Continuous Flow Nonthermal CO2 Processing: The Lethal Effects of Subcritical and Supercritical CO2 on Total Microbial Populations and Bacterial Spores in Raw Milk

Abstract

The effect of pressurized (<50MPa) CO2 as a nonthermal process for bacterial reduction in raw skim milk was examined using a unique pressurized continuous flow system. The lethal effects of subcritical and super-critical CO2 applied at different temperatures and pressures toward total native psychrotrophic microbial populations, total inoculated Pseudomonas fluorescens, and total inoculated spore populations were studied and compared. Pressures between 10.3 and 48.3MPa; temperatures of 15, 30, 35, and 40°C; and CO2 concentrations of 0, 3, 66, and 132 g/kg of milk were studied. For both native populations and inoculated P. fluorescens, greater total microbial lethality was observed under supercritical CO2 conditions than under subcritical CO2 conditions. At 30°C, there was no effect on total microbial lethality of increasing pressure up to 20.7MPa with either 66 or 132 g/kg of CO2; at 35°C, there was a positive relationship between pressure and lethality at CO2 levels of 132 g/kg, but no relationship at 66 g/kg of CO2. For total microbial populations and P. fluorescens, CO2 applied at 132 g/kg at 30°C and pressures of 10.3 to 20.7MPa resulted in an average standard plate count reduction of 3.81 and 2.93 log, respectively; at 35°C and 20.7MPa, maximum reductions achieved were 5.36 and 5.02 log, respectively. For both total microbial populations and inoculated P. fluorescens, CO2 exhibited a greater overall lethal effect at 132 g/kg than at 66 g/kg and a greater effect at 35°C than at 30°C. At 24.1 and 48.3MPa and 40°C, microbial lethality in raw aged milk treated with 3 g/kg of CO2 was not significantly different than that observed for uncarbonated milk; lethality achieved in milk treated with 132 g/kg of CO2 was significantly higher than that achieved in these 2 low-level CO2 treatments. No treatment studied had any significant impact on spore populations. Our work shows that, using the studied system, pressurized CO2 results in greater microbial lethality in milk above critical temperatures than below and suggests that a critical concentration threshold level of CO2 is required for lethal effects. Our work also suggests that supercritical CO2 processing in a continuous flow system can achieve reductions in some microbial populations equal to or better than that typically achieved during high-temperature, short-time pasteurization.