Tartaric Acid

Tartaric acid is a naturally occurring α-hydroxycarboxylic acid widely distributed in fruits, particularly grapes and tamarinds, and long recognised for its importance in food chemistry, winemaking, and organic synthesis. Its diprotic and dihydroxy structure makes it chemically versatile, while its salts—especially potassium bitartrate—play significant roles in culinary and industrial applications. Historically central to the development of stereochemistry, tartaric acid has been the subject of extensive scientific study since the eighteenth century.

Chemical Characteristics and Natural Occurrence

Tartaric acid appears as a white crystalline solid and contributes a distinctive sour taste when added to food. Its naturally occurring form is 2R,3R-tartaric acid, historically called dextro- or L-tartaric acid, and is the most abundant stereoisomer in nature. The compound is diprotic, capable of donating two protons, and is structurally a dihydroxyl derivative of succinic acid. The acid and its salts occur widely in grapes, bananas, tamarinds, avocados, and citrus fruits; trace amounts are also found in berries such as cranberries and in plants including Pelargonium species and Phaseolus vulgaris.
In winemaking, potassium bitartrate crystallises naturally during fermentation, forming “wine diamonds” that may adhere to corks or the inside of bottles. Although harmless, the crystals are sometimes removed by cold stabilisation. Historically, tartrates collected from wine barrels served as a major industrial source for potassium bitartrate.

Historical Discovery and Early Investigations

Although winemakers had long been familiar with tartaric deposits, the chemical isolation of tartaric acid was achieved in 1769 by Carl Wilhelm Scheele. The significance of the compound expanded in the nineteenth century when it became pivotal to the discovery of chirality. In 1832, Jean-Baptiste Biot demonstrated that tartaric acid rotated plane-polarised light, an observation that revealed its optical activity.
Further advances came from Louis Pasteur, who in 1847 examined the crystalline forms of sodium ammonium tartrate. By manually separating mirror-image crystals, he isolated pure samples of the enantiomers, thereby providing the first evidence of molecular chirality. This work fundamentally shaped the field of stereochemistry and contributed to modern understanding of molecular asymmetry.

Stereochemistry and Crystal Forms

Tartaric acid possesses several stereoisomers: the naturally occurring 2R,3R form; its enantiomer 2S,3S; and a meso form designated 2R,3S (or 2S,3R). Modern nomenclature replaces the archaic “dextro” and “levo” terms with R/S descriptors. Racemic tartaric acid, a 1:1 mixture of the two enantiomers, crystallises in monoclinic or triclinic forms, whereas anhydrous meso-tartaric acid exhibits both triclinic and orthorhombic polymorphs. Hydrated meso-tartaric acid crystallises in distinct monoclinic and triclinic forms depending on the temperature of crystallisation.
These crystal structures have served as key examples in mineralogical and chemical studies since the nineteenth century, including the extensive documentation of P. Groth in Chemische Krystallographie.

Industrial Production of Tartaric Acid

The predominant industrial form is the naturally occurring L-tartaric acid, extracted chiefly from the lees of wine fermentation. Potassium bitartrate recovered from fermentation residues is converted to calcium tartrate by treatment with calcium hydroxide, often with supplemental calcium sulfate to increase yields. Subsequent treatment of calcium tartrate with sulfuric acid regenerates free tartaric acid.
Racemic tartaric acid, by contrast, is synthesised through multi-step chemical processes, beginning with the epoxidation of maleic acid using hydrogen peroxide, followed by hydrolysis to yield tartaric acid derivatives. Meso-tartaric acid arises as a by-product when dextro-tartaric acid is heated in water at elevated temperatures or through reactions involving compounds such as dibromosuccinic acid.

Chemical Reactivity and Derivatives

Tartaric acid undergoes diverse reactions. Oxidation with hydrogen peroxide in the presence of ferrous salts yields dihydroxymaleic acid, which can be further oxidised to tartronic acid. Chemically, tartaric acid complexes readily with transition metals; for example, in Fehling’s solution it binds Cu(II), preventing precipitation of copper hydroxide.
A wide variety of derivatives have industrial, scientific, and pharmaceutical relevance:

• Potassium bitartrate (cream of tartar): used in cooking and as a stabiliser.
• Potassium sodium tartrate: valued for its piezoelectric properties.
• Antimony potassium tartrate: a classic resolving agent in stereochemical studies.
• Diisopropyl tartrate: widely employed in asymmetric synthesis, particularly in Sharpless epoxidation.

Toxicology and Safety

Although tartaric acid is a natural food component and widely consumed, high doses can inhibit malic acid production, functioning as a muscle toxin. The median lethal dose (LD₅₀) values, approximately 7.5 g per kg body weight for humans based on extrapolated estimates, indicate that ordinary dietary exposure poses no risk. As a food additive, it is designated as E334 and used primarily as an antioxidant and acidity regulator.
Tartaric acid solutions also possess notable cleaning capabilities; for example, cream of tartar suspensions effectively remove copper(II) oxide from metal surfaces by forming soluble tartrate complexes.

Functional Role in Winemaking

The compound is essential in enology, where it stabilises acidity and inhibits the growth of undesirable microbes during must fermentation. Tartaric acid contributes tartness and affects flavour balance in the finished wine, complementing other organic acids such as malic and citric acids. Its role in maintaining appropriate pH levels is critical for wine preservation and quality.

Occurrence in Fruits and Plant Materials

Beyond grapes and tamarinds, notable concentrations of tartaric acid are present in fruits such as bananas, avocados, apples, cherries, papayas, peaches, pears, pineapples, strawberries, mangoes, citrus fruits, and Opuntia ficus-indica. Trace levels occur in cranberries and related berries, with studies highlighting its variation across plant tissues. Leaves and pods of several species also contain measurable quantities.