Lactic souring is the great danger always associated with stuck fermentations, and any significant amount of residual sugar places wine in jeopardy. Consequently, stuck fermentations should be restarted promptly before lactic bacteria can multiply to excessive levels. This is why some winemakers prefer to inoculate their wines with ML bacteria late in the alcohol fermentation when very little sugar remains in the wine. Restarting a stuck fermentation of this kind can be very difficult. Large amounts of acetic acid are in the wine, and acetic acid is toxic to wine yeast.
Large wineries remove the acetic acid by using a special reverse osmosis process.
When the excess acetic acid is gone, new yeast is added to restart fermentation, but reverse osmosis is a complicated process, and applying it to small quantities of wine is seldom feasible. Small wineries and home winemakers deal with lactic souring problems primarily by prevention. This is one reason 30 to 50 milligrams per liter of sulfur dioxide should be added when the grapes are crushed. Then tartaric acid is added to the crushed grapes, and the pH is lower to less than 3.5 before fermentation is started. Acetaldehyde Production Candida mycoderma can oxidize ethyl alcohol in the wine into a very volatile liquid called acetaldehyde.