[Research on pretreatment methods for cystine determination in milk powder]

To establish a rapid, accurate and safe pretreatment method for the determination of cystine in milk powder.

Samples were oxidized at 50 ℃ for 10 min after adding performic acid, and then the reaction was terminated with ethanol as oxidation terminator. The reaction solution was concentrate to dryness by vacuum centrifugation at 50 ℃. The residue was hydrolyzed at 145 ℃ for 4 h after adding 15 mL hydrochloric acid(6 mol/L). Quantitatively transfer the hydrolysate to 25 mL volumetric flask, rinsing with water, and dilute to volume. A proper amount of hydrolysate was concentrated to dryness by vacuum centrifugation at 50 ℃, dissolved in sodium citrate buffer, filtered, and determined by amino acid analyzer.

A rapid detection method of cystine in milk powder was realized in this study. Cysteic acid were linearly correlated within the set range, and the correlation coefficients were more than 0.999. This method was applied to the determination of milk powder reference material SRM1849A. The result was within the range of values, and the precision was all less than 4%; The detection limit of cystine was 0.001%, and the limit of quantitation was 0.003%. The applicability of this approach was validated by a comparison study with milk powder reference material(SRM 1869). To explore the scalability of the method, fish meal and soybean meal were measured for six times under the above conditions according to the classification of animals and plants. Meanwhile, milk powder, fish meal and soybean meal were also measured according to GB/T 15399-2018 as reference value. The relative deviation of the average values measured by both method were all less than 6%.

This method is simple and rapid, with good repeatability and high accuracy, and less harm to the operators and experimental equipment. It can be used for the rapid detection of cystine in milk powder, and it also has applicability to other complex samples.