Purpose: The purpose of this study was to evaluate survival of thermally-stressed Salmonella in a model food system (peanut paste) with various combinations of aw and fat levels.
Methods: Four model peanut pastes with combinations of two different levels of fat (47 and 56%) and aw (0.3 and 0.6) were inoculated with a dry inoculum of Salmonella Tennessee, Salmonella Typhimurium DT104 or Enterococcus faecium. Inoculated pastes were heat-treated at 75ºC for 30-50 min to obtain approximately 1.0-log reduction of each organism and stored at 20 ± 1ºC for ~ 1 year. Monthly monitoring of survivors was conducted using tryptic soy agar plate counts.
Results: Greater log-reductions of Salmonella Tennessee, Salmonella Typhimurium and E. faecium occurred in pastes at higher aw than lower aw, regardless of the fat levels during 10 months of storage. Salmonella Tennessee showed a ~1.7 log CFU/g reduction in 0.3 aw pastes at both fat levels; a greater reduction (2.7 and 2.3) was observed in the 0.6 aw pastes at 47 and 56% fat, respectively. Salmonella Typhimurium showed a similar survival trend, with enhanced survival occurring in 0.3 aw pastes when compared to the 0.6 aw pastes. While a similar trend occurred in E. faecium, higher survival rates were observed for the surrogate in all tested conditions; the decline of E. faecium in all formulations was within the range of 1.1-1.8 log.
Significance: The study provides information on the influence of fat on Salmonella survival in low aw foods. Higher levels of fat in a peanut paste matrix appear to decrease the survival of Salmonella.