A certain amount of acidity is expected and desirable in wine. Red wines typically have a pH of 3.3 to 3.6; white wines are usually slightly more acidic. Some grapes, and the musts made from those grapes, however, may contain high levels of organic acids, such that the pH is too low (i.e., <3.5).Wines made from those grapes will suffer from excess acidity, a serious and readily noticeable flavor defect.
Of the organic acids ordinarily present in grapes, malic acid is particularly important because of its ability to influence pH.This is because malic acid is a four carbon dicarboxylic acid, meaning it contains two carboxylic acid groups and can release or donate two protons. Thus, musts containing high concentrations (0.8% to 1.0%) of malic acid are acidic and have a low pH. High malic acid concentrations are especially common in grapes grown in cooler, more northern climates, such as those in Oregon, Washington, northern California, and New York.Although many of the vineyards in Europe are located in warmer regions and produce grapes with less malic acid (a situation that may lead to the opposite problem— too little acidity), grapes from Germany, Switzerland, and even some regions in France can still contain significantly high malic acid levels. Also, some grape cultivars ordinarily contain more malic acid than others.
One way to reduce the malic acid levels and to "deacidify" the wine is to promote the biological decomposition of malic acid. This deacidification process occurs via the malolac-tic fermentation pathway that is performed by specific species and strains of lactic acid bacte-ria.These bacteria may be naturally present in wine and may, therefore, initiate the fermentation on their own. It has now become common to add selected malolactic strains, in the form of a pure culture, directly to the must.
The malolactic fermentation has been the subject of extensive research in the last two decades. The pathway was initially thought to involve two separate reactions (Figure 10-4A), in which malate is first decarboxylated by an NAD-dependent decarboxylase yielding pyru-vate and reduced NADH. In the second reaction, pyruvate serves as the electron acceptor yielding lactic acid. Is it now known that the reaction is catalyzed instead by a single malolactic enzyme that decarboxylates malic acid directly to lactic acid (Figure 10-4B).Although this enzyme requires NAD (and manganese), no intermediate is formed.The net effect of the malolactic reaction is that malic acid, a dicarboxylic acid, is converted to lactic acid, a monocarboxylic acid, thereby reducing the acidity of the wine.
Several lactic acid bacteria have the enzymatic capacity to perform this fermentation. The most well-studied are species of Oenococ-cus, particularly Oenococcus oeni (formerly Leuconostoc oenos). Several species of Lacto-bacillus also have malolactic activity.Although some of these bacteria are found naturally in musts, commercial cultures are now available and are commonly used.
The malolactic fermentation was one that interested microbiologists, not only because of its industrial importance, but also because there seemed to be no obvious reason for bacteria to perform this conversion. In other words, how do malolactic bacteria benefit, or more to the point, how do they gain energy, from the conversion of malic acid to lactic acid? The pathway, after all, contains no substrate level phosphorylation step that would lead to ATP formation, nor is there a change in the redox potential. As it turns out, there is a means of generating ATP via the malolactic pathway, but it is indirect.
As shown in Figure 10-5,malic acid is transported into the cell via one of two ways. In
Figure 10-4. The Malolactic Reaction. The conversion of malic acid to lactic acid was originally thought to be catalyzed by two separate reactions, with pyruvic acid formed as an intermediate, and with NAD required as a co-factor (A). It is now well established that the malolactic reaction occurs as a single step, with lactic acid formed directly from malic acid via a decarboxylation reaction catalyzed by malolactic enzyme (B). No intermediate is formed.
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