Page 17 - MALOLACTIC FERMENTATION – IMPORTANCE OF WINE LACTIC ACID BACTERIA IN WINEMAKING

THE CHEMISTRY OF MALOLACTIC FERMENTATION
actual changes in wine acidity and pH attributable to MLF depend on other factors, including the buffering capacity of the wine as well as the initial pH (Boulton et al. 1998). In general, the overall decrease in wine acidity resulting from MLF can vary from 1 to 3 g/L, and pH may rise by 0.1 to 0.3 pH units (Davis et al. 1985). Wines produced from grapes cultivated in cool cli- mate viticultural areas contain a naturally high level of malic acid of up to approximately 8 g/L, and are considered to benefit from such an acid reduction. On the other hand, wines pro- duced from grapes grown in warm-to-hot regions have lower total acidity (4.5 to 5.5 g/L), and a further reduction in acidity from MLF can have a negative impact on wine quality, causing a flat taste and a greater predisposition to bacterial spoilage (Rankine 1972). Nevertheless, MLF can be desired in such wines to confer a degree of biological stability and/or to impart flavour complexity, necessitating the use of acidulants to adjust wine acidity and pH to acceptable levels after MLF. The increase in wine pH accompanying MLF can also influence wine colour.
• Flavour changes
Although there has been conjecture over the contribution of MLF to the sensory properties of wine (Davis et al. 1985), more recent research has provided greater insight into specific sen- sory changes associated with the growth and metabolic activity of malolactic bacteria in wine. It is clear that different strains of malolactic bacteria may increase or decrease the intensity of certain wine aroma and flavour attributes, and such changes are strain dependent (Bartowsky and Henschke 1995, and Bartowsky et al. 2011). In addition to deacidification, the flavour attributes imparted by MLF can be described as buttery, lactic, nutty, yeasty, oaky, sweaty and earthy. MLF may also impact fruity and vegetative aromas, as well as the mouthfeel of wine (Henick-Kling 1993, Henick-Kling et al. 1993, and Laurent et al. 1994). Mechanisms by which malolactic bacteria can influence wine flavour may include (i) removal of existing flavour compounds by metabolism and adsorption to the cell wall, (ii) production of new bacterial-derived flavour compounds from the metabolism of sugars, amino acids and other substrates, and (iii) metabolism and modification of grape- and yeast-derived secondary me- tabolites to end products having greater or lesser sensory impact (Bartowsky and Henschke 1995). Importantly, the net impact of MLF on wine sensory properties will depend on factors such as bacterial strain characteristics, varietal aroma intensity of the wine and vinification techniques employed (Henick-Kling 1995). The following sections outline some of the im- portant flavour compounds and sensory effects associated with the metabolism of malolactic bacteria in wine.
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