by Sally Colby

Wine consultant Denise Gardner says it’s important for winemakers to understand malolactic fermentation (MLF) and how it can influence wine at various stages of production. Gardner describes MLF as the biological conversion of malic acid to lactic acid, and says it’s considered a biological process because bacteria are converting the acid.

Gardner explained that acid can be adjusted in many ways – juice acid adjustments, primary fermentation, MLF and wine acid adjustments, blending and product additions that don’t necessarily change the acid content but influence the perception of acidity.

“Malolactic fermentation offers more than just an acid management tool,” said Gardner. “It can contribute to a lot of other factors in wine, including changes in red wine color and changes in the aroma and flavor, which ultimately influence complexity and wine style.”

Gardner said even if a winemaker is not using MLF, a 2013 study showed that most commercial wine yeasts also consume malic acid. “You’ll see the segregation of malic acid even after primary fermentation,” she said. “The study showed that the percent of malic acid consumed or reduced ranged from 6.8 to 40 percent,” she said. “That’s almost half of the malic acid for some of these strains.” Gardner added that if the winemaker’s goal is to retain as much malic acid as possible, avoid yeasts that have higher consumption rates of malic acid. “The consumption of malic acid during primary fermentation will ultimately alter the mouth feel of wine, and alter the pH and PA as well.”

During MLF, pH rises because the chemical structure is changed. “This increase in pH is significant because it affects wine stability,” said Gardner. “Any time we alter pH, there’s going to be the secondary effect of stability in the wine. That means pH has influence over a lot of the chemical and microbiological factors in wine – color, the chemical charge of tannins, different ions, tartrate stability, protein stability, sulfur dioxide stability and biological stability.”

Because the chemical structure of acid is changed through MLF, there’s a decrease in titratable acidity (TA). Gardner said this decrease in TA is important because consumers ultimately perceive it as sourness.

The MLF process can be either spontaneous/native fermentation or inoculated/controlled fermentation. In spontaneous or native fermentation, any lactic acid bacteria (LAB) species can consume the malic acid in wine in whatever time or way best serves them. “This is a fairly unpredictable process,” said Gardner. “You’re leaving it up to the native populations of lactic acid bacteria on your grapes or collected somewhere in your winery, and you don’t really know when MLF is going to start or end. The flavor influences are unpredictable because there’s a wide range of secondary flavors that come from the MLF process.”

The spontaneous process can produce enhanced fruity flavors or enhanced vegetative flavors that may not be desirable for some wines. The process can also result in different mouth feel structures that change the flavor and perception of the wine, and ultimately, wine quality. “When going through a native MLF, especially if you don’t know which species dominate the winery, there is unpredictability in the process,” said Gardner.

A controlled MLF usually involves inoculation with the lactic acid bacteria Oenococcus oeni, or O. oeni, although not exclusively. Gardner said this strain is fairly predictable, and is the LAB strain of choice for commercial application. “It’s a little more resistant to different environmental factors in the wine,” she said. “There are strains you can select for high diacetyl or that buttery perception versus low or even no diacetyl by the time MLF is complete. With this control, you also get more persistent wine quality because you’re adjusting where that process is going and you know which strain is completing the process.”

Strains of LAB that contribute to MLF generally have similar properties. “They’re gram-positive bacteria and have a fixed cell membrane that can retain the red-violet stain,” said Gardner. “They’re non-mobile – they stay in one location and do their thing. They’re not sporulating, and are considered facultative anaerobes. While LAB can survive in oxygenated and non-oxygenated environments, they typically prefer lower oxygen environments.” Gardner added that the optimum temperature for malolactic bacteria (MLB) is 68 to 86 degrees F. “That’s where lack of oxygen can come into play – you can better create an environment for the growth and survival of MLB as opposed to spoilage bacteria.”

Microorganisms are naturally-occurring on grapes and come in from the vineyard. However, most research has shown that these bacteria enter the winery at low numbers compared to native yeasts and native acetic acid bacteria. “When you bring in native lactic acid bacteria, it’s almost always dominated by the Lactobacillus strain,” said Gardner. “This is different than what is used for inoculation.”

Gardner said commercial lactic acid bacteria strains are almost always O. oeni because they’re more reliable in a wine environment. “They tend to have more consistent growth and a more consistent outcome of using malic acid during the MLF process,” she said. “They are also more tolerant of low pH conditions, which is important for some wines and ciders.”

One major disadvantage of commercially inoculated MLF is potential loss of red wine color intensity. Another issue with commercial, and sometimes native, MLF is the issue of MLF getting “stuck.” “Dryness is usually considered about 300 mg/L of malic acid,” said Gardner. “It’s essential to measure that to see if you’re at that point. It isn’t uncommon for MLF to stop, then after bottling restart.” Gardner said if this happens, there are several options: adjust the wine temperature, remove wine from lees because the lees can pull out nutrients essential for MLB to continue growth, add MLB nutrient or conduct analytical testing to find out what’s happening in the wine and ensure MLF is complete.

Gardner said MLF timing is a stylistic decision based on the winemaker’s preference or logistics. “There is no standard in terms of when you should inoculate for MLF,” said Gardner. “MLF timing can be co-inoculation or simultaneous with wine yeast – when you’re pitching yeast, you’re also pitching the malolactic bacteria.”

Advantages of co-inoculation include better bacteria acclimatization (because they prefer lower alcohol) and the introduction of rising alcohol over time. “There is some evidence that co-inoculation with wine yeast increases fruit flavor intensity,” said Gardner. “That’s been of interest to regions that struggle with intense fruit flavors in their wines. It helps minimize spoilage risk because there is no down time – you pitch both yeast and bacteria at the same time and never have a moment when something isn’t happening in the wine during which a spoilage microorganism can outcompete the proliferant fermentation.”

The practical aspect of co-inoculation is that it can decrease the total fermentation production time. Instead of a sequential situation where the wine goes through primary, then racking, then MLF (which can take a month to two, depending on yeast and processing technique), the process can be reduced to 10 to 14 days. However, MLB can be antagonistic toward yeast growth, which causes stuck primary fermentation. Gardner suggests winemakers contact their suppliers and request compatible yeast and bacteria strains.