Advantages of ohmic cooking in the kilohertz-range - part I: Impact of conductivity and frequency on the heating uniformity of potatoes

The heating uniformity of potato cubes and whole potato tubers was investigated during cooking by ohmic heating (OH) in the low (12 kHz) and high (300 kHz) kHz range. Potatoes were heated in aqueous NaCl solutions with various conductivities (1–5 mS/cm). Conductivities and cell disintegration index of the tissue were determined in dependency of the temperature and the frequency at different points in the tubers. Heating uniformity was characterized by analyzing the temperature profiles with up to four thermocouples and by thermal imaging. Heating behavior of peeled and unpeeled tubers were compared to evaluate the impact of the skin on the OH treatment. For the higher frequency applied, increased electrical conductivities in the potato tissue were observed. At 300 kHz a better heating uniformity, with lower temperature differences within the potato samples compared to OH at 12 kHz was observed. The results provide a better understanding of the effect of high frequency electric fields on the heating uniformity and the conductivity of intact potato tissue.Industrial relevanceOhmic heating (OH) is an alternative heating technology with the potential to reduce processing times and energy consumption. It can overcome limitations in the heat transfer particularly given for large-volume solid foods and provide a uniform heating. However, limited attention has been given to the effects of the product and process parameters on the heating uniformity during OH treatments of solid foods. The present paper highlights the importance of the electrical conductivity and the frequency of the applied electric field on the heating uniformity. The obtained results contribute to the understanding of the interaction of the electric field and the food matrix, which is the basis for the implementation of tailored and targeted processing concepts in the industry.     

Electrical Properties of Fish Mince During Multi-frequency Ohmic Heating

A multi-frequency ohmic heating system with 30 Hz～1 MHz range which could deliver 250 watts was developed for measuring electrical conductivity and absolute dielectric loss of food samples. Pacific whiting surimi paste and stabilized mince in the 20～70°C range were tested at frequencies from 55 Hz to 200 kHz. Sample impedance decreased slightly with frequency. The DC electrical conductivity (σ_dc) and absolute dielectric loss (ε″) of Pacific whiting surimi paste increased with temperature and salt concentration; adc and ε″ of the stabilized mince increased with temperature. Empirical models of electrical properties for surimi paste (moisture content 79% and salt at 1, 2 or 3%) and stabilized mince (77% moisture and 0.74% salt) were derived. Electrolytic corrosion diminished with frequency.     

Ohmic tempering using a high frequency ohmic heating and model food of minced tuna based on Allaska pollock surimi – Evaluation of electrical conductivities

Inexpensive and practical model foods are necessary for developing ohmic tempering (OT) of tuna to reduce experimental costs. In this study, the effects of salt addition (0, 0.25, 0.5, 1.0, and 2.0%) in pollock surimi within the temperature range of −40 to 10 °C and frequency range of 50 Hz to 20 kHz, and adjusting voltage were evaluated to develop a model food for the OT of minced tuna by comparing their electrical conductivity (EC) values and temperature distribution. The EC values of the model food increased with the salt content, temperature, and frequency. The adjustment of voltage during OT in three steps (400 V, −35 to −20 °C; 200 V, −20 to −10 °C; 100 V, −10 to −4 °C), helped improve the uniformity of the temperature distribution of the samples. The EC values, time-temperature profiles, and temperature distribution results of the model food after the addition of 0.5% salt were the closest to the minced tuna during the 20 kHz OT process. The results laid a foundation for the development of inexpensive and effective frozen model foods for OT applications especially at high frequencies.     

淀粉溶液的通电加热研究

本文研究了食盐浓度、淀粉浓度和电压对淀粉溶液的电导率和通电加热速率的影响;研究了通电加热对淀粉糊化特性的影响。结果表明:淀粉溶液在通电加热过程中,加热速率和电导率随食盐浓度的增大而增大;随电压的升高而增大;随淀粉浓度的增大而减小。而电导率在达到糊化温度之前随温度的升高而增大,在糊化温度之后随温度的升高而降低。淀粉溶液在通电加热时其糊化特性无明显变化。     

Effect of Ohmic Heating of Soymilk on Urease Inactivation and Kinetic Analysis in Holding Time

