Continuous ohmic heating system for the pasteurization of fermented red pepper paste

Accepted Manuscript Continuous ohmic heating system for the pasteurization of fermented red pepper paste

Won-Il Cho, Eun-Jung Kim, Hee-Jeong Hwang, Yun-Hwan Cha, Hee Soon Cheon, Jun-Bong Choi, Myong-Soo Chung PII: DOI: Reference:

S1466-8564(16)30702-0 doi: 10.1016/j.ifset.2017.07.020 INNFOO 1803

To appear in:

Innovative Food Science and Emerging Technologies

Received date: Revised date: Accepted date:

16 November 2016 8 July 2017 8 July 2017

Please cite this article as: Won-Il Cho, Eun-Jung Kim, Hee-Jeong Hwang, Yun-Hwan Cha, Hee Soon Cheon, Jun-Bong Choi, Myong-Soo Chung , Continuous ohmic heating system for the pasteurization of fermented red pepper paste, Innovative Food Science and Emerging Technologies (2017), doi: 10.1016/j.ifset.2017.07.020

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Original Research Article

Continuous Ohmic Heating System for the Pasteurization of

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Fermented Red Pepper Paste

Won-Il Cho a, Eun-Jung Kim b, Hee-Jeong Hwang b, Yun-Hwan Cha c,

Dept. of Food Science and Engineering, Ewha Womans University, Seoul 03766, Korea c

Dept. of Food and Nutrition, SoongEui Women’s College, Seoul 04628, Korea

Graduate School of Hotel & Tourism, The University of Suwon, Gyeonggi-do 18323, Korea

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e

R&D center, Seoul Perfumery Corporation, Seoul 06533, Korea

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CJ Foods R&D, CJCheiljedang Corp.,Gyeonggi-do 16495, Korea

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a

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Hee Soon Cheon d, Jun-Bong Choi e, Myong-Soo Chung b,*

Short version of title: Continuous ohmic heating for pasteurization of fermented red

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pepper paste

Corresponding author at: Department of Food Science and Engineering, Ewha Womans University, 52 Ewhayeodae-gil, Seodaemun-gu, Seoul 03766, Korea. Tel.: +82 2 3277 4508. Email address: [email protected] (MS. Chung).

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ABSTRACT

Ohmic heating (OH) is a processing in which heat is generated inside foods by the passage of

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electric current. This study aimed to determine the optimal conditions for pasteurizing the Gochujang Korean-style fermented food containing hot pepper paste using a continuous OH

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system. The effective conditions for OH involved three pairs of electrodes connected diagonally

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and excitation over the voltage range of 50–100 V. The selected conditions provided heating above 100°C without scale buildup. A microbial inactivation with a 2 log reduction was achieved

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by the OH, with the processing time being shorter than for conventional heating (CH). OH was

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available for 57 cycles during 1 cycle of CH, and it could process 104.4 kilotonnes of product. The consistency index related to thickening properties was markedly higher for OH than for CH.

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This study has confirmed that continuous OH is suitable for the heat-based sterilization of highly

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viscous foods.

Keywords: continuous ohmic heating system; highly viscous foods; red pepper paste;

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Gochujang; pasteurization

1. Introduction The heat-based sterilization of viscous foods by conventional methods using steam or boiling water can lead to undesirable deteriorations in quality, including changes in color, destruction of nutrients, and decreased flavor. Such deteriorations are associated with the long time required to 2

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increase the temperature at the coldest point, which is normally the center of the largest particle, since this may result in overheating of the remaining particles and the surrounding liquid. Ohmic heating (OH), which is also referred to as Joule heating, electrical resistance heating, and electroconductive heating, is an innovative heating method used in the food industry for

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processing a broad range of food products (Samaramyake, Sastry, & Zhang, 2005; Sastry, 2008). OH of food products involves passing alternating current through them, which generates internal

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heat as a result of their intrinsic electrical resistance. The amount of heat released is proportional

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to the square of the current, which is known as Joule’s first law (Sarang, Sastry, & Knipe, 2008).

