Chapter 7

Malolactic Fermentation in Wine

Inoculated Malolactic Fermentation

Although some wineries continue the tradition of using native microflora, winemakers increasingly inoculate grape must or wine with lactic acid bacteria starter cultures to ensure the successful completion of malolactic fermentation; and to reduce the risks associated with spontaneous malolactic fermentation. The advantages of induction of malolactic fermentation by inoculation with selected strains of lactic acid bacteria are twofold. First, to have better control over the time and speed of malic acid conversion, and, second, to have a more consistent influence on wine flavor and quality.

Timing of Inoculation

The best time for inoculation with wine lactic acid bacteria (LAB) is the topic of much debate. There are three possible inoculation scenarios for malolactic fermentation: co-inoculated with Saccharomyces cerevisiae yeast at the beginning of fermentation or later during alcoholic fermentation, or sequentially after alcohol fermentation. In summary, there are a number of options available regarding the timing of malolactic fermentation. There is no universal right time as you must consider several different factors when making this decision such as the chemical composition of the grape must, fermentation conditions, and style of wine.

Co-Inoculation

Some winemakers believe co-inoculation during alcohol fermentation is best for inducing malolactic fermentation. The bacterial inoculum is either added with the yeast inoculum at the beginning of alcoholic fermentation or within 24 or 48 hours of yeast inoculation. Inoculation 24 or 48 hours after yeast addition ensures that the selected bacteria find all the key nutrients needed in the grape must, and can slowly adapt to the increasing alcohol content during the fermentation.

Sequential Inoculation

Traditionally, it has been recommended that wines should be inoculated with malolactic bacteria after the completion of the alcoholic fermentation. There are a number of reasons why sequential inoculation is preferred by most winemakers. First, it is logistically advantageous to separate the two processes of alcoholic fermentation and malolactic fermentation. Second, there is concern that the addition of Oenococcus oeni into juice or must (rather than wine) can lead to an increase in volatile acidity and D-lactic acid since O. oeni can convert sugar into acetic acid, a situation which is referred to as pique lactique (Rib�rau-Gayon et al., 2006a).

Sensory Impact of Timing of Inoculation

Timing of inoculation, interaction with the wine yeast, presence of precursors promoting the production of aroma compounds, pH, and temperature conditions are all criteria that modulate the aromatic expression in wines. Choosing a wine bacteria and its timing of inoculation have become a key factor to take into consideration for developing a specific wine profile.

Starter Cultures

Winemakers often inoculate grape must or wine with commercial starter cultures to reduce the risk of indigenous lactic acid bacteria, promote the rapid start and completion of malolactic fermentation, and also encourage a positive flavor contribution by the lactic acid bacteria. A starter culture is a term that refers to the process of expanding the volume of a culture by inoculating a volume of media and allowing it to incubate. The incubation period allows the culture to consume nutrients and carbohydrates in order to multiply. Once the culture has been allowed to expand (or multiply), it is ready to inoculate a larger volume of must. Commercial malolactic bacterial cultures are available for use as an inoculum available in the freeze-dried (lyophilized), frozen, or liquid states.

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