Term for the conversion of malic acid into the milder lactic acid (Latin malum = apple, lac = milk), which plays an alternative role in the production of fruit juice, wine and sparkling wine(champagne, sparkling wine). The process is also known as biological acid degradation (BSA), bacterial malic acid degradation, microbial acid degradation or apple lactic acid fermentation, as carbon dioxide is also released. However, it is not a real fermentation, but was previously thought to be. Around 1890, Dr Hermann Müller-Thurgau (1850-1927), who was working in Geisenheim at the time, correctly suspected bacteria as the cause. Dr Wenzel Seifert (1862-1942) was able to identify the acid-degrading bacterium at the Klosterneuburg Viticulture Institute in 1903 and named it "Micrococcus malolacticus".
Start of the process
This natural process can take 10 to 40 days. It is initiated spontaneously from around 16 °Celsius by existing or added bacteria (Oenococcus oeni). However, the process can also be started by stirring lees, by blending with a wine already in the BSA process or (usually the case) by adding starter cultures (special lactic acid bacteria). At this point, sulphurisation must be avoided because the sensitive starter cultures would stop working. The optimum wine temperature for this process is around 22 °Celsius. Ideally, the wine should have an acidity or pH value of at least 3.2 pH (the lactic acid bacteria cannot multiply below this level) and a residual sugar content of less than 20 g/litre.
The malic acid is converted first, then the citric acid and finally the residual sugar. Around 1 to 1.2 grams of lactic acid are produced from 2 grams of malic acid. This reduces the total acidity by around 1 gram/litre. If malolactic fermentation is not desired, the wine must be removed from the lees, clarified and sulphurised as quickly as possible after fermentation. This can also be prevented by cooling and reductive ageing. The process can also be regulated or interrupted by adding the enzyme lysozyme.
White wines optional, red wines obligatory
BSA is mostly used for red wines, but always before barrique ageing. The advantages are a reduction in malic acid, more body due to the lactic acid, less sulphur required and better microbiological stability. White wines acquire a buttery flavour or a malolactic note and lose their fruitiness. For this reason, white wines made from bouquet varieties do not undergo BSA. In contrast to red wine, malic acid is desirable in white wine as a tangy, refreshing component. In Switzerland, however, a BSA is common for white wines. Certain grape varieties such as Chardonnay benefit from this, which is utilised in Chablis. It is also common to subject only part of the wine to BSA and then blend it with the other part.
Possible wine faults
Wine defects such as bitterness, acetic souring, geranium, lime tint, mannitol t int or lactic acid tint (yoghurt tint, sour taste) are possible if the process is not carried out correctly. Colour loss and taste impairments can also occur. The complex process must therefore be closely monitored by measuring the malic acid content. Uncontrolled or excessive acid degradation must be categorised as acidity drop (wine defect). Once the malolactic fermentation is complete, sulphurisation and clarification may only take place after one to two weeks. Oxygen ingress must be prevented (full containers). Allergenic substances such as histamine and tyramine can be formed if the process is not carried out correctly.
Further information
For more information on the regulation of acidity in wine, see also the keywords deacidification and acidification. For information on the production of alcoholic beverages, see Champagne (sparkling wines), Distillation (distillates), Spirits (types), Winemaking (wines and wine types) and Wine law (wine law issues).
Graphic: By user Yikrazuul, Own work, CC BY-SA 3.0, Link
