Scholarly article on topic 'Recent developments and trends in thermal blanching – A comprehensive review'

Recent developments and trends in thermal blanching – A comprehensive review Academic research paper on "Agriculture, forestry, and fisheries"

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{"Thermal blanching" / "Hot water blanching" / "Microwave blanching" / "Steam blanching" / "Ohmic blanching" / "Infrared blanching"}

Abstract of research paper on Agriculture, forestry, and fisheries, author of scientific article — Hong-Wei Xiao, Zhongli Pan, Li-Zhen Deng, Hamed M. El-Mashad, Xu-Hai Yang, et al.

Abstract 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 frequently used to evaluate blanching process is summarized. After that, the principles, applications and limitations of current thermal blanching methods, which include conventional hot water blanching, steam blanching, microwave blanching, ohmic blanching, and infrared blanching are outlined. Finally, future trends are identified and discussed.

Academic research paper on topic "Recent developments and trends in thermal blanching – A comprehensive review"

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Recent developments and trends in thermal blanching-a comprehensive review

Hong-Wei Xiao, ZhongliPan, Li-Zhen Deng, Hamed M. El-Mashad, Xu-Hai Yang, Arun S. Mujumdar, Zhen-Jiang Gao, QianZhang



S2214-3173(16)30091-9 INPA 74

Information Processing in Agriculture

Received Date: 22 August 2016

Revised Date: 31 December 2016

Accepted Date: 6 February 2017

Please cite this article as: H-W. Xiao, ZhongliPan, L-Z. Deng, H.M. El-Mashad, X-H. Yang, A.S. Mujumdar, Z-J. Gao, QianZhang, Recent developments and trends in thermal blanching-a comprehensive review, Information Processing in Agriculture (2017), doi:

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Recent developments and trends in thermal blanching-a comprehensive review

Hong-Wei Xiaoa, ZhongliPanc,f, Li-Zhen Denga, Hamed M. El-Mashadd, Xu-Hai Yangb, Aran S. Mujumdare, Zhen-Jiang Gaoa, QianZhangb'*

a College of Engineering, China Agricultural University, P.O. Box 194,17 QinghuaDon 100083, China

onglu, Beiji

■i 832001, China


b College of Mechanical and Electrical Engineering, ShiheziUniversity, Shihezi

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

d Department of Agricultural Engineering, Mansoura University, Mansoura, Egypt; e Department of Bioresource Engineering, McGill University, Ste. Anne de Bellevue, Quebec, Canada. f Healthy Processed Foods Research Unit, USDA-ARS, 800 Buchanan St., Albany, CA 94710, USA

ch Unit, U

E-mail addr

* Corresponding authors. Tel.:+86 10 62736978; Fax: +86 10 62736978 (Q. Zhang)


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, purposes of blanching, which include inactivating enzymes, enhancing drying rate and proi 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 frequently used to evaluate blanching process is summarized. After that, the principles, applications and limitations of current thermal blanching methods, which include conventional hot water blanching, steam blanching, microwave blanching, ohmic blanching, and infrared blanching are outlined. Finally, future trends are identified and discussed.

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1. Introduction

ing; hot water t anching

Keywords: thermal blanching; hot water blanching, microwave blanching;steam blanching;ohmic blanching; infrared bla

Blanching is a thermal treatment that is usually performed prior to food processes such as drying,

ing, frying, and canning [1, 2]. It is essential to preserve the product quality during the long-term

storage because it inactivates the enzymes and destroys microorganisms that might contaminate raw

vegetables and fruits during production, harvesting and transportation [3, 4]. Blanching involves

heating vegetables and fruits rapidly to a predetermined temperature and maintaining it for a specified

amount of time, typically 1 to less than 10 min. Then blanched product is either rapidly cooled or

passed immediately to a next process. The time required for blanching a product depends on the time

required for inactivation of peroxidase and polyphenoloxidase enzymes.

Numerous studies have been carried out for optimizing the operational parameters and design of blanching processes for different vegetables and fruits. The objectives of this article were to review (1) the purposes of blanching; (2) applied methods for evaluating blanching process; (3) the principles,

application performance' and limitations of the existing thermal blanching technologies such as hot

water, steam, microwave, and infrared blanching; and (4) research needs and future prospective of thermal blanching.

2.The purposes of blanching

The purposes of blanching are shown in Fig. 1.

