This type of acid is mainly found in yoghurt, sour cucumbers and sauerkraut and is very important in plant, animal and human metabolism (produced by muscle activity). In wine, it is one of the non-volatile acids (tending towards medium volatile ) and is present in amounts of 0.8 to 3.3 g/l. In small quantities it is produced as a by-product of alcoholic fermentation. However, it is mainly produced during malolactic fer mentation (biological acid degradation) through the conversion of malic acid by lactic acid bacteria, as well as small amounts of citric acid and sugar. See a list of wine constituents under total extract.