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Pasteurization of fermented red pepper paste by ohmic heating
Pasteurization of fermented red Pepper Paste by ohmic heating Won-Il Cho, Myong-Soo Chung PII: DOI: Reference:
S1466-8564(16)30003-0 doi: 10.1016/j.ifset.2016.01.015 INNFOO 1470
To appear in:
Innovative Food Science and Emerging Technologies
Received date: Revised date: Accepted date:
1 November 2015 20 January 2016 23 January 2016
Please cite this article as: Cho, W.-I. & Chung, M.-S., Pasteurization of fermented red Pepper Paste by ohmic heating, Innovative Food Science and Emerging Technologies (2016), doi: 10.1016/j.ifset.2016.01.015
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Original Research Article
Pasteurization of Fermented Red Pepper Paste
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by Ohmic Heating
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CJ Foods R&D, CJCheiljedang Corp., Seoul 152-050, KOREA
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Department of Food Science and Engineering, Ewha Womans University, Seoul 120-750, KOREA
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Won-Il Cho a, Myong-Soo Chung b,*
Short version of title: Pasteurization of fermented red pepper paste by ohmic heating
Corresponding author at: Department of Food Science and Engineering, Ewha Womans University, 52 Ewhayeodae-gil, Seodaemun-gu, Seoul 120-750, Korea. Tel.: +82 2 3277 4508. Email address: [email protected] (MS. Chung).
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ABSTRACT
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Ohmic heating was applied to a Korean traditional fermented food containing red
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pepper paste, called Gochujang with low thermal conductivity (0.458 w/M·K), by varying frequencies (40-20,000 Hz) and applied voltages (20-60 V). Contrary to
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conduction heating, the entire sample was heated uniformly, and the specific heating rate was found to be highly dependent on the frequency, peaking at 5 kHz and 60 V. The
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results showed that complex differential equation and the Runge-Kutta fourth-order method are suitable for simulating the temperature profile during ohmic heating. The
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effective deactivation of vegetable cells of Bacillus strains on fermented red pepper
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paste by ohmic heating was indicated by a 99.7% reduction, compared with conduction heating for 8 min at 100C producing a 81.9% reduction. The organoleptic and
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physicochemical qualities of the samples pasteurized by ohmic heating were nearly the
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same as those of raw samples, and higher than those of conventionally heated samples.
Keywords: ohmic heating, fermented red pepper paste, electrical model, frequency, pasteurization
1. Introduction
Heat transfer with driving forces based on a temperature gradient is generally used for the heating of foods. The quality deterioration of liquid foodstuffs with low viscosity after high-temperature exposure can be minimized by using high-temperature, short-
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time sterilization (HTST) or ultra-high temperature (UHT) sterilization. However,
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various problems are encountered when applying conduction heating to high-viscosity
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foods and foods comprising solid–liquid mixtures. Gochujang is a Korean traditional
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fermented food in the form of a red pepper paste that can have a high viscosity, making the use of a heat-exchange plate difficult. Moreover, its low heat-conduction coefficient means that heating needs to be applied for a long time in the sterilization process. These
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phenomena can result in overheating of the heating surface, producing a deterioration of
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food quality via the generation of off-flavors and decoloring reactions (Kim & Kwon, 2001; Lim, Kim, Kim, Mok, & Park, 2001; Yoo, 2001).
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Given that the high viscosity of fermented red pepper paste restricts the usefulness of
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HTST and UHT sterilization, sterilization processes involving a tube-type heat exchanger and chemical treatments such as adding ethyl alcohol and sorbic acid have
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been commonly used. However, heating at 70–80C using a tube-type heat exchanger is not sufficiently effective at decreasing microbes, and most of consumers object to the
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addition of preservatives such as sorbic acid reactions (Kim & Kwon, 2001; Lim, Kim, Kim, Mok, & Park, 2001; Yoo, 2001). Fermented red pepper paste is one of the most suitable materials for sterilization by ohmic heating. The electrical conductivity of food means that heat will be generated within its internal electrical resistance when alternating current (AC) is passed through it, thereby representing the conversion of electrical energy into heat energy; this heating method is called ohmic heating (Lee, Lee, Koh, & Lee, 2000; Parrott, 1992; Sastry & Sevugan, 1992). Microwave heating also involves the generation of heat by the conversion of electrical energy into heat inside food, via the vibration of water molecules on dipole rotation and ionic polarization of ions in a food. However, 3
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microwave heating tends to produce nonuniform increases in temperature due to (1) the
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limited penetration depth of the microwave irradiation and (2) the difficulty of ensuring
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irradiation by a uniform electromagnetic field at such high frequencies (especially in
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domestic microwave ovens). In contrast, ohmic heating has no limitations regarding the penetration depth as long as the inherent electrical resistance of the food is not too high. Moreover, liquids and solids on ohmic heating are heated simultaneously without
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requiring a stirring or mixing process of conventional heating (De Alwis & Fryer,
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1990b).
Ohmic heating has been studied in various investigations of its application in
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commercial processes, such as for the sterilization of paste foodstuff with high
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viscosities and solid–liquid mixtures, the cooking and sterilization of seafoods such as surimi, and the thawing of frozen foods. Examples of developed equipment include the
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continuous ohmic heating system of a group in Cambridge and APV Baker in United Kingdom, the Joule heating system in Japan for the sterilization and molding of surimi,
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and the meat emulsion heating system and thawing machine for frozen fish blocks in Russia (De Alwis & Fryer, 1990a; Parrott, 1992; Wang & Sastry, 1993). The present study designed and implemented a novel sterilization process based on a static ohmic heating system with low-frequency AC at the laboratory scale for fermented red pepper paste with a low thermal conductivity (0.458 W/m∙K) (Chang & Chun, 1982). The developed system was used to investigate the mechanisms and characteristics underlying the induction of ohmic heating and then, tested the pasteurization effect against microorganisms in fermented red pepper paste.
2. Materials and methods 4
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2.1. Sample preparation
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Gochujang manufactured by a food processing company (Jinmi Food, Seoul, Korea) was used as the experimental sample and stored in a refrigerator (4C). The sample contained wheat flour, red pepper powder, milled glutinous rice, salt, corn syrup and
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water. Compositions of water, protein, fat, carbohydrate and ash of the sample were
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43%, 8%, 4%, 27% and 18%, respectively, that can affect the heat generation on ohmic heating. The important properties of fermented red pepper paste were organoleptic
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quality based on taste, color and mouthfeel related to viscosity.