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Systems for implementing OH usually involve electrodes being placed inside the food, with electricity passed through it using various voltage and current combinations, and the food being

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heated by the resulting dissipation of electrical energy. Most foods contain ionic components such as salts and acids that allow the food to conduct electric current and thereby rapidly and

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evenly generate ohmic heat (Sarang, Sastry, & Knipe, 2008).

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In contrast to conventional heating (CH), where the heat of a hot surface is conducted from the outside of the food to its inside, OH induces heat within the entire mass of the food uniformly

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(Leizerson & Shimoni, 2005; Sarang, Sastry, & Knipe, 2008). The success of OH depends on the rate of heat generation of the system, the electrical conductivity of the food, and the method by

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which the food flows through the system (Leizerson & Shimoni, 2005a). OH has several advantages over CH, one of which is providing uniform heating without a temperature gradient, with the absence of a cold spots making it possible to rapidly heat an entire sample. OH makes it possible to heat both the solid particles and the liquid in a two-phase food material extremely rapidly and uniformly, resulting in less thermal damage than when using CH, which inherently relies on heat transfer (Icier & Bozkurt, 2011). In addition, the absence of a hot 3

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surface in OH reduces fouling problems and thermal damage to the product, and results in the rapid production of a high-quality product with minimal structural, nutritional, or organoleptic changes (Leizerson & Shimoni, 2005b). These advantages mean that OH is considered suitable for high-viscosity foods and multiphase

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foods (Samaramyake, Sastry, & Zhang, 2005; Sastry, 2008). Gochujang is a Korean traditional fermented food containing red pepper paste with high viscosity that may support many

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microorganisms including heat-resistant spores of Bacillus strains. The application of an

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effective sterilization process is therefore necessary for the commercial production of safe

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processed Gochujang. Various studies have investigated methods for extending the storage period of Gochujang, including by adding garlic, alcohol, chitosan, K-sorbate, and mustard to

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the matured paste, and pasteurization, retorting, and OH (Kim & Kwon, 2001). It was found out that the heat treatment conditions that produced adequate microbial inactivation when

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Gochujang was placed inside a 15-mm thick retort pouch were 85, 45, and 35 min at 110°C,

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115°C, and 120°C, respectively. And the extra-high-pressure treatment (a nonthermal treatment method) at 680 MPa and 49–73°C for 30 min resulted in 0–3 log reductions of microbes

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Gochujang (Lim, Kim, Kim, Mok, & Park, 2001). The heat sterilization of highly viscous paste foods such as Gochujang is difficult using CH

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due to their low thermal conductivity and the resulting long heating time required to deliver heat to the center of a food sample. The excessive external heating caused the destruction of nutritional components and resulted in a burnt smell, discoloration, and tissue degradation (Cho, Kim, Kim, & Pyun, 1994). OH has been recognized as a promising alternative to traditional food thermal processing particularly due to the shorter treatment time, which is critical for maintaining the quality of high-viscosity food as Gochujang (Kim, 4

Kim, Park, Cho, & Han,

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1996; Shynkaryk, & Sastry, 2012). The objectives of this study were (1) to determine the pasteurization conditions of Gochujang utilizing a continuous OH system and (2) to characterize the rheological properties, color values,

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and microbial inactivation that can be achieved by OH.

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2. Materials and methods

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2.1. Sample preparation

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Gochujang that had been manufactured by the Sunchang Food Company (Sunchang-gun, Jeollabuk-do, Korea) was obtained from a local market. This type of Gochujang contained red pepper powder, chopped garlic and onion, starch syrup, gelatinized rice flours, salt, water, and L-

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monosodium glutamate. All samples were stored at room temperature in a sealed state before being subjected to OH processing.

