This is part of the fermentation process, in which the malic acid in the wine is converted into lactic acid and carbon dioxide. A wine undergoing this change will have an odd, flattish smell that can definitely evoke the smell of milk and will certainly not be presenting itself adequately for tasting. While the wine is in a large vat or in a cask, the malo-lactic fermentation can be observed and it passes quickly. It can, however, present many problems if it takes place once the wine has been bottled, for then the sealed container – the bottle – may be unable to contain the fizziness of the wine and the corks may blow or, at best, the wine may be strange and unsatisfactory to taste.
Certain wines seem to progress without difficulty through the various stages of fermentation, especially today, when the skill of the winemaker enables their evolution to be observed and controlled. Other wines are definitely difficult as regards their malo-lactic fermentation, chiefly because, unless the process takes place actually during the first fermentation or follows it within a fairly short time, it appears difficult to know when it may occur. There is no reason for the ordinary winedrinker to be much concerned with this aspect of winemaking, although the chemist will probably be interested. But, when young wines are being tasted from vat or cask, the possible presence of the malo-lactic fermentation should be remembered. Ideally, most winemakers prefer always to get the malo-lactic fermentation over early in a wine’s life. In some cases, they suppress it so as to achieve a greater – ie. more definite – acidity in the ultimate wine.