Improvement of the Conditions for the Determination of Total Nitrogen in Fish Meal in Kjeldahl Method and Its Comparison with Dumas Method

The determination of total nitrogen in fish meal by the Kjeldahl method under the Official Method of Feed Analysis in Japan was studied in comparison to the Dumas method. There were significant differences in several samples between the methods. In order to obtain the optimum digestive condition, the added amounts of catalyst and the boiling period after the liquid became clear were examined using L-lysine-HCl. The optimum conditions for the digestion of 1 g of fish meal were found to be adding 30 mL of H2SO4 and 10 g of catalyst (9 g K2SO4, 1 g CuSO4 center dot 5H(2)O), and boiling for 120 min after a clear liquid is obtained in traditional digestion by a gas heating device. Two sets of data by the Dumas and Kjeldahl method under the optimum digestive condition were tested with a two-sided Welch test. The mean values indicate that the Dumas method tends to produce a result that is slightly higher than the Kjeldahl method, however, there were no significant differences between these methods. The Dumas method had excellent precision, and was as reliable as the Kjeldahl method under the optimum digestive condition. The differences are unlikely to be attributable to any contribution from inorganic N, such as nitrate and nitrite, but are related to incomplete digestion by the Kjeldahl method under non-optimum conditions as shown by poor recoveries of organic nitrogen compounds such as lysine in fish meal.