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2.2. Continuous ohmic heating system

The continuous ohmic heating system consisted of a monoplunger pump, a product tank, a power supply with voltage generator (DLC-10K300, Daelimec Co., Paju, Gyeonggi-Do, Korea), a heating cell, a data logger, and a computer (Fig. 1). To implement a continuous system, the monoplunger pump (KRH 20-1, Kraft Precision Industry Co., Seoul, Korea) was used to move the sample from the product tank to the collection container. The monoplunger pump operates at 5

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a pressure of 6 kgf/cm2 with a correspondingly high head and it is suitable for viscous liquids and paste foods. The products are conveyed by a screw inside the stator and driven into the outlet pipe by a helical rotor. After the sample was in the heating cell, the heating process was started by turning on the power supplier (capacity: 30 kVA) to the voltage generator connected to the

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electrodes attached to the heating cell. The supplied power was a sine-wave alternating current at 60 Hz, and the voltage was controlled by a variable transformer over a range of 0–300 V. The

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sample was moving while being heated inside the cell installed with several carbon electrodes,

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and a data logger (Hydra 2625A, Fluke, Everett, WA, USA) linked to a computer recorded

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essential data such as the voltage, current, and temperature during the processing. A thermocouple (T type, Shinhan, Seoul, Korea) inside the heating cell was used to measure the

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temperature of the heated sample. In order to observe effects of each electrode, thermocouples were located at inlet, outlet, and in-between of the electrodes as shown in Fig. 1(E). The

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temperature was measured continuously and recorded at 5 sec intervals by data logger linked to a

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computer.

The heating cell with electrodes is one of the important part of the ohmic heater because it is

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an electrical source for heating. The cell was made with polyether ether ketone (PEEK) as it is thermoplastic to resist for impact and abrasion, and has high mechanical strength over a wide

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temperature range. As shown in Fig.1, 2 rectangular-shaped short cells (cell size: 300 × 100 × 100 mm, W × L × H) with 2 electrodes (electrode size: 150 × 30 × 10 mm) and 1 rectangularshaped long cell (cell size: 600 × 100 × 100 mm) with 4 electrodes (electrode size: 150 × 30 × 10 mm) that can be attached or detached were used for this study. Electrodes made with carbon have excellent electrical conductivity and resistant against acid and base component.

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2.3. Experimental design for ohmic heating

Several OH conditions were selected by adjusting the following factors in a statistical factorial design: heating-cell combination, electrode length, voltage change, and the speed of flow. First,

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two types of heating cell were used: two short cells containing a pair of electrodes, and one long

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cell containing two pairs of electrodes. This provided the following cell combinations: short cell, short–short cell, short–long cell, short–short–long cell, and long cell. Second, the effect of

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electrode length was investigated in the long cell. This cell had two pairs of electrodes, which

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were connected either diagonally (/ type) or in a straight line (I type). Third, voltages of 30, 50, 70, and 100 V were applied. The temperature history of the heating system was measured for

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each of the cell combination. The input energy was calculated using the following equation:

(1)

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Energy (J) = Voltage (V) × Current (A) × Heating time (sec)

The obtained data were used to select the conditions that increased the temperature to over

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100°C with the lowest energy consumption. Selected conditions for treatments of primary ohmic heating as cell combinations, voltage, heating time and temperature, and heating rate were shown

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in detail in Table 1. Based on preliminary studies on stability of sample flow in our continuous ohmic heating system, pump flow rate was fixed as 1.3 cm/sec by controlling pump speed. As shown in Table 1, the heating time for each condition was calculated by considering total lengths (60 cm for OH3, OH5 and OH7, and 45 cm for OH9) of sample passing the electrode as following equation:

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Heating time (sec) = length of electrode (cm) / flow rate (cm/sec)

(2)

2.4. Conventional heating method

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The CH experiments applied the commonly used industrial method of batch pasteurization of

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Gochujang using CH in contrast to OH. For CH treatment, 700 mL of Gochujang was placed in a 1-L beaker and then subjected to the thermal treatment via conduction and forced convection

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heating using a circulating water bath (VS-1991W PID controller, Vision Science, Daejeon,

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Korea) with agitation at 60 rpm in a laboratory-scale experiment. Gochujang with a low thermal conductivity (0.458 W/m∙K) was heated at 100°C for 91 min, which produced a heating rate of

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0.013°C/sec (Chang, & Chun, 1982).

