'Malolactic Fermentation' (ML) describes a fermentation by malolactic bacterial culture
(leuconostoc oenos) that are able to convert malic acid from grapes into lactic acid. It occurs alongside, and in addition to regular fermentation, and can be desirable for two reasons: Reducing excess acidity. By converting the relatively harsh tasting malic acid into the softer lactic, ML softens the flavor of the wine. Adding complexity. In addition to converting the acid, malolactic bacteria can add a component of 'buttery' flavor (diacetyl), along with more complex flavors and aromas. Uncontrolled ML is very undesirable. The same bacteria responsible for reducing acidity are responsible for the production of sauerkraut, whose flavors and aromas are not what you would expect in a fine wine. In addition, if malolactic bacteria work in the presence of potassium sorbate (a preservative in kit wines and some commercial wines) it will produce geraniol, a compound that smells like a cross between ripe salmon and rotting geraniums.
Which wines should get malolactic fermentation? Generally, only wines made from grapes get malolactic treatment. There are two reasons for this, the first being that the bacteria needs a small amount of grape pulp and solids to get a foothold in the wine. Second, it is only really the top quality wines that merit malolactic treatment, and these are most often sold in the form of grapes. Wines made from concentrate are generally unsuitable for malolactic treatment. Not only are the acidity levels usually balanced toward the low side for early drinking, the levels of solids in concentrate are virtually nil. In addition, some concentrates may have small amounts of sorbate in them, causing geraniol problems.
The treatment criteria for wines are as follows: The wine should be one where complexity is desired over fruitiness. ML tends to emphasize fermentation aromas and flavors and reduce the fresh fruity flavors of a wine. If this doesn't suit your palate, you should reconsider treatment. The wine must have an alcohol level below 14%. Malolactic bacteria are not especially alcohol tolerant, and a wine with a higher alcohol content will not support it. The wine must have a pH of 3.3 or greater. If you don't know what the pH is, it will be necessary to have it tested before attempting malolactic inoculation. The wine must have some grape pulp or yeast sediment to promote the growth of the bacteria. If it is clear, MLF will be very difficult to start.
What form does the malolactic bacteria come in? There are two main types of malolactic culture available, refrigerated liquid culture, and freeze dried powder. They are both perfectly good choices for inoculating your grapes. They both have to be kept refrigerated at all times. They both come with instructions for use, which, while fine for inoculation in a laboratory setting, are somewhat confusing for the home user.
What are some of the dangers of malolactic fermentation? In addition to transforming malic acid into lactic, leuconostoc oenos also transforms citric acid into acetic acid, which is the acid that gives vinegar it's distinctive flavor. This usually isn't a problem, unless the must has been treated with citric acid. Also, because it reduces fruity flavors, there are few white wines that will benefit from ML, and some reds are best left fruity as well.
Because ML fermentation can cause the pH of the wine to rise, it may make it more susceptible to other forms of bacterial infection. You may need to adjust sulphite levels to compensate.
Partial ML, or ongoing malolactic fermentation in a wine that is to be bottled can cause the wine to carbonate (become fizzy), break the bottle with CO2 pressure, push out the cork, and spoil the flavor. You must be certain that ML is complete and/or arrested before bottling (see below).
How do I go about starting a malolactic fermentation? Assuming you have purchased good quality grapes, you have tested the acid and pH, and are confident that going through ML will produce a better wine for you, the first step is to purchase malolactic bacterial culture
. If you are only inoculating 23 litres of wine, you don't have to make a starter culture with either the freeze dried or liquid cultures. With the liquid culture package, when the grapes are almost finished fermentation (below) simply match the temperature of the package with the temperature of the grapes (it should be 20-25°Celsius) and add it. Within two to three weeks the ML will be well under way, and should be complete within two months.
With the freeze-dried culture it is necessary to make a sterile suspension and rehydrate the bacteria. For two grams of culture, prepare 25 ml of distilled water at 25-30° Celsius in a sanitised cup and sprinkle the culture on top. After 15 minutes, stir with a sanitised spoon and add to your fermenting must. Again it is important that the wort be below 5°Brix, so as not to slow the ML.
If you are inoculating more than 23 litres, you should purchase the culture about two weeks before your wine is nearly fermented dry, to allow time to make a starter culture. Judging the point when your wine will be two weeks away from dryness may seem difficult on first blush, but a good rule of thumb would be to purchase the culture the week before you receive your grapes.
A culture from liquid is prepared thus: obtain 2½ litres of fresh grape juice. Choose a grape juice with less than 15 PPM of SO2, and with less than 20°Brix of sugar content. If you can't find an available juice with these characteristics, it is possible to change the pH and sugar content of a less suitable juice by either diluting with water or adding a pH adjuster. Make sure you use only grape juice: apple juice or grape concentrate is not suitable.
Add about 1 gram of yeast energizer to the juice. This will help speed cell growth. Ensure the juice is between 20-25°C and inoculate with a package of malolactic friendly yeast (Epernay is ideal) and allow to sit for 2 days, and inoculate with liquid malolactic culture and maintain a temperature of 20-25°C.
After 12 days the culture will be ready. 2½ litres is sufficient for 120 litres of wine. If you have more than 120 litres, you can culture up the amount you need by simply adding your starter culture to larger amounts of suitable juice. You will need approximately ½ litre for every 23 litres of wine.
When is malolactic fermentation finished? If the wine is kept at relatively warm temperatures (above 20°C) the fermentation will be complete within two months.
Once you are satisfied that your ML is finished, you can stabilize your wine by adding sulphites to the level of approximately 50 PPM. This will prevent the re-growth of any leuconostoc bacteria, and will prevent oxidation. It is a good idea to test the pH and acidity of your wine at this point as well. Because ML can cause the pH to rise, you may need to adjust it.