Part 2: Bacterial Fermentation

(Readers please note: This is the second in a series of wine-related topics that have been selected in response to questions from home winemakers.)


Home winemaker’s question or inquiry:I’ve heard and read that something called malolactic fermentation should be considered in the red wines that I make. I understand that when it’s done properly, it is supposed to improve the ’mouthfeel’ and overall quality of my finished wine. If all this is true, I’d like to try doing it to my red wines. However, I am uncertain as to how I should proceed. Help!”


Discussion: An excellent question. To begin, many home winemakers will say that they have heard or read something about malolactic (mah-low-lack-tick) fermentation, but don’t really know a lot about the process. At this point in the learning curve it’s a good place to have a ‘learning break’ on this topic.

While not trying to make a biochemist out of anyone, what follows is a bit of information that all home winemakers should have as part of their overall winemaking knowledge base.


First, there are different kinds of fermentations, and malolactic and alcoholic fermentations are two examples. Malolactic fermentation (hereafter referred to as MLF) is a kind of fermentation that is performed by bacteria, not yeast. In this case, the goal of this exercise is to reduce the amount of a naturally-occurring acid called malic acid, which is found in fresh wine grapes, fresh wine grape juice and certain fruits, such as apples. Malic acid is a sharp- tasting acid that many consumers find objectionable when present in excessive amounts. That said, if this is the case, then we should try to reduce the amount of this acid if we want people to drink (and enjoy) our wine. Here’s where MLF comes in handy. When properly performed, MLF works by converting the harsh-tasting malic acid into a softer, more agreeable tasting acid called lactic acid. For example, lactic acid is often found in certain dairy products such as milk. In the case of our wine, the amount of harsh malic acid is reduced by the bacteria while converting it into the more pleasant-tasting lactic acid, thus resulting in a softer, fuller, more pleasant-tasting wine.


MLF also has a beneficial side effect. During the conversion process, the bacteria produce small amounts of a compound called di-acetyl (die-ah-see-tel) which has a butter-like aroma and creamy mouth feel, both very desirable chacteristics in red wines and also a few whites like chardonnay. (FYI – di-acetyl is the primary compound responsible for the “buttery” aroma in both naturally- occurring butter and artificial butter-like products.)

The ‘how to’ – - Steps for doing MLF on our wine

(Author’s note: The following guidelines are just that: guidelines. The steps involved usually depend on which expert or author you are talking to. However, please note that these suggestions follow widely used and accepted winemaking practices that usually results in an improved, quality product.)

MLF for improved taste, acidity reduction, improved balance as well as stability in the presence of spoilage bacteria can be encouraged in several ways1:

  1. Use of low levels of SO2 at crush (not more than 20 – 30 mg/L), and during MLF. Also, remember that if the SO2 levels are too high the bacteria will not work.
  2. Higher pH (>3.2) improves MLF. While important, many home winemakers don’t check pH routinely. Hence another potential problem if the pH dips below 3.2-3.12.
  3. Temperatures above 64 degrees Fahrenheit and below 77 degrees Fahrenheit are best for MLF. Temperatures at or above 86 degrees usually result in cell death while temperatures below 64 degrees will usually inhibit growth.
  4. Adding MLF-specific nutrients at the start of MLF is always recommended.
  5. Inoculation with healthy population of prepared malolactic bacteria. With advanced technology available today, newer preparations of ready-to-use bacteria make this step easy. Sizes available include individual packets of freeze-dried bacteria, enough for 6 gallons and larger packets that will safely inoculate 66 gallons. Costs for both sizes are modest and inoculation consists of directly adding the individual packet to the wine. Once the packet is opened, it should be used completely as soon as possible. While it’s true that ML bacteria work best in a low to no-oxygen environment, prolonged exposure to oxygen can be fatal to the little critters. Hence, keep this in mind when preparing to use ML Bacteria. Unopened packets are best kept in the refrigerator.
  6. Answers to questions about the length of time required to complete MLF come up routinely. The answer is that it may take anywhere from 4 weeks to 2 months, and the only way to know for sure is to perform a test on our wine called a chromatograph. Here, a small sample of our wine is taken and placed on a special sheet of filter paper along with several known standards for comparison. The paper is then placed in a developing chamber with a special developer solvent and allowed to stand for several hours, which is then followed by a drying period. The dried test paper is then held up to a strong light. If the test was correctly performed, the results are clearly visible on the dried paper, telling us if the MLF has been completed or not.
  7. While MLF can be performed just about any time prior to bottling, it is more likely to be successful if it doesn’t have to fight with the yeast trying to perform alcoholic fermentation at the same time. Many winemakers take an easier route and add the bacteria just as the new wine is going into secondary fermentation, where the yeast have pretty much done the bulk of their work and ML bacteria can then work at its own pace.

When MLF is not desired3

There are times when MLF is not desirable. These reasons include:

  1. Retaining fruitiness or aromatics that are beneficial to the character of our wine.
  2. Preserving balance between acids, aromatics, color, taste and other desirable characteristics.
  3. Working with kit wines. In this case the acid ratio has been properly adjusted and stabilized by the manufacturer. Doing MLF here might ruin our wine by destroying the balance that was achieved at the factory, resulting in a wine with a flabby, washed out taste.

Preserving our wine and discouraging Malolactic bacteria (MLB) growth after the wine has been bottled can be done by:

  1. Using a sufficient amount of SO2 at crush and throughout the winemaking process to inhibit the growth of MLB.
  2. Not inoculating the wine with cultured MLB.
  3. Maintaining a low pH to discourage the growth of MLB
  4. Keeping the wine at or below 59 degrees Fahrenheit, which will discourage the growth of any MLB. Remember however, that MLB cells will remain viable.
  5. Use of a bacterial cell wall inhibitor such as lysozyme to prevent the growth of MLB.



  1. MLB will inhabit (permanently) the oak wood or tartrate layers found in cooperage. Any wine not processed through MLF or not intended to undergo MLF cannot be safely made or stored in any barrel that previously contained MLF wine.
  2. Unless prevented, MLF can occur at any time, including after the wine has been bottled. Such an occurrence can be disastrous, including bottle explosions due to the pressure of carbon dioxide produced by fermenting MLB, sedimentation, effervescence and off flavors and odors.
  3. The use of Potassium sorbate has no effect on MLB and in fact should be discouraged. This is due to the fact that if fermentation occurs in the bottle after the addition of sorbate, the result can again be disastrous due to the formation of a foul smelling compound associated with rotting geraniums which cannot be reversed.




  1. Trela, Brent. 2005. Malolactic fermentation & monitoring. University of California, Davis press, Topic 6.2.2. Quality Control and Analysis in Winemaking. 13-24.
  2. Bousbouras, G.E, and R.E. Kunkee. 1971. Effect of pH on malolactic fermentation in wine. Am.J.Enol. Vitic. 22:121-126
  3. Davis, C.R., Widbowo,R.,Eschenbruch., T.H. Lee, and G.H.Fleet. 1985. Practical implications of malolactic fermentation in wine. J. Appl. Bacteriol. 63, 513-521.