Malolactic Fermentation

Malolactic fermentation is the conversion of malic acid into lactic acid within a must or wine, facilitated by lactic bacteria, commonly Oenococcus oeni (Decanter). Decanter notes that it is technically a bacterial conversion rather than a fermentation, because it does not use yeast. The process softens acidity by converting harsh-tasting malic acid into softer lactic acid, and it raises the wine's pH. The AWRI describes MLF as a secondary bacterial fermentation carried out in most red wines and some white and sparkling wines, and as crucial to microbiologically stabilise most red wines: it removes malic acid that can be a carbon source for yeast and bacterial growth, leading to spoilage, spritz, and unwanted flavours.

• The AWRI describes the reaction as L-malic acid being decarboxylated to form L-lactic acid
• Most often occurs after alcoholic fermentation, but can run concurrently (co-inoculation)
• Malic acid can give two protons and lactic acid one, so lactic acid contributes half as much to titratable acidity and pH is expected to rise (AWRI)
• Decanter adds that MLF helps stabilise a wine by preventing a spontaneous MLF later, potentially after bottling

The AWRI names three main genera of lactic acid bacteria connected with grape must and wine: Lactobacillus, Oenococcus, and Pediococcus. Oenococcus oeni (formerly known as Leuconostoc oenos) is the species preferred by winemakers because it ferments relatively rapidly and produces few off-odours, although all three genera can be associated with spoilage. Low numbers of lactic acid bacteria are commonly found on sound fruit, and these are often the bacteria that conduct MLF when it proceeds naturally. Winemakers commonly inoculate with commercial O. oeni starter cultures, including freeze-dried preparations, because natural or wild MLF can be unpredictable in both time of onset and impact on wine quality (AWRI).

• Commercial malolactic starter cultures are generally strains of O. oeni, and starter cultures of Lactobacillus plantarum are also becoming available for red winemaking (AWRI, 2024)
• Co-inoculation adds the bacteria at the start of alcoholic fermentation, typically around 18 to 24 hours after yeast inoculation, and can give a shorter overall fermentation time (AWRI)
• The AWRI lists high levels of certain medium-chain fatty acids derived from yeast among the factors that inhibit MLF, and advises selecting a compatible yeast and bacteria combination
• For wines with a potential ethanol content above 15-16% v/v, the AWRI recommends an ethanol-tolerant strain of malolactic starter culture

The AWRI lists the sensory terms used to describe MLF influences on wine as 'buttery', 'vanilla-like', 'nutty', 'spicy', 'fruity', 'vegetative', 'toasty', 'fuller' and 'rounded'. One of the major compounds O. oeni produces during MLF is diacetyl, formed through the metabolism of citric acid, which contributes buttery or butterscotch aroma and flavour; the AWRI gives its sensory threshold in red wine as 0.9-2.8 mg/L. Diacetyl thresholds differ between white and red wines, and the AWRI says diacetyl formation can be managed through winemaking choices including strain selection, inoculation rate and timing, and oxygen exposure. Butteriness can also be masked by strong oak or very fruity characters, according to the AWRI.

• Oregon State University Extension notes that diacetyl can be objectionable at high concentrations (above 5 mg/L) but may be desirable at lower concentrations, depending on the wine style
• The AWRI links lower diacetyl to a high O. oeni inoculation rate, a shorter MLF, 25°C rather than 18°C, anaerobic conditions, and long yeast lees contact
• In AWRI-cited trials on Shiraz and Cabernet Sauvignon, red wines made with co-inoculation were rated more fruity by a sensory panel, and co-inoculated MLF would be expected to give lower buttery character
• Oregon State University Extension found that red wine colour loss from MLF, due mainly to lower polymeric pigments, occurs whether MLF is simultaneous or sequential

Decanter notes that red wines more commonly benefit from MLF than whites, and that many red wines undergo MLF in barrels, which can also promote smoky and spicy notes. For white wines, the decision is stylistic: Decanter notes that Chardonnay and Viognier routinely go through malo, while MLF is generally undesirable in aromatic, high-acid whites such as Riesling and Sauvignon Blanc. Decanter also notes that reds from very warm climates that lack natural acidity do not benefit from the process. Winemakers can inhibit MLF by adding SO₂ to wines after fermentation or by using enzymes such as lysozyme (Decanter). WSET notes that the choice often comes down to house style, and that producers who want fresh wines block malolactic fermentation to retain bright acidity.

