Pulsed Ohmic Heating Research Articles

Effect of particle size and concentration on foodborne pathogen inactivation by pulsed ohmic heating in tomato sauce

This research investigated the effects of the size and concentration of diced tomato particles in tomato sauce on the performance of pulsed ohmic heating (POH). Samples were prepared by adding 5 and 10 g of tomato particles (square shapes with sides of 2.5, 5, 10, and 15 mm) to 20 and 15 g of tomato sauces, and tomato sauce without particles was used as the control group. As the size and concentration of tomato particles increased, the passage of electric current was hindered, resulting in a decrease in electrical conductivity and consequently a deceleration in the heating rate of POH. Through infrared camera image analysis, it was determined that tomato particles formed cold areas in the tomato sauce, inhibiting uniform heating of POH. Because particles reduced efficiency of POH, there was an inverse relationship between the size and content of particles and the reduction of pathogens. At 20% tomato particles, the reduction levels of Escherichia coli O157:H7 were 1.60, 1.85, 2.12, 2.28, and 3.07 log CFU/ml for tomato sauces with particle sizes of 2.5, 5, 10, and 15 mm and without particles, respectively. When the size of the tomato particles was the same, increasing the particle concentration in the sauce from 20 to 40% resulted in an average reduction of 0.47 in the inactivation levels of E. coli O157:H7 by POH. Therefore, the size and concentration of particles dispersed in food should be contemplated when designing a POH process to inactivate harmful bacteria in food.

Development and investigation of ultrasound-assisted pulsed ohmic heating for inactivation of foodborne pathogens in milk with different fat content

The central objective of this research was to develop an ultrasound-assisted pulsed ohmic heating (POH) system for inactivation of food-borne pathogens in phosphate buffered saline (PBS) and milk with 0–3.6% fat and investigate its bactericidal effect. Combining ultrasound with POH did not significantly affect the temperature profile of samples. Both POH alone and ultrasound-assisted POH took 120 s to heat PBS 60℃. Milk with 0, 1, and 3.6% fat was heated to 60℃ by POH alone and ultrasound-assisted POH after 335, 475, and 525 s, respectively. This is because the electrical conductivity of the samples was the same for POH alone and ultrasound-assisted POH. Despite identical temperature profiles, ultrasound-assisted POH exerted a synergistic effect on the reduction of Escherichia coli O157:H7, Salmonella Typhimurium, Listeria monocytogenes, and Staphylococcus aureus. In particular, the inactivation level of S. Typhimurium in PBS subjected to ultrasound-assisted POH treatment for 120 s corresponding to a treatment temperature of 60℃ was 3.73 log units higher than the sum of each treatment alone. A propidium iodide assay, intracellular protein measurements, and scanning electron microscopy revealed that ultrasound-assisted POH treatment provoked lethal cell membrane damage and leakage of intracellular proteins. Meanwhile, fat in milk reduced the efficacy of the bacterial inactivation of the ultrasound-assisted POH system due to its low electrical conductivity and sonoprotective effect. After ultrasound-assisted POH treatment at 60℃, there were no significant differences (P > 0.05) in the pH, color, and apparent viscosity of milk between the untreated and treated group.

Combination system of pulsed ohmic heating and 365-nm UVA light-emitting diodes to enhance inactivation of foodborne pathogens in phosphate-buffered saline, milk, and orange juice

This study developed and investigated the combination system of pulsed ohmic heating (POH) and ultraviolet A light-emitting diode (UVA-LED) to enhance inactivation of pathogens in phosphate-buffered saline, milk, and orange juice. When Escherichia coli O157:H7 and Salmonella Typhimurium (6–8 log CFU/ml) in samples were subjected to the simultaneous combination of POH and UVA-LED at 50–65°C, the reduction levels increased >1 log CFU/ml compared with each treatment alone. After pathogens in PBS were subjected to the combination treatment at 65°C (50 s), the degree of intracellular ROS generation and lipid peroxidation were increased approximately 1.6 times than each treatment alone. Moreover, propidium iodide uptake values, which indicated cell membrane damage, for E. coli O157:H7 were 0.92, 14.22, and 19.92 after UVA-LED, POH, and their combination treatment, respectively. Therefore, the combination of POH and UVA-LED may possibly be applied as an effective hurdle technology for food safety. Industrial relevanceIt was identified that the combination treatment induced the generation of intracellular ROS; thereby causing lipid peroxidation and cell membrane damage of pathogens. However, the combination system has no effect on DNA damage of pathogens. In addition, there was no significant difference in the content of vitamin and malondialdehyde in milk and orange juice before and after the combination treatment. Thus, this research provides a fundamental database for the applicability of the combination system of POH and UVA-LED to the pasteurization process in the food industry.

