In general, where biological molecules have optical isomers, only one of the isomers or forms will be active biologically. The other will be unaffected by the enzymes in living cells. The meso form of the molecule is not affected by polarized light.
The fourth form — the DL mixture, is not a single molecule, but a mixture of equal amounts of D and L isomers. It does not rotate polarized light (like the meso form) because the rotation of light by the D and L forms is equal in amount but opposite in direction. It is possible to separate the DL mixture into the two isomers, each of which does rotate light. In the 1840s, Louis Pasteur determined that each of the two isomers of tartaric acid rotated light in opposite directions, and the meso form was inactive in this respect. He also separated by hand, crystals of the racemic mixture to show that it was made of equal amounts of the D and L forms, making it different than the meso form of tartaric acid.
Tartaric acid is found in cream of tartar, which is used in making candies and frostings for cakes. Tartaric acid is also used in baking powder where it serves as the source of acid that reacts with sodium bicarbonate (baking soda). This reaction produces carbon dioxide gas and lets products “rise,” but it does so without the “yeast” taste that can result from using active yeast cultures as a source of the carbon dioxide gas.
Tartaric acid is used in silvering mirrors, tanning leather, and in the making of Rochelle Salt, which is sometimes used as a laxative. Blue prints are made with ferric tartarte as the source of the blue ink.
In medical analysis, tartaric acid is used to make solutions for the determination of glucose. Common esters of tartaric acid are diethyl tartarate and dibutyl tartrate. Both are made by reacting tartaric acid with the appropriate alcohol, ethanol or n-butanol. In the reaction the Hydrogen of the COOH acid group is replaced with an ethyl group (diethyl tartarate) or butyl group (dibutyl tartarate. These esters are used in manufacturing lacquer and in dyeing textiles.