Evaluation of energy efficacy and texture of ohmically cooked noodles

Accepted Manuscript Evaluation of Energy Efficacy and Texture of Ohmically Cooked Noodles

Yeon-Ji Jo, Sung Hee Park PII:

S0260-8774(19)30008-1

DOI:

10.1016/j.jfoodeng.2019.01.002

Reference:

JFOE 9500

To appear in:

Journal of Food Engineering

Received Date:

27 February 2018

Accepted Date:

03 January 2019

Please cite this article as: Yeon-Ji Jo, Sung Hee Park, Evaluation of Energy Efficacy and Texture of Ohmically Cooked Noodles, Journal of Food Engineering (2019), doi: 10.1016/j.jfoodeng. 2019.01.002

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ACCEPTED MANUSCRIPT

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Evaluation of Energy Efficacy and Texture of Ohmically Cooked Noodles

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Yeon-Ji Joa, Sung Hee Parkb*

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a Department

of Biomedical Science and Engineering, Konkuk University, Seoul 05029, South Korea,

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bDepartment

of Marine Food Science and Technology, Gangneung-Wonju National

University, Gangneung-si, Gangwon-do 25457, South Korea

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Running title: Ohmic heating for cooking of instant noodles

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*Corresponding

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Department of Marine Food Science and Technology, Gangneung-Wonju National

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University, 7 Jukheon-gil, Gangneung-si, Gangwon-do 25457, South Korea

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Phone: +82-33-640-2347, Fax: +82-33-640-2850

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E-mail: [email protected]

author. Sung Hee Park

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Abstract

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The feasibility of ohmic heating for cooking instant noodles was evaluated using a

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customized ohmic system. Temperature come-up time, heat transfer ratio (HTnls), system

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performance coefficient (SPC), and textural qualities were evaluated as a function of

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different electric fields (10, 12.5, 15, and 17.5 V/cm) and temperature holding times (30,

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60, 90, and 120 s). Temperature come-up time to 100°C was 3.9±0.1, 2.5±0.1, 2.1±0.2,

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and 1.3±0.1 min at electric fields of 10, 12.5, 15 and 17.5 V/cm, respectively.

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Temperature come-up time decreased significantly with an increase in electric field. The

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highest HTnls of 0.89 was observed at 15 V/cm. An electric field of 15 V/cm with a 90 s

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holding time yielded the greatest SPC of 0.63±0.05 and the most preferable textural

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qualities for hardness. Our study shows the potential of ohmic heating to rapidly cook

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instant noodles with good textural qualities and energy efficiency.

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Key words: ohmic heating; noodles; heat transfer; system performance coefficient; energy; texture

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1. Introduction

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Instant noodles have gained increasing popularity globally because of various

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advantages including convenience, longer shelf-life, and affordable price (Jang et al.,

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2016). Traditionally, noodles are cooked in water boiled on a gas stove or electric kettle.

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However, boiling cold water is time-and energy-consuming. Furthermore, boiling starchy

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food is time-consuming, e.g., noodles and rice. A processing technique that to achieve

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cooking in a short period is desirable (Xue et al., 2008). Alternative cooking methods for

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noodles could save cooking time and energy and improve the quality of the cooked

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noodles. Lifestyle changes have also increased the demand for a cooking technique that

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minimizes cooking time (Xue et al., 2008). The advanced utensils of cooking methods

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significantly affect the quality attributes and physicochemical composition of cooked

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foods (Jittanit et al., 2017). In response to consumer demand, commercial food

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processing units are investigating numerous advanced thermal processing including

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ohmic heating and microwave heating (Park et al., 2014). A more homogenous heating

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can be achieved, since ohmic heating is volumetric heating wherein the entire volume of

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food is heated simultaneously (Tornberg, 2013; Turp et al., 2016).

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Ohmic heating is a potential energy-based, time-saving technique to cook noodles.

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In ohmic heating, an alternating electric current is passed through materials to primarily

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heat the materials via conversion of electrical energy to thermal energy, which generally

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results in a rapid and uniform temperature increase in food (Cappato et al., 2017;

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Leizerson and Shimoni, 2005; Mercali et al., 2014; Wongsa-Ngasri and Sastry, 2015).

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The amount of heat dissipated is directly associated with the applied electric field

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and the electrical conductivity of the product or of individual product fractions, as

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determined on the basis of Ohm's law (Jaeger et al., 2016; Varghese et al., 2014). Food

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products with lower electrical conductivities are heated slower than those with higher

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electrical conductivity if the identical electric field strength is applied (Jittanit et al.,

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2017).

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Ohmic heating may offer numerous advantages including quicker cooking, less

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power consumption, and a safer food product (Ito et al., 2014). Changes in the cooking

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process influence the overall digestibility of noodles (Ye and Sui, 2016). In general, a

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shorter noodle cooking time will improve the textural attributes of the noodles, with a

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chewy and resilient bite without surface stickiness (Jang et al., 2016).

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In this study, the applicability of ohmic heating to cook non-fried instant noodles

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was evaluated. With an increase in health concerns among consumers worldwide, the

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demand for low fat, non-fried instant noodles has increased rapidly (Wang et al., 2011).

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These noodles are generally produced by molding the dough into a sheet and cutting it

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into strips after mixing wheat flour with salt water (Inazu et al., 2002). Non-expanded

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and non-fried noodles require prolonged cooking times. A technical challenge in

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improving the quality attributes of noodles is to shorten the cooking time (Jang et al.,

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2016). An electric field enhances water diffusion through the foodstuff during ohmic

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heating (Jittanit et al., 2017; Kanjanapongkul, 2017; Kemp and Fryer, 2007). Considering

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the significance of noodle cooking, energy efficiency, and textural quality, this study

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aimed to the purposes of this study were (a) evaluate the effect of ohmic electric field

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strength and holding time on noodle cooking time, (b) estimate the heat transfer ratio and 4

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system performance coefficient (SPC) as a function of the electric field, and (c)

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determine the textural quality of ohmically cooked noodles.

