Effects of ohmic and microwave cooking on textural softening and physical properties of rice

Accepted Manuscript Effect of ohmic and microwave cooking on textural softening and physical properties of rice Mohsen Gavahian, Yan-Hwa Chu, Asgar Farahnaky PII:

S0260-8774(18)30398-4

DOI:

10.1016/j.jfoodeng.2018.09.010

Reference:

JFOE 9397

To appear in:

Journal of Food Engineering

Received Date: 21 June 2018 Revised Date:

10 September 2018

Accepted Date: 11 September 2018

Please cite this article as: Gavahian, M., Chu, Y.-H., Farahnaky, A., Effect of ohmic and microwave cooking on textural softening and physical properties of rice, Journal of Food Engineering (2018), doi: https://doi.org/10.1016/j.jfoodeng.2018.09.010. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

1

Effect of ohmic and microwave cooking on textural softening and physical properties of

2

rice

7

8 9 10

11

No. 331 Shih-Pin Rd., Hsinchu, 30062, Taiwan, ROC b

Southern Taiwan Service, Food Industry Research and Development Institute, No. 331

Shih-Pin Rd., Hsinchu, 30062, Taiwan, ROC c

RI PT

6

Product and Process Research Center, Food Industry Research and Development Institute,

SC

5

a

School of Biomedical Sciences, ARC Industrial Transformation Training Centre for

M AN U

4

Mohsen Gavahian*a,b, Yan-Hwa Chu a, Asgar Farahnaky c

Functional Grains and Graham Centre for Agricultural Innovation, Charles Sturt University, Wagga Wagga, NSW, Australia

* Corresponding author: [email protected]; [email protected]

12

TE D

3

Abstract

14

The effects of two volumetric heating methods, ohmic and microwave, on consumed energy and

15

physical properties, such as color, texture, and hydration, of a rice recipe (rice-water ratio of 1:15)

16

were investigated and the results were compared to that of the hotplate cooking method. The

17

textural parameters were analyzed using texture profile analysis and fitted into a previously

18

suggested equation to obtain the texture softening rate (K) and residual constant (A) values.

19

Although the color values were negatively affected by ohmic heating, this processing method

20

resulted in greater softening rates with the K value of 0.4 as compared to that of the traditional

21

method (0.2). In addition, ohmic heating consumed 69 % of the required energy in the

AC C

EP

13

1

ACCEPTED MANUSCRIPT

microwave process for a 50 % decrease of the initial hardness of the rice grains and eliminated

23

the fouling issue of the hotplate cooking. This innovative method could be a promising

24

alternative to traditional rice cooking methods after further improvements in its safety and design.

RI PT

22

25

Keywords

27

Energy-saving; Microwave; Ohmic heating; Rice; Texture.

SC

26

28

M AN U

Nomenclature

residual texture (a constant texture parameter)

a*

redness-greenness

AG

the number of attached grains to the heating surface

B

color intensity

b*

yellowness-blueness

Co

cohesiveness

Cw

chewiness

df

degrees of freedom

F0 Fexp Ft

EP

TE D

A

AC C

29

the textural parameter at time 0

the calculated textural parameter through experiment

the predicted textural parameter at time t

Ha

hardness

I

the electric current (A)

K

the constant rate

2

ACCEPTED MANUSCRIPT

gap between the electrodes (m),

L*

lightness

R2

coefficient of determination

RSS

residual sum of squares

S

surface area of the electrodes (m2).

SEE

sum of squared errors,

Sp

springiness

T

process time (min)

TG

total number of processed grains in each experiment

the applied voltage (V)

X

a texture parameter in the mathematical model

σ

specific electrical conductivity (S/m)

TE D

V

30

SC

M AN U

TPA texture profile analysis

RI PT

L

1 Introduction

32

Rice is the most widely consumed cereal and the staple food for about half of the world's

33

population (Muthayya et al., 2014). The heating method involved in the cooking process of the

34

grains affects the quality parameters such as texture (Ramesh et al., 2000; Srisawas & Jindal,

35

2007; Tamura et al., 2014). Although each region of the world may have its own traditional rice

36

cooking styles, generally, the two most common techniques for rice cooking are the optimum

37

water level and the excess water procedures. In the optimum water level, which is the common

38

practice in many East Asian countries, rice is cooked with a precisely-measured volume of water

39

in the presence of steam (as the heat transfer medium) in a rice cooker (rice steamer). On the

40

other hand, the former rice cooking procedure, which is the traditional rice cooking method in

AC C

EP

31

3

ACCEPTED MANUSCRIPT

many Middle Eastern countries, includes heating a mixture of rice and a large amounts of salted

42

water in a cooking pot using a conventional heating source, such as direct flame or an electric hot

43

plate, and draining away the remaining water after an appropriate cooking time, i.e., after

44

reducing the firmness of rice grains to the proper level (Daomukda et al., 2011; Proctor, 2018). It

45

was reported that the excess water cooking style can reduce the arsenic, i.e. a naturally occurring

46

carcinogen metalloid element (Gray et al., 2016). This boiled rice could be subjected to another

47

process, such as steaming, depending on the desired cuisine (Roden, 2008). The heating source

48

for this traditional cooking method is usually flame or hotplate wherein the thermal energy needs

49

to transfer from a heating surface to the cooking pot, and then the rice-water mixtures through

50

thermal conduction and convection. These modes of heat transfers are known to be time- and

51

energy- intensive with limited energy efficiency (Havahoian et al., 2012). On the other hand,

52

recent research showed that volumetric heating methods, such as microwave and ohmic heating,

53

offer several advantages, including shorter process time and saving energy (Gavahian et al., 2015;

54

Ekezie et al., 2017). The energy saving aspects of microwave and ohmic cooking of rice in

55

comparison to electric rice cooker were well illustrated by Lakshmi et al. (2007) and

56

Kanjanapongkul (2017), respectively.

57

Although both microwave and ohmic heating are categorized within the group of electro-heat

58

techniques, they convert electrical energy into thermal energy in different ways. Microwave

59

energy is a form of electromagnetic energy with a frequency range of 300 MHz to 300 GHz and

60

a food application range of 915 MHz and 2.45 GHz. (Al-Harahsheh & Kingman, 2004; Meda et

61

al., 2017). On the other hand, ohmic heating occurs whenever an alternating electric current

62

passes through a matter and energy is converted from electric to thermal according to Joule’s law.

63

A wide range of frequencies can be applied to food materials in an ohmic process. Recent

AC C

EP

TE D

M AN U

SC

RI PT

41

4

ACCEPTED MANUSCRIPT

research showed that using higher frequencies can reduce the electrochemical reactions between

65

food and electrodes (Ramaswamy et al., 2014; Gavahian & Farahnaky, 2018) and some

66

manufacturers (e.g. Emmepiemme SRL) use high-frequency power generators to reduce these

67

unpleasant reactions. However, some of the ohmic systems are also operating at the frequency of

68

50 or 60 Hz (Gavahian, Chu & Sastry, 2018) as these are the most common frequencies that are

69

available for food industries and research centers in some regions of the world and converting

70

them to higher frequencies require additional equipment, i.e., frequency converter, which is an

71

additional expense (Marra et al., 2009; Varghese et al., 2014; Wang & Farid, 2015; Schavemaker

72

& Van der Sluis, 2017). Both microwave and ohmic methods heat food materials through dipolar

73

rotations and ionic mobility and the main difference in heating mechanisms of these techniques

74

is that with ohmic heating, the ionic components are dominant, and dipole rotation effects are

75

relatively small but in microwave heating, the dipole effects become significant (Al-Harahsheh

76

& Kingman, 2004; Meda et al., 2017; Gavahian et al., 2015; Ekezie et al., 2017; Gavahian, Chu

77

& Sastry, 2018; Ramaswamy et al., 2014). Food texture and kinetic studies of this process is

78

essential in the proper design of a thermal process to enhance the product quality and reduce

79

process time and consumed energy (Farahnaky et al., 2012; Ling et al., 2015). As one of the first

80

attempts to evaluate the applicability of replacing conventional rice cooker with an ohmic heater,

81

Jittanit et al. (2017) designed and developed an ohmic heater for cooking long grain rice

82

(Jasmine variety) according to the optimum water level cooking procedures and reported that

83

ohmic heating can be considered as an energy-saving alternative to the tradition rice cooker

84

which is a common appliance in many East Asian countries. In addition, they measured the

85

texture of the cooked Jasmine rice and observed that the textural properties of the rice cooked by

AC C

EP

TE D

M AN U

SC

RI PT

64

5

ACCEPTED MANUSCRIPT

ohmic equipment were different from that of prepared by electric rice steamer (Jittanit et al.,

87

2017).