Intensified inactivation efficacy of pulsed ohmic heating for pathogens in soybean milk due to sodium lactate

Pulsed ohmic heating (POH) is a promising alternative to conventional heat processing for microbial safety. It has been reported that the efficacy of POH depends on the concentration of electrolytes. This research investigated the effect of sodium lactate as an electrolyte on the efficacy of POH with respect to the heating rate and inactivation of pathogens in PBS and soybean milk. When PBS and soybean milk with 0–3% sodium lactate were subjected to POH, as the sodium lactate content increased, the electrical conductivity increased, thereby accelerating the heating rate of POH. For instance, the heating rate of POH was 0.3 and 1.3°C/s for 0 and 3% of sodium lactate in soybean milk. Reductions in pathogens increased as the sodium lactate content increased. When 3% of sodium lactate was added to PBS or soybean milk, the bacterial reduction was more than 1 log unit compared to 0%. We hypothesized that sodium lactate intensified the damage to bacterial membrane by accelerating the heating rate of POH and amplifying the electric current; thus, the extent of bacterial cell membrane damage was determined using propidium iodide (PI) and transmission electron microscopy (TEM). The PI uptake values of pathogens significantly (P < 0.05) increased as the sodium lactate content increased, indicating that the membrane damage increased in proportion to the sodium lactate content. Furthermore, the analysis of TEM images of Escherichia coli O157:H7 and Listeria monocytogenes showed fatal morphological cell rupture at 3% sodium lactate. After soybean milk with 0–3% sodium lactate was treated with POH, no electrode corrosion was observed, and pH and color of samples were not changed. Thus, this research corroborated that sodium lactate intensifies the efficacy of POH without electrode corrosion and quality degradation of soybean milk.

Application of continuous-type pulsed ohmic heating system for inactivation of foodborne pathogens in buffered peptone water and tomato juice

The purpose of this study was to inactivate Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes by continuous-type pulsed ohmic heating in buffered peptone water (BPW) and tomato juice. First, BPW inoculated with the three pathogens were treated at different flow rates (0.2–0.4 LPM) and treatment voltages (9.43–12.14 Vrms/cm). Both heating rate of BPW and reduction rates of pathogens increased corresponding to decreased flow rate. Accordingly, higher numbers of pathogens survived at a higher flow rate (0.4 LPM). Increasing treatment voltage was an effective way to inactivate pathogens at 0.4 LPM, but the heating rate overly accelerated with increasing voltage adversely affecting food quality. Alternatively, increasing initial temperature by preheating can help inactivate pathogens in the early treatment stage without affecting heating rate. From the BPW experiments, we identified that treatment conditions such as flow rate, voltage, and initial temperature are important factors determining pathogen inactivation performance of continuous-type ohmic heating. When applied to tomato juice, 5 log reductions of all three pathogens were achieved by applying 12.14Vrms/cm ohmic heating with 0.2 LPM flow rate after preheating sample to 50 °C with a water bath. Quality aspects of color and lycopene content were observed, and a∗ and b∗ values decreased after treatment. Because preheating with additional equipment is inconvenient and occupies valuable space, we developed sequential three cylinder type ohmic heating. By applying the developed sequential ohmic heating, 5 log reductions were achieved for all three pathogens without preheating under the same treatment conditions. Therefore, we concluded that sequential continuous-type ohmic heating can be used utizied effectively to control foodborne pathogens by the juice industry.

Application of low frequency pulsed ohmic heating for inactivation of foodborne pathogens and MS-2 phage in buffered peptone water and tomato juice

The purpose of this study was to inactivate foodborne pathogens effectively by ohmic heating in buffered peptone water and tomato juice without causing electrode corrosion and quality degradation. Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes were used as representative foodborne pathogens and MS-2 phage was used as a norovirus surrogate. Buffered peptone water and tomato juice inoculated with pathogens were treated with pulsed ohmic heating at different frequencies (0.06–1 kHz). Propidium iodide uptake values of bacterial pathogens were significantly (p < 0.05) larger at 0.06–0.5 kHz than at 1 kHz, and sub-lethal injury of pathogenic bacteria was reduced by decreasing frequency. MS-2 phage was inactivated more effectively at low frequency, and was more sensitive to acidic conditions than pathogenic bacteria. Electrode corrosion and quality degradation of tomato juice were not observed regardless of frequency. This study suggests that low frequency pulsed ohmic heating is applicable to inactivate foodborne pathogens effectively without causing electrode corrosion and quality degradation in tomato juice.

