Wine, acids

Carboxylic acids present no exceptional problems in reversed phase analysis, although detectability may be a limitation in the analysis of simple fatty acids. Wine acids, including succinic, acetic, citric, lactic, malic, and tartaric,... 

To establish its commercial value. In this case great importance attaches to the examination of the objective characters and to the determination of some of the more important constituents, e.g., the alcohol, extract, sugars (for sweet wines), acidity and intensity of colour (for vins de oupage). 

Many LAB found in wines can improve wine quality by metabolizing malic acid to lactic acid in a process called malolactic fermentation (MLF). This fermentation is an enzyme-mediated decarboxylation of the dicarbox-ylic acid, L (—) malic acid, to the monocarboxylic L (+) lactic acid (Amerine et al., 1980 Kunkee, 1967 Lonvaud-Funel, 1999). MLF decreases wine acidity and is particularly important in wines produced from grapes grown in cool climates, which often have high acidity (Beelman and Gallander, 1979 Kunkee, 1967, 1974). 

CRUDE WINE ACID AJc. A sigil given by GESSMA N 1906 is probably late... 

Mono-acids react with ethanol to form only neutral esters, whereas di-acids may produce one neutral and one acid ester (e.g. ethyl tartrate and ethyl tartaric acid). On average, wine contains approximately the same quantity of neutral and acid esters. The latter contribute to wine acidity. 

The acids present in a given wine are determined by the grape variety, climate, presence of gray rot [Botrytis cinerea), yeasts, bacteria, and various treatments to which the wine may be subjected (sulfur dioxide, ascorbic acid, acidification, desacidifica-tion). There are at least 50 different acids in wine ranging in concentration from 1 or 2gl (tartaric, malic, succinic acids) to hundreds of mgl (citric, lactic, acetic acids) to tens of mgl (pyruvic, shikimic acids). However, tartaric, malic (depending on the MLF), acetic, and succinic acids constitute 80-90% of total complement of wine acids. 

Chemical methods also exist for the various organic wine acids. For tartaric acid, the OIV describes a method involving sample cleanup on an ion-exchange column followed by reaction between tartaric acid and vanadic acid to give a red complex that is measured spectrophotometrically. This reaction may be incorporated into a flow injection system, which eliminates the need for the ion-exchange sample cleanup step. Lactic acid is first oxidized to ethanol and measured by colorimetry following reaction with ni-troprusside and piperidine for citric acid the chemical method requires controlled oxidation to form acetone which is subsequently separated by distillation and determined by iodometric titration. 

Wine acids partition themselves in a chromatographic system according to their relative affinities for the mobile (solvent) and stationary (paper) phases. Usually, the system is operated in an ascending manner however, descending chromatography is also a possibility (as is thin layer) at a significant additional cost. 

Tartaric, malic, lactic wine acid standards (0.3%)... 

Table 1-1. Rf Values for wine acids in ra-butanol, formic acid, and water mobile phase.

The time required for movement of the solvent front is not critical and depends largely on the paper solid phase and temperature. To facilitate maximum separation of wine acids, the solvent front should be allow to move as close to the top edge as feasible. 

In wine, Hansenula exists as part of the film yeast community, where it utilizes ethanol, glycerol, and wine acids in the production of acetic acid and acetaldehyde as well as esters of which ethyl acetate and 3-methylbu-tylacetate may be the most notable (Sponholz and Dittrich, 1974). Acid utilization by H. anomala may be substantial, resulting in measurable decreased titritable acidity and upward pH shifts (Sponholz, 1993). 

Table 15.3. R[ values for wine acids separated using paper chromatography and solvent 1.

The production of the enzyme for direct use in wines is of no use, since this protein is rapidly inhibited by diverse substances in wine—acids, alcohol and polyphenols. The malolactic reaction takes place at the interior of the bacterium in a medium protected from inhibitors by the bacterial membrane. The degradation rate of mahc acid is Hmited by its transport speed in the interior of the cell. Although the optimal pH for enzyme activity is around 6.0, it is around 3.0 to 3.5 for whole cells of O. oeni. At this pH, mahc acid penetrates more easily into the bacterium than at higher pHs. 

Evidently, adaptation phenomena (probably similar to those described for lactic bacteria) occur, ensuring their ethanol tolerance in wine. Acidity and ethanol concentration simultaneously influence the physiology and the resistance of acetic acid bacteria. 

Malolactic fermentation is both relatively simple and extremely important in practice, and all sensible winemaking and red wine storage techniques take its existence and laws into account. It is an important element in premium wines, even in complete maturity years. In addition, it regulates wine quality from year to year. The less ripe the grapes and therefore the higher the malic acid concentration, the more malolactic fermentation lowers wine acidity. The differences in acidity of wines from the same region are much smaller than those of the corresponding musts. 

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