88

To the best of our knowledge, there is no report on the applicability of replacing hotplate, as a

89

traditional heating source, with volumetric heating techniques, i.e., microwave and ohmic

90

heating, for cooking short grain rice based on the Middle Eastern cooking style (the excess water

91

procedures). In this study, ohmic and microwave heating were utilized to cook rice kernels in

92

excess water and the changes in the grain’s textural properties, color, hydration, size, and the

93

energy consumption were evaluated and compared to those of the conventional heating method.

94

In addition, this study, for the first time, had fitted the experimental textural data during cooking

95

process into a mathematical model to propose a model for predicting the firmness of the rice

96

grains while conventional, microwave or ohmic cooking in the excess volume of salted water

97

which is the determining parameter for cooking time in the Middle Eastern cooking style.

98

Therefore, the objectives of this research were: to evaluate the efficacy of ohmic and microwave

99

heating in Middle Eastern-style cooking of rice in comparison with the traditional heating

100

method (hotplate) and to evaluate the changes in physical and textural properties of the rice

101

grains during these thermal processes.

EP

TE D

M AN U

SC

RI PT

86

AC C

102 103

2 Materials and methods

104

2.1 Raw material

105

White short grain Asian rice (Oryza sativa) was purchased from San-Hao Rice Company (New

106

Taipei, Taiwan). The identity of the rice (Chinese variety name: Tainan 11, NPGRC No.:

107

05A05731) was confirmed by the experts from the supplier and also Food Industry Research and

6

ACCEPTED MANUSCRIPT

108

Development Institute (FIRDI), Taiwan. This rice had a moisture content of 14 (w/w %) with a

109

grain average surface area of 10.3±1.3 mm2 (n=40).

RI PT

110

2.2 Cooking procedures

112

Rice was cooked according to the traditional Middle-Eastern cooking style, as reported by

113

Halder et al. (2014) with small modifications. This cooking method involves soaking, boiling

114

and rinsing. Briefly, 25 g rice was soaked in 375 g salted water (rice-water ratio: 1:15)

115

containing 0.5 w/v % sodium chloride for 30 min at 28±1 °C, and then was subjected to 7.5 min

116

ohmic, 15 min microwave or 15 min conventional heating. These cooking times were defined

117

according to the required heating time for about 90 % reduction in rice grain hardness. The same

118

Pyrex heating chamber 16×11×6 cm LWH (=length× width× height) was used in all cooking

119

experiments and the weight of the sample (rice + salted water) was monitored during the cooking

120

process using a digital scale (Sartorius ED6202S, AG, Germany) to measure the evaporation rate,

121

i.e., the changes in the sample weight over process time. The input voltage was constantly

122

controlled during all the cooking processes, i. e., microwave, ohmic, and conventional methods,

123

to run the process at a constant power of 1.4±0.1 kW using the manual voltage-adjusting

124

function of the power supply (SPA-1103, Satech Power, Taiwan).

125

The ohmic process was conducted using an ohmic heater designed and developed

126

Southern Taiwan Service Center, FIRDI, Taiwan (Figure 1) which consists of a 3 KVA power

127

supply (SPA-1103, Satech Power, Taiwan), a digital multimeter (GDM-8342, GW-Instek, Good

128

Will Instrument, Taiwan), and two titanium electrodes (7×8 cm LW and 0.2 cm thickness). The

129

electrodes located on both sides of the Pyrex chamber and a constant power of 1.4±0.1 KW (60

130

Hz) was applied to the rice-water mixture for 3, 4.5, 6, and 7.5 min. The Input voltage, electrical

AC C

EP

TE D

M AN U

SC

111

7

at the

ACCEPTED MANUSCRIPT

current, and the temperature were recorded during the thermal process by a digital multimeter

132

equipped with a T-type thermometer.

133

Microwave process was run at the frequency of 2.45 GHz using an inverter microwave oven

134

(NN-SD681, Panasonic, China). The Pyrex vessel containing salted water and rice was located

135

on a glass turntable plate of the microwave oven. The same multimeter as the ohmic process was

136

used to record the consumed energy. The sample temperature was monitored using the GDM-

137

8342 digital multimeter as described for the ohmic cooking experiments. The samples were

138

subjected to microwave radiation for 3, 6, 9, 12, and 15 minutes.

139

An efficient hotplate (halogen cooktop) (HD-4415, Philips, Taiwan) was used for the

140

conventional cooking process. The Pyrex heating chamber was located on the top of the hotplate

141

heater’s halogen lamps and thermal energy was applied for the same duration as the microwave

142

process. The process parameters were also recorded as described for the ohmic treatment.

TE D

M AN U

SC

RI PT

131

143

2.3 Electrical conductivity and energy consumption measurement

145

An AXE-1 digital power meter (Winling Technology Inc., Taiwan) was used to record the

146

energy consumption data. Moreover, the electrical data, such as voltage, electric current, and

147

power, obtained from the monitoring system of the power supply (Satechpower, Taiwan) and a

148

digital multimeter (GDM-8342, GW-Instek, Good Will Instrument, Taiwan). These data along

149

with the manually recorded contact area between the salted water and electrodes, i.e., the cross-

150

sectional area of the mixture, over process time, were used to calculate the electrical conductivity

151

(S/m) of the sample with three replications (Eq. (1)) (Palaniappan & Sastry, 1991):

152

AC C

EP

144

=



(1) 8

ACCEPTED MANUSCRIPT

where σ is the specific electrical conductivity (S/m), L is the gap between the electrodes (m), and

154

S is the surface area of the electrodes which is in contact with the sample (m2). V is the applied

155

voltage (V) and I is the electric current (A).

RI PT

153

156

2.4 Texture analysis

158

The texture of rice grains before and after thermal processes was examined by a texture analyzer

159

(TA.XT Plus, Stable Micro System Corp., UK). The rice samples were gently rinsed with tap

160

water (1 liter/min) for 3 min, cooled to 28±1°C, and rinsed. The rice grains were put in a

161

convenient baggie (100×70mm) and sealed to prevent moisture loss during texture evaluation.

162

Sampling was performed based on kernels sampling method as described by other researchers

163

(Lyon, et al., 2000; Miao et al., 2016) with small modifications. Briefly, four intact rice kernels

164

were selected randomly and arranged in a single-grain layer on the center of the clean flat

165

aluminum base of the texture analyzer (Lyon et al., 2000). The textural experiments were

166

repeated for twelve times.

