Non-casein nitrogen

Initially, it was believed that milk contained only one type of protein but about 100 years ago it was shown that the proteins in milk could be fractionated into two well-defined groups. On acidification to pH 4.6 (the isoelectric pH) at around 30°C, about 80% of the total protein in bovine milk precipitates out of solution this fraction is now called casein. The protein which remains soluble under these conditions is referred to as whey or serum protein or non-casein nitrogen. The pioneering work in this area was done by the German scientist, Hammarsten, and consequently isoelectric (acid) casein is sometimes referred to as casein nach Hammarsten. 

About 20% of the total protein of bovine milk belongs to a group of proteins generally referred to as whey or serum proteins or non-casein nitrogen. Acid and rennet wheys also contain casein-derived peptides both contain proteose-peptones, produced by plasmin, mainly from /J-casein, and the latter also contains (glyco)macropeptides produced by rennets from K-casein. These peptides are excluded from the present discussion. 

NCN, Non-casein nitrogen NPN, non-protein nitrogen TN, total nitrogen. 

In addition to the caseins and whey proteins, milk contains two other groups of proteins or protein-like material, i.e. the proteose-peptone fraction and the non-protein nitrogen (NPN) fraction. These fractions were recognized as early as 1938 by Rowland but until recently very little was... 

Total nitrogen = Kjeldahl I Casein = Kjeldahl I-Kjeldahl n Non-protein nitrogen = Kjeldahl III... 

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