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2. Material and methods

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2.1. Instant noodle sample preparation

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Japanese Udon-type instant noodles were purchased from a local market. The

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Udon-type noodles were selected, since they have an appropriate diameter (2.5 mm) and

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elastic structure for thermocouple insertion. One bag of instant noodles contains noodle

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strands (225 g), soup powder (10 g), and dried vegetable-meat flakes (5 g). For one batch

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ohmic heating experiment, one-quarter of a bag of the instant noodle mixture (noodle: 56

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g, soup powder: 2.5 g, dried vegetable-meat flakes: 1.25 g) was added to 87.5 ml of

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distilled water.

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2.2. Ohmic heating system

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Figure 1 illustrates the laboratory-scale self-customized ohmic heating system for

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cooking instant noodles. A rectangular waterproof plastic food container (76×46×97 mm;

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Lock & Lock, Seoul, Korea) was used as the ohmic container. Two square-type titanium

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electrodes (75×45 mm2, thickness: 1 mm) were placed at both ends of the ohmic

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container and were separated by 9.5 cm. Titanium electrodes were selected to minimize

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the electrode chemical reaction (Samaranayake and Sastry, 2005). An AC power supply

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(HCS-2SD50; Hanchang Trans, Busan, Korea) provided the electric field (V/cm) applied 5

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across the sample for ohmic heating. To estimate the heating and energy efficacy,

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temperature, voltage, and current were measured and recorded every 3 s, using a Data

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Acquisition System (DAQ, 34970A; Agilent Technologies, Santa Clara, CA, USA).

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2.3. Ohmic heating treatment

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The prepared instant noodle mixture and distilled water described above were

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placed in the ohmic container. Two 0.25 mm-diameter K-type thermocouples (TFIR-003-

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50; Omega Engineering, Stamford, CT, USA) were used to measure the temperature of

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the noodles (Tnl) and the soup (Ts). One thermocouple was inserted into the horizontal

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center of the instant noodle strand and the other was positioned in the noodle soup. The

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temperature of instant noodle mixture was equilibrated to 20℃ before ohmic heating. The

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AC power supply provided four different electric fields of 10, 12.5, 15, and 17.5 V/cm

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across the ohmic container until the soup was ohmically heated up to the target

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temperature of 100°C. This temperature was maintained for the holding time of 30, 60,

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90, and 120 s during which ohmic cooking of the noodles occurred. A target soup

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temperature of 100℃ was maintained during the holding time through the on/off function

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of the proportional-integral-derivative (PID) controller (ITC-100; Inkbird, Shenzhen,

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China). A K-type thermocouple of the PID controller was used to measure the soup

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temperature. The power supply was turned off once the soup temperature approached

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100℃. It was intermittently turned on when soup temperature decreased to below 99℃.

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2.4. Traditional method of noodle cooking using an electric kettle

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We cooked noodles via the traditional manner using an electric kettle for reference.

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Water (87.5 ml) was boiled in the electric kettle (KEK-MS120, Zhongsha Meisu

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Electrical Co., China) and then the instant noodle mixture (noodle: 56 g, soup powder:

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2.5 g, dried vegetable-meat flakes: 1.25 g) was cooked for 2 min in the boiling water, per

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the manufacturer’s instructions.

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2.5. Heating transfer ratio (HTnls) and empirical modeling fitting

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The HTnls was estimated the heat transfer between the noodles and the soup as a

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function of the electric field (10, 12.5, 15, and 17.5 V/cm) and holding time (30, 60, 90,

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and 120 s). To calculate the HTnls, thermal doses of the noodles (TDnl) and soup (TDs)

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were defined as the amount of heat applied to the noodles and soup from the initial (ti) to

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the final holding time (tf) (Fig. 2b, A↔C), respectively (Park et al., 2014). TDnl and TDs

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were calculated through integration of an arbitrary noodle temperature (Tnl) and soup

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temperature (Ts) during the come-up time, as shown in Fig. 2b. Eq. 1, and Eq. 2

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represented the mathematical function to calculate the TDnl and TDs.

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tf

TDnl   Tnl dt  ti

(Tnl ,0  Tnl ,1 )  t 2



(Tnl ,1  Tnl ,2 )  t 2

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7

  

(Tnl ,n 1  Tnl ,n )  t 2

(Eq. 1)

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tf

(Ts ,0  Ts ,1 )  t

(Ts ,1  Ts ,2 )  t

(Ts ,n 1  Ts ,n )  t

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TDs   Ts dt 

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The aforementioned function was calculated using the trapezoidal numerical

ti

2



2

  

2

(Eq. 2)

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integration of MATLAB software (Version 7.9.0.529; Mathworks Inc., Natick, MA, USA)

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as suggested by Park et al. (2014). In our study, the Ts increased earlier than that of Tnl;

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thus, the heat transfer ratio (HTnls) was calculated as shown in Eq. 3:

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HTnls 

TDnl TDs

(Eq. 3)

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The calculated HTnls was empirically fit to a second-order multivariate polynomial

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regression as a function of the electric field (∇V, V/cm) and holding time (t, s) as shown

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in Eq. 4 using Statistical Analysis System (SAS) software (version 9.1.3, SAS Institute

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Inc., Cary, NC, USA). Equation 4 presents the estimated coefficients (β0, β1, β2, β3, β4, β5)

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of the estimated empirical model.

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HTnls   0  1 V   2  t  3 V 2   4  t 2  5 V  t  

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2.6. Determination of system performance coefficient (SPC)

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(Eq. 4)

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The SPC was calculated as the conversion ratio of total volumetric ohmic internal

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energy dose ( Evd , J) to the amount of energy in the form of heat (J) to increase the

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sample temperature to target temperature (Qtaken, J). SPC is commonly used to evaluate

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the energy efficacy of ohmic heating (Darvishi et al., 2012; Darvishi et al., 2013; Icier

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and Ilicali, 2004; Icier and Ilicali, 2005; Park et al., 2017). SPC was calculated during an

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increase in soup temperature to 100°C (temperature come-up time; Figure 2b, A↔B),

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since this is the period of the greatest electrical energy consumption during ohmic heating.

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The amount of energy in the form of heat required to increase the sample

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temperature to target temperature ( Qtaken , J) was calculated considering the increase in

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the temperature of both soup and noodles, as described by Icier and Ilicali (2005), and as

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shown in Eq. 5:

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Qtaken =ms  C p , s  (Tis -T fs )+mnl  C p ,nl  (Tinl -T fnl )

(Eq. 5)

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where Qtaken is the amount of energy in the form of heat (J), ms is the soup mass (kg),

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mnl is the noodle mass (kg), C p ,s is the specific heat of the soup (J·kg-1·K-1), C p ,nl is the

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specific heat of the noodles (J·kg-1·K-1), Tis is the initial temperature of the soup (°C), T fs

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is the final target temperature (100°C) of the soup, Tinl is the initial temperature of the

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noodles (°C), and T fnl is the final temperature of the noodles when soup temperature

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approached 100°C.

