Spectrophotometric method for the determination of sorbic acid in various food samples with iron(III) and 2-thiobarbituric acid as reagents

Abstract

A simple, rapid and accurate spectrophotometric method has been developed for the determination of sorbic acid in various food samples based on the oxidation of sorbic acid by iron(III) at 100 °C to malonaldehyde, which then reacts with 2-thiobarbituric acid to form a reddish brown product. The optimum experimental conditions for colour development have been assessed. Absorbance measurements were made at 529 nm in the presence of 0.4%m/V citric acid. The calibration graph was linear for 0–6 µg ml^–1 of sorbic acid with a slope of 0.131 A µg^–1 ml. The recoveries of sorbic acid at concentrations of 164–557 µg ml^–1 ranged from 96 to 103%. The relative standard deviations of ten replicate determinations of sorbic acid in a synthetic cream soda sample spiked with 573 µg ml^–1 of sorbic acid and in an onion juice sample containing 82 µg ml^–1 of sorbic acid were 1.6 and 1.9%, respectively. Interferences from several common food additives can be minimised by extracting sorbic acid with diethyl ether and then back-extracting the acid with sodium hydrogen carbonate. The method has been applied successfully to the determination of sorbic acid in a wide range of food samples including beverages, cake, cake mate, garlic bread sprinkle, onion juice, oyster flavoured sauce and grenadine syrup.