167

Texture profile analysis (TPA) test was conducted according to the procedure reported by Tian et

168

al. (2014). A compression plate (SMS P/35, Stable Micro System Corp., UK) with a diameter of

169

35 mm was located at 20 mm above the base and employed to run a two-cycle compression test

170

with 50 % strain. The crosshead pretest speed was set at 5 mm/s, while the test speed and post-

171

test speed were set at 1 mm/s with a trigger force of 5 g (Zhu et al., 2010). This process was

172

repeated at least 12 times for each sample using different rice grains and the average values of

173

hardness (Ha), cohesiveness (Co), and springiness (Sp) were collected for statistical analysis. In

174

addition, the chewiness (Cw) of the sample was calculated by Eq. (2):

AC C

EP

TE D

M AN U

SC

157

9

ACCEPTED MANUSCRIPT

175

=

×

×

(2)

In addition, the hardness and chewiness data were modeled to study the kinetics of texture

177

softening which is an important parameter in thermal processing design. A common approach in

178

food softening studies is expressing the change of a texture parameter, such as hardness, over

179

process time by a first order equation, i.e., Eq. (3) (Verlinden et al., 1996): = −kX

(3)

SC

180

RI PT

176

wherein t is the process time and k is the constant rate. X is a texture parameter and was

182

calculated according to Eqs. (4) and (5).

183

X t = 0 = X − 0

184

X = X exp

185

The presented model in Eq. (3) has been updated by including an additional constant i.e., “A

186

parameter” (Farahnaky et al., 2012). This parameter defines the residual texture which is the

187

final remaining texture at the end of the process (Eq. (6)).

188

F = A + F exp

189

wherein Ft is the predicted textural parameter at time t and F0 the textural parameter at time 0.

190

Considering the variations in the hardness and chewiness of the rice grains over cooking time

191

(Figure 6) and applicability of the updated version of the texture softening model for ohmic and

192

microwave cooking (Farahnaky et al., 2012; Kamali & Farahnaky, 2015), the Eq. 6 was

193

employed to fit the texture data of the current study. Fitting the experimental data into the

194

suggested model was performed using the Solver function of Microsoft Excel 2010 (Microsoft

195

Corporation, United States) by minimizing the residual sum of squares (RSS) between the

196

measured and predicted data (McMinn, 2006). The k and A parameters were then calculated and

(4) (5)

(6)

AC C

EP

$≥0

TE D

M AN U

181

10

ACCEPTED MANUSCRIPT

statistically analyzed to compare the textural softening of the rice grain subjected to different

198

cooking methods. In addition, the accuracy of the proposed model was evaluated by comparing

199

the predicted and actual data to obtain the coefficient of determination (R2) and the sum of

200

squared errors (SEE) (McMinn, 2006; Jamali et al., 2006).

201

RI PT

197

2.5 Grain size and cooking loss measurements

203

At least 20 intact grains were randomly selected, converted to digital images by a multifunction

204

printer-scanner (DocuCentre-V C3374, Fuji Xerox, Japan) and their surface area was evaluated

205

by ImageJ software (1.51j8, National Institutes of Health, USA) (Fang et al., 2015).

206

The heating chamber was rinsed with the water pressure of 2×105 N/m2 (PLG water pressure

207

Gauge, PLG Air Tools, China) after thermal treatments, and the number of remaining rice (the

208

grains which needed scrubbing to remove from the heating surface, i.e., thermally damaged rice)

209

in the heating chamber was recorded and expressed as the cooking loss percentage according to

210

the Eq. (7):

211

% loss =

212

wherein AG is the number of attached grains to the heating surface and TG is the total number of

213

processed grains. TG was calculated in triplicate by manually counting the number of rice grains

214

in a 25 g rice sample.

× 100

TE D

M AN U

,-

EP

*+

AC C

215

SC

202

11

(7)

ACCEPTED MANUSCRIPT

2.6 Hydration degree

217

The hydration degree of the samples was determined as described by Tian et al. (2014) and Wani

218

et al. (2017). In brief, unprocessed and processed samples were immediately wrapped in filter

219

paper to remove the surface water. Five grams of these grains were placed in an infra-red

220

moisture determination balance (FD-600 Kett Electric Laboratory, Japan) and the moisture

221

contents of samples (g/100 g dry basis) were determined in triplicates at 105 °C for 99 minutes.

SC

RI PT

216

223

M AN U

222

2.7 Color values determination

225

The color of soaked and cooked rice samples was determined by a colorimeter (ZE 400 Nippon

226

Denshoku, Japan). In this regard, color coordinates for the extent of lightness (L*), redness-

227

greenness (a*), and yellowness-blueness (b*) were collected in triplicate. The color intensity (B)

228

was also calculated from the Eq. (8) (Roy et al., 2008):

229

/=0

2

EP

+ 3∗

(8)

AC C

230

∗ 2

TE D

224

231

2.8 Statistical analysis

232

Statistical analysis was performed using SPSS Statistics V. 23.0 (IBM, USA). The ANOVA test

233

was carried out to identify the effects of cooking methods on dependent variables, such as energy

234

consumption, heating rate, and textural and visual properties of the rice grains. The differences

235

between the means of these parameters were compared with Post Hoc-Duncan test at a

236

confidence interval of 95 %. 12

ACCEPTED MANUSCRIPT

237

3 Results and discussion

239

3.1 Electrical conductivity

240

Understanding the variations in the electrical conductivity of the food materials as influenced by

241

process temperature and sample volume should be considered while designing an ohmic cooking

242

process. The electrical conductivity of the food materials usually increases at high temperatures

243

(Sarang et al., 2008). Similarly, the electrical conductivity of the rice-water mixture increased

244

with temperature (Figure 2). The electrical conductivity of the sample was about 1.5 S/m at the

245

beginning and increased steadily to 3.5 S/m with temperature increase and then leveled off at the

246

gelatinization temperature i.e., about 85°C and the heating time of 2 min (see Figure 2 and

247

Figure 4). It has been reported that the electrical conductivity of starch dispersions decreases at

248

gelatinization point (Li et al., 2004; Gally et al., 2016). In this study, rice was heated in the

249

presence of the excessive amount of water (ratio: 1/15) and the liquid phase (salted water)

250

affected the overall electrical conductivity of the sample. According to Figure 2, electrical

251

conductivity growth slowed down noticeably during starch gelatinization, i.e., between the

252

process times of 2 and 3 minutes. This reduction in the electrical conductivity was followed by a

253

sharper decrease which may be related to the escape of water molecules from the mixture at high

254

temperatures i.e., after reaching the boiling point or changes in the properties of rice grains and

255

rice and water mixture due to ohmic cooking. Water evaporation affected the sample volume and

256

composition which decreased the surface area (S) and increased the electrical conductance. By

257

elimination of H2O molecules from the system, the concentration of sodium (Na+) and chloride

258

(Cl-) ions increased which is supposed to increase the electrical conductance of the sample.

259

However, reduction in water molecules can be translated into an increase in the concentration of

AC C

EP

TE D

M AN U

SC

RI PT

238

13

ACCEPTED MANUSCRIPT

rice grains. Therefore, the overall electrical conductivity became more similar to that of rice

261

grains as the evaporation continued. Evaporation of 250 g water along with starch gelatinization

262

resulted in a 50 % reduction in the electrical conductivity of the rice-water mixture (from 3 to 1.5

263

S/m) at 100 °C. According to the reported data, the variation in the electrical conductivity of

264

materials should be considered in the process design of open ohmic systems. The possibility of

265

the process runaway and a slowing down process should be taken into account during the come-

266

up time and after that, respectively.