Quantification of metal release from stainless steel electrodes during conventional and pulsed ohmic heating

Abstract Electrochemical reactions at the electrode-solution interface of an ohmic heater can be avoided or significantly limited by choosing appropriate processing conditions in relation to the food properties. In the present work the effect of the electrical parameters (electric field strength and frequency of the applied current signal) and product factors (halides concentration, electrical conductivity and pH) on metal release from stainless steel (type AISI 316 L) electrodes of a batch ohmic heater was investigated. In each experiment, the concentrations of the main constituents of stainless steel (iron, chromium and nickel) released in the heating medium were detected by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and Atomic Absorption Spectrophotometry (AAS). Results showed that the rate of metal release from the electrodes to the heating medium depends on frequency and applied field strength. However, the use of ohmic heating at a higher frequency than conventional (50 Hz) can significantly (p ≤ 0.05) reduce the flux of metal ions from stainless steel electrodes. Moreover, it was also demonstrated that electrochemical phenomena occurring at the electrode-solution interface strongly depend on the composition, pH and electrical conductivity of the heating medium. Industrial relevance The magnitude of electrode material released into the heating medium during ohmic processing depends on many factors, whose effects should be known in order to define optimal treatment conditions, electrode material and food properties able to avoid or minimize the undesired phenomenon of contamination of the food product, electrode-fouling and electrode corrosion. This paper contributes to clarifying the effects of electric field strength applied as well as electrical conductivity, pH, and presence of halides in the heating medium on electrode corrosion or release of electrode materials during high frequency (25 kHz, bipolar square wave) pulsed power OH in comparison with conventional (50 Hz sine wave) OH. Interestingly, the use of sufficiently large frequencies may avoid or reduce the extent of electrochemical reactions at the electrode interface, minimizing corrosion and leakage of metals to the heating medium, even when electrode material of low cost and electrochemically active like stainless steel is used.

Extraction of Polyphenols from Red Grape Pomace Assisted by Pulsed Ohmic Heating

The present work is devoted to the investigation of the effect of pulsed ohmic heating (POH) on cells membrane damage and intensification of polyphenols extraction from red grape pomace. Untreated, POH-treated and freeze-thawed samples were compared. The effects of electric field strength (E = 100–800 V/cm) and the percentage of ethanol in water (E/W = 0–50 %) on polyphenols extraction were discussed. Measurements of electrical conductivity and electric energy consumption were performed for POH pretreatment optimization. Results show that POH treatment results in cells membrane denaturation. This permeabilization increases with the elevation of electric field strength and temperature. POH pretreatment accelerates the extraction kinetics of total polyphenols from grape pomace. Freeze-thawed samples are always accompanied with a high degree of cell damage and high concentration of polyphenols in the extract. The highest extraction yields were obtained with a POH pretreatment at 400 V/cm followed by a diffusion step for 60 min at 50 °C and with a solvent composed of 30 % of ethanol in water. In these conditions, the polyphenol content was 36 % more than untreated samples. The proposed technique (POH pretreatment) appears to be promising for future industrial applications of polyphenols extraction from pomace.

Ohmic sterilization inside a multi-layered laminate pouch for long-duration space missions

A pouch that can be used to ohmically reheat and sterilize food and later reused to stabilize waste will significantly reduce the Equivalent System Mass during a long term space mission. Reheating of food and stabilization of waste have been successfully done in the past using a V-shaped electrode pouch using pulsed ohmic heating, however improvement of field distribution was needed. The main goals of this work were to develop an improved pouch design to facilitate uniform ohmic heating of food; to model ohmic sterilization within the pouch; and to verify the model through an inoculated pack study.The pouch was redesigned to improve the electric potential distribution and hence heating uniformity. A temperature distribution study on a pouch with 227g of tomato soup showed the presence of cold regions at the non-electrode sides due to channeling of current through a hotter and more conductive centre. A much more uniform temperature distribution was obtained the use of two external strip heaters installed along these sides. The ohmic heating process inside the pouch was simulated in 3D to predict possible cold and hot spots. The simulated heating profile was in good agreement with the measured values confirming the efficacy of the model. An inoculated pack study using Geobacillus stearothermophilus spores (ATCC 5973) confirmed the mathematical model. This technology provides a new method of sterilization inside sealed pouches/packets using ohmic heating.

