Detectability Levels of Four Beta-Lactam Antibiotics in Eight Milk Products Using the AOAC Bacillus stearothermophilus Disc Assay

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Abstract
The 50% dectability level (ED50) of the Bacillus stearothermophilus disc assay in raw, pasteurized whole, protein-fortified lowfat, lowfat, and skim milks, half-and-half, heavy cream and goat's milk was determined for penicillin G, ampicillin, cloxacillin and cephapirin. Results demonstrate a lower level of detectability with PM agar than with A4 agar for ampicillin, cloxacillin and penicillin. Ranges of detection using PM agar at 64°C were 0.0025 to 0.0042 IU/ml (0.0016 to 0.0026 μg/ml) for penicillin G, 0.0021 to 0.0042 μg/ml for ampicillin, 0.0030 to 0.0059 μg/ml for cephapirin and 0.0167 to 0.0334 μg/ml for cloxacillin. Liquid penicillinase is recommended when performing the confirmation test for beta-lactam identification.

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