Submitted to: International Journal of Food Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 23, 2009
Publication Date: January 1, 2010
Citation: Yuk, H., Geveke, D.J., Zhang, H.Q. 2010. Efficacy of supercritical carbon dioxide for nonthermal inactivation of Escherichia coli K12 in apple cider. International Journal of Food Microbiology. 138:91-99. Interpretive Summary: Microbial contamination has been a major concern in the apple cider industry since a foodborne outbreak occurred in 1996. Heat treatment at 72 to 88C is the most effective way to kill bacteria in apple cider; however, this method may cause side effects such as destruction of nutrients and generation of off-flavors. To overcome these disadvantages of heat treatment, several technologies that kill bacteria at low temperature have been developed. Among them, supercritical carbon dioxide is a relatively new process that has many advantages including safety and consumer acceptance. This study evaluated the effect of supercritical carbon dioxide on killing Escherichia coli in apple cider. Results showed that the killing effects were improved at high carbon dioxide concentrations and temperatures. Almost complete kill of E. coli was obtained at all tested temperatures (34, 38, and 42C). Supercritical carbon dioxide also injured the small fraction of surviving E. coli and microscopic imaging showed that the bacteria were damaged. Storage tests at room temperature showed that the cider remained free of E. coli for 28 days. This research showed that supercritical carbon dioxide can be used to pasteurize apple cider at low temperatures.
Technical Abstract: This study evaluated the efficacy of a supercritical carbon dioxide (SCCO2) system with a gas-liquid porous metal contactor for eliminating Escherichia coli K12 in apple cider. Pasteurized, preservative-free apple cider was inoculated with E. coli K12 and processed using the SCCO2 system at CO2 concentrations of 0 – 10% (wt %, g CO2/100 g product), outlet temperatures of 34, 38, and 42 C, a system pressure of 7.6 MPa, and a flow rate of 1 L/min. Increased CO2 concentrations and temperatures significantly (P < 0.05) enhanced the bactericidal effect, resulting in a maximum reduction of 7.31-log CFU/ml at 8% CO2 and 42 C. A response surface model indicated that minimum CO2 concentrations of 9.9% at 34 C, 7.4% at 38 C, and 5.4% at 42 C are needed to achieve a 5 log reduction of E. coli K12 in apple cider. SEM observations showed morphological changes in the cell envelop after SCCO2 processing. At a processing condition of 8% and 38 C, the reduction of E. coli was 6.03 log and the sublethal injury of the survivors was 84%. The regrowth or survival of E. coli in SCCO2 processed apple cider was not observed during storage for 28 days at 4, 8, and 20 C. Thus this study showed the potential of SCCO2 processing with a gas-liquid porous metal contactor for the nonthermal pasteurization of apple cider.