Lactic acid, is an organic compound belonging to the family of carboxylic acids, it occurs naturally in the blood (in the form of its salts, called lactates) when, during exercise, glycogen is broken down in the muscles. It can be converted back to glycogen in the liver. Known to be difficult for babies to metabolise. Also occurs naturally in sour milk as the result of bacterialogical activity. It is usually obtained commercially by heating and fermenting carbohydrates such as sucrose, molasses, starch, or whey.

As a food additive it increases the antioxidant properties of other substances and can be used to add a bitter taste. In brewing it is used to reduce losses of carbohydrates from germinated barley and for adding to the malt slurry to assist in making a beer of consistent quality.

It is the commonest acidic constituent of fermented milk products such as sour milk, cheese, and buttermilk. Aso used in carbonated drinks, jams, jellies, soft margarine, marmalade, infant milks and cereals, pickled red cabbage, salad dressings, sweets, tartare sauce and many tinned products, such as babyfoods, mackerel, pears, sardines, strawberries and tomatoes.

Vegetarians should be aware that as it is a naturally occurring animal product, it could conceivably be of animal origin.