Evaluation of Dye Reduction Tests for Manufacturing-Grade Bulk-Tank Milk

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Abstract
Resazurin reduction test results and standard plate counts with incubation at 32[degree]C were compared on 670 manufacturing-grade bulk-tank milk samples obtained at all seasons of the year. Thirty-three % of the samples had standard plate counts at least twice as high as would be expected from the resazurin test results for classifying milk. Samples placed in resazurin class-1 included samples with standard plate counts from less than 50,000/ml to over 21,000,000/ml, Samples placed in resazurin class-1 would be expected to have plate counts of 200,000/ml or less. Psychrophilic, thermoduric, or a combination of both types of bacteria usually constituted a high proportion of microflora of the milk when the results of the resazurin and standard plate count results did not agree. Representative psychrophilic cultures had poor resazurin reducing ability. Eleven of the 26 cultures with plate counts exceeding 1,000,000/ml were placed in resazurin class-1. Cultures placed in this class had plate counts ranging from 1,500,000-42,000,000/ml. A bath temperature of 28oC for the resazurin test did not improve the test results when compared to the standard plate count. Similar patterns were obtained in comparing the methylene-blue test with the standard plate count. The resazurin test was slightly more sensitive to higher bacterial population that the methylene-blue test. Reduction of 2,3,5-triphenyltetrazolium chloride did not accurately predict the plate count of milk.

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