several years. Usually tannoid,... 

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. 

At present the structures of the wine-derived-tannins are practically unknown and a great part of the structures identified so far have been only demonstrated in wine model studies. Hence, the influence, or contribution, of wine tannins to astringency is far from being ascertained. For that reason, it is crucial to make efforts to clarify those structures and the mechanisms involved in their formation. Moreover, the role of oxygen is not fully understood and it is important to know how to deal with it during winemaking to control oxidation, and therefore improve the wine taste characteristics. 

Nicolini, G., Larcher, R. and Bontempo, L. (2004b) Micro- and trace-element composition of tannins used in winemaking. J. Commodity Sci., 43(1), 21-29. 

The B- and A-type procyanidins and proanthocyanidins (condensed or nonhydrolyzable tannins, Fig. 6.4) are polymers of flavan-3-ols present in the skin and seeds of the grape berry. In winemaking, they are transferred to the wine, and the sensorial characteristics of astringency and bitterness of wine are linked to the galloylation degree (DG) and degree of polymerization (PD) of flavan-3-ols (Cheynier and Rigaud, 1986 Vidal et al., 2003). 

While the identity of the relevant yeast metabolites in the fermented medium sampled at day 2 needs to be clarified, previously published data have provided some evidence about the role of acetaldehyde-mediated condensation of catechin with MSG (13-20). We therefore aimed to extend these model studies and to confirm chemical formation of pigmented polymers from condensed tannins, which are commercially used in red winemaking, and anthocyanins. The model reactions were conducted with vatying concentrations of acetaldehyde and SO2 as shown in Table 2 and analysed by HPLC after 2, 4 and 7 days. After 7 days visible precipitation of unidentified material started to occur in presence of acetaldehyde and the reactions were discontinued. 

The model experiments involving condensed tannins used for winemaking thus provided confirmatoiy evidence to preceding research in which catechin or oligomeric procyanidins had been employed to characterize the newly formed pigments and colour stabilisation in the presence of acetaldehyde (13-20). 

The impact of the protective colloid effect on the bitartrate stabilization of a wine varies according to the winemaking methods used. Red wines have a higher phenol content than white wines, and then-condensed tannins have a strong inhibiting effect. 

Condensed tannins, especially procyanidins and catechins, present in aU of the solid parts of grape bunches (skin, seeds, stalk), are dissolved in the wine when it is left on the skins. Concentrations in red wine vary according to grape variety and, to an even greater extent, winemaking methods. Values are between 1 and 4 g/1. In dry white wine, the quality of settling determines the tannin... 

Tannin concentrations range from 1 to 4 g/1 in red wine and from 10 to 200 mg/1 in white wine. These values depend on the type of grapes, their degree of ripeness, the effects of rot (if any) and winemaking techniques. 

In red winemaking, maceration (Volume 1, Section 12.5) must be modulated according to the type of grapes. Research into Bordeaux grape varieties indicates that a distinction should be made between the extraction of anthocyanins and tannins ... 

In many European wine growing areas where large quantities of tannins are extracted during the winemaking process, the wines are turbid, unrefined, and aggressive, requiring several years aging to clarify and acquire finesse. This is the only case in which precipitation of polymerized tannins makes the wine softer. 

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