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Heat loss to the surroundings was considered as shown in Eq. 6 during temperature come-up time during ohmic cooking.

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Qloss =  htw  Atw  Tavtw  hbw  Abw  Tavbw  2  hsw  Asw  Tavsw  2  hsew  Asew  Tavsew   tcu 181

1/4 1/4    Tavtw   Tavbw   A   T  0.59   A   T 1.32    tw avtw bw avbw     L   L    t = 1/4 1/4   cu  T  T     avsw avsew  2 1.37     Asw  Tavsw  2 1.37     Asew  Tavsew  L L      

(Eq. 6)

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where Qloss is the heat loss from each wall (top, bottom, side) of ohmic (76×46×97 mm;

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Lock & Lock Seoul, Korea) container to surrounding by natural convection, htw is the

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convective heat transfer coefficient (W/m2·K) of top wall (horizontal plate where heat

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plate is facing up) at the ohmic container, hbw is the convective heat transfer coefficient

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(W/m2·K) of bottom wall (horizontal plate where heated plate is facing down) at the

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ohmic container, hsw is the convective heat transfer coefficient (W/m2·K) of side wall

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(vertical plane) at the ohmic container, hsew is the convective heat transfer coefficient

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(W/m2·K) of electrode side wall (vertical plane) at the ohmic container, Atw is the area of

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top wall at the ohmic container, Abw is the area of bottom wall at the ohmic container,

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Asw is the area of side wall at the ohmic container, Asew is the area of electrode side wall

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at the ohmic container, Tavtw is the average temperature driving force of top wall

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estimated from initial wall temperature, final wall temperature and ambient air

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temperature, Tavbw is the average temperature driving force of the bottom wall estimated

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from initial wall temperature, final wall temperature, and ambient air temperature, Tavsw

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is the average temperature driving force of side wall estimated from initial wall

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temperature, final wall temperature, and ambient air temperature, Tavsew is the average

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temperature driving force of electrode side wall estimated from initial wall temperature,

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final wall temperature, and ambient air temperature (Icier and Ilicali, 2005).

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Heat transfer coefficients of the top wall, bottom wall, side wall, and side wall

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with the electrode were calculated from the simplified equations for natural convection

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with an appropriate Gr×Pr number range (Geankoplis, 1993; Icier and Ilicali, 2005). K-

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type thermocouple was installed on each top wall, bottom wall, side wall, and side wall

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with electrodes of the ohmic container to measure the initial and final wall temperature

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during ohmic come-up time. These data were utilized to estimate the average temperature

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driving force ( T ), which is the average of initial wall temperature, final wall

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temperature, and ambient air temperature (Darvishi et al., 2012 ; Icier and Ilicali, 2005).

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Convective heat transfer coefficients of top wall, side wall, electrode side wall and

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bottom wall were 10.05, 10.38, 10.34 and 9.46 W/m2·K.

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In our study, C p ,s was substituted by 4184 J·kg-1·K-1, which is considered to have a

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value similar to that of water. C p ,nl was substituted by 3100 J·kg-1·K-1, which is the same 11

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as the reported value of waxy starch at 60% moisture content since Udon assessed in this

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study was prepared from waxy starch at 60% moisture content (Tan et al., 2004). In our

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study, the specific heat of C p ,s and C p ,nl were assumed to be independent of

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temperature, as suggested by Icier and Ilicali (2005).

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 , W·m-3) was calculated as Ohmic internal energy generation rate per volume ( Q ie

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a function of squared electric field (V/m) and electrical conductivity (S/m) as shown in

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Eq. 7 (Li and Zhang, 2010):

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Q ie =  V

2

(Eq. 7)

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Electrical conductivity (σ, S/m) of the material was determined from the cell

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constant (k, m-1), voltage (V), and current (A) data. The cell constant (k, m-1) considered

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the sample dimension where sample length (L, m) was divided by cross-sectional area (A,

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m2). In our study, the sample length (L, m) was 0.0950 m, which is the distance between

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the electrodes. The cross-sectional area (A, m2) was equal to the area of the rectangular

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titanium electrodes (0.075×0.045 m) and was calculated as 0.0034 m2. Electrical

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conductivity (σ, S/m) was determined as shown in Eq. 8:

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 =k 

I V

(Eq. 8)

 , W/m3) is expressed with a Thus, ohmic internal energy generation rate ( Q ie combination of Eq. 7 and 8 as shown in Eq. 9: I Q ie =k   V V

2

12

(Eq. 9)

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Total volumetric ohmic internal energy dose (Evd, J) of the noodles and soup was

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 , W/m3), sample estimated by considering the ohmic internal energy generation rate ( Q ie

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volume (v, m3), and their integration versus time (s) as shown in Eq. 10: tf Evd   Q ie dt ti

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tf

 k ti

I 2  V dt V

 I 0   I1 I1 I2 2 2 2 2   V0   V1  t0~1    V1   V2   t1~2  V1 V2 k  V0  V1     v (Eq. 10)    2        I 2  V2 2  I 3  V3 2   t2~3    I n 1  Vn 1 2  I n  Vn 2   tn 1~ n    V2  V3 Vn   Vn 1  236 237 238

The aforementioned function was calculated using the aforementioned trapezoidal numerical integration of MATLAB software, as suggested by Park et al. (2014).

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Finally, the SPC was calculated as shown in Eq. 11 in consideration of Qtaken, Evd

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and Qloss. SPC has been used to estimate the efficacy of ohmic heating for the

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temperature increase of foods (Darvishi et al., 2012; Darvishi et al., 2013; Icier and Ilicali,

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2004; Icier and Ilicali, 2005; Park et al., 2017).

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SPC 

Qtaken Evd  Qloss

(Eq. 11)

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Once the ohmic internal energy is totally converted to the temperature increase (heat), the

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value of SPC is 1, with the value decreasing with low energy conversion status.

