turning juice to wine
There are 2 predominant types of acid in wine grapes (both of relatively equal strength), tartaric and malic. Tartaric is completely biologically stable, both on and off the vine. As grapes ripen the amount of TTA stays the same (though the concentration will go down and up with berry swell and shrivel). Malic, on the other hand is respired during the night cycle of photosynthesis, and declines as the berries ripen late in the season. The cooler the area for a given grape, the higher the malic acid will be in the harvested berries. In a region like ours, malate will account for roughly 1/2 of the total acidity in the fruit.
Malolactic fermentation is carried out by various types of malic acid bacteria, in our case a strain we introduce in the winery to ensure completion of the fermentation. There are 3 important outcomes of this fermentation. First, malic acid is converted to lactic acid (hence the name). The significance of this is that malic acid has 2 acid groups on the molecule (like tartaric), and lactic has only one, hence you are losing 1/2 the acidity due to malic acid in the wine, or about 1/4 of the total. In very cool places like Burgundy and Marin, this acid reduction is important to the final balance of the wine. In hotter places like Napa, the acidity is much lower to begin with, so ML might produce a flat, flabby wine. Our acidity is often higher after ML than a wine from a hotter place is before it.
The second significance of MLF is that as a byproduct of their fermentation the ML bacteria produce a compound called diacetyl. Diacetyl smells like butter, so the buttery aroma of some wines is the result of malolactic fermentation. Different bacteria strains will produce varying amounts of the compound, and different wines and techniques will show it differently, so this is an area the winemaker crafts according to taste and the particular fruit.
The third result of MLF is microbial stability in the wine. Once primary fermentation yeast has used up all the sugar in a wine, and the ML bacteria have completed their work, the levels of usable nutrients are very low in the wine so that the wine is far more stable than one that hasn't undergone MLF. If one doesn't undergo MLF the wine must be membrane filtered for microbial stability. If you do, and many other factors are also met, the wine can then be bottled unfiltered, which if often our preference.