Welcome to part 6 of Terroir & the chemistry of wine. We have already covered soil and climate, acids and pH, phenols, aroma compounds and the alcoholic fermentation itself. Now the bacteria step onto the stage. Malolactic fermentation (MLF) is the process that most often decides whether a wine feels sharp and tense or round and soft in the mouth, and it is governed by a microbiology far more capricious than that of yeast.
Here we go in depth with the chemistry, with the organisms that do the work, and with the tools the winemaker has to promote or curb the process. It is a part of vinification where small margins in pH, alcohol and sulphur decide the outcome.
What you will learn
- What malolactic fermentation actually is on a chemical level, and which cofactors the reaction requires
- What role the lactic acid bacteria play, and why Oenococcus oeni thrives where others die
- How MLF changes the wine's acidity, pH, flavour and texture
- When the process is desired, and which tools are used to promote or avoid it
What happens chemically
Malolactic fermentation is the name of a bacterial conversion of L-malic acid into L-lactic acid. Despite the name, it is not a fermentation in the classic sense, but an enzymatic decarboxylation: a carboxylic acid molecule is released as carbon dioxide along the way.
The entire reaction is catalysed by a single enzyme, malate decarboxylase, which requires NAD and Mn++ as cofactors. It is worth noting that this is a single enzymatic pathway. That explains why the process is relatively predictable in its chemical result, even though it is hard to control microbiologically.
L-malic acid is a hard, divalent acid. L-lactic acid is softer and monovalent. When one replaces the other, the total titratable acidity falls, and the wine's pH typically rises by 0.2 to 0.5 units. At the same time, carbon dioxide is released. The amount of malic acid available depends greatly on the climate: must from a cool climate contains 2 to 5 g/L L-malic acid, while must from a warm climate lies below 2 g/L. That difference explains why MLF has greater practical importance in cool growing regions.
Energy and transport
For the bacterium there is more at stake than simply removing acid. The rate of MLF is regulated by the transport of malate into the cell via an L-malate-specific permease. When the external malate concentration exceeds 5 mM, malate-dependent ATP is formed during the decarboxylation. The bacterium thus gains a small energy yield from the process, which is part of the explanation for why it invests in it at all. The energy economy is moreover sensitive to the sugar level: under low glucose concentration (2 g/L) the ATP yield is 55%, against 10% under high glucose concentration (10 g/L).
The lactic acid bacteria
Wine is a hostile environment for most microorganisms. Low pH, alcohol and sulphur make existence short for most. Oenococcus oeni is the lactic acid bacterium best adapted to precisely these harsh conditions, and it is therefore the typical agent behind MLF.
The wine-associated lactic acid bacteria are nutritionally fastidious and slow-growing organisms. They require complex growth media containing, among other things, pantothenic acid in order to thrive. This is part of the explanation for why MLF often only gets going after the alcoholic fermentation, when the yeast has left nutrients behind and conditions have stabilised. An important threshold point is cell density: significant conversion of L-malate does not typically occur until the cell density exceeds 1×10^6 cells/mL. Until the population is built up, not much really happens.
MLF can proceed both during and after the alcoholic fermentation. When it is set in motion at the same time as the yeast, one speaks of simultaneous MLF. It is a choice with consequences, because the bacteria must then compete in an environment where the alcohol is rising.
Spontaneous or controlled
Previously spontaneous MLF was the norm, but paradoxically spontaneous MLF becomes more unpredictable as hygiene at the winery improves. It typically appears in wines above pH 3.4. To gain more control, commercial freeze-dried, concentrated starter cultures were developed in the mid-1990s, which removed the need for reactivation and pre-adaptation. That gave the winemaker the ability to inoculate at a chosen moment rather than wait for nature.
Effect on acidity, flavour and texture
The most tangible effect is the deacidification. When the hard malic acid is converted into the softer lactic acid, the wine is perceived as rounder and less sharp. The rise in pH accompanies this. This is precisely why MLF is so central in cool regions, where the must can carry a considerable amount of malic acid.
But MLF is more than deacidification. Lactic acid bacteria also convert citric acid, which is found in wine in concentrations of 0.1 to 0.7 g/L. This side reaction contributes to aroma changes and is part of the flavour modification the process can give. The aroma complex that arises during fermentative conversion by bacteria belongs to the fermentative aromas we touched on in the part about wine's aroma compounds.
