Secondary Fermentation (Malolactic Fermentation)
Secondary fermentation is otherwise known as Malolactic Fermentation.
Malolactic fermentation or 'conversion' is accomplished by lactic acid bacteria (such as Oenococcus oeni), which consume malic acid to liberate energy. This can occur naturally. However, in commercial wine making, malolactic conversion typically is initiated by an inoculation of desirable bacteria. This prevents undesirable bacterial strains from producing off-flavors. Conversely, commercial winemakers actively prevent malolactic conversion when it is not desired, to prevent accidental initiation and maintain a tarter, more acidic profile in the finished wine. In winemaking, malolactic conversion is generally encouraged in many red wines and some white wines, particularly those that are aged in oak. A prominent example of this is the prevalence of malolactic fermentation in chardonnays. Unoaked white wines and fruit wines generally should not undergo malolactic conversion.
Chemically, malolactic fermentation is a decarboxylation, which means that carbon dioxide is liberated in the process.
Sometimes malolactic conversion can occur unintentionally after the wine is bottled. This is almost always a fault, and the result is a slightly carbonated wine that typically tastes bad. The carbonation from this type of change should not be confused with benign carbonation, known as spritz.
Because it consumes malic acid, which is present at the time the grapes are crushed, malolactic conversion can take place at any time during or after alcoholic fermentation. A wine undergoing malolactic conversion will be cloudy due to the presence of bacteria, and may have the curious smell of buttered popcorn, due to the production of diacetyl.
Red Wines/White Wines to Undergo Malolactic Fermentation
In both the commercial and amature world of wine making you are far better off to control Secondary or Malolactic Fermentation if you can. Because Malolactic Fermentation can occur at any stage during the wine making process it is advisable to start it either at the same time as primary fermentation or immediately after it. The main reason to do this is because after your wine has gone through both primary and secondary fermentation you will want to make sure it is well protected with sulphites as soon as possible. As long as your wine is not protected by a safe level of sulphites it is suseptable to contamination. I will explain the addition of sulphites in more detail in the next section.
There are a few strains of lactic acid bacteria to choose from when you are ready to innoculate your wine to begin malolactic fermenation. One of the best known strains is called Oenococcus oeni and is used by most commercial wineries, however, there are some new strains on the market. Contact one of your local wine making suppliers to find out what your options are. There are several very good suppliers listed on this website that offer these wine making supplies.
Not only will you need a lactic acid bacteria, but you are recommended to also add in another additive known as Opti-Malo (there are other additives such as this one but this is the best known and trusted product). The purpose of Opti-Malo is to help kick start the lactic acid bacteria and feed it to help it carry out it's job - convert malic acid to lactic acid.
Once primary fermentation has slowed down or is nearly complete you should 'rack' (otherwise known as syphon) your wine into a carboy, demijon or another glass or stainless steel container that can be sealed off from oxygen exposure. If you are using a carboy or demijon this is carried out by using a bung and airlock. Once you have racked your wine into a carboy (being as careful as possible not to transfer over the sediment at the bottom of the primary fermentation vessel) After the first racking of the wine you will notice in a day or two that there is quite a bit of sediment now at the bottom of the carboy. At this point you should rack your wine again into a clean, sanitized carboy (remember to avoid splashing and minimize oxygen exposure as much as you can). This is referred to as racking off the 'gross lees'. This will help to eliminate a lot of the stringent and 'yeast' taste from your wine later.
At this point now that you have removed quite a bit of the sediment from the wine you can add the Opti-Malo. Make sure this is mixed in well and let it sit for approximately 12 hours. After the waiting period add in the lactic acid bacteria (hydrated for best results). Gently stir into the wine. Put the bung and airlock on your carboy and now wait.
In the next few days to a week you should notice very small pin sized bubbles forming around the ring of the top of the wine. This is an indication that secondary fermenation is underway. Keep an eye on these small bubbles. Secondary fermentation can take weeks or even up to months to complete. Once you notice that the small bubbles have disappeared it is now time to move onto the next step.
Avoiding Malolactic Fermentation
Avoiding Malolactic Fermentation can be a tricky endeavour because as stated above malolactic fermentation can occuring at any stage during the production of alcohol. But don't worry - there is a secret to help the amature wine maker to avoid this dreaded fault. That secret is called Lysozyme.
Discovered in the 1920s and used for decades in the pharmaceutical, dairy and cheese industries, this natural protein has come into increasing use in the wine industry in the past decade. Lysozyme now available in home-scale quantities, giving amateur winemakers a means of reducing sulfite use while still protecting the health of their wines.(1)
Lysozyme is an enzyme found in the protective fluids (tears, saliva and mucus) of most animals. Commercial lysozyme is isolated from egg whites. Lysozyme kills certain types of bacteria by attacking their cell walls. It’s effective against most eubacteria, but not against archaebacteria or eukaryotes. (Eubacteria are “ordinary” bacteria; archaebacteria are thermal-vent bacteria and their relatives. Eukaryotes include animals, plants, protists and fungi, including yeast.)(1)
Lysozyme works against a range of lactic-acid bacteria that can affect wine stability, including Oenococcus, Pediococcus and Lactobacillus, all of which produce off-odors in wine. Lysozyme also kills Leuconostoc, the bacteria responsible for transforming malic acid into lactic acid during malolactic fermentation. It does not interfere with yeast.(1)
Once primary fermentation is complete, malolactic activity can be blocked with an addition of 250–500 ppm of lysozyme. This dosage will also stop a malolactic fermentation in progress, resulting in a partial malolactic conversion. For wines that received a small lysozyme treatment at the crusher, inoculation with a malolactic starter should work normally after the alcoholic fermentation has run to completion.(1)
Because lysozyme is a protein, it can contribute to protein haze in some wines when they are stored at warmer temperatures. Its killing action can also result in some precipitation of the lees. Precipitation, if it does appear, normally shows up in a few days and the wine can simply be racked for clarification. Be cautious and allow at least a couple weeks between a final lysozyme addition and bottling.(1)
1. (T. Patterson, WineMaker Magazine, 2002)