Direct determination of free tryptophan contents in soy sauces and its application as an index of soy sauce adulteration

A simple and rapid HPLC method using UV detector for determination of tryptophan (Trp) contents in pure fermented soy sauces, acid-HVP, and commercial soy sauces was developed. The method requires only simple sample pretreatment. The limit of detection (LOD) is estimated at 1 mg/L (signal-to-noise ratio of 3), and the recovery yields ranged from 92% to 108%. The analysis showed that Trp in all of the analysed acid-HVP samples was not detected and there was obvious detection of Trp (ranged from 136.4 to 261.8 mg/L) in all of the analysed fermented soy sauce samples. The observations suggest that Trp is a practical index of pure fermented soy sauce. The absence of Trp contents or lower level of Trp contents than that in pure fermented soy sauces suggests the soy sauces are not pure fermented soy sauces or presence of soy sauce adulteration. Simultaneous determination of levulinic acid (LV) in samples may be a good help in making assessment of soy sauce adulteration with acid-HVP.