A non-protein nitrogen index for discriminating raw milk protein adulteration via the Kjeldahl method
In this paper, we have developed the concept of the non-protein nitrogen (NPN) index defined as a ratio coefficient of NPN content and crude protein nitrogen (CPN) content in milk. To demonstrate the NPN index, the NPN and CPN in a series of milk samples were determined by using the Kjeldahl method and the trichloroacetic acid precipitation method (TCA). The NPN index of raw milk was chosen for accurately discriminating milk adulteration with ammonium chloride, ammonium sulfate, urea, melamine, and whey powder. The results showed that all of the NPN indices of adulterated milk fell outside the discriminating ranges, which were supposed to be from 0.8084 to 1.4987, unless the amount of melamine was less than 0.2 mg kg−1 in adulterated milk. All the results suggested that the NPN index could be used to accurately detect milk adulteration with nitrogen-rich substances and provided an efficient way to monitor milk quality.