Jeff Gross, age 13, of Nampa, Ids., for his question:

HOW IS LACTIC ACID FORMED?

Lactic acid is the common organic acid found in milk and other dairy foods that have turned sour. Lactic acid is formed naturally by the fermentation of lactos or milk sugar. Its name comes from the Latin word lac, which means milk.

A Swedish chemist named Karl Scheele in 1780 was the first to isolate the lactic acid from sour milk.

Lactic acid also occurs in sauerkraut, pickles nad beer. Commercially, the acid is prepared by fermentation of cornstarch, glucose, molasses or whey, which is the watery part of milk. It is widely used in foods and beverages to bring out flavor.

The tanning industry uses crude lactic acid to remove lime from hides. The textile industry uses it in dyeing wool. It combines with alcohols to form lactate esters, used as lacquer solvents.

Lactic acid is a colorless or yellowish syrupy liquid that mixes readily with water. It has several forms that differ only in the arrangement of the atoms.