• Oregon State University Extension notes that for cool-climate wines with high malic acid, the decrease in acidity is essential to wine balance
• Decanter notes that the bacteria like a warm environment (above 16°C), which traditionally meant waiting until the spring after harvest; modern cellars and tanks can be heated to start MLF when desired
• MLF in the bottle is a spoilage problem, producing dissolved carbon dioxide, haze, and sediment; the AWRI names sterile filtration followed by sterile bottling as the best way to ensure no microbiological activity after bottling
• Without sterile filtration, the AWRI advises more than 0.6 mg/L molecular SO₂ just after bottling; at pH 3.5 that means at least 30 mg/L free SO₂

In Champagne, the Comité Champagne describes MLF as optional and entirely up to each winegrower, and says it introduces notes of brioche and butter. In California, WSET notes that Chardonnay is known for full use of malolactic fermentation, lees contact, and oak barrel fermentation and maturation, giving a full-bodied, toasty, buttery typical style, though not the only style the state produces. Domaine Leflaive's own page for its Puligny-Montrachet lists alcoholic and malolactic fermentation in oak casks. Decanter reports that in Bordeaux in 2014, châteaux used various techniques, including MLF, to control high acidity in white wines made from Semillon and Sauvignon Blanc.

• Domaine Leflaive, Puligny-Montrachet: alcoholic and malolactic fermentation in oak casks, then 12 months in barrels on lees and 10 months in stainless steel tank on fine lees (producer's site)
• Champagne: growers may carry out MLF fully or only partially, or avoid it altogether, depending on the wine they wish to make (Comité Champagne)
• Chardonnay: WSET notes that some winemakers block malolactic fermentation and use stainless steel instead of oak for a purer expression of the variety's character
• Chablis: WSET contrasts a cool-climate Chablis with pronounced acidity against an oak-aged Chardonnay, shaped by MLF and barrel time, that feels rounder, softer, and more buttery

The AWRI identifies alcohol, pH, temperature, and SO₂ concentration as the main compositional factors that determine MLF success, and notes that the factors are additive: as more of them become unfavourable, MLF becomes increasingly difficult. Pesticide residues, high residual copper, and high levels of certain yeast-derived medium-chain fatty acids can also inhibit MLF. For a successful MLF, Oregon State University Extension recommends no SO₂ additions until MLF is complete, a temperature of 18-22°C (64-71°F), and considering ML nutrients for lots that have been problematic in the past. Regular measurement of L-malic acid lets delayed or stuck MLF be detected so a rescue starter culture can be used quickly (AWRI). If MLF stalls, the AWRI recommends confirming the critical wine parameters and adjusting them where possible, then re-inoculating with a freeze-dried O. oeni culture acclimatised to the harsh wine conditions.

• The AWRI's unfavourable conditions for MLF in red wine: ethanol above 16% v/v, temperature below 16°C or above 25°C, pH below 3.3, and total SO₂ above 40 mg/L
• Temperatures above 25°C slow MLF and increase the risk of bacterial spoilage and volatile acidity (AWRI)
• For wines where direct inoculation has failed or conditions are particularly unfavourable, the AWRI protocol rehydrates freeze-dried O. oeni, grows it in a grape juice activation medium, then acclimatises it by adding 10 L of the problem wine to the 10 L starter before inoculating 1,000 L
• Delays in post-MLF stabilisation may raise volatile acidity and expose the wine to spoilage by acetic acid bacteria and Brettanomyces (AWRI)
• Once MLF is complete, a 40-50 mg/L SO₂ addition helps kill residual lactic acid bacteria, and storage below 18°C also inhibits their growth (AWRI)

Primary sources (the Australian Wine Research Institute, Oregon State University Extension, the Comité Champagne, WSET, and Domaine Leflaive for its own winemaking): https://www.awri.com.au/wp-content/uploads/2020/09/MLF-in-red-wine.pdf, https://www.awri.com.au/wp-content/uploads/2011/06/Malolactic-fermentation.pdf, https://www.awri.com.au/wp-content/uploads/mlf_modulation_AWRI_fact_sheet.pdf, https://www.awri.com.au/wp-content/uploads/2011/06/Avoiding-spoilage-from-LAB.pdf, https://www.awri.com.au/industry_support/winemaking_resources/wine_fermentation/mlf-starter-culture/, https://www.awri.com.au/industry_support/winemaking_resources/fining-stabilities/hazes_and_deposits/microbiological/, https://extension.oregonstate.edu/food/wine-beer/conducting-successful-malolactic-fermentation, https://extension.oregonstate.edu/catalog/em-9641-simultaneous-malolactic-fermentation-it-right-option-your-wine, https://www.champagne.fr/en/from-vine-to-wine/wine-making/malolactic-fermentation-clarification, https://www.wsetglobal.com/knowledge-centre/blog/2026/understanding-acidity-in-wine, https://www.wsetglobal.com/knowledge-centre/blog/2022/may/24/behind-the-grape-chardonnay-in-the-us, https://www.leflaive.fr/en_US/wine/puligny-montrachet. Secondary: https://www.decanter.com/learn/what-is-malolactic-fermentation-51591/. Last verified 2026-09-28