As with the yeast fermentation the malolactic fermentation may be spontaneous and caused by winery resident bacteria or those coming in on the grapes. Alternately commercial malolactic inocula may be used to initiate and conduct the malolactic fermentation. Commercial preparations are generally of Oenococcus oeni(formerly called Lecuonsotoc oenos). Although the direct consequences of the malolactic conversion are the same regardless of the organism involved, the spectrum of other flavor-active compounds produced may differ. As discussed in the section on the metabolism of lactic acid bacteria some bacteria are homolactic, making primarily lactic acid from sugar while others are heterolactic and will produce acetate, ethanol and carbon dioxide in addition to lactate. Different species and strains also display dissimilarity in the use of amino acid compounds and end products generated there from. Thus the lactic acid bacterium present can have an impact on the profile of the wine beyond that of the malolactic conversion.
Commercial versus Indigenous Strains
Indigenous lactic acid bacteria found in wines are generally either members of the genera Lactobacillus, Oenococcus or Pediococcus. Members of these genera display tolerance to acidity and ethanol and are therefore capable of growth in wine. In general the lactic acid bacteria are found in nutrient-rich environments and while wine contains nutrients it is by no means nutrient-rich. The species found in wine tend to be those that can tolerate environments that are not generally high in nutrients. Lactobacillus is the dominant species found on the surface of the grapes. Indigenous malolactic conversions occurring in grape juice or must are often conducted by Lactobacillus. Pediococcus can be found in some vineyards but is not universally present at harvest. Lactobacillus, Oenococcus and Pediococcus can all become established winery residents. Indigenous malolactic conversions occurring in the finished wine may be conducted by any one of these organisms or by a combination of organisms. Which organism dominates depends upon the conditions of the wine: pH, level of ethanol, presence of growth substrates and growth factors, absence of yeast inhibitors. Low pH (below 3.5) favors the growth of Oenococcus over the other two genera.