In terms of texture, the result is often experienced as a softer, more creamy mouthfeel. Combined with the higher pH, the wine's whole balance changes character, which makes MLF a stylistic decision just as much as a technical one. If MLF is to take place in wood, a smoky complexity can develop, and this is one of the reasons why barrel choice and MLF are often considered together. More on precisely that in the next part.
Controlling, promoting or avoiding it
Promoting MLF is essentially about giving the bacteria living conditions they can cope with. Classically, lactic acid bacteria require warmth, low sulphur levels, a pH between 3 and 4 and the natural grape nutrients. Inoculation with a starter culture gives temporal control, while simultaneous MLF can be chosen where one wants the process finished early.
Avoiding MLF requires working against the bacteria's tolerance limits. Several factors pull in that direction:
- pH. A wine below pH 3.5 inhibits the growth of spoilage bacteria, including species of Lactobacillus and Pediococcus. Low pH is thus a barrier in itself.
- Sulphur (SO2). Molecular SO2 has an antimicrobial effect. The proportion is strongly pH-dependent: at pH 3 the molecular fraction makes up 5 to 10%, while it approaches zero at pH 4. SO2 furthermore induces a stress response in O. oeni and can lower ATPase activity by up to 70% at concentrations of 20 to 40 mg/L.
- Alcohol. Ethanol concentrations above 14% inhibit both bacterial growth and the initiation of MLF.
Enzymes and antimicrobial agents
Beyond the natural barriers, there are more targeted tools. Lysozyme cleaves the β(1-4) glycosidic bonds between N-acetylmuramic acid and N-acetylglucosamine in the peptidoglycan layer of Gram-positive bacteria. That makes it effective against Gram-positive lactic acid bacteria, but not against Gram-negative spoilage bacteria or yeast. It is typically added at 250 to 500 mg/L to prevent or delay MLF.
Other agents work by other routes. Dimethyl dicarbonate (DMDC) inactivates glycolytic enzymes and hydrolyses into methanol and carbon dioxide within one to five hours. Nisin, produced by Lactococcus lactis, alters the bacteria's cell membranes so that cytoplasm leaks out and the proton motive force is destroyed. Finally, there are bacteriophages against O. oeni, which have the potential to produce 100 to 200 offspring per infection cycle, and which can thereby collapse a bacterial population.
In short
- MLF is an enzymatic decarboxylation of L-malic acid into L-lactic acid, catalysed by malate decarboxylase with NAD and Mn++ as cofactors.
- The result is deacidification, a pH rise of 0.2 to 0.5 units, release of carbon dioxide and a softer texture and flavour.
- Oenococcus oeni is the best-adapted agent, but lactic acid bacteria are fastidious and slow, and significant conversion requires cell densities above 1×10^6 cells/mL.
- MLF is promoted with warmth, low sulphur, pH between 3 and 4 and possibly starter cultures. It is avoided with low pH, SO2, high alcohol, lysozyme, DMDC or other agents.
- MLF is both a technical and a stylistic decision, not least in cool climates with high malic acid content.
Frequently asked questions
Why is MLF more important in cool climates?
Because must from a cool climate contains more L-malic acid (2 to 5 g/L) than must from a warm climate (below 2 g/L). There is simply more hard acid to soften, so the deacidification effect is more noticeable and more often desired.
Why has spontaneous MLF become more unpredictable?
Paradoxically, it becomes more unpredictable the better the hygiene at the winery is, because the natural population of lactic acid bacteria is reduced. That is why freeze-dried starter cultures were developed in the mid-1990s, so the winemaker can inoculate in a controlled way.
Ready for the next step?
MLF and the barrel are closely connected, among other things because the process in contact with wood can add smoky complexity. It is a natural transition to the next part of the series, Barrel ageing and maturation, where we look at what the vessel itself does to the wine over time.
When you taste your way through wines, it is worth noticing the round, creamy mouthfeel that MLF often gives. Do drop by our selection and try to set the theory against your own taste. And remember the simple truth behind all the chemistry: the best pairing is the wine you like with the food you like.