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Kjeldahl Method. A widely used analytical procedure for determining the nitrogen content of a product, such as flour (q.v.), to arrive at its protein (q.v.) content. In carrying out this method, a sample of flour is heated in concentrated sulfuric acid to reduce its protein nitrogen to ammonium sulfate from which ammonia (q.v.) is liberated and distilled off into standard acid (q.v.) and titrated (q.v.). The protein content of flour is calculated by multiplying the nitrogen content by the factor 5.7.