2.1 Inactivaction of quality-deterioration enzymes

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Enzymatic reactions cause deterioration of fruits and vegetables during the transportation, storage and processing [5]. The main purpose of blanching is to inactivate quality-changing enzymes responsible for deterioration reactions that contribute to off-flavors, odors, undesirable color and texture, and breakdown of nutrients. Another purpose is to destruct microorganisms contaminating produce. Therefore, stabilization of texture and nutritional quality could be achieved during processing and storage [6, 7]. Kidmose and Martens [8] reported that un-blanched frozen carrots had an off-taste caused by the release of fatty acids due to esterases activity. Ramesh et al. [9, 10] observed that the carotenoid in blanched red chili dramatically increased ascompared to un-blanched red chili.

2.2 Enhancing dehydration rates and product quality

The quality and drying rate of product depend not only on the drying conditions, but also on other processes performed before and after drying [11]. For some fruits such as plums and grapes, a natural

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waxy layer covers fruit surfaces and hinders moisture transfer during drying. Blanching increases the drying and dehydration rates by changing physical properties of the products, which can improve their quality attributes. The improvement in product quality resulted from the increased permeability of cell membranes, which in turn increases the rate of moisture removal [12]. Dev et al. [13] applied microwaveas a pretreatment of grape before drying, to replace the traditional chemical pretreatments. Results indicated that the drying time of the microwaved grapes was reduced by 20% as compared to the un-pretreated ones. Moreover, the total soluble solids of the samples treated by microwave were higher than those pretreated with chemical solution. The traditional blanching methods such as hot water blanching or steam blanching can also increase the dehydration rate [14]. Rocha et al. [15] found that steam blanching significantly increased the drying rate of basil. Similarly, Ramesh et al. [ 10] observed that after steam blanching, the drying rate of pericarp increased due to


also observed that the effective diffusivity of moisture increased by more than two orders of magnitude due to steam blanching treatments [16].

Compared to the samples dried directly without blanching, Rocha et al. [15] and Singh et al. [17] found that blanching treatments resulted in better retention of chlorophyll in basil, marjoram and rosemary. Ramesh et al. [10] attributed the high quality of steam blanched products to the better retention of due to the low oxygen atmosphere. Hossain et al.[18] observed a faster drying rate and higher color value in red chilli samples that have been blanched.


2.3 Removing pesticide residues and toxic constituents

Pesticides are commonly used for controlling wild grasses and diseases in farming to obtain a better crop yield. Pesticide residues could be found on fruits and vegetables that are semi-processed or

consumed raw [19]. Residual pesticides in agricultural products threaten human health with toxic effects varying from mild diseases such as headaches and nausea to serious diseases like cancer. Therefore, removing pesticide residues in fruits and vegetables is vital for human health. Blanching plays an important role in the reduction of pesticide residues on vegetables and fruits. This reduction

could be due to degradation of the toxic substance or washing and leaching of the toxins into the

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microwave blanching and in-pack sterilization processing on the removal of five pesticide residues (deuteratedethylenethiourea, ethylenethiourea, deltamethrin, 3,5-dichloroaniline, boscalid) in spinach. Results showed that, among various processing, hot water blanching was the most effective way to remove the five pesticide residues by 10-70%, while microwave blanching without water reduced pesticide residues by a maximum of 39%, washing with tap water reduced residues by 10-50%.

2.4 Expelling air entrapped inside plant tissues

Blanching can expel air entrapped inside plant tissues, especially intercellular gas. This is a vital step prior to canning b ching can prevent the expansion of air during processing, as well as

reduce strain on the containers and the risk of misshapen cans and faulty seams. Furthermore, removing the gas from blanched pear tissues resulted in better texture as well as softer and more transparent

tissues [21]. In addition, removing oxygen from the tissue reduces oxidation of the product and

corrosion of the materials used for cans manufacturing.

2.5 Minimizing non-enzymatic browning reactions

Non-enzymatic browning, especially Maillard reaction or caramelization, occurs in food during frying, cooking, drying, and storage. This reaction could lead to the loss of product color. Maillard reaction

and/or caramelization browning reaction depends on the reducing sugar content of the products [22]. Therefore, decreasing the reducing sugar content in a product by blanching can reduce browning and improve product color. Pimpaporn et al. [23] found that hot water blanching pretreatment had a more significant effect on reducing the red color of the potato chips than the pretreatments using freezing and the immersion in monoglyceride or glycerol.