SC

RI PT

260

M AN U

267

3.2 Evaporation rate and temperature profile

269

The results showed that the evaporation rate in ohmic heating (33.7±0.6 mL/min) is greater than

270

that of microwave (19.3±0.3 mL/min) and the traditional cooking method (20.0±0.7 mL/min)

271

(Figure 3). Ohmic heating is a volumetric method which generates heat directly inside the

272

material. The effective heating mechanisms resulted in a high thermal efficiency (near 100 %)

273

and a higher evaporation rate than the traditional heating method. On the other hand, higher

274

evaporation rate was also expected in the microwave heating as a form of volumetric heating,

275

considering the greater rate of temperature increase as compared to the traditional method.

276

However, the results revealed that there was no significant difference between the evaporation

277

rate of the microwave and the traditional cooking method. It is generally believed that

278

microwave can generate heat at a higher rate and evaporate water faster than the traditional

279

method. However, the microwave process requires being isolated from the environment due to

280

safety considerations. Applying thermal energy to the rice-water mixture evaporates water

281

molecules into the surrounding area i.e., free air in the conventional method and a limited area

282

inside the microwave box. As a result, the partial pressure of water molecules inside the

AC C

EP

TE D

268

14

ACCEPTED MANUSCRIPT

microwave box increased over process time, which reduces the volatility of water molecules and

284

decreases the evaporation rate in the current study. Nevertheless, it should be noted that it is

285

possible to design systems that will cause air flow over the sample, even in a microwave cavity,

286

to optimize the process design for industrial applications.

287

In addition, starch gelatinization was affected by the rate of temperature increase in the

288

microwave process. According to Figure 4, the rate of temperature increase was higher at the

289

beginning of the microwave heating process as compared to that of after gelatinization

290

temperature. This delay in reaching the boiling point slowed down the heating process and

291

resulted in a lower rate of temperature increase and evaporation rate, as compared to that of

292

ohmic heating. This observation was similar to Zhang et al. (2009). They reported that the

293

temperature of a thick starch solution increased sharply in the first minute of microwave heating

294

which followed by a temporary plateau near gelatinization temperature, and then a gradual

295

increase. The plateau was not observed in the temperature profile of the microwave rice cooking

296

as the rice grains were mixed with plenty of water. This difference in time-temperature profiles is

297

related to differences in product composition and experiment conditions. The effect of

298

microwave radiation on starch is discussed in details elsewhere (Braşoveanu & Nemţanu, 2014).

SC

M AN U

TE D

EP

AC C

299

RI PT

283

300

3.3 Energy consumption

301

As presented in Figure 6 and Table 1, 5.1±0.1, 7.3±0.1 6.9±0.5 minutes and 0.70±0.02,

302

1.02±0.02, and 1.01±0.03 watt-hour were required for 50 % reduction in the hardness of soaked

303

rice by ohmic, microwave, and traditional cooking, respectively. These observations indicated

304

that not only ohmic heating accelerated the cooking process but also saved about 31 % of the

305

consumed energy as compared to the traditional hotplate method (Table 1; Figure 4). This 15

ACCEPTED MANUSCRIPT

finding was in a good agreement with previous reports on the economic aspect of ohmic heating

307

(Wang & Sastry, 2002; Pereira & Vicente, 2010; Gavahian et al., 2012; Ramaswamy et al., 2014;

308

Gavahian & Farahnaky, 2018). Kanjanapongkul (2017) reported that replacing the electric rice

309

cooker with ohmic cooker can save about 75 % of cooking energy. Likewise, Jittanit et al. (2017)

310

reported that ohmic heating is an energy-saving process for cooking rice (according to the

311

East Asian method) compared to the rice cooker. Ohmic heating generates heat directly inside

312

the material, shortens the process time and reduces the consumed energy (Gavahian et al., 2018).

313

While in ohmic system heating occurs due to the passage of an electrical current through

314

materials and direct energy conversion, microwave heating rate depends on the materials ability

315

to absorb the generated waves i.e., dielectric loss factor (Pereira &Vicente, 2010). Therefore,

316

electrical conductivity and dielectric loss factor are the most important parameters that affect the

317

heating rate in the ohmic and microwave processes, respectively.

318

Similar to this study, Lakshmi et al. (2007) assessed the applicability of replacing two

319

conventional cooking appliances (electric rice cooker and pressure cooker) with microwave and

320

reported that microwave cooking neither decreased the energy consumption nor greatly

321

shortened the cooking time. According to the authors, while the required cooking time for all the

322

studied methods was in the range of 20–24 min, electric rice cooker was the most energy

323

efficient appliance. In addition, these authors showed that the microwave generation efficiency

324

of the magnetron (energy conversion from electrical to microwave) was about 50 % which is

325

disappointing compared to that of ohmic heating, i.e., about 100 % (Ramaswamy et al., 2014).

326

The conducted study by Lakshmi et al. (2007) also revealed the microwave absorbance

327

efficiency of the rice-water mixture could be as low as 68 %. Furthermore, while some expensive

328

materials, such as quartz, are microwave transparent, the common low-cost materials that can be

AC C

EP

TE D

M AN U

SC

RI PT

306

16

ACCEPTED MANUSCRIPT

used for microwave processing of foods, such as Pyrex with dielectric constant of about 3-10, are

330

known to absorb microwaves to some extent practically when they are in low-qualities, i.e.

331

contains some impurities (Robinson et al., 2010; Lu et al., 2017). Geometrical parameters were

332

also reported to affect the efficiency of the microwave heating process (Uyar et al., 2014). In this

333

study, the Pyrex vessel was involved in the heating process by adsorbing some the applied

334

energy through either microwave or conventional heater and slowed down the cooking process.

335

On the other hand, ohmic energy directly generated heat inside the rice-water mixture. The

336

above-mentioned reasons could be also the case for the low rate of temperature increase in the

337

microwave process. Therefore, ohmic heating was superior to both microwave and conventional

338

cooking method in terms of cooking time and consumed energy.

339

According to the literature (Gavahian et al., 2012; Gavahian et al., 2018), a higher energy

340

consumption is related to a higher release of greenhouse gases to the atmosphere, as combustion

341

of fossil fuels is one of the main sources of energy in the world. Therefore, replacing the

342

traditional rice cooking method by the energy-saving ohmic heating can indirectly protect the

343

environment by reducing the production of one of the leading causes of global warming i.e.,

344

greenhouse gases. However, it should be mentioned that, if traditional heating is directly done

345

with burning fossil fuels, e.g. natural gas, the energy use will be far less, because conversion to

346

electricity has a limited efficiency. Future studies may compare the energy consumption and

347

environmental impacts of volumetric heating methods, such as microwave and ohmic, and the

348

traditional heating cooking system which uses the heat of combustion of fossil fuels.

AC C

EP

TE D

M AN U

SC

RI PT

329

349

17

ACCEPTED MANUSCRIPT

3.4 Textural changes

351

The textural parameters, including hardness, were obtained from the compression force vs. time

352

curves of the TPA and the chewiness of the samples was calculated accordingly. The changes in

353

the hardness and chewiness of the rice samples as a function of cooking time are presented in

354

Figure 5. All the cooking procedures decreased the hardness and chewiness of the rice samples

355

over process time. A greater softening rate at the beginning of the cooking process followed by a

356

slower texture softening was observed for all the cooking methods. Similar textural softening

357

trends were reported by other researchers for variation in the texture of several food materials

358

over cooking time (Farahnaky et al., 2012; Peng et al., 2014; Jobe et al., 2016). Starch is the

359

major component of rice and its structure, gelatinization temperature, swelling, and

360

macromolecular leaching out of starch granules are among the determining parameters of the

361

cooked rice texture and the starch granules lose their crystalline order, absorb water and swell

362

during rice cooking, and amylose leaches out of the granules (Li et al., 2016). Likewise, the

363

hardness and the chewiness of the rice samples decrease over ohmic, microwave, and hotplate

364

process time in this study (Figure 5).

