November 30, 2010
Yesterday, I talked about different acids in wines and cider, such as malic acid and lactic acid. It is actually possible, using bacteria, to convert malic acid to lactic acid. The process is called malolatic fermentation (MLF), and the result is a smoother, less sharp tasting wine with a higher pH, sometimes tasting mildly of butter.
MLF is a secondary fermentation that happens with wine and cider. However, it is caused by bacteria instead of yeast, and it does not make alcohol. Instead, the bacteria eat malic acid and convert it to lactic acid and carbon dioxide. Lactic acid bacteria eat or break the bonds with one of the two acid components that malic acid has, AKA the carboxylic acid COOH. The COO is released as CO2, and the extra H is redistributed into the remaining molecule. The result is a milder lactic molecule with only one set of COOH acid compounds, raising the pH. In turn, it tastes less sharp and smoother than the previous product.
There are several different bacteria that will cause MLF, including prefered Oenococcus oeni, though others can also cause MLF, but they can also create other infections as well.
MLF processes will happen naturally unless sulfites are regularly used to prevent it, or if the pH is too low. If a winemaker harvests in the fall and starts fermentation, the MLF process usually begins in the spring when things warm up again. However, in a controlled environment, lactic acid bacteria are sometimes introduced to start the winemaking process sooner to ensure that it happens, that there is no alcohol to kill the bacteria, and to create a more shelf stable wine.
The results of MLF in wine is that it tastes a little smoother with less bite than the original wine because there is now less acid. However, because lactic acid is associated with dairy products, the increased lactic acid can give a dairy like taste. Also, once a wine goes though MLF, the yeast reproduction inhibitor potassium sorbate cannot be used to stabilize the wine, or it will cause the wine to have an unpleasant geranium odor. If MLF happens after bottling, the lactic acid can cause the wine to taste like cured meats.
Wine makers always do MLF on red wines, partly because red wines are also stored in oak barrels. The oak barrels start to have a little bit of lactic acid bacteria living in the wood after being exposed to them in the wine, so the next batch of wine will become exposed to the bacteria from the wood. Also, red wines can still create a puckering sensation with a higher pH due to having more tannins from being allowed to ferment with the grape skins. White wines, because they are more delicate and can be overpowered in taste, usually do not go though MLF or oak aging, or in the case of Reisling, it tastes more refreshing from the sharper malic acid. The exception to not doing MLF on white wine is Chardonnay, which are sometimes described as tasting buttery as a result of the MLF. It is becoming easier to find Chardonnay aged in stainless steel without MLF, which are described as being more green apple flavored. Must be the malic acid.
Cider makers have the option of doing MLF. It can enhance the flavor, especially if it was too acidic, and it can create a more stable product from having to worry if MLF will happen. However, the MLF bacteria and the conditions in which MLF likes to occur – high pH and low sulfites – is the same conditions in which some cider sicknesses such as rope. Andrew Lea describes rope like this, “When the cider is poured, it assumes the consistency of a light oil or of a slimy ropy texture like raw egg-white, although the flavour is not much affected. This is due to the slow growth of certain forms of lactic-acid bacteria which produce polysaccharide gels (similar to those formed by related bacteria during yoghurt-making and which provide its texture).” When it comes to making perry, MLF is usually avoided because the citric acid in the pears can be converted by the lactic acid bacteria to acetic acid, the acid of vinegar.
MLF can not be recommended in these cases:
- In general, fruit wines and most white wines should not go through MLF unless put in an oak barrel.
- It is not recommended for sweet wines, especially if potassium sorbate is to be used.
- Because MLF converts a strong malic acid to a weak lactic acid, the pH raises. This can also allow bacteria to start growing.
- The sulfite level needed to do MLF is low. This can allow bacteria to start growing, and low sulfite level would allow some oxidization.
- It increases the cost to make wine as it requires more time to make the wine.
So really, sometimes MLF really enhances a wine, and sometimes it can make a wine spoil.
- Purdue University: The Joy of Malolactic Fermentation
- EnologyAccess.org – The Malolactic Fermentation
- Buttery bacteria: Malolactic fermentation and you
- An Introduction to Malolatic Fermentation in Wine
- Pictures of malic acid molecules and lactic acid molecules