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6.2 Introduction 15 8 6.3 Materials and Methods 160 6.3.1 Meat samples 6.3.2 Microbial recovery studies 6.3.3 Microbiological analyses 6.3.4 FCM analyses of spoiling pork patties 6.3.5 FCM analysis of retail samples 6.3.6 Quantification of bacterial cells using FCM 6.3.7 Evaluation of column materials 6.3.8 Clarification of meat suspensions for column material evaluation 6.3.8 Clarification of meat suspensions for flow cytometry 6.3.9 Flow cytometry 165 6.3.10 Staining protocol 165 6.3.12 Statistics 165 6.4 Results 165 6.4.1 Evaluation of column materials 165 6.4.2 Quantification of bacterial cells in meat homogenate using FCM 167

Beef was packaged under CO* containing < 500 ppm O2 (MAP), vacuum, or air and stored at 0, 2, or 4°C. Samples were analyzed weekly for bacterial numbers using anaerobic plate count (pre-reduced medium and anaerobic chamber),-aerobic plate count, and anaerobic iar plate count (MAP samoles only) methods. For both MAP and ;ac&m packaged samples, ihe anaeiobic plate count was consistently greater than the aerobic plate count and for MAP samples the anaerobic plate count was consistently higher than the anaerobic jar plate count. Differences between plating methods were most frequent during the latter third of 0 and 2°C storage. Anaerobic isolates from MAP samples were most often lactic acid cocci and staphylococci. No clostridia were isolated from any of the treatments.

Frozen pork patties, thawed overnight at 0 degrees C or temperature abused through storage at 15 degrees C for 24 h, were packaged using both vacuum and air packaging methods. Immediately after packaging, both sets of patties were irradiated at 0, 0.5, 1, and 2 kGy. All the samples were stored at 2 degrees C and were analyzed for populations of mesophilic, psychrotrophic, and lactic acid bacteria every 3 days for 30 days. By using a mesophilic population of 10(7) cells/g as a criteria for spoilage, fresh pork patties receiving a dose of 0 kGy had shelf lives of 11 and 16 days with air and vacuum packaging methods, respectively, whereas temperature-abused patties had a shelf life of 7 days with both air and vacuum packaging methods. Both fresh and abused patties that received a dose of 2 kGy had shelf lives that were greater than 30 days at 2 degrees C with both air and vacuum packaging methods. Descriptive models based on the Gompertz equation for mesophilic, psychrotrophic, and lactic acid bacteria were developed, and the generation time and lag-phase duration for each bacterial population were calculated.