365

The textural softening parameters, including k and A values, were obtained for hardness and

366

chewiness from a fitted texture data model (Table 2). The softening rate constants (k values) of

367

the ohmic process were greater than those of the microwave and hotplate cooking methods which

368

suggest that ohmic heating is a more effective thermal process for rice cooking, as compared to

369

microwave and classical cooking methods. The hardness K value of ohmic heating (about 0.4)

370

was almost two times greater than that of microwave (0.2) and hotplate (0.2) methods. Similarly,

371

the K values for the chewiness for the rice grain subjected to ohmic heating, microwave, and

372

hotplate methods were about 0.6, 0.3, and 0.3, respectively. These observations were in line with

AC C

EP

TE D

M AN U

SC

RI PT

350

18

ACCEPTED MANUSCRIPT

the previous reports on accelerating textural softening of food material by ohmic heating

374

(Farahnaky, et al., 2012; Kamali & Farahnaky, 2015). Dielectric properties of materials affect the

375

heating rate in a microwave process. Although increasing salt concentration can enhance the

376

microwave heating rate, both low concentration salted water and cereal grains have low

377

dielectric constants when compared to other food materials, such as egg (Al-Harahsheh &

378

Kingman, 2004; Meda et al., 2017). This could be the reason for low heating rate and low

379

textural softening rate of microwave-treated rice grains in the present study. Ohmic heating can

380

generate heat directly inside the materials and is able to rapidly heat the electroconductive

381

materials, such as salted water (Ramaswamy et al., 2014; Gavahian et al., 2012). It should be

382

noted that the cooking time for ohmic heating was shorter than that of microwave and traditional

383

cooking in this study which could be the reason of the high A value of the ohmic treated sample

384

(Table 2). The initial hardness value of the soaked rice was 5125±238 g which was decreased to

385

747±33, 1282±96 g and 1117±80 g upon a six-minute ohmic, microwave and traditional process,

386

respectively. However, the additional cooking time in traditional and microwave cooking method

387

resulted in lower residual texture values of their final products (Table 2).

SC

M AN U

TE D

EP

388

RI PT

373

3.5 Grains hydration and expansion during cooking

390

Figure 6 presents the changes in the grains size during different cooking procedures. The

391

increase in the surface area of ohmic treated samples is slightly higher than other samples at the

392

beginning of the thermal process which could be related to a higher heating rate and quicker

393

water uptake in the ohmic process. Research showed that the swelling of rice grains is a

394

temperature dependent phenomenon (Shanthilal & Anandharamakrishnan, 2013) and the

395

temperature history of the sample while cooking can affect the swelling rate of the grains. While

AC C

389

19

ACCEPTED MANUSCRIPT

the ohmic treated rice grains reached the gelatinization temperature in about 2 minutes, the rice

397

grains subjected to microwave and hotplate needed to wait for long times to experience starch

398

gelatinization (i.e. about 3 and 4 minutes, respectively). The gelatinization temperature of

399

Chinese rice cultivars ranged from 65 to 85°C (Wang et al., 2010). High temperatures can initiate

400

starch gelatinization by affecting the crystalline structure of the granules, breaking the hydrogen

401

bonds and releasing the amylose and amylopectin into the surrounding area. This phenomenon

402

increases the amount of water intake by starch granules, resulting in an increase in the size of the

403

grains and their moisture contents (An & King, 2007; Kong et al., 2015). It should be noted that

404

in this study, rice grains were soaked in the salted water before cooking (similar to that of Middle

405

Eastern cooking style) and the migration of water molecules, sodium, and chloride ions into the

406

grains enhanced the electrical conductivity which increased the possibility of direct ohmic

407

heating on rice grains. This internal heating along with the high-temperature raise of the grains’

408

surrounding area (salted water) provided suitable conditions for starch gelatinization process.

409

The release of the amylose and amylopectin molecules at high temperatures might also affect the

410

electrical conductivity and the heating rate of the sample (An & King, 2007). It was reported that

411

an increase in the electrical conductivity of the media enhances the ohmic heating rate (Sakr &

412

Liu, 2007; Gavahian et al., 2017). On the other hand, the availability of free water and process

413

duration can affect the gelatinization degree. Due to the short process time, the swelling values

414

of the ohmic treated samples did not grow as much as that of the microwave and traditional

415

methods. The moisture content of about 60 % and the surface area of 28 mm2 were observed for

416

all the samples after cooking for 7.5 min. However, the microwave- and conventional- treated

417

samples exhibited a moisture content of 70 % and a surface area of 30 mm2 after an additional

418

7.5 min thermal process. Therefore, to reach the same moisture content as the traditional method,

AC C

EP

TE D

M AN U

SC

RI PT

396

20

ACCEPTED MANUSCRIPT

an extended ohmic process might be required. However, it should be noted that the higher rice-

420

water ratio due to higher evaporation rate in the ohmic process might also affect the observed

421

moisture contents in this study.

RI PT

419

422

3.6 Color intensity and lightness analysis

424

The previous studies showed that a reduced color intensity of cooked rice enhances the consumer

425

acceptance of the product and boosts its market value (Tian et al., 2014). Therefore, the

426

industrial adaption of alternative cooking methods of rice requires an investigation of their

427

ability in retaining the color intensity of the cooked rice.

428

This study revealed that the lightness and color intensity of rice samples decreased over process

429

time in all the cooking procedures (Figure 7). The lightness of the rice decreased from 82.9±0.1

430

to 78.6±0.1 upon a 7.5 min ohmic process which was similar to that of the microwave and

431

conventional methods. However, the additional cooking time to complete the cooking process by

432

microwave and hotplate method further decreased the lightness of the rice to 76.6±0.8 and

433

75.6±1.0, respectively. The same trend was observed for the color intensity of the samples. The

434

color intensity of all samples reduced from 9.9±0.2 to about 5 after 7.5 minutes of cooking.

435

However, longer process time resulted in the reduced values of color intensities for both

436

microwave and traditional heating methods (4.0+0.2 and 3.2+0.7, respectively). The higher

437

evaporation rate in the ohmic process and the lower moisture content of the ohmic-treated grains

438

might also affect the reported color values.

439

It was previously reported that other processes, such as high-pressure processing, can affect the

440

lightness and color intensity of the rice (Yu et al., 2017). According to Figure 7, cooking rice by

AC C

EP

TE D

M AN U

SC

423

21

ACCEPTED MANUSCRIPT

ohmic heating may negatively impact the color of the sample and reduce the product

442

acceptability. It should be noted that running ohmic heating at lower power may provide a longer

443

process time and further reduce the lightness and color intensity of the rice while reducing the

444

process energy due to internal heat generation. In addition, it was previously reported that

445

electrochemical reactions and electrode corrosion may affect the color of the processed food by

446

ohmic heating (Mercali et al., 2014) which could be the case for the observed color variations in

447

the rice grains that were cooked by ohmic heating. Further investigations regarding the effects of

448

the input power, electrode corrosion, and electrochemical reactions on the color of rice grain

449

along with a comprehensive sensory evaluation are required to confirm this highlighted

450

drawback of cooking rice by ohmic heating in this study.

M AN U

SC

RI PT

441

451

3.7 Rice grain loss

453

Ohmic and microwave heating generated heat directly within the materials and did not rely on

454

the classical modes of heat transfer. On the other hand, cooking rice on a hotplate necessitates

455

thermal penetration from the heating surface which resulted in the temperature gradient along the

456

vertical direction of the heating chamber i.e., an elevated temperature is expected at the bottom

457

of the heating chamber wherein there is a direct contact between the heating plate and the Pyrex

458

vessel which could be considered as the border of conductive and convective heat transfer area.

