Determination of the Dry Heat-resistance of Salmonella spp. and Eschericha coli O157:H7 Dried on to Stainless Steel Surfaces in Low Loisture Food Production Environments

Introduction: It is known that the dry heat resistance of members of the pathogenic species Salmonella enterica and some other vegetative organisms is greatly increased at low water activity, potentially allowing resistant populations to survive thermal decontamination processes.  This affect is known to vary between different low moisture foods.  However, very little published data is currently available concerning the heat resistance of such organisms under low moisture conditions in dry food processing environments.

Purpose: To determine the heat resistance characteristics of Salmonella Typhimurium ATCC 14028 and a desiccation-resistant strain of Salmonella Enteritidis across a 20°C temperature range, providing data demonstrating the effect of dry heat on Salmonella and Escherichia coli O157:H7 ATCC 43888 on stainless steel surfaces. To facilitate the design of thermal treatments which may potentially allow decontamination of sections of process lines, equipment and food contact surfaces.

Methods: Salmonella and Escherichia coli O157:H7 strains were dried onto the surface of stainless steel discs and exposed to dry heat on the surface of an aluminium block in an oil bath at 95, 100, 105, 110 and 115°C (for Salmonella spp.) or 75, 80, 85, 90 and 95°C (for Escherichia coli).  In all cases, survivor curves were used to calculate D and z values for the organisms tested.

Results: Analysis of the death kinetics of Salmonella Typhimurium ATCC 14028 and S. Enteritidis on stainless steel discs exposed to dry heat has given z-values for these strains of 24.4 and 28.7C° respectively.  Dry heat D-values on stainless steel at 105°C were found to be 70.5 and 51.4 minutes respectively.   Initial testing of Escherichia coli O157:H7 and Cronobacter sakazakii on stainless steel gave D-values of <3 minutes and 9.9 minutes respectively at 95°C.

Significance: When compared with previously published D-values determined for S. Typhimurium ATCC 14028 in low a_w nuts & seeds (102.4-242.1 minutes, Campden BRI R&D report no. 244, 2013), D-values have been found to be lower on stainless steel, but still considerably higher than might be expected for Salmonella spp. heated under moist conditions (>6 log reductions after 2 minutes at 70°C).  Initial testing of C. sakazakii and E. coli O157:H7 on stainless steel suggests that these organisms may not be as heat resistant as Salmonella under low a_w conditions.