Lactic bacteria

Lactic bacteria are naturally present in soils, in the intestinal and vaginal flora of animals, and in decomposing plants. These microscopic organisms produce lactic acid by fermenting carbonaceous substrates such as sugars (glucose, lactose, etc.) and acid (fatty acids, malic acid, citric acid, etc.). This fermentative pathway is often accompanied by the production of secondary metabolites, such as antimicrobial compounds, aromatic molecules.

The fermentation power of lactic acid bacteria has been known for thousands of years. Their simultaneous action on the taste and preservation of food has given lacto-fermented products a place of honour on our plates: yoghurts, cheese, bread, sausage, olives, sauerkraut, beer, cider, wine... But also soy sauce, nuoc-mâm, kimchi or miso! Five major families of lactic bacteria are responsible for all these delights:

• Lactobacillaceae,
• Streptococcaceae,
• Enterococcaceae,
• Bifidobacteriaceae,
• Leuconostocaceae.

Malic acid is the main carbon substrate fermented by the lactic acid bacteria present in grape must:this is known as malolactic fermentation. The resulting lactic acid is weaker than the latter, resulting in a phenomenon of wine deacidification, the central aim of MLF. This drop in total acidity gives way to more suppleness and roundness on tasting. Malolactic fermentation has thus become a stage in the winemaking process:

• essential for making red wines,
• optional for white and rose wines, whose varietal aromas it sometimes distorts.

But the action of lactic acid bacteria on the must, and therefore on the wine, doesn't stop there. They can also impact organoleptic properties by modifying color and aromas. The latter can evolve following the production of secondary metabolites by lactic acid bacteria, such as diacetyl with its buttery odor, or certain substrates at the origin of aromatic esters.

Native lactic bacteria

The genera Lactobacillus, Leuconostoc, Pediococcus and Œnococcus are the most common natural contaminants of grape must. Rich in sugars and acids, the latter is nevertheless not favorable to them due to the presence of yeasts that colonize it. Their fermentative activity gradually transforms it into an environment hostile to the indigenous lactic acid bacteria, whose population dwindles. Malolactic fermentation therefore becomes very random.

Exogenous lactic bacteria

These are mainly selected, laboratory-grown strains of Œnococcus œni bacteria. More resistant, they survive until alcoholic fermentation ends and the yeast dies by autolysis, releasing molecules that encourage bacterial growth. These exogenous bacteria are sold in the form of freeze-dried microbial biomasses.

Malolactic yeast is added to the must either by bacterial inoculation with or without acclimatization, or by co-inoculation at the same time as the yeast. These methods ensure the control of malolactic fermentation. They thus limit the risk of microbiological deviations due to certain bacteria, and preserve the organoleptic characteristics and therefore the quality of the wines.