Tartaric acid is a white crystalline organic acid that occurs naturally in plants, particularly grapes, bananas, and tamarinds. Tartaric acid also deposits in wine casks during alcoholic fermentation of grape juice (See References 1). In Japan, tartaric acids can be legally used in food products to provide a sour taste provided that they are declared on the label (See References 2).
What is tartaric acid?
Tartaric acid comes in two forms known as the (RR) and the (SS) isomers of tartaric acid. The two forms or isomers of tartaric acid are the same molecule with atoms or groups attached to same partners. The difference is in how those atoms or groups are arranged in space. The (RR) and (SS) isomers are actually non superimposable mirror images of one another, as such the tartaric acid molecule is known as a chiral molecule. The word chiral comes from the Greek for hand, because, just like your left and right hand, these molecules cannot be superimposed (See References 3).
Any molecule that contains a carbon atom with four different groups attached to it, can give rise to chiral molecules, simply based on the way the groups arrange themselves around the carbon atom in space. The carbon atom itself is called the stereogenic center. Molecules may have more than one stereogenic center, and in fact tartaric acid has two (See References 3).
Both forms of tartaric acid have identical properties, such as melting point, boiling point, density, solubility and various types of spectra. The only way they differ is in their chiral properties which essentially means that they rotate plane polarized light in opposite directions (clockwise or anticlockwise). The only place where chiral molecules behave differently is in biology, because biological reactions normally involve other chiral molecules. Almost all enzymes are chiral and therefore can distinguish between left and right. For example the enzyme lactic acid dehydrogenase will oxidize one isomer of lactic acid to pyruvic acid, but will have absolutely no effect on its mirror image (See References 3).
What is racemic acid?
Racemic acid is actually a mixture that contains equal amounts (50:50) of the two isomers, though in the mid nineteenth century when the French scientist Louis Pasteur (1822-1895) was doing his research, it was initially thought that racemic acid simply consisted of the mirror image molecules of tartaric acid. So what we now know as (RR) tartaric acid and (SS) tartaric acid, was then known as tartaric acid and racemic acid respectively (See References 4).
What Louis Pasteur found
Pasteur laid the basis for the field of stereochemistry, with his discovery of chiral molecules. He prepared salts of what he called tartaric acid and racemic acid. Racemic acid (initially known as paratartaric acid) was obtained from crude tartar around 1819 by Paul Kestner, a French manufacturer of tartaric acid, who mistook it for oxalic acid. In 1828 French chemist Joseph Louis Gay-Lussac showed that it has the same chemical composition as tartaric acid, and thus named it racemic acid (acide racémique, from the Latin racemus, meaning a bunch of grapes) (See References 4).
Pasteur noticed that the crystals of the sodium ammonium salt of tartaric acid were not symmetrical, ie their shape displayed “handedness”. Furthermore, when Pasteur examined the crystals of the same salt of what he called racemic acid, he found that they too were asymmetrical but that this time some of the crystals were left handed and some right handed. Using just a magnifying glass and a pair of tweezers, he separated the crystals into two piles and found that racemic acid was not really a single substance, but a 50:50 mixture of the two isomers of tartaric acid. (See References 4). We now define a racemic mixture as a 50:50 mixture of isomers of a molecule, and the term racemic acid is no longer used. In 1955 organic chemists Cahn, Ingold and Prelog came up with the R-S convention for labeling isomeric molecules, which is today known as the Cahn-Ingold-Prelog system (See References 3).