The Malolactic Choice: The Case For and Against Converting Malic Acid to Lactic Acid

What malolactic conversion is

Alcoholic fermentation is the work of yeast, which turns grape sugar into ethanol. Malolactic conversion is a separate event carried out by bacteria. This secondary bacterial fermentation takes place in most red wines and in some white and sparkling wines. The main organism is Oenococcus oeni, a lactic acid bacterium that copes with the harsh conditions of wine: high alcohol, low pH and few nutrients.

Its role has three parts: it lowers acidity, it stabilises the wine by removing L-malic acid, a potential food source for microbes, and it shapes aroma and flavour. Each effect can be welcome in one wine and unwelcome in another, which is why MLC is a decision rather than a default. The sections below take the three effects one at a time, then look at how cellars encourage MLC or hold it back.

Acidity and pH: one acid group fewer

The bacteria's malolactic enzyme removes one carboxyl group from L-malic acid as carbon dioxide, leaving L-lactic acid: L-malic acid → L-lactic acid + CO₂. Malic acid carries two carboxyl (acid) groups; lactic acid carries one. Exchanging one for the other lowers the titratable acidity, raises the pH and leaves the wine tasting less sharp.

The size of the change depends on how much malic acid the wine held at the start, because the reaction can only convert what is there. The fall in acidity and rise in pH therefore differ from wine to wine.

Diacetyl: where the buttery note comes from

Diacetyl (2,3-butanedione) is the compound behind the buttery character that some malolactic bacteria can give a wine. It can form during both alcoholic fermentation and malolactic conversion; lactic acid bacteria produce it as they metabolise the citric acid in wine.

Whether diacetyl is noticeable depends on the wine. In a 1995 study by Martineau, Acree and Henick-Kling, a tasting panel detected diacetyl at 0.2 mg/L in Chardonnay, 0.9 mg/L in Pinot Noir and 2.8 mg/L in Cabernet Sauvignon, whereas earlier work had quoted a single threshold of 2–3 mg/L. An amount that goes unnoticed in a Cabernet Sauvignon can therefore be noticeable in a Chardonnay.

The winemaker has levers. The choice of Oenococcus oeni strain and the timing of inoculation can steer MLC towards buttery or towards fruity, berry-like characters. In a study of cool-climate wines, spontaneous MLC produced the most diacetyl of the induction techniques compared.

Stability: why most red wines complete MLC

For most red wines the main reason is microbial stability. Malic acid left in a wine can later feed yeast and bacteria, leading to spoilage, spritz (a slight fizz) and unwanted flavours. Completing MLC in the cellar removes that food source before the wine is bottled.

Whether MLC has finished is checked by testing for the malic acid that remains, not by tasting.

What lets MLC start, and what stops it

Four wine factors decide whether MLC succeeds: alcohol, pH, temperature and sulphur 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. Below 16 °C or above 25 °C, below pH 3.1, above 40 mg/L total SO₂ or above 14% v/v alcohol, conditions are unfavourable.

Lactic acid bacteria are highly sensitive to the molecular form of SO₂, so a wine meant to undergo MLC should contain no detectable free or molecular SO₂. The pH range is a compromise: at lower pH, SO₂ is more toxic to the bacteria; at higher pH, spoilage organisms grow more easily. Read the other way, the same list shows which conditions hold MLC back.

Encourage or hold back

In white and sparkling wines the choice is open. Because MLC lowers acidity and can add a buttery note, a winemaker who wants to keep the sharper acidity and primary fruit of a white may avoid it, while one who wants softer acidity or a buttery character may encourage it. Neither is a rule for a grape variety.

To hold MLC back, a cellar keeps the wine in unfavourable conditions, notably below 16 °C and above 40 mg/L total SO₂. These make MLC unlikely to succeed rather than ruling it out. A wine that has not been through MLC also keeps its malic acid, which is a potential food source for yeast and bacteria, so the stability question described above does not go away.

When MLC is wanted, timing is a further choice: bacteria can be added after alcoholic fermentation, so that the two fermentations run in sequence, or during it, in co-inoculation, so that they overlap.

Reading MLC in the glass

A wine that has been through MLC usually shows lower, softer acidity. If the strain and conditions produced enough diacetyl, there may be a buttery note, and in Chardonnay a small amount can be enough to notice. A wine that has not been through MLC keeps its malic acid and tastes sharper.

A buttery aroma does not prove MLC, and its absence does not prove that MLC was avoided: diacetyl can also form during alcoholic fermentation, and some bacterial strains are chosen for fruity rather than buttery characters. Treat aroma as a clue, acidity as the stronger signal, and the producer's technical notes, where published, as the answer.

Practice question 1: the reaction and its effects

Question: Write the overall reaction of malolactic conversion and explain its three main effects on a wine.

Model answer: L-malic acid → L-lactic acid + CO₂, carried out mainly by Oenococcus oeni. (1) Acidity: malic acid has two carboxyl groups and lactic acid one, so titratable acidity falls and pH rises; the size of the change depends on how much malic acid was present. (2) Stability: malic acid, a potential food source for yeast and bacteria, is removed, which protects the wine against later spoilage, spritz and unwanted flavours. (3) Aroma and flavour: bacteria can make diacetyl from citric acid, giving a buttery note; how noticeable it is depends on the wine (0.2 mg/L in Chardonnay against 2.8 mg/L in Cabernet Sauvignon in one panel study).

Mark scheme (10 marks): the reaction and the organism (3); the change in acidity and pH with its reason (3); stability (2); diacetyl and its dependence on wine type (2).

Practice question 2: avoiding or encouraging MLC

Question: A winery makes an aromatic, high-acid white and a barrel-fermented Chardonnay. Explain why it might avoid MLC in the first and encourage it in the second, and name two cellar conditions that hold MLC back, with their values.

Model answer: In the aromatic white the aim is to keep sharp acidity and primary fruit; MLC would lower the acidity and could add a buttery note that competes with the fruit. In the Chardonnay, MLC softens the acidity, can add a buttery note that is easy to notice in this wine (a panel detected diacetyl at 0.2 mg/L in Chardonnay) and removes malic acid before bottling. To hold MLC back: keep the wine cool (below 16 °C is unfavourable) and keep SO₂ up (above 40 mg/L total SO₂ is unfavourable). These conditions make MLC unlikely rather than impossible, and the malic acid left in the aromatic white remains a potential food source for yeast and bacteria.

Mark scheme (10 marks): the style reason for each wine (4); the two conditions with their values (4); recognising that they do not guarantee that MLC never happens and that the malic acid remains (2).