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.
Winemakers have always experienced a decrease in the colour of red wines after MLF. It can partially be attributed to the pH increase during MLF, but the influence of Oenococcus oeni has never been researched intensively. Recent research in the USA tried to clarify the influence of MLF on the colour of Pinot noir. The wine was made with standard cellar practices and sterile filtered prior to MLF. After the filtration it was inoculated with three different Oenococcus oeni strains and compared with a non-inoculated control. After completion of MLF, different colour components were measured and the wine was also matured for nine months. Wine was also made with simultaneous alcoholic and malolactic fermentation. Lower colour (measured as absorbance at 520 nm) was observed in all the wines with MLF, in comparison with the control sample. The reduced colour persisted throughout ageing. The lower colour in the MLF wines corresponded with decreased polymeric pigment and higher concentrations of the more unstable monomeric anthocyanins. Analyses also confirmed that acetaldehyde and pyruvic acid are decomposed by Oenococcus oeni during MLF. When acetaldehyde was added to the wine after MLF, the colour difference between the control wine and MLF was smaller.
Winemakers believe that colour losses during MLF can be prevented by delaying the MLF. In the American research it was delayed for up to six months, but it did not have a significant influence on the colour loss (absorbance at 520 nm) during MLF. The polymeric pigment and monomeric anthocyanin concentration did however differ less from the control. It was most probably due to the acetaldehyde remaining in the wine for a longer period. From a practical point of view the delay of the MLF for several months can be risky, because SO2 additions cannot be made before MLF is not completed. Wine must consequently be kept at a low temperature to minimise microbial growth (Osborne & Burns, 2015).
Osborne, James & Burns, Tresider, 2015. Malolactic fermentation and red wine colour. Wine & Viticulture Journal, September/October 2015