Lactic Acid, , the acid contained in sour milk. In the pure state it is a colourless, transparent, syrupy liquid, of specific gravity 1.215. It is without smell, has a sharp acid taste, and is miscible with water, alcohol, and ether. It is formed in milk by the fermentation of the milk-sugar under the influence of an organised ferment. On a large scale it is usually prepared from cane-sugar in the following manner : 7 lb. of cane-sugar and oz. tartaric acid are dissolved in 4 gal. of water and allowed to stand for a few days ; then 4 oz. of rotten cheese rubbed up in a gallon of sour milk, and lb. of zinc oxide (zinc white) are added, and the mixture is thoroughly stirred and kept at a temperature of about F. for eight or ten days. The liquid is boiled to stop the fermentation, filtered, and evaporated till the zinc lactate which it contains crystallises ; this is then re-dissolved in water, decomposed with sulphuretted hydrogen, the mixture filtered to free it from zinc sulphide, and evaporated on a water-bath.
Lactic acid occurs very widely distributed as a product of the natural fermentation of sour vegetable materials, such as sauerkraut ; it is also found in the stomach and intestines. An isomeric acid of the same composition but slightly different properties, called sarco- or para-lactic acid, occurs as a product of waste of animal tissues, and is found in muscle of all kinds, especially after violent exertion or artificial tetanus. Sarco-lactic acid has been frequently detected in blood soon after its removal from the body, in quantities usually below 1 part per 1000, but it apparently does not occur in normal healthy blood while in the body.
The tests for lactic acid are not very satisfactory. On addition of lead acetate and alcoholic ammonia to a solution containing lactic acid an insoluble lead lactate, , is precipitated as a white powder. The properties and amount of water of crystallisation of the zinc lactates are also characteristic. Most of the lactates are crystalline and soluble in water.