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3 Recent developments and trends in thermal4 blanching – A comprehensive review

56

7 Hong-Wei Xiao a, Zhongli Pan c,f, Li-Zhen Deng a, Hamed M. El-Mashad d, Xu-Hai Yang b,8 Arun S. Mujumdar e, Zhen-Jiang Gao a, Qian Zhang b,*

9 aCollege of Engineering, China Agricultural University, P.O. Box 194, 17 Qinghua Donglu, Beijing 100083, China

10 bCollege of Mechanical and Electrical Engineering, Shihezi University, Shihezi 832001, China

11 cDepartment of Biological and Agricultural Engineering, University of California, One Shields Avenue, Davis, CA 95616, USA

12 dDepartment of Agricultural Engineering, Mansoura University, Mansoura, Egypt

13 eDepartment of Bioresource Engineering, McGill University, Ste. Anne de Bellevue, Quebec, Canada

14 fHealthy Processed Foods Research Unit, USDA-ARS, 800 Buchanan St., Albany, CA 94710, USA15

16

171 9 A R T I C L E I N F O

20 Article history:

21 Received 22 August 2016

22 Received in revised form

23 31 December 2016

24 Accepted 6 February 2017

25 Available online xxxx

26 Keywords:

27 Thermal blanching

28 Hot water blanching

29 Microwave blanching

30 Steam blanching

31 Ohmic blanching

32 Infrared blanching33

A B S T R A C T

Thermal blanching is an essential operation for many fruits and vegetables processing. It

not only contributes to the inactivation of polyphenol oxidase (PPO), peroxidase (POD),

but also affects other quality attributes of products. Herein we review the current status

of thermal blanching. Firstly, the purposes of blanching, which include inactivating

enzymes, enhancing drying rate and product quality, removing pesticide residues and toxic

constituents, expelling air in plant tissues, decreasing microbial load, are examined. Then,

the reason to why indicators such as POD and PPO, ascorbic acid, color, and texture are fre-

quently used to evaluate blanching process is summarized. After that, the principles, appli-

cations and limitations of current thermal blanching methods, which include conventional

hot water blanching, steam blanching, microwave blanching, ohmic blanching, and infra-

red blanching are outlined. Finally, future trends are identified and discussed.

� 2017 China Agricultural University. Publishing services by Elsevier B.V. This is an open

access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-

nd/4.0/).

49

50

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

2. The purposes of blanching. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

2.1. Inactivaction of quality-deterioration enzymes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

2.2. Enhancing dehydration rates and product quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

2.3. Removing pesticide residues and toxic constituents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

2.4. Expelling air entrapped inside plant tissues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

2.5. Minimizing non-enzymatic browning reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

http://dx.doi.org/10.1016/j.inpa.2017.02.0012214-3173 � 2017 China Agricultural University. Publishing services by Elsevier B.V.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

* Corresponding author.E-mail address: [email protected] (Q. Zhang).

Peer review under responsibility of China Agricultural University.

Avai lab le a t www.sc ienced i rec t .com

INFORMATION PROCESSING IN AGRICULTURE XXX (2017) XXX–XXX

journal homepage: www.elsev ier .com/ locate / inpa

Please cite this article in press as: Xiao H-W et al. Recent developments and trends in thermal blanching – A comprehensive review. Info Proc Agri(2017), http://dx.doi.org/10.1016/j.inpa.2017.02.001

INPA 74 No. of Pages 29, Model 7

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2.6. Decreasing microbial load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

2.7. Peeling of products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

2.8. Increasing extraction efficiency of bioactive compounds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

2.9. Other purposes of blanching. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

3. Assessment of the effectiveness of blanching process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

3.1. Activity of peroxidase (POD) and polyphenol oxidase (PPO) enzymes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

3.2. Ascorbic acid as an indicator to evaluate nutrients loss during blanching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

3.3. Color as an indicator of product quality change during blanching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

3.4. Texture as indicator of the effect of blanching on product physical properties. . . . . . . . . . . . . . . . . . . . . . . . . . . 00

4. The traditional hot water blanching technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

4.1. Hot water blanching processing and its application. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

4.1.1. Pepper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

4.1.2. Brussels sprouts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

4.1.3. Almond . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

4.1.4. Potato chips. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

4.1.5. Carrot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

4.1.6. Others . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

4.2. Limitations of hot water blanching. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

4.2.1. Losses of nutrients during blanching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

4.2.2. Wastewater from blanching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

5. The emerging and innovative blanching technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

5.1. Steam blanching and high-humidity hot air impingement blanching (HHAIB) . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

5.1.1. The principle of steam blanching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

5.1.2. Applications of steam blanching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

5.1.3. Limitations of steam blanching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

5.2. Microwave blanching. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

5.2.1. The operation principle and advantages of microwave heating. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

5.2.2. Applications of microwave blanching. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

5.2.3. Limitations of microwave blanching. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

5.3. Ohmic blanching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

5.3.1. The principle of ohmic blanching. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

5.3.2. Applications of ohmic blanching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

5.3.3. Limitations of ohmic blanching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

5.4. Infrared blanching. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

5.4.1. The principle of infrared heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

5.4.2. Applications of infrared blanching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

5.4.3. Limitations of infrared blanching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

6. Future trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

6.1. Investigations on products microstructure change during thermal blanching . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

6.2. Development of new hybrid technologies for blanching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

6.3. Evaluation and enhancing the sustainability of thermal blanching using life cycle assessment (LCA). . . . . . . . . 00

7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

8. Uncited references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

105 1. Introduction

106 Blanching is a thermal treatment that is usually performed

107 prior to food processes such as drying, freezing, frying, and

108 canning [1,2]. It is essential to preserve the product quality

109 during the long-term storage because it inactivates the

110 enzymes and destroys microorganisms that might contami-

111 nate raw vegetables and fruits during production, harvesting

112 and transportation [3,4]. Blanching involves heating vegeta-

113 bles and fruits rapidly to a predetermined temperature and

114 maintaining it for a specified amount of time, typically 1 to

115 less than 10 min. Then blanched product is either rapidly

116 cooled or passed immediately to a next process. The time

117required for blanching a product depends on the time

118required for inactivation of peroxidase and polyphenoloxi-

119dase enzymes.

120Numerous studies have been carried out for optimizing

121the operational parameters and design of blanching pro-

122cesses for different vegetables and fruits. The objectives

123of this article were to review (1) the purposes of blanching;

124(2) applied methods for evaluating blanching process; (3)

125the principles, application performance, and limitations of

126the existing thermal blanching technologies such as hot

127water, steam, microwave, and infrared blanching; and (4)

128research needs and future prospective of thermal

129blanching.

2 I n f o r m a t i o n P r o c e s s i n g i n A g r i c u l t u r e x x x ( 2 0 1 7 ) x x x –x x x



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130 2. The purposes of blanching

131 The purposes of blanching are shown in Fig. 1.

132 2.1. Inactivaction of quality-deterioration enzymes

133 Enzymatic reactions cause deterioration of fruits and vegeta-

134 bles during the transportation, storage and processing [5]. The

135 main purpose of blanching is to inactivate quality-changing

136 enzymes responsible for deterioration reactions that con-

137 tribute to off-flavors, odors, undesirable color and texture,

138 and breakdown of nutrients. Another purpose is to destruct

139 microorganisms contaminating produce. Therefore, stabiliza-

140 tion of texture and nutritional quality could be achieved dur-

141 ing processing and storage [6,7]. Kidmose and Martens [8]

142 reported that un-blanched frozen carrots had an off-taste

143 caused by the release of fatty acids due to esterases activity.

144 Ramesh et al. [9,10] observed that the carotenoid in blanched

145 red chili dramatically increased as compared to un-blanched

146 red chili.

147 2.2. Enhancing dehydration rates and product quality

148 The quality and drying rate of product depend not only on the

149 drying conditions, but also on other processes performed

150 before and after drying [11]. For some fruits such as plums

151 and grapes, a natural waxy layer covers fruit surfaces and

152 hinders moisture transfer during drying. Blanching increases

153 the drying and dehydration rates by changing physical prop-

154 erties of the products, which can improve their quality attri-

155 butes. The improvement in product quality resulted from

156 the increased permeability of cell membranes, which in turn

157 increases the rate of moisture removal [12].

158 Dev et al. [13] applied microwaves a pretreatment of grape

159 before drying, to replace the traditional chemical pretreat-

160 ments. Results indicated that the drying time of the micro-

161 waved grapes was reduced by 20% as compared to the un-

162 pretreated ones. Moreover, the total soluble solids of the

163samples treated by microwave were higher than those pre-

164treated with chemical solution. The traditional blanching

165methods such as hot water blanching or steam blanching

166can also increase the dehydration rate [14]. Rocha et al. [15]

167found that steam blanching significantly increased the drying

168rate of basil. Similarly, Ramesh et al. [10] observed that after

169steam blanching, the drying rate of pericarp increased due

170to higher cell wall destruction, resulting in a less resistance

171to moisture movement during drying. It was also observed

172that the effective diffusivity of moisture increased by more

173than two orders of magnitude due to steam blanching treat-

174ments [16].

175Compared to the samples dried directly without blanch-

176ing, Rocha et al. [15] and Singh et al. [17] found that blanching

177treatments resulted in better retention of chlorophyll in basil,

178marjoram and rosemary. Ramesh et al. [10] attributed the

179high quality of steam blanched products to the better reten-

180tion of vitamins due to the low oxygen atmosphere. Hossain

181et al. [18] observed a faster drying rate and higher color value

182in red chilli samples that have been blanched.

1832.3. Removing pesticide residues and toxic constituents

184Pesticides are commonly used for controlling wild grasses and

185diseases in farming to obtain a better crop yield. Pesticide

186residues could be found on fruits and vegetables that are

187semi-processed or consumed raw [19]. Residual pesticides in

188agricultural products threaten human health with toxic

189effects varying from mild diseases such as headaches and

190nausea to serious diseases like cancer. Therefore, removing

191pesticide residues in fruits and vegetables is vital for human

192health. Blanching plays an important role in the reduction

193of pesticide residues on vegetables and fruits. This reduction

194could be due to degradation of the toxic substance or washing

195and leaching of the toxins into the blanching water. Bon-

196nechere et al. [20] assessed the effects of washing, hot water

197blanching, microwave blanching and in-pack sterilization

198processing on the removal of five pesticide residues (deuterat-

Fig. 1 – The purposes of blanching.

I n f o r m a t i o n P r o c e s s i n g i n A g r i c u l t u r e x x x ( 2 0 1 7 ) x x x –x x x 3



20 February 2017


199 edethylenethiourea, ethylenethiourea, deltamethrin, 3,5-

200 dichloroaniline, boscalid) in spinach. Results showed that,

201 among various processing, hot water blanching was the most

202 effective way to remove the five pesticide residues by 10–70%,

203 while microwave blanching without water reduced pesticide

204 residues by a maximum of 39%, washing with tap water

205 reduced residues by 10–50%.

206 2.4. Expelling air entrapped inside plant tissues

207 Blanching can expel air entrapped inside plant tissues, espe-

208 cially intercellular gas. This is a vital step prior to canning

209 because blanching can prevent the expansion of air during

210 processing, as well as reduce strain on the containers and

211 the risk of misshapen cans and faulty seams. Furthermore,

212 removing the gas from blanched pear tissues resulted in bet-

213 ter texture as well as softer and more transparent tissues [21].

214 In addition, removing oxygen from the tissue reduces oxida-

215 tion of the product and corrosion of the materials used for

216 cans manufacturing.

217 2.5. Minimizing non-enzymatic browning reactions

218 Non-enzymatic browning, especially Maillard reaction or

219 caramelization, occurs in food during frying, cooking, drying,

220 and storage. This reaction could lead to the loss of product

221 color. Maillard reaction and/or caramelization browning reac-

222 tion depends on the reducing sugar content of the products

223 [22]. Therefore, decreasing the reducing sugar content in a

224 product by blanching can reduce browning and improve pro-

225 duct color. Pimpaporn et al. [23] found that hot water blanch-

226 ing pretreatment had a more significant effect on reducing

227 the red color of the potato chips than the pretreatments using

228 freezing and the immersion in monoglyceride or glycerol.

229 2.6. Decreasing microbial load

230 Microorganisms contaminate foods causing food spoilage and

231 poisoning. Therefore, inactivation or inhibition of microbial

232 growth is essential to assure safe and disease risk free foods.

233 Microbial inactivation can be achieved using thermal tech-

234 nologies such as microwave, radio frequency treatment,

235 ohmic heating, or non-thermal technologies such as high

236 pressure, ozone, ultraviolet light (UV), gamma or X-ray irradi-

237 ation, chlorine or iodine solutions, ultrasound, and pulsed

238 electric fields. Conventional peroxidase (POD) and polyphenol

239 oxidase (PPO) enzymes inactivation and microbial inactiva-

240 tion are two separate processes and have drawbacks of low

241 energy efficiency and long processing time. Recently, thermal

242 decontaminated food products are safer for consumers than

243 chemically and irradiated ones. Thermal blanching of some

244 products can simultaneously achieve inactivation of both

245 enzymes and microorganisms. This could avoid cross-

246 contamination or re-contamination, increase energy effi-

247 ciency, and reduce processing time.

