Insufficient antilisterial capacity of low inoculum Lactobacillus cultures on long-term stored meats at 4 °C

Two of the 210 lactobacilli strains isolated from chilled meats produced antilisterial bacteriocins: Lactobacillus sake 265 (Lb 265) and Lactobacillus casei 52 (Lb 52). Factors affecting antilisterial effectiveness of these and two other bacteriocin-producing (Bac+) strains (Lactobacillus sake 706, Lb 706; and Lactobacillus sake 148, Lb 148) at refrigeration temperature (4 °C) were studied in laboratory media and meat systems. At both 4 °C and 25 °C, these Bac+ strains grown in buffered MRS broths (pH 5.4 or 6.5) showed longer lag phases and shorter generation times than Listeria monocytogenes (mixture of strains NCTC 7973 and two food derived strains, L70 and L72) when grown in buffered BHI broths at the same pH values. These differences were more significant at 4 °C than at 25 °C. The highest concentrations of bacteriocin in MRS broth were produced at 25 °C and 4 °C by strain Lb 265 and Lb 706, respectively. Generally, production of bacteriocins was more efficient at lower pH (in buffered MRS broths of pH 5.4 and unbuffered MRS broths), than at higher pH (in buffered broths of pH 6.5). On vacuum packaged, raw beef (pH 5.3–5.4) initial numbers of L. monocytogenes (103/g) did not change significantly during 23-days storage at 4 °C, when inoculated either alone or in the presence of Bac+ strains inoculated at initial levels of 103/g. On vacuum packaged emulsion-type of sausages (pH 6.4) inoculated with L. monocytogenes and stored at 4 °C for 23 days growth was not significantly affected by addition of Bac+ strains at initial levels of 103/g. These results indicated that amounts of bacteriocins produced in situ by low initial numbers (103/g) of the protective strains tested were not sufficient to inhibit and/or reduce L. monocytogencs on these chilled meats, where high initial numbers of lactic acid bacteria are not desirable for product quality resons. To achieve these effects, higher concentrations of active (free) bacteriocins in meats must be provided.