After Primary Alcoholic (Yeast) Fermentation…
Red wines, and some white wines, are allowed and indeed encouraged to go through “Malo”, a secondary malolactic fermentation where harsh, unstable malic acid is converted into stable lactic acid and in the process slightly deacidifying the wine, softening the wine by reducing bitterness whilst enhancing fruitfulness & body and stabilising the wine for bottling.
This process occurs due to the action of a bacteria Oenococcis oeni that transforms each gram of malic acid (green apple acid) into two-thirds of a gram of lactic acid (found in milk, butter, cheese and yogurt) and one-third of a gram of carbon dioxide.
In the past successful malolactic fermentation was a somewhat hit or miss affair relying on naturally occurring strains of the Oenococcis oeni bacteria, which frequently could not fully complete the acid conversion due to intolerance to high levels of acid and alcohol in the wine.
To verify that malolactic fermentation has completed we test using thin-layer chromatography whether the wine still has any malic acid. The photograph below shows four samples: one the left is reference malic acid and next to it reference lactic acid and on the right two wine samples. The test like splitting colours of a dye or ink on blotting paper using water, allows the malic and lactic acid in the wine to be separated.
As you can see the first wine sample shows no matching bright dot with the malic acid reference, but a matching dot with the lactic acid reference and so we know this wine has completed malolactic fermentation. Once complete the wine can be racked and sulfur dixoxide added to protect against spoilage microbes such as Lactobacillus, Pediococcus and Brettanomyces.
You may have noticed a third bright dot above the lactic acid one, this represents another acid, succinic (amber) acid, which in Europe has been used an a natural antibiotic for centuries.
Today’s winemakers can rely on inoculating wines with selected Oenococcis oeni stains that survive the chemical conditions found in wine which Louis Pasteur remarked as being “the most hygienic of beverages”.