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2.5. Microbial inactivation

The microbiological stability of each condition of the continuous OH and CH treatments was

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assessed based on the total bacterial growth. Each treated sample (10 g) was collected in a sterile Stomacher bag with distilled water and pummeled for 1 min at 6 strokes/sec with a Stomacher

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bag (HBM-400A, Tianjin Hengao Technology Development Co., Tianjin, China). Serial dilutions were then performed using distilled water, and the produced diluted samples were spread on Plate Count Agar (PCA; Difco Laboratories, Detroit, MI, USA). The PCA plates were incubated at 37°C for 24 hr, and then the number of viable cells was calculated in CFU/mL on plates with 30–300 countable colonies.

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2.6. Viscosity measurement

The viscosity of each sample was measured in a rotational viscometer (Brookfield Viscometer LVDH-II+Pro, Brookfield Engineering, Middleborough, MA, USA). To ensure temperature

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control, the sample was kept in a water bath at 25±1°C. All samples were tested at 0.1–1.0 rpm

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with a No. 64 spindle. The obtained data were used to calculate the K (consistency index) and n (flow behavior index) values to allow comparisons with the viscosity values. First, the shear rate

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and shear stress were calculated with the basic equations for a rotational viscometer [Equations

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(3) and (4)], and then those values were substituted into power-law Equation (5). Equation (6) was made by linearizing Equation (5). The data from these equations were used to construct a

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trend line and calculate the K and n values:

(3) (4)

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(5)

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(6)

where γ is the shear rate (1/sec), τ is the shear stress (Pa), r1is the spindle radius (cm), r2 is the container radius (cm), ω is the angular velocity of the spindle (rad/sec) = (2π/60)∙N (where N is in rpm), M is the torque input by the instrument (dyne/cm) = 673.7 dyne/cm∙torque (%), L is the effective length of the spindle (cm), K is the consistency index (Pa∙secn), and n is the flow behavior index. 9

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2.7. Color measurement

The color values of Gochujang were measured as the L value (brightness), a value [(+) redness

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/ (–) greenness], and b value [(+) yellowness / (–) blueness] according to the Hunter Lab Color

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Space using a spectrophotometer (Color Quest XE, Hunter Associates Laboratory, Reston, VA, USA). The color difference (ΔE) between continuous OH- and CH-treated Gochujang was

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quantified using the following equation:

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∆E = [(L1 – L0)2 + (a1 – a0)2 + (b1 – b0)2]1/2

where L1, a1, and b1 are the color parameters for treated Gochujang, and L0, a0, and b0 are those

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for raw Gochujang.

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2.8. Statistical analysis

The statistical analyses were performed using the SAS/STAT statistical software (Statistical

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Analysis System Ver. 9.3, SAS Institute, Cary, North Carolina, USA). The results are expressed as mean and standard deviation values (n=3) for each treatment condition. Differences were analyzed with a cutoff for significance of p<0.05 in the one-way ANOVA procedure and Duncan’s multiple-range test.

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3. Results and discussion

3.1. Determination of ohmic heating conditions

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This study selected 9 conditions that combined the number of heating cells, the distance

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between electrodes, the voltage variable, and the speed of flow that did not result in any scale

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buildup, and with a pump speed of 2.5 Hz, and calculated the input energy (Fig. 2). In deriving the above results, the 120 examination trials of ohmic heating were conducted by preliminary

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adjusting the following factors in a 3³ factorial design with 4 replications (108 trials): heatingcell combination, electrode length, and voltage change, and a 22 factorial design with 3

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replications (12 trials): heating-cell combination and the speed of flow. As shown in Fig. 2 and Table 1, the temperature range of 100–105°C was not large, but it was confirmed that there was

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maximum difference of about 3.5-fold in the OH input energy: from 38.5 kJ for cell combination

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7 (OH7) to 132.7 kJ for OH8. The total input energies for OH3, OH5, OH7, and OH9 were 45.0, 51.3, 38.5, and 47.6 kJ, respectively.

