Predictable and unpredictable survival of foodborne pathogens during non-isothermal heating

In previous work, extreme survival of various bacterial species during cooking was reported when attached to chicken meat. In this paper the effects of an extremely high challenge temperature on survival of Salmonella Typhimurium and Campylobacter jejuni, attached to chicken breast fillets or pork to test for matrix effects are reported. Survival was predicted, using standard D- and z-values from the literature, and compared to experimentally obtained data. Attached to meat, both S. Typhimurium and C. jejuni survived longer than predicted, longer when attached to chicken meat than when attached to pork. Additionally, the effect of non-isothermal heating on survival of almonella in buffer is described. In buffer, when slowly heated, Salmonella died off as predicted. When Salmonella was heated in buffer according to a heating profile mimicking that of the surface of meat in boiling water, it appeared that cells died off much slower than predicted. It is shown that the thermal characteristics of Salmonella surviving the first 35 s of fast heating had changed. After these 35 s, remaining Salmonella survived for minutes, even at a challenge temperature of 90 °C. During heating, cell size decline was observed. A loss of intracellular water during cooking might have resulted in smaller, dehydrated cells, in cells with altered thermal resistance characteristics. This could explain why the use of standard D-and z-values did not allow the correct prediction of survival of Salmonella during fast heating in buffer, or during cooking, being attached to the surface of meat. Many factors affect the level of heat resistance of bacteria. The results of this and a former study show that attachment to meat contributes to an increased level of heat resistance of bacteria. A fast heating process further contributes to the increased level of heat resistance possibly as the result of changed thermal characteristics due to a loss of water.