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2.2. Experimental apparatus
A self-designed ohmic heating system was used in the experiments. The power
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supply of the ohmic heating apparatus consisted of a function generator to generate sine and square waves from 40 Hz to 20 kHz, and an amplifier that can output 95-volt signals. An automatic multimeter, oscilloscope, and electrical conductance meter were used for the analysis and calculation of data. The heating cell (85 x 45 x 0.5 mm, W x L x T) was constructed from an upward-opening polypropylene box (90 x 90 x 50 mm, W x L x D), and aluminum was used as the electrode material. To ensure safety during the experiments, the heating cell was installed in a Pyrex box. The electrical conductivity of liquid and paste foodstuffs were measured with an electrical conductor meter (CM-2A, Tokyo TOA Electronics, Tokyo, Japan). The current and voltage applied to the food during ohmic heating were measured using two digital 5
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multimeters (3500T, DM 303 TR, HC, Seoul, Korea), and the resistance of the food was
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calculated by applying Ohm’s law to the measured current and voltage values. The
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waveforms produced by the function generator were observed on a two-channel 50-
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MHz oscilloscope (MO-1254 A, Meguro, Tokyo, Japan), including to measure their frequencies. A thermistor with a thermocouple (T type, Shinhan, Seoul, Korea) was
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variability within the sample was 2℃.
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used to measure center temperature on heated sample. The maximum temperature
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2.3. Experimental procedure
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The sample was placed inside the heating cell of the ohmic heating of batch type apparatus, and then its ohmic heating characteristics as electric conductivity and heating
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rate were examined at various frequencies (40-20,000 Hz) and applied voltages (20-60 V). The effects of the internal ohmic heat generation of the frequency and voltage on the
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pasteurization of fermented red pepper paste were investigated. Also, polynomial approximation, complex differential equation and the Runge-Kutta fourth-order method were used to simulate the temperature profile of fermented red pepper paste on various frequencies and voltages during ohmic heating. An experiment was also performed to implement conventional conduction heating, involving measuring the temperatures when the sample packaged in an aluminum box of the same size as the ohmic heating cell was immersed in water at 70–100C. Sensory evaluation and analysis for physiochemical properties were conducted for comparing between products applying ohmic heating and conventional conduction heating.
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2.4. Measurements of pH and acidity
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In order to measure pH of homogeneous fermented red pepper paste, 3 g of the each
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sample was mixed with 30 mL of distilled water and the diluted solution was centrifuged at 1,400g with a centrifuge (1248R, GYROZEN, Seoul, Korea). The pH of the supernatant was measured by a pH meter (420A, Orino, Tokyo, Japan) at room
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temperature.
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The acidity of the sample was analyzed by a quantitative method with lactic acid. A 10-mL aliquot of supernatant of the sample obtained by centrifugation was diluted with
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10 mL of distilled water, and then titration of sample was performed by end-point
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checking as the retention of light redness during 30 s with the addition of 0.5 mL of 1% phenolphthalein and 0.1 N NaOH:
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Lactic acid (%) = [titration volume of 0.1 N NaOH (%) Factor of NaOH 0.009] / (1)
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[weight of sample (g)]
2.5. Color measurement
The color values of samples were measured by a spectrophotometer (UV-120-02, Shimadzu, Tokyo, Japan). A 0.5-g sample and 20 mL of acetone were mixed for 10 min, and then the OD value was measured at 460 nm. The color value of Gochujang was calculated as Color value = [OD value / (weight of sample (mg))] 1000 dilution multiple
2.6. Sensory evaluation 7
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The organoleptic characteristics of the samples were determined by a trained panel
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consisting of 10 students in the Department of Food Engineering, Yonsei University.
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After completing three training sessions related to descriptive profiling, the multiple comparison test was conducted for evaluating sensory attributes such as taste, color, flavor, texture, and overall acceptability of the fermented red pepper paste. All samples
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were the same weight and each was served on a randomly coded plate and water was
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provided to the panelists to cleanse the palate after tasting each sample. The panelists rated the preference of sensory attributes from 1 (extremely bad) to 5 (extremely good)
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for each sample on a 5-point hedonic scale.
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2.7. Microbiological analysis
In order to identify the viability of microorganisms in fermented red pepper paste, 10
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g of each sample was placed in 100 mL of sterile distilled water and pummeled for 3 min at 9 h/s with a Stomacher (HBM-400A, Tianjin Hengao, Tianjin, China). The mixture was serial diluted and spread on plate count agar (PCA, Difco Lab., Detroit, MI, USA) and incubated for 24 h at 37C. After the incubation, the number of viable cells related to total bacterial growth was quantified in CFU/mL; the number of colonies that could be counted was 30–300 per culture plate. A Korean traditional fermented food containing red pepper paste with high viscosity have many kinds of microorganism including heat-resistant spores of Bacillus strains. Therefore, the inactivation effect of the ohmic heaing and boiling against vegetative cell of Bacillus spp. was measured by a standard colony counting on tryptic soy broth agar (TSA Difco Lab., Detroit, MI, USA) 8
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plates.
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2.8. Statistical analysis
All of the data are expressed as mean and standard deviation values from five replicate measurements for treatment conditions. In order to verify significant
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differences (p<0.05) between the samples, the statistical analysis of one-way ANOVA
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procedure and Duncan’s multiple-range test were conducted using Minitab (version
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MTB13, Minitab, Pennsylvania, USA).
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3. Results and discussion
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3.1. Electrical properties of red pepper paste
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The heating rate in conventional conduction heating depends on the heat conductivity of the sample, whereas the most important factor in ohmic heating is the electrical conductivity. The electrical conductivity of the fermented red pepper paste when electric current is not being passed through it was 1.865 S/m. The electrical conductivity varies not only with the food structure and constituents, but also with the heating time and temperature on Fig. 2 (Halden, De Alwis, & Fryer, 1990; Nistor, Stãnciuc, Andronoiu, Mocanu, & Botez, 2015). The specific heating rate (in C/g∙s) increased linearly with voltage at 60 Hz, and increased rapidly with the frequency (when this was higher than 1 kHz) for a constant voltage (Fig. 2). The heating rate in fermented red pepper paste peaked at 5 kHz. 9
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The mechanism of ohmic heating achieved by the application of AC (50–60 Hz, 110–
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220 V) can be clearly explained by an electrical resistor model (Shiby Varghese,
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Pandey, Radhakrishna, & Bawa, 2014;. Zareifard, Ramaswamy, Trigui, & Marcotte,
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2003), but this model is not valid for the heating mechanism when applying lowfrequency AC since it does not accurately reflect how increases in temperature are related to changes of frequency.