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The heating rates for OH3, OH5, OH7 and OH9 above 85°C decreased in proportion to the reduction of the electrical conductivity compared to under 85°C (Table 1). It has been reported

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that the heating rate in ohmic heating process is directly proportional to the square of the electric field strength and the electrical conductivity related to migration of ion (Sastry, & Palaniappan, 1992; Castro, Teixeira, Salengke, Sastry, & Vicente, 2003; Tulsiyan, Sarang, & Sastry, 2008). In this study, the electrical conductivity of Gochujang seemed to be affected by changes in physicochemical properties due to the gelatinization of some starch materials and the change in moisture content above 85°C. It could be considered that the change of the moisture content 11

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affects to the migration of the polar ions related to the heat generation in ohmic heating process and provides significant effect to the heating rate. Since the ohmic heater used in the experiment was not in a sealed condition in which pressurization could occur, the final temperature of heating could not increase by more than 100-

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105℃. Therefore, the final heating temperature was similar for all experimental conditions, even

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if the voltage conditions were different on OH3, OH5 and OH7 with cell combinations as S-SL(/) (Table 1).

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Effective heating was achieved with less scale buildup of Gochujang for the I type than for the

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/ type. Moreover, since the I type had a smaller effective area than the / type, the I type that involved a current of more than 20 A resulted in scale buildup of Gochujang around the electrode.

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Also, conditions involving the use of three pairs of electrodes in a step-by-step manner demonstrated favorable heating characteristics. Therefore, the study selected OH3, OH5, OH7,

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scale buildup of Gochujang.

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and OH9 as conditions with large temperature increases relative to the input energy and without

The heating time for OH3, OH5, and OH7 with the same number of electrodes and distance

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between them was 46.2 sec, and it was confirmed that 68.9 kilotonnes of food product could be treated per hour, based on y = 8.529x – 2.203 (R2 = 0.977). Meanwhile, the OH9 condition with

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different electrode distances from the three conditions above showed a shorter heating time of 34.6 sec and could treat 60.9 tonnes of Gochujang per hour, based on y = 7.851x – 2.712 (R2 = 0.999). The four conditions of OH3, OH5, OH7, and OH9 were therefore selected as the optimal OH processing conditions based on the results of this study.

3.2. Microbial inactivation 12

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Gochujang was treated under the OH3, OH5, OH7, and OH9 conditions, and the total counts of Gochujang for CH without and with the water bath are shown in Fig. 3. The total count of Gochujang for each condition of OH treatment decreased by 0.7–1.5 log: 1.5 log, 1.0 log, 1.0 log,

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and 0.7 log for OH3, OH5, OH7, and OH9, respectively. The microbe inactivation mechanism of OH mainly involves heating, as for general methods of heat sterilization, but it has been reported

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that electroporation of microbial cells by the electric current due to the buildup of electrical

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charges and the formation of pores across the cells can also be involved under some conditions

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(Cho, Yousef, & Sastry,.1999; Yoon, Lee, Kim, & Lee, 2002).

In conclusion, 91 min of CH treatment was required to reach 100°C, while for the four optimal

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OH treatment conditions this temperature was achieved in 28.35 and 46.21 sec (Table 1). Moreover, despite the long CH process, the inactivation effect was lower than for OH treatment.

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CH treatment produced an inactivation with a 0.65 log reduction. This difference was mainly due

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to the uniform heating achieved by internal OH, with some additional nonthermal effects of electroporation. OH produced effective inactivation within a shorter time than when using CH,

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which means that OH is a superior heating process in terms of energy efficiency.

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3.3. Rheological properties

Viscosity is one of the most important factors influencing the heating of highly viscous fluids. The relation between sheer stress (σ) and shear rate showed non-Newtonian features with yield stress occurring under all of the conditions (Fig. 4). The flow behavior index (n)—which shows the trend for shear thinning—was less than 1, and was higher for CH and OH than for raw 13

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samples, although the difference was not statistically significant (Table 2A). Therefore, the Herschel-Bulkley model formula related to non-Newtonian fluids was applied, which revealed the trend of a pseudoplastic fluid (shear thinning) with yield stress occurring irrespective of the viscosity value. The observed shear-thinning movement was attributed to particle aggregation

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from particle interactions, since Gochujang consists of solid particles dispersed in a continuous liquid. In particular, the shear-thinning movement appears in Gochujang due to it containing

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glutinous rice flour with large molecular weights, and it is known that the flour plays a very

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important role in the stability of the network structure of the paste during storage (Lee, Hwang,

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Choi, & Lee, 2003).

