🍷 Oenococcus oeni

Oenococcus oeni is a member of the lactic acid bacteria (LAB) family and the main bacterium responsible for conducting malolactic fermentation (MLF) in wine, according to the Australian Wine Research Institute (AWRI). It owes this role to its ability to survive the harsh conditions of wine (high alcohol, low pH and low nutrients) and to its production of desirable wine sensory attributes.

MLF is a secondary bacterial fermentation carried out in most red wines and some white and sparkling wines. It often occurs naturally after primary fermentation or can be induced by inoculation with a selected bacterial strain. MLF removes the malic acid that could otherwise be a carbon source for yeast and bacterial growth, leading to spoilage, spritz and unwanted flavours.

The AWRI names four main factors for success: alcohol, pH, temperature and sulfur dioxide (SO2). Favourable conditions are 18-22 C, pH 3.3-3.5, total SO2 below 30 mg/L and alcohol below 13% v/v. Lactic acid bacteria, including O. oeni, are highly sensitive to molecular SO2.

Three main LAB genera are connected with grape must and wine: Lactobacillus, Oenococcus and Pediococcus. O. oeni is the species most commonly used for MLF, but all three can be associated with spoilage and wine faults. O. oeni is linked to acidification, 'geranium' character (when sorbic acid is added as a preservative) and mousiness. A molecular SO2 concentration of 0.8 mg/L inhibits LAB growth.