We recently received calls from dairy producers who wondered why they were being paid for "true protein" and not the percentage of total protein in their milk. The following is a brief summary of what has happened.
The Kjeldahl test has historically been used to measure the amount of protein in milk. This test, however, does not directly measure the percentage of protein, but the nitrogen content of the milk. The Kjeldahl analysis presumed that "all the nitrogen found in milk is contained in the protein." The Kjeldahl nitrogen analysis test can still be used to measure the amount of protein, but adjustments are needed to determine the amount of non-protein nitrogen (NPN) so the true protein can be measured. Non-protein nitrogen is made up of compounds such as urea. The summation of true protein and NPN equals the old "total" protein
In practice, the Kjeldahl test is not the common method used to measure protein in milk today. Milk infrared analyzers have become the most common test instruments used to determine the amount of true protein. These analyzers eliminate essentially all of the measurement error in determining the amount of usable protein in milk.
Vernal Packard, a food scientist at the University of Minnesota, summarized the reasons why true protein should be used instead of total protein. Some of the reasons he outlined include the following:
� NPN has no value in cheese yield. Thus, it has no place in payment for milk that is used to make cheese, which is the primary product in the western milk marketing order.
� If all plants do not pay on the same basis there is a source of confusion. The changes in the Milk Marketing Order reform that were implemented on January 1, 2000, made payment on true protein rather than crude or total protein the standard. Also, as of May 1, 2000, all DHIA laboratories now test for true protein.
� The level of NPN is not uniform in all milk. The percentage of NPN varies by breed of animal (Jerseys generally have the highest percentage of "true" protein and lowest percentage of NPN), season (the percentage of NPN is generally highest in the summer) and area or region of the country. The use of true protein rather than the total protein allows pricing that reflects these differences.
The change from total crude or total protein to "true" protein allows plants to pay for only that portion of the milk that is usable protein. It should therefore be viewed as a positive step in receiving equitable payment for milk in the marketplace.©