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An electrical equivalent circuit comprising a resistor and dielectric can explain the
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heating mechanism of fermented red pepper paste during ohmic heating with lowfrequency AC. The R value corresponds to the resistance for leakage current by food components with properties of an electrical conductor. In raw foods with intact cell
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walls, the plasma membrane and extracellular fluid contain Na2+ and Ca2+ ions that act as electrical conductors. In paste foods such as sauces and jam without an internal
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structure, polarized electrolytes such as salt, amino acids, and proteins act as conductors. In contrast, the r value was the resistance based on delay of polarization of electrolytes.
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The electrolytes in an ideal capacitor can rapidly orientate and polarize to effect a lossless current upon the application of a voltage. However, the food components acting as electrolytes in fermented red pepper paste cannot rapidly orientate on electrical polarization due to their high molecular weight and the unique vibrations induced by ohmic heating at frequencies higher than 5 kHz. The orientation delay of electrolytes is called anomalous dispersion, and generates energy losses that result in the rapid internal heating of foods. The electrical properties of fermented red pepper paste were calculated using the following equations: Pε'' = Wε'' = Cp·m·ΔT/Δt = P·tan(δ) = V·I·tan(δ)
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tan(δ) = 1/(ω·Cr), ω = 2πf, XR = 1/(ω·C)
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ε' = C·d/s, ε'' = ε'/(ω·Cr)
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where P and W are the electrical energy (W), ε'' is the dielectric loss, Cp is the heat
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capacity (J/kg·K), m is mass (kg), ΔT is temperature gradient (K), Δt is time gradient
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(s), tan(δ) is a dielectric dissipation factor, V is voltage (V), I is electric current (A), ω is the angular velocity (ω), C is the capacitance (F), r is internal resistance (Ω), f is the frequency (Hz), XR is the capacitive reactance (Ω), ε' is the dielectric constant, d is
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distance of parallel plate (m), and s is surface of parallel plate (m2). The electrical
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capacitance of fermented red pepper paste decreases gradually as the frequency of the applied stimulation increases, but the electrical conductivity related to the generation of
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heating also gradually increases.
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3.2. Simulation of the temperature induced by ohmic heating
Simulating the temperature profile is very important for optimizing the sterilization
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conditions during ohmic heating (De Alwis & Fryer, 1990a; Lekwauwa & Hayakawa, 1986; Zaror, Pyle, & Molnar, 1993). The present study investigated a model for simulating the temperature changes for various frequencies and voltages. The approximation formula of the temperature dependence of electrical conductivity in fermented red pepper paste was obtained through polynomial approximation based on experimental data. The approximation formula for AC at 60 Hz was K = –1.598 + 0.154·T – 0.043·T2 + 0.000054·T3 + 2.3210–7·T4
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The correlation curve of low-frequency AC at 500 Hz was described by K = –5.760 + 0.722·T – 0.035·T2 + 0.000898·T3 – 0.000012·T4 + 8.78310–8·T5 – 2.50610–10·T6
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The relationship between electrical conductivity and frequency was expressed as K = 0.312 – 0.000015·f + 8.1810–8·f 2 – 4710–12·f 3
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This equation indicates the presence of a strong correlation as the correlation coefficient
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higher than 0.95 (p<0.05).
The temperature of foods comprising a mixture of liquid and solid was simulated using the numerical analysis of the following complex differential equation:
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ΔT/Δt = 1/(ρCp)·Δ/Δx·(λxΔT/Δx) + 1/(ρCp)·Δ/Δy·(λyΔT/Δy) +∑Vi2·ki/(ρCp)
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where ρ is the density (kg/m3), λ is the thermal conductivity (W/m·K), and ki is the individual electrical conductivity (S/m). However, for the ohmic heating of fermented
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red pepper paste, the following simpler equation could be applied to simulate the
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temperature because the heating due to conduction and convection can be considered to be negligible in a homogeneous sample: (10)
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ΔT/Δt = ∑Vi2·ki/(ρCp)
To simulate the temperature profile of fermented red pepper paste during ohmic
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heating, the change in the electrical conductivity dependence on temperature was calculated based on the equations 6 and 7. And then, the electrical conductivity for changes in frequency was calculated using the equation 8. Finally, the increase in temperature with heating time was simulated by the equation 10 and the Runge-Kutta fourth-order method as shown in Figs. 3 and 4. The error value was relatively low, at 2– 4%, indicating the validity of the proposed simulation model.
3.3. Pasteurization effect by ohmic heating
To ensure both safety and quality it is necessary to check for the generation of new 12
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electrolytes during ohmic heating. In general, direct-current stimulation by an electric
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field will induce a large amount of electrolysis that will effect a change in the pH,
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whereas AC stimulation could result in a lower degree of electrolysis due to the
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associated rapid changes in the electric field (Cha, 2014; Yang, Han, Lee, Park, & ., Kim, 2014). The phenomenon of electrolysis hardly ever happened when applying ohmic heating to fermented red pepper paste at various frequencies in the present study,
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as indicated by the pH of the sample treated by ohmic heating being very similar to that
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of the raw sample (Fig. 5A).
The analysis data for the properties of fermented red pepper paste with conventional
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conduction heating at 80C for 7 min and with ohmic heating at 40 Hz to 20 kHz, 20–60
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V, and 90–100C for 2 min are presented in Fig. 5B–D. From result of this study, it is interesting to note the dramatic improvement in sensory quality between ohmic heating
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and conduction heating. The sensory qualities such as taste, flavor, texture and color on sensory evaluation were better for ohmic heating than for conduction heating despite the
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higher temperature used during the former. The results in Table 1 indicate that the scores for each organoleptic characteristics were higher about 0.3-0.5 points with significant different level for ohmic heating (p<0.05). However, there was no significant difference in the organoleptic quality on overall acceptability based on the consideration of its taste, color and mouthfeel of the raw samples and the sample treated with ohmic heating (p<0.05). The pasteurization effects on fermented red pepper paste of ohmic heating at 60 Hz and 30 V and of conduction heating in hot water were investigated. The samples were heated to 100C using the two methods over 150 s, and then the number of viable cells was counted for different heating times. The results in Fig. 6 indicate that the 13
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pasteurization effect on fermented red pepper paste was better for ohmic heating than
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for conventional conduction heating. Ohmic heating was effective in deactivating
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vegetative cells of Bacillus strains on fermented red pepper paste, as indicated by a 2.5
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log reduction (4.7106 CFU/g decreased to 1.4104 CFU/g), compared with conduction heating at 100C for 8 min producing a 81.9% reduction. This difference was due to the uniform internal heating induced by ohmic heating. Although main effect for
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inactivation microorganisms by ohmic heating is heat itself, additional non-thermal
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effects of electroporation have been reported in which electrical charges can build up and form pores across microbial cells (Yoon, Lee, Kim, & Lee, 2002). The quality of the
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samples pasteurized by ohmic heating was nearly same as that of raw samples, and
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higher than that for conventional heating. The influence of frequency on the pasteurization effects of ohmic heating on
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fermented red pepper paste was determined while maintaining a temperature of 100C in order to exclude temperature effects (Sun et al., 2011; Yoon, Lee, Kim, & Lee, 2002).