459

Consequently, in addition to uneven heating, the thermal damage to the rice grains which are

460

located in this area is unavoidable in a prolonged conventional thermal process (Figure 8). It was

461

observed that the number of attached rice to the heating chamber increased over process time in

462

the conventional method and 4.3±0.2 % of the product lost after a 15-min heating of the rice-

463

water mixture by the classical method (TG was 1785±18). It should be noted that the higher ratio

AC C

EP

TE D

452

22

ACCEPTED MANUSCRIPT

of rice to water can further decrease the overall thermal conductivity of the mixture and result in

465

a higher temperature gradient and a higher product loss in some other recipes. In addition,

466

replacing the Pyrex heating chamber with other materials, such as stainless steel, can also

467

increase the amount of product loss due to the difference in their surface properties. Adhesion of

468

food constitutes and the resulted fouling is a challenge for food industries and considerable

469

efforts have been made to produce anti-adhesive materials for heating surface and cooking

470

utensils (Zorita et al., 2010). However, In addition to the cost, research revealed the migration of

471

hazardous substances from non-stick coatings layers to food materials (Golja et al., 2017). Using

472

volumetric heating methods instead of traditional heating is another solution for this issue.

473

According to the findings of this study, both microwave and ohmic heating did not induce any

474

product loss which is due to the lack of a hot surface for heat transfer. This observation

475

highlighted the advantages of volumetric heating methods over tedious classical cooking in terms

476

of product loss.

TE D

477

M AN U

SC

RI PT

464

4. Conclusion

479

Traditional, microwave, and ohmic heating changed the physical properties of the soaked rice

480

cooked according to the Middle Eastern cooking style differently. While microwave did not

481

accelerate the cooking process, ohmic heating addressed several drawbacks of the traditional

482

cooking method, such as long process time, high energy consumption and fouling. Ohmic

483

cooking was shown to be energy efficient and reduced the come up time of the conventional

484

cooking method by 48 %. The constant rate values showed that ohmic cooking can soften rice

485

grains at a greater rate, as compared to other cooking methods. Comparing to microwave

AC C

EP

478

23

ACCEPTED MANUSCRIPT

486

cooking, not only innovative ohmic cooking consumed less energy and saved process time, but

487

also does not need any radiation isolating box for operational safety.

RI PT

488

Acknowledgements

490

This research was supported by the Ministry of Economic Affairs, project no. 106-EC-17-A-22-

491

0171, Taiwan, Republic of China.

SC

489

492

References

494 495

Al-Harahsheh, M., & Kingman, S. review. Hydrometallurgy, 73(3-4), 189-203.

496

An, H. J., & King, J. M. (2007). Thermal characteristics of ohmically heated rice starch and rice

497

flours. Journal of Food Science, 72(1). C084-C088.

498

Braşoveanu, M., & Nemţanu, M. R. (2014). Behaviour of starch exposed to microwave radiation

499

treatment. Starch‐Stärke, 66(1-2), 3-14.

500

Daomukda, N., Moongngarm, A., Payakapol, L., & Noisuwan, A. (2011). Effect of cooking

501

methods on physicochemical properties of brown rice. In 2nd international conference on

502

environmental science and technology (IPCBEE) (Vol. 6). (pp.V1-1-V1-4). IACSIT Press,

503

Singapore.

504

Ekezie, F. G. C., Sun, D. W., Han, Z., & Cheng, J. H. (2017). Microwave-assisted food

505

processing technologies for enhancing product quality and process efficiency: A review of recent

506

developments. Trends in Food Science & Technology. 67, 58-69.

M AN U

493

(2004).

Microwave-assisted

leaching-a

AC C

EP

TE D

W.

24

ACCEPTED MANUSCRIPT

Fang, C., Hu, X., Sun, C., Duan, B., Xie, L., & Zhou, P. (2015). Simultaneous determination of

508

multi rice quality parameters using image analysis method. Food analytical methods, 8(1), 70-78.

509

Farahnaky, A., Azizi, R., & Gavahian, M. (2012). Accelerated texture softening of some root

510

vegetables by ohmic heating. Journal of Food Engineering, 113(2), 275-280.

511

Gally, T., Rouaud, O., Jury, V., & Le-Bail, A. (2016). Bread baking using ohmic heating

512

technology; a comprehensive study based on experiments and modelling. Journal of Food

513

Engineering, 190, 176-184.

514

Gavahian, M., & Farahnaky, A. (2018). Ohmic-assisted hydrodistillation technology: A

515

review. Trends in Food Science & Technology. 72, 153-161.

516

Gavahian, M., Chu Y-H & Sastry, S. (2018). Extraction from food and natural products by

517

moderate

518

Comprehensive Reviews in Food Science and Food. (in press). doi: 10/1111/15414337.12362

519

Gavahian, M., Farahnaky, A., Farhoosh, R., Javidnia, K., & Shahidi, F. (2015). Extraction of

520

essential oils from Mentha piperita using advanced techniques: Microwave versus ohmic assisted

521

hydrodistillation. Food and Bioproducts Processing, 94, 50-58.

522

Gavahian, M., Farahnaky, A., Javidnia, K., & Majzoobi, M. (2012). Comparison of ohmic-

523

assisted hydrodistillation with traditional hydrodistillation for the extraction of essential oils

524

from Thymus vulgaris L. Innovative Food Science & Emerging Technologies, 14, 85-91.

525

Gavahian, M., Farhoosh, R., Javidnia, K., Shahidi, F., Golmakani, M. T., & Farahnaky, A.

526

(2017). Effects of Electrolyte Concentration and Ultrasound Pretreatment on Ohmic-Assisted

M AN U

field:

mechanisms,

benefits

and

potential

industrial

applications.

AC C

EP

TE D

electric

SC

RI PT

507

25

ACCEPTED MANUSCRIPT

Hydrodistillation of Essential Oils from Mentha piperita L. International Journal of Food

528

Engineering, 13(10). (in press) doi: 10.1515/ijfe-2017-0010

529

Gavahian, M., Lee, Y. T., & Chu, Y. H. (2018). Ohmic-assisted hydrodistillation of citronella oil

530

from Taiwanese citronella grass: Impact on the essential oil and extraction medium. Innovative

531

Food Science & Emerging Technologies. 48, 33-41.

532

Golja, V., Dražić, G., Lorenzetti, M., Vidmar, J., Ščančar, J., Zalaznik, M., Kalin, M., & Novak,

533

S. (2017). Characterisation of food contact non-stick coatings containing TiO2 nanoparticles and

534

study of their possible release into food. Food Additives & Contaminants: Part A, 34(3), 421-433.

535

Gray, P. J., Conklin, S. D., Todorov, T. I., & Kasko, S. M. (2016). Cooking rice in excess water

536

reduces both arsenic and enriched vitamins in the cooked grain. Food Additives & Contaminants:

537

Part A, 33(1), 78-85.

538

Halder, D., Biswas, A., Šlejkovec, Z., Chatterjee, D., Nriagu, J., Jacks, G., & Bhattacharya, P.

539

(2014). Arsenic species in raw and cooked rice: Implications for human health in rural

540

Bengal. Science of the Total Environment, 497, 200-208.

541

Jamali, A., Kouhila, M., Mohamed, L. A., Idlimam, A., & Lamharrar, A. (2006). Moisture

542

adsorption–desorption isotherms of Citrus reticulata leaves at three temperatures. Journal of

543

Food Engineering, 77(1), 71-78.

