Citric acid is a tricarboxylic acid with the formula C[sub]6[/sub]H[sub]8[/sub]O[sub]7[/sub].H[sub]2[/sub]O occurring in plants, especially citrus fruits (in lemons up to 8%) and in animals. The commercial product melts at 153[sup]o[/sup]C. Its main uses are based on its being a tricarboxylic acid.
Lactic acid C[sub]3[/sub]H[sub]6[/sub]O[sub]3[/sub], 2-hydroxypropanoic, CH[sub]3[/sub]CHOHCOOH, is a hygroscopic alpha-hydroxy acid is the primary acid component in sour milk. Since it contains both hydroxyl and carboxyl functional groups it may undergo self-esterification and polymerization. It occurs in three optically isomeric liquid forms: L form, in blood and muscle tissue as a product of glucose and glycogen metabolism; D form, obtained by fermentation of glucose; and DL form, a racemic mixture present in foods prepared by bacterial fermentation, and also made synthetically.
For more information consult any organic chemistry book.