IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 42, NO. 10, OCTOBER 2014 3161 Performance of Electrode Materials During Food Processing by Pulsed Electric Fields Ahmed Gad, Student Member, IEEE, Shesha H. Jayaram, Fellow, IEEE , and Mark Pritzker Abstract— Physical and electrical contact between liquid food and the metallic electrodes during the pulsed electric field process for microbial inactivation is unavoidable and causes some metallic ions to be released from the electrodes into the processed food. In this paper, the resistance to the release of metallic ions from electrodes coated with four different materials (Ag, Cr, Ni, and Ti) is compared. Ti has been found to be the most effective of these metals and is compared more extensively with widely used food-grade stainless steel. When the electrodes are made of Ti, the concentration of metal released is always found to be below the 32-ppb detection limit. Index Terms— Electrochemical processes, electrodes, electro- static processes, food industry, metals, pulsed power systems. I. I NTRODUCTION P ROCESSING of liquid food products by pulsed electric fields (PEFs) provides a nonthermal preservation method with high retention levels of nutrients and fresh-like taste. Studies on microbial inactivation or shelf-life extension or both have demonstrated the effectiveness of this method [1], [2]. However, the undesirable release of metallic ions from the electrodes during PEF processing can affect food safety and taste [3], degrade food compounds/color [4], and shorten the electrode lifetime [5], [6]. Attempts to eliminate, or at least reduce, the release of metallic ions during PEF processing include: 1) delivering zero net charges to the electrodes [7], [8]; 2) applying rel- atively short pulses [9], [10]; 3) generating oscillatory pulse shapes [5], [6]; 4) coating with conductive-plastic films [11]; and 5) utilizing ceramic-coated electrodes [12]. The PEF systems commonly use stainless steel electrodes [5]–[10]. In pulsed ohmic heating systems, another technique for micro- bial inactivation in which the electric pulses with much lower amplitudes are applied, the use of titanium and platinized- titanium electrodes showed superior performance compared with stainless steel and graphite ones [13]. To the best of our knowledge, in none of the previous PEF studies was the use of such metallic-coated electrodes investigated. The objective of this paper is to reduce the release of metallic ions using electrode materials, which may have higher Manuscript received October 31, 2013; revised February 6, 2014; accepted March 16, 2014. Date of publication March 28, 2014; date of current version October 21, 2014. This work was supported by the Natural Sciences and Engineering Research Council of Canada. A. Gad and S. H. Jayaram are with the Department of Electrical and Computer Engineering, University of Waterloo, Waterloo, ON N2L 3G1, Canada (e-mail: agad@uwaterloo.ca; jayaram@uwaterloo.ca). M. Pritzker is with the Department of Chemical Engineering, University of Waterloo, Waterloo, ON N2L 3G1, Canada (e-mail: pritzker@uwaterloo.ca). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPS.2014.2312711 resistance to PEF processing. In particular, experiments are conducted with real food products (i.e., milk and orange juice) because they should provide more accurate and industrially relevant results than those obtained with buffer solutions having the same conductivity and pH [6]. Preliminarily, the performances of chromium-, nickel-, silver-, and titanium- coated electrodes are compared with each other under typical PEF conditions. Then, a whole-titanium electrode is tested against a conventional stainless steel electrode during the processing of milk (neutral) and orange juice (acidic). II. ELECTROCHEMICAL PHENOMENA With the exception of a few attempts to avoid the phys- ical/electrical contact between liquid food and the metallic electrodes [10], [12], [14], electric pulses are typically applied to the liquid food products via two metallic electrodes. Hence, the PEF processing zone represents an electrochemical cell, specifically an electrolytic cell because an external source of electrical energy is applied to drive electrical current through the cell. This current involves electrons in the conductive solid electrodes and ions in the conductive liquid electrolyte. Electrochemical reactions take place at the electrode/ electrolyte interfaces and enable the transition from electronic transport to ionic transport to take place. A variety of electrochemical reactions at each electrode can lead to damage or breakdown of food compounds, electrolysis of water, gas production, and/or release of metallic ions from the electrodes. Since the electrode materials used in this paper belong to the transition metals, the release of dissolved metals during PEF processing is considered to occur predominantly by anodic oxidation of the electrodes. The onset of a given anodic half-cell reaction i occurs when the applied electrode potential E exceeds the Nernst potential E i of the half-cell reaction. Once an electrode reaction occurs, current corre- sponding to the rate of this reaction flows across the cell. When more than one half-cell reaction occurs at an electrode, the total current passing through the cell is distributed among the half-cell reactions depending on their relative rates. The current transferred by a given half-cell reaction depends on the electrode potential difference E – E i , the kinetic para- meters associated with the reaction, and the temperature. Faraday’s law governs the relationship between the number of moles n ij of a species j produced by the reaction i and the cumulative electrical charge Q i transferred by the reaction over a period of time, particularly n ij = Q i F · z ij ∀ E ≥ E i 0093-3813 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. 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