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A Technical Guide for Wine Producers
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The influence of malolactic fermentation on the volatile composition of red wines
The most basic definition of malolactic fermentation (MLF) is the bacteriological conversion of malic acid to lactic acid, accompanied by a decrease in the total acid content and an increase in the pH of the wine. It is also known that MLF changes the mouthfeel and flavour profile of wines. However, very little detail is known about the latter.
The volatile composition of wine is influenced, inter alia, by the alcoholic fermentation, MLF and wood maturation. The former two processes are particularly influenced by the Saccharomyces and Oenococcus organisms respectively. It is common knowledge that Saccharomyces cerevisiae, which is mostly responsible for alcoholic fermentation, may form significant quantities of flavourants such as higher alcohols, fatty acid ethyl esters and acetate esters in the course of the process. Although it is generally accepted that MLF goes hand in hand with changes in the flavour composition of wines, knowledge about the factors that influence this process is limited. Considerable quantities of diacethyl, which is responsible for the buttery character in wines, are formed during MLF, but different esters may be formed during this process. MLF bacteria are also able to form flavour compounds from flavourless precursors and break down flavourants that are formed during the alcoholic fermentation. However, the bulk of the research on this topic has been done using model solutions and not wine.
Recently research was conducted in Italy on the indigenous red cultivar, Aglianico. Wines were fermented using Lallemand D47 yeast strain and four different MLF cultures were then used for MLF. The wines were matured in bottles for two months following completion of the MLF before they were compared. The four different MLF strains caused an increase in the quality of ethyl butanoate, ethyl hexanoate, ethyl octanoate, ethyl decanoate, acetate esters and ethyl-3-hydroxy butanoate during MLF. The latter compound is especially associated with the strawberry character of Pinot noir. MLF did not influence the quality of ethyl-2-methyl butanoate and ethyl-3-methyl butanoate, which play an important role in the flavour of red wines. Other volatile compounds that are formed during alcoholic fermentation, such as the qualities of higher alcohols and fatty acids, were generally not influenced by MLF. The quality of 3-(methylthio)-1-propanol, which is associated with the boiled cabbage character of wines and considered an off-odour in high concentrations, was increased during MLF by two of the MLF strains. In all instances the concentrations thus formed exceeded the detectable threshold value of the compound.
It has long been known that glycocidic flavour precursors are able to play an important role in the flavour of grapes or wine. The hydrolysis of such precursors may result in the release of terpenes and norisoprenoids, which may contribute to the cultivar character of wines. MLF bacteria are also able to effect this hydrolysis process. For example linalol, an important colourant, especially in white wines, and farnesol, an important flavour component of Tempranillo and Grenache grapes, both increased during the MLF.
The concentrations of two volatile phenols, 4-vinyl phenol and 4-vinyl guaiacol (which is also associated with Brettanomyces), increased in two of the MLF yeast strains during MLF.
In conclusion, research has shown that Oenococcus oeni has the potential to change the flavour profile of red wines during MLF through the flavourant formation of the bacteriological metabolism, or through the hydrolysis of glycocidic flavour precursors that derive from the grapes (Ugliano & Molo, 2007).
Reference
Ugliano, M. & Molo, L. 2007. Malolactic fermentation and wine flavour: Changes in the volatile composition of red wine following malolactic fermentation with four commercial strains of Oenococcus oeni. The Australian & New Zealand Grapegrower & Winemaker, Annual Technical Issue: 53 - 60.
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