248 De La Vega-Miranda et al. [24] found that microwave

249 blanching of the fresh jalapeno peppers and coriander foliage

250 could achieve a 4–5 log reduction in Salmonella typhimurium.

251 Jabbar et al. [25] found a significant decrease in yeast and

252 mold grown on carrot after blanching with combined hot

253 water and ultrasound treatment.

2542.7. Peeling of products

255Fruits and vegetables peeling is an important operation in

256food processing. Peeling is sometimes performed manually

257for some products such as tomato, potato and peanut. How-

258ever, manual peeling is tedious, laborious, time consuming

259and subject to human error and inconsistency. Therefore,

260thermal, mechanical and chemical peeling methods are often

261applied. Although it is highly automated and efficient,

262mechanical peeling often causes higher peeling loss due to

263the difficulties in controlling peeling depth for varying pro-

264duct shapes and sizes. Moreover, chemical peeling methods

265have health and safety considerations and produce chemical

266and organic contaminated wastewater that is always costly to

267treat and dispose. Therefore, they are restricted in some

268countries. Steam peeling, on the other hand, has less environ-

269mental pollution and low peeling losses. Garrote et al. [26]

270applied steam blanching to peeling potatoes and asparagus.

271Results showed that steam peeling of asparagus followed by

272an adiabatic holding time after steam exhausting and before

273water cooling could sufficiently inactivate peroxidase with a

274peeling time of 20 s and one cycle; for potato, at a peeling time

275of 36 s was a good peeling quality obtained at one or two

276cycles, the yield was approximately 90% with three cycles.

277Yu et al. [27] removed the pink-red skin of peanut by boiling

278water blanching for 2 min.

2792.8. Increasing extraction efficiency of bioactive280compounds

281Thermal blanching can cause structural changes in plant tis-

282sues such as disruption of cell membranes, loosening of the

283hemi-cellulose, cellulose and pectin networks, and alternat-

284ing cell wall porosity. These can improve the extraction of

285bioactive compounds [28].

286Gliszczynska-Swiglo et al. [29] found that, after 10 min

287steam blanching, the total polyphenol content extracted from

288broccoli increased by 52% compared with untreated samples.

289The authors attributed this phenomenon to thermal disrup-

290tion of the polyphenol–protein complexes. Stamatopoulos

291et al. [30] observed that after 10 min of steam blanching, the

292extraction yield of oleuropein from olive leaves increased

293from 25- to 35-fold compared to the un-blanched sample.

294Moreover, the antioxidant activity increased from 4 to 13

295times. Although the effect of hot water blanching was not

296as great as steam blanching due to a leaching effect, it was

297also found that hot water blanching significantly increased

298oleuropein yields and antioxidant activity when compared

299with un-blanched ones. Similarly, Hiranvarachat et al. [31]

300found that the contents of b-carotene, total carotenoids, and

301antioxidant activities of blanched carrots were significantly

302higher than those of the un-blanched samples.

3032.9. Other purposes of blanching

304Blanching can also clean the surface of plants, kill parasites

305and its eggs, remove damaged or discolored seeds, foreign

306material and dust of fruits and vegetables. Blanching of pota-

307toes chips prior to frying can reduce the oil uptake because




20 February 2017


308 blanching gelatinizes the surface starch and forms a compact

309 appearance with less pores and air cells [32].

310 3. Assessment of the effectiveness of311 blanching process

312 3.1. Activity of peroxidase (POD) and polyphenol oxidase313 (PPO) enzymes

314 The effectiveness of blanching is usually judged by the inacti-

315 vation degree of peroxidase (POD) and polyphenol oxidase

316 (PPO) enzymes because they are easily measured compared

317 to other enzymes. The POD is a heme-containing enzyme that

318 commonly found in plant. It can catalyze a large number of

319 reactions that are closely associated with quality deteriora-

320 tion in raw and un-blanched products [3]. POD enzyme can

321 be combined with endogenous hydrogen peroxide to produce

322 free radicals that react with a wide range of food constituents

323 including ascorbic acid, carotenoids and fatty acids. This can

324 cause undesirable changes in products, such as color and fla-

325 vor loss, as well as nutrients degradation [33–35]. POD is the

326 most heat stable enzyme within the enzyme group responsi-

327 ble for quality deterioration during processing and storage of

328 fruits and vegetables [2,7]. It is well documented that the

329 destruction of POD assures the inactivation of other enzymes

330 responsible for the deterioration of food quality [36]. Polyphe-

331 nol oxidase (PPO) is another enzyme commonly used as an

332 indicator for the effectiveness of blanching process. PPO is

333 present in nearly all plant tissues, and can also be found in

334 fungi, bacteria, and insects [37,38]. Containing four atoms of

335 copper per molecule and binding site for two aromatic com-

336 pounds and oxygen, PPO can catalyze the O-hydroxylation

337 of O-monophenols to O-diphenols and produce O-quinones

338 (a kind of substance with black, brown, or red). The latter is

339 responsible for fruit and vegetable browning reactions that

340 causes undesirable quality changes [1,39]. POD is the most

341 heat-resistant enzyme and requires a long-time blanching

342 for complete inactivation (i.e., over blanching). This could

343 cause heavy loss of nutrients and increase the cost of energy

344 [40]. A comparison of the inactivation kinetics of POD and PPO

345 in potato during blanching is shown in Fig. 2. On the other

346 hand, research demonstrated that the quality of blanched

347 and frozen product is better if there is some POD activity left

348 after the blanching [41]. It was suggested that optimal blanch-

349 ing should attain 3–10% as a residual of peroxidase activity.

350 These activity residuals were sufficient to prevent any deteri-

351 oration in fruits and vegetables [41–43].

352 3.2. Ascorbic acid as an indicator to evaluate nutrients353 loss during blanching

354 Thermal blanching has negative effects on heat sensitive

355 nutrient contents, texture, and color of products. Therefore,

356 it is essential to correlate the adequate enzymatic inactiva-

357 tion by the thermal blanching and nutrients loss, undesirable

358 color changes, and texture degradation of the products.

359 Ascorbic acid is an important substance found in almost all

360 fruits and vegetables. It does not only prevent diseases such

361 as scurvy, lung, bladder, and prostate cancers, but can also

362be used as a biological antioxidant to delay the aging process

363[44,45]. In addition, ascorbic acid can combine with other

364antioxidants, including vitamin E, b-carotene, and selenium,

365to provide a synergistic antihypertensive effect [46]. Ascorbic

366acid is water soluble that makes it prone for leaching from

367cells. It is thermally labile, pH-, metal- ion-, and light-

368sensitive, and can be degraded by ascorbic acid oxidase

369[3,47]. Therefore, ascorbic acid is usually selected as the most

370frequently measured nutrient to evaluate the nutrients loss

371during blanching process. The preservation of ascorbic acid

372after blanching is a good indicator for the preservation of

373other nutrients [48,49].

374While the main mechanisms of ascorbic acid loss during

375steam, infrared, or microwave blanching could be enzymatic

376oxidation and thermal degradation, the main mechanism of

377ascorbic acid losses during hot water blanching is leaching

378or diffusion from the plant to the blanching water [6,50,51].

379The loss of ascorbic acid during hot water blanching strongly

380depends on the blanching temperature and time. Aguero

381et al. [52] found that at high temperature and short time

382resulted in higher ascorbic acid retention. Ramesh et al.

383[53] found that the vitamin C retention was significantly

384higher in microwave-blanched spinach, bell pepper, and car-

385rots than those blanched with hot water. This was due to the

386low leaching losses of vitamin C in the microwave

387blanching.

3883.3. Color as an indicator of product quality change during389blanching

390Color is one of the most important appearance attributes.

391Undesirable changes in color of food may lead to a decrease

392in consumer’s acceptance and market value [54,55]. The color

393of raw materials or final products can be associated with

394other quality attributes, such as freshness, sensory, nutri-

395tional, visual, and non-visual defects. It also has a good corre-

396lation with the antioxidant abilities, oxidation and Maillard

397reactions, and controls them indirectly [56–59]. The color

398intensity was considered as a reliable indicator of high nutri-

399tional value of carrots during hot water blanching [42]. Color

400is often used as an indicator to evaluate severity of the heat

401treatment and to predict the corresponding quality degrada-

402tion caused by blanching process.

403Krokida et al. [32] studied the effect of sulfite pretreatment,

404water and steam blanching pretreatment on the color of

405dehydrated apples, bananas, potatoes, and carrots. Non-

406pretreated dried materials showed extensive browning, indi-

407cated by a significant drop in the lightness of color and an

408increase in redness and yellowness. Sulfite pretreatment pre-

409vented significant color deterioration, while water and steam

410blanching prevented enzymatic browning during convective

411drying. Color can be used as a critical parameter to optimize

412carrot quality attributes during hot water blanching [42]. Xiao

413et al. [60] studied the effect of different superheated steam

414blanching time on color preservation of yam slices. When

415the blanching time was increased from 0 to 9 min, the white-

416ness index of dried yam slices increased from about 44 to 71,

417then dropped to 61 as the blanching time was prolonged to

41811 min.




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419 3.4. Texture as indicator of the effect of blanching on420 product physical properties

421 Product texture is a primary indicator of product quality for

422 consumers [54,61]. The texture of food determines the phy-

423 sic–chemical characteristics of the cell wall, and it indicates

424 how they change during processing [42]. In general, thermal

425 blanching significantly reduces the final textural properties

426 of the cell structure of fruits and vegetables. The softening

427 of the final textural properties of the product is due to both

428 turgor loss caused by cell membrane disruption and changes

429 in cell wall polymers, especially the pectic substances [62].

430 Greve et al. [62] observed that the tissue firmness of carrot

431 was quickly lost during the first few minutes when carrot was

432 blanched at 90 �C, it mainly due to the loss of cellular turgor

433 and cell wall integrity during hot water blanching. Song

434 et al. [63] investigated the effect of three hot water blanching

435 conditions (80 �C for 30 min, 90 �C for 20 min, and 100 �C for

436 10 min) on the color, texture, nutrient content, and sensory

437 value of soybeans. It was found that the hardness of the sam-

438 ple was decreased from 468.9 to 283.8 g (breaking force) as the

439 blanching temperature increased from 80 to 100 �C. The

440 increase in softness of soybeans during blanching was proba-

441 bly due to the gelatinization of starch granules and the forma-

442 tion of soluble pectic substances. Sila et al. [64] found that

443 increasing blanching time enhanced the softening of carrots.

444 They attributed the softening effect to pectin solubility prop-

445 erties and the accompanying depolymerisation mechanisms.

446 Fraeye et al. [65] found that thermal blanching caused a

447 strong decrease in firmness and major tissue disruption of

448 strawberries. Goncalves et al. [42] examined the texture

449 change kinetics of carrot slices during hot water blanching.

450 They found that the firmness of carrot rapidly decreased with

451the increase in blanching time (10–15 min) and temperature

452(75–90 �C) until it was at the residual texture level.

4534. The traditional hot water blanching454technology

4554.1. Hot water blanching processing and its application

456Hot water blanching is the most popular and commercially

457adopted blanching method, as it is simple to establish and

458easy to operate [4]. In a typical hot water blanching, products

459are immersed in hot water (70 to 100 �C) for several minutes.

460Then blanched samples are drained and cooled before being

461sent to the next processing operation. In general, after a cer-

462tain amount of blanching time, the blanching water needs to

463be replenished as it becomes saturatedwith nutrients leached

464from the products. This step does not only consume high

465amounts of water and energy [66]. In order to preserve the

466color of product and inactivate microbial activity, sodium sul-

467fite and sodium metabisulfite are often added to the blanch-

468ing water. This makes it more difficult to deal with the

469wastewater generated from the blanching operation.

4704.1.1. Pepper471In order to produce high quality paprika and chili powders,

472immediate and complete inactivation of endogenous

473enzymes is a necessary prerequisite. Under humid condi-

474tions, the deteriorative enzymes such as POD, PPO, and

475lipoxygenase (LOX) can negatively affect taste, pungency,

476color intensity, and color stability during long-term storage.

477Schweiggert et al. [67] determined residual activities of POD,

478PPO, and LOX in paprika and chili powder after immediate

479hot water and steam blanching. Chili was blanched using

Fig. 2 – The comparation of inactivation kinetics of POD and PPO in potato during blanching (from Sotome et al. [136] with

some changes).




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480 hot water and steam at 80 �C for 10 min, 90 �C for 5 and

481 10 min, or 100 �C for 5 and 10 min. Paprika pods were

482 blanched at 90 �C for 1 and 5 min in water and at 100 �C for

483 5 min in water and steam, respectively. It was found that

484 POD activities decreased by approximately 98% in chili and

485 paprika powder, while PPO showed the lowest heat stability

486 and was completely inactivated by heating at 80 �C for

487 10 min. It was observed that LOX inactivation was also largely

488 accomplished by blanching at 90 �C for 5 min and 100 �C for

489 5 min [67].

