Comparison of AOAC, Microwave and Vacuum Oven Methods for Determining Total Solids in Milk

Abstract

A collaborative study was conducted on fluid milk for total solids measurement by a microwave oven and the AOAC method. The microwave oven method performed better than the AOAC method in repeatability and reproducibility of total solids measurement. In addition, comparative tests of the AOAC, microwave and vacuum oven methods showed high correlation coefficients between the methods. It was also observed that the microwave and vacuum oven methods consistently measured about 0.2o/'o higher in total solids than did the AOAC method. This tendency may be attributed to the browning which occurred with the AOAC method. California dairy processors pay dairy farmers for milk on the basis of fat and solids-not-fat contents. There are several methods of measuring solids-not-fat. One of the methods is to obtain solids-not-fat by subtracting fat from total solids measured by the AOAC method (2}. However, the AOAC method requires about 4 h in the procedures. The California dairy industry has considered the microwave oven method as a potential rapid method for use in the payment of milk and standardizing milk products. To evaluate the microwave oven method, collaborative tests were conducted on fluid milk for total solids measurement by the microwave oven and AOAC methods. The California Department of Food and Agriculture Dairy Laboratory and one industry laboratory participated in the tests. From the collaborative test results, repeatability and reproducibility (5) ofthe AOAC and microwave oven methods were estimated. In addition, the State Laboratory made comparative tests of the AOAC. microwave and vacuum oven methods for correlations between pairs of the methods. MATERIALS AND METHODS Collaborative study Seventeen samples of fluid milk were prepared by the State Laboratory and shipped to the industry laboratory with instructions for the analyses. The milk samples ranged from 9.59 to l4.16"7o total solids. All analyses were in duplicate.