Though benzoic acid is used in cosmetics, dyes, plastics and insect repellents, it is most commonly included in food products as a preservative. The earliest mention of benzoic acid appears from the 16th century. The substance received its name from gum benzoin, the plant from whose resin it was first derived. In the 19th century, benzoic acid was synthesized from coal tar. Today it is manufactured from toluene, a petroleum byproduct.
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Encyclopedia Britannica describes benzoic acid as a colorless organic compound. It is classified as a carboxylic acid, which means it is formed of a carbon bonded to an oxygen atom and a hydroxyl group (-OH). It is weakly acidic, with a pH of 2.8. In normal conditions it has a white, flaky appearance that actually consists of small, needle-like crystals.
Both benzoic acid and sodium benzoate, its salt form, have inhibitory effects on the growth of yeast, a major cause of food spoilage. In particular, yeast is particularly devastating in foods and drinks with a low pH and a high sugar content. A study published in 1991 in Applied and Environmental Microbiology described the metabolic disruption benzoic acid wreaks upon yeast cells. By slowing the yeast’s ability to ferment sugars, benzoic acid starves the yeast of energy and prevents its growth.
Because benzoic acid is toxic, the amount of benzoates that can be added to foods is carefully controlled. Codex Alimentarius, an international treaty dictating food safety standards, limits the amount of benzoic acid or sodium benzoate to 0.05 to 0.1 percent by volume. Most foods are allowed no more than 1,000 mg per kilogram. Liquid egg products, diet foods, chewing gum and processed vegetables are among the foods with the highest amount of benzoate legally allowed.
There is some concern that benzoates added to some types of soft drinks might produce benzene, a dangerous carcinogen and environmental pollutant. Benzene exposure typically comes from breathing exhaust fumes, but it can also be produced from the interaction of benzoic acid and ascorbic acid, a precursor of vitamin C. The pH of the beverage, the temperature at which it is stored and the extent of exposure to UV light could all affect the extent to which benzene is produced. Current data, however, is insufficient to indicate reliably whether benzene is formed as result in the interaction of ascorbic and benzoic acids in common products.
A 2004 study by the Department of Biochemistry and Food Science of the Hebrew University of Jerusalem described a novel way to use benzoates to prevent food spoilage. If yeast is starved of nitrogen, it nevertheless can power itself through a catabolic process in which it converts some of its own internal structures into energy. Benzoic acid effectively inhibits this metabolic process at lower concentrations than it takes to directly inhibit fermentation. By combining these two approaches, meaningful prevention of food spoilage could be obtained with lower levels of preservatives.