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The sterility of ohmic heating was constant within 0.01% as the frequency was varied, indicating that stimulation at different frequencies had no addition effect except for the bactericidal effects associated with changes in temperature (Fig. 7). The mechanism of high frequency sterilization has been identified as microbial inactivation by rapid dielectric heating, the non-thermal sterilizing effect on the electromagnetic field has not been clearly revealed (Curet, Rouaud, & Boillereaux, 2013).
4. Conclusions
The temperature of fermented red pepper paste produced by ohmic heating as 14
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simulated using the proposed electrical model comprising a resistor and capacitor, and
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the approximation equation for the electrical conductivity dependence on frequency and
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temperature were in good agreement with the experimental data (Lebovka, Shynkaryk,
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& Vorobiev, 2006; Shynkaryk, Ji, Alvarez, & Sastry, 2010). The phenomenon of electrolysis that causes problems with food safety was not observed when applying ohmic heating to fermented red pepper paste at different frequencies in this study. The
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pasteurization effect was greater for ohmic heating than for conventional conduction
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heating due to the uniform internal heating produce by low-frequency AC. The effective deactivation of vegetative cells of Bacillus strains was indicated by a 99.7% reduction,
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compared with conduction heating at 100C for 8 min producing a 81.9% reduction.
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The quality of the fermented red pepper paste treated by ohmic heating was nearly the
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same as that of raw samples, and higher than that for conventional heating.
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Fig. 1. Schematic diagram of ohmic heating system of batch type.
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Fig. 2. The variance of electric conductivity and specific heating rate with voltage (at 40 Hz) and frequency (at 20 V) of alternating current during ohmic heating of fermented red pepper paste [square wave, experimental sample: 40 g, 25 (H) 65 (W) 20 (D) mm3]. Data are mean and standard deviation values.
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Fig. 3. Simulated temperature profile of fermented red pepper paste produced by ohmic heating at different frequencies [20 V, square wave, experimental sample: 40 g, 25 (H) 65 (W) 20 (D) mm3]. △, 40 Hz; ▲, 1 kHz; ■, 5 kHz; ○, 10 kHz; □,
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boiling; solid line, simulated temperature; dashed line, boiling. Data are mean and standard deviation values.
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Fig. 4. Simulated temperature profile of fermented red pepper paste produced by ohmic heating at different voltages (60 Hz, sine wave, sample: 40 g, electrode distance: 20 mm). ■, 5 V/cm; □, 10 V/cm; △, 15 V/cm; ●, 20 V/cm; ▲, boiling; solid
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line, simulated temperature; dashed line, boiling. Data are mean and standard deviation values.
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(B)
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(A)
(C)
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Fig. 5. Comparison of physiochemical properties between applying ohmic heating and conventional conduction heating to fermented red pepper paste. Data are mean and standard deviation values.
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Fig. 6. Temporal changes in viable cell counts for fermented red pepper paste during ohmic heating and conventional conduction heating (heating temperature: 100C). ▲,
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△, conventional heating; ■, □, ohmic heating; solid line, viable cell number; dashed
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line, temperature. Data are mean and standard deviation values.
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Fig. 7. Effect of the frequency of ohmic heating on the pasteurization of fermented red
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pepper paste. Data are mean and standard deviation values. ST is not treated samples.
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Table 1. Comparisons of sensory evaluation of fermented red pepper paste with ohmic heating
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and conduction heating. Preference score (5-point hedonic scale)1) Taste
Flavor
Color
Texture
4.0±0.2a 3.9±0.1a 3.6±0.2b
3.9±0.1a 3.8±0.2a 3.4±0.3b
4.0±0.1a 3.9±0.1a 3.6±0.2b
3.9±0.2a 3.8±0.1a 3.5±0.1b
3.8±0.2a 3.8±0.1a 3.5±0.2b
Raw sample Ohmic heating Conduction heating 1)
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Overall acceptability
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Methods
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Values are expressed as mean ± standard deviation (n=10). Values marked above mean ± standard deviation with different letters are significantly different by ANOVA with Duncan’s multiple range test at p<0.05.
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Industrial Relevance
The present study designed and implemented a novel sterilization process based on a
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static ohmic heating system with low-frequency AC at the laboratory scale for
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fermented red pepper paste with a low thermal conductivity (0.458 W/m∙K).
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The developed system was used to investigate the mechanisms and characteristics
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underlying the induction of ohmic heating and then, tested the pasteurization effect
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against microorganisms in fermented red pepper paste.
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Comparing with conventional heating processes, ohmic heating could provide rapid and uniform heating, thereby is more suitable for pasteurization and sterilization of viscous foods as fermented red pepper paste on industrial thermal processing.
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Highlights
Ohmic heating enables very rapid heating of highly viscous paste foods containing red
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pepper with low thermal conductivity.
The specific heating rate was found to be highly dependent on the frequency, peaking at
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5kHz.
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The effective deactivation of vegetable cells of Bacillus strains by ohmic heating was
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indicated by a 99.7% reduction.
The quality of the samples by ohmic heating was higher than that of those heated conventionally.
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S1466-8564(16)30003-0 doi: 10.1016/j.ifset.2016.01.015 INNFOO 1470
To appear in:
Innovative Food Science and Emerging Technologies
Received date: Revised date: Accepted date:
1 November 2015 20 January 2016 23 January 2016
Please cite this article as: Cho, W.-I. & Chung, M.-S., Pasteurization of fermented red Pepper Paste by ohmic heating, Innovative Food Science and Emerging Technologies (2016), doi: 10.1016/j.ifset.2016.01.015
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Original Research Article
Pasteurization of Fermented Red Pepper Paste
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by Ohmic Heating
a
CJ Foods R&D, CJCheiljedang Corp., Seoul 152-050, KOREA
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Department of Food Science and Engineering, Ewha Womans University, Seoul 120-750, KOREA
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b
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Won-Il Cho a, Myong-Soo Chung b,*
Short version of title: Pasteurization of fermented red pepper paste by ohmic heating
Corresponding author at: Department of Food Science and Engineering, Ewha Womans University, 52 Ewhayeodae-gil, Seodaemun-gu, Seoul 120-750, Korea. Tel.: +82 2 3277 4508. Email address: [email protected] (MS. Chung).
