Lactic Acid, a product of the decomposition of any kind of sugar in solution, induced by the presence of certain albuminous ferments, as diastase exposed for some time in solution to the air. Milk contains both the elements for the production of this acid, sugar of milk and albuminous caseine. Its change to sour milk is called the lactic fermentation, and lactic acid is a product of this change. It was in sour milk that the acid was originally discovered by Scheele, whence he named it lactic; but it has since been obtained from the juices of many vegetables, and from the fluids of the stomach and flesh of animals. As milk turns, the coagulum which is formed is a combination of lactic acid and caseine. If the lactic acid be taken up by bicarbonate of soda, the caseine set free induces further fermentation, and more lactic acid is formed from the sugar of milk; and so by adding more soda the process may be kept up until all the sugar of milk is converted into lactic acid. If a succeeding fermentation be allowed to take place, butyric acid is produced. The composition of lactic acid is expressed by the formula C3H6O3, and it has the same centesimal composition as sugar of milk.
When concentrated in vacuo over sulphuric acid, lactic acid is obtained in the form of a sirupy colorless fluid, of specific gravity 1.215 and very sour. At a temperature of 266° F. it yields water and becomes an anhydrous solid (dilactic acid), which dissolves sparingly in water, but readily in alcohol and ether. Lactide is a crystalline substance, of composition C3H4O2, produced by subjecting the acid to a temperature of 482°; it is now called lactic anhydride. - In the animal economy lactic acid is thought to play an important part from its property of dissolving large quantities of freshly precipitated phosphate of lime; and this has led to its prescription in medicine with the view of its removing phosphatic deposits in the urine, as well as to hold in solution phosphate of lime when given as a medicine. It has also been recommended in certain forms of dyspepsia. It has been proposed as a local application to dissolve the false membranes of croup, being applied in liquid form, or as a spray from an "atomizer;" but the success obtained with this treatment by other observers has not equalled that claimed by the original proposer.
The acid may be conveniently prepared by evaporating sour milk to one eighth its bulk, filtering, adding lime, again filtering, separating the crystals of lactate of lime which form, purifying these by redissolv-ing and recrystallizing, and finally decomposing the salt by means of oxalic acidand recovering the lactic acid by filtering. But it is best obtained by dissolving 8 parts of cane sugar in about 50 parts of water, and fermenting by 1 part caseine and 3 parts chalk, and decomposing the lime salt by sulphuric acid. - The salts formed by this acid with bases are called lactates. The only important one is the lactate of iron, which is much employed in medicine as a stimulant and tonic. It is prepared by digesting lactic acid and iron filings at a gentle heat on a sand bath for five or six hours, and then allowing the liquor to boil. It is then filtered, concentrated, and allowed to cool and crystallize. The crystals are drained in a funnel, washed with alcohol, dried rapidly, and transferred to a bottle, which must be well stopped. Lactate of iron when pure is in white crystalline plates. It has an acid reaction, is soluble in 12 parts of boiling water, and the solution soon becomes yellow from the iron passing to a higher degree of oxidation.
When sold in a powdered state, it is apt to be adulterated; for this reason it should be purchased in the crystals. The medicinal applications may be in the form of lozenges or sirup. In one of the Paris hospitals it has been introduced into bread, hence known as chalybeate bread, a grain of lactate of iron in each ounce, which does not injuriously affect the taste or quality of the bread. This is given to patients suffering from chlorosis, and in other forms the medicine has proved beneficial in this disease. It is observed that it decidedly increases the appetite. Sarcolactic acid is the variety which is obtained from the juice of flesh; paralactic acid also exists.