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

This study describes the evaluation of potentially more sensitive methods f or the recovery of Salmonella cells injured by heating (54 to 60 degreesC) at different water activity values (0.65 to 0.90, reduced using equal porti ons of glucose and fructose). These methods included gradual rehydration, t he use of diluting media with added solutes or blood, the addition of blood to plating agar, and the use of different incubation temperatures and time s. Gradual rehydration of cells that had been challenged at low water activ ity (0.65 and 0.70) and high temperature markedly improved recovery, measur ed as a >50% increase in the time to obtain a 3-log(10) reduction in cell n umbers, 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 a pproximately 38% more cells than nutrient agar. Prolonged incubation of aga r plates allowed recovery of injured Salmonella cells that presumably had e xtended lag periods, with significantly higher recovery rates after 48 h in cubation at 37 degreesC than after 24 h (P = 0.05). This work highlights th at by recovering Salmonella using a method specific to the nature of the in jury, a better prediction of food safety and the success of food processing can be made.