To verify the effect of the ohmic heating on the urease activity in the soymilk, the ohmic heating methods with the different electrical field conditions (the frequency and the voltage ranging from 50 to 10 kHz and from 160 to 220 V, respectively) were employed. The results showed that if the value of the urease activity measured with the quantitative spectrophotometry method was lower than 16.8 IU, the urease activity measured with the qualitative method was negative. The urease activity of the sample ohmically heated was significantly lower than that of the sample conventionally heated (P < 0.01) at the same target temperature. It was concluded that the electrical field enhanced the urease inactivation. In addition, the inactivation kinetics of the urease in the soymilk could be described with a biphasic model during holding time at a target temperature. Thus, it was concluded that the urease in the soymilk would contain 2 isoenzymes, one is the thermolabile fraction, the other the thermostable fraction, and that the thermostable isoenzyme could not be completely inactivated when the holding time increased, whether the soymilk was cooked with the conventional method or with the ohmic heating method. Therefore, the electric field had no effect on the inactivation of the thermostable isoenzyme of the urease.     

Ohmic Heating Behaviour of Cabbage and Daikon Radish

Shredded cabbage (50 % v/v) and Daikon radish cubes (57 % v/v) with different salt concentrations (0.15, 0.5, 1, 1.5, and 1.85 %) were heated from 30 to 70 °C in a static ohmic heating cell at different voltages (65, 80, 100, 120, and 135 V) and frequencies (60, 2070, 5030, 7990, and 10,000 Hz) to evaluate their ohmic heating behaviour. Radish heated under 1.5 % salt, 120 V and 7990 Hz or 1 % salt, 135 V and 5030 Hz conditions gave the shortest heating time of 6 min from 30 to 70 °C, and cabbage gave the longest time of 128 min at 0.15 % salt, 100 V, and 5030 Hz. Regression models of heating rate as a quadratic function of the sample temperature gave R² >0.98. The general trend observed was that the magnitude of the heating rate increased with frequency at high voltage but decreased at low voltage for cabbage, while the opposite trend was observed for radish. Heating was more efficient at higher salt concentration and applied voltage. Radish heated more rapidly than cabbage. A slight slope change was observed in all cases between 50 and 60 °C. The response surface models revealed linear, cross products and quadratic effects to be significant with R ² over 0.98.     

Salt diffusion into vegetable tissue as a pretreatment for ohmic heating: Electrical conductivity profiles and vacuum infusion studies

The potential for salt diffusion as a pretreatment for ohmic heating was investigated by soaking vegetable (potato) tissue in aqueous NaCl solutions of various concentrations. Electrical conductivity profiles were found to be sensitive to salt concentration profiles at concentrations above 0·01 g/cm³, but below this concentration conductivity increases were negligible. Vacuum infusion was effective only in increasing salt concentrations in outer layers, indicating that its usefulness was primarily limited to small particles. Electrical conductivity-temperature relations were found to be linear for low-concentration NaCl-diffused samples, but at higher salt concentrations, the curve became nonlinear, possibly as a result of salt equilibration during the heating process.     

CONDUCTIVITIES AND OHMIC HEATING OF MEAT EMULSION BATTERS

The conductive nature of meat emulsions and their ohmic heating rates were investigated by examining model and commercial meat batters. Results indicate that salt or other electrolytic ingredients were essential for ohmic heating, and for a given batter, an increase in salt content increased electrical conductivity and increased ohmic heating rate (P<0.001). Conversely, an increase in fat content caused a reduction in conductivity (P<0.001) leading to an increase in ohmic heating time (P<0.001). Results of a study examining the impact of nonelectrolytic dry ingredients, such as spices, potato starches and soya protein isolates on the conductivity and ohmic heating rates of emulsion batters was inconclusive and further work is needed before definite conclusions can be made in this regard. Overall results suggest ohmic heating could be used for cooking meat batters but would not be as effective for cooking meats to which no salt or other electrolytes have been added.     

Advantages of ohmic cooking in the kilohertz range – Part II: Impact on textural changes and approaches to improve heating uniformity

