The effects of irradiation dose (0, 0.5 and 1.0 kGy), various gas atmospheres and storage temperature (5, 15 and 25$ sp circ$C) on the physical, chemical, microbiological, and organoleptic changes in fresh pork were studied using factorial design experiments. The effects on toxin production by Clostridium botulinum were also investigated using challenge studies. Shelf life could be extended to 21 d when product was packaged in 0% O$ sb2$, irradiated at 1.0 kGy and stored at 5$ sp circ$C compared to 4 d for control samples. While the presence of O$ sb2$ in the package headspace enhanced the antimicrobial effects of low-dose irradiation, it adversely affected the organoleptic qualities of pork. Botulinum toxin was detected after only 2 d in all inoculated treatments stored at 25$ sp circ$C. At 15$ sp circ$C, toxin was produced faster when pork was initially packaged with O$ sb2$ or low levels of CO$ sb2$ (15-30%) as compared to 100% N$ sb2$. Higher levels of CO$ sb2$ (45-75%) delayed toxin production. In most treatments, spoilage preceded toxigenesis. Models were developed relating the above factors to the time until toxin production and to the probability of toxigenesis. Temperature, initial O$ sb2$ and irradiation were all significant factors.

Listeria monocytogenes is a pathogenic, psychrotrophic microorganism that is ubiquitous in nature. L. monocytogenes has been isolated from numerous meat products, both fresh and processed, the incidence of contamination varying greatly. The ability of Listeria to grow in meats depends on temperature, pH, water activity (a$ sb{ rm w}$), nutrients, species and numbers of competing microorganisms, gaseous conditions, and levels of additional barriers. Therefore, methods to control the growth of L.monocytogenes are of great importance to food processors since this organism can grow under a wide range of environmental and storage conditions. Two methods of control, in conjunction with temperature, were studied in this project: (i) modified atmosphere packaging (MAP) and (ii) chitosan, to determine the optimum levels of these "hurdles" needed to effectively control the outgrowth of L.monocytogenes in both model broth and agar systems and in fresh pork loin. On the basis of these preliminary studies, a combination of chitosan as a dipping solution and modified atmosphere packaging were investigated to control the growth of L.monocytogenes in fresh pork loin. Pork loin samples were dipped in a 0.2% chitosan solution for 60 seconds and packaged under various atmospheres in Cryovac bags and stored at 5, 10 and 15$ sp circ$C up to 28 days. Samples were monitored for physical, chemical and microbiological changes throughout the storage period. Optimum control over the growth of L. monocytogenes was achieved using a combination of 100%N$ sb2$ + an Ageless FX oxygen absorbent and dipping in a 0.2% chitosan solution. Based on these studies, a combination of 0.2% chitosan and MAP could be used to extend the shelf life of pork without adversely affecting color, odor and exudate loss while inhibiting the growth of the pathogenic microorganism, L.monocytogenes. (Abstract shortened by UMI.)

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