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Evaluation of Dye Reduction Tests for Manufacturing-Grade Bulk-Tank Milk^{1, 2,}

Department of Dairy and Food Industry, Iowa State University, Ames

ABSTRACT

Early work (5, 6, 9), using a 1-hr reduction time and a 3-hr reduction time (7) indicated that milk could be graded simply and accurately by the resazurin test. Good correlation was found between the standard plate method and the resazurin test results. At that time, the holding temperature of the milk on the farm commonly was above 10 C, and microorganisms able to reduce resazurin apparently predominated. Jones and Davis (8) indicated that in practically every case the organisms capable of souring milk reduced resazurin rapidly.

Recently, work (17) has shown that the resazurin test was quite unsuitable for bacterialogical control of bulk milk collection in Scotland. Watt (18) also indicated that, as a basic raw milk test, resazurin reduction was inadequate. He reported that it bore no relationship to the plate count and that it allowed many unsanitary practices to escape detection.

Other workers (3, 4, 11, 16) reported experiments using tetrazolium compounds for determining the bacteriological quality of raw or pasteurized milk.

¹ Journal Paper No. J-5057 of the Iowa Agricultural and Home Economics Experiment Station, Ames, Iowa. Project No. 1343.

² This project was supported in part by a grant from the Dairy Products Marketing Association, Incorporated.

³ Present address: Department of Dairy Science, University of Arizona, Tucson, Arizona.