THE EFFECT OF FREQUENCY AND WAVE FORM ON THE ELECTRICAL CONDUCTIVITY‐TEMPERATURE PROFILES OF TURNIP TISSUE¹

Abstract

Electrical conductivity (σ) – temperature (T) curves were determined as a function of frequency and wave form of alternating current. Turnip cylinders were sandwiched between titanium electrodes and heated ohmically to 100C using four frequencies (4, 10, 25 and 60 Hz) and three wave forms (square, sine and sawtooth). The σ‐T curve shifted upward at 4 Hz for all wave forms. Curses were not significantly different at 10, 25 and 60 Hz for sawtooth and sine wave forms, while those for square waves were not significantly different at any of the frequencies tested. A comparison of wave forms showed that σ‐T curves for square waves were shifted downward significantly with respect to sine and sawtooth waves. Heating rate increased with decreasing frequency. Changes in vegetable tissue cellular structure are believed to be responsible for these results. Further research is necessary to characterize these mechanisms.

Effect of Frequency and Waveform on Inactivation of Escherichia coli O157:H7 and Salmonella enterica Serovar Typhimurium in Salsa by Ohmic Heating

“…We observed that the heating rate of salsa for a 60-Hz square wave was significantly (P Ͻ 0.05) lower than for 60-Hz sine and sawtooth waves. This result was in agreement with other research studies which showed that square wave was less efficient than the sine wave at 60 Hz (14,15). These investigators mentioned that changing of the waveform affected electrical conductivity, and this effect increased as the frequency decreased (14).…”

“…This result was in agreement with other research studies which showed that square wave was less efficient than the sine wave at 60 Hz (14,15). These investigators mentioned that changing of the waveform affected electrical conductivity, and this effect increased as the frequency decreased (14). For this reason, no significant (P Ͼ 0.05) difference in the ohmic heating rate between waveforms was observed as frequency increased above 500 Hz.…”

“…Our results agree with the equation: more heat was generated as a result of higher electrical conductivity of salsa re- sulting from higher applied frequencies. In contrast to our results, some studies reported that heating rate increased with decreasing frequency due to the resulting reduction of impedance (12)(13)(14). However, such a reduction of impedance, which acts as the resistance of several factors in the alternating current circuit, can result in more rapid heat generation by increasing the electrical conductivity shown in the as equation 1.…”

“…To prevent undesirable electrochemical reactions between electrodes and solid or viscous liquid products, increasing the frequency or changing the waveform of alternating current has been suggested (10). There have been some efforts to clarify the impact of frequency and waveform of alternating current on ohmic heating rates of solid or semisolid foods (11)(12)(13)(14)(15). However, there has been no comprehensive research on the effects of frequency on electrode corrosion, electrical conductivity and microbial inactivation in solid-liquid food mixtures.…”

Development of an experimental procedure for real time investigation of diffusion in foods

“…Two Titanium wires, 5 mm long and 3 mm in diameter coated with 50 g of Platinum/m 2 (Magneto Special Anodes, NL) were placed in either side of this cavity and used as electrodes. Previous work has shown that metals, such as stainless steel, used as electrodes, cause electrolysis effects [18]. At 50 Hz, the Ti-Pt materials have no significant electrocatalytic properties towards foods [19].…”