Effectiveness of temperature modification in decreasing the bias in milk fat test results between the Babcock and...

Both the Babcock (AOAC Method 989.04, revised Final Action 2000) and modified Mojonnier ether extraction (AOAC Method 989.05) methods are used in the dairy industry to determine the fat content of milk. Prior to revision in 1997, the Babcock method gave consistently higher fat test results than did the ether extraction. In 1997, a modification of the Babcock method was introduced to bring the results of the Babcock test into closer agreement with the ether extraction. The Babcock method was modified by lowering the temperatures used at various points in the method from about 57.5 to 48 degrees C to increase the density of the material in the Babcock column. A collaborative study of the modification indicated it was successful in bringing the Babcock and ether extraction results into agreement but suggested that performance of the modified method was not as good as that of the unmodified method. In the present study, substantial evidence is presented to validate the success of the Babcock modification in bringing test results into agreement with ether extraction, and to document that temperature modification does not adversely affect method performance. Data were evaluated from an on-going proficiency testing program where 8-15 laboratories tested 7 milk samples in blind duplicate once every 2 months. Laboratories used the unmodified method from 1995 through 1996 and the modified method from 1998 through 1999. Compared with ether extraction, test results from the unmodified Babcock test were consistently higher by an average of 0.022% fat. For the modified Babcock test, average test results were -0.003% fat lower than with ether extraction and not significantly different from zero. AOAC method performance statistics (within- and between-laboratory precision) were equivalent for both the unmodified (Sr = 0.027, SR = 0.041, RSDr = 0.73%, RSDR= 1.08%) and modified (Sr = 0.023, SR = 0.038, RSDr = 0.60%, RSDR = 1.02%) Babcock methods. Modification of the Babcock method was successful in bringing test results into agreement with those of ether extraction.