What’s New with Malolactic Fermentation

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: james.osborne@oregonstate.edu