Determination of Nonprotein Nitrogen Components of Milk by Nuclear Magnetic Resonance

Nonprotein nitrogen compounds may be added to milk by unscrupulous producers to boost the apparent protein concentration. These compounds are nephrotoxic and may cause urinary disease. Nuclear magnetic resonance (NMR) was used for the simultaneous determination of urea, melamine, guanidine, biuret, and diaminoguanidine in milk. The nitrogen-containing compounds were accurately quantified with recoveries between 80 and 110% with relative standard deviations less than 6%. The limit of quantitation was 2.0 mg/L for melamine and urea; 8.0 mg/L for guanidine, biuret, and diaminoguanidine. The urea in milk was quantified by NMR, and the concentration varied with the production region. However, the level did not exceed the maximum safe level of 620 mg/L.