The determination of total nitrogen in soy sauce by the Kjeldahl method using traditional digestion and distillation, or using block digestion and steam distillation, has been studied as a candidate to be proposed to the Codex Alimentarius Commission. In order to obtain the optimum digestive condition, the added amounts of H
2SO
4, K
2SO
4 and CuSO
4•5H
2O catalysts, the way of adding the catalyst, and the boiling period after the liquid became clear were examined. The optimum conditions for the digestion of 1 ml soy sauce were found to be adding 10 ml of H
2SO
4, 10 g of K
2SO
4, and 1 ml of 20% CuSO
4•5H
2O, and boiling for 80 min after a clear liquid is obtained in traditional digestion. Eight grams of K
2SO
4 were found to be appropriate in block digestion. Samples were measured by weight, and the total nitrogen was calculated on a % (w/w) basis to avoid the effect of the soy sauce viscosity. The nitrogen content was converted into % (w/v) by using the density of each sample. These sampling and digestion methods were compared with the JAS grading method for three kinds of soy sauce, lysine and (NH
4)
2SO
4 solutions. The results yielded a significantly higher nitrogen content in full-bodied soy sauce; also, the recoveries (98 to 100%) for lysine and (NH
4)
2SO
4 solutions, had lower RSD values than did that of the JAS grading method, and were regarded as being more accurate.
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