Thermal resistance of Salmonella senftenberg 775W in dry animal feeds

The thermal resistance of Salmonella senftenberg 775W was determined in meat and bone meal and chick starter contaminated by broth culture and by a simulated-natural method employing cells grown and dried in wet meat and bone meal base. Data obtained from modified thermal-death-time (TDT) tubes indicated that, except for initial fast killing rates, the heat resistance of S. senftenberg 775W in dry feeds is an exponential function of heating time, as in liquid media. The z values obtained (18 to 20 F) were higher than for vegetative cells heated in liquid media and similar to those of the more heat-resistant spores. Heat resistance in feeds was markedly higher with contamination by the simulated-natural method than by broth cultures. Increasing feed moisture levels decreased heat resistance, with a declining effect starting between 15 and 20% moisture. Results obtained with a laboratory-model steam feed conditioner indicated that the data obtained from TDT tubes are generally applicable to requirements for industrial-feed pasteurization. It is suggested that feed pasteurization can be accomplished by processing conditions which reduce the salmonella population by five log cycles. Processing feed at a moisture level of 15% or greater and a temperature of 190 F is indicated. Data are provided to allow prediction of thermal destruction under varying moisture and temperature conditions by use of the formula log D2= log D1+(1/z)( T1- T2).