Evaluation of digestion procedures in Kjeldahl method to quantify total nitrogen in analyses applied to animal nutrition/Avaliacao da etapa de digestao do metodo de Kjeldahl para quantificacao do nitrogenio total em analises voltadas a nutricao animal

The effects of the salt-to-metal catalyst ratio and amount of digestion mixture on total nitrogen content in different materials using the Kjeldahl method were evaluated. Four samples with low nitrogen contents and four samples with high nitrogen contents were analyzed. The study evaluated two ratios of salt (sodium sulfate) to metal catalyst (copper sulfate) in the digestion mixture (10:1 and 20:1) and three amounts of digestion mixture per sample (1.0, 1.5, and 2.0 g 200 [mg.sup.-1]). There was an interaction between low-nitrogen material and amount of digestion mixture on nitrogen contents. Samples of cattle feces and corn presented higher nitrogen content when 1.5 and 2.0 g of digestion mixture were used. The high-nitrogen materials presented higher nitrogen contents when 2.0 g of the digestion mixture was used. However, there was an interaction between high-nitrogen material and the ratio of the digestion mixture components. The cattle carcass sample showed higher nitrogen content when the 20:1 ratio was used. The digestion mixture amount in the Kjeldahl method must be 2.0 g with a salt-to-metal catalyst ratio of 20:1 when samples of approximately 200 mg are analyzed. Keywords: crude protein, digestion mixture, feed analysis. Avaliaram-se os efeitos da quantidade de mistura digestora e da razao dos componentes da mistura digestora utilizada no metodo de Kjeldahl para quantificacao do nitrogenio total em diferentes materiais. Foram utilizadas quatro amostras com baixo teor de nitrogenio e quatro amostras com alto teor de nitrogenio. Avaliou-se a comninacao de duas razoes entre sal (sulfato de sodio) e catalisador metalico (sulfato de cobre) na mistura digestora (10:1 e 20:1) e tres quantidades de mistura digestora por amostra (1,0; 1,5 e 2,0 g 200 [mg.sup.-1]). Para os materiais de baixo nitrogenio houve interacao entre o material analisado e a quantidade de mistura digestora. Amostras de fezes bovinas e milho apresentaram maiores teores de nitrogenio com o uso de 1,5 e 2,0 g de mistura digestora. Para os materiais de alto nitrogenio verificou-se maior teor de nitrogenio com o uso de 2,0 g de mistura digestora. No entanto, houve interacao entre material avaliado e razao na mistura digestora para amostras de alto nitrogenio. Amostra de carcaca bovina apresentou maior teor de nitrogenio utilizando-se a razao 20:1. A quantidade de mistura digestora para o metodo de Kjeldahl deve ser de 2,0 g com razao entre sal e catalisador metalico de 20:1 para amostras