Dr. James Osborne, Associate Professor and Enology Extension Specialist, OSU
The malolactic fermentation (MLF) is a vital step in the production of cool climate red wines as well as some white wines. But despite its importance, MLF often gets taken for granted and just considered a step to reduce wine acidity. However, MLF is much more than just a biological de-acidification process and can have a number of other impacts on wine quality. Our lab has been conducting a number of projects over recent years investigating various aspects of MLF. One project is investigating interactions between Oenococcus oeni and the spoilage yeast Brettanomyces bruxellensis. An interesting result from this study was discovering that some O. oeni strains were capable of increasing the concentration of the volatile phenol precursors p-coumaric acid and ferulic acid. These pre-cursor compounds are found in grapes and wine mainly bound to a tartaric acid and in this form are not utilized by Brettanomyces. However, some O. oeni strains can remove the tartaric acid through the action of an enzyme, cinnamic esterase, and release free p-coumaric and ferulic acid that Brettanomyces can then metabolize to 4-ethylphenol and 4-ethyl guaiacol. This finding has led to the labelling of many commercial O. oeni strains as either cinnamic esterase (+) or (-) with the recommendation being to avoid use of cinnamic esterase (+) strains in situations where the wine may be at risk for Brettanomyces spoilage.
An additional area of research has been determining the effect of MLF on red wine color. We know that MLF changes wine pH which can cause a shift in red color, but were there other impacts on color due to MLF? Our lab demonstrated that independent of pH change, MLF results in a loss of color and lower formation of polymeric pigments. Results from a number of studies showed that this color loss was likely due to the metabolism of acetaldehyde by O. oeni. Acetaldehyde plays a key role in the development of polymeric pigments and so metabolism of acetaldehyde during MLF reduced formation of these color compounds. Delaying MLF was shown to help mitigate this color loss but delaying MLF for long periods is risky from a microbial spoilage point of view, as SO2 cannot be added to the wine until MLF is complete. Additional strategies to mitigate color loss due to MLF are currently being explored. One such strategy is the use of ML bacteria that do not metabolize acetaldehyde. To date, all O. oeni strains screened can metabolize acetaldehyde but other lactic acid bacterial species such as Lactobacillus look more promising. There has been renewed interest in using certain Lactobacillus species and strains to conduct MLF. In particular, homofermentative species of Lactobacillus have been studied as potential ML starter cultures. These bacteria do not produce acetic acid from glucose metabolism and so could be used for conducting concurrent alcoholic and malolactic fermentations without the risk of increased acetic acid. Currently, there are commercially produced L. platarum cultures available outside of the USA for use in winemaking. However, at this time these cultures are not available for winemaking use in the USA. The use of concurrent alcoholic and malolactic fermentation is one final area our lab has been studying. While there are obvious time advantages to conducting alcoholic and malolactic fermentation at the same time, there are still some concerns over the impact on wine quality, particularly for red wines. We recently completed a study investigating how the timing of MLF impacts Chardonnay aroma and mouthfeel and will be continuing work in this area focused on concurrent fermentations of red wines. As we continue to study malolactic bacteria, we are gaining a better appreciation for the impact they can have on wine quality and potential new strategies for their use. For additional information on any of the studies we have conducted on MLF please contact me at: email@example.com