Cooling raw milk : change in the spoilage potential of contaminating Pseudomonas

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.