The psychrotrophic microflora of raw milk from a Cornell University herd was examined and the three most frequently occurring isolates (Pseudomonas species) were subjected to oxygen concentrations of 1 to 12 ppm and temperatures of 3 to 9°C in growth studies in raw milk. At 3°C, a reduction in oxygen level from 9–12 to 1–3 ppm resulted in a 63% increase in generation time for Pseudomonas fluorescens. However, the reduction in growth temperature from 9 to 3°C at 9–12 ppm oxygen produced a 280% generation time increase for P. fluorescens. Similar observations were made for the other isolates. An analysis of variance revealed a significant interaction between the effects of oxygen and temperature on growth of the isolates.

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1Growth on SPC agar in·2 d at 42°C, hydrolysis of gelatin within 7 d at 32°C, growth on cetrimide agar in 2 d at 32°C, production of brown pigment, anaerobic utilization of dextrose in 3 d at 32°C, production of alkaline conditions when grown on xylose in 3 d at 32°C, and susceptibility to penicillin G and polymixin B.