RELATIONSHIP BETWEEN THE KEEPING QUALITY PROPERTIES OF GRADE A RAW MILK AND STANDARD PLATE, THERMODURIC, AND COLIFORM COUNTS1

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Abstract
Summary The usefulness of a keeping quality test for evaluating the quality of a Grade A raw milk supply has been investigated. One hundred bulk tank truck and 81 individual producer samples were examined for keeping quality properties (development of off-odor, “leathery-like” fat, or gas during storage at 20–21 C for 16 to 40 hours) and for standard plate count (SPC), thermoduric count (TC), and coliform count (CC). Multiple correlations observed between the keeping quality properties and the bacterial counts of the milk were: off-odor vs. SPC, TC, and CC, 0.46; “leathery-like” fat vs. SPC, TC, and CC, 0.42; gas vs. SPC, TC, and CC, 0.38; SPC vs. off-odor, “leathery-like” fat, and gas, 0.49; TC vs. off-odor, “leathery-like” fat, and gas, 0.33; and CC vs. off-odor, “leathery-like” fat, and gas, 0.53. Using 100,000/ml for SPC, 1,000/ml for TC and 1,000/ml for CC as upper limits for Grade A milk, over 75% of the samples exceeding one or more of these limits were detected by the keeping quality test after 16 hours incubation. Longer incubation periods resulted in the detection of higher percentages of samples which exceeded one or more of the bacterial standards. Based on results obtained, speed of obtaining results, and simplicity of the test, it is believed that this raw milk keeping quality test merits consideration as a supplement to other routine quality control procedures, particularly for plants having minimum laboratory facilities.

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