490 4.1.2. Brussels sprouts491 Lisiewska et al. [68] evaluated the hot water blanching of

492 Brussels sprouts at 96–98 �C hot water for 5 min. After blanch-

493 ing, samples were cooled in cold water and left to drip on

494 sieves for 30 min. The total amino acids decreased from

495 2783 mg/100 g in fresh samples to 2345 mg/100 g in the

496 blanched samples. There was less of a decrease in amino

497 acids caused by hot water blanching of Brussels sprouts when

498 compared to the cassava leaves and broccoli [69]. The differ-

499 ences can be attributed to hot water blanching conditions

500 such as the ratio of material to water, blanching time, temper-

501 ature, and product properties [68].

502 4.1.3. Almond503 Harris et al. [70] studied the effect of hot water blanching for

504 12 min at different temperatures (60, 70, 80, and 88 �C) on the

505 removal of the pellicle from the almond kernels. They also

506 evaluated the survival of Salmonella Enteritidis PT 30, Sal-

507 monella Senftenberg 775 W and Enterococcus faecalis on whole

508 almond kernels before and after hot water blanching. The ini-

509 tial microorganism load on almonds was5 log CFU/g. It was

510 observed that neither Salmonella serovar could be recovered

511 after blanching at 88 �C for 2 min. Currently, in almond indus-

512 try, the almonds are submerged in hot water (85–100 �C) for 2513 to 5 min [71]. Therefore, these findings provided more data

514 and information to validate almond industry blanching

515 processes.

516 4.1.4. Potato chips517 Potato chips are often produced by deep-frying that resulted

518 in final products with an oil content of up to 45% (w.b.) [72].

519 A high fat and caloric diet can cause serious health diseases,

520 especially cardiovascular disease. In addition, a high oil con-

521 tent not only increases the production cost, but also often

522 makes the chips greasy or oily. Therefore, alternative tech-

523 nologies are needed to produce potato chips with reduced

524 oil content and desired color and texture.

525 Pimpaporn et al. [23] studied the influence of various pre-

526 treatment methods on the low-pressure superheated steam

527 drying kinetics and quality of dried potato chips. It was

528 observed that combining hot water blanching with freezing

529 was the most suitable methods of pretreatment for producing

530 good quality potato chips. Furthermore, Kingcam et al. [73]

531 studied the effect of three pretreatments (hot water blanching

532 and then freezing for 24 h, hot water blanching and then

533 repeated freezing/thawing either for 3 or 5 cycles) on the

534 degree of starch retrogradation. The pre-treated sampleswere

535 then dried through low-pressure superheated steam drying,

536 and the effects of three pretreatments on the degree of

537pcrystallinity of dried potato chips were studied. This investi-

538gation found that an increase in the degree of starch retrogra-

539dation led to higher degree of crystallinity of dried potato

540chips.

5414.1.5. Carrot542The food-borne illness outbreaks have increased in recent

543years due to the consumption of raw or processed products

544polluted by microorganisms, such as Salmonella in fresh veg-

545etables and fruits, and Listeria monocytogenesin ready to eat

546meat [74]. No detection strategy can guarantee food safety,

547so in order to best protect consumers multiple prevention

548efforts should be enhanced [75]. Reducing microbial load dur-

549ing food processing operation is a critical step for food safety.

550Dipersio et al. [76] evaluated the effect of different blanching

551methods on the inactivating of Salmonella during prepara-

552tion, home-type (60 �C, 6 h) dehydration and storage of carrot

553slices. The studied methods were namely steam blanching

554(88 �C, 10 min), hot water blanching (88 �C, 4 min), hot water

555blanching (88 �C, 4 min) combined with 0.105% or 0.21% citric

556acid solution. It was observed that bacterial populations were

557reduced by 3.8–4.1, 4.6–5.1 and 4.2–4.6 log cfu/g immediately

558following steam, hot water, and hot water combined with

559citric acid blanching, respectively. Additionally, after drying

560for 6 h, the total reductions were 4.0–5.0 log cfu/g after steam

561blanching, 4.1–4.6 log cfu/g after hot water blanching, and

5624.9–5.4 log cfu/g after hot water combined with citric acid

563blanching [76]. Hot water blanching at 88 �C for 4 min com-

564bined with 0.21% citric acid blanching was proposed as the

565best pretreatment method for inactivating Salmonella.

5664.1.6. Others567Bureau et al. [77] explored the effects of boiling water, steam-

568ing, high pressure, and microwave pretreatment on quality of

56913 vegetables including green bean, pea, brussels sprout, leek

570(slices), broccoli, zucchini (slices), spinach branch, hashed

571spinach, yellow French bean, cauliflower, mushroom, carrot

572(slices). It was found that boiling water cooking resulted a

573higher loss of total ascorbic acid loss (average of �51% on

574fresh matter) than other three.

5754.2. Limitations of hot water blanching

5764.2.1. Losses of nutrients during blanching577The loss of nutrients during hot water blanching is caused

578mainly by leaching or diffusion [4]. All water-soluble nutri-

579ents, such as vitamins, flavors, minerals, carbohydrates, sug-

580ars, and proteins, can leach out from plant tissues to the

581blanching water. In addition, hot water blanching can also

582lead to degradation of some thermal sensitive substances

583such as ascorbic acid, aroma and flavor compounds. It was

584found that about 8% of tissues and 3% of total solids were lost

585after hot water blanching of carrots for 10 min at 70 �C [78].

586Mukherjee and Chattopadhyay [4] observed that more than

58710% of solid was lost after 129 s of hot water blanching of

588potato at 100 �C. Haase and Weber [79] investigated the effect

589of cutting, hot water blanching, par-frying and freezing, and

590final frying step, on the loss of ascorbic acid in French fries.

591Ascorbic acid content decreased from 94.6 to 69.7 mg/100 g

592dry matter. The reduction of ascorbic acid during blanching




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593 was also reported in broccoli and cauliflower [80]. Ismail et al.

594 [81] observed 20% loss of total phenolic content in cabbage

595 after 1 min of blanching in boiling water. The degradation of

596 total phenolic compounds during blanching (100 �C, 1 min)

597 of swap cabbage, spinach, shallots and kale was 26%, 14%,

598 13% and 12%, respectively [82]. Gawlik-Dziki [83] demon-

599 strated that boiling water treatment significantly reduced

600 the polyphenol content of fresh broccoli. Similarly, Sikora

601 et al. [84] reported a significant decrease in total polyphenol

602 and antioxidant components in thermal water processed

603 broccoli.

604 Garrote et al. [50] reported that the loss of ascorbic acid

605 during hot water blanching was entirely a diffusion-

606 controlled phenomenon. The apparent diffusion coefficient

607 of ascorbic acid in potato tissues increases as the blanching

608 temperature increased. Lin et al. [48] performed hot water

609 blanching (90 �C, 7 min) prior to the drying of carrot slices to

610 inactivate ascorbic acid oxidase and prevent its enzymatic

611 degradation in the subsequent processes. The results indi-

612 cated that a substantial loss of vitamin C content from 770

613 to 443 lg/g solid occurred, probably due to leaching, during

614 the blanching.

615 The leaching or diffusion of ascorbic acid in hot water

616 blanching process can be positively influenced by the solid

617 content of the water; therefore, the recycled water with a high

618 content will lead to less loss [6]. This assertion has been con-

619 firmed by Arroqui et al. [84], who observed that the retention

620 of ascorbic acid was higher when potatoes were blanched in

621 recycled hot water than when they were blanched in distilled

622 water.

623 4.2.2. Wastewater from blanching624 The discharged wastewater from hot water blanching contain

625 high concentrations of biochemical, soluble solids, and chem-

626 ical oxygen demand due to leaching and dissolution of sug-

627 ars, proteins, carbohydrates and water-soluble minerals.

628 This wastewater can cause environmental pollution, e.g.

629 eutrophication [85], if not well treated before discharge. Hot

630 water blanching is a water-intensive industry. To alleviate

631 the problems of the traditional hot water blanching method,

632 new energy efficient blanching technologies are being devel-

633 oped and applied.

634 5. The emerging and innovative blanching635 technologies

636 New blanching technologies with higher energy efficiency,

637 less nutrient loss and less environmental impacts are being

638 developed and applied. The principles, characteristics, the

639 current status of application, and the challenges or limita-

640 tions of several emerging blanching technologies are identi-

641 fied and discussed in the following sections. The emerging

642 and innovative blanching technologies include high-

643 humidity hot air impingement blanching (HHAIB), micro-

644 wave, ohmic, and infrared combined with hot air blanching.

645 The principles, characteristics, current status of application,

646 and challenges or limitations of these emerging blanching

647 technologies are identified and discussed in the following

648 sections.

6495.1. Steam blanching and high-humidity hot air650impingement blanching (HHAIB)

6515.1.1. The principle of steam blanching652Superheated steam is commonly used as a heating media for

653blanching due to its high enthalpy contents. During the early

654stage of steam blanching, it condenses on the surface of the

655products and a large amount of latent heat transfers to the

656material because product temperature is lower than that of

657steam. The temperature of the products gradually increases

658until reaching the critical temperature of enzymes or organ-

659isms activity, after which they are inactivated.

660It is believed that the steam blanching is relatively inex-

661pensive and retains most minerals and water-soluble compo-

662nents when compared with water blanching due to the

663negligible leaching effects [86]. On the other hand, during

664the steam blanching process, softening of the tissue and

665undesirable quality changes often resulted a long heating

666time due to the lower heat transfer in steam blanching than

667hot water blanching, especially when the velocity of the

668steam is very low.

6695.1.2. Applications of steam blanching6705.1.2.1. Spinach leaves. Teng and Chen [87] found that the

671application of boiling water and microwave blanching on spi-

672nach, which is then followed by steam and baking blanching,

673resulted in the highest degradation rate of both chlorophylls a

674and b. In addition, pyrochlorophylls a and b were detected in

675spinach leaves after being steam blanched for 30 min or

676microwave (at 700 Wand 2450 MHz) blanched for 1 min. How-

677ever, the authors found that pyropheophytins a and b were

678not formed until steam or microwave blanching took place

679for 30 or 5 min, respectively. The authors concluded that

680steam blanching favors the formation of pyropheophytins,

681whereas microwave blanching favors the formation of

682pyrochlorophylls.

6835.1.2.2. Kiwifruit. To optimize blanching process, Llano

684et al. [88] studied the effect of steam blanching on mechanical

685and biochemical properties of kiwifruit. The changes in the

686microstructure during the blanching were determined by

687transmission electron microscopy (TEM) and fluorescence

688microscopy (FM). It was found that after 5 min of blanching,

689tissue became yellow–brown probably due to chlorophyll

690degradation and loss in vitality of the cell membrane in the

691outer pericarp tissue. TEM analysis has also indicated that

692plasmodesmatal areas have lost the stain intensity when

693compared with raw tissue. The firmness of kiwifruit

694decreased with the increase of blanching time and decrease

695in residual tissue elasticity, which coincided with membrane

696damage.

6975.1.2.3. Potato. Sotome et al. [89] compared the effects of

698hot water blanching (HWB), superheated steam blanching

699(SHS), and superheated steam combined with spraying of

700hot water microdroplets blanching (SHS +WMD) on the color,

701texture, and microstructure of potato. The potato blanched in

702hot water became soft and brittle, and its brightness and

703chromatic quality decreased due to the absorption of water

704and dissolution of solid content to the water. On the contrary,




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705 these quality degradation was hindered using SHS and SHS

706 +WMD blanching. Furthermore, while the weight of potato

707 was kept almost constant during the SHS +WMD blanching,

708 it was 3.3% after SHS blanching for 16 min. SHS +WMD

709 blanching also significantly reducedwater loss during blanch-

710 ing when compared to SHS blanching.

711 Liu and Scanlon [90] blanched potato strips in a steam-

712 heated kettle at temperatures ranging from 62.8 to 90.6 �C713 for 2–20 min. Results showed that at low temperatures (<74 �714 C), blanching time had little effect on the texture of blanched

715 strips, while at high temperatures (�74 �C), the texture soft-

716 ened as blanching time increased [91]. In order to provide

717 information to operators to manipulate the blanching pro-

718 cess, a quantitative description model of the texture changes

719 during steam blanching operation was developed.

720 5.1.2.4. Mango slices. Ndiaye et al. [91] studied the effect of

721 saturated steam blanching of mango slices (1 cm thick) at 94

722 ± 1 �C for 0, 1, 3, 5, and 7 min on the color and the activation of

723 PPO and POD enzymes. They found that PPO and POD were

724 completely inactivated after 5 and 7 min of steam blanching,

725 respectively. If the blanching time exceeded 5 min, color loss

726 became more serious.

727 5.1.2.5. Garlic slices. Peeled garlic suffers undesirable

728 changes in quality, such as rapid browning, due to PPO and

729 POD, which can be inactivated using thermal blanching. For

730 garlic slices, Fante and Norena [92] investigated the effects

731 of hot water blanching at 80 and 90 �C and steam blanching

732 at a temperature of 100 �C on the inactivation kinetics of

733 PPO, POD, and inulinase, as well as the color. Results showed

734 that blanching in steam for 4 min was the best treatment that

735 achieved no changes in texture and reduced the enzymatic

736 activities of POD, PPO, and inulinase by 93.53%, 92.15% and

737 81.96%, respectively. Prolonged hot water blanching could

738 lead to serious undesirable changes in the products such as

739 color degradation, nutrients and texture loss.

