by Charl Theron

The conversion of malic acid to the weaker lactic acid during malolactic fermentation (MLF) is not the only important change that occurs during this process. Other intermediate and co-products are also formed, which will have a secondary influence on the wine composition. The decrease in the colour is one such product, but the reason for it is unknown.

Wine quality and its sensory description are determined by factors like flavour, taste and mouthfeel, but especially in the case of red wines the colour is also an important factor. This is especially applicable in the case of light coloured red wines like Pinot noir. Much research was consequently done to determine which viticultural and cellar practices will optimise the colour. The colour of red wines is not only determined by the anthocyanin concentration of the wine. After being extracted from the skins, the anthocyanins can react with different compounds in the wine to form more complex colour compounds. They may for example react with tannins to form polymeric pigments. This formation can be accelerated by reactions with acetaldehyde. Stable anthocyanin derivative products can also be formed by reactions with pyruvic acid. All these pigments are more resistant to oxidation and bleaching by sulphur dioxide (SO2), and also tend to increase during wine maturation.

It is known that winemaking factors like fermentation temperature and extended maceration influence the formation of polymeric pigments. Yeast can also influence the colour of red wines by the adsorption of anthocyanins to their cell walls or by the formation of acetaldehyde and pyruvic acid. Yeast strains differ regarding the acetaldehyde concentration which is formed and will consequently have different effects on the colour of red wines. Other micro-organisms can also influence the colour of red wines. It is for example known that Oenococcus oeni decomposes acetaldehyde and pyruvic acid during MLF …