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ABSTRACT
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Ohmic heating was applied to a Korean traditional fermented food containing red
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pepper paste, called Gochujang with low thermal conductivity (0.458 w/M·K), by varying frequencies (40-20,000 Hz) and applied voltages (20-60 V). Contrary to
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conduction heating, the entire sample was heated uniformly, and the specific heating rate was found to be highly dependent on the frequency, peaking at 5 kHz and 60 V. The
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results showed that complex differential equation and the Runge-Kutta fourth-order method are suitable for simulating the temperature profile during ohmic heating. The
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effective deactivation of vegetable cells of Bacillus strains on fermented red pepper
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paste by ohmic heating was indicated by a 99.7% reduction, compared with conduction heating for 8 min at 100C producing a 81.9% reduction. The organoleptic and
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physicochemical qualities of the samples pasteurized by ohmic heating were nearly the
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same as those of raw samples, and higher than those of conventionally heated samples.
Keywords: ohmic heating, fermented red pepper paste, electrical model, frequency, pasteurization
1. Introduction
Heat transfer with driving forces based on a temperature gradient is generally used for the heating of foods. The quality deterioration of liquid foodstuffs with low viscosity after high-temperature exposure can be minimized by using high-temperature, short-
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time sterilization (HTST) or ultra-high temperature (UHT) sterilization. However,
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various problems are encountered when applying conduction heating to high-viscosity
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foods and foods comprising solid–liquid mixtures. Gochujang is a Korean traditional
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fermented food in the form of a red pepper paste that can have a high viscosity, making the use of a heat-exchange plate difficult. Moreover, its low heat-conduction coefficient means that heating needs to be applied for a long time in the sterilization process. These
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phenomena can result in overheating of the heating surface, producing a deterioration of
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food quality via the generation of off-flavors and decoloring reactions (Kim & Kwon, 2001; Lim, Kim, Kim, Mok, & Park, 2001; Yoo, 2001).
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Given that the high viscosity of fermented red pepper paste restricts the usefulness of
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HTST and UHT sterilization, sterilization processes involving a tube-type heat exchanger and chemical treatments such as adding ethyl alcohol and sorbic acid have
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been commonly used. However, heating at 70–80C using a tube-type heat exchanger is not sufficiently effective at decreasing microbes, and most of consumers object to the
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addition of preservatives such as sorbic acid reactions (Kim & Kwon, 2001; Lim, Kim, Kim, Mok, & Park, 2001; Yoo, 2001). Fermented red pepper paste is one of the most suitable materials for sterilization by ohmic heating. The electrical conductivity of food means that heat will be generated within its internal electrical resistance when alternating current (AC) is passed through it, thereby representing the conversion of electrical energy into heat energy; this heating method is called ohmic heating (Lee, Lee, Koh, & Lee, 2000; Parrott, 1992; Sastry & Sevugan, 1992). Microwave heating also involves the generation of heat by the conversion of electrical energy into heat inside food, via the vibration of water molecules on dipole rotation and ionic polarization of ions in a food. However, 3
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microwave heating tends to produce nonuniform increases in temperature due to (1) the
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limited penetration depth of the microwave irradiation and (2) the difficulty of ensuring
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irradiation by a uniform electromagnetic field at such high frequencies (especially in
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domestic microwave ovens). In contrast, ohmic heating has no limitations regarding the penetration depth as long as the inherent electrical resistance of the food is not too high. Moreover, liquids and solids on ohmic heating are heated simultaneously without
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requiring a stirring or mixing process of conventional heating (De Alwis & Fryer,
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1990b).
Ohmic heating has been studied in various investigations of its application in
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commercial processes, such as for the sterilization of paste foodstuff with high
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viscosities and solid–liquid mixtures, the cooking and sterilization of seafoods such as surimi, and the thawing of frozen foods. Examples of developed equipment include the
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continuous ohmic heating system of a group in Cambridge and APV Baker in United Kingdom, the Joule heating system in Japan for the sterilization and molding of surimi,
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and the meat emulsion heating system and thawing machine for frozen fish blocks in Russia (De Alwis & Fryer, 1990a; Parrott, 1992; Wang & Sastry, 1993). The present study designed and implemented a novel sterilization process based on a static ohmic heating system with low-frequency AC at the laboratory scale for fermented red pepper paste with a low thermal conductivity (0.458 W/m∙K) (Chang & Chun, 1982). The developed system was used to investigate the mechanisms and characteristics underlying the induction of ohmic heating and then, tested the pasteurization effect against microorganisms in fermented red pepper paste.
2. Materials and methods 4
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2.1. Sample preparation
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Gochujang manufactured by a food processing company (Jinmi Food, Seoul, Korea) was used as the experimental sample and stored in a refrigerator (4C). The sample contained wheat flour, red pepper powder, milled glutinous rice, salt, corn syrup and
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water. Compositions of water, protein, fat, carbohydrate and ash of the sample were
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43%, 8%, 4%, 27% and 18%, respectively, that can affect the heat generation on ohmic heating. The important properties of fermented red pepper paste were organoleptic
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quality based on taste, color and mouthfeel related to viscosity.
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2.2. Experimental apparatus
A self-designed ohmic heating system was used in the experiments. The power
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supply of the ohmic heating apparatus consisted of a function generator to generate sine and square waves from 40 Hz to 20 kHz, and an amplifier that can output 95-volt signals. An automatic multimeter, oscilloscope, and electrical conductance meter were used for the analysis and calculation of data. The heating cell (85 x 45 x 0.5 mm, W x L x T) was constructed from an upward-opening polypropylene box (90 x 90 x 50 mm, W x L x D), and aluminum was used as the electrode material. To ensure safety during the experiments, the heating cell was installed in a Pyrex box. The electrical conductivity of liquid and paste foodstuffs were measured with an electrical conductor meter (CM-2A, Tokyo TOA Electronics, Tokyo, Japan). The current and voltage applied to the food during ohmic heating were measured using two digital 5
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multimeters (3500T, DM 303 TR, HC, Seoul, Korea), and the resistance of the food was
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calculated by applying Ohm’s law to the measured current and voltage values. The
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waveforms produced by the function generator were observed on a two-channel 50-
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MHz oscilloscope (MO-1254 A, Meguro, Tokyo, Japan), including to measure their frequencies. A thermistor with a thermocouple (T type, Shinhan, Seoul, Korea) was
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variability within the sample was 2℃.
