Abstract: A new method for quickly detecting the content of amino acid nitrogen in brewed soy sauce has been established for grade identification. Several Chinese brands of soy sauce were tested using natural food colorants as chromogenic agents according to the national standard GB 18186-2000 Fermented Soy Sauce. According to the law of conservation of mass, the concentration of NaOH and the amount of substances needed to detect all grades of soy sauce products were systematically investigated. The results showed that the purple sweet potato colorant exhibited red, orange-red or orange-gray colors for qualified soy sauce. The developed method was simple, convenient, intuitive, accurate, and practical, and took a few minutes to give results consistent with those from the national standard method GB 2009.235-2016 Detection of Amino Acid Nitrogen in Food.

Key words: brewed soy sauce; amino acid nitrogen; rapid identification