Nitrogen contents in food: a comparison between the Kjeldahl and Hach methods

Abstract

The objective of the present work is to compare the nitrogen contents measured by the fast method of Hach and the traditional Kjeldahl method, allowing for the influence of the varia- tion in structure and macronutrient in the food content. Foods with a wide range of protein and moisture contents were selected. The nitrogen content of 25 food samples was measured with both methods. Some studies have compared the effectiveness of the Hach methods with that of the Kjel- dahl methods. This study emphasizes the evaluation of the digestion stage of both methods. In order to carry out an analytical quality control and to determine the accuracy and precision of both methods, two reference samples, elaborated by the Institute of Nutrition of the Univer- sity of Mahidol, Thailand, were also analyzed. One was based on fish flour and the other on a mix of cereal and soybean products. The variance and linear regression between both meth- ods were analyzed. The Tukey multiple comparison test was used to compare the means when significant differences were found in the variance analysis (Statgraphics). The correlation coefficient between the results obtained in both methods was 99%, suggesting that the Hach method can be used as an alternative to the Kjeldahl method. Despite its being slightly less accurate and precise than the Kjeldahl method, the Hach method exhibits some advantajes, such as a lower consumption of reagents, a smaller sample size and mainly, the shorter time required.