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2.7. Texture profile analysis (TPA)

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TPA of ohmically cooked noodle strands was conducted using a TX-XT Plus

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Texture Analyzer (Texture Technology Corp., Brewster, NY, USA) with certain

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modifications of a previous method (Wang et al., 2011). Ohmically cooked noodle

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strands were removed from the ohmic container immediately after treatment and were

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then placed on a stainless-steel mesh to eliminate soup from the noodle strands. The

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noodle sample was placed on a stainless-steel plate for the TPA. Each noodle strand was

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cut using a model TA-47 W pasta blade (Texture Technology Corp.). TPA settings were

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as follows: pretest speed at 5 mm/s, test speed at 3.30 mm/s, post-test speed at 1.00 mm/s,

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target distance at 2 mm, time at 2.0 s, and trigger type of auto. Cooked noodle strands

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displayed an average thickness of 2.9 mm; hence, the target distance (cutting depth) was

259

set as 2 mm, which represented 70% of the noodle thickness. TPA measurements

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included hardness (N), cohesiveness, springiness, and gumminess (N): hardness (force

261

necessary to attain a given deformation, maximum force), cohesiveness (adheres to itself

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under some compressive or tensile stress, Area 2/Area 1), springiness (degree to which a

263

product returns to its original shape once it has been compressed, Length 2/Length 1),

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and gumminess (simulated energy required to disintegrate a semisolid food to a steady

265

state, hardness×cohesiveness) (Klinmalai et al., 2017; Turp et al., 2016).

266

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2.8. Statistical analyses

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Data were analyzed using the Statistical Analysis System (SAS) software (version

269

9.1.3, SAS Institute Inc.). Statistical analyses were performed using analysis of variance

270

(ANOVA) for multiple comparisons. Fisher’s least-significant difference (LSD)

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procedures were used for multiple comparisons among treatments at the 95% confidence

272

interval (P<0.05). All the ohmic treatments were replicated five times.

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3. Results and discussion

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3.1. Temperature histories of soup and noodles at different electric fields

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Figure 2a shows the temperature histories of the ohmically heated soup at different

277

electric fields (10, 12.5, 15, and 17.5 V/cm). As expected, an increase in electric field

278

intensity induced a faster temperature come-up as a function of the elevated voltage

279

gradient (V/cm). Electric fields at 10, 12.5, 15, and 17.5 V/cm yielded temperature come-

280

up time to 100°C of 3.9±0.3, 2.5±0.1, 2.1±0.2, and 1.3±0.1 min, respectively. In ohmic

281

heating, the rate of increase of the temperature was associated with enhanced voltage

282

(Lee et al., 2012; Yoon et al., 2002). Since the electrical energy per treatment time, which

283

is converted to heat energy, depends on the voltage gradient and the current passing

284

through the sample, the temperature increase at any instant is higher at higher voltage

285

gradients (Darvishi et al., 2012).

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Figure 2b shows the results of the comparison of the representative temperature

287

histories between soup and noodles at 15 V/cm and the 90 s holding time. Soup

15

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temperature increased before that of the noodles at all tested ohmic heating treatments.

289

For example, when the soup temperature approached 100°C (Tfs) within 114 s (A↔B)

290

during electric field application at 15 V/cm, the noodle temperature approached 51.9°C

291

(Tfnl), thereby representing a temperature difference (ΔT) of 48.1°C. The rate of ohmic

292

heating is proportional to the electrical conductivity increase (Sastry and Palaniappan,

293

1992). In our study, a more rapid increase in soup temperature reflected a higher

294

electrical conductivity of the soup than that of noodles, since soup contains various ions

295

including sodium chloride as well as seasonings. The movements of ions and subsequent

296

amplification of electrical conductivity will expedite the temperature increase during

297

ohmic heating (Jittanit et al., 2017; Shirsat et al., 2004).

298

In our study, the traditional method of cooking noodles with an electric kettle was

299

compared to ohmic heating as described in the section 2.4. Boiling of water in the electric

300

kettle lasted 3.6±0.3 min; thereafter, the noodles were cooked for 2 min per the

301

manufacturer’s instructions. The total cooking time was 5.6±0.3 min. Although SPC and

302

textural qualities are discussed in a later section, ohmic heating at 15 V/cm and 90 s

303

holding time yielded the best SPC and textural qualities. In case of ohmic heating at 15

304

V/cm and 90 s, the temperature come-up time was 2.1 min; holding time, 1.5 min.

305

Subsequently, ohmic heating at 15 V/cm and 90 s holding time resulted in a total cooking

306

time of 3.6 min, which is more rapid than the (5.3 min) traditional method of cooking.

307

Our study showed the potential of ohmic heating, which could save time in cooking

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noodles.

16

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309

3.2. Heat transfer ratio (HTnls) between noodle and soup

310

Figure 3 shows the HTnls as a function of elevated electric fields (10, 12.5, 15, and

311

17.5 V/cm) and prolonged holding time (30, 60, 90, and 120 s). The lowest HTnls of

312

0.29±0.03 was obtained at 10 V/cm and 30 s holding time. The highest HTnls of

313

0.89±0.01 was obtained at 15 V/cm and a holding time of 60 s, and at 15 V/cm and a

314

holding time of 120 s. HTnls was significantly increased as a function of prolonged

315

holding time at a low electric field intensity of 10 V/cm (P<0.05). Temperature come-up

316

time was relatively longer (3.9±0.3 min) at the low electric field intensity of 10 V/cm;

317

thus, the noodles cooked slowly during the prolonged holding time. In contrast, no

318

significant difference of HTnls was observed among the tested holding times at 15 V/cm

319

(P>0.05), except for the 30 s holding time. Among the electric fields assessed, 15 V/cm

320

yielded a good HTnls. For example, although the holding time was as short as 30 s at 15

321

V/cm, the HTnls was high (0.83±0.02). A high HTnls indicates enhanced heat transfer

322

between the soup and noodles. Noodles cooked rapidly at high HTnls displayed improved

323

textural qualities. The association between HTnls and textural qualities is discussed in

324

section 3.4.

325

Table 2 presents the empirical polynomial regression parameters (β0-5) predicting

326

HTnls as a function of the electric field (∇V, V/cm) and holding time (t, min) calculated

327

in Eq. 5. The polynomial model indicated a significantly positive linear coefficient, with

328

a β1 value 0.462792 for electric field strength (∇V; P<0.05), implying enhanced heat

329

transfer between the noodles and the soup with an increase in electric field intensity.

