BEHAVIOR OF LISTERIA-MONOCYTOGENES ON VACUUM-PACKED BEEF

Vacuum-packed beef with a pH of < 5.8 which had been artifically contaminated with 10(3) Listeria monocytogenes/cm2 showed no change in its listeriae count during 9 weeks storage at 2 and 4-degree-C. Storage at 7-degrees-C at a pH of 5.8 led to a rise in the listerae count by two powers of ten between the 3rd and the 7th day. Vacuum-packed beef with a pH of > 6.0 underwent a growth of listeriae of about one power of ten during the first week of storage at 2 and 4-degrees-C. At a storage temperature of 7-degrees-C the L. monocytogenes count increased during 3 days by about one power of ten and by about three powers of ten at 10-degrees-C. In samples with a pH below 5.8 and held at 2 and 4-degrees-C the lactic acid bacteria developed within a few days to become the dominant flora and suppressed the growth of L. monocytogenes, but Brochothrix thermosphacta and the Enterobacteriaceae were dominant on vacuum-packed meat at higher pH values (pH > 6.0). DFD meat is therefore unsuitable for storage under vacuum because of the danger of a buildup of L. monocytogenes. On the other hand there is no danger of a growth of listeriae in pieces of vacuum-packed beef with a pH of less than 5.8 and at storage temperatures of about 2-degrees-C.