Ask Jim: All about malolactic fermentation
By Jim Chatto
Can you please explain what malolactic fermentation is and why winemakers speak of it with such reverence? Is it really that important? And what is partial malo? (if it’s so great, why stop it?)
– V Rollins
Malolactic Fermentation (MLF) is a natural process, being, quite simply the metabolisation of malic acid to lactic acid by lactic acid bacteria. Lactic acid bacteria are well known as the microorganisms associated with yoghurt production.
MLF serves two main roles in winemaking, the first being modification of flavour. Malic by comparison is a much stronger acid than lactic. Therefore when MLF occurs there is a net reduction in the acidity of the wine, thus the wine tastes softer. MLF also imparts flavours to the wine, which can add to a wines complexity; most obvious is a buttery character from the MLF byproduct diacetyl.
The second role of MLF is that of microbial stability. The vast majority of red wine undergoes complete MLF in order to eliminate any possibility of MLF occurring in the bottle and leading to hazes, off flavours and spritzyness. MLF also depletes the wine of an array of nutrients that could otherwise be utilised by spoilage microorganisms. In other words, a wine that has completed MLF will be far less susceptible to spoilage.
Winemakers often speak of MLF with such reverence because it is a critical step and can quite often be difficult to control compared to alcoholic fermentation.
The term ‘partial malo’ is a self explanatory yet rather vague term. A partial MLF could be anywhere from 0.1% to 99.9% complete. A partial MLF is generally only desired in white wines, particularly Chardonnay, when the winemaker wishes to achieve a degree of the softening of acidity and or flavour characters without dominating the other characters of the wine.
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