Organic acids comprise the second most plentiful non-water constituent in must. Although the organic acids are present at relatively modest concentrations, typically ranging from 1% to 2%, their effect on wine quality is extremely im-portant.These acids are responsible for the low and well buffered pH of the must and the wine (usually between 3.0 and 3.5).That wine is so well preserved is due not only to the ethanol and low pH, but also to the presence of organic acids that have powerful antimicrobial activities. At these pH values, pathogenic and other microorganisms are inhibited, including Salmonella, Escherichia coli, and Clostridium spp. The exceptions are aciduric and acidophilic organisms such as lactic acid bacteria, acetic acid-producing bacteria, and some fungi and yeast. In addition to antimicrobial effects, low pH also stabilizes the anthocyanins that give red wine its color, inhibits oxidation reactions, and contributes desirable flavor.
Wine chemists generally categorize wine acids into two general groups.Volatile acids are those that are volatilized or removed by steam treatment; those that remain are considered fixed acids.There are little, if any, volatile acids in must. During the ethanolic fermentation, small and usually inconsequential amounts (<0.5 g/L) of acetic acid and other short- and medium-chain fatty acids may be produced. However, if higher concentrations are produced, either by oxidation of ethanol by bacteria or as an end product of microbial metabolism, the wine will suffer serious defects and may be unsalable (as discussed later).
The main fixed acids, depending on the grape, condition, and maturity, are tartaric acid and malic acid. The ratio of these two acids is usually about 1:5 (tartaric:malic), but it can be reversed in some grapes. These acids are important in wine for several reasons. Since malic acid contains two carboxylic acid groups, it contributes more protons in solutions, and makes the must more acidic. If the malic acid concentration is too low, as might occur in overly mature grapes grown in warm climates, the wine pH will be too high. The wine will lack the desirable acid flavor and may be more prone to spoilage by bacteria. In contrast, although a minimum acidity is desirable in wine, excess acidity is also a defect and results in an inferior sour-tasting wine. Musts obtained from grapes grown in cool climates may contain high levels of malic acid, and are, therefore, problematic. A natural, biological method for deacidifying wine is commonly used for such musts (see below). Finally, it should be noted that other important fixed acids may be present in wine, including succinic acid and lactic acid, both which are produced during the wine fermentation by yeast or bacteria.
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