ABSTRACTA set of 70 organisms consisting of 9 lactic acid bacteria, 14 non‐lactic gram positives, 16 yeasts, 6 molds, 12 pseudomonads, and 13 nonpseudomonad gram negative bacteria was tested on plate count agar (PCA) at pH 5.0 to 8.0 and in the presence of varying concentrations of diacetyl. All organisms were more sensitive to diacetyl at a pH of 7.0 and below than at pH 8.0 with the greatest sensitivity at pH 5.0. The lactic acid bacteria were the most resistant while the pseudomonads were the most sensitive. Excluding the lactic acid bacteria, 300 ppm completely inhibited 95, 90, 85, and 43 respectively at pH 5.0, 6.0, 7.0, and 8.0. Using PCA at pH 6.0, three Gram negative bacteria were inhibited by 86 ppm in pour plates while 6 of 7 nonlactic cultures were inhibited by 172 ppm. The compound was shown to be effective in fresh ground bee
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