ABSTRACT
A group of 80 Pseudomonas spp. strains isolated from raw milk shortly after milking was compared to another group of 81 obtained from the same sample after incubating it at 7°C for 3 days. Comparison of both collections of strains included growth rates at 7°C and 21°C and production of extracellular proteinase, lipase, and siderophores. The strains selected after cold incubation showed an average to-fold higher growth rate at 7°C, 1,000-fold more proteolytic activity, and 280-fold more lipolytic activity than those found before the incubation. At 21°C, however, they grew half as quickly as the strains isolated before the incubation. In all but one of the 161 Pseudomonas strains tested, there was some production of siderophores, and yields were only moderately increased in the strains obtained after incubation of the milk at 7°C. These changes in spoilage-related properties took place while global Pseudomonas counts increased only 13-fold. Distribution frequencies of the variables tested, their correlation coefficients, and regression models are shown.
ABSTRACT
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.
