Page 12 - MALOLACTIC FERMENTATION – IMPORTANCE OF WINE LACTIC ACID BACTERIA IN WINEMAKING

MALOLACTIC FERMENTATION IN WINE
SPOILAGE BY LACTIC ACID BACTERIA
MLF is not always beneficial and can be responsible for undesirable changes to the sensory properties of wine. As previously mentioned, several species of LAB may conduct MLF. MLF that occurs at a pH below 3.5 and is induced by catalogued, commercial strains is generally conducted by O. oeni, which is less likely to generate off-odours than indigenous MLB strains. Excessive amounts of acetic acid, as well as buttery, cheesy, milky, metallic or earthy odours, are usually present when indigenous strains of Pediococcus sp. and/or Lactobacillus sp. cat- alyze MLF at pH levels in excess of 3.5.
LAB can produce biogenic amines by the decarboxylation of amino acids. Histamine, de- rived from the decarboxylation of histidine, is felt to cause a reaction in sensitive individuals if the wine contains more than 0.1 mg/L. Of the LAB, Pediococcus sp. and heterofermentative Lactobacillus brevis are regarded as the two biggest producers of histamine (Lonvaud-Funel 2001). Both these genera are usually found in wines with a pH above 3.5; consequently bio- genic amine synthesis appears to be more prevalent in wines exhibiting a high pH. If bacterial strains capable of producing biogenic amines are known to be present, the winemaker should inoculate the wine with a selected malolactic starter culture capable of replacing the indige- nous LAB. Bacteria have a slight capacity to form histamine only during their active growth phase. However, it has been shown (Lonvaud-Funel 2001) that a non-proliferating bacterial flora can develop considerable amounts of histamine. Therefore, the bacterial population of a wine should be eliminated by SO2 addition followed by clarification as soon as the MLF has completed; this is especially important in high pH wines. Other undesired sensory changes, due to the metabolism of indigenous LAB, include mousy taints, colour changes and ropiness.
THE CONTROL OF MALOLACTIC FERMENTATION Prevention
If MLF is not desired, the growth of LAB in grape must or wine should be suppressed by removing or inactivating the bacteria present. Although MLF is occasionally difficult to induce, the prevention of LAB can also be difficult. The addition of 50 to 100 mg/L of SO2 to the must, depending on the pH, destroys more than 90% of the viable bacteria present. The effect of SO2 is dependent on wine pH, with SO2 being more effective at lower pH levels. In high pH wines, a combination of SO2 and lysozyme, or lysozyme alone, should be considered. Inhi- bition of MLF can be accomplished by the following conditions... (Complete text available in printed version of the book.)
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