Crude Protein (%) = Nitrogen (%) × Conversion Factor
The Kjeldahl method, developed by Johan Kjeldahl in 1883, measures the total nitrogen content of a sample by acid digestion, distillation, and titration. Crude protein is estimated by multiplying nitrogen content by a conversion factor that accounts for nitrogen's average proportion in proteins. The general factor of 6.25 assumes protein contains 16% nitrogen (1/0.16 ≈ 6.25). Different foods have different amino acid profiles, so specific factors exist: dairy 6.38, wheat 5.70, rice 5.95, soy 5.71. The term "crude" protein is used because the method measures all nitrogen — including non-protein nitrogen from nucleic acids, urea, ammonia, and alkaloids. Despite this limitation, Kjeldahl remains the official reference method for protein determination in food analysis worldwide.