Sorbic acid, or 2,4-hexadienoic acid, is a natural organic compound used as a food preservative. It has the chemical formulaCH3(CH)4CO2H and the structureH3C−CH=CH−CH=CH−C(=O)OH. It is a colourless solid that is slightly soluble in water and sublimes readily. It was first isolated from the unripe berries of the Sorbus aucuparia (rowan tree), hence its name.[1]
Production
The traditional route to sorbic acid involves condensation of malonic acid and crotonaldehyde.[2] It can also be prepared from isomeric hexadienoic acids, which are available via a nickel-catalyzed reaction of allyl chloride, acetylene, and carbon monoxide. The route used commercially, however, is from crotonaldehyde and ketene.[3] An estimated 30,000 tons are produced annually.[1]
History
Sorbic acid was isolated in 1859 by distillation of rowanberry oil by A. W. von Hofmann.[4] This affords parasorbic acid, the lactone of sorbic acid, which he converted to sorbic acid by hydrolysis. Its antimicrobial activities were discovered in the late 1930s and 1940s, and it became commercially available in the late 1940s and 1950s. Beginning in the 1980s, sorbic acid and its salts were used as inhibitors of Clostridium botulinum in meat products to replace the use of nitrites, which can produce carcinogenic nitrosamines.[5]
Properties and uses
With a pKa of 4.76, sorbic acid is about as acidic as acetic acid.
Sorbic acid and its salts, especially potassium sorbate and calcium sorbate, are antimicrobial agents often used as preservatives in food and drinks to prevent the growth of mold, yeast, and fungi. In general the salts are preferred over the acid form because they are more soluble in water, but the active form is the acid. The optimal pH for the antimicrobial activity is below pH 6.5. Sorbates are generally used at concentrations of 0.025% to 0.10%. Adding sorbate salts to food will, however, raise the pH of the food slightly so the pH may need to be adjusted to assure safety. It is found in foods such as various kinds of cheese, bread, muffins, donuts, pies, cookies, protein bars, syrups, lemonades, fruit juices, dried meats, sausages, nuggets, burgers, sandwiches, tacos, pizzas, smoked fish, margarine, sauces, soups, and more.[6]
^Ashford's Dictionary of Industrial Chemicals, Third edition, 2011, page 8482
^Hofmann, A.W. (1859). "Neue flüchtige Säure der Vogelbeeren" [New volatile acid of rowan berries]. Annalen der Chemie und Pharmacie (in German). 110 (2): 129–140. doi:10.1002/jlac.18591100202. Hofmann named sorbic acid on p. 133: "Ich schlage für die krystallinische Säure den Namen Sorbinsäure vor, wodurch ein alter Name der in den Vogelbeeren gefundenen Aepfelsäure neue Bedeutung gewinnt." (For the crystalline acid, I suggest the name "sorbic acid", whereby an old name of the malic acid that's found in rowan berries gains new meaning.)