330

During ohmic heating, a high electric field intensity would induce rapid heating and 17

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331

subsequent expedited heat transfer between the noodles and the soup. We obtained a

332

significant positive linear coefficient of β2 of 0.009153 for holding time (t; P<0.05).

333

However, the numerical magnitude of β2 was markedly smaller than that of β1, indicating

334

that the electric field strength is more influential than holding time. In the quadratic

335

function of the electric field (∇V2) and holding time (t2), both β3 and β4 displayed

336

negative coefficients (-0.000426 and -0.014338, respectively) with significance (P<0.05).

337

Both quadratic parameters of β3 and β4 were very small numbers, indicating that a non-

338

linear function is a weak function for electric field strength and holding time. The linear

339

combination (β5) of the electric field (∇V) and holding time (t) did not yield significant

340

results in the probability test (P>0.05). In the empirical model fitting, it is assumed that

341

electric field strength (∇V, V/cm) is the most influential function to increase heat

342

transfer from soup to noodles associated with rapid heating in an elevated electric field.

343

3.3. Amount of energy in the form of heat (Qtaken, J), total volumetric ohmic internal

344

energy dose (Evd, J), heat loss (J), and SPC

345

Figure 4 shows a representative temperature history at 15 V/cm and a holding time

346

of 120 s of soup (Ts) and noodles (Tnl) versus the ohmic internal energy generation rate

347

 , W·m-3). The data were utilized to calculate the volumetric ohmic per volume ( Q ie

348

internal energy dose (Evd, J), amount of energy in the form of heat ( Qtaken , J), and system

349

performance coefficient (SPC) at 15 V/cm and 120 s holding time. The temperature of

350

the soup (Ts) increased from the initial temperature (Tis) of 25°C to final target

351

temperature (Tfs) of 100°C within 105 s (A↔B). During this period, the Tnl increased 18

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352

from the initial temperature (Tinl) to the final target temperature (Tfnl) of 68°C. The

353

increases in the soup (ΔTs) and noodles (ΔTnl) temperatures were 75 and 43°C,

354

respectively. As described in Eq. 5, the amount of energy in the form of heat (Qtaken, J)

355

was calculated using temperature increases (ΔTs, ΔTnl) of the soup and noodles and the

356

soup mass (ms, kg), noodle mass (mnl, kg), specific heat of soup ( C p ,s ,J·kg-1·K-1), and

357

specific heat of noodles ( C p ,nl ,J·kg-1·K-1). Calculated Qtaken values were tabulated as a

358

function of the elevated electric fields (Table 1). An electric field of 10 V/cm resulted in

359

Qtaken of 29605±197 J. Elevation of the electric field to 12.5 V/cm produced a Qtaken of

360

31970±947 J. The highest Qtaken was 36607±1592 J at the 15 V/cm electric field. The 17.5

361

V/cm electric field yielded a lower Qtaken (34326±1496 J) in comparison to that of 15

362

V/cm. The highest Qtaken at 15 V/cm is attributed to the combined effect of enhanced heat

363

transfer from soup to noodles and minimized heat loss. Qtaken values are governed by final

364

soup (liquid) temperature and noodle temperature. Although final soup temperature was

365

identical to 100℃ during the temperature come-up time, noodle temperature differed at

366

different electric field intensities. Noodle temperature approached 43.3±1.2, 57.2±5.6,

367

84.4±9.3, and 62.1±8.8℃ at 10, 12.5, 15, and 17.5 V/cm during temperature come-up

368

time of soup to 100℃ , respectively. 15 V/cm induced the most efficient increase in

369

noodle temperature; hence, it resulted in the highest Qtaken value. Therefore, 15 V/cm

370

resulted in the highest Qtaken (36607±1592 J) in combination with an appropriate

371

temperature come-up time and heat transfer from soup to noodles. In the scope of our

19

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372

study, 15 V/cm was the optimum electric field strength to effectively secure the energy in

373

the form of heat during ohmic heating. Its efficacy will be discussed subsequently in

374

relation to SPC.

375

 , W/m3) continuously Ohmic internal energy generation rate per volume ( Q ie

376

 was calculated increased during the temperature come–up time of ohmic heating. Q ie

377

with a combination of electrical conductivity (σ, S/m) and electric field (V/m) across the

378

 was responsible for increased electrical conductivity in sample. Increasing Q ie

379

accordance with an increase in temperature. Palaniappan and Sastry (1991) reported that

380

the electrical conductivities of tomato and orange juice increased linearly with

381

temperature. Increasing electrical conductivity of liquids at elevated temperature can be

382

explained by the reduced drag for the movement of ions (Palaniappan and Sastry, 1991).

383

Many studies have reported an increase in the electrical conductivity of liquids and solid

384

foods with an increased temperature during ohmic heating (Darvishi et al., 2012; Darvishi

385

et al., 2013; Icier and Ilicali, 2004; Icier & Ilicali, 2005; Kanjanapongkul, 2017; Sarang et

386

al., 2008).

387

The total volumetric ohmic internal energy dose (Evd, J) from noodles and soup

388

 , W·m-3), was calculated by considering the ohmic internal energy generation rate ( Q ie

389

sample volume (v, m-3), and their integration versus time (s), as shown in Eq. 10 and

390

tabulated in Table 1. Evd ranged from 49793±2135 J to 51460±3659 J; however, there

391

was no significant difference (P>0.05) among the electric fields (10, 12.5, 15, and 17.5

392

V/cm). At a low electric field intensity of 10 V/cm, the internal energy generation rate

20

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393

 , W·m-3) slowly increased with a lower slope for a long time. At a high per volume ( Q ie

394

 rapidly increased with a higher slope for a longer electric field intensity of 17.5 V/cm Q ie

395

time. Therefore, no difference was observed in total volumetric ohmic internal energy

396

 was integrated versus time. dose (Evd, J) among treatments when Q ie

397

The traditional manner of cooking noodles, using an electric kettle consumed an

398

electrical energy of energy of 81000±5400 J during water boiling. Subsequently, it took

399

the electrical energy of 129600±3600 J for 2 min cooking at boiling water. Therefore, the

400

total electrical energy spent in traditional noodle cooking was 210600±4762 J. Ohmic

401

heating at 15 V/cm spent the electrical energy of 50859 J (Table 1). Internal heat

402

generation of ohmic cooking could save the electrical energy by approximate 76% as

403

compared to cooking in an electrical kettle. Ohmic heating provided 82–97% of energy

404

saving while reducing the heating times by 90–95% compared to conventional heating

405

(Darvishi et al., 2013; De Halleux et al., 2005).

