Crude Protein Calculator

Crude protein is not a measurement of protein at all. It is a measurement of nitrogen scaled by a factor. Total nitrogen is determined by Kjeldahl digestion or by Dumas combustion and multiplied by a conversion factor to give a protein figure. The default factor of 6.25 embeds the assumption that protein averages 16 percent nitrogen, since 100 divided by 16 equals 6.25. Amino acid composition makes that assumption approximate, so commodity-specific factors are used: 6.38 for milk, 5.70 for wheat, 5.71 for soybean, 5.95 for rice, and 5.55 for gelatin. Choosing the wrong factor moves the answer by up to 10 percent, which matters when soybean meal is traded on a 44 or 48 percent protein basis and wheat flour is graded around an 11.5 percent threshold for bread making. Because the method counts every nitrogen atom, non-protein nitrogen inflates the result. Free amino acids, nucleotides, urea, betaine, nitrate in leafy vegetables and forages, and urea deliberately added to ruminant feed all register as protein, so crude protein typically exceeds true protein by 5 to 15 percent, and considerably more in silage or nitrate-accumulating crops. Results should also be stated on a defined basis, since a feed at 42.9 percent crude protein as received is 48.2 percent on a dry matter basis at 11 percent moisture. The method says nothing about amino acid balance or digestibility, which is why swine and poultry rations are increasingly formulated on digestible lysine and other limiting amino acids rather than on crude protein alone.

Johan Kjeldahl developed the nitrogen determination that bears his name at the Carlsberg Laboratory in Copenhagen, publishing it in 1883. He was studying protein changes during malting and brewing and needed something faster and more reliable than the combustion methods then available. His procedure of digestion in hot concentrated sulfuric acid to convert organic nitrogen to ammonium, liberation of ammonia with excess alkali, distillation, and titration proved robust enough that it remains an official reference method more than 140 years later, with catalysts and block digestors the main modernisations. The 6.25 factor traces to Gerardus Johannes Mulder, who in the late 1830s analysed protein substances and reported a nitrogen content near 16 percent, an average that was carried forward as a universal constant. D. Breese Jones of the US Department of Agriculture challenged that universality in Circular 183, published in 1931, tabulating separate factors for individual foods based on their measured nitrogen contents, and that is the source of the wheat, milk, and soy values still in use. The alternative combustion route goes back further still, to Jean-Baptiste Dumas in 1831, but only became practical for routine work with automated instruments from the 1980s onward. The limits of nitrogen-based protein testing were exposed publicly when melamine, a nitrogen-rich industrial chemical, was added to milk and pet food products in 2007 and 2008 to fake high protein readings.