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used to measure center temperature on heated sample. The maximum temperature
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2.3. Experimental procedure
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The sample was placed inside the heating cell of the ohmic heating of batch type apparatus, and then its ohmic heating characteristics as electric conductivity and heating
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rate were examined at various frequencies (40-20,000 Hz) and applied voltages (20-60 V). The effects of the internal ohmic heat generation of the frequency and voltage on the
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pasteurization of fermented red pepper paste were investigated. Also, polynomial approximation, complex differential equation and the Runge-Kutta fourth-order method were used to simulate the temperature profile of fermented red pepper paste on various frequencies and voltages during ohmic heating. An experiment was also performed to implement conventional conduction heating, involving measuring the temperatures when the sample packaged in an aluminum box of the same size as the ohmic heating cell was immersed in water at 70–100C. Sensory evaluation and analysis for physiochemical properties were conducted for comparing between products applying ohmic heating and conventional conduction heating.
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2.4. Measurements of pH and acidity
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In order to measure pH of homogeneous fermented red pepper paste, 3 g of the each
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sample was mixed with 30 mL of distilled water and the diluted solution was centrifuged at 1,400g with a centrifuge (1248R, GYROZEN, Seoul, Korea). The pH of the supernatant was measured by a pH meter (420A, Orino, Tokyo, Japan) at room
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temperature.
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The acidity of the sample was analyzed by a quantitative method with lactic acid. A 10-mL aliquot of supernatant of the sample obtained by centrifugation was diluted with
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10 mL of distilled water, and then titration of sample was performed by end-point
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checking as the retention of light redness during 30 s with the addition of 0.5 mL of 1% phenolphthalein and 0.1 N NaOH:
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Lactic acid (%) = [titration volume of 0.1 N NaOH (%) Factor of NaOH 0.009] / (1)
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[weight of sample (g)]
2.5. Color measurement
The color values of samples were measured by a spectrophotometer (UV-120-02, Shimadzu, Tokyo, Japan). A 0.5-g sample and 20 mL of acetone were mixed for 10 min, and then the OD value was measured at 460 nm. The color value of Gochujang was calculated as Color value = [OD value / (weight of sample (mg))] 1000 dilution multiple
2.6. Sensory evaluation 7
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The organoleptic characteristics of the samples were determined by a trained panel
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consisting of 10 students in the Department of Food Engineering, Yonsei University.
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After completing three training sessions related to descriptive profiling, the multiple comparison test was conducted for evaluating sensory attributes such as taste, color, flavor, texture, and overall acceptability of the fermented red pepper paste. All samples
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were the same weight and each was served on a randomly coded plate and water was
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provided to the panelists to cleanse the palate after tasting each sample. The panelists rated the preference of sensory attributes from 1 (extremely bad) to 5 (extremely good)
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for each sample on a 5-point hedonic scale.
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2.7. Microbiological analysis
In order to identify the viability of microorganisms in fermented red pepper paste, 10
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g of each sample was placed in 100 mL of sterile distilled water and pummeled for 3 min at 9 h/s with a Stomacher (HBM-400A, Tianjin Hengao, Tianjin, China). The mixture was serial diluted and spread on plate count agar (PCA, Difco Lab., Detroit, MI, USA) and incubated for 24 h at 37C. After the incubation, the number of viable cells related to total bacterial growth was quantified in CFU/mL; the number of colonies that could be counted was 30–300 per culture plate. A Korean traditional fermented food containing red pepper paste with high viscosity have many kinds of microorganism including heat-resistant spores of Bacillus strains. Therefore, the inactivation effect of the ohmic heaing and boiling against vegetative cell of Bacillus spp. was measured by a standard colony counting on tryptic soy broth agar (TSA Difco Lab., Detroit, MI, USA) 8
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plates.
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2.8. Statistical analysis
All of the data are expressed as mean and standard deviation values from five replicate measurements for treatment conditions. In order to verify significant
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differences (p<0.05) between the samples, the statistical analysis of one-way ANOVA
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procedure and Duncan’s multiple-range test were conducted using Minitab (version
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MTB13, Minitab, Pennsylvania, USA).
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3. Results and discussion
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3.1. Electrical properties of red pepper paste
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The heating rate in conventional conduction heating depends on the heat conductivity of the sample, whereas the most important factor in ohmic heating is the electrical conductivity. The electrical conductivity of the fermented red pepper paste when electric current is not being passed through it was 1.865 S/m. The electrical conductivity varies not only with the food structure and constituents, but also with the heating time and temperature on Fig. 2 (Halden, De Alwis, & Fryer, 1990; Nistor, Stãnciuc, Andronoiu, Mocanu, & Botez, 2015). The specific heating rate (in C/g∙s) increased linearly with voltage at 60 Hz, and increased rapidly with the frequency (when this was higher than 1 kHz) for a constant voltage (Fig. 2). The heating rate in fermented red pepper paste peaked at 5 kHz. 9
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The mechanism of ohmic heating achieved by the application of AC (50–60 Hz, 110–
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220 V) can be clearly explained by an electrical resistor model (Shiby Varghese,
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Pandey, Radhakrishna, & Bawa, 2014;. Zareifard, Ramaswamy, Trigui, & Marcotte,
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2003), but this model is not valid for the heating mechanism when applying lowfrequency AC since it does not accurately reflect how increases in temperature are related to changes of frequency.
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An electrical equivalent circuit comprising a resistor and dielectric can explain the
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heating mechanism of fermented red pepper paste during ohmic heating with lowfrequency AC. The R value corresponds to the resistance for leakage current by food components with properties of an electrical conductor. In raw foods with intact cell
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walls, the plasma membrane and extracellular fluid contain Na2+ and Ca2+ ions that act as electrical conductors. In paste foods such as sauces and jam without an internal
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structure, polarized electrolytes such as salt, amino acids, and proteins act as conductors. In contrast, the r value was the resistance based on delay of polarization of electrolytes.
