Factors effecting survival of Escherichia coli O157:H7 during freezing on meat

Summary

Escherichia coli O157:H7 is the major human pathogen of current concern with respect to fresh meat.
Freezing is widely used in the Australian meat industry to preserve manufacturing cuts during transport over long distances, particularly to Australia's major trading partners.
Since freezing is known to damage and inactivate bacteria it has been suggested that this process may add an extra margin of safety to product with respect to E. coli O157:H7 as compared to unfrozen cuts.
Results of various studies investigating the effect of freezing on the survival of E. coli O157:H7 on beef have been contradictory due to difficulty in obtaining controlled data. For this reason a previous MLA project developed a laboratory based method for obtaining controlled data on E. coli O157:H7 during freezing on beef. Using this method it was established that there was no significant reduction in numbers of this pathogen on beef during simulations of industry freezing profiles.
In all cases the inoculum applied to the meat in the previous project was from a 24 hour culture grown in nutrient broth at 37ºC.
Escherichia coli O157:H7 cells present on frozen boxed beef may be in varying physiological states due to exposure to different growth conditions and stresses, or to being in different phases of growth. These stresses, whether occurring before or together with freezing, may influence their ability to survive on meat.
For this reason this study was undertaken to establish the effect of variations in growth conditions and stress on the ability of E. coli O157:H7 to survive freezing under controlled conditions established in the previous project. This report addresses the effect of six of these:water activitygrowth temperaturedifferent pHgrowth phase of culturesgrowth in liquid or on solid mediumfreezing on fat or muscle