Bacterial cross-contamination of meat during liquid nitrogen immersion freezing

Prerigor beef carcass surface tissue (BCT) was used to simulate lamb carcasses on a processing line with a 15-min liquid nitrogen (LN) immersion freezing step, and the potential for the dissemination of bacteria during freezing was examined. Streptomycin-resistant strains of Listeria innocua and Escherichia coli O157:H7 spiked into a fecal slurry were inoculated onto BCT pieces that were introduced into the freezing process to represent contaminated carcasses. Following this introduction, subsequently frozen uninoculated BCT, LN, and LN containers were examined for the inoculated organisms. In the first study, BCT samples were inoculated with ca. 7 log CFU/cm2 of both L. innocua and E. coli O157:H7, spray washed with water and frozen, distributed among uninoculated BCT, in LN for 15 min. In two separate trials, L. innocua was recovered by enrichment from all uninoculated BCT and LN samples. E. coli O157:H7 was also recovered from uninoculated BCT and LN, but this cross-contamination was more sporadic. Both species were recovered from the LN container following freezing. Attempts to enumerate cross-contaminating bacteria in the second trial indicated that contaminating levels were low (< 1.0 CFU/cm2 BCT). In a second study, a 2.0% lactic acid spray wash was used to reduce further the numbers of L. innocua introduced into the freezing system and resulted in fewer positive samples, although this organism was still recovered from many uninoculated BCT samples. When either bacterium was inoculated at lower initial levels (1.35 to 1.77 log CFU/cm2) and BCT was water or 2.0% lactic acid spray washed prior to freezing, neither L. innocua nor E. coli O157:H7 was recoverable by enrichment from uninoculated BCT, LN, or from the freezing container. Results demonstrate that bacterial cross-contamination of meat during LN immersion freezing can occur but indicate that the use of good sanitation practices and product with low microbial numbers can limit this occurrence.