Enhanced Extraction from Solid Foods and Biosuspensions by Pulsed Electrical Energy

The purpose of this article is to review matters dealing with application of electrical pulsed energy (pulsed electric field, pulsed ohmic heating and high voltage electrical discharges) for enhancing the solid–liquid extraction in the food industry. The quality of products (e.g., purity, color, texture, flavor and nutritients) extracted from solid foods (sugar beets, apples, grapes, etc.) and quality of proteins and polysaccharides extracted from biosuspensions (aqueous suspensions of yeast, e.coli cells, etc.) may be degraded by conventional mechanical, thermal or chemical pretreatment. The traditional treatments are restricted by temperature elevation and require high energy consumptions; moreover, chemical methods are based on application of unsafe organic solvents, enzymes and detergents. Physical treatments assisted by electrical pulsed energy are shown to be very remarkable for enhancing the solvent and pressure extraction and dehydration processes and of high interest for the food industry. This article provides an overview of the pulsed energy extraction technologies showing promise for commercial food processing.

Migration of electrode components during ohmic heating of foods in retort pouches

During space missions, life support technologies must satisfy mission constraints, including maximizing safety and acceptability of food and minimizing crew time, storage volume, power, water usage, and maintenance down-time. Ohmic heating appears to be a feasible potential solution to meet food reheating and waste sterilization requirements; however, it is necessary to verify if electrochemical reactions such as gas generation and electrode corrosion occur. A pulsed ohmic heating system and reusable pouch were tested to ensure the food safety and quality by minimizing the undesired electrochemical reactions. Results of gas generation during ohmic heating show that stainless steel is far superior to aluminum since there were no noticeable gas bubbles even at the temperature of 70 °C. A study of migration of the major key metal ions from stainless steel, such as Fe, Cr, Ni, Mn, and Mo, measured by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) shows that pulsed ohmic heating produced comparable or lower migration of most targeted metal ions, compared to the conventional retorting when electrodes were present. The intakes of individual metal contaminants evaluated with respect to a typical meal (8 oz) after ohmic treatment were, as a maximum, 13.5% of recently published upper-level daily dietary exposure estimates. Consequently, pouches with stainless steel electrodes powered by a pulsed ohmic heater shows promise as a potential reheating and sterilization technology for space missions. Ohmic heating has been used for many years in different industries and proved to be a promising food processing technology due to its rapid, yet uniform and high energy efficient heating capability. This paper demonstrates the potential of direct ohmic heating for foods in retort pouches at the household level as well as the mass production level, ensuring food safety by suppressing the metal ions migration far less than the dietary exposure estimates.

Pulsed Ohmic Heating–A Novel Technique for Minimization of Electrochemical Reactions During Processing

ABSTRACTMinimization of electrochemical reactions during ohmic heating would be desirable. This study examines a pulsed ohmic heating technique to determine its effect on electrochemical reactions. Effects of pulse parameters, such as frequency, pulse width, and delay time were studied, in comparison with conventional (60 Hz, sine wave) ohmic heating using various electrode materials. Analyses of electrode corrosion, hydrogen gas generation, and pH change of the heating media were performed. The results suggest that pulsed ohmic heating is capable of significantly (P &lt; 0.05) reducing the electrochemical reactions of stainless steel, titanium, and platinized‐titanium electrodes, compared with conventional 60 Hz ohmic heating. The importance of allowing enough delay time for discharge of the electrical double layers after each pulse input is emphasized.

MODELING AND OPTIMIZATION OF OHMIC HEATING OF FOODS INSIDE A FLEXIBLE PACKAGE

ABSTRACT A pulsed ohmic heating system and flexible package for food reheating and sterilization were developed to minimize Equivalent System Mass during long‐duration space missions. A package made of flexible pouch materials was powered through a pair of metal foil electrodes extending out. Preliminary tests of the package within an ohmic heating enclosure show that International Space Shuttle menu items such as chicken noodle soup and black beans could be heated using pulsed ohmic heating technology. The electrical conductivities of selected samples ranged between 0.01 and 0.03 S/cm. A 2‐D thermal‐electric model was developed using commercial CFD software Fluent to optimize the design and layout of electrodes to ensure uniform heating of the material. A static system under the assumption of there being no fluid motion in a microgravity environment was implemented. A package configuration with V‐shaped electrodes with dimensionless width of 0.147 was validated to be most appropriate for uniform heating while minimizing the cold zone to 2% of total area. The effect of field overshoot near the electrode edge is expected to be crucial to determine the uniformity of heating.

Rapid thermal annealing of n-(Cd, Hg)Te crystals

We studied native defect formation in n-(Cd, Hg)Te bulk crystals and thin layers by non-equilibrium techniques, such as pulse laser irradiation and pulsed ohmic heating. The short-term annealing was performed in a vacuum by passing current directly through a sample. The temperature during annealing in a vacuum was measured by registering the Planck black-body radiation from the sample surface. A surface thin-film thermometer was used for temperature monitoring during the laser irradiation.