740 5.1.2.6. Fresh broccoli. Roy et al. [86] investigated the effect

741 of steam blanching on the total antioxidant activity of fresh

742 broccoli by determining oxygen radical capacity (ORAC) and

743 the reactive oxygen species (ROS). It was found that the steam

744 blanching increased the total ORAC value by 2.3-fold. The

745 hydrophilic part of a steam blanched broccoli had a signifi-

746 cant reduction of 2,2-azobis [2-amidinopropane] dihydrochlo-

747 ride (AAPH) induced intracellular ROS level when compared

748 to that of the fresh samples. Furthermore, the total phenolic

749 content and total flavonoid content increased after steam

750 blanching.

751 5.1.2.7. Blueberries. Rossi et al. [93] evaluated the effect of

752 steam blanching on the inactivation of PPO before milling of

753 blueberry fruits, as well as the recovery of total and individual

754 anthocyanins and total cinnamates that are important radical

755 scavengers of blueberry juices. It was observed that the steam

756 blanching resulted in a significant increase in the recovery of

757 anthocyanins in blueberry juice. Additionally, the juice pro-

758 duced from blanched fruits was bluer and less red than that

759 obtained from un-blanched fruits. The authors attributed this

760 phenomenon to the positive effect of thermal blanching on

761the extraction of the most soluble anthocyanin pigments,

762which are the most intense blue.

763Brambilla et al. [94] studied the effect of steam blanching

764on the quality attributes of frozen blueberry purees in terms

765of color, monomeric anthocyanin pigments (MAP), and total

766phenolic compounds (TPC). The steam blanching increased

767MAP and TPC contents by 11.3% and 51.6%), respectively as

768compared to the un-blanched samples.

7695.1.2.8. Vegetable soybean. Saldivar et al. [95] compared

770steam hot water blanching at 100 �C for 10 min of shelled

771green soybean seeds in order to identify the proper technol-

772ogy that preserves its sugar content. Steam blanching pre-

773served soluble sugars in both green pods and seeds. Soluble

774sugars decreased in soybean seeds during water blanching

775due to leaching. The presence of pods effectively prevented

776the leaching of sugars in water blanching.

7775.1.2.9. Cabbage. Drying does not completely destroy

778microorganisms contaminating vegetables and fruits. A pre-

779treatment is always needed prior to drying to ensure the

780deactivation of microorganisms, especially pathogens such

781as Salmonella [96]. Phungamngoen et al. [96] pre-treated cab-

782bage before hot air drying, vacuum drying (10 kPa) or low-

783pressure superheated steam drying (10 kPa) at 60 �C. Cabbage784sampleswere pre-treated either by soaking in 0.5% (v/v) acetic

785acid for 5 min, blanching in hot water for 4 min, or blanching

786in saturated steam for 2 min. They found that the Salmonella

787load decreased from the initial level of 6.4 log cfu/g to 1.6, 3.8,

788and 3.6 log cfu/g after being pre-treated by soaking in acetic

789acid, hot water blanching, and steam blanching, respectively.

790It was postulated that heat accumulated during thermal

791blanching might damage cell membranes and cause protein

792denaturation of bacterial cells.

793The production of value-added functional dietary fibre (DF)

794from white cabbage was proposed because the high concen-

795tration of dietary fibre and glucosinolates. The production of

796DF from cabbage leaves involves thermal blanching and dry-

797ing processes. Tanongkankit et al. [97] found that steam

798blanching better preserved glucosinolates than hot water

799blanching. Steam blanching of the outer cabbage leaves prior

800to slicing in combination with vacuum drying at 80 �C was the

801most favourite processing step for the production of DF.

8025.1.3. Limitations of steam blanching803Steam blanching carried out in thick layers on moving

804belts, often resulted in non-uniform blanching effects

805[4,98]. It needs longer blanching time than hot water and

806therefore it affect the capacity and the economics of pro-

807cessing. Selman [78] found that hot water blanching of car-

808rots achieved higher degree of POD inactivation than steam

809blanching. Although steam blanching avoids the leaching of

810nutrients in the blanching medium, it sometimes could

811cause weight loss and the formation of a dried layer on

812product surface due to evaporation of water. Sotome

813et al. [89] found that employing a hot water spray system

814on blanching of potato achieved a constant weight as com-

815pared to superheated steam blanching, while dipping the

816sample in water before steam blanching reduced water loss

817during steam blanching.




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818 Recently, new blanching techniques such as high-

819 humidity hot air impingement blanching (HHAIB) technology

820 have been developed. In the HHAIB, advantages of steam and

821 impingement technology are combined, resulting in a uni-

822 form, rapid, wastewater free, and efficient processing [99].

823 Compared to traditional hot water blanching, HHAIB can

824 extensively reduce loss of water-soluble nutrients. Moreover,

825 HHAIB is more efficient than traditional superheated steam

826 blanching because it has high heat transfer rates [99]. For

827 these advantages, HHAIB was used to blanch yam slices to

828 prevent browning and to maintain color [61]. HHAIB was

829 applied to increase drying rates of grape [100], obtain desired

830 color and texture in sweet potato bar [44], denature the auto-

831 lyze enzyme in sea cucumber blanching [101], reduce micro-

832 bial load in fresh-cut lettuce and poultry products [102,103],

833 and inactivate polyphenol oxidase in apple quarters [104].

834 HHAIB was appliedto blanch of red peppers, the results

835 showed that HHAIB pre-treatment effectively denatured PPO

836 and increased drying rate of pepper [105]. Xiao et al. [99] also

837 presented a comprehensive review on HHAIB.

838 5.2. Microwave blanching

839 5.2.1. The operation principle and advantages of microwave840 heating841 Microwaves are electromagnetic waves with wavelengths

842 ranging from 1 mm to 1 m that have corresponding frequen-

843 cies ranging from 300 MHz to 300 GHz [106]. Microwaves have

844 many uses in modern society including communication,

845 radar, radio astronomy, navigation, and food processing. For

846 industrial, scientific and medical (ISM) heating applications,

847 only 915 MHz and 2450 MHz microwaves are allowed because

848 the Federal Communications Commission (FCC) of USAwants

849 to prevent those devices from interfering with communica-

850 tion signals.

851 In microwave heating, heated materials absorb microwave

852 energy and convert it into heat by dielectric heating effect

853 caused by molecular dipole rotation and agitation of charged

854 ions within a high-frequency alternating electric field [107].

855 Specifically, when the oscillating electric field interacts with

856 high water content materials, the permanently polarized-

857 dipolar molecules particularly water molecules will align

858 themselves in the direction of the electromagnetic field alter-

859 nates at 915 or 2450 MHz [106]. The internal resistance due

860 rotating molecules that push, pull, and collide with other

861 adjacent molecules or atoms, produces volumetric heating

862 [108]. Agitation of charged ions in the alternating electrical

863 field also contributes to microwave heating, more so at

864 915 MHz than 2450 MHz. Microwave heating not only takes

865 place on the surface of wet biological materials, but also

866 within them. In conventional thermal processing, energy is

867 transferred by conduction from the product surface to the

868 inner part. This depends mainly on temperature gradient

869 and the thermal conductivity of the product.

870 Compared to conventional heating methods applied in the

871 food industry, microwave heating has several advantages

872 such as volumetric heating, high heating rates and short pro-

873 cessing times. Therefore, it has been successfully used in dry-

874 ing, pasteurization, blanching, thawing, tempering, baking,

875 etc. [10,100,109]. One of the most important features of

876microwave blanching is that it involves direct interaction

877between the electromagnetic field and food materials for

878heating generation. Thus, compared to that in conventional

879hot water blanching, the amount of nutrients loss by leaching

880is significantly reduced [8,53,110]. For example, the ascorbic

881acid retention was found to be higher in green beans, peas,

882and carrots blanched by microwave than those blanched by

883hot water [41]. In addition, microwave heating is rapid, very

884energy efficient, easy to install and clean-up, and requires a

885short start-up time, etc. [111].

8865.2.2. Applications of microwave blanching8875.2.2.1. Carrot slices. Kidmose and Martens [8] compared the

888influence of microwave blanching with that of steam and

889hot water blanching on dry matter losses and quality attri-

890butes of carrot slices in terms of texture, microstructure, sug-

891ars and carotene contents and drip losses. The microwave

892blanching was performed using a continuous conveyer micro-

893wave oven with 4 magnetrons (power of 1.25 kW for each

894magnetron) at 2450 MHz and a conveyer speed of 0.5 m/min.

895Steam blanching was carried out by a steamer at 90 �C for

8963 min; the water blanching was performed in a jacket vessel

897at 90 �C for 4 min. No significant difference was found in the

898carrots texture after the three blanching methods. However,

899the microwave-blanched sample had a significantly different

900appearance from those blanched by steam or hot water. The

901microwave-blanched samples had a texture composed of a

902patchwork of groups of well-preserved cells, layers of col-

903lapsed and sunken cells. This was believed to be caused by

904high internal vapor pressure when water was converted into

905steam during the microwave blanching process. The dry mat-

906ter, sucrose, and carotene of the samples blanched by micro-

907wave were significantly higher than those of the steam

908blanched samples. Hot water blanching yielded samples with

909the least amount of these substances. In conclusion, although

910the microwave blanching did not improve the texture of pro-

911duct when compared to steam and hot water blanching

912method, it enhanced the nutritional quality.

913Mild blanching conditions aremore appropriate in mitigat-

914ing the negative effect of microwave on the texture and

915microstructure of the products. Lemmens et al. [112], con-

916firmed the findings with the blanching of carrots using strong

917microwave blanching (90 �C, 1 min) and mild microwave

918blanching (60 �C, 40 min).They found that the microstructure

919of the samples before and after blanching (as shown in Fig. 3)

920illustrated that the raw carrots have an intact cell structure

921with well-defined and well-organized individual cells. The

922microstructure of samples blanched in mild microwave was

923more similar to the fresh ones as compared to the samples

924blanched in strong microwave, which caused the cell wells

925to disappear and different cells to melt together [112].

9265.2.2.2. Mushroom. The shelf life of minimally processed

927mushroom is limited to a few days due to the enzymatic

928browning during storage. Inactivation of enzymes that cause

929browning such as PPO through thermal blanching, application

930of antioxidants, or enzyme inhibitors is essential to prevent

931enzymatic browning. Microwave blanching has been explored

932as an alternative method for industrial blanching of mush-

933rooms. Direct application of microwave energy to an entire




20 February 2017


934 mushroom was found to unsuitable because the large tem-

935 perature gradients generated within the samples during heat-

936 ing can result in internal water vaporization, which is

937 associated with damage in the texture of mushrooms [113].

938 In order to solve the abovementioned problems, Devece

939 et al. [114] explored the effect of combining microwave heat-

940 ing at 85 �C for different times and then immediately

941 immersed in a 92 �C water bath for 20 s. Results clearly

942 showed that this new blanching method completely inacti-

943 vated PPO in 2 min. However, conventional hot water blanch-

944 ing needed more than 6 min. Product browning and the loss

945 of antioxidant contents were significantly lower in the sam-

946 ples blanched by the combined microwave and water method

947 than microwave or hot water blanching [114].

948 5.2.2.3. Asparagus. Kidmose and Kaack [115] compared

949 the effects of microwave, hot water and steam blanching on

950 the toughness and vitamin C content of asparagus. Similar

951 or greater toughness and lower vitamin C were obtained by

952 microwave than by steam and hot water blanching. Sun

953 et al. [116] studied the effect of microwave-circulated water

954 blanching on the antioxidant content and color of asparagus,

955 while comparing it to hot water blanching and steam blanch-

956 ing. It was observed that there was no significant difference in

957 texture and rutin content of asparagus blanched by these

958 methods. In addition, compared to steam blanching and hot

959 water blanching, the microwave-circulated water blanching

960 obtained higher antioxidant activity and better retention of

961 green color [116]. This work confirmed that microwave-

962 circulated water blanching has better advantages over con-

963 ventional hot water and steam blanching. It is shown to be

964 a potential alternative blanching method for asparagus.

965 5.2.2.4. Artichokes. Ihl et al. [117] evaluated the effect of

966 microwave-, steam-, and boiling water blanching on chloro-

967 phyllase inactivation, color changes, and loss of ascorbic acid

968 in artichokes. It took 2, 6, and 8 min for microwave, steam,

969 and boiling water blanching, respectively, to completely inac-

970 tivate chlorophyllase. Microwave and boiling water blanching

971 were best in preserving the original perceptual color of arti-

972 chokes, while the steam blanched sample showed lower val-

973 ues for lightness index, hue angle and chroma. Microwave

974 blanching did not cause a significant loss in ascorbic acid

975 when compared to the 16.7% decrease in ascorbic acid with

976 boiling water blanching. In view of chlorophyllase inactiva-

977 tion, color changes, and ascorbic acid loss, this investigation

978 clearly showed that microwave blanching is a more suitable

979 method for blanching artichokes when comparedwith boiling

980 water and/or steam blanching.

