A shorter version of this article first appeared in Wine and Spirit in 2007, since merged into Harpers Wine and Spirit.
Good bacteria exist in wine as well as in probiotic formulations. Their greatest vinous contribution is the malolactic fermentation (MLF or ‘the malo’). This winemaking tool was pretty much a mystery until the middle of the 20th century. It was just observed in spring that wines maturing in tank and barrel started bubbling, in tune with the rising of the sap in the vines. In fact, these various species of lactic acid bacteria only come out to play in warmer rather than cooler weather, and they really get partying over 18°C, so winter in the naturally cool European regions was a hibernation period for them.
MLF lowers acidity, increases microbial stability and changes the flavour profile
Now the science of MLF is understood, it can be controlled. Wine can be inoculated with blends of bacteria to reduce some of the unpredictability of the malo process. Additionally, if the winemaker wants to speed through the MLF straight after the alcoholic fermentation, s/he can manage the temperature to encourage the bugs to do their business.
At its most basic malic acid (think crisp, crunchy apples) is transformed in softer, rounder lactic acid (think milk), with carbon dioxide as a by-product, which dissipates into the air during wine maturation. If MLF occurs in bottle, then a spritz will develop, which probably isn’t very tasty in a red wine and it likely to be considered a fault.
The three fundamental effects of MLF are a lowering of acidity; an increase in microbial stability, because the bacteria have done their work, and the addition of alternative flavour compounds such as diacetyl which gives a buttery character. In concentrations of 1 to 4 mg/l diacetyl can be desirable, over 5mg and it’s a bit like slathering your tongue with the real thing.
On the down side, there can be a loss of fruit and varietal definition due to the breakdown of fruit esters by the bacteria, and if acidity is lowered too much this can encourage the bad boys of bacteria to crash the party which spoils the wine.
South Africa’s Napier Vineyard winemaker Leon Bester said: “To avoid MLF add sulphur and chill to 10°C. If this isn’t enough, the last option is to take the wine off its lees” (the dead yeast cells left after fermentation which provide nutrients for the bacteria). Lees are good for building weight and texture in a wine, so winemakers often want to keep the wine on them for as long as possible.
Malo goes better with some grape varieties than others. Chardonnay has great affinity with MLF. But malo is not always a good idea for varieties such as riesling and sauvignon blanc. Whilst they do both have high natural acidity, the focus of flavour for the vast majority of wines made from these grapes is their primary fruit characters uninfluenced by techniques that might detract from this pristine expression, such as oak, or indeed, MLF.
Most reds undergo MLF. With their lower acidity, and lower sulphur regime it is more challenging to prevent it. And anyway, the extra degree of smoothness and suppleness provided by the MLF is desirable in reds. Whether the MLF is done in tank or barrel is one of the variables here. It’s less effort in tank, as the process must be individually controlled in each vessel. Oak barrels are smaller, so it is a time-consuming process, but the advantage is MLF in barrel creates a softer, rounder mouthfeel.