Page 9 - MALOLACTIC FERMENTATION – IMPORTANCE OF WINE LACTIC ACID BACTERIA IN WINEMAKING

FACTORS INFLUENCING THE SURVIVAL AND GROWTH OF LACTIC ACID BACTERIA
The survival and growth of LAB in wine can be influenced by the following factors:
• The chemical and physical composition of the wine
• Factors associated with vinification
• The interaction between LAB and other micro-organisms
Wine pH exerts a strong selective action that largely determines which strain(s) of LAB will be present. It influences their viability, rate of growth, speed of L-malic acid degradation and metabolic behaviour. A wine pH of 3.5 is pivotal because below this value wine microflora are easier to control. Although MLF is more difficult to induce at lower pH levels, under these conditions wine LAB will conduct MLF mostly with no deleterious effects on wine quality. SO2 strongly inhibits the growth of LAB, but the sensitivity of LAB to SO2 varies, and SO2 is more inhibitory at low pH. Both the growth of LAB and the progress of MLF are increasingly inhibited as alcohol concentrations rise above 6%, with 14.5% v/v the upper limit tolerated by most MLB. Wine LAB are mesophilic, with an optimal growth temperature between 15° and 30°C (59° and 86°F). The rate of bacterial growth and speed of MLF are strongly inhibited by lower temperatures.
Certain winery practices, such as juice and wine clarification, can remove a large portion of the LAB. This will reduce bacterial growth and its resulting effect on wine quality (Henick-Kling 1988). During clarification, some nutrients and suspended particles that are stimulatory to bacteria growth will be removed. Wines made by thermo-vinification have been reported as being less suitable for MLF. The timing of the inoculation with MLB influences the kinetics of MLF.
With respect to interactions with other wine organisms, the presence of indigenous yeast and/or bacterial microflora, along with the selected yeast used to conduct the AF, may intro- duce the possibility of an antagonistic effect and/or synergistic relationships. In some minor cases, however, there may be no observable effect. In winemaking, interactions between MLB and yeasts, other fungi, acetic acid bacteria, bacteriophages or other LAB are known (Ribéreau-Gayon et al. 2000). The antagonistic effect attributed to yeast has been explained by nutrient competition, SO2 production or the presence of medium-chain fatty acids, all of which are capable of inhibiting MLB growth. Conversely, yeast may favour the growth of LAB and stimulate the MLF. During extended lees contact with wine, the process of yeast auto- lysis releases vitamins and amino acids. This results in nutrient enrichment and subsequent
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