Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 31, 2008
Publication Date: January 3, 2009
Citation: Geveke, D.J., Gurtler, J., Zhang, H.Q. 2009. Inactivation of Lactobacillus plantarum in apple cider using radio frequency electric fields. Journal of Food Protection. 72(3):656-661. Interpretive Summary: Apple cider has been pasteurized at low temperatures using a novel technology employing radio frequency electric fields (RFEF). However, most of the previous studies on RFEF pasteurization used gram negative bacteria and none used gram positive bacteria. The present study is the first to RFEF process Lactobacillus plantarum, a gram positive bacterium. The population of L. plantarum in apple cider was reduced by 90% following RFEF processing at 50 C. Raising the RFEF temperature to 55 C killed more than 99% of the L. plantarum and injured 94% of the survivors. Refrigerating the cider for 5 weeks killed nearly all of the remaining bacteria. This provides the first evidence that nonthermal RFEF processing is capable of killing a gram positive bacterium and that the surviving cells die off during refrigerated storage. The results of this study and previous studies point to the possibility of using RFEF processing to pasteurize apple cider containing gram negative and gram positive bacteria.
Technical Abstract: Radio frequency electric fields (RFEF) processing is effective at inactivating Gram negative bacteria in fruit juices at moderately low temperatures, but has yet to be shown to be effective at reducing Gram positive bacteria. Lactobacillus plantarum ATCC 49445, a Gram positive bacterium, was inoculated into apple cider and the cider was RFEF processed at the following conditions: field strength 0.15 to 15 kV/cm; temperature 45 to 55 C, frequency 5 to 65 kHz, treatment time 170 microseconds, and holding time 5 to 50 s. In addition, the effect of refrigerating the inoculated cider prior to RFEF processing, the extent of sublethal injury, and the effect of storing the treated cider for 35 d was investigated. The population of L. plantarum in apple cider was reduced by 1.0 log following RFEF processing at 15 kV/cm, 20 kHz, and 50 C with a hold time of 5 s. There is a synergistic effect between RFEF and heat above 50 C. Inactivation significantly (P < 0.05) increased as the frequency was decreased from 65 kHz to 5 kHz. There was little inactivation at field strengths below 8 kV/cm. Above this “critical” level, inactivation increased linearly with increasing field strength. Holding the cider at 55 C following RFEF treatment for 5 and 50 s resulted in reductions of 2.5 and 3.1 log, respectively. The sublethal injury of L. plantarum was 1.4 log. The population of processed L. plantarum cells stored at 4 C steadily and significantly (P < 0.05) declined over a 35 d period from 4.5 log to 0.9 log cfu/ml. The electrical energy density for the RFEF process was 51 J/ml of cider. This provides the first evidence that nonthermal RFEF processing is capable of inactivating a Gram positive bacterium and that the surviving cells die off during refrigerated storage.