Precision and Accuracy of Infrared Milk Analysis

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Abstract
Estimates of precision and accuracy of milk analysis by the IR method are reported for comparison studies done at the University of Guelph, the Ontario Central Milk Testing Laboratory in Guelph, the British Columbia Department of Agriculture Laboratory in Vancouver, B.C., and the Dairyman’s Cooperative Creamery Association in Tulare, Calif. The standard reference methods used were: the Mojonnier method for fat, a semimicro Kjeldahl method for protein, a polarimetric method for lactose, the USDA lactometer method for solids-not-fat, and the AOAC method for total solids. Mean differences of 0.01 or less and standard deviations of difference between means of duplicate tests of the order of ±0.03 have been reported for fat, protein, and lactose; whereas, for solids-not-fat or total solids, mean differences have been about 0.015% and standard deviations of difference about ±0.09%. Best results have been obtained when calibrations have been routinely checked with a reference standard method, and have been adjusted when accumulated calibration data showed either the slope or level calibration to be in need of adjustment. Since the IR method essentially reproduces the results which would be obtained by the standard analytical method used for calibration of the IR instrument, significant differences between the results produced by different laboratories with standard methods will cause significant differences between the IR results produced by the same laboratories. This has been substantiated by the results of a collaborative study in which 5 pasteurized homogenized and 5 raw milk samples were analyzed by both standard and reference methods at the above mentioned laboratories and at Ross Laboratories in Columbus, Ohio. Although regression analysis showed that the calibrations at the various laboratories were producing good estimates of the standard values, there nevertheless were statistically significant differences between the mean values produced by these laboratories for both the standard and IR methods. The IR milk analyzer method for the analysis of milk has been adopted as official first action.

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