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. Technically, it is 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. MLF is a secondary bacterial fermentation carried out in most red wines and some white and sparkling wines, and it is 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.

• In the reaction, L-malic acid is 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
• MLF also helps stabilise a wine by preventing a spontaneous MLF later, potentially after bottling

Three main genera of lactic acid bacteria are 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.

• Commercial malolactic starter cultures are generally strains of O. oeni, and starter cultures of Lactobacillus plantarum are also becoming available for red winemaking
• 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
• High levels of certain medium-chain fatty acids derived from yeast are among the factors that inhibit MLF, so selecting a compatible yeast and bacteria combination matters
• For wines with a potential ethanol content above 15-16% v/v, an ethanol-tolerant strain of malolactic starter culture is recommended

The sensory terms used to describe MLF influences on wine include '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; its sensory threshold in red wine is 0.9-2.8 mg/L. Diacetyl thresholds differ between white and red wines, and 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.

• Diacetyl can be objectionable at high concentrations (above 5 mg/L) but may be desirable at lower concentrations, depending on the wine style
• Lower diacetyl is linked to a high O. oeni inoculation rate, a shorter MLF, 25°C rather than 18°C, anaerobic conditions, and long yeast lees contact
• In 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
• Red wine colour loss from MLF, due mainly to lower polymeric pigments, occurs whether MLF is simultaneous or sequential

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

• For cool-climate wines with high malic acid, the decrease in acidity is essential to wine balance
• 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; sterile filtration followed by sterile bottling is the best way to ensure no microbiological activity after bottling
• Without sterile filtration, more than 0.6 mg/L molecular SO₂ is advised just after bottling; at pH 3.5 that means at least 30 mg/L free SO₂

In Champagne, MLF is optional and entirely up to each winegrower, and it introduces notes of brioche and butter. In California, 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 Puligny-Montrachet goes through alcoholic and malolactic fermentation in oak casks. 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
• Champagne: growers may carry out MLF fully or only partially, or avoid it altogether, depending on the wine they wish to make
• Chardonnay: some winemakers block malolactic fermentation and use stainless steel instead of oak for a purer expression of the variety's character
• Chablis: a cool-climate Chablis should show pronounced acidity, while an oak-aged Chardonnay shaped by MLF and barrel time should feel rounder, softer, and more buttery

The main compositional factors that determine MLF success are alcohol, pH, temperature, and SO₂ concentration, and 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. Criteria for a successful MLF include 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, and a malic acid result of 0.1 g/L or less is low enough to consider MLF complete. If MLF stalls, the critical wine parameters should be confirmed and adjusted where possible before re-inoculating with a freeze-dried O. oeni culture acclimatised to the harsh wine conditions.

• 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
• For wines where direct inoculation has failed or conditions are particularly unfavourable, a rescue protocol prepares a grape juice activation medium, rehydrates freeze-dried O. oeni, 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
• 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

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 2026-10-04