Determination of lactic, acetic, succinic, and citric acids in table olives by HPLC/UV

This paper describes a procedure performed by high performance liquid chromatography/UV detection for quantification of the major carboxylic acids in table olives (lactic, acetic, succinic, and citric acids); derivatization of carboxylic acids with 0-(4-nitrobenzil)-N,N'diisopropylisourea (PBNDI) was performed. The sample preparation involved deproteination with ethanol and the use of strong cation-exchange resin (Dowex 50W-X8) to liberate the free carboxylic acids. The same resin was used to remove the excess of derivatizing reagent. The chromatographic separation was achieved using reverse-phase column C18 (ODS). The mobile phase used was a gradient of water and acetonitrile at a flow-rate of 1 mL/min. The effluent was monitored using a UV detector at 265 nm. The method allowed for well-resolved peaks of lactic, acetic, succinic, and citric acids in less than 25 min. Precision and recovery assays were performed with good results for the acids under study. Nineteen samples of table olives available on the Portuguese market, including nine samples of green olives and 10 samples of black olives, were successfully monitored applying this methodology. The concentrations of carboxylic acids are expressed in percentage of moist olive pulp and ranged from not detected to 477.3 mg of lactic acid/100 g of moist olive pulp and 9.43 to 232.1 mg of acetic acid/100 g of moist olive pulp. Citric acid was detected only in two samples of green table olives with concentrations of 24.7 and 188.3 mg/100g of moist olive pulp. Succinic acid was detected in five samples of green table olives ranging from 10.1 to 25.8 mg/100 g moist olive pulp, and in two samples of black table olives with concentrations of 10.7 and 29.4 mg/100 g of moist olive pulp.