The volatile acidity of a Vin is consisted the part of the fatty-acids pertaining to the acetic series which is in the wine either in a free state, or at the salified state.

The official method of analysis imposes a separation of the volatile acids by drive on the steam then correction of the vapors. The acidity of the Sulfur dioxide free and distilled compound is not taken into account in volatile acidity and must be cut off from the acidity of the Distillat, as well as the acidity of the sorbic acid possibly present, lactic and of the Carbonic gas.

Volatile acidity is expressed in mEq.L-1 or g.L-1 acid chosen conventionally by each country for its interior use (Acid sulphuric in France with MM = 98).

A wine of name of controlled origin is “merchant” if his volatile acidity does not exceed 0,9 g.L-1 expressed in $H\_2\; SO\_4$, that is to say 1,35 g.L-1 in tartaric acid and 1,1 g.L-1 in acetic acid.

The alcoholic fermentation of a Moût leads to the normal formation from 0,2 to 0,3 g.L-1 $H\_2\; SO\_4$ of volatile acidity according to the conditions of medium (hygiene, potential alcohol, level of washing, temperature of fermentation…) and the stock of Yeast. The kinetics of production of the acetic Acid by yeast is irregular: the peak is located into 3 and 5° actual alcoholic strength, for then dropping. This property of consumption of the acetic acid by yeast of fermentation can be made profitable by the vinificator “to pass by again” of the wines whose volatile acidity is too high (Super technique “four”).

The acetic Bactéries Aérobie S also form acetic Acid by oxidation of the ethanol: this phenomenon has place during all the conservation of the wine and conduit in the extreme cases with the “acetic Piqûre”.

The lactic Bactéries Anaérobie S produce small quantities of volatile acidity (between 0,10 and 0,20 g.L-1 $H\_2\; SO\_4$) during the malolactic Fermentation (FML), in particular at the end of the FML by consumption of the citric Acid . The latter is naturally present in the wine but can be also added to the setting out of bottle to avoid the ferric Casse because it is a chelating iron. In addition, these same bacteria can degrade the Sucre S residual of the wine and produce great quantities of acetic acid: it is the “lactic Piqûre” dreaded by the vinificator and which is often consecutive with a FML badly controlled afterwards with a stop of alcoholic fermentation. This accident is more frequent the hot years when the cellars are insufficiently equipped for the control with the temperatures with fermentation.

Others Micro-organism S contaminants of the wine such as the pédioccoques ones (Bacteria) or the Brettanomyces yeasts are also able to produce significant quantities of acetic acid and thus to make increase the volatile acidity of a wine.