Crude and Protein Nitrogen Bases for Protein Measurement and Their Impact on Current Testing Accuracy

¹ Northeast Dairy Foods Research Center, Department of Food Science, Cornell University, Ithaca, NY 14853

As the value of milk protein increases, accuracy of measurement of milk protein content becomes more important. Traditionally, milk protein content has been estimated from total N. However, about 5 to 6% of milk total N is not associated with protein and varies among farms, regions, and seasons. Currently, most milk protein testing is done with infrared milk analyzers using Kjeldahl total N as a basis for protein calibration. Analytical errors occur as a result of using total N as the basis for protein calibration of infrared analyzers instead of true protein. These errors are caused by two factors: differences in mean NPN as a percentage of total N from one set of calibration samples to the next and differences in NPN as a percentage of total N between samples within each set of calibration milks. Examples of the magnitude of errors created by these factors are calculated using actual data from industry laboratories. The accuracy of protein testing using infrared analyzers for payment and record-keeping purposes could be improved substantially by using true protein as a basis for calibration instead of total N.

Key Words: protein • infrared milk analysis • nonprotein nitrogen

Submitted on September 30, 1991
Accepted on February 10, 1992

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