Malolactic fermentation: lactic acid bacteria
In the first part of our article series on malolactic fermentation, Westgarth Wines wine specialist Maurizio Broggi discusses lactic acid bacteria and how it affects the final wine’s character.
Malolactic fermentation is the process by which sharp malic acid is converted to softer lactic acid using lactic acid bacteria. Malolactic fermentation typically occurs after alcoholic fermentation and has significant consequences for the style, acidity, and stability of the wine. The process of malolactic fermentation (often abbreviated as MLF) is also referred to as malolactic conversion or secondary fermentation.
Lactic acid bacteria
Unlike alcoholic fermentation where yeasts carry out the fermentation, malolactic fermentation is carried out by a specific group of bacteria, the lactic acid bacteria which degrade malic acid into lactic acid and at the same time produce carbon dioxide. The three main genera are Oenococcus, Lactobacillus, and Pediococcus. Lactic acid bacteria tend to be inhibited by low temperatures, low pH, alcohol, and sulfur dioxide.
The genus Oenococcus, and particularly the species Oenococcus oeni, is the lactic acid bacteria of primary interest for winemakers. It is more tolerant than other genera to the acidic environment typical of wine as well as to alcohol and sulfur dioxide. It is also able to function with low levels of nutrients. Equally important, Oenococcus oeni produces positive effects such as adding flavors, complexity, and texture to the wine.
Lactobacillus and Pediococcus
By comparison, Lactobacillus and Pediococcus are considered spoilage organisms because they can lead to the production of faults and off-flavors if allowed to grow in wine. Fortunately, at typical wine pH (below 3.5), Lactobacillus and Pediococcus are inhibited, thus Oenococcus oeni is usually the only lactic acid bacteria active during malolactic fermentation.
Spontaneous malolactic fermentation
Often, malolactic fermentation takes place spontaneously thanks to the presence of indigenous lactic acid bacteria on grapes, those present in the cellar, and on winery surfaces – especially oak barrels.
Lactic acid bacteria on grapes tends to significantly decrease in population due to the unsuitable conditions in grape juice and fermenting wine (e.g., sulfur dioxide, high acidity, high sugars, and increasing alcohol content). However, vessels such as oak barrels, particularly where malolactic fermentation has occurred in previous vintages, will likely host lactic acid bacteria that will encourage the onset of malolactic fermentation once the wine has been transferred to barrels.
Cultured lactic acid bacteria
Conveniently, as with yeasts for alcoholic fermentation, winemakers can directly inoculate the wine with specific strains of commercially available cultured lactic acid bacteria. These are often better suited to tolerate specific wine conditions, such as higher alcohol or acidity, or more likely to produce desired sensory characteristics (e.g., butter flavors). For some winemakers, inoculation of cultured lactic acid bacteria is the preferred option as it gets the job done faster and in a more predictable, and therefore controlled, manner.