Submitted to: Journal of Food Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/28/2007
Publication Date: 8/2/2007
Citation: Geveke, D.J., Brunkhorst, C. 2007. Radio Frequency Electric Fields Inactivation of Escherichia coli in Apple Cider. Journal of Food Engineering. 85:215-221. Interpretive Summary: The radio frequency electric fields (RFEF) process has been shown to inactivate 99.3 % of Escherichia coli in juice at moderately low temperatures, but the FDA requires a 99.999 % reduction. The RFEF process was modified to increase the electric field and treatment time in order to improve the microbial inactivation. Models that relate the electric field and treatment time to the level of microbial inactivation were developed to better understand the process. In addition, the energy efficiency of the process was investigated to provide operating cost information to industry. The modified RFEF process now meets the FDA requirement for pasteurization of apple cider.
Technical Abstract: Apple cider has been implicated in several outbreaks. Thermal pasteurization eliminates this threat, but it can detrimentally affect the quality of the cider. A nonthermal process using radio frequency electric fields (RFEF) was developed to pasteurize cider. An 80 kW RFEF pilot plant system was used to process cider at flow rates of 1.5 and 1.9 l/min. Escherichia coli K12 in apple cider was exposed to electric field strengths of 20 to 30 kV/cm at frequencies of 21, 30, and 41 kHz. Treatment times varied from 140 to 420 microseconds. Electrical energy costs were calculated using the measured voltages and currents. Energy balances were performed using the inlet and outlet temperatures. RFEF processing at an outlet temperature of 60 C reduced the population of E. coli by 4.8 log, whereas thermal processing at the same conditions had no effect. Varying the frequency between 21 and 41 kHz had no effect on the level of microbial inactivation; however, increasing the treatment time, field strength and outlet temperature enhanced inactivation. The inactivation data at 20 kV/cm and 60 C follow first order kinetics with a calculated D values of 74 microseconds. The inactivation data are represented well by the electric field strength model; the calculated critical electric field strength for 60 C was 4.0 kV/cm. The electrical energy for RFEF pasteurization was 260 J/ml. The electrical cost was $0.0050/l of apple cider. Processing temperature had the greatest influence on energy efficiency. A RFEF nonthermal process has been developed to pasteurize apple cider. The effect of varying processing conditions on energy efficiency was investigated and the optimum electrical cost appears to be minor. In addition, the RFEF process can be correlated using first order kinetic models.