The impact of ohmic heating (OH) in the low (12 kHz) and high (300 kHz) kilohertz-range on textural changes and texture uniformity of potato cubes and whole potato tubers was investigated. Potatoes were heated in aqueous NaCl solution with a conductivity of 2.5 mS/cm. Texture was characterized by cutting and puncture tests and the energy (mJ) necessary to cut or penetrate the samples was found to be a suitable indicator. The impact of the initial temperature of the surrounding liquid on the heating uniformity was analyzed by starting the cooking treatment in cold (20 ± 2 °C) and in hot (90 ± 5 °C) NaCl solution. To further improve the heating efficiency, the conductivity was adjusted during the treatment. With an increasing power input during OH, increasing texture softening rates were observed. Compared to conventional cooking, an up to 8-times faster texture softening rate occurred. Treatments at 300 kHz resulted in a better texture uniformity compared to OH at 12 kHz. Compared to the conventional cooking, OH treatments resulted in significant (p ≤ 0.05) higher weight losses and a higher texture softening at similar cooking degrees. By adjustment of the liquid conductivity and power input during the treatment, textural properties were improved, and weight loss could be reduced.Industrial relevanceOhmic heating (OH) is an alternative heating technology with the potential to reduce processing times and energy consumption. It can overcome limitations in the heat transfer particularly given for large-volume solid foods and provide a uniform heating. However, limited attention has been given to the effects of the product and process parameters on the heating uniformity during OH treatments of solid foods. The present paper highlights the importance of the electrical conductivity and the frequency of the applied electric field on the texture uniformity. The obtained results contribute to the understanding of the interaction of the electric field and the food matrix, which is the basis for the implementation of tailored and targeted processing concepts in the industry.     

Quality and consumer acceptance of radio frequency and traditional heat pasteurised kiwi puree during storage

To explore the feasibility of using radio frequency (RF) heating as a method to process kiwi puree, this study compared the effects of RF heating with traditional heat pasteurisation (TP) on various properties of kiwi puree. RF treatment completely inactivate of microbes in kiwi puree, with total aerobic bacteria counts (TAC) decreased by 4.81 log CFU mL^?1 and yeast and mould counts (YMC) decreased by 2.62 log CFU mL^?1, which are similar to TP treatment. During the 7 weeks’ storage, RF‐treated puree showed retarded growth of microbes. The pH, titratable acidity and °Brix values of RF‐treated samples were significantly different from those of the control. The vitamin C, total phenolic compounds and antioxidant capacity of RF‐treated samples were significantly higher than those of TP‐treated samples during storage. RF‐treated sample retained better colour than TP‐treated sample throughout storage. The customer satisfaction analysis indicated that consumers prefer RF‐treated kiwi puree than TP‐treated samples. These findings suggest that RF heating has potential applications in processing of kiwi puree or similar products.     

EFFECTS OF OHMIC PRETREATMENT ON OIL UPTAKE OF POTATO SLICES DURING FRYING AND SUBSEQUENT COOLING

The effect of ohmic pretreatment on the oil uptake of potato slices during frying and subsequent cooling was investigated. The treated potato slices were heated by either directly sandwiching the slices between two electrodes or by heating them in a 0.11% salt solution. The treatment was carried out using an alternating current field of 35.5 V/cm. Results of this study indicate that oil uptake during frying and subsequent cooling of potato slices was decreased by the ohmic pretreatment without involving a liquid medium (direct sandwiching). However, when the ohmic pretreatment was carried out in a liquid medium, the effect was not evident. This might be due to water absorption during the treatment, which increased initial moisture content as well as porosity of the treated samples.     

Dielectric Properties of Potato Puree in Microwave Frequency Range as Influenced by Concentration and Temperature

Dielectric properties of potato flour-water dispersions (slurry) were measured in the frequency range of 500–2500 MHz by the open-ended coaxial probe method using a network analyzer as a function of concentration (10–25% w/w) and temperature (20–75^oC). Both commercial and laboratory prepared samples of potato flours were used. Results indicated that the dielectric constant (ε′) decreased with temperature and frequency while increased with concentration. The loss factor (ε″) increased with frequency and concentration; however, temperature showed mixed effect. Both ε′ and ε″ data in conventionally used microwave frequencies (915 and 2450 MHz) were studied as function of concentration and temperature for two sample types using a response surface methodology and found to follow 2^nd order polynomial models. Temperature and concentration contributed significantly on dielectric spectra of potato slurry and the sample source had some effect. A change in ε′ and ε″ above 70^oC could be attributed by starch gelatinization. Penetration depth (D_p) decreased with an increase in frequency and non-systematic with temperature. Addition of salt substantially reduced D_p of potato slurry.     

