Questions Pertaining to Malolactic Fermentation in Wine
By: Denise M. Gardner
What is Malolactic Fermentation?
Malolactic fermentation, MLF, is a bacterial fermentation, which converts malic acid to lactic acid. Malic acid, the acid affiliated with apples is perceptibly harsher than lactic acid, which is the primary acid in milk, and perceptibly softer than malic acid.
What is the difference between a native MLF and one that is inoculated?
MLF can occur spontaneously through the native microflora (i.e., lactic acid bacteria) that comes in on the fruit from the vineyard. Most native malolactic bacteria (MLB) strains are in the Lactobacillus genera. These bacterial populations are easily manipulated by pH changes and are sensitive to general environmental changes (e.g. increases in alcohol).
However, due to the fact that native or natural MLB is unpredictable and may contribute to several affiliated off-flavors of the wine, many winemakers choose to inoculate for MLF. The most common bacteria used for commercial inoculation is Oenococcos oeni, which can be purchased through a number of commercial suppliers. Many suppliers offer several different strain selections to accommodate varying wine conditions (e.g. high or low pH, high alcohol environments, use of sulfur dioxide, etc.). Commercial strains are bred to be reliable, consistent, and tolerant of low pH wines. They also contribute less flavor changes to the wine, if specific chains are selected by the winemaker, compared to many native bacterial strains.
One of the biggest problems affiliated with commercial MLF strains is the loss of color in red wines between primary and secondary fermentation. While we will touch briefly on color stability next week, several studies (Gerbaux and Briffox 2003, Morenzoni and Specht 2005, Burns and Osborne 2013) have noted the issues affiliated with red wine color stability since commercial strains have become more prevalent. Earlier this year, José Santos from Enartis Vinquiry discussed various red wine color stability issues during his “Harvest Preparation” workshop. You can read more about his discussion here.
When should wines be inoculated for MLF?
Currently, inoculation and timing of inoculation for MLF is a stylistic decision made by the winemaker. There are several options available for any given wine:
- Following primary fermentation: Bacteria inoculation occurs after the completion of primary fermentation. This has become the standard way to use MLB.
- Co-Fermentation of yeast and bacteria: Bacteria inoculation occurs after inoculation of yeast for primary fermentation, but before primary fermentation is complete.
- Co-Inoculation or simultaneous inoculation: Here, yeast for primary fermentation and bacteria for MLF are inoculated at the same time to the must.
There are some advantages and concerns for co-fermentation or co-inoculation. In terms of advantages, previous research has highlighted how MLF bacteria can utilize remaining primary fermentation macromolecules (Guilloux-Benatier and Feuillat 1991), including mannoproteins (Alexandre et al. 2004) for their own nutritional gain. Beelman and Kunkee (1985) and King and Beelman (1986) found that bacteria acclimate to the changing wine conditions (i.e., increases in ethanol content) while the yeast strain proliferated to complete primary fermentation. Another advantage includes a reduction in total production time from the start of primary fermentation to the end of MLF, which can be beneficial to wineries with space or tank constraints.
In contrast, some research has pointed towards antagonistic relationships between yeast and bacteria, causing an increase in stuck fermentations. Typically, those studies also found an increase in volatile acidity. As mentioned previously, there has been an anecdotal observation of color intensity reduction in red wines, in general, utilizing commercial MLB strains. However, this observation has not yet been linked to timing of inoculation for MLF (Burns and Osborne 2013).
Based on the varying results from past research, it is advised that producers that utilize co-fermentation practices discuss strain selections, of both yeast and bacteria, with their supplier. There is a general review online, authored by Dr. Sibylle Krieger from Lallemand, written in 2007 regarding strain selection and MLF stylistic approaches, which you can read here.
Alexandre, H. et al. (2004) Saccharomyces cerevisiae – Oenococcus oeni interactions in wine: current knowledge and perspectives. Int. J. Food Micro. 93: 141-154.
Beelman, R.B. and R.E. Kunkee. (1987) Inducing simultaneous malolactic/alcoholic fermentation. Practical Winery & Vineyard Management. July/August 1987. pg. 44-56.
Burns, T.R. and J.P. Osborne. (2013) Impact of Malolactic Fermentation on the Color and Color Stability of Pinot noir and Merlot Wine. Am. J. Enol. Vitic. 64:3, pg. 370-377.
Gerbaux, V. and C. Briffox. (2003) Influence de l’ensemencement en bacteries lactiques sur l’evolution de la couleur des vins de Pinot Noir pendant l’elevage. Revue des Înologues. 103:19-23.
Guilloux-Benatier, M. and M. Feuillat. (1991) Utilisation d’adjuvants d’origine levurienne pour améliorer l’ensemencement des vins en bactéries sélectionnées. Rev. Fr. Oenol. 132:51-55.
King, S.W. and R.B. Beelman. (1986) Metabolic interactions between Saccharomyces cerevisiae and Leuconostoc oenos in a model grape juice/wine system. Am. J. Enol. Vitic. 37(1): 53-60.
Morenzoni, R. and K. Scully Specht (Eds.). (2005) Malolactic Fermentation in Wine: Understanding the Science and the Practice. Lallemand Inc.