Optimum Conditions for the Determination of Total Nitrogen in Vinegar by the Kjeldahl Method

Abstract

The determination of total nitrogen in vinegar (komekuro-zu) by the Kjeldahl method was studied for a revision of the Japanese Agricultural Standard (JAS). Based on the previous JAS analysis, optimization was performed for the Kjeldahl method using either an electric heating device and a traditional distillation system, or a digestion block instrument and a steam distillation unit. The optimum digestive conditions were examined including the volume of sulfuric acid, the amount of potassium sulfate, the volume and the addition method of a 30% hydrogen peroxide solution, the types of catalysts, and the heating period after the liquid became clear. The optimum conditions for the digestion of vinegar involved the addition of 2 catalyst tablets (9.0 g K₂SO₄, 1.0 g CuSO₄•5H₂O), 15 mL of sulfuric acid, and 10 mL of a 30% hydrogen peroxide solution. In digesting the vinegar, the heating period after the liquid became clear was determined to be 120 min by the electric heating device and 90 min by the digestion block instrument. This method yielded high recoveries of nitrogen from lysine-HCl (more than 97%), and a lower RSD of total nitrogen in vinegar (1.0% or less). This method was more convenient than the previous JAS method, by making block digestion and steam distillation available.