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The electrolytes in an ideal capacitor can rapidly orientate and polarize to effect a lossless current upon the application of a voltage. However, the food components acting as electrolytes in fermented red pepper paste cannot rapidly orientate on electrical polarization due to their high molecular weight and the unique vibrations induced by ohmic heating at frequencies higher than 5 kHz. The orientation delay of electrolytes is called anomalous dispersion, and generates energy losses that result in the rapid internal heating of foods. The electrical properties of fermented red pepper paste were calculated using the following equations: Pε'' = Wε'' = Cp·m·ΔT/Δt = P·tan(δ) = V·I·tan(δ)
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tan(δ) = 1/(ω·Cr), ω = 2πf, XR = 1/(ω·C)
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ε' = C·d/s, ε'' = ε'/(ω·Cr)
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where P and W are the electrical energy (W), ε'' is the dielectric loss, Cp is the heat
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capacity (J/kg·K), m is mass (kg), ΔT is temperature gradient (K), Δt is time gradient
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(s), tan(δ) is a dielectric dissipation factor, V is voltage (V), I is electric current (A), ω is the angular velocity (ω), C is the capacitance (F), r is internal resistance (Ω), f is the frequency (Hz), XR is the capacitive reactance (Ω), ε' is the dielectric constant, d is
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distance of parallel plate (m), and s is surface of parallel plate (m2). The electrical
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capacitance of fermented red pepper paste decreases gradually as the frequency of the applied stimulation increases, but the electrical conductivity related to the generation of
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heating also gradually increases.
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3.2. Simulation of the temperature induced by ohmic heating
Simulating the temperature profile is very important for optimizing the sterilization
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conditions during ohmic heating (De Alwis & Fryer, 1990a; Lekwauwa & Hayakawa, 1986; Zaror, Pyle, & Molnar, 1993). The present study investigated a model for simulating the temperature changes for various frequencies and voltages. The approximation formula of the temperature dependence of electrical conductivity in fermented red pepper paste was obtained through polynomial approximation based on experimental data. The approximation formula for AC at 60 Hz was K = –1.598 + 0.154·T – 0.043·T2 + 0.000054·T3 + 2.3210–7·T4
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The correlation curve of low-frequency AC at 500 Hz was described by K = –5.760 + 0.722·T – 0.035·T2 + 0.000898·T3 – 0.000012·T4 + 8.78310–8·T5 – 2.50610–10·T6
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The relationship between electrical conductivity and frequency was expressed as K = 0.312 – 0.000015·f + 8.1810–8·f 2 – 4710–12·f 3
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(8)
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This equation indicates the presence of a strong correlation as the correlation coefficient
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higher than 0.95 (p<0.05).
The temperature of foods comprising a mixture of liquid and solid was simulated using the numerical analysis of the following complex differential equation:
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ΔT/Δt = 1/(ρCp)·Δ/Δx·(λxΔT/Δx) + 1/(ρCp)·Δ/Δy·(λyΔT/Δy) +∑Vi2·ki/(ρCp)
(9)
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where ρ is the density (kg/m3), λ is the thermal conductivity (W/m·K), and ki is the individual electrical conductivity (S/m). However, for the ohmic heating of fermented
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red pepper paste, the following simpler equation could be applied to simulate the
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temperature because the heating due to conduction and convection can be considered to be negligible in a homogeneous sample: (10)
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ΔT/Δt = ∑Vi2·ki/(ρCp)
To simulate the temperature profile of fermented red pepper paste during ohmic
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heating, the change in the electrical conductivity dependence on temperature was calculated based on the equations 6 and 7. And then, the electrical conductivity for changes in frequency was calculated using the equation 8. Finally, the increase in temperature with heating time was simulated by the equation 10 and the Runge-Kutta fourth-order method as shown in Figs. 3 and 4. The error value was relatively low, at 2– 4%, indicating the validity of the proposed simulation model.
3.3. Pasteurization effect by ohmic heating
To ensure both safety and quality it is necessary to check for the generation of new 12
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electrolytes during ohmic heating. In general, direct-current stimulation by an electric
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field will induce a large amount of electrolysis that will effect a change in the pH,
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whereas AC stimulation could result in a lower degree of electrolysis due to the
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associated rapid changes in the electric field (Cha, 2014; Yang, Han, Lee, Park, & ., Kim, 2014). The phenomenon of electrolysis hardly ever happened when applying ohmic heating to fermented red pepper paste at various frequencies in the present study,
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as indicated by the pH of the sample treated by ohmic heating being very similar to that
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of the raw sample (Fig. 5A).
The analysis data for the properties of fermented red pepper paste with conventional
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conduction heating at 80C for 7 min and with ohmic heating at 40 Hz to 20 kHz, 20–60
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V, and 90–100C for 2 min are presented in Fig. 5B–D. From result of this study, it is interesting to note the dramatic improvement in sensory quality between ohmic heating
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and conduction heating. The sensory qualities such as taste, flavor, texture and color on sensory evaluation were better for ohmic heating than for conduction heating despite the
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higher temperature used during the former. The results in Table 1 indicate that the scores for each organoleptic characteristics were higher about 0.3-0.5 points with significant different level for ohmic heating (p<0.05). However, there was no significant difference in the organoleptic quality on overall acceptability based on the consideration of its taste, color and mouthfeel of the raw samples and the sample treated with ohmic heating (p<0.05). The pasteurization effects on fermented red pepper paste of ohmic heating at 60 Hz and 30 V and of conduction heating in hot water were investigated. The samples were heated to 100C using the two methods over 150 s, and then the number of viable cells was counted for different heating times. The results in Fig. 6 indicate that the 13
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pasteurization effect on fermented red pepper paste was better for ohmic heating than
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for conventional conduction heating. Ohmic heating was effective in deactivating
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vegetative cells of Bacillus strains on fermented red pepper paste, as indicated by a 2.5
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log reduction (4.7106 CFU/g decreased to 1.4104 CFU/g), compared with conduction heating at 100C for 8 min producing a 81.9% reduction. This difference was due to the uniform internal heating induced by ohmic heating. Although main effect for
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inactivation microorganisms by ohmic heating is heat itself, additional non-thermal
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effects of electroporation have been reported in which electrical charges can build up and form pores across microbial cells (Yoon, Lee, Kim, & Lee, 2002). The quality of the
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samples pasteurized by ohmic heating was nearly same as that of raw samples, and
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higher than that for conventional heating. The influence of frequency on the pasteurization effects of ohmic heating on
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fermented red pepper paste was determined while maintaining a temperature of 100C in order to exclude temperature effects (Sun et al., 2011; Yoon, Lee, Kim, & Lee, 2002).
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The sterility of ohmic heating was constant within 0.01% as the frequency was varied, indicating that stimulation at different frequencies had no addition effect except for the bactericidal effects associated with changes in temperature (Fig. 7). The mechanism of high frequency sterilization has been identified as microbial inactivation by rapid dielectric heating, the non-thermal sterilizing effect on the electromagnetic field has not been clearly revealed (Curet, Rouaud, & Boillereaux, 2013).
