Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 29, 1998
Publication Date: N/A
Interpretive Summary: Studies were conducted to determine if cross-contamination of bacteria can occur between meat frozen in a vat of LN. Beef carcass tissue (BCT) was used to represent lamb carcasses on a processing line that included a 15-min LN immersion freezing step. Escherichia coli O157:H7 and/or Listeria innocua were used to contaminate BCT. These contaminated carcasses were frozen first, then were followed in the freezing process by uninoculated BCT representing clean carcasses. After freezing in LN, the uninoculated BCT, the LN, and the LN containers were examined for the presence of the bacteria. In the first study, BCT was inoculated with high levels of the bacteria, spray washed with water, and frozen. L. innocua was found on all uninoculated BCT and LN samples, and E. coli was found on most of the uninoculated BCT samples. However, the levels of these contaminating bacteria were low (<1.0 cell per square centimeter). In a second study, a lactic acid spray wash was used to reduce the numbers of L. innocua introduced into the LN, and resulted in fewer samples containing L. innocua. When either bacterium was inoculated at low levels, neither L. innocua nor E. coli was found on uninoculated samples or surfaces. These experiments demonstrate that bacterial cross-contamination of meat can occur during freezing by immersion in LN, but indicate that good cleaning practices and the use of food products with low microbial numbers can limit this occurrence.
Technical Abstract: Beef carcass 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 dissemination of bacteria during freezing was examined. Listeria innocua and Escherichia coli O157:H7 were inoculated onto BCT, which were introduced into the freezing process to represent contaminated carcasses. Subsequently frozen uninoculated BCT, LN, and LN containers were examined for the presence of these organisms. BCT samples were inoculated with 7 log CFU/cm**2 of both bacteria, spray washed with water (32 deg C, 80 psi, 12 s), and frozen, distributed among uninoculated BCT, in LN for 15 min. In two trials, L. innocua was recovered by enrichment from all uninoculated BCT and LN samples. E. coli was also recovered from uninoculated BCT, although this recovery was more sporadic, and was not recovered from LN. Both bacteria were recovered from the LN container. Enumeration of cross-contaminating bacteria indicated that contaminating levels were low (<1.0 CFU/cm**2 BCT). In a second study, a 2.0% lactic acid spray wash was used to reduce the numbers of L. innocua introduced into the system and resulted in fewer positive samples, although this organism was still recovered from many uninoculated BCT samples. When either bacterium was inoculated at levels of 1.35-1.77 log CFU/cm**2 and BCT was water or lactic acid spray washed before freezing, neither was recoverable from uninoculated BCT, LN, or from the container. Results demonstrate that bacterial cross-contamination of meat during LN immersion freezing can occur, but that the use of good sanitation practices and product with low microbial numbers can limit this occurrence.