The electrical impedance of media is altered with chemical changes brought about by microbial metabolism and growth. Time required to bring about readily detectable change (detection time — DT) is a function of the initial levels of microorganisms in the sample. DTs were compared to Standard Plate Counts for 407 milk samples — homogenized, low fat, skim and raw. Using the criterion that a sample of pasteurized milk with a DT of 7 h or less was indicative of a plate count of 10,000/ml or greater, 323 of 380 samples were correctly classified. For raw milk, the DT was 10 h to resolve samples into greater or less than 10,000 organisms per ml. Results of a preliminary study on estimation of psychrotrophs in pasteurized milk showed that impedance monitoring at 21 C provided a 22-h screen correctly classifying 88% of the samples into categories of more than or less than 1,000 organisms per ml. Better agreement (91%) in a shorter time (13.7 h) was obtained with a screen for 10,000 organisms. Finally, for the first 22 samples analyzed, keeping quality data on pasteurized milk have correlated better with post-pasteurization impedance measurements than with either post-pasteurization total counts or psychrotrophic counts.

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