As indicated in Table 2A, the consistency index (K) increased regardless of the heat treatment

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method compared to the raw product, but sharply increased during OH treatment compared to CH. The K increase affected the hydration capacity and the thickening properties (Gujral,

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3.4. Color values

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peaking for the OH3 condition.

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Sharma, & Singh, 2002). In the present study it was found out that K was high for OH treatment,

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The color values differed significantly with the treatment conditions, with L, a, and b decreasing under all conditions (Table 2B). First, the effect of L value, which refers to lightness, was decreased by heat treatment to 0.39–2.83, compared with the value for the raw sample of 32.77. Both CH and OH resulted in darkening, but the CH-treated water bath produced the lowest value. Second, the a and b values were also lowest for the CH-treated water bath. The ΔE value for CH was higher than for all of the OH conditions, with the ΔE values decreasing in the 14

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following order: CH, OH5, OH7, OH3, and OH9. CH resulted in huge changes in the color values compared with OH under any conditions such as treatment temperature or time. That is, CH treatment took longer than OH to reach the desired temperature due to the slower heating rate, which resulted in ΔE being highest for CH treatment.

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The second highest ΔE value was for the OH5 condition, which had the same treatment time as OH3 but produced the highest final treatment temperature of all of the OH conditions. OH9

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resulted in the lowest value for ΔE, and achieved the same temperature as the final treatment

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with OH3 and OH7, but required a shorter time to reach that temperature. The ΔE values were

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influenced more by the treatment temperature than by treatment time for OH treatment, whereas CH was influenced more by treatment time than by the treatment temperature. In 1999, Shie and

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Park similarly reported that higher processing temperatures and longer heating times could result in deterioration of the color quality of surimi seafood, because the whiteness of this product

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decreased during heat treatment and white meat is generally perceived as being of higher quality

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(Shie, & Park, 1999).

It is considered that a major cause for the decline in color values is the starch present in

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Gochujang being broken down into dextrin and reducing sugars by long-duration heat treatment, and that these products react with basic amino acids via the Maillard reaction ( Leizerson, &

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Shimoni, 2005a, 2005b).

4. Conclusions

In conclusion, the most effective conditions for OH of Gochujang involved three pairs of electrodes, since they resulted in a larger effective treatment area, and a voltage of 50–100 V. 15

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When Gochujang was heated in a step-by-step manner using three pairs of electrodes and a current of less than 20 A, there was no scale buildup. Gochujang scale appeared above 100 V, while the heating rate was slow below 50 V. The rapid and uniform OH produced a microbial inactivation effect of a 2 log reduction despite the shorter processing time than for batch-type CH.

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In future complementary experiments we will investigate the pasteurization effects of continuous-type CH by installing a steam pipe on the outer side of the tube in a continuous

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ohmic heater.

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(E)

Fig. 1. Schematic (A) and flow (B) diagrams of the continuous ohmic heating (OH) system, dimensions of the short (C) and long (D) heating cell, and thermocouple location (E) ((B) ①

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Input ② Monoplunger pump ③,④,⑤ Cell heater ⑥ Output).

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Fig. 2. Comparison of input energies for various OH conditions. Cell combinations refer to eight small–small–large diagonal types [(1) 30-70-150 V, (2) 30-30-150 V, (3) 50-70-100 V, (4) 30-30-200 V, (5) 70-70-100 V, (6) 30-30-250 V, (7) 50-50-100 V, and (8) 30-30-300 V]

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and one small–large straight type [(9) 70-70-70 V]. All data are mean and standard deviation

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values for triplicate measurements. Bars labeled with different letters are significantly different by ANOVA with Duncan’s multiple-range test at p<0.05.

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D

Fig. 3. Comparison of the microbial inactivation effects for various OH conditions. Cell

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combinations refer to three small–small–large diagonal types [(3) 50-70-100 V, (5) 70-70-100 V, and (7) 50-50-100 V], one small–large straight type [(9) 70-70-70 V], and conventional

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heating (CH). All data are mean and standard deviation values for triplicate measurements. Bars labeled with different letters are significantly different by ANOVA with Duncan’s

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multiple-range test at p<0.05.

