What is the difference between crude protein and true protein?

Crude protein, sometimes called total protein, is estimated from measuring the total nitrogen content of milk by Kjeldahl analysis. Nitrogen is multiplied by 6.38 to express the results on a protein equivalent basis. The total amount of nitrogen in milk, however, comes from both protein and non- protein nitrogen sources. True protein re- flects only the nitrogen associated with protein and does not include the nitrogen from non-protein sources.

What is non-protein nitrogen?

This is a normal part of milk. The non- protein nitrogen (NPN) fraction is com- posed of urea and other low molecular weight nitrogen-containing compounds such as creatine and creatinine. About 50% of the NPN in milk is urea, and variation in NPN is attributed primarily to variation in urea content. Non-protein nitrogen has little nutritional value and does not con- tribute to cheese yield. Therefore, it does not have the same economic value as “true” milk protein to either the processor or the consumer.

How are crude protein and true protein measured?

Kjeldahl nitrogen analysis forms the basis for the reference tests for both crude and true protein. In both cases, nitrogen is multiplied by 6.38 to express the results on a protein-equivalent basis. Milk infra- red analyzers are the most common test- ing instruments used for determination of  protein for payment testing. They are cali- brated using results from Kjeldahl refer- ence testing. These instruments detect a signal generated from the protein mol- ecules. In simple terms, the machines “see” protein, but cannot see NPN substances.

Why change the basis for measurement of the protein concentration in milk from crude protein to true protein?

In the past, most electronic milk testing equipment were calibrated on a crude pro- tein basis. This created problems because, although the NPN varied, the machine could not measure this variation. By cali- brating on crude protein, a certain amount of error was inevitable when the machine attempted to predict something it could not measure. The direction and magnitude of these errors are not easily predicted, as NPN is not well correlated with either crude or true protein level. These errors are eliminated when true protein is used as the basis for calibration, because the electronic testing instruments can directly detect the protein signal.