544

Jobe, B., Rattan, N. S., & Ramaswamy, H. S. (2016). Kinetics of Quality Attributes of Potato

545

Particulates during Cooking Process. International Journal of Food Engineering, 12(1), 27-35.

AC C

EP

TE D

M AN U

SC

RI PT

527

26

ACCEPTED MANUSCRIPT

Kamali, E., & Farahnaky, A. (2015). Ohmic‐Assisted Texture Softening of Cabbage, Turnip,

547

Potato and Radish in Comparison with Microwave and Conventional Heating. Journal of Texture

548

Studies, 46(1), 12-21.

549

Kanjanapongkul, K. (2017). Rice cooking using ohmic heating: Determination of electrical

550

conductivity, water diffusion and cooking energy. Journal of Food Engineering, 192, 1-10.

551

Lakshmi, S., Chakkaravarthi, A., Subramanian, R., & Singh, V. (2007). Energy consumption in

552

microwave cooking of rice and its comparison with other domestic appliances. Journal of Food

553

Engineering, 78(2), 715-722.

554

Li, H., Prakash, S., Nicholson, T. M., Fitzgerald, M. A., & Gilbert, R. G. (2016). Instrumental

555

measurement of cooked rice texture by dynamic rheological testing and its relation to the fine

556

structure of rice starch. Carbohydrate Polymers, 146, 253-263.

557

Li, L. T., Li, Z., & Tatsumi, E. (2004). Determination of starch gelatinization temperature by

558

ohmic heating. Journal of Food Engineering, 62(2), 113-120.

559

Ling, B., Tang, J., Kong, F., Mitcham, E. J., & Wang, S. (2015). Kinetics of food quality

560

changes during thermal processing: a review. Food and Bioprocess Technology, 8(2), 343-358.

561

Lu, K., Nguang, S. K., Ji, S., & Wei, L. (2017). Design of auto frequency tuning capacitive

562

power transfer system based on class-E2 dc/dc converter. IET Power Electronics, 10(12),

563

1588-1595.

564

Lyon, B. G., Champagne, E. T., Vinyard, B. T., & Windham, W. R. (2000). Sensory and

565

instrumental relationships of texture of cooked rice from selected cultivars and postharvest

566

handling practices. Cereal Chemistry, 77(1), 64-69.

AC C

EP

TE D

M AN U

SC

RI PT

546

27

ACCEPTED MANUSCRIPT

Marra, F., Zhang, L., & Lyng, J. G. (2009). Radio frequency treatment of foods: review of recent

568

advances. Journal of Food Engineering, 91(4), 497-508.

569

McMinn, W. A. M. (2006). Thin-layer modelling of the convective, microwave, microwave-

570

convective

571

Engineering, 72(2), 113-123.

572

Meda, V., Orsat, V., & Raghavan, V. (2017). Microwave heating and the dielectric properties of

573

foods. In The Microwave Processing of Foods (Second Edition) (pp. 23-43).

574

Mercali, G. D., Schwartz, S., Marczak, L. D. F., Tessaro, I. C., & Sastry, S. (2014). Ascorbic

575

acid degradation and color changes in acerola pulp during ohmic heating: Effect of electric field

576

frequency. Journal of Food Engineering, 123, 1-7.

microwave-vacuum

drying

of

lactose

powder. Journal

of

Food

M AN U

SC

and

RI PT

567

577

Miao, W., Wang, L., Xu, X., & Pan, S. (2016). Evaluation of cooked rice texture using a novel

579

sampling technique. Measurement, 89, 21-27.

580

Miao, W., Wang, L., Xu, X., & Pan, S. (2016). Evaluation of cooked rice texture using a novel

581

sampling technique. Measurement, 89, 21-27.

582

Muthayya, S., Sugimoto, J. D., Montgomery, S., & Maberly, G. F. (2014). An overview of

583

global rice production, supply, trade, and consumption. Annals of the New York Academy of

584

Sciences, 1324(1), 7-14.

585

Palaniappan, S., & Sastry, S. K. (1991). Electrical conductivities of selected solid foods during

586

ohmic heating. Journal of Food Process Engineering, 14(3), 221-236.

AC C

EP

TE D

578

28

ACCEPTED MANUSCRIPT

Peng, J., Tang, J., Barrett, D. M., Sablani, S. S., & Powers, J. R. (2014). Kinetics of carrot

588

texture degradation under pasteurization conditions. Journal of Food Engineering, 122, 84-91.

589

Pereira, R. N., & Vicente, A. A. (2010). Environmental impact of novel thermal and non-thermal

590

technologies in food processing. Food Research International, 43(7), 1936-1943.

591

Proctor, A. (Ed.). (2018). Alternatives to Conventional Food Processing. 2nd Edition. Royal

592

Society of Chemistry. Page 101.

593

Ramaswamy, H. S., Marcotte, M., Sastry, S., & Abdelrahim, K. (Eds.). (2014). Ohmic heating in

594

food processing. CRC press.

595

Robinson, J., Kingman, S., Irvine, D., Licence, P., Smith, A., Dimitrakis, G., ... & Kappe, C. O.

596

(2010). Understanding microwave heating effects in single mode type cavities—theory and

597

experiment. Physical Chemistry Chemical Physics, 12(18), 4750-4758.

598

Roden, C. (2008). The new book of Middle Eastern food. Alfred A. Knopf Inc. New York:

599

United States. Pp. 334-364.

600

Roy, P., Ijiri, T., Okadome, H., Nei, D., Orikasa, T., Nakamura, N., & Shiina, T. (2008). Effect

601

of processing conditions on overall energy consumption and quality of rice (Oryza sativa L.).

602

Journal of Food Engineering, 89(3), 343-348.

603

Sakr, M., & Liu, S. (2014). A comprehensive review on applications of ohmic heating

604

(OH). Renewable and Sustainable Energy Reviews, 39, 262-269.

605

Sarang, S., Sastry, S. K., & Knipe, L. (2008). Electrical conductivity of fruits and meats during

606

ohmic heating. Journal of Food Engineering, 87(3), 351-356.

AC C

EP

TE D

M AN U

SC

RI PT

587

29

ACCEPTED MANUSCRIPT

Schavemaker, P., & Van der Sluis, L. (2017). Electrical power system essentials. John Wiley &

608

Sons. PP. 1-10.

609

Shanthilal, J., & Anandharamakrishnan, C. (2013). Computational and numerical modeling of

610

rice hydration and dehydration: A review. Trends in food science & technology, 31(2), 100-117.

611

Tamura, M., Nagai, T., Hidaka, Y., Noda, T., Yokoe, M., & Ogawa, Y. (2014). Changes in

612

histological tissue structure and textural characteristics of rice grain during cooking

613

process. Food structure, 1(2), 164-170.

614

Tian, Y., Zhao, J., Xie, Z., Wang, J., Xu, X., & Jin, Z. (2014). Effect of different pressure-

615

soaking treatments on color, texture, morphology and retrogradation properties of cooked

616

rice. LWT-Food Science and Technology, 55(1), 368-373.

617

Uyar, R., Erdogdu, F., Sarghini, F., & Marra, F. (2016). Computer simulation of radio-frequency

618

heating applied to block-shaped foods: analysis on the role of geometrical parameters. Food and

619

Bioproducts Processing, 98, 310-319.

620

Varghese, K. S., Pandey, M. C., Radhakrishna, K., & Bawa, A. S. (2014). Technology,

621

applications and modelling of ohmic heating: a review. Journal of food science and

622

technology, 51(10), 2304-2317.

