Malolactic Fermentation—An introduction to MLF

By David A. Gerling

Malolactic fermentation (MLF) has become a common practice in winemaking in recent years, though it likely occurred naturally throughout the history of winemaking. Inoculation with malolactic bacteria (MLB) started commercially in the last two to three decades and is now done on almost all premium red wines, and on certain white wines such as Chardonnay. Home winemakers too can change the flavor profile of their reds and Chardonnays to make them mellower and less fruit forward by introducing MLF.Glass_of_red_wine_from_top_Photoxpress.jpg

MLF is the transformation of malic acid (green apple taste) into the softer-tasting lactic acid plus carbon dioxide gas, by either cultured or wild MLB. Lactic acid is so named because it is also found in milk products.

MLB occur naturally in the air and on the grapes. The good MLB are from the Leuconostoc genus, the most common one being L. oenos. This genus is also named Oenococcus in some of the newer books. Bad MLB cause off flavors in wine or spoil the wine altogether. Bad MLB include bacteria from the Lactobacillus genus, which give the wine a spoiled milk taste, and bacteria from the Pediococcus genus, which give the wine an oily texture, a buttery or cheesy smell (from the large quantity of diacetyl produced), acetic acid and a mousy taint.

Since two of the three MLB are undesirable, winemakers usually kill all of the MLB by sulfiting at crushing, adding 45–50 ppm free sulfur for red grapes and 70–80 ppm for white grapes. Then you can inoculate after alcohol fermentation with a pure strain of the desired MLB.

Potassium sorbate should not be used if an MLF has been done, since the MLB use the sorbate as a nutrient and would cause a geranium flavor and smell.

Getting MLF started

MLF can be started as soon as alcoholic fermentation is completed. This is the most widely used method as the two fermentations are kept separate with no competition for nutrients. That also leaves no unfermented sugar for the MLB to metabolize, reducing the risk of volatile acids (VA), the main one being acetic acid (vinegar).

Recommended freeze-dried MLB cultures to use include Lalvin 31, Enoferm Alpha, Lalvin Bacchus O. oenes, and Viniflora CH 35. Two direct-inoculation liquid cultures are Wyeast Laboratories Vintner’s Choice and White Labs WLP675.

Presque Isle sells the freeze-dried Lalvin Bacchus O. oenes and Viniflora CH 35. Vinquiry has freeze-dried and liquid MLB cultures.

Here are the factors favorable for MLF:

  1. Select a wine yeast strain that is compatible with MLF. Some strains of Saccharomyces cerevisiae work well, while strains of Saccharomyces bayanus have been known to suppress MLF because they tend to produce more sulfur dioxide than other yeast.
  2. The pH should be higher than the prescribed minimum pH. Oenococcus oenus can live at a pH of 3.0–3.3, but would do better at a higher pH. (Lactobacillus and Pediococcus bacterial growth is almost impossible below Ph 3.3, but at pH 3.6 they grow well.)
  3. Make sure the wine’s free SO2 is less than the maximum prescribed by the culture’s manufacturer. Sulfur dioxide is a very effective inhibition agent against MLB growth, even in the bound form.
  4. The temperature should be 68–70 degrees F. Below 59 degrees F the growth of MLB is very near zero.
  5. Feed the MLB with plenty of nutrients such as Lallemand’s Opti-Malo or Presque Isle’s MicroEssentials Oenos.
  6. Keep oxygen away from the wine. Most MLB grow and survive better with very little or no oxygen.
  7. Get a full set of manufactures’ instructions when you buy an MLB culture and strictly follow them.

MLF generates tiny bubbles of carbon dioxide gas. Shake the carboy; a rush of small bubbles is a good sign of MLF. When the bubbles slow, then stop, MLF is usually done.

However, paper chromatography is the best way to test for MLF completion (see photos). Kits, such as the ones sold by Presque Isle Wine Cellars, have all of the materials and chemicals you need to do a chromatograph. Detailed instructions are included. Chromatography involves the use of dangerous chemicals, so always conduct the test in a well-ventilated area (the solvent has a very powerful and irritating smell) and follow the instructions very carefully.

chromatograph_before.jpgThree wine samples (dark spots at right) plus malic and lactic acids were just placed on the paper, which is now ready to be put in the solvent. 

 

 

chromatograph_after.jpgA chromatograph after being put in the solvent, removed and allowed to dry. This one had two wine samples (dark spots at right) and shows that the malic acid has been changed into lactic acid

 

 

After MLF is complete, the wine can be racked and sulfited for a free SO2 of 40–50 ppm. Also check the TA and pH, as explained below.

Controlling TA in a wine with MLF

During MLF, malic acid will be converted into roughly equal proportions of lactic acid and carbon dioxide gas, resulting in a TA decrease equal to one-half the malic acid concentration. The reduction in TA will be in the range of 1–3 g/l (.1%–.3%) and the pH will increase .1–.2 units. So you will want the TA to start .1%–.3% higher and the pH .1–.2 units lower than you want to end up with. If you need to increase the TA or lower the pH before MLF, do so before the start of alcoholic fermentation, using tartaric acid.

After MLF is complete, if the pH and TA are in the range you want (pH of 3.3–3.5 and TA of .6%–.7%) you can cold stabilize the wine. But if the pH is too high and the TA too low, you can add tartaric acid before cold stabilizing. Tartaric acid additions will affect the pH more than the TA, so the pH will be lowered more than the TA is raised.

After cold stabilization the pH will go down if the pH was 3.7 or lower at the start of cold stabilization. It will go up if the starting pH was above 3.7. The TA will also come down some.

You will want your red wines’ final pH to be 3.5 or lower for wine stability, and the TA to be .65%–.75%. Taste your wines and adjust them to the way you like them.

If you don’t want MLF in your wine

MLF is not desirable for all wines, so here’s what to do if you don’t want it:

  1. Add free sulfur dioxide (SO2) beyond the maximum before the start of alcoholic fermentation, usually 45–50 ppm for reds and 70–80 ppm for whites.
  2. Keep the temperature below 60 degrees F after alcoholic fermentation.
  3. Keep the pH below 3.5.
  4. Add lysozyme, a naturally occurring enzyme that breaks down the cell walls of the MLB, killing them.
  5. Filter your wine through a .45u or smaller filter, which will remove the MLB and most of the yeast.

For more information

If you’re ready to try something new in your winemaking and you think your reds and Chardonnays could benefit from mellowing their taste, why not give MLF a try? This article was written with information from these books in my home winemaking library, which are available for club members to borrow:

Techniques in Home Winemaking by Daniel Pambianchi

Home Winemaking Step by Step by Jon Iverson

Concepts in Wine Chemistry by Dr. Yair Margalit, Ph.D.

Concepts in Wine Technology by Dr. Yair Margalit, Ph.D.

Photo credits: Wine in glass © photoxpress.com. All other photos by David A. Gerling.

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