Tartaric acid

Tartaric acid is a white crystalline dicarboxylic acid.

Tartaric acid was first isolated from potassium tartrate, known to the ancients as Louis Pasteur continued this research in 1847 by investigating the shapes of tartaric acid crystals, which he found to be asymmetric. Pasteur was the first to produce a pure sample of levotartaric acid.

Stereochemistry

Naturally-occurring tartaric acid is chiral, meaning that it has molecules that are non-superimposable on their mirror-images. It is a useful raw material in organic chemistry for the synthesis of other chiral molecules. The naturally occurring form of the acid is L-(+)-tartaric acid or dextrotartaric acid. The mirror-image (enantiomeric) form, levotartaric acid or D-(−)-tartaric acid, and the achiral form, mesotartaric acid, can be made artificially. Note, that the dextro and levo prefixes are not related to the D/L configuration (which is derived from the reference D- or L-diastereomer of both of them.

A rarely occurring optically inactive form of tartaric acid, DL-tartaric acid is a 1:1 mixture of the levo and dextro forms. It is distinct from mesotartaric acid and was called racemic acid (from Latin racemus - "a bunch of grapes"). The word racemic later changed its meaning, becoming a general term for 1:1 enantiomeric mixtures - racemates. ­­

Derivatives

  Important derivatives of tartaric acid include its salts, Cream of tartar (potassium bitartrate), Rochelle salt (potassium sodium tartrate, a mild laxative) and tartar emetic (antimony potassium tartrate).

Tartaric acid is a muscle toxin, which works by inhibiting the production of malic acid, and in high doses causes paralysis and death. The minimum recorded fatal dose for a human is about 7,5 grams/kg. In spite of that, it is included in many foods, especially sour-tasting sweets. As a food additive, tartaric acid is used as an E number E334, tartrates are other additives serving as antioxidants or emulsifiers.

When cream of tartar is added to water, a suspension results which serves to clean copper coins very well. This is due to the fact that the tartrate solution can dissolve the layer of copper(II) oxide present on the surface of the coin. The resulting Copper(II)-tartrate complex that results is easily soluble in water.

Tartaric acid in wine

  Tartaric acid may be most immediately recognizable to wine drinkers as the source of "wine diamonds," the small potassium bitartrate crystals that sometimes form spontaneously on the cork. These "tartrates" are harmless, despite sometimes being mistaken for broken glass, and are prevented in many wines through cold stabilization. The tartrates that remain on the inside of aging barrels were at one time a major industrial source of potassium bitartrate.

However, tartaric acid plays an important role chemically, lowering the pH of fermenting "must" to a level where many undesirable spoilage bacteria cannot live, and acting as a preservative after malic acids also play a role. The modern practice of extended hang time, where grapes are allowed to sit on the vine nearly until they become raisins, can dramatically reduce the taste of tartaric acid in a wine, leaving it smoother but also potentially less compatible with food.