Reference and routine methods for measurements of N fractions [in milk]
1994
Barbano, D.M. (Cornell Univ., Ithaca (USA). Northeast Dairy Foods Research Center)
As the value of milk protein increases, accuracy of measurement of milk protein content becomes more important. This presentation provides a summary of the status of reference and routine analytical methods for measurement of the protein content of milk and dairy foods, a focus on special factors that influence the accuracy of protein testing for milk payment, and an identification of some of the needs in protein testing method development and standardization; The analytical reference for milk protein determination has been the measurement of total nitrogen by the Kjeldahl method multiplied by 6.38. Today, Kjeldahl factors can be calculated more accurately from the known amino acid sequences of individual proteins in milk and this is discussed. It has been known for a long time that not all of the nitrogen present in milk is associated with protein. Nonprotein nitrogen (NPN) present in milk accounts for, on average, about 5-6 % of the total nitrogen. Nonprotein nitrogen content of milk varies from farm to farm, region to region and seasonally. Historically, the dairy industry has measured the total nitrogen (TN) content of milk. However, milk TN includes both true protein (TP) nitrogen and NPN. A new official Kjeldahl reference method for direct measurement of the TP content of milk is available. Currently, most routine milk protein testing is done with infrared milk analyzers using Kjeldahl TN multiplied by 6.38 as a basis for protein calibration. Analytical errors occur as a result of using TN as the basis for protein calibration of infrared analyzers instead of TP. Explanations of cause of these analytical errors and possible approaches for their elimination are provided.
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