Malolactic fermentation
The second fermentation: bacteria turn sharp malic acid into softer lactic acid.
What happens
Malolactic fermentation is the decarboxylation of L-malic acid into L-lactic acid and carbon dioxide by lactic acid bacteria.1 Its main agent in wine is Oenococcus oeni.1
L-malic acid → L-lactic acid + CO₂1
The OIV notes that it lowers titratable and actual acidity, raising pH, and makes the wine biologically more stable. During it the wine is kept at about 18 °C.2
A hard life
O. oeni must survive more than 13% alcohol and a pH below 3.5. At 12% ethanol it spent 30 times more ATP just to stay alive.1
Winemakers who want to keep the malic acid can block the process: the OIV allows fumaric acid at 300 to 600 mg/L for this.3
Sources
- Genome-Scale Reconstruction of the Metabolic Network in Oenococcus oeni to Assess Wine Malolactic Fermentation (Mendoza et al., Frontiers in Microbiology 8). Frontiers in Microbiology, 2017. level 3 · checked 2026-10-13
- International Code of Oenological Practices, II.3.1.2.3 Microbiological de-acidification by lactic acid bacteria. OIV. level 2 · checked 2026-10-13
- International Code of Oenological Practices, II.3.4.23 Treatment with fumaric acid to inhibit malolactic fermentation. OIV. level 2 · checked 2026-10-13