2.6 Decreasing microbial load

Microorganisms contaminate foods causing food spoilage and poisoning. Therefore, inactivation or inhibition of microbial growth is essential to assure safe and disease risk free foods. Microbial inactivation can be achieved using thermal technologies such as microwave, radio frequency treatment, ohmic heating, or non-thermal technologies such as high pressure, ozone, ultraviolet light (UV), gamma or X-ray irradiation, chlorine or iodine solutions, ultrasound, and pulsed electric fields. Conventional peroxidase (POD) and polyphenol oxidase (PPO) enzymes inactivation and microbial inactivation are two separate processes and have drawbacks of low energy efficiency and long processing time. Recently, thermal decontaminated food products are safer for consumers than

chemically and irradiated ones. Thermal blanching of some products can simultaneously achieve

inactivation of both enzymes and microorganisms. This could avoid cross-contamination or inactivation of both enzymes and microorganisms. This could avoid cross-contamination or

re-contamination, increase energy efficiency, and reduce processing time.

De La Vega-Miranda et al. [24] found that microwave blanching of the fresh jalapeno peppers and coriander foliage could achieve a 4-5 log reduction in Salmonella typhimurium. Jabbar et al. [25] found a significant decrease in yeast and mold grown on carrot after blanching with combined hot water and ultrasound treatment.

2.7 Peeling of products

Fruits and vegetables peeling is an important operation in food processing. Peeling is sometimes performed manually for some products such as tomato, potato and peanut. However, manual peeling is tedious, laborious, time consuming and subject to human error and inconsistency. Therefore, thermal, mechanical and chemical peeling methods are often applied. Although it is highly automated and efficient, mechanical peeling often causes higher peeling loss due to the difficulties in controlling peeling depth for varying product shapes and sizes. Moreover, chemical peeling methods have health and safety considerations and produce chemical and organic contaminated wastewater that is always costly to treat and dispose. Therefore, they are restricted in some countries. Steam peeling, on the other hand, has less environmental pollution and low peeling losses. Garrote et al. [26] applied steam blanching to peeling potatoes and asparagus. Results showed that steam peeling of asparagus followed by an adiabatic holding time after steam exhausting and before water cooling could sufficiently inactivate peroxidase with a peeling time of 20 s and one cycle; for potato, at a peeling time of 36 s was a good peeling quality obtained at one or two cycles, the yield was approximately 90% with three cycles.Yu et al. [27] removed the pink-red skin of peanut by boiling water blanching for 2 min.

2.8 Increasing extraction efficiency of bioactive compounds

Thermal blanching can cause structural changes in plant tissues such as disruption of cell membranes, loosening of the hemi-cellulose, cellulose and pectin networks, and alternating cell wall porosity. These can improve the extraction of bioactive compounds [28].

Gliszczynska-Swiglo et al. [29] found that, after 10 min steam blanching, the total polyphenol content

extracted from broccoli increased by 52% compared with untreated samples. The authors attributed this

phenomenon to thermal disruption of the polyphenol-protein complexes. Stamatopoulos et al. [30]

observed that after 10 min of steam blanching, the extraction yield of oleuropein from olive leaves increased from 25- to 35-fold compared to the un-blanched sample. Moreover, the antioxidant activity increased from 4 to 13 times. Although the effect of hot water blanching was not as great as steam blanching due to a leaching effect, it was also found that hot water blanching significantly increased oleuropein yields and antioxidant activity when compared with un-blanched ones. Similarly,

Hiranvarachat et al. [31] found that the contents of P-carotene, total carotenoids, and antioxidant activities of blanched carrots were significantly higher than those of the un-blanched samples.

2.9 Other purposes of blanching

Blanching can also clean the surface of plants, kill parasites and its eggs, remove damaged or discolored seeds, foreign material and dust of fruits and vegetables. Blanching of potatoes chips prior to frying can reduce the oil uptake because blanching gelatinizes the surface starch and forms a compact appearance with less pores and air cells [32].

3. Assessment of the effectiveness of blanching process

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

The effectiveness of blanching is usually judged by the inactivation degree of peroxidase (POD) and polyphenol oxidase (PPO) enzymes because they are easily measured compared to other enzymes. The POD is a heme-containing enzyme that commonly