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(A)

(B)

Fig. 4. Plot of shear stress vs. shear rate for various OH treatment conditions [(A) ■ : raw, ▲ : CH, ● : OH3, (B) ■ : OH3, ▲ : OH5, ● : OH7, ◆ : OH9]. All data are mean and standard deviation values for triplicate measurements. 22

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Table 1. The conditions for ohmic heating treatments as cell combinations, voltage, heating time and temperature, and heating rate.

Cell Combinations*

OH1

S-S-L(/)

30-70-150

46.2

OH2

S-S-L(/)

30-30-150

46.2

OH3

S-S-L(/)

50-70-100

46.2

OH4

S-S-L(/)

30-30-200

46.2

OH5

S-S-L(/)

70-70-100

OH6

S-S-L(/)

30-30-250

OH7

S-S-L(/)

50-50-100

OH8

S-S-L(/)

OH9

S-L(I)

under

above

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85℃

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0.49

1.30

0.58

104

1.27

0.59

105

1.98

0.38

46.2

105

1.49

0.39

46.2

105

2.76

0.27

46.2

104

1.10

0.77

30-30-300

46.2

104

3.73

0.20

70-70-70

34.6

104

1.47

0.90

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103

Heating rate (℃/sec)

1.52

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S (short cell), L (long cell), / type: diagonally line, I type: straight line

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Voltage (V)

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Max. Heating Temperature time (sec) (℃)

Ohmic heating conditions

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Table 2. (A) Consistency index (K), flow behavior index (n), and R2 values. None of the means are significantly different (p<0.05). (B) Color values for each condition. All data are mean±standard deviation values for triplicate measurements. CH, conventional heating; OH,

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ohmic heating.

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(A) K, n, and R2 values

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a 21.81±0.06 a 18.46±0.05 b 20.84±0.06 a 19.81±0.13 c 20.20±0.14 c 21.15±0.01 a

0.7310±0.09 a 0.7865±0.05 a 0.8385±0.01 a 0.8193±0.02 a 0.8429±0.01 a 0.8385±0.01 a

0.9822 0.9592 0.9743 0.9766 0.9792 0.9592

b 13.66±0.04 a 11.43±0.03 b 12.71±0.02 c 11.92±0.15 c 12.13±0.08 c 12.92±0.06 a

ΔE 4.92±0.01 a 1.48±0.01 b 3.01±0.09 c 2.43±0.10 c 1.07±0.06 b

Values labeled with different letters are significantly different by ANOVA with Duncan’s multiple-range test at p<0.05.

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Raw CH OH3 OH5 OH7 OH9

L 32.77±0.02 a 29.94±0.01 b 32.18±0.02 a 31.35±0.09 a 31.78±0.08 a 32.38±0.02 a

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(B) Color values

R2

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379914±35640 a 387692±54825 a 427105±11267 b 419142±15267 b 418688±18636 b 419083±28836 b

Raw CH OH3 OH5 OH7 OH9

n (-)

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K (Pa∙secn)

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Industrial Relevance

Ohmic heating (OH) is a food processing technique in which heat is generated inside food

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products by the passage of alternating electric current. This study aimed to determine the optimal

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paste using a self-designed continuous OH system.

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conditions for pasteurizing the Gochujang Korean-style fermented food containing hot pepper

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The effective conditions for OH involved three pairs of electrodes connected diagonally and

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100°C without scale buildup.

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excitation over the voltage range of 50–100 V. The selected conditions provided heating above

The rapid and uniform OH produced a microbial inactivation effect of a 2 log reduction despite

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the shorter processing time than for batch-type conventional heating (CH).

OH was available for 57 cycles during 1 cycle of CH, and it could process 104.4 kilotonnes of food products. And the continuous OH is suitable for the heat-based sterilization of highly viscous foods and could result in better energy efficiency and be suitable for implementation in 25

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industrial applications.

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Highlights

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The continuous ohmic heating is suitable for sterilization of highly viscous foods.

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The effective conditions involved three pairs of electrodes connected diagonally.

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A microbial inactivation with a 2 log reduction was achieved by ohmic heating (OH).

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OH was available for 57 cycles during 1 cycle of conventional heating (CH).

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The consistency index related to thickening was markedly higher for OH than for CH.

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