Testing for MLF
December 1, 2010
Yesterday, I talked about malolatic fermentation (MLF). I mentioned that MLF that happens in a bottle will cause the wine to taste like cured meats, so how does one know when an malolatic fermentation is done to know when it is safe to bottle?
The answer is to perform a malolatic chromatography test to see if all of the malic acid has been converted to lactic acid. That is to say, all of the malic acid from the wine is gone, and there is a strong presence of lactic acid.
Let me back up and explain chromatography, which can be experienced by doing a child’s experiment for color. Take a coffee filter and cut it into strips. Take a marker, preferably not a prime color, and make a line about 2” from the bottom on the strip. Black, purple, and green work well. Place the tip of the coffee filter in water, and watch the water rise up the coffee filter. When it hits the ink, the ink will begin to travel with the water, but because the ink is a combination of colors, it will begin to separate into the different colors. The colors with more attraction to the paper instead of the water will separate out first (eHow).
Testing for acids in wine is pretty much the same process, except there are a few more controls in place. First, with a pencil and trying not to touch the special paper too much with your hands, a line is drawn across the bottom of the page with evenly spaced X on the line to represent sample points, and then at the top of the page, label what each sample will be placed on the X. The first three X need to be control points, where laboratory samples of tartaric, malic, and lactic acid will be dropped, and the remaining Xs will be wine samples. The drops are usually placed using different capillary pipetts, and the goal is to get as much sample in the smallest area possible so that they do not bleed into each other. One technique is to place a drop of one sample, move onto the next samples, and then come back to the first sample after it has had a chance to partially dry. From there, being careful not to touch the paper too much, the paper is rolled and stapled into a tube.
Instead of using water for the test, there is a special chemical solvent used, which has noxious fumes. Preferably working outside or in a well ventilated area, a small amount of the solvent is placed in a large jar, in which the paper with the samples is then placed, and then covered with a lid. The solvent will begin to climb the paper much like the water climbed the coffee filter in the child’s experiment. Instead of carrying ink, it will carry acids. Depending on the paper, this test can take 8-12 hours, and the solvent should not be allowed to reach all the way to the top, or it could “spill over,” carrying the acids back to the bottom. The solvent is normally reusable unless this happens, as it is then contaminated. When the solvent reaches the top, the paper is pulled out of the jar and allowed to dry.
Tartaric acid is usually the first to fall out, remaining towards the bottom. Malic acids will be next, and lactic acid will be last, almost at the top of the paper. The case above has all the different spots labeled, though they were a bit sloppy and did not make control samples of tartaric, malic, and lactic acids. All of the samples have tartaric at the bottom, which is to be expected of grape wines. The third and fifth sample are missing malic acid spots, indicating they are completely done with MLF as there is no more malic acid present. The remaining samples still have malic acid spots, so they are not done with MLF, though perhaps the first one is nearing completion.
Here is another example that used four control points, including citric acid, though the control points got a bit of an echo going on. Of the four wine samples, only the second one appears to really have any malic acid, though the other ones are not as empty in the malic area as I would like. Still, I would consider the other one finished with MLF.
One flaw with this test is that there is no way to measure the quantity of malic acid left in the wine sample. It can only indicate a presence of a given acid. The papers can be kept in gallon sized zip lock bags for long term storage, allowing you to compare tests to see how MLF is progressing, or to discover that it is stuck.
- Acid Control in Winemaking Part Two: Reducing Acid with Malolactic Fermentation
- Indian Creek Winery Malolactic Fermentation
- Malolactic Chromatography Testing