981 5.2.2.5. Peas. Lin and Brewer [111] evaluated the effects of

982 direct and indirect (i.e., product in bags) microwave-, steam-,

983 and boiling water blanching prior to freezing of manually

984 shelled peas. Direct microwave blanching was conducted by

985 immersing peas in water for 4 min; indirect microwave

986 blanching was conducted by immersing packed peas in plas-

987 tic bags in water for 4 min; steam and boiling water blanching

988 was conducted for 4 min. After blanching, the samples were

989 frozen. The quality attributes of frozen products include per-

990 oxidase activity, ascorbic acid content, visual appearance,

991color, aroma, flavor, and texture were determined after stor-

992age for 0, 6, 12 weeks at -18 �C. The results showed that no sig-

993nificant differences were found among the studied blanching

994methods in the reduction peroxidase activity that was deter-

995mined to be 97%. After the storage for 6 or 12 weeks, steam

996blanched peas retained the maximum ascorbic acid, while

997boiling water-blanched samples contained the least. Peas

998blanched by both microwave methods had more breakage

999and splitting appearance compared to boiling water and

1000steam blanched ones. The authors attributed this phe-

1001nomenon to the non-uniform heating characteristics of

1002microwave, especially for the round shape materials [111].

1003In terms of color, both microwave blanching methods had

1004equivalent lightness and were darker compared to the other

1005blanched ones. There was no significant difference among

1006blanching methods on greenness/redness (a*), blueness/yel-

1007lowness (b*), grassy, grainy or earthy aromas, and sweet, fru-

1008ity, or buttery flavors. Unblanched peas had the most

1009umami flavor, while the microwave blanched ones had the

1010least. With respect to texture, steam blanched peas were

1011not as tough as the unblanched control samples. However,

1012they were tougher than the samples blanched by steam or

1013boiling water. Although the better chemical and sensory attri-

1014butes (color, aroma, and flavor) obtained with microwave

1015than conventional blanching methods, microwave blanching

1016produces poor visual appearance and loss of physical integ-

1017rity. More investigations are needed to overcome these short-

1018coming on peas and other products.

10195.2.2.6. Herbs and spices. Drying of herbs and spices is

1020essential to extend their shelf life. This is because low mois-

1021ture contents prevent the growth and reproduction of

1022microorganisms that cause decay. Blanching is a crucial step

1023before drying to inactivate enzymes. Application of a suitable

1024blanching technology with a selection of appropriate condi-

1025tions are of great importance, since blanching directly affects

1026the quality of the dried product in terms of its physical and

1027nutritional property.

1028Singh et al. [17] evaluated the effects microwave

1029(2450 MHz, 800 W), boiling water, and steam blanching meth-

1030ods on the quality attributes of marjoram and rosemary in

1031terms of volatile oil, color, texture, and chlorophyll and ascor-

1032bic acid contents. Immediately after blanching, the samples

1033were dried by microwave (2450 MHz, 800 W). Marjoram, cut

1034into segments of 5 cm in length, and rosemary leaves were

1035blanched for 1 min. Prior to microwave radiation the herbs

1036werewetted with minimum amount of water. Volatile oil con-

1037tent in marjoram was almost lost in all studied methods. The

1038authors attributed the losses to the delicate nature of the herb

1039with soft stem and flower buds and also to the presence of

1040low boiling and more volatile non-oxygenated terpene hydro-

1041carbons. Microwave and hot water blanching of rosemary had

1042a 47.5% reduction in volatile oil and the steam blanching

1043resulted in a 62.5% reduction in volatile oil. The blanching

1044had more of a positive effect in retaining the original green

1045color of the fresh herbs than with direct drying. For both

1046herbs, hot water blanched samples had the best color (i.e.,

1047Chlorophyll retain), followed by microwave blanched sam-

1048ples, and lastly, the steam blanched samples. Steam blanch-

1049ing resulted in softer products than the other two blanching




20 February 2017


1050 methods. Maximum retention of ascorbic acids in both herbs

1051 was obtained with microwave, followed by steam, and hot

1052 water. The latter methods caused had lower retention of

1053 ascorbic acid due to leaching in surrounding water and ther-

1054 mal breakdown during blanching [17].

1055 Dorantes-Alvarez et al. [5] used microwave blanching

1056 without water for 10, 15, 20, 25, and 30 s to evaluate the

1057 changes in antioxidant activity of pepper, when treating

1058 with microwaves to inactivate PPO enzymes. After micro-

1059 wave blanching, the phenolic compounds of the products

1060 were reduced by 20.8% (from 9.6 to 7.6 mg/g peppers in

1061 dry weight basis), whereas the antioxidant activity was

1062increased by 44.8% (from 29 to 42 lM de trolox/g peppers

1063in dry weight basis). It is likely that microwave blanching

1064not only inactivates enzymes, but also induces the forma-

1065tion of derivatives of phenolics, which enhances the

1066antioxidant activity of the products after being blanched

1067[5].

10685.2.3. Limitations of microwave blanching1069Despite being energy efficient and requiring less time, micro-

1070wave blanching has some drawbacks that could limit its

1071application.

(A) Fresh carrot (B) Mild microwave blanched (C) Strong microwave blanched

Fig. 3 – The microstructure of the fresh and blanched carrot samples under different conditions [74].

Fig. 4 – The time–temperature profile and temperature distribution in different part during 915 MHz continuous microwave

processing of packages of red bell peppers [70].




20 February 2017


1072 5.2.3.1. Loss water during blanching. During microwave

1073 blanching, moisture in vegetables may evaporate. High inten-

1074 sity microwave power may cause cells folding and destruction

1075 of product microstructure [8]. To reduce water loss during

1076 blanching and increase heat absorption, vegetables may be

1077 heated while immersed water. However, water-soluble nutri-

1078 ents can be lost through leaching or diffusion to blanching

1079 water.

1080 5.2.3.2. Penetration depth of microwave is limited. The pen-

1081 etration depth of microwave in a sample is a function of its

1082 dielectric properties, which determines the temperature dis-

1083 tribution within the material [118]. The dielectric properties

1084 (e) of a product is strongly dependent on the dielectric con-

1085 stant (e0), which is a measure of the ability food material to

1086 store electromagnetic energy, and the dielectric loss factor

1087 (e00), which determines the ability of the material to dissipate

1088 electromagnetic energy after being heated [119]. The penetra-

1089 tion depth (dP) of microwave into material can be determined

1090 using the following equation [120]:1091

dp ¼ k

2pffiffiffiffiffiffi2e0

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1þ e00

e0

� �2s

� 1

24

35

�12

ð1Þ10931093

1094 where dP is the penetration depth, k is the wavelength of

1095 microwaves, e0 is the dielectric constant, e00 is the loss factor.

1096 During microwave heating the loss factors decreased with

1097 moisture reduction, so the conversion of microwave energy

1098 into heat is reduced at lower moisture contents. It has been

1099 determined that the microwave penetration depth for whey

1100 protein is about 12 mm at 915 MHz at 20 �C [121], for mashed

1101 potato sample (82.7% moisture content) is 1.6 cm [118], for

1102 sweet potato, red bell pepper, and broccolior is about 1.5–

1103 3.5 cm [120].

1104In addition, the penetration depth of dielectric heating

1105decreases as frequency increases. It was observed that pene-

1106tration depths in radio frequency range (27 and 40 MHz) are

1107several times as that in microwave frequencies (915 and

11082450 MHz) at each corresponding temperature [121]. There-

1109fore, it is recommended that for larger or thick product radio

1110frequency technology is suitable while for the small or thin

1111samples microwave heating is better.

11125.2.3.3. Non-uniform heating. Microwave heating mainly

1113depends on the conversion of electromagnetic energy into

1114heat via friction of dipolar molecules, especially water mole-

1115cules, and ions that follow the oscillating electrical field at

1116very high frequencies [106].

1117However, since there is an uneven distribution of moisture

1118and ions in different parts of the samples, the microwave

1119heating also ends up being non-uniform. With the microwave

1120applicator producing a non-uniform microwave field, the

1121uneven energy distribution caused hot and cold points in

1122the sample. Furthermore, the limited penetration depth made

1123the heating with microwave more inhomogeneous. All of

1124these factors cause large temperature variations when it

1125comes to processing large and bulky materials. Koskiniemi

1126et al. [120] used a 915 MHz and 4 kW continuous microwave

1127system with a residence time of 4 min to pasteurize packaged

1128acidified vegetables. It was found that the heating of the pack-

1129age was non-uniform. There was a hot spot of about 95 �C and

1130cold area of approximately 80 �C, as shown in Fig. 4. Walde

1131et al. [122] also found that microwave drying of mushroom

1132resulted in the charring of edges due to non-uniform heating.

11335.2.3.4. Difficulties to precisely control blanching1134temperature. The effective conversion of electrical energy

1135in a microwave applicator to thermal energy depends largely

Fig. 5 – A schematic diagram of the principle of ohmic heating [48].




20 February 2017


1136 on the dielectric properties of products, especially the dielec-

1137 tric loss. The dielectric properties are mainly determined by

1138 the chemical composition, structure and density of the prod-

1139 ucts [107]. Water content and its state (free or bound water),

1140 along with ionic contents, play important roles in determin-

1141 ing the dielectric properties of the products. Often water dis-

1142 tribution in a product and ionic concentration in vegetables

1143 may not be uniform; this will cause non-uniform heating.

1144 This complication is confounded with standing waves in

1145 microwave heating cavities, as common design for domestic

1146 and industrial microwave heating system, causing unpre-

1147 dictable hot and cold spots in the material during microwave

1148 heating. In addition, the energy decreases rapidly as the

1149 microwave penetrate the product, and the penetration depth

1150 of microwave is limited. Due to the non-uniform distribution

1151 of water in the product, standing wave effect, and rapid decay

1152 of microwave within heated foods, it hard to predict and pre-

1153 cisely control the temperature; this results in overheating or

1154 inadequate heating during blanching. These challenges can

1155 be mitigated with a proper microwave system design for

1156vegetables that have consistent compositions and are packed

1157in well-defined geometries (e.g., diced carrots in vacuum

1158sealed bags).

11595.3. Ohmic blanching

11605.3.1. The principle of ohmic blanching1161The ohmic heating is also known as Joule heating, electrical

1162resistance heating, or electro-heating. During ohmic heating,

1163food products are placed between two electrodes. Food prod-

1164ucts behave as an electrical resistance, in which heat is gen-

1165erated and product temperature rapidly increases [123,124].

1166The principle of ohmic heating is shown in Fig. 5. The heat

1167generated inside the food depends mainly on the current

1168induced and the electrical conductivity of the product [125].

1169Ohmic heating has several advantages, such as fast and uni-

1170form heating. Therefore, ohmic heating systems can achieve

1171a mild thermal treatment, instant shutdown and no residual

1172heat transfer after shut off of the current, low operation costs,

1173high energy conversion efficiencies, and less problems of sur-

Fig. 6 – Infrared combined with hot air system-front view (A) and side view (B) [57].




20 February 2017


Table 1 – Application of emerging and innovative blanching technologies.

Blanching technology Product Processing conditions Main findings References

Steam blanching Spinach leaves Steamed over boilingwaterfor 7.5, 15, 30, 45, or60 min

Steaming favored theformation of pheophytinsa and b, while microwavecooking favored that ofpyrochlorophyllsa and b

[87]

Kiwifruit Water vapor atatmosphericpressure (99.8 �C) andholdon a predeterminedheatingtime

Tissue became yellow–brown after 5 min of blanching,plasmodesmatal areas havelost the stain intensity, firmnessdecreased with the increase ofblanching time

[88]

Potato Superheated steam(SHS),spray of hot watermicro-droplets (WMD):steam was set to 115 �C,total water supply ratewasset to 3.0 kg/h

The soft and brittle,and brightness and chromaticquality decreased of potato werehindered by SHS +WMD andSHS,whencompared to blanching in hotwater;SHS +WMD also significantlyreducedwater loss

[89]

Potatostrips Steam temperaturesfrom62.8 �C to 90.6 �C andperiods of time from 2 to20 min were usedd.

Low temperatures (<74 �C),blanching time had little effecton the texture product,while at high temperatures(�74 �C), the texture softened asblanching time increased

[90]

Mango slices Steamed at 94 ± 1 �C for0, 1, 3, 5, and 7 min

PPO and POD were completelyinactivated after 5 and 7 min ofsteam blanching, respectively

[91]

Garlic slices Steam temperature at100 �C, for 1, 2, 4, 6, 8and 10 min

Compared towater blanching (80and 90 �C) and other duration,steam blanchingfor 4 min was the best treatmentthat achieved no changes intexture and reduced theenzymatic activities of POD, PPO,and inulinase by 93.53%, 92.15%and 81.96%, respectively

[92]

Broccoli Steamed for 5 and 10min, respectively

When compared to freshsamples, the total ORAC valueincreased by 2.3-fold, the2, 2-azobis [2-amidinopropane]dihydrochloride inducedintracellular ROS levelreduced, the total phenoliccontent and total flavonoidcontent increased after steamblanching

[86]

Blueberries Steamed for 3 min The recovery of anthocyaninsincreased, the juice was bluerand less red thanthat obtained from un-blanchedfruits.

