Proteolytic behavior of isolated Pseudomonas spp. from refrigerated raw milk in different concentrations and storage temperatures

Abstract

The objective of the work was to evaluate the multiplication capacity and proteolytic activity of different Pseudomonas spp. cell counts inoculated in milk and storaged under different temperature. Strains isolated from refrigerated raw milk (RRM) were confirmed at genus level by Polymerase Chain Reaction (PCR). The Pseudomonas spp. was cultured in cephalothin-sodium fusidate-cetrimide (CFC) agar-base (30?C for 48 h) until it reached 2 log and 6 log CFU mL-1. Three of eight strains confirmed as Pseudomonas spp were inoculated in sterile reconstituted whole milk powder and incubated at 2°C, 4°C, and 8°C for 96 h. Primary proteolysis indices was determined by the Kjeldahl method. When taking into account the effect of storage time in Pseudomonas spp. population, it was found that the initial population (2 log CFU mL-1) showed significant difference in growth rates only from 0 h to 24 h, keeping at the same levels along 96 h. When a higher initial population was incubated (6 log CFU mL-1), it was not observed a significant difference for times tested. Related to the effect of storage time in proteolysis index, it was not observed a significant difference in samples inoculated with 2 and 6 log CFU mL-1 Pseudomonas spp. When we analyzed the influence of storage temperature on the bacterial multiplication, there was a significant difference in the Pseudomonas spp. population only between 2°C and 8°C after 96 h of milk storage with 2 log CFU/mL of initial inoculum. If we consider the temperature effect in the primary proteolysis index, there were significant differences at the inoculum of 2 log CFU mL-1 where the primary proteolysis at 24 h was lower at 2°C than at 8ºC. Low temperatures or short storage time had no influence on Pseudomonas spp. enumeration or in the primary proteolysis index when high initial contaminations are observed. At lower Pseudomonas spp. initial population, the smaller storage time tested influenced the population control, and linked with the reduction in the storage temperature, lower proteolysis index were observed.