Changes in milk native content of several carbon and nitrogen sources were studied, along with growth at 7°C of a Pseudomonas fluorescens strain. The aim was to characterize the particular compositional environment in milk in which psychrotrophic bacteria produce their extracellular proteinases. Glucose and lactate were depleted from milk, pyruvate and gluconate were significantly diminished, but citrate was mostly unused when proteinase was first detected by the Hide Powder Azure assay, the psychrotrophic count being around 1010 CFU/ml. At that stage, levels of ammonia, amino acids and short peptides had just started to rise and only about 20% of the original urea had been consumed. A procedure to anticipate, in cold stored raw milk batches, the time for production of extracellular proteinase, on the basis of sensitive lactate and ammonia determination, is suggested.

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