During the fermentation of soy sauce, nitrogen constituent is
one of the important parameters used for judging the quality of
soy sauce. According to the Chinese National Standard (Xu, 2000),
Grade A soy sauce should contain total nitrogen and amino
nitrogen of more than 1.30 and 0.70 g/100 ml, respectively. As
can be seen from Fig. 2c and d, a rapid increase in total and amino
nitrogen contents occurred from the initial stage of 5 days to the
period of 60 days in both types of soy sauces.