406

Estimated heat losses from the top, bottom, side and the side with electrodes were

407

tabulated in table 1. As shown in Eq. 6, heat loss was estimated through a simplified

408

estimation convective heat transfer coefficient (h, W·m-2·K-1), area of each wall, and

409

average temperature driving force (ΔTav). Among electric fields assessed, a lower electric

410

field yielded a greater increase in temperature on the walls during temperature come-up

411

time. For example, 10 V/cm indicated a final temperature of 79.5±6.1, 74.2±6.1, and

412

64.8℃±5.5℃ at side wall, bottom wall and electrode side wall, respectively. Whereas,

21

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413

17.5 V/cm indicated the final temperature of 61.7± 6.7, 60.5± 6.4, and 59.2℃±6.5℃ at

414

side wall, bottom wall and electrode side wall, respectively. A low electric field yielded a

415

longer temperature come-up time, suggesting that slow temperature come-up time at low

416

electric field allows for more heat loss to the surroundings. Estimated convective heat

417

transfer coefficients ranged from 4.45 W·m-2·K-1to 10.38 W·m-2·K-1. The bottom wall

418

showed the lowest hbw of 4.45±0.01 W·m-2·K-1, whereas the side wall with the electrode

419

showed the highest values of 10.38 W·m-2·K-1. Marra et al. (2009) reported the overall

420

heat transfer coefficient ranged from 5 W·m-2·K-1 to 50 W·m-2·K-1 during ohmic heating

421

of potato. It is evident that heat loss occurs to the cell wall and electrode during ohmic

422

heating of surfaces, which must be reduced by improving the system design such as

423

thermal insulation and heating tape application (Marra et al., 2009; Zell et al., 2011).

424

Overshooting of electric field strength near the electrode edges resulted in a greater

425

temperature increase with an enhanced ohmic heating effect (Jun and Sastry, 2005). Heat

426

loss indicated the 13937±53, 8924±21, 7474±37, and 4598±20 J at 10, 12.5, 15, and

427

17.5 V/cm, respectively. A low electric field intensity yielded a longer temperature come-

428

up time; thus, it would allow for greater heat loss to the surroundings. The energy loss

429

represents the heat required to heat the test cell, electrodes, etc., and heat loss to the

430

surroundings via natural convection and the portion of the generated heat used for

431

purposes other than heating the liquid, i.e., chemical reaction (Darvishi et al., 2012). As

432

the voltage gradient increased from 6 V/cm to 14 V/cm, specific energy losses decreased

22

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433

from 1.49 to 0.62 MJ/kg during ohmic heating of tomato paste (Torkian Boldaji et al.,

434

2015). The present results suggest that rapid temperature come-up time is desirable to

435

minimize heat loss to the surroundings during ohmic heating.

436

SPC values during ohmic heating are markedly dependent on the electric field

437

strength applied to the sample (Icier and Ilicali, 2005; Park et al., 2017). The electric field

438

strength of 10 V/cm yielded an SPC of 0.46±0.02, which indicate that 46% of the

439

electrical energy was converted to thermal energy to heat the sample. SPC increased up

440

to 0.63±0.05 at the electric field strength of 15 V/cm and decreased to 0.58±0.02 at the

441

field strength of 17.5 V/cm. SPC strongly depends on the voltage gradient (electric field)

442

during ohmic heating (Darvishi et al., 2013). SPC values range from 0.47-0.92 during

443

ohmic heating (Icier and Ilicali, 2005). In this study, all the electrical energy was not

444

completely changed to heat since SPC was < 1. A portion of ohmic electrical energy is

445

used for physical, chemical, and electrochemical changes of food during ohmic heating

446

(Assiry et al., 2003; Darvishi et al., 2013; Icier and Ilicali, 2005) and heat loss (4598-

447

13937 J) to the surroundings estimated in our study. We postulate that a minor portion of

448

the ohmic internal energy generation was also used for the gelatinization of starch

449

granules in the noodles other than for an increase in temperature. When starch granules

450

are gelatinized in excess water, there is a phase change from an ordered to a disordered

451

configuration; simultaneously, other phenomena are observed, including the uptake of

452

heat by starch granules and loss of birefringence (Li et al., 2004). Gelatinization enthalpy

453

of starch ranges from 0.807 to 1.591 J/g during the heating process (Coral et al., 2009).

454

Per the scope of our experiment, the most efficient SPC was 0.63±0.06 at 15 V/cm

23

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455

among the tested electric fields. According to energy conversation equation, electrical

456

energy input is converted to heat for temperature increase of material, electrochemical

457

reactions, heat loss to surrounding, accompanying circuits and device in the ohmic

458

heating (Min et al., 2007; Yin et al., 2018). Electrical conductivity of foods has the

459

significant effect on SPC. These multiple factors would influence on the SPC.

460

We suggest that 15 V/cm is the optimum electric field to cook noodles in an

461

energy-efficient manner. The appropriate electric field for SPC should be determined on

462

the basis of the ohmic heater design and properties of foods.

463

464

3.4. Texture profile analysis (TPA)

465

Figure 5 shows the results of TPA analyses of hardness (N) and gumminess (N) of

466

ohmically cooked noodles at electric fields strengths of 10, 12.5, 15, and 17.5 V/cm and

467

holding times of 30, 60, 90, and 120 s. TPA is one of the most widely accepted

468

instrumental methods to estimate the sensory texture attributes of cooked noodles (Baik,

469

2010; Ross, 2006; Wang et al., 2011). Although several textural parameters (hardness,

470

cohesiveness, springiness, and gumminess) were analyzed, only hardness (N) and

471

gumminess (N) yielded a significant difference among treatments (P<0.05). Springiness

472

ranged from 4.96 to 5.25; however, there was no significant difference among tested

473

electric fields and holding times (P>0.05). No significant difference in springiness could

474

be attributed to the small diameter of noodles at 2.5 mm. For cohesiveness, it showed

475

very similar trends to those of hardness (data not shown). Gumminess is the combination

24

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476

of hardness and cohesiveness (hardness ×cohesiveness), gumminess (N) could represent

477

the experimental data of cohesiveness when those have similar values.

