Page 16 - MALOLACTIC FERMENTATION – IMPORTANCE OF WINE LACTIC ACID BACTERIA IN WINEMAKING

MALOLACTIC FERMENTATION IN WINE
medium. This gradient, combined with a specific ATPase in the cell membrane, facilitates the generation of energy available for transport processes in the form of ATP. The synthesis of one ATP requires the entry of three protons through the membrane-bound ATPase (Cox and Henick-Kling 1989 and 1990, Henick-Kling 1993 and 1995, and Versari et al. 1999).
Malic and citric acids do not serve as the sole energy sources for the growth of LAB (Liu et al. 1995). Consequently, malolactic bacteria require sugars as a carbon source. However, under conditions of limiting sugar availability or of low pH, which inhibit sugar metabolism, energy (ATP) generated from MLF is beneficial to cell growth (Henick-Kling 1993). Another, but minor (<1%), activity of the malolactic enzyme has also been suggested to stimulate the metabolic activity and initial growth rates of wine LAB. This secondary malolactic enzyme activity (Morenzoni 1974) catalyzes the following reaction.
L-malic acid + NAD+ Pyruvic acid + NADH2 + CO2
Figure 2. Secondary malolactic enzyme reaction
The very small amounts of pyruvic acid and NADH2 generated by this secondary malolactic activity are considered to stimulate the initial stages of glucose metabolism and initial growth rates through the provision of hydrogen acceptors (Kunkee 1991, and Boulton et al. 1998).
In addition to the role of malolactic bacteria in conducting MLF, certain yeasts, including Schizosaccharomyces pombe, are also capable of catabolizing malic acid. However, this me- tabolism is not a true MLF since malic acid is metabolized to ethanol (Mayer and Temperli 1963, Bidan et al. 1974). Despite its potential for wine deacidification, drawbacks to using maloethanolic fermentation in yeast by species of Schizosaccharomyces include the forma- tion of undesirable flavour compounds, such as hydrogen sulphide (Bidan et al. 1974, Gal- lander 1977, Rankine 1966, Snow and Gallander 1979, and Davis et al. 1985).
THE EFFECTS OF MALOLACTIC FERMENTATION ON OVERALL WINE COMPOSITION AND QUALITY
• Acidity reduction
For each molecule of L-malic acid catabolized to the weaker L-lactic acid through MLF, there is a stoichiometric loss of a carboxyl group and corresponding reduction in wine acidity. In addition to the dependency of such effects on the initial concentration of malic acid, the
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