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 pa thogenic strains of Listeria monocytogenes, reference strain NCTC 7973 , food strain L70, and clinical strain L94, were Stored at 4 degrees C in phosphate-buffered saline, pH 5.5, containing a combination of sor bate (0.2% wt/vol) and nisin (40 IU/ml). After 4 weeks, hemolysin prod uction by the strains had ceased, their subsequent lag phases at 37 de grees C were extended from an initial 1.23 to 1.32 h to a final 7.13 t o 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 thre e strains at approximately 5 log CFU/cm(2) and the surface of half the steaks was treated with the antimicrobial solution 1.0% sorbate plus 1,000 IU of nisin per mi. The meat was packaged under vacuum or 100% c arbon dioxide and stored at 4 degrees 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 tr eated 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 degrees C was unaffecte d. On treated CO2-packaged meat L. monocytogenes decreased during stor age to the extent that 89.3% of the initial pathogen load was eliminat ed, and for surviving cells the lag phase at 37 degrees C was extended . Treatment with the sorbate-nisin combination did not significantly a ffect pathogenicity of the L. monocytogenes cocktail recovered from va cuum-or carbon dioxide-packages after storage, in contrast to the in v itro study, where pathogenicity was clearly attenuated. The reason for this difference is unknown.