Improving recovery of Salmonella enterica serovar typhimurium DT104 cells injured by heating at different water activity values

Abstract

This study describes the evaluation of potentially more sensitive methods for the recovery of Salmonella cells injured by heating (54 to 60°C) at different water activity values (0.65 to 0.90, reduced using equal portions of glucose and fructose). These methods included gradual rehydration, the use of diluting media with added solutes or blood, the addition of blood to plating agar, and the use of different incubation temperatures and times. Gradual rehydration of cells that had been challenged at low water activity (0.65 and 0.70) and high temperature markedly improved recovery, measured as a >50% increase in the time to obtain a 3-log10 reduction in cell numbers, compared to dilution into media with a high water activity. Adding sucrose, glycerol, or blood to the diluting media (maximal recovery diluent) did not improve recovery, but a plating agar containing blood recovered approximately 38% more cells than nutrient agar. Prolonged incubation of agar plates allowed recovery of injured Salmonella cells that presumably had extended lag periods, with significantly higher recovery rates after 48 h incubation at 37°C than after 24 h (P=0.05). This work highlights that by recovering Salmonella using a method specific to the nature of the injury, a better prediction of food safety and the success of food processing can be made.