[93]

Blueberry purees Steamed for 3 min The steam blanching increasedMAP and TPC contents by 11.3%and 51.6%),respectively as compared to theun-blanched samples

[94]




20 February 2017


Table 1 – (continued)


Vegetable soybean Steamed at 100 �C for 10 min Soluble sugars in both greenpods and seeds after steamblanching weresignificantly higher (16–333%)than water blanched samples

[95]

Cabbage Blanching in saturated steamfor 2 min

The Salmonella loaddecreased from the initiallevel of 6.4 log cfu/g to 3.6log cfu/g after being steamblanching, dried blanchedsamples exhibitedgreener and darker color thanuntreated samples

[96]

Cabbage Suspended over boilingwater for 1 min

Total glucosinolates retentionwas up to 92.40% of steamblanched sample, thatmuch higher than hot waterblanched (95 ± 2 �C for 2 min)with 52.92

[97]

High-humidity hot airimpingement blanching(HHAIB)

Red pepper Air velocity of 14.0 ± 0.5 m/s,temperature of 110 �C, andhot airrelative humidityof 35–40%, for 30, 60, 90, 120,150, 180, 210, and 240 s

PPO residual activity wasdecreased to 7% after 120 s,drying time was reduced up to7 hwhen blanching for 120 s,created superficial micro-cracks

[105]

Red pepper Air velocity of 14.0 ± 0.5 m/s,temperature of 110 �C for 1,2 and 3 min.

HHAIB maintained higher redpigments, ascorbic acidretention, total antioxidantactivity and DPPHvalues, as compared with thesamples blanched by hotwater blanching, reduceddrying time for4.0 h (blanching for 2 and3 min) compared to untreatedones

[165]

Yam slice Superheated steam at 120 �Cand 35% relative humidity,and air outlet velocityat 10.0 m/s, for 3, 6, 9 and12 min, respectively.

When blanching for 6 min,drying time was reduced by35%, whiteness index wasincreased by 50%

[60]

Sweet potato Superheated steam at 120 �Cand 35% relative humidity,and air velocity was10.0 m/s for 3 and 5 minseparately

Increased drying time by 11%and 44%, but obtained ahomogeneous compactstructure, softer texture,and desirable color

[43]

Sea cucumber Superheated steam atrelative humidity 10–50%,temperature 90–200 �C, airvelocity 3–20 m/s. for 5–40 min

Autolytic enzyme wascompletely inactivated, thecolor and shape of theproducts were improved

[101]

Apple four different temperatures:90, 100, 110 and 120 �C, airvelocity at 15.0 m/sand relative humidity was40–45%

The time PPO totallyinactivated (38 cm quarters,90 �C – 7 min, 100 �C – 6 min,110 �C – 5 min, 120 �C – 5 min)was shorter than infrared(7 min), the percentageretention of vitamin C was(11.29%, 10.79%, 7.78% and4.48%,respectively) higher thanwater blanching of asparagus(3–8%)

[104]




20 February 2017




Seedless grape Different temperatures (90,100, 110, and 120 �C) andseveral durations (30, 60, 90,and 120 s)

PPO residual activity lowerthan 10%, the drying timesreduced 12–25 h at dryingtemperatures of 55–75 �C,yielded desirable green–yellow or green raisins whenblanching of 110 �C for 90 s

[100]

Microwave blanching Carrot slices A continuous conveyermicrowave oven with 4magnetrons (power of1.25 kW foreach magnetron) and aconveyer speed of 0.5 m/min

Microwave blanching resultedin enhanced nutritionalquality as compared to waterand steamblanching, for instance, thecontent of dry matter, sucroseand carotene were 11%–39%higherthan steam and waterblanching

[8]

Carrotpieces Microwave temperature of60, 90 �C, for 1 or 40 min,respectively

Mild microwave blanching(60 �C)is recommended, itdecreases the degree ofmethoxylation, andmaintained themicrostructure well

[112]

Carrots A frequency of 2450 Hz, atlow power of 450 W, for10 min

Microwaving led to a goodtexture with losing only 39.8 %of the initial energy requiredto cut raw samples,while boiling water and steamblanching exhibited higherpercentage values (91.1 and92.5 %, respectively)

[166]

Mushroom Microwave heating at 85 �Cfor different times and thenimmediately immersedin a 92 �C water bath for 20 s

The PPO completelyinactivated in 2 min, muchlowed than conventional hotwater blanching, whichneeded more than 6 min

[114]

Broccoli Microwave oven working at2450 MHz–900W, for 40, 50,60, 70 and 80 s, respectively

The total inactivation of PODwas achieved in 80 s used themicrowave treatment, shorterthan steam and boiling water(needed 90 and120 s, respectively); increasedof ascorbic acid content,contrary to water and steam,which decrease the ascorbicacid by 47% and 25%,respectively

[167]

Sweet Potato Microwave oven with inputpower: 1200W, output 700 W,at maximum power

Microwave required the leastof time (60 s) to inactive POD,compared to steam (110 s) orboiling water (130 s); reduceddrying time by 44%,more efficient than steam orboiling water (22%); reducedthe reduction of anthocyanin(59.34%), compared to steam(53.55%) and boiling water(40.37%) samples

[168]

Asparagus Used a rotating plate in astationary cavity microwaveoven with a microwavepoweroutput of 556 W, blanched for3 and 0.5 min

Spears with similar or highershear force values and lowervitamin C were obtained bymicrowave blanching than bysteam (at 90 �C for 6 or 1 min)and water (at 90 �C for 7.5 or1 min)

[115]




20 February 2017




Peas Microwave-blanched in aPyrex beaker (80 mL of water;4 min; 800 W); ormicrowave-blanched in a bag(80 mL of water, 4 min)

Both microwave treatmentsreduced peroxidase activity by97% compared with controls

[111]

Rosemary Microwave radiation(2450 MHz, 800 W)

Microwave and boiling waterblanching of rosemary had a47.5% reduction in volatile oiland the steam blanchingresulted in a 62.5%reduction in volatile oil;microwave blanching retainedthe original green color of thefresh sample than with directdrying

[17]

Red pepper 650, 750 and 900W for 100 s The residual activity values ofPPO and POD are 9.80% and16.43%, respectively; reduceddrying time for 3.5 h, at powerof 900 W for 100 s

[165]

Pepper Used a microwave oven,duration for 10, 15, 20, 25 and30 s

PPO was inactivated, thephenolic compounds of theproducts were reduced by20.8%, and the antioxidantactivity was increased by44.8% after blanched for 20 s

[5]

Ohmic blanching Artichoke heads Useda constant gradientvoltage of 24 V/cm, once thetemperature of 80 ± 2 �Cwas reached in the core ofthe artichokes, the sampleswere maintained atthis temperature for thefollowing holding times: 0,60, 120, 180, 240, and 300 s

The ohmic blanchinginactivated POD and PPO at ahigher rate than conventionalboiling blanching, with a totalinactivation time of 360 s and480 s, respectively, higherfirmness, 24% and 53% higherof proteins and polyphenolcontent than conventionalones, respectively

[132]

Acerola pulp The solids content of thepulp (2–8 g/100 g) and theheating voltage (120–200 V),samples were heated to 85 �Cfor 3 min

The ohmic heatingexperiments carried out at lowvoltages ( < 140 V) exhibitedascorbic acid degradationsimilar to the conventionalheating (0 V), high voltagegradients induced greaterascorbic acid degradation

[133]

Strawberry Used ohmic heating at 60 Hz,manual transform 0–240 V,temperature at 30 �C, 40 �Cand 50 �C for 300 min, as theratio of solution to fruit was3:1 (w/w)

Ohmic heating enhancedmass transference kinetics ofsample during osmoticdehydration, larger amount ofsolute gain (0.181–0.262) thanunheated (0.149–0.223),induced changes in the shapeand thickness of the middlelamellae and increasedcellular breakage

[140]

Tomato juice Used 24 V/cm electric fieldstrength, held at 90 �C (thecenter of the sample) for 15,30, 45, and 60 s

PG (Polygalacturonase) andPME (Pectin methyl esterase)enzyme inactivation achievedby the OH (1 min.) was similaras compared to CT (hot waterbath of 90 �C) of 5 min; theascorbic acid of OH was 29–51% higher than CT; OH Pastewas more viscous and brightred than CT treatment

[169]




20 February 2017




Vegetable baby purees(40% carrots, 20% peas,15% zucchini, 0.1% saltand 24.9% water)

Used 25 kHz high voltagefrom the regular 50 Hznetwork, temperature at129 �C for 11 min

Ohmic heating did not haveeffect on the total amino acidcontent; contrarily, forconventional retortsterilization (spraying hotwater on the jars at 129 �C for10 min), the content of totalessential and non-essentialamino acids significantlydecreased in 35% and 9%,respectively

[170]

Sugarcane juice Used 25 V to the electrodes(electric field intensity 3.57–4.39 V/cm), temperatures of60, 70 and 80 �C for 2.0, 3.0,3.5 and 4.0 min

PPO was almost totallyinactivated, the degradation oftotal phenolics was between11 and 23% and thedegradation of total flavonoidsvaried from 20 to 38%, both for6 and 12 min, and similar tohot water blanching

[171]

Acerola pulp Used 30 V (60 Hz offrequency), temperature at80, 85, 90 and 95 �C for 0, 10,20, 30, 40, 50 and 60 min,respectively

Ascorbic acid and carotenoiddegradation was similarbetween IR with and withoutthe application of the electricfield.

[172]

Blueberry pulp Applied the voltage (160, 172,200, 228 and 240 V),maintainsample at temperature of90 �C for 2 min

The degradation ofanthocyanin ohmic heating(5.7–14.7%) was lower orsimilar to conventionalheating (7.2%)

[138]

Apples Heating temperature at 30 �C, 40 �C and 50 �C for 90 min,as the ratio of solution tofruit was 3:1 (w/w), with analternating current at 60 Hzand 100 V, generating anelectric field of 13 V/cm

Combined with ohmic heatingat 50 �C was the best processfor dehydrating apples: PPOwas completely inactivated;the water loss and the solidgain were 48% and 37% greaterthan untreated, respectively;the firmness was 68% higherthan untreated

[140]

Infrared blanching Appleslices Radiation intensity (3000,4000 and 5000 W/m2), slicethickness (5, 9 and 13 mm)and processing time (2, 5, 7,10, 15 and 20 min).

It took 2–15 min to achieve90% inactivation of POD inapple slices with thicknessesof 5–13 mm used thecontinuous heating mode

[151]

Appleslices Appleslices with threedifferent thicknesses, 5, 9,and 13 mm, were heatedused infrared for up to10 min at 4000 W/m2 IRintensity

The process of simultaneousdry blanching anddehydration of apple slicesunder IR heating can bepredicted with the first-orderkinetics

[154]

Carrot slices Infrared radiation (chambermaintained at 180–240 �C) for8–15 min

IR blanching reduced themoisture content by 13–23%;increased the retention ofvitamin C (62%), water (43%)and steam (49%) blanching;took about 45% lesser dryingtime and possessed �5%higher rehydration moisture,compared to water blanched–hot air dried samples

[155]




20 February 2017


1174 face fouling [126]. Ohmic heating has extensive potential

1175 applications in food industry, such as blanching, evaporation,

1176 dehydration, fermentation, extraction, sterilization, and pas-

1177 teurization [127,128]. The frequency of applied voltage

1178 strongly influences the performance of ohmic heating. It

1179 was found that the heating rate decreased with increasing

1180 of the frequency [129], so low frequency is frequently used.

1181 Compared to conventional hot water blanching, ohmic

1182 blanching requires a shorter time due to it volumetric heating

1183 characteristics. In addition, it yields better product quality as

1184 it reduces solids and nutrients leaching and preserves color

1185 and texture [130,131]. Furthermore, it can be used for blanch-

1186 ing vegetables and fruits with alarger volume, which are diffi-

1187 cult to be blanched using conventional hot water blanching

1188 that could cause quality degradation due to its low conduc-

1189 tion and convention heat transfer rate.

1190 5.3.2. Applications of ohmic blanching1191 5.3.2.1. Artichoke heads. Guidaet al. [132] compared theeffects

1192 of ohmic blanching with hot water blanching of artichoke

1193 heads on the inactivation of POD and PPO enzymes, total pro-

1194 tein and bioactive compounds, and texture and color degrada-

1195 tions. Results showed that comparedwithhotwater blanching,

1196 ohmic blanching inactivated both enzymes at a lower blanch-

1197 ing time and preserved the texture and color. In addition, total

1198 protein and polyphenolic contents, immediately after blanch-

1199 ing as well as after three months of canning storage, were

1200 higher than those of the hot water blanched ones.

1201 5.3.2.2. Carrot, red beet and golden carrot. The effects of

1202 ohmic blanching on kinetics of textural softening of cylindri-

1203cal pieces of carrot roots, red beet and golden carrots were

1204compared with that of hot water and microwave blanching

1205[127]. It was found that ohmic heating resulted in greater soft-

1206ening rates and weight losses and significantly less firm prod-

1207ucts than those blanched with either hot water or microwave

1208blanching methods. This work indicated ohmic blanching

1209may be not a suitable technology for blanching of the selected

1210vegetables.

