Carcase chilling is an integral component of meat processing that influences microbial safety. Microbes on the carcase surface typically encounter not only a shift in temperature, but also a lowering of water activity as the carcase exterior dries from exposure to the outside environment. In vitro studies with STEC E. coli O157:H7 Sakai show a significant (2 log) decrease in culturable cell numbers when exposed to non-lethal combined osmotic and cold shock. Cells recover rapidly, before transiently losing culturability for a second time. The cells then enter a lag phase before resuming growth at a rate expected for the conditions.
Cells are thought to be injured rather than dead because the rate of increase in culturability post-shock is far higher than the rate at which the cells can replicate under the conditions. This hypothesis was tested by culturing post-shock cells at a temperature that precluded their gowth. Nonetheless, CFUs increased under these conditions, indicating recovery of culturability of cells rather than growth. Aggregation and filamentation of cells were also eliminated, via microscopy, as the predominant cause of reduction of CFUs.
It was theorized that injury to the cell membrane could explain these observations, as both cold and osmotic shock cause membrane disruption and fragility. To test this, flow cytometry was employed to assess a number of parameters using different stains: PI (membrane integrity), DiBAC (Membrane potential), dihydroethium (oxidation stress) and CFDAse (Cellular respiration). Membrane damage, oxidative stress and physical parameters such as cell size and volume were correlated with the culturability of combined shock cells on conventional recovery media.