478

The lowest hardness was 1.163±0.192 N at 10 V/cm and 30 s holding time among

479

the tested experimental conditions. The present results suggest that the noodles were

480

insufficiently cooked at 10 V/cm and 30 s and displayed a low N value. The desired

481

hardness of noodle varies with the noodle type and regional preference (Wang et al.,

482

2011). Hardness of ohmically cooked noodles was increased (1.353 N) during a

483

prolonged holding time of 120 s at the electrical field strength of 10 V/cm. With an

484

increase in cooking time, starch granules swell and voids appear, which alters the textural

485

properties (Ye and Sui, 2016). During cooking of noodles, a network displaying firmness

486

that in the order of that of the structural elements themselves is gradually established

487

through the processes of starch swelling and gelatinization and protein coagulation (Sui et

488

al., 2006). In contrast, when the holding time was increased from 90 s to 120 s at 17.5

489

V/cm, hardness decreased from 1.538±0.140 N to 1.279±0.125 N. An excessive holding

490

time would lead to overcooking of noodles and diminish their chewy texture.

491

Undercooking of noodles induces the insufficient absorption of water to produce a coarse

492

and hard texture, whereas overcooked noodles absorb excessive water, leading to a sticky

493

and soft texture (Jin et al., 1994; Ye and Sui, 2016). Optimally cooked noodles have a

494

chewy and resilient bite without surface stickiness (Miskelly and Moss, 1985; Sui et al.,

495

2006). In our study, the hardness of ohmically cooked noodles increased as a function of

496

elevated electric fields up to 15 V/cm and then decreased 17.5 V/cm. The high electric

497

field at 17.5 V/cm showed a rapid temperature come-up time up to 100°C of 1.3±0.1 min, 25

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498

which could hinder precise temperature control. The desired noodle hardness varies with

499

the noodle type and regional preference (Wang et al., 2011). Generally, undercooked

500

noodles are not preferred by consumers owing to their unpleasant raw doughy taste, while

501

overcooked noodles are difficult to handle with either chopsticks or a fork (Sui et al.,

502

2006). One of the technical challenges in improving the quality attributes of noodles is to

503

shorten their cooking time to yield a product with a chewy, resilient bite without surface

504

stickiness (Jang et al., 2016). In the present study, it was expected that the ideal textural

505

quality of noodle for consumer appeal could be similar to that of the traditional method of

506

cooking of noodles in accordance with the manufacturer’s cooking instructions. The

507

traditional method of cooking produces a hardness of 1.528±0.271 N. Ohmic heating at

508

15 V/cm and 90 s holding time resulted in a hardness of 1.534±0.189 N, which is highly

509

similar to that of the traditional method of cooking noodles.

510

Changes in gumminess (N) are presented in Figure 5b. The gumminess of a noodle

511

is the energy (amplified force) requirement to disintegrate or masticate the noodle (Guo

512

et al., 2003). Hardness has been positively correlated with gumminess and chewiness (Li

513

et al., 2017). Presently, gumminess ranged from 0.377 N to 0.427 N at a holding time of

514

30 s and no significant difference was observed among different electric fields at this

515

holding time. This would not be sufficient to cook noodles to produce the desirable

516

chewy and resilient texture. Gumminess displayed an increasing trend as a function of

517

prolonged holding time and an elevated electric field intensity. Noodles cooked in boiling

518

water in accordance with the manufacturer’s cooking instructions (traditional method)

519

yielded a gumminess value of 0.658±0.097 N. The conditions of 15 V/cm and a 90 s

520

holding time yielded the most similar gumminess value of 0.606±0.037 N. Per the scope 26

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521

of our experiment, considering both textural qualities and SPC for energy efficacy, an

522

electrical field strength of 15 V/cm and a 90 s holding time was considered the optimal

523

ohmic heating condition for cooking noodles.

524

525

4. Conclusions

526

The present study shows the potential of ohmic heating to cook instant noodles

527

with a variation of electric fields and temperature holding times. Ohmic heating at an

528

electric field strength of 15 V/cm displayed promising energy efficacy in terms of SPC

529

where 63% of electrical energy could be efficiently converted to thermal energy to cook

530

noodles and soup. Ohmic heating could save the time required to boil the water and

531

subsequently cook the noodles, since the temperature rapidly increased up to 100°C

532

within 1.3 min. In terms of textural qualities, the preferable hardness and gumminess

533

were obtained at an electrical field strength of 15 V/cm, which yielded a value similar to

534

that observed when cooking in accordance with the manufacturer’s cooking instructions.

535

Per the scope of our experiment, 15 V/cm was the optimum electric field strength for

536

energy efficacy, heat transfer, and textural qualities. Appropriate electric fields differ

537

with differences in food matrices, ingredients, cooking purpose and ohmic heater design.

538

Considering modern-day lifestyles, ohmic heating can be considered popular for the

539

warming or cooking of convenience foods, meal replacement food, and outdoor foods.

540

Further investigations for the use of ohmic heating technology for various convenience

27

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541

foods, such as instant noodles, rapidly prepared rice, and retort pouch products are

542

warranted.

543

Acknowledgments

544

This study was supported by Basic Science Research Program through the National

545

Research Foundation of Korea (NRF) funded by the Ministry of Education of the

546

Republic of Korea (No.2018R1A6A1A03023584 and No. 2017R1C1B5017458).

