Acetoin, also known as acetyl methyl carbinol or 3-hydroxybutanone, is a ketone, an organic molecule with the formula C4H8O2. It is part of the chemical composition of wines, which sometimes have a pleasant buttery smell when tasted.

Acetoin formation in wine

Oenologists have long been aware of the presence of acetoin in wine, whether white, red or rosé. Its concentration is often low, averaging 10 mg/l in still wines and 25 mg/l in sparkling wines. But when and how does acetoin appear in wine?

During alcoholic fermentation

Some of the yeasts involved in this stage of the winemaking process will synthesise diacetyl, a volatile ketone with a characteristic milky, butter-like odour. However, most of this diacetyl will be reduced to other acetoin compounds: butane-2,3-diol and acetoin. The concentration of these molecules in the wine therefore depends on the type of yeast used by the winemaker. Saccharomyces strains, exogenous yeasts frequently inoculated to guide alcoholic fermentation, produce very few.

During malolactic fermentation

During this second fermentation, some of the lactic acid bacteria present in the must transform some of the citric acid (citrate) in the wine into various acetoin substances: diacetyl, butane-2,3-diol and acetoin. This metabolic reaction occurs in response to a need to deacidify the environment in which the lactic acid bacteria are present.

However, the acetoin produced in this way will be consumed by these micro-organisms at the end of the malolactic fermentation process to ensure the survival of the bacterial population. In short, malolactic fermentation has very little effect on a wine's acetoin content. As for ageing, it too has very little impact.

Organoleptic benefits of acetoin

In oenology

The odour perception threshold for this volatile compound is reached when its concentration exceeds 150 mg per litre of wine. Acetoin then has a masking effect on fresh fruit notes, and an intensifying effect on jammy fruit notes. But while its buttery aroma, which is much sweeter than that of diacetyl, is beneficial to the aromatic profile of some wines, particularly whites, this is not very common: the majority have an acetoin content below this threshold.

It should be pointed out, however, that too high a concentration of this molecule alters the organoleptic quality of a wine. Its excessive production during vinification may be due to the presence of acetoin-producing yeast strains such as Schizosaccharomyces or Kloeckera apiculata. Excessive O2 during alcoholic fermentation and microbial contamination of the must or wine can also be responsible.

In the food and perfume industry

Acetoin is a chemical compound used as a flavouring agent in various food preparations, particularly bakery products. . Its slight buttery flavour also makes it an ingredient in margarine, to which it gives a richer taste. It is also found in the aromatic composition of popcorn. . This molecule is also present as an additive in tobacco and in the caramel flavoured liquids used in electronic cigarettes.