In simple layperson’s terms MLF in wine in the transformation of the ‘tart’ malic acid bacteria (similar to that in a green apple) into ‘softer’ lactic acid bacteria.
This transformation has the result of reducing the primary fruit-forward profile of a wine and allowing more complex textures and flavors to emerge. Depending on the style of the finished wine, the winemaker will want to encourage or discourage MLF.
In most white wine grapes the primary fruit profile is welcomed in the finished wine. These grapes are known as ‘aromatic white grape varieties’ and examples are Riesling, Gewvürtraminer, Viognier, Pinot Gris. Therefore the winemaker will not allow MLF to happen, by adding sulphur after the alcoholic fermentation in order to kill and trap the malic acid bacteria. That is why wines with these grape varieties are always dominated by their fruit-forward profile.
Usually, Sauvignon Blanc is included in this group. Think of the village of Sancerre in the Loire Valley, the home of French Sauvignon Blanc. The wines from Sancerre are not allowed to go through MLF and therefore the green fruits, citrus, and herbaceous notes are dominant in the profile.
The complexity and character of White Burgundy , generally using the Chardonnay grape, means it is generally standard practice to allow the wines to undergo MLF. Diacetyl is one byproduct and in small quantities this imparts notes of butter, popcorn, nuts etc. It is important to note that this does not mean that the fruit profile is diminished. In Burgundy expression of ‘Terroir’ and balance is the key goal so the winemaker will not allow any of these MLF elements to dominate the wines, but they are present and detectable, increasing in intensity with age.
Burgundy wine is governed by a certain philosophy as well as a complex set of rules but Burgundy is also full of exceptions !!
One such place is the Village of Saint Bris. Here is the only place in Burgundy allowed to make wine using Sauvignon Blanc. Actually if you look on a map, the village of Sancerre is not too far away from Saint Bris. Therefore you have a dichotomy The region of Burgundy, famed for its complexity which is achieved partly through the use of MLF and Sauvignon Blanc, a grape that sees its best expressions through a simpler winemaking process which forbids MLF.