As the primary alcoholic fermentation nears completion, most winemakers add a small amount of Oenococcus oeni bacteria to red wines and some whites, like Chardonnay, to naturally convert the malic acid to lactic acid. Carbon dioxide and other trace metabolites are produced as well. We usually refer to the whole process simply as “ML”.
O. oeni is naturally present on the skins of grapes, as are several other strains of bacteria (not all of them good). Other common examples of lactic acid bacteria are Pediococcus and Lactobacillus which often accompany over-ripe or rotten fruit. These species easily convert malic acid to lactic acid, but they also produce significant amounts of acetic acid (vinegar), and will irreparably diminish the quality of the wine. We want to avoid that.
Together with alcoholic fermentation, ML helps create a “nutrient desert” where three key energy sources for spoilage organisms like sugar, amino acids and malic acid are naturally eliminated by consumption. The other big energy source for bad bugs is oxygen, and we use several methods in the cellar to reduce exposure throughout the life of the wine.
The photograph here shows the paper chromatography method I use for monitoring ML. Paper chromatography is a pretty simple analytical method that separates individual molecules dissolved in a solvent. The process is pretty simple: a tiny drop of wine from each barrel is placed incrementally at the bottom of the chromatography paper. The sheet is then rolled vertically into a tube, stapled together at each end and placed standing up in a big pickle jar with a small amount of solvent at the bottom. As the solvent carries the wine spots up the paper by capillary movement (the mobile phase), the dissolved acid molecules eventually fall out of solution, based on their solubility, and bind to the paper. In this example, the presence of three organic acid components are represented as distinct spots aligned in a column: tartaric acid (bottom spot), malic acid (middle) and lactic acid (top). The single spot on the far left confirms a malic acid standard. The next 8 columns to the right show the migration as the drops of wine wick up the paper. Notice the third analyte from the left (GFVF) is missing the middle spot. This is what I’m looking for. It means that most, if not all, of the malic acid has been converted to lactic acid and ML is finished. The result is confirmed using a precise but relatively expensive enzymatic spectroscopy method, so we only want to do it once if possible. I’ll continue to monitor the other barrels using paper chromatography throughout the winter.