Methyl Sulfide Production by Aerobacter aerogenes in Milk

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Abstract
In 3 trails in which A. aerogenes was inoculated in vacuum heat-treated milk and incubated at 5 C for 7 days, methyl sulfide was produced. The milk was prepared from commercial pasteurized homogenized milk that was steam-distilled at 63 C for 90 min. at 26 cm of Hg and cooled to 20-25 C. The concentration of methyl sulfide, attained a maximum of 500-800 ppb when bacterial counts reached between 1 and 100 million/ml. This occurred between the 3rd and the 5th day after inoculations of 3000-4000/ml. During the 7 day incubation periods, counts in control milk changed from 50-200/ml at the time of inoculation to 500-9000/ml at the end of the trail. The concentrations of methyl sulfide were well above flavor threshold levels of 115 ppb in homogenized milk. There were off flavors in the inoculated milk when measured concentrations of methyl sulfide surpassed this threshold level. This flavor defect was demonstrated in commercial milk samples from which A. aerogenes was isolated.

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