Antilisterial Effects of a Sorbate-Nisin Combination In Vitro and on Packaged Beef at Refrigeration Temperature

The antilisterial effects of a sorbate-nisin combination were assessed in vitro and on beef at refrigeration temperature. Three hemolytic pathogenic strains of Listeria monocytogenes , reference strain NCTC 7973, food strain L70, and clinical strain L94, were stored at 4°C in phosphate-buffered saline, pH 5.5, containing a combination of sorbate (0.2% wt/vol) and nisin (40 IU/ml). After 4 weeks, hemolysin production by the strains had ceased, their subsequent lag phases at 37°C were extended from an initial 1.23 to 1.32 h to a final 7.13 to 8.06 h and their pathogenicity for chick embryos had decreased from an initial 93.3 to 95.5% to a final 43.3 to 60.0%. Sterile beef steaks of normal pH (5.4 to 5.5) were inoculated with a cocktail of the three strains at approximately 5 log CFU/cm² and the surface of half the steaks was treated with the antimicrobial solution 1.0% sorbate plus 1,000 IU of nisin per ml. The meat was packaged under vacuum or 100% carbon dioxide and stored at 4°C for 4 weeks. On untreated meat, L. monocytogenes grew by 1.79 log cycles in vacuum packages, but in CO₂ packages the initial population decreased by 0.54 log cycle. On treated vacuum-packaged meat, L. monocytogenes decreased during storage to the extent that 96.5% of the initial pathogen load was eliminated, but the lag phase of the remaining cells at 37°C was unaffected. On treated CO₂-packaged meat L. monocytogenes decreased during storage to the extent that 89.3% of the initial pathogen load was eliminated, and for surviving cells the lag phase at 37°C was extended. Treatment with the sorbate-nisin combination did not significantly affect pathogenicity of the L. monocytogenes cocktail recovered from vacuum- or carbon dioxide-packages after storage, in contrast to the in vitro study, where pathogenicity was clearly attenuated. The reason for this difference is unknown.