Purpose: The purpose of this study was to quantify the effect of temperature and humidity on the inactivation of Salmonella and a common non-pathogenic surrogate (Enterococcus faecium) in a low-moisture powder on stainless steel.
Methods: A soy-based protein powder was inoculated (~109 CFU/g) with a 3-strain cocktail of Salmonella (Enteritidis PT30, Tennessee, and Enteritidis PT4) or E. faecium (NRRL B-2354), adhered in a thin-layer (~0.5 mm) to stainless steel coupons, equilibrated to ~0.175 aw, then heated in a pilot-scale moist-air impingement oven (Tdb = 93, 121, or 149°C; Tdp = ~33, 60, or 71°C; vair = 1.2 m/s; 3 reps). Surviving Enterococcus and Salmonella were enumerated by plating recovered samples on deMan, Rogosa, and Sharpe agar or modified trypticase soy agar, respectively. Protein residue temperatures were measured using surface thermocouples on the coupons, and used to calculate D-values for both organisms.
Results: For Tdb = 121°C, and Tdp of ~33, 60, or 71°C, the D-values for Salmonella and E. faecium were 34, 21, 9 and 20, 18, 10 min, respectively. For all but the highest humidity (71°C) the E. faecium inactivation rate was higher (P < 0.05) than for Salmonella. At Tdp of ~33°C, increasing Tdb from 121 to 149°C reduced (P < 0.05) the Salmonella D-value from 34 to 1.5 minutes (R2= 0.98, 0.98).
Significance: Air temperature and humidity both significantly affected inactivation rates for Salmonella and E. faecium; however, utilization of the surrogate for process validation would require comparison to Salmonella resistance at the specific process conditions.