No abstract

The thermal inactivation kinetics of Salmonella enteritidis PT4 between 49 and 60°C were investigated. Using procedures designed to eliminate methodological artifacts, we found that the death kinetics deviated from the accepted model of first-order inactivation. When we used high-density stationary-phase populations and sensitive enumeration, the survivor curves at 60°C were reproducibly biphasic. The decimal reduction time at 60°C (D 60°C) of the tail subpopulation was more than four times that of the majority population. This difference decreased with decreasing temperature; i.e., the survivor curves became more linear, but the proportion of tail cells remained a constant proportion of the initial population, about 1 in 104 to 105. Z plots (log D versus temperature) for the two populations showed that the D values coincided at 51°C, indicating that the survivor curves should be linear at this temperature, and this was confirmed experimentally. Investigations into the nature of the tails ruled out genotypic differences between the populations and protection due to leakage from early heat casualties. Heating of cells at 59°C in the presence of 5 or 100 μg of chloramphenicol per ml resulted in reductions in the levels of tailing. These reductions were greatest at the higher chloramphenicol concentration. Our results indicate that de novo protein synthesis of heat shock proteins is responsible for the observed tailing. Chemostat-cultured cells heated at 60°C also produced biphasic survivor curves in all but one instance. Cells with higher growth rates were more heat sensitive, but tailing was comparable with batch cultures. Starved cells (no dilution input) displayed linear inactivation kinetics, suggesting that during starvation a rapid heat shock response cannot be initiated.

No abstract

A Salmonella Assay using the Vitek Immunodiagnostic Assay System (VIDAS) was compared with a conventional cultural method (CCM) for the detection of salmonellas in 141 samples of artificially and naturally contaminated foods. There was an overall agreement of 92.9% between the methods. The productivity of the VIDAS Salmonella Assay (VSA) was not improved using an alternative enrichment protocol for the detection of Salmonella in 12 raw meat samples. The sensitivity and specificity of the VSA was assessed using pure cultures of salmonellas and non-salmonellas. The detection limit was 1.8 x 10(6) salmonellas ml-1 in M-broth and some Citrobacter freundii strains gave false-positive results. Using an immunomagnetic separation (IMS) technique and an abbreviated cultural enrichment, the VSA results could be obtained a day earlier than the standard VSA method.

Several organic acids have been accepted by the EU as non toxic food grade additives, to control spoilage and maintain fresh appearance in raw meat. Research has confirmed the antibacterial effect of certain organic acids such as acetic, propionic, sorbic, fumaric and citric acids on fresh beef (Anderson 1992; Dickson and Siragusa, 1994, Podolak et al., 1996). However many of these trials have been focused on spraying the acids onto the surface of meat, while little attention has been given to evaluating their effectiveness when mixed into ground beef. The work described here deals with the effect of a mixture of organic acids on the microbial contamination and appearance of minced meat.Forequarter muscles from three beef carcasses were removed 48h after slaughter. The meat from each carcass was cut into cubes and kept separate during storage for a further 24h at 2 °C. Half of the meat from each carcass was treated with a commercial preparation (BOMBAL, Van Hees; FISPAK Ltd.) containing the following organic acids; sodium acetate, sodium ascorbate, citric acid and ascorbic acid at a rate of 5g BOMBAL/kg of diced meat.