28

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570 V

547

571 548 Nomenclature

(m2)

cross sectional area

550 Cp

specific heat (J/kg·K)

551 E total volumetric ohmic internal 552 energy (E) 553 h

convective heat transfer

554

coefficients (W·m-2·K-1)

555 HT

heat transfer ratio

556 I

current (A)

557 k

cell constant (m-1)

558 L

length (m) or

559

electrode distance (m)

561

mass (kg)

Q The amount of energy in the form of

562 heat (J)

563

Q

564

V

electric field (V/m), electric

572 field (V/cm)

549 A

560 m

voltage (V)

Ohmic generation rate per volume (W·m-3)

573 v

volume (m3)

574 β

parameters in the empirical model

575 σ

electrical conductivity (S/m)

576 577 Subscript 578 av

average

579 bw

bottom wall

580 d

dose

581 f

final

582 fnl

final noodle

583 fs

final soup

584 i

initial

585 ie

internal energy

586 inl

initial noodle

587 is

initial soup

588 loss

loss to the surrounding

589 nl

noodle

565 SPC

system performance coefficient

590 nls

noodle and soup

566 T

temperature (°C)

591 s

soup

567 TD

thermal dose

592 sew

568 TPA

texture profile analysis

593 sw

569 t

time (s), time (min)

594 taken 29

side wall with electrode side wall heat taken

ACCEPTED MANUSCRIPT

595

622 tret

treated sample

596

623 tw

top wall

597

624 v

volume

598

625 626 627 628

599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621

30

0,1,2,3,…,n subinterval in trapezoidal numerical integration or parameters (intercept & slope) in the empirical model fitting

ACCEPTED MANUSCRIPT

629

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37

ACCEPTED MANUSCRIPT

Data Acquisition System Current

Voltage

Temperature

: Electrical

wiring

: Thermocouple

wiring

Thermocouple

Current Sensor

Electrode

Noodle & Soup Ohmic Container

Safety Fuse

AC Power Supplier (0-220 Vac, 60 Hz)

Electrode

Grounding

Figure 1. A schematic diagram of ohmic heating system for instant noodle cooking.

.

ACCEPTED MANUSCRIPT

120

Temperature (°C)

15 V/cm

100 80

17.5 V/cm

10 V/cm

12.5 V/cm

60 40 20 0 0

50

100

150

200

250

Time (s)

300

350

400

(a)

120

Tfs

100

Temperature (°C)

80 Ts

60 40

Tfnl

Tis

20

Tnl

Tinl

0 0 A

50

100

150 B

200

250

C

Time (s) (b) Figure 2. (a) Temperature histories of soup during ohmic heating at different electric fields (10, 12.5, 15 and 17.5 V/cm) and (b) representative comparison of temperature histories between soup and noodles at 15 V/cm electric field application.

ACCEPTED MANUSCRIPT

Heat transfer ratio (HTnls)

1.0 0.9

a

abcd

0.8

fg

0.7

cde

ef

abc bcde cde

g

a def

abc

fg

h

0.6 0.5 0.4 0.3

i j

0.2 0.1 0.0 30

60

Time (s)

90

120

Figure 3. Heat Transfer ratio (HTnls) between noodles and soup as a function of electric field (V/cm) and holding time (s). 10 V/cm, 12.5 V/cm, 15 V/cm, 17.5 V/cm.

a-jMeans

at P<0.05.

(±Standard deviation) with a different letter are significantly different

ACCEPTED MANUSCRIPT

3000000

Temperature (°C)

Q ie (W/m3)

100

2500000

80

2000000 Ts

60

1500000 Tnl

40

1000000

20

500000

0

0 0 A

50

100 B

150

200

250

300

Ohmic internal energy generation rate er volume ( W/m3)

120

Time (s)

Figure 4. Temperature histories of soup and noodles versus volumetric internal energy generation rate (W/m3) at 15 V/cm and 120 s holding time.

ACCEPTED MANUSCRIPT

2.0

a

Hardness (N)

1.6 1.2

d d

cd d

d

d

bc bcd

b b bcd d

bc bcd

cd

0.8 0.4 0.0 30

60 90 Holding time (s)

120

(a) 0.9 Gumminess (N)

0.8

a

0.7

ab

0.6 0.5 0.4

cd cd

d d

bcd cd cd d

bc d

cd

bc cd

cd

0.3 0.2 0.1 0.0 30

60 90 Holding time (s)

120

(b) Figure 5. (a) Hardness (N) and (b) Gumminess (N) of ohmically cooked noodles as a function of electric field (V/cm) and holding time (s). 10 V/cm, 12.5 V/cm, 15 V/cm ,

17.5 V/cm .a-dMeans (± Standard deviation) with a different letter are

significantly different at P<0.05.

ACCEPTED MANUSCRIPT

► Investigate the potential of ohmic heating to cook instant noodles as a function of electric fields (10-17.5 V/cm) and holding times (30, 60, 90 & 120 s). ► Ohmic heating enabled rapid cooking of instant noodles using electric field and internal energy generation. ► Energy efficacy of ohmic heating was promising since most of electrical energy was converted to heat. ► Heat transfer between noodles and soup was expedited at higher electric field application. ► Ohmic heating showed its potential to produce good textural quality of noodles.

Table 1. Comparison of temperature come-up time to 100°C (min), amount of energy in the form of heat (Qtaken, J), total volumetric ohmic internal energy dose (Evd, J), and system performance coefficient (SPC) during temperature come-up time to among different electric fields (10, 12.5, 15 and 17.5 V/cm) Come up time (min)

Amount of energy in the form of heat (Qtaken, J)

Total volumetric ohmic internal energy dose (Evd, J)

Heat loss (J)

System performance coefficient (SPC)

10 V/cm

3.9±0.3a

29605±197ca

49793±2135a

13937±53a

0.46±0.02ba

12.5 V/cm

2.5±0.1b

31970±947bc

48944±4665a

8924±21b

0.56±0.06ab

15 V/cm

2.1±0.2c

36607±1592aa

50859±2209a

7474±37c

0.63±0.05aa

17.5 V/cm

1.3±0.1d

34326±1496ab

51460±3659a

4598±20d

0.58±0.02a

a-dMeans

(±Standard deviation) with a different letter in the same column are significantly different at P<0.05.

Table 2. Estimated coefficients and probability testing of fitted polynomial parameters for heat transfer ratio (Htr) as a function of electric

field

( V ,

V/cm)

and

temperature

holding

time

(t,

min)

during

hydrothermal

processing

( H tr   0  1 V   2  t  3 V 2   4  t 2  5 V  t   )

β0 -3.001902 0.0001

Coefficients Pr > |t| R2 values SEE (ε)* *Standard error of the estimate.

β1 0.462792 0.0001

β2 0.009153 0.0013

β3 -0.000426 0.0016 0.81 0.09

β4 -0.014338 0.0001

β5 -0.000009 0.5086