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Mark II Pro-Milk Tester for Estimation of Protein Percentage in Plant Milk Supplies¹

Central Milk Testing Laboratory, Ministry of Agriculture and Food, Guelph, Ontario, Canada
Department of Food Science, University of Guelph, Guelph, Ontario, Canada

ABSTRACT

The Mark II Pro-Milk Tester was evaluated for its accuracy, repeatability, reliability, and operational limitations. During 1 yr the instrument calibration was checked continually with preanalyzed calibration milks prepared by combining individual cow milks at specific fat contents. Protein ranged from 2.96 to 4.24% by a semi-micro Kjeldahl method. Plant milk samples (833) were analyzed with the instrument and protein of these samples was from 2.75 to 3.65% by the Kjeldahl method.

From regression analysis calibration maintained with calibration milks was unsuitable for use with the plant milks. For the 1 yr the standard deviation of difference between Kjeldahl and Pro-Milk results for calibration milks was .031 whereas for the plant milks it was .074. Repeatability of the Pro-Milk test ( ± .0110) was about the same as with the Kjeldahl (± .0114), and mean difference between Kjeldahl and Pro-Milk were small both for calibration milks (.0007) and plant milks (.0049). The larger standard deviation of difference with plant milks, therefore, was the result of a different requirement for calibration slope.

Six adjustments of calibration were required during the year, three with changing from one dye lot to another and three with the same dye lot. As there was no noticeable drift in instrument read-out at any time, changes in calibration could be required because of differences in purity of dye or because of seasonal changes in mean dye binding capacity of milk proteins. Calibration of the Pro-Milk Tester needs to be monitored continually and its results adjusted to conform to those of an official method for the type of milk to be analyzed routinely with the instrument.

¹ This research was initiated and financed by the Ontario Ministry of Agriculture and Food, Milk Commission of Ontario. Instrument used was provided by courtesy of De Laval Company Limited, Peterborough, Ontario, Canada.