12115.3.2.3. Acerola pulp. Acerola fruit (Malpighiaemarginata D.

1212C.), a tropical fruit, is a rich source of health-promoting com-

1213pounds, such as vitamin C, anthocyanins, carotenoids, and

1214elements. Mercali et al. [133] performed an investigation to

1215explore the effect of pulp’ssolids content (2–8 g/100 g) and

1216heating voltage (120–200 V) on vitamin C degradation of acer-

1217ola pulp during the ohmic heating process. It was found that

1218the vitamin C degradation ranged from 3.08% to 10.63%,

1219which was significantly influenced by the applied voltage

1220and the solids content of the pulp during ohmic heating. In

1221the case of voltage gradient, it was observed that an increase

1222in the voltage gradient from 120 to 200 V led to an increase in

1223the vitamin C degradation from 2.0% to 5.1% [133]. Ohmic

1224heating at low voltage gradients exhibited vitamin C degrada-

1225tion similar to that with conventional heating, while higher

1226voltage gradients accelerated vitamin C degradation. The lat-

1227ter was attributed to the occurrence of electrochemical reac-

1228tions that yielded oxygen, which enhanced vitamin C

1229deterioration. The effect of electric field frequency on ascorbic

1230acid degradation and color changes in acerola pulp during

1231ohmic heating was explored and compared with the conven-

1232tional thermostatic water heating [134]. It was found that



Carrots slices Carrotslice surfacetemperature (85, 90 and 95 �C), carrot slice thickness (3, 5and 7 mm) and processingtime (2, 4, 7, 10, 15, 20 and30 min)

IR blanching process whichproduced 1 log reduction inPOD activity has resulted inmoisture reduction from 40.2to 88.8 g/100 g, overall colorchange (DE) from 3.17 to 5.13and retention of vitamin Cfrom 56.92 to 77.34 g/100 gcompared to control

[173]

Mango Power sequences 8%–100%,blanching at 65 �C for 10 min(LTST) or 90 �C for 2 min(HTST)

PPO was completelyinactivated, AAO hadremained 30% and 9%–15%after LTST and HTST; IRblanching had higher VCretention of 88.3 ± 1.0% (HTST)and 69.2 ± 2.9% (LTLT),compared with waterblanching 61.4 ± 5.3% (HTST)and 50.7 ± 9.6% (LTLT); reducedby nearly 23%–28% drying timecompared to untreated

[174]

Red pepper 80 �C for 1, 2 and 3 min The residual activity of PPOand POD are 12.18% and16.75%, respectively, reduceddrying time for 4.0 h, whentreated for 3 min

[165]




20 February 2017


1233 greater ascorbic acid degradation and more color changes

1234 occurredwhen the sampleswere blanched at low electric field

1235 frequency (10 Hz). Ohmic and conventional heating processes

1236 at 100 Hz demonstrated similar degradation rates of ascorbic

1237 acid and similar color changes [134]. Mercali et al. [135] exper-

1238 imentally compared the effect of ohmic heating and conven-

1239 tional hot water heating on the degradation kinetics of

1240 anthocyanins in acerola pulp at temperatures ranging from

1241 75 to 90 �C. It was found that there was no significant differ-

1242 ence between both heating methods on the degradation rate

1243 constants at the same temperature. This may indicate that

1244 similar mechanisms of degradation occurred with both ohmic

1245 and conventional heating.

1246 5.3.2.4. Strawberries. The effects of ohmic heating and

1247 vacuum impregnation on the osmotic dehydration kinetics

1248 and microstructure of strawberries were investigated by

1249 determining water loss, solid gain, color, and firmness of

1250 the products [136]. It was found that the greatest amount of

1251 solute gain occurred with the treatment that combines osmo-

1252 tic with ohmic heating, along with vacuum impregnation.

1253 This indicates that ohmic heating and vacuum impregnation

1254 can enhance mass transfer during osmotic dehydration of

1255 strawberry [136]. However, a loss of firmness was found in

1256 the samples pretreated with ohmic heating and vacuum

1257 impregnation at a higher temperature of 50 �C. This was

1258 mainly because the destruction of the microstructure. These

1259 findings indicated that the application of ohmic heating and

1260 vacuum impregnation can enhance mass transfer and

1261 improve quality attributes when performed at a lower

1262 temperature.

1263 In addition, to evaluate the influence of different electric

1264 field strengths (9.2, 13, 17 v/cm) on the effect of osmotic dehy-

1265 dration combined with ohmic heating and vacuum impregna-

1266 tion combined with ohmic heating on physiochemical and

1267 quality attributes of strawberry as well as on microbial stabil-

1268 ity of starage samples at 5 and 10 �C, another investigation

1269 was carried out with a 65% (w/w) sucrose solution at 30 �C1270 [137]. It was found that the vacuum impregnation combined

1271 with ohmic heating at 13 V/cm produced products with the

1272 greatest solute gain, least loss in firmness and least color

1273 degradation. Furthermore, the shelf life of products processed

1274 under this condition and stored at 5 �C was extended from

1275 12 d (control samples) to 25 d [137].

1276 5.3.2.4.1 Blueberry pulp. Blueberry is becoming more and

1277 more popular, as it has health benefits because it is high in

1278 anthocyanins, which are potent antioxidants that have high

1279 radical-scavenging activities. Blanching is an essential step

1280 for blueberry processing to extend its shelf life through the

1281 inactivation of primary enzymes that contribute to quality

1282 deterioration and anthocyanin degradation. Ohmic heating

1283 was used to blanch blueberry pulp, and the optimal process-

1284 ing conditions were identified [138]. It was observed that the

1285 degradation of anthocyanins increased with the increase of

1286 voltage and solids content. In addition, when ohmic blanch-

1287 ing under low voltage gradients, the percent of anthocyanin

1288 degradation was similar or even lower than that obtained

1289 with conventional hot water blanching. The authors attribu-

1290 ted the higher degradation of anthocyanins under high volt-

1291 age gradients to the electrochemical reactions catalyzed by

1292the oxygen that was generated by water electrolysis [138].

1293The findings in this work highlighted the parameters for the

1294optimization of ohmic heating and the need to use inert

1295material in electrode and electrode coating to limit water

1296electrolysis and mitigate nutrients degradation.

12975.3.2.4.2 Milk, fruit and vegetable juices. In order to inactiva-

1298tion of alkaline phosphatase, pectin methylesterase and per-

1299oxidase, ohmic heating of milk, fruits and vegetable juices

1300was performed at several incubation temperatures compared

1301with conventional indirect heating [139]. It was found that

1302compared with inactivation by conventional indirect heating,

1303ohmic heating enhanced the rate of enzymes inactivation in

1304food materials. Furthermore, the kinetic parameters had

1305changed, while inactivation mechanisms remained the same.

1306In addition, the peroxidase in vegetable juices was more

1307prone to destabilization with ohmic heating [139]. It was also

1308found that only the activation entropy, not the activation

1309enthalpy, is different for ohmic heating, indicated that a

1310cause of its decreased stability was not due to the modifica-

1311tion of enzyme tertiary structure by the electric field.

13125.3.2.4.3Apples. The enzymatic browning and spoilage

1313caused by polyphenoloxidase (PPO) activity in fruits and

1314vegetables during processing and storage is a great problem

1315for the food industry. Moreno et al. [140] investigated the

1316effects of combining ohmic heating and osmotic dehydra-

1317tion with vacuum impregnation on PPO inactivation, phys-

1318ical properties and microbial stability of apples stored at 5

1319or 10 �C. It was found that there was a complete inactiva-

1320tion of PPO, and the least change in firmness and color

1321was obtained with the vacuum impregnation combined

1322with ohmic heating treatment at 50 �C. In addition, the

1323shelf life of the products was extended by more than

13244 weeks when stored at 5 �C.

13255.3.3. Limitations of ohmic blanching13265.3.3.1. Difficulty in controlling the blanching1327temperature. Electric conductivity is a crucial factor that affect

1328the performance of ohmic heating. However, the electrical

1329conductivity is a temperature dependent [141]. Therefore, in

1330order to control the blanching temperature precisely, it is nec-

1331essary to develop a real-time temperature monitoring system

1332and a reliable feedback control technology to adjust the sup-

1333ply power according to the temperature change of the pro-

1334cessed products. To improve the performance of ohmic

1335heating, Zell et al. [142] designed a triple-point probe to mon-

1336itor temperature changes during the blanching.

13375.3.3.2. Generating oxygen and hydrogen. The frequency of

1338applied voltage strongly influences the performance of

1339ohmic heating. It was found that the heating rate

1340decreased with increasing of the frequency [129]. Conven-

1341tional ohmic heating underlow frequency alternative cur-

1342rent ranging between 50 to 60 Hz, could generate oxygen

1343and hydrogen from the electrolyzation of water [144].

1344Degradation of nutrients in ohmic heating was attributed

1345to the generation of oxygen and the anode and hydrogen

1346at the cathode during the electrolysis of water. Sarkis

1347et al. [138] found that the molecular oxygen produced

1348through water electrolysis enhances oxidation of the

1349anthocyanin. Mercali et al. [134] also observed that the




20 February 2017


1350 use of low electric field frequency (10 Hz) led to greater

1351 ascorbic acid degradation and more color changes in acer-

1352 ola pulp, which may be due to the catalytic action of oxy-

1353 gen released by the electrolysis of water.

1354 5.3.3.3. Corrosion and erosion of electrodes. As for cellular

1355 foodstuffs such as vegetables, the cell membrane is an

1356 electrical insulator, so pure water is not a good conductor

1357 of electricity. As a result, metal ions or acidic solutions

1358 are often used to increase the electric conductivity [128].

1359 However, the added ionic substances such as acids and

1360 salts accelerate corrosion and erosion of electrodes. It

1361 was found that the electrode materials suffered intense

1362 electrode corrosion at pH 3.5 [144]. In addition, the added

1363 salts and acids can influence the quality attributes, espe-

1364 cially the flavor of products. Unfortunately, it is difficult

1365 to alleviate the problems associated with solutions contain-

1366 ing salts and acids.

1367 5.4. Infrared blanching

1368 5.4.1. The principle of infrared heating1369 Infrared heating is generated by the electromagnetic radia-

1370 tion that falls between the regions of visible light waves

1371 (0.38–0.78 lm) and microwaves (1–1000 mm) [109]. Unlike

1372 thermal conduction or convection, infrared radiation heat

1373 can propagate through both vacuum and atmosphere. It is

1374 absorbed by molecules of food components through the

1375 mechanism of rotational-vibrational movements that pro-

1376 duces heat [109].

1377 Infrared heating is dependent on the wavelength of the

1378 radiation. According to the ISO 20473 scheme (ISO

1379 20473:2007, ISO), infrared heaters can be classified into three

1380 regions: near infrared (NIR) with wavelengths between 0.78

1381 to 3 lm, mid infrared (MIR) with wavelengths between 3 to

1382 50 lm, and far infrared (FIR) with wavelengths between 50

1383 to 1000 lm [145]. The wavelength of infrared is determined

1384 by the temperature of the radiation body; the higher the tem-

1385 perature, the shorter the wavelength. Water, proteins,

1386 starches, and fats, which are the main components of food,

1387 absorb far infrared energy better than near infrared energy

1388 [146]. In addition, the penetration depth of infrared radiation

1389 strongly depends on the composition and structure of the

1390 food, and the radiation wavelengths. The longer the wave-

1391 length of radiation, the deeper its penetration depth. There-

1392 fore, in food processing far infrared heating is frequently

1393 used.

1394 The heat transfer rate and efficiency are higher for infra-

1395 red than conventional heating under similar conditions. This

1396 implies that infrared heating can shorten the heating time

1397 and save energy. The intermittent infrared drying with an

1398 energy input of 10 W/m2 is equivalent to convective drying

1399 with a heat transfer coefficient of 200 W/(m2 K) [147]. Infrared

1400 predominantly heats opaque, absorbent objects, rather than

1401 the air around them. Therefore, in infrared heating, the ambi-

1402 ent temperature can be kept at normal levels, which reduces

1403 energy consumption. In addition, infrared heating is multi-

1404 functional and can be used in drying, baking, roasting, pas-

1405 teurization, thawing and blanching. It is a space-saving,

1406environmentally friendly, easy to operate, simple to con-

1407struct, and a contactless heating method.

14085.4.2. Applications of infrared blanching1409Infrared (IR) blanching is a new blanching technology that is

1410applied to inactivate enzymes and simultaneously removes

1411a certain amount of moisture in fruits and vegetables

1412[148,149]. Compared to conventional heating systems, infra-

1413red blanching has higher energy efficiency, shorter process

1414time, larger heat transfer coefficient, and also, it accommo-

1415dates convective, conductive, and microwave heating. Infra-

1416red blanching can achieve the purposes of conventional

1417blanching and drying in one simple step.

1418Infrared blanching can work in two heatingmodes: contin-

1419uous and intermittent heating. In the case of continuous

1420mode, the infrared radiation intensity is kept constant. The

1421continuous infrared heating mode is suitable for quick

1422enzyme inactivation, as it delivers a high constant energy to

1423products [150]. For example, Zhu and Pan [151] found that it

1424took 2–15 min to achieve 90% inactivation of POD in apple

1425slices with thicknesses of 5–13 mm using the continuous

1426heating mode. Intermittent heating can be performed by

1427operating the infrared radiation using on and off modes dur-

1428ing the blanching process. This saves energy and yields good

1429quality products, since the desired processing temperature

1430can be maintained [152].With these advantages, infrared

1431blanching has been applied to several fruits and vegetables.

