Determination of 10 Synthetic Colorants in Glutinous Rice Products by High Performance Liquid Chromatography after Sample Enzymolysis

The samples of glutinous rice products (5 g) were taken, and the enzymatic hydrolysis was carried out in a 60℃ water bath for 6 h with 150 mg of medium temperature amylase (2 000-7 000 U·g^-1), then the solution of enzymatic hydrolysis to be tested was obtained. 1 g of polyamide powder (0.075-0.15 mm) mixed into porridge was added into the above solution, and filtered through G3 vertical dissolution funnel. The filter cake was successively washed with 60℃ water (10 mL×3) and eluted with a mixture of ethanol, ammonia and water at volume ratio of 7:2:1 (15 mL×3). The obtained eluent was heated and concentrated to about 5 mL. The concentrated solution was made its volume up to 20 mL with water and then centrifuged. 10 synthetic colorants in the supernatant were determined by high performance liquid chromatography (HPLC). C₁₈ chromatographic column was used as the stationary phase, the mixture of methanol and 20 mmol·L^-1 ammonium acetate solution at different volume ratio was used as the mobile phase for gradient elution, and the diode array detector was at 430 nm for detection of citrine, 500 nm for detection of sunset yellow, new red, ponceau 4R, amaranth, allura red, azorubine and etythrosine B, and 600 nm for detection of indigotin and brilliant blue. As shown by the results, linear relationships between values of peak area and mass concentration of 10 synthetic colorants were found in the same ranges of 0.2-20.0 mg·L^-1, with detection limits (3S/N) in the range of 0.19-0.43 mg·kg^-1. Test for recovery was made by standard addition method, giving results in the range of 76.9%-94.0%, with RSDs (n=6) of the determined values less than 4.0%. This method was applied for analysis of 10 batches of glutinous rice products on the market, and citrine, sunset yellow and brilliant blue were detected respectively in 3 batches of samples. Compared with the determination results of national standard GB 5009.35-2016, this method could effectively overcome the false negative results of this kind of food.