Introduction to Malo-Lactic Cultures
A malo-lactic fermentation (MLF) is a bacterial fermentation by Leuconostoc oenos or Lactobacillus spp, which converts malic acid into lactic acid and carbon dioxide.
Wines which have gone through MLF will typically be 0.2% to 0.4% lower in total acidity, 0.2 to 0.3 higher in pH and will be softer in character. Fruitiness will be reduced but the wine will be more complex.
M-L fermentation is often very desirable in dry reds and in such dry white wines as Chardonnay and Viognier. Because it does reduce fruitiness, it is almost never desirable for 'fruity' Germanic style wines such as Riesling or Gewurztraminer nor in any sweet wine.
IF a wine is excessively high in acid, an MLF may be really good way to reduce that acid to a better level. M-L fermentation can produce a wine that has more complex vinous aromas and can improve biological stability in the wine. If an MLF is encouraged, do not add potassium sorbate or potassium metabisulfite until the MLF is complete.
You may still want to have an MLF occur with a low acid red wine to get complexity, however, an acid adjustment upward may be needed. M-L fermentation can occur spontaneously, but not always by desirable bacterial strains. In these cases, off odors are likely to develop, especially in wines with higher pH.
MLF is more likely to occur if the wine is stored in an oak container (especially one in which MLF's had occured in prior batches). M-L fermentation is inhibited when free SO2 levels are above 20 ppm, the temperature is below 60° F, fumaric acid or lactozyme (lysozyme) has been added, or the pH is below 3.20.
There are differing opinions as to when is the ideal time to inoculate wine with M-L bacteria. Some say toward the end or at the end of the yeast fermentation. There seems to be a growing trend towards co-inoculation which means doing the yeast fermetnattion and the MLF at about the same time.
The M-L bacteria may be added just a few days after the yeast was added. The addition of an organic nutrient especially for malolactic bacteria (such as Micro Essentials) will improve the growth conditions for the bacteria and will encourage a faster, more successful MLF. The nutrient use is certainly good but not always required. It is very difficult at best to see a MLF occur.
In addition to the other parameters mentioned, the MLF should never exceed 77° F as that can kill the bacteria. Optimum temperature is 68° to 72° F. If all conditions are optimal, an MLF should take about 4 weeks to complete. It may not be wise to try to stretch a culture to grow to do larger gallonage than designed because the bacteria is slow to grow.