Evaluation of the growth of Salmonella enterica in infant milk formulas under different preparation and storage conditions

Abstract

Salmonella enterica is one of the most important pathogens associated with infant milk formulas (IMFs). In this study, the survival and growth of an S. enterica serovar Typhi strain in two reconstituted IMFs (F1 and F2) under different conditions of preparation and storage, similar to those used in hospital lactaries, was evaluated. For both IMFs inoculated at room temperature, there was 3.3 log increase in the population of S. enterica by the first 24 h of incubation and > 4.5 log increase at the end of 72 h. At 4ºC, in the first 48 h of incubation, there was a slight increase in population (< 1.0 log) and after 72 h, growth was similar to that observed at room temperature. A heated water bath was used to test the effect of thermal treatment methods on bacterial viability. At 60ºC for 5 min, there was a reduction of slightly more than 1 log CFU/mL; however, at 60ºC for 10 min, the observed decrease was 2.8 log for F1 and 2.3 log for F2. At 70ºC for 5 min, there was a reduction of approximately 3 log CFU/ml for both IMFs and for 10 min, this reduction was approximately 1 log higher. Heating in a microwave oven was the most efficient way of reducing populations of Salmonella in the IMFs, as bacterial cell counts were not detected after any of the treatments used. Our results suggest that IMF contamination during the preparation step may support bacterial growth even under refrigeration if kept for a prolonged time and that some thermal treatment methods are not sufficient for the complete inhibition of S. enterica