5.6.1 Review of microorganisms that cause wine defects during wine ageing

Acetic acid bacteria

Acetobacter aceti bacteria develop when oxygen is present. They are the most common spoilage microorganisms that result in elevated volatile acidity via acetic acid production (vinegar odour) from ethanol. Following acetic acid bacteria proliferation, the wine is referred to as having vinegar taint. Acetic acid is perceptible from 0.7 gH2SO4/L.
Ethyl acetate is also produced. The latter leads to an aroma of nail polish, and increases sensations of burning and harshness in the wine. It is noticeable from 0.15 g/L. Monitoring must be rigorous because both these elements are detrimental to wines' organoleptic quality.
Legislation is strict on this subject, if the acetic acid concentration exceeds a certain level the wine is considered to be unmarketable. This value depends on wine type of wine and appellation. The maximum OIV value is 0.98 gH2SO4/L.

Brettanomyces bruxellensis

B. Bruxellensis is perhaps one of the most difficult spoilage microorganisms to manage. This yeast has low nutrient requirements and is sometimes resistant to high concentrations of free SO2 or high levels of ethanol and relatively low pH. Due to its significant capacity to adapt to the environment, it is difficult to control. Moreover, it is present everywhere, firstly on the grapes themselves and above all on winemaking equipment from previous contamination.

These yeasts produce ethyl-phenols from phenolic acids (ferulic, caffeic and p-coumaric acids), more precisely 4-ethyl-phenol and 4-ethyl-gaiacol. The aromas range from leather to stables or horse sweat and gouache. Even at low concentrations, a masking effect on other wine aromas is observed.
Furthermore, this yeast can potentially be a source of mouse taint due to tetrahydropyridine production. This taste can also be produced by lactic bacteria via lysine breakdown. It can also produce acetic acid when it breaks down sugars. Finally, in rare cases, biogenic amines can be produced.
Due to its high adaptability and minimal requirements, B. bruxellensis can potentially colonize the environment during any fermentation issues (premature ending of alcoholic fermentation, significant lag time between AF and MLF).
The first management measure is therefore to ensure a rapid, vigorous alcoholic fermentation. The same applies to malolactic fermentation, the gap between these two fermentations constituting a period of significant risk.

Lactic acid bacteria

The consequences of the growth of lactic acid bacteria are detailed in the section on Malolactic fermentation.