In meat juice medium, aerobic spoilage bacteria utilized the following substrates in the order shown: Pseudomonos, glucose, amino acids, lactic acid; Acinetobacter, amino acids, lactic acid: Enterobacter, glucose, glucose‐6‐phosphate, amino acids; Microbacterium thermosphactum, glucose, glutamate. All the bacteria grew at their maximum rate utilizing the first and second substrates, but the growth rates declined when these were exhausted. The growth rate of Acinetobacter was reduced at pH 5·7 and below. All other species grew at their maximum rate within the pH range 5·5–7·0. On meat pseudomonads grew faster than the other species at all temperatures between 2° and 15°C. Interactions between any two species were observed only when one organism had attained its maximum cell density. Substrate exhaustion at the meat surface did not limit bacterial growth and it is suggested that the maximum cell density of aerobic spoilage cultures is determined by oxygen limitation of growth.

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Joints of beef were stored in packaging films with oxygen permeabilities ranging from 0–920 ml/m224 h/atm at 25 °C and 100% r.h. The storage life of the ‘vacuum‐packaged’ meat, as assessed by discolouration and the development of putrefactive odours, was inversely related to film permeability; the best results were obtained with meat which received ‘zero oxygen’ treatment. The growth rates and final counts of Pseudomonas spp. increased with increasing film permeability; the storage life of the meat corresponded with the time taken for the counts of the organism to reach ca. 106/cm2 for putrefactive odours to be produced. Although their growth rate was unaffected, the final counts of Brochothrix thermosphacta also increased with increasing film permeability. These results are discussed in terms of the influence of film permeability on the inhibitory effects of Lactobacillus spp., whose numbers were unaffected by the permeability of the film used, and the substrates in the meat available to the bacteria.

The inhibitory effect of the lactic acid in meat on gram-negative psychrotrophs appears to be due mainly to the decrease in pH, not to action of the undissociated acid. Species of Pseudomonas were essentially unaffected by the pH of normal meat. Other gram-negative psychrotrophs isolated from a meatworks included a large number of strains which would not grow on meat of normal pH at chill temperatures. Raising either the pH or the incubation temperature allowed many of the pH-sensitive strains to initiate growth. However, the growth rates of pH-insensitive strains were not affected by increasing the pH, and there were no significant differences in the composition of the spoilage floras which developed on chilled meat of normal and high pH.

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