Malolactic Fermentation
Have you ever wondered why some wines have a crisp, almost tart acidity to them (think Pinot Gris or a dry Riesling) while others have a more mellow creaminess feel (Chardonnay). This is due to a process called malolactic fermentation. To many people, fermentation means the conversion of sugar to alcohol. In winemaking we call this the primary fermentation. Once the primary fermentation is finished the wine will undergo a secondary fermentation called malolactic fermentation.
During malolactic fermentation the tart, crisp, almost harsh malic acid that is naturally present in grapes is converted to a softer, richer, and often buttery acid called lactic acid. On a side note malic acid is the primary acid found in green apples while lactic acid is the primary acid found in milk. If you compare the taste of a tart green apple to creamy milk you kind of get a sense of how important malolactic fermentation is to the flavor of wine.
Malolactic fermentation is not desired on all wines though. What would Pinot Gris be like without it’s crispness? Certainly not the summer wine that it is. Conversely, a red wine such as Merlot or Pinot Noir would not have the richness, smoothness, or complexity that make these wines desirable.
Malolactic fermentation can last for months if it is cold (like Idaho) and is very slow and a very non-vigorous fermentation. Often times it is not even noticable. Because of this the only reliable way to monitor the fermentation is through chromatography. This is a process of separating the two different acids on a sheet of paper. Basically a small amount of the sample wine is placed on the bottom of a sheet of chromatography paper. The paper is placed in a special solvent and as the paper becomes soaked by the solvent, the acids are separated. When the paper dries there is a yellow dot if the acid is present. This is best shown through the photos below:
The sample wine must be put on the paper in a very precise manner. By just placing a drop in one location, allowing it to dry, placing another drop in the same location and repeating 7-10 times the wine becomes concentrated in a very small “dot”. This is important later on so there is enough contrast that the acid can be seen on the paper.
Here is the chromatography paper after the sample wine has been placed. It is hard to see in this photo but the samples from left to right are: Malic Acid (for reference), Lactic Acid (for reference), 2008 Cabernet, 2008 Merlot, 2008 Petit Verdot, 2008 Malbec, and another 2008 Cab.
Here the chromatography paper with the wine samples is soaking up the solvent. After about 12 hours the solvent will have climbed to the top of the paper, separating the acids on it’s way.
12 hours later: The solvent has soaked the entire paper, time to take it out and let it dry.
This is the chromatography paper after it dries…and this is where it all starts to make sense. The yellow dots represent the different acids. The left most sample is the Malic acid control. So any dots on this page that are at the same height indicate that the wine still has Malic acid and is still going through fermentation. The next sample to the right shows a very faint dot at the top. This is the Lactic acid control. The next five samples are various wines. You can see that they all show the Lactic acid dot, indicating that malolactic fermentation is occurring. All but the middle sample, the 2007 Syrah that we are about to bottle, show that malolactic fermentation is still occurring, although probably very close to finishing in the first two samples due to the faintness of the dots.