In the old days we would use paper chromatography to monitor the completion of malolactic fermentation (MLF). We dotted little drips of the sample wine, along with liquid standards of malic and lactic acids, onto a piece of paper. When dry, we rolled up the paper and stuck it in a big mayonnaise jar full of poisonous, smelly solvent (butanol, formic acid and colored indicator bromocresol green). The solvent traveled up the paper, taking some of the dried wine residue with it. Once the solvent reached the top of the paper, we removed it from the jar, and hung it up to dry (even more smelly and toxic!) until we could “read” it. By comparing the presence or absence of malic acid represented by a little yellow dot on the sheet, we could tell if our wine was MLF complete or not.
As you might be guessing, I’m done with this method. Some people still use it, but I happen to have pets and small kids around and so would never keep something as toxic as that infamously nasty chromatography solvent on my property. You must wear eye protection, gloves and have a lab-grade fume hood to work properly with it. You can certainly spend the money on the setup and chemicals but as the solvents are toxic, I choose to send my samples to a commercial wine laboratory once it seems that I no longer hear or see any bubbles ticking away in my barrels, indicating MLF is still ongoing. That way you get an exact number and hey presto, no smelly toxic mess for me to worry about. The commercial laboratories generally use specific enzymatic and spectrophotometric methods to quantitatively assess the level of malic and lactic acids in the wine.