14325.4.2.1. Apple. Zhu et al. [153] evaluated the effectiveness

1433of dipping treatments on reducing enzymatic browning of

1434apple cubes before the infrared blanching process. It was

1435found that the combination of any two chemicals among

1436the three chemicals (ascorbic acid, citric acid, and calcium

1437chloride) tested could effectively reduce browning rate.

1438Results showed that dipping apple cubes in 0.5% ascorbic acid

1439and 0.5% citric acid for 5 min was the most favorable pretreat-

1440ment that maximally preserved product color and texture and

1441avoided excessive solid loss conditions for blanching apple

1442cubes.

1443In addition, Lin et al. [154] developed an infrared blanching

1444and drying process to improve the quality of apple slices.

1445Apple slices were blanched under infrared radiation for

144610 min with an intensity of 4000 W/m2. Heat and mass trans-

1447fer models were developed to predict temperature and mois-

1448ture profiles and enzyme inactivation rate during blanching

1449and dehydration. It was found that thinner apple slices were

1450more suited to the models than the thicker slices, which

1451might be due to ununiformed temperature distributions

1452within the thicker slices [154]. Furthermore, Zhu et al. [150]

1453applied intermittent infrared heating for blanching and dry-

1454ing of apple slices. A three-factor factorial experimental

1455design was performed to evaluate the effects of processing

1456parameters including apple slice surface temperature (70,

145775, and 80 �C), slice thickness (5, 9, and 13 mm) and process-

1458ing time (2, 5, 7, 10, 15, 20, 30, and 40 min) on drying rate, dry-

1459ing kinetics, and final product quality in terms of surface

1460color, moisture reduction, and PPO and POD activities. It

1461was found that the intermittent heating was generally slower

1462than continuous heating, resulting in greater moisture reduc-

1463tion, but a similar overall surface color change [150].




20 February 2017


1464 5.4.2.2. Carrot slices. Vishwanathan et al. [155] employed

1465 intermittent infrared blanching and a combination of infrared

1466 and air drying to dry carrot slices. The performance was com-

1467 pared with conventional water or steam blanching and hot air

1468 drying in terms of POD inactivation kinetics, vitamin C reten-

1469 tion and rehydration characteristics. A maximum vitamin C

1470 retention of 62% was observed in the samples blanched by

1471 infrared blanching, while vitamin C retention was 49% and

1472 43% for steam and hot water blanching, respectively. How-

1473 ever, the blanching time required to inactivate the POD

1474 enzyme to the desired level was 3, 5, and 15 min for steam,

1475 hot water, and infrared blanching, respectively. In the case

1476 of drying time, it was found that infrared blanched samples

1477 dried by the combined drying mode took approximately 45%

1478 less time than that of the hot water blanched and hot air dried

1479 samples. It was also found that the samples blanched by

1480 infrared heating and then dried by the combined method

1481 had a rehydration rate of about 5% higher than the ones

1482 blanched by hot water and then dried by hot air. This

1483 investigation illustrated that infrared blanching and infrared

1484 combined with hot air drying not only reduced the processing

1485 time, but also obtained better product quality when com-

1486 pared with the traditional method involving hot water

1487 blanching followed by hot air drying [155].

1488 Although the benefits of IR blanching in destroying

1489 enzymes that deteriorate quality, however, it can also nega-

1490 tively affect tissue cell membrane disruption, protein denatu-

1491 ration, turgor loss, deteriorated firmness and crispness.

1492 Galindo et al. [156] compared the blanching of carrot slices

1493 for 7 s with radiant energy in the far infrared region at a radi-

1494 ant surface of 810 K, for 5 to 30 s in boiling water. It was

1495 observed that cell damage was superficial and less pro-

1496 nounced in infrared blanching than that in boiling water

1497 blanching for 5 s. The carrot slices blanched by infrared heat-

1498 ing maintained higher quality in terms of tissue strength than

1499 that blanched in boiling water.

1500 5.4.3. Limitations of infrared blanching1501 Although the aforementioned advantages of infrared blanch-

1502 ing, it does have some limitations such as surface deteriora-

1503 tion due to overheating, non-uniform heating due to the

1504 poor penetration, oxidation, charring due to the surface tem-

1505 perature of food products increasing rapidly and overheating

1506 with time, and low yields due to loss water.

1507 5.4.3.1. Poor heat penetration. Infrared radiation cannot

1508 penetrate deep in product with only a few millimeters below

1509 the surface of the sample [109]. This makes the IR blanching

1510 not suitable for the thick samples such as potato cubes and

1511 apple quarters. Therefore, in order to alleviate this problem

1512 and enlarge the application of infrared blanching technology,

1513 the combination of infrared technology with microwave and

1514 other traditional conductive and convective modes of heating

1515 were proposed. For example, Hebbar and Ramesh [157]

1516 designed a combined system of continuous infrared and hot

1517 air (shown as Fig. 6) that can be used for different operations,

1518 with minor changes in design, such as drying, blanching,

1519 roasting, and baking of food materials. It was found that the

1520 combination of infrared and hot air might be a better alterna-

1521 tive to IR processing as it provides the synergistic effect,

1522resulting in an efficient thermal process and giving the syner-

1523gistic effect [158]. Furthermore, the combined infrared with

1524hot air technology reduced the drying time of products by

152548% and saved more energy (63%) when compared with hot

1526air drying [157].

1527In addition to combining IR with other heating methods,

1528intermittent heating was also suggested to mitigate the prob-

1529lem of limited penetration of infrared heating, since the

1530desired processing temperature can be obtained [159].

15315.4.3.2. Un-uniform heating. According to the black body

1532radiation law, the radiation energy that the material can

1533absorb is negatively associatedwith the square of the distance

1534between the sample surface and the radiator. The wide varia-

1535tion of energy absorbed in different parts of food means the

1536penetration capacity is poor. These factors cause the un-

1537uniform heating of infrared radiation. Generally, the surface

1538temperature of food products increases rapidly and heat is

1539transferred to the inner part by conduction. However, due to

1540the poor penetration, the sample temperature decreases as

1541the sample depth increases. Therefore, this may cause over-

1542heating and even charring on the surface of produce, while

1543the inner part is insufficiently heated to inactivate PPO and

1544POD. Hence, infrared blanching is suitable for the leafy vegeta-

1545bles with thin thickness. It should be noted that even for the

1546thin food samples, external agitation or moved belt is needed

1547to expose all parts of the food to uniform radiation so that it

1548could be possible to alleviate the un-uniform heating problem.

15495.4.3.3. Serious water loss and surface color1550degradation. Infrared blanching technology is still not

1551widely used in industry, since many technical problems occur

1552during the process. One of them is that it can cause a large

1553percentage (up to 49%) of water loss during blanching and

1554severe surface color degradation, especially when it comes

1555to thick samples and requiring a high inactivation rate (over

155690%) of POD [151]. Water loss is a serious problem for certain

1557fruits and vegetables. It can negatively affect product quality

1558such as poor texture, reduced size and undesirable color. In

1559this case, alternative means of infrared blanching with high

1560efficient blanching and minimum moisture dehydration are

1561very tempting for the food industry.

1562For the convenience of comparation, different blanching

1563technologies and their applications are summarized in Table 1.

15646. Future trends

15656.1. Investigations on products microstructure change1566during thermal blanching

1567Microstructure of a material determines its macroscopic

1568properties [160,161]. The change of product microstructure

1569is needed to enhance the understanding of the mecha-

1570nisms of the changes in food texture, and mechanical per-

1571formance of the products. In addition, the information on

1572change in microstructure is essential for better process

1573control and improving product appearance by optimizing

1574the blanching parameters. Research is needed in the fol-

1575lowing areas:




20 February 2017


1576 - Determine the kinetics of the ultra-structure change during

1577 the blanching of fruits and vegetables using atomic force

1578 microscope (AFM), environmental scanning electronmicro-

1579 scopy (ESEM), or transmission electron microscope (TEM)

1580 - Explore the relationship between microstructure change

1581 and macro-properties such as texture, the mechanical per-

1582 formance of the products

1583 - Elucidate the relationship between microstructure changes,

1584 extraction of bioactive compounds, moisture transfer, air

1585 elimination, and microbial inactivation

1586 - Determine the degradation kinetics of cell wall polysac-

1587 charides such as pectin, cellulose and hemicelluloses

1588 under different thermal blanching conditions

1589 - Develop quantitative methodologies to evaluate and

1590 describe microstructure change.

1591

1592

1593

1594 6.2. Development of new hybrid technologies for blanching

1595 Application of only one thermal blanching method is some-

1596 times not very effective in inactivating enzymes while

1597 maintaining product quality. Hybrid technologies that com-

1598 bine two of the blanching technologies could alleviate the

1599 shortcomings of using single technology. There is a need

1600 to develop new hybrid technologies for blanching in order

1601 to achieve uniform heating, minimizing loss of nutrients,

1602 increasing energy efficiency, and reducing pollution. Several

1603 hybrid technologies have been proposed:

1604 - Combining traditional thermal blanching with ultrasound

1605 can significantly accelerate the heat transfer rate, reducing

1606 the blanching time accordingly, reducing nutrition loss,

1607 and increasing energy efficiency. Combining these two

1608 technologies can successfully accelerate the inactivation

1609 of PPO and POD enzymes by cavitation phenomena and

1610 enhance mass transfer [162]. Combining hot water and

1611 ultrasound blanching could significantly improve the qual-

1612 ity of the color pigments, contents, chlorogenic acid and

1613 mineral elements, as well as reduced the microbial popu-

1614 lation [25].

1615 - Steam blanching combined with vacuum could increase

1616 the penetration of the superheated steam in the product

1617 that could reduce the blanching time due to the improve-

1618 ment of heat transfer coefficient between product and

1619 steam. The vacuum could be provided by a vacuum pump

1620 that removes the air in the blanching chamber.

1621 - Radio-frequency heating with lower frequencies (13.56,

1622 27.12, and 40.68) and longer wavelengths, could penetrates

1623 into products deeper than microwave heating. Therefore,

1624 radio frequency heating is more suitable for thick fruits

1625 and vegetables. Research is needed to evaluate the combi-

1626 nation of radio frequency heating other blanching meth-

1627 ods to replace microwave blanching.

1628

1629 6.3. Evaluation and enhancing the sustainability of1630 thermal blanching using life cycle assessment (LCA)

1631 The consumption of energy, the emissions of the greenhouse

1632 gas, and the environmentally safe disposal methods of wastes

1633are among the challenges for all people all over the world.

1634Thermal blanching is an energy intensive process with the

1635production of wastewater from steam and hot water meth-

1636ods. Reducing the energy consumption in blanching and cost

1637effective wastewater treatment of wastewater are important

1638to increase the profits of food processing, reduce CO2 emis-

1639sion, and promotes sustainable development of the industry.

1640Life cycle assessment (LCA) is a powerful tool to evaluate

1641the sustainability of product or a process. It compiles and

1642evaluates inputs and outputs and the potential environmen-

1643tal impacts of a product, or a process throughout its life cycle

1644[163,164]. LCA can help in identifying the most environmen-

1645tally friendly blanching process, enhancing energy efficiency,

1646and identifying the best wastewater management method.

1647There is a need to evaluate the LCA for different blanching

1648technologies ‘‘from cradle to grave”.

16497. Conclusions

1650Blanching is a very important unit operation in fruits and veg-

1651etables processing. It not only affects the inactivation of PPO,

1652POD, but also affects other quality attributes of products. Ther-

1653mal blanching can inactivate enzymes present in products,

1654enhance dehydration rate, remove pesticide residue, and

1655reduce microbial load. The indicators that are frequently used

1656to assess include POD and PPO enzymes, ascorbic acid and

1657nutrient contents, color and texture. The conventional water

1658and steam blanching methods are mature technologies that

1659are being applied inmany food processors. However they need

1660a lot of energy, negatively affect the nutrient contents, and pro-

1661duce highly polluted wastewater. There are other emerging

1662thermal blanching technologies, including HHAIB-,

1663microwave-, ohmic-, and infrared blanching that have also

1664advantages and shortcomings. Several future research trends

1665needs are discussed and identified.

1666In today’s advanced food processing technologies, the

1667trend is to minimize nutrients loss, environment load and

1668cost of production; and to maximize nutrients retention, sus-

1669tainability of the process, and energy efficiency to produce

1670better quality products. Therefore, selecting a suitable

1671blanching technology that could achieve the desire product

1672quality and reduce the negative environmental foot print is

1673crucial for food products. To select a suitable blanching tech-

1674nology, it is crucial to understand the mechanisms of differ-

1675ent blanching technologies; physical and chemical

1676properties of the products; and the effect of different tech-

1677nologies on the quality attributes of final products, and the

1678environment. Due to the variations of the properties among

1679different fruits and vegetables no single blanching technology

1680can be effectively applied for all products.

16818. Uncited references

1682[64,143]

1683Acknowledgements

1684This research is supported by the National Natural Science

1685Foundation of China (No. 31360399), the National Key Tech-




20 February 2017


1686 nology R&D Program of China during the Twelfth Five-year

1687 Plan Period (2015BAD19B010201) and the Chinese Universities

1688 Scientific Fund (No. 2011JS018). We thank Prof. Tang Juming

1689 (Washington State University) for his useful discussion and

1690 comments on the current work.

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