What is malolactic fermentation and why is it carried out?
Malolactic fermentation is carried out by lactic bacteria belonging to the genera Lactobacillus, Pedicoccus and Leuconostoc, whose metabolism transforms malic acid, naturally present in wines, into lactic acid. The main impact of malolactic fermentation on any wine is to reduce its acidity, lactic acid being a weaker acid than malic one, although in reality lactic bacteria tend to metabolise not only malic acid but also any residual sugars still present in the wine after alcoholic fermentation. The wine will therefore be softer on the palate, providing an even, rounder sensation that is well suited to structured red wines and some white wines for ageing. Malic acid, on the other hand, provides a distinctly more acidic sensation, similar to that of an unripe apple, and is usually liked in young white wines as it provides freshness and longevity. It will therefore be the producer’s choice to favour malolactic fermentation or inhibit it altogether depending on the wine he intends to make.
Why it is important to favour malolactic fermentation early in the autumn
In order for malolactic fermentation to take place, a wine must have a very low SO2 content and a certain ‘turbidity’ to favour the action of the bacteria. For this purpose, it is good practice to add the ‘fine lees’ from alcoholic fermentation to the wine. However, keeping a wine in these conditions until the following spring is risky, as unwanted fermentation by yeasts (brettanomyces) or bacteria can be favoured, causing dangerous deviations in smell and taste. For the same reasons, it is advisable at the end of malolactic fermentation to proceed with a sulphiting of the wines to inhibit undesirable yeasts and bacteria.
How to start malolactic fermentation in autumn
When you want to start malolactic fermentation, keep the sulphur content very low, combine the fine lees of fermentation and keep the wine temperature around 18-20 °C. One can also proceed with an inoculation with selected bacteria and nutrients. It is also important to have a Ph value above 3.2, otherwise a slight deacidification may be necessary.
How long does malolactic fermentation last
The development of malolactic fermentation is monitored by constantly checking the malic and lactic acid values and lasts between three and six weeks, after which the wine is decanted and sulphited.
Heating the wine
The main problem obviously lies in maintaining the temperature of the wine at around 18-20 °C. For this purpose, large industrial cellars use insulated tanks connected to thermal systems capable of heating and cooling the wine. In ‘artisan cellars’, heating bands adaptable to simple stainless steel or wooden tanks solve the problem very well. This system, combined with a thermostat, allows the desired temperature to be set and maintained for the duration of malolactic fermentation. There is therefore no need to replace the containers already existing in the cellar and it also allows malolactic fermentation to be carried out in wood, combining the functionality of the heating bands with the advantages of barriques and tonneaux in terms of both oxygen micro-exchange and the transfer of noble oak tannins.
Carlo Torelli
Studente Facoltà di Enologia e Viticoltura di Grugliasco