623

Verlinden, B. E., barsy, T., baerdemaeker, J., & deltour, R. (1996). Modelling the mechanical

624

and histological properties of carrot tissue during cooking in relation to texture and cell wall

625

changes. Journal of Texture Studies, 27(1), 15-28.

626

Wang, L., Xie, B., Shi, J., Xue, S., Deng, Q., Wei, Y., & Tian, B. (2010). Physicochemical

627

properties and structure of starches from Chinese rice cultivars. Food Hydrocolloids, 24(2-3),

628

208-216.

AC C

EP

TE D

M AN U

SC

RI PT

607

30

ACCEPTED MANUSCRIPT

Wang, R., & Farid, M. M. (2015). Corrosion and health aspects in ohmic cooking of beef meat

630

patties. Journal of Food Engineering, 146, 17-22.

631

Wang, W. C., & Sastry, S. K. (2002). Effects of moderate electrothermal treatments on juice

632

yield from cellular tissue. Innovative Food Science & Emerging Technologies, 3(4), 371-377.

633

Wani, I. A., Sogi, D. S., Wani, A. A., & Gill, B. S. (2017). Physical and cooking characteristics

634

of some Indian kidney bean (Phaseolus vulgaris L.) cultivars. Journal of the Saudi Society of

635

Agricultural Sciences, 16(1), 7-15.

636

Yu, Y., Pan, F., Ramaswamy, H. S., Zhu, S., Yu, L., & Zhang, Q. (2017). Effect of soaking and

637

single/two cycle high pressure treatment on water absorption, color, morphology and cooked

638

texture of brown rice. Journal of food science and technology, 54(6), 1655-1664.

639

Zhang, J., Wang, Z. W., & Shi, X. M. (2009). Effect of microwave heat/moisture treatment on

640

physicochemical properties of Canna edulis Ker starch. Journal of the Science of Food and

641

Agriculture, 89(4), 653-664.

642

Zorita, S., Niquet, C., Bonhoure, J. P., Robert, N., & Tessier, F. J. (2010). Optimisation of a

643

model food mixture using response surface methodology to evaluate the anti‐adhesive properties

644

of cooking materials. International Journal of Food Science & Technology, 45(12), 2494-2501.

646

SC

M AN U

TE D

EP

AC C

645

RI PT

629

647

31

ACCEPTED MANUSCRIPT

Tables

649

Table 1. The effect of ohmic, microwave and conventional heating on the rate of temperature

650

increase, come up time, evaporation rate and consumed energy. Ohmic

RI PT

648

Microwave

Time to boiling point 3.8±0.1c

Hotplate

6.6±0.3b

7.3±0.1a

16.6±0.8 b

Mean

evaporation 33.7±0.6 a

19.3±0.3 b

rate (mL/min) energy** 0.70±0.02b

Consumed

*

652

**

653

texture data. .

20.0±0.7b

1.02±0.02a

(Wh) 651

14.2±0.4 c

M AN U

25.5±0.3a

Heating rate (°C/s)

SC

(min)

1.01±0.03a

TE D

The same letters indicate that the means are not significantly different (p < 0.05). To 50 % reduction in the texture hardness i.e., reaching 969 g. data calculated from the

EP

654

Table 2. Changes in rate of texture softening (K) and residual texture (A) of different cooking

656

methods as determined from hardness and chewiness parameters.

AC C

655

Hardness

K

Ohmic

A

Microwave 0.224±0.013

657

*

RSS

0.358±0.038a 100.2±66.5 a 88.4˟103 b

Hotplate

Chewiness

100. 4±21.8

0.221±0.006b 61.9±22. 3a

a

3

986.3˟10

2

SEE

R

K

81.8

0.999

0.550±0.036 a 46.2±14.3 a 6.9˟103

288.5 0.982

1269.4˟103 399.3 0.973

0.264±0.010

A

b

22.2±8.2

0.279±0.008b 5.3±4.5c

RSS

b

3

SEE

R2

25.0

0.999

17.3˟10

124.6 0.987

6.9˟103

125.2 0.988

The same letters indicate that the means are not significantly different (p < 0.05).

658

32

ACCEPTED MANUSCRIPT

Figures

M AN U

SC

RI PT

659

TE D

660

Fig. 1. Schematic representation of the ohmic cooker used in this research for rice cooking in

662

excess water.

AC C

663

EP

661

33

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

664

668

669

TE D

667

EP

666

Fig. 2. Electrical conductivity of rice-water mixture as affected by process time and temperature.

AC C

665

34

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

670

674

TE D

673

EP

672

Fig 3. Evaporation rate of heating media during rice cooking.

AC C

671

35

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

675 676

Fig 4. Heating curves for rice-water mixture during ohmic, microwave, and conventional heating.

EP AC C

678

TE D

677

36

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

679

(a)

AC C

EP

TE D

680

37

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

681

(b)

683

Fig. 5. Variations in hardness (a) and chewiness of the rice grains as affected by ohmic,

684

microwave, and hotplate cooking methods.

EP

686

AC C

685

TE D

682

38

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

687

(a)

AC C

EP

TE D

688

689 39

ACCEPTED MANUSCRIPT

(b)

691

Fig. 6. Changes in the moisture content (a) and the surface area (b) of the rice grains during

692

ohmic, microwave and, conventional thermal process.

RI PT

690

695

(a)

AC C

694

EP

TE D

M AN U

SC

693

40

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

696

(b)

698

Fig. 7. Variations in the lightness (a) and color intensity (b) of rice sample during thermal

699

process

AC C

EP

TE D

697

41

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

700

Fig. 8. The kinetic of rice grain attachment to the heating surface in the traditional cooking

702

method. The number of attached grains for both microwave and ohmic heating were zero during

703

these processes and were not shown in this figure.

EP

705

AC C

704

TE D

701

42

ACCEPTED MANUSCRIPT

List of captions

707

List of Tables

708

Table 1. The effect of ohmic, microwave and conventional heating on the rate of temperature

709

increase, come up time, evaporation rate.

710

Table 2. Changes in rate of texture softening (K) and residual texture (A) of different cooking

711

methods as determined from hardness and chewiness parameters.

SC

RI PT

706

M AN U

712

List of Figures

714

Fig. 1. Schematic representation of the ohmic cooker used in this research for rice cooking in

715

excess water.

716

Fig. 2. Electrical conductivity of rice-water mixture as affected by process time and temperature.

717

Fig 3. Evaporation rate of heating media during rice cooking.

718

Fig 4. Heating curves for rice-water mixture during ohmic, microwave, and conventional heating

719

Fig. 5. Variations in hardness (a) and chewiness of the rice grains as affected by ohmic,

720

microwave, and hotplate cooking methods.

721

Fig. 6. Changes in the moisture content (a) and the surface area (b) of the rice grains during

722

ohmic, microwave and, conventional thermal process

723

Fig. 7. Variations in the lightness (a) and color intensity (b) of rice sample during thermal

724

process

AC C

EP

TE D

713

43

ACCEPTED MANUSCRIPT

Fig. 8. The kinetic of rice grain attachment to the heating surface in the traditional cooking

726

method.The number of attached grains for both microwave and ohmic heating were zero during

727

these processes and were not shown in this figure.

RI PT

725

AC C

EP

TE D

M AN U

SC

728

44

ACCEPTED MANUSCRIPT Highlights•

Rice was successfully cooked by ohmic and microwave heating Volumetric and conventional methods yielded grains with similar

RI PT

physical properties

Ohmic cooking resulted in a greater textural softening rate

AC C

EP

TE D

M AN U

SC

Ohmic heating is a time- and energy-saving method for rice cooking