Malolactic Fermentation

This describes the malolactic bacteria (leuconostoc oenos) created process by which the harsh malic acid in grapes is converted into softer lactic acid and carbonic gas during grape fermentation. This process is part of the winemaker's arsenal of options to make wine better.

Why

Well, it really is all about creating a better tasting wine. The primary reason for using malolactic fermentation is to reduce malic acidic levels in wine. Reducing excess malic acidity, which has a stronger harsh taste, can soften wine's flavor. This process effectively reduces the wine's overall natural acidity, and therefore removes certain undesirable flavor components.
Here is another way to look at this. Malic was derived from malum, a latin term which describes the tartness of green apples; While lactic, derived from lactis, describes milk. Again, the transformation from a tart acid to a milky acid, directly affecting its overall taste structure.
However, there is also another taste dimension provided. Using just the right amount of malolactic bacteria can add special complexity components of flavor and aroma to the wine. It also produces a chemical called diacetyl, which expresses itself as the buttery aroma and flavor many chardonnay lovers treasure.

Useful for Home Winemaking

Dealing with this process is advisable for only very seasoned and experienced winemakers, even then only with very special care and planning. This bacteria culture is typically only used on high quality wines, of course, made with grapes, and not concentrate. This is a laboratorial focused process and can be purchased in both freeze-dried powder and liquid versions.

from: http://www.winedefinitions.com/