A Rapid Method for Direct Determination of Levulinic Acid in Soy Sauce

Abstract

A simple and rapid method was developed to determine the levulinic acid level in soy sauce using megapore polar column (CP-Wax, 30 m × 0.53 mm) with splitless direct injection gas chromatography. Direct quantitative analysis of levulinic acid in soy sauce was carried out without any sample pretreatment procedure. A water-soluble compound, 1,6-hexanediol, was used as the internal standard. The detection limit for levulinic acid was 10 μg/mL. Recoveries from soy sauce and pickled condiment liquid were performed by spiking 2.5, 5.0, or 10.0 mg to 1 mL of test samples and were found to be at the range of 98∼103% with coefficients of variation less than 6.3%. Fourty-six food samples including animal and vegetable protein hydrolysate, pickle condiment liquid, and soy sauce were analyzed using the proposed method. Levulinic acid contents in pickled condiment liquid were found to be 2.1∼4.7 mg/mL and in soy sauce were 7.8∼24.5 mg/mL, which were higher than CNS regulated level (1.0 mg/mL), indicating commercial pickles and soy sauce might be adulterated with vegetable protein hydrolysate. These results were inconsistent with the labeling of "100% fermented soy sauce" on the packages.