Various factors influence the amount of diacetyl, acetoin and 2,3-butanediol produced during fermentation impacting the buttery aroma.
Malolactic fermentation (MLF) is a secondary fermentation carried out by lactic acid bacteria (LAB). This process can occur spontaneously or can be induced by using MLF starter cultures. Currently, the commercially available MLF starter cultures belong to the species Oenococcus oeni and Lactobacillus plantarum. The use of starter cultures to induce MLF is preferred to avoid the risks associated with spontaneous MLF. The starter cultures can be inoculated simultaneously with the yeast, known as co-inoculation, or after the completion of alcoholic fermentation, known as sequential inoculation. MLF is a desirable process as the decarboxylation of l-malic acid to l-lactic acid and carbon dioxide decreases the acidity and increases the microbial stability of wine. This process also influences the organoleptic properties of wine.
In addition to malic acid, some MLF starter cultures can also degrade citric acid usually present in grape must at concentrations of 0.031 g/ℓ to 0.42 g/ℓ. The metabolism of citric acid leads to the production of acetate, d-lactate, diacetyl, acetoin and 2,3-butanediol (Figure 1). The production of acetate is one of the reasons for the 0.1 g/ℓ to 0.3 g/ℓ volatile acidity increase during MLF as citric acid metabolism is linked to malic acid degradation.
When present at low concentrations, diacetyl can contribute to the complexity of wine. Diacetyl has a buttery aroma which contributes to wine complexity when present at concentrations above its sensory threshold value of 0.2 mg/ℓ to 2.8 mg/ℓ. However, high diacetyl concentrations above 5 mg/ℓ can give rise to an overwhelming buttery aroma that masks the fruity and/or vegetative aromas in wines. Diacetyl can be reduced to the less sensory active acetoin and 2,3-butanediol (Figure 1) with much higher sensory thresholds of 150 mg/ℓ and 600 mg/ℓ, respectively. The reduction of diacetyl to these compounds is therefore encouraged during winemaking if a buttery style wine is not wanted. Several factors influence this reduction, as well as the sensory perception of diacetyl in wines.
A few of these factors include:
Composition of grape must
The grape must composition influences the concentrations, as well as the sensory perception of diacetyl. There are three main components of grape must that can influence the diacetyl concentrations during fermentation. These components are:
Diacetyl is more rapidly reduced to acetoin during the fermentation of grapes from warm climate regions that have a higher pH. Wines from these regions might therefore have less diacetyl than wines from cool climate regions that are usually associated with a low pH.
- Citric acid concentration
Grape must with a higher citric acid concentration leads to increasing concentrations of acetate, d-lactate, diacetyl, acetoin and 2,3-butanediol. Excess acetate and d-lactate causes over acidification and inhibits bacterial growth thus prolonging MLF. A longer MLF duration can result in more diacetyl being produced during the fermentation.
- Phenolic compounds
Several studies have previously indicated that diacetyl in white wines was less stable and more likely to be reduced to acetoin and 2,3-butanediol than in red wines. However, the buttery aroma of diacetyl is more likely to occur in white wines than in red wines, due to the presence of phenolic compounds such as p-coumaric, caffeic, ferulic, gallic and protocatechuic acid. These phenolic compounds lower the buttery aroma in red wines by binding to diacetyl.