A METHOD TO CHARACTERIZE ELECTRODE CORROSION DURING OHMIC HEATING

A method was developed to characterize visual electrode corrosion during ohmic heating of a model system. Stainless steel 304 electrodes were energized in a 2% salt solution at room temperature of 24±1C for 10 min. The effects of Alternating Current (AC) electrical frequency and current density were examined in a frequency range of 55 to 5000 Hz and current density range of 1200 to 3500 A/m². The ratio of colorimeter values of lightness (L)/yellowness (b) was used to quantify the degree of visual corrosion. Corrosion was most serious at low AC electrical frequencies of 55, 100, 200 Hz and at high current density of 3500 A/m². At AC electrical frequency values above 5000Hz, corrosion reduced dramatically even as current density was increased to 3500 A/m² and heating time extended to 1–1/2 h.     

CHANGES IN ELECTRICAL CONDUCTIVITY OF SELECTED VEGETABLES DURING MULTIPLE THERMAL TREATMENTS1

Electrical conductivity (σ) is the most important parameter in ohmic heating. Although data exist on its changes during ohmic heating, limited information is available about preheated foods. In this study, the conductivity changes of raw vegetable samples (potato, carrot, and yam) in cyclic ohmic heating and samples preheated by conventional heating prior to ohmic heating were investigated. In cyclic ohmic heating, cylindrical vegetable samples were subjected to three repeated cycles of ohmic heating (40 V/cm, 60Hz) to 80C, and cooling to 25C. Fresh samples were also preheated by conventional heating to 80C, then subjected to ohmic heating for comparison. Specific heats changed by cycles, although moisture content remained constant in all cases. The results show that in cyclic ohmic heating, the heating rate increased by cycles. Samples preheated by either conventional or ohmic heating showed a higher heating rate than raw materials. Electrical conductivity data during ohmic heating showed that preheated vegetables have higher conductivities than fresh ones, and a tendency of increase by cycles was found.     

Analysis of heat transfer during ohmic processing of a solid food

To produce a safe cooked food product it is necessary to ensure a uniform heating process. The aim of this study was to develop a mathematical model of a solid food material undergoing heating in a cylindrical batch ohmic heating cell. Temperature profiles and temperature distribution of the ohmic heating process were simulated and analysed via experimental and mathematical modelling which incorporated appropriate electromagnetic and thermal phenomena. Temperature profiles were measured at nine different symmetrically arranged locations inside the cell. The material was ohmically heated imposing a voltage of 100 V, while electrical field and thermal equations were solved for experimental and theoretical models by the use of FEMLAB, a finite element software. Reconstituted potato was chosen to represent a uniform solid food material and physical and electrical properties were determined prior to the experiment as a function of temperature. The simulation provided a good correlation between the experimental and the mathematical model. No cold spots within the product were detected but both experimental and model data analysis showed slightly cold regions and heat losses to the electrode and cell surfaces. The designed model could be used to optimize the cell shape and electrode configurations and to validate and ensure safe pasteurisation processes for other solid food materials.     

Mathematical modeling of ohmic heating of two-component foods with non-uniform electric properties at high frequencies

The ohmic heating (OH) of two-component foods (consisting of mashed potato and mashed potato with 1% NaCl) configured using four different filling patterns (parallel, series, and two concentric patterns) was investigated experimentally and by modeling. The electrical conductivities of the samples at 50 Hz and 20 kHz were obtained in the range of 20–85 °C. The differences between the thermal behaviors of the two-component foods were analyzed using internal thermal images and temperature profiles from four positions. A three-dimensional OH model for high frequencies was developed based on electric field analysis using Maxwell's equations. The model agreed sufficiently with the measured results and was able to confirm that the current applied to the sample follows the path that presents minimum electrical resistance, which is independent of the filling pattern configuration. Analysis of the heat flux density provided a better understanding of the heating behavior of two-component foods during OH.Industrial relevanceAlthough several mathematical approaches used to simulate the OH process of solid foods have been published in recent years, the simulation of temperature and electric field distributions using simplified methods, such as by the solution of Laplace's equation, is still a problem for two-component solid foods with special configurations of the internal components treated using an OH process at high frequencies. This problem can be solved using a more fundamental solution based on Maxwell's equations and electric field analysis via computer simulation. A comprehensive mathematical simulation of the thermal behavior and the effect of the orientation of the current within the two-component foods can be used to accurately predict the heat during OH. Therefore, the modeling of heating patterns of complex foods can assist in the design of food sterilization and pasteurization processes. Moreover, it will contribute in industrial applications for a better design of OH systems and electrode configurations for rapid and uniform heating.     

