Effect of water activity on thermal resistance of Salmonella krefeld in liquid medium and on rawhide surface

The thermal resistivity of Salmonella krefeld at 60 and 65 °C as affected by water activity (0.99–0.60) was investigated in a liquid medium viz. Trypticase Soy Broth (TSB) and on rawhide surface. A reduction of the aw of the liquid medium resulted in an increase in the heat resistance of Salmonella krefeld. At a particular water activity the type of solute played a significant role with Salmonella krefeld exhibiting much higher heat sensitivity in the media with glycerol compared to the media with sucrose. The heat resistance of Salmonella krefeld inoculated onto the surface of rawhide was also investigated during hot air drying at 60 and 65 °C at two initial levels of aw, 0.99 and 0.90, prior to drying as determined at 250 °C. The rate of water loss or aw decrease during drying was not significantly influenced by the initial water activity but was clearly dependent on the drying temperature with all survival curves displaying an upward concavity. Salmonella on rawhide with an initial aw of 0.90 exhibited higher heat tolerance than with an initial aw of 0.99 during a prolonged heating period with the initial aw significantly affecting the heat resistance particularly at higher drying temperatures.