4. Conclusions
The temperature of fermented red pepper paste produced by ohmic heating as 14
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simulated using the proposed electrical model comprising a resistor and capacitor, and
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the approximation equation for the electrical conductivity dependence on frequency and
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temperature were in good agreement with the experimental data (Lebovka, Shynkaryk,
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& Vorobiev, 2006; Shynkaryk, Ji, Alvarez, & Sastry, 2010). The phenomenon of electrolysis that causes problems with food safety was not observed when applying ohmic heating to fermented red pepper paste at different frequencies in this study. The
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pasteurization effect was greater for ohmic heating than for conventional conduction
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heating due to the uniform internal heating produce by low-frequency AC. The effective deactivation of vegetative cells of Bacillus strains was indicated by a 99.7% reduction,
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compared with conduction heating at 100C for 8 min producing a 81.9% reduction.
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The quality of the fermented red pepper paste treated by ohmic heating was nearly the
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same as that of raw samples, and higher than that for conventional heating.
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Sastry, S. K., & Sevugan, P. (1992). Ohmic heating of liquid-particles mixtures. Food
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Technology, 46(12), 64–67.
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Sun, H., Masuda, F., Kawamura, S., Himoto, J. I., Asano, K., & Kimura, T. (2011). Effect of electric current of ohmic heating on nonthermal injury to Streptococcus
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thermophilus in milk. Journal of Food Process Engineering, 34(3), 878–892. Wang, W. C., & Sastry, S. K. (1993). Salt diffusion into vegetable tissue as a
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pretreatment for ohmic heating: Electrical conductivity profiles and vacuum infusion studies. Journal of Food Engineering, 20(4), 299–309. Yang, J. W., Han, D. S., Lee, C. H., Park, S. J., & Kim, Y. E. (2014). Evaluation on the quality of fresh, conventionally heated and ohmically heated mulberry fruit juice. Journal of the East Asian Society of Dietary Life, 24(1), 80–91. Yoo, B. (2001). Rheological properties of hot pepper-soybean paste. Journal of Texture Studies, 32(4), 307–318. Yoon, S. W., Lee, C. Y. J., Kim, K. M., & Lee, C. H. (2002). Leakage of cellular materials from Saccharomyces cerevisiae by ohmic heating. Journal of Microbiology and Biotechnology, 12(2), 183–188. 17
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Zareifard, M. R., Ramaswamy, H. S., Trigui, M., & Marcotte, M. (2003). Ohmic heating
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Science and Emerging Technologies, 4(1), 45–55.
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behaviour and electrical conductivity of two-phase food systems. Innovative Food
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Zaror, C. A., Pyle, D. L., & Molnar, G. (1993). Mathematical modeling of an ohmic
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heating sterilizer. Journal of Food Engineering, 19(1), 33–53.
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Fig. 1. Schematic diagram of ohmic heating system of batch type.
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Fig. 2. The variance of electric conductivity and specific heating rate with voltage (at 40 Hz) and frequency (at 20 V) of alternating current during ohmic heating of fermented red pepper paste [square wave, experimental sample: 40 g, 25 (H) 65 (W) 20 (D) mm3]. Data are mean and standard deviation values.
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Fig. 3. Simulated temperature profile of fermented red pepper paste produced by ohmic heating at different frequencies [20 V, square wave, experimental sample: 40 g, 25 (H) 65 (W) 20 (D) mm3]. △, 40 Hz; ▲, 1 kHz; ■, 5 kHz; ○, 10 kHz; □,
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boiling; solid line, simulated temperature; dashed line, boiling. Data are mean and standard deviation values.
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Fig. 4. Simulated temperature profile of fermented red pepper paste produced by ohmic heating at different voltages (60 Hz, sine wave, sample: 40 g, electrode distance: 20 mm). ■, 5 V/cm; □, 10 V/cm; △, 15 V/cm; ●, 20 V/cm; ▲, boiling; solid
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line, simulated temperature; dashed line, boiling. Data are mean and standard deviation values.
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Fig. 5. Comparison of physiochemical properties between applying ohmic heating and conventional conduction heating to fermented red pepper paste. Data are mean and standard deviation values.
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Fig. 6. Temporal changes in viable cell counts for fermented red pepper paste during ohmic heating and conventional conduction heating (heating temperature: 100C). ▲,
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△, conventional heating; ■, □, ohmic heating; solid line, viable cell number; dashed
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line, temperature. Data are mean and standard deviation values.
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Fig. 7. Effect of the frequency of ohmic heating on the pasteurization of fermented red
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pepper paste. Data are mean and standard deviation values. ST is not treated samples.
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Table 1. Comparisons of sensory evaluation of fermented red pepper paste with ohmic heating
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and conduction heating. Preference score (5-point hedonic scale)1) Taste
Flavor
Color
Texture
4.0±0.2a 3.9±0.1a 3.6±0.2b
3.9±0.1a 3.8±0.2a 3.4±0.3b
4.0±0.1a 3.9±0.1a 3.6±0.2b
3.9±0.2a 3.8±0.1a 3.5±0.1b
3.8±0.2a 3.8±0.1a 3.5±0.2b
Raw sample Ohmic heating Conduction heating 1)
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Overall acceptability
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Methods
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Values are expressed as mean ± standard deviation (n=10). Values marked above mean ± standard deviation with different letters are significantly different by ANOVA with Duncan’s multiple range test at p<0.05.
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Industrial Relevance
The present study designed and implemented a novel sterilization process based on a
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static ohmic heating system with low-frequency AC at the laboratory scale for
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fermented red pepper paste with a low thermal conductivity (0.458 W/m∙K).
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The developed system was used to investigate the mechanisms and characteristics
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underlying the induction of ohmic heating and then, tested the pasteurization effect
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against microorganisms in fermented red pepper paste.
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Comparing with conventional heating processes, ohmic heating could provide rapid and uniform heating, thereby is more suitable for pasteurization and sterilization of viscous foods as fermented red pepper paste on industrial thermal processing.
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Highlights
Ohmic heating enables very rapid heating of highly viscous paste foods containing red
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pepper with low thermal conductivity.
The specific heating rate was found to be highly dependent on the frequency, peaking at
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5kHz.
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The effective deactivation of vegetable cells of Bacillus strains by ohmic heating was
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indicated by a 99.7% reduction.
The quality of the samples by ohmic heating was higher than that of those heated conventionally.
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