Electrical conductivity and physical properties of surimi-potato starch under ohmic heating

ABSTRACT:  Electrical conductivities of Alaska pollock surimi mixed with native and pregelled potato starch at different concentrations (0%, 3%, and 9%) were measured at different moisture contents (75% and 81%) using a multifrequency ohmic heating system. Surimi-starch paste was tested up to 80 °C at frequencies from 55 Hz to 20 KHz and at alternating currents of 4.3 and 15.5 V/cm voltage gradient. Electrical conductivity increased when moisture content, applied frequency, and applied voltage increased, but decreased when starch concentration increased. Electrical conductivity was correlated linearly with temperature ( R ²≈ 0.99). Electrical conductivity pattern (magnitude) changed when temperature increased, which was clearly seen after 55 °C in the native potato starch system, especially at high concentration. This confirms that starch gelatinization that occurred during heating affects the electrical conductivity. Whiteness and texture properties decreased with an increase of starch concentration and a decrease of moisture content.     

Electrical conductivity of particle–fluid mixtures in ohmic heating: Measurement and simulation

The electrical conductivity (EC) of food systems is a key parameter of the ohmic heating process. Ohmic heating experiments were conducted using a Teflon cylindrical cell. Electrical conductivity values were evaluated for the ohmic processing (25–125 °C) of blanched, 2-cm cubic particles (carrot, potato, radish, beef muscle, pork muscle and commercial ham) dispersed in carrier fluid (5% w/w starch–water solution with 0.15–1.5% w/w salt concentrations). Assuming an analogy between thermal conductivity (TC) and EC five existing TC theoretical models (series, parallel, two forms of Maxwell–Eucken models and effective medium theory) were applied and evaluated for EC of various solid–fluid structural systems. Parallel and Series models are not practically applicable in real food system, however they can indicate the useful limit values of highest and lowest EC among all two-phase structures. The first Maxwell–Eucken model, which describes solid particles dispersed in continuous liquid, showed the best agreement between predicted EC values and experimental data. This model provided a useful and relatively simple new approach to predict the effective EC for mixtures of different types of solid food particles immersed in liquid food.     

Modeling of ohmic heating patterns of multiphase food products using computational fluid dynamics codes

Modeling of the thermal behavior of multiphase food products with various electrical conductivities under ohmic heating has been a challenge. Distortion of electric field due to heterogeneous food properties and electrical conductivity distribution should be taken into consideration for accurate prediction of the thermal performance of ohmic heaters. The objective of this study was to model ohmic heating pattern of solid–liquid food complex that contain three different solid particles with substantially different electrical conductivities and 3% NaCl solution. The solid food samples used in this experiment were potato, meat, and carrot, which are less conductive than carrier medium. The transient heating patterns of each solid food and carrier medium were simulated using computational fluid dynamics (CFD) codes with user defined functions (UDFs) for electric field equations. The predicted temperature values were in good agreement with the experimental data with the maximum prediction error of 6 °C. Hot spots existed on the continuous phase in zones perpendicular to the solid cubes and cold spots were in between the particles where the current density lacks. CFD model prediction of detailed thermal profiles of multiphase food mixtures under the static ohmic heating will assist in designing of a continuous flow ohmic heater with pursuit of heating uniformity, furthermore, ensuring food safety and quality.     

Pressure Ohmic Thawing: a Feasible Approach for the Rapid Thawing of Frozen Meat and Its Effects on Quality Attributes

Rapid thawing has its significance to minimize the changes in quality attributes and microbial growth of frozen foods. This study investigated the potential of pressure ohmic thawing (POT) which utilize the synergy of high-pressure thawing and ohmic thawing. POT refers to the thawing of frozen foods using ohmic thawing (40 V/cm) under elevated pressures (200 MPa). Custom developed POT system was composed of flexible ohmic cell, pressure chamber, and electric field supplying system. Frozen beef samples in POT cells that were thawed with POT (40 V/cm, 200 MPa) were compared to control samples thawed using conventional, ohmic, and pressure-assisted thawing (PAT). POT had the shortest thawing time (0.8 min) followed by ohmic (5.5 min), PAT (11.5 min), and conventional thawing (43.3 min). POT and PAT applications decreased melting points of frozen samples from ?1.6 °C at 0.1 MPa to ?27 °C at 200 MPa. In POT, the in situ electrical conductivity increased (0.2–1.0 S/m), which combined with decreased melting points possibly enabled quicker thawing. POT minimized changes in shear force tests and peak forces close to that of raw sample as compared to controls. Our study demonstrates the benefits of POT applications for rapid thawing without affecting meat quality.