Lactic acid bacteria in the quality improvement and depreciation of wine

The winemaking process includes two main steps: lactic acid bacteria are re sponsible for the malolactic fermentation which follows the alcoholic ferme ntation by yeasts. Both types of microorganisms are present on grapes and o n cellar equipment. Yeasts are better adapted to growth in grape must than lactic acid bacteria, so the alcoholic fermentation starts quickly. In must , up to ten lactic acid bacteria species can be identified. They belong to the Lactobacillus, Pediococcus, Leuconostoc and Oenococcus genera. Througho ut alcoholic fermentation, a natural selection occurs and finally the domin ant species is O. oeni, due to interactions between yeasts and bacteria and between bacteria themselves. After bacterial growth, when the population i s over 10(6)CFU/ml, malolactic transformation is the obvious change in wine composition. However, many other substrates can be metabolized. Some like remaining sugars and citric acid are always assimilated by lactic acid bact eria, thus providing them with energy and carbon. Other substrates such as some amino acids may be used following pathways restricted to strains carry ing the adequate enzymes. Some strains can also produce exopolysaccharides. All these transformations greatly influence the sensory and hygienic quali ty of wine. Malic acid transformation is encouraged because it induces deac idification. Diacetyl produced from citric acid is also helpful to some ext ent. Sensory analyses show that many other reactions change the aromas and make malolactic fermentation beneficial, but they are as yet unknown. On th e contrary, an excess of acetic acid, the synthesis of glucane, biogenic am ines and precursors of ethylcarbamate are undesirable. Fortunately, lactic acid bacteria normally multiply in dry wines; moreover some of these activi ties are not widespread. Moreover, the most striking trait of wine lactic a cid bacteria is their capacity to adapt to a hostile environment. The mecha nisms for this are not yet completely elucidated . Molecular biology has pr ovided some explanations for the behaviour and the metabolism of bacteria i n wine. New tools are now available to detect the presence of desirable and undesirable strains. Even if much remains unknown, winemakers and oenologi sts can nowadays better control the process. By acting upon the diverse mic roflora and grape musts, they are more able to produce healthy and pleasant wines.