Nonthermal inactivation and sublethal injury of Lactobacillus plantarum in apple cider by a pilot plant scale continuous supercritical carbon dioxide system

The objective of this study was to evaluate the efficacy of supercritical carbon dioxide (SCCO2) for inactivating Lactobacillus plantarum in apple cider using a continuous system with a gas-liquid metal contactor. Pasteurized apple cider without preservatives was inoculated with L. plantarum and processed using a SCCO2 system at a CO2 concentration range of 0–12% (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. Processing with SCCO2 significantly (P < 0.05) enhanced inactivation of L. plantarum in apple cider, resulting in a 5 log reduction with 8% CO2 at 42 °C. The response surface model indicated that both CO2 concentration and temperature contributed to the microbial inactivation. The extent of sublethal injury in surviving cells in processed apple cider increased as CO2 concentration and processing temperature increased, however the percent injury dramatically decreased during SCCO2 processing at 42 °C. Structural damage in cell membranes after SCCO2 processing was observed by SEM. Refrigeration (4 °C) after SCCO2 processing effectively inhibited the re-growth of surviving L. plantarum during storage for 28 days. Thus this study suggests that SCCO2 processing is effective in eliminating L. plantarum and could be applicable for nonthermal pasteurization of apple cider.

Research highlights
► SCCO2 was effective in killing L. plantarum in apple cider at low temperature.
► Sublethal injury increased as CO2 concentration and processing temperature increased.
► 4 °C inhibited the re-growth of surviving L. plantarum during storage for 28 days.