Lactic disease

When alcoholic fermentation is too slow or when it stops, conditions are favourable for bacterial development. LAB ferment different quantities of sugars that have not been totally fermented by yeasts and produce acetic acid and D-lactic acid. This alteration is called Lactic disease" (piqure lactique) and is characterised by a high volatile acidity that depreciates the wine. If this volatile acidity exceeds the limit of 1 g/L, the wine is unmarketable (Lonvaud-Funel 1999). This spoilage also occurs in fortified wine where O. oeni, L. hilgardii, L. fructivorans and L. plantarum are active in spite of very high ethanol contents. 

The spontaneous evolution of a mixture of species corresponds with the selection of those best adapted to wine—which is a hostile acidic and alcoholic environment. The composition of the plasmic membrane, and the various mechanisms, that permit it to react to the aggressiveness of the medium, seem to influence this adaptation. Certain species or strains may also differ in their ability to carry out these transformations. Strains of Lfructivorans adapt better to ethanol than L. plantarum and L. hilgardii, due to a more effective modification of their fatty acids (unsaturation and chain length) (Kalmar, 1995). Unsurprisingly, strains of this species are often identified in fortified wines tainted by lactic disease with an alcohol content between 15 and 20% volume. 

Lactic disease, therefore, occurs when environmental conditions are favorable to bacterial growth, even though yeasts have not yet... 

A posteriori, the diagnosis of lactic disease is based on the nature of the products of the bacterial metabolism (Section 14.2.3). Wines presenting an elevated volatile acidity can also have been the site of acetic bacterial multiplication or a metabolic deviation of yeasts. Yet when they produce acetic... 

In wines, the first step in preventing lactic disease is the proper sulfiting of grapes, especially when they are very ripe. The corresponding mnsts are more subject to stuck fermentations than others (Section 3.8.1). The winemaker mnst react accordingly and, if need be, use additives snch as nitrogen, vitamins and yeast hulls whose effectiveness is clearly established. Of conrse, elemental operations, notably aeration and temperatnre control, must also be scrupulously respected. 

The catabohsm of sugars, malic acid and citric acid are normal occurrences during fermentation. Lactic disease only exists if O. oeni mnlfiplies prematnrely. Many other transformations also occnr and some depend on the nature of the strain. Malolacfic fermentation has been confirmed to canse chromatic changes in wines and a decrease in their color, while stabilizing it. 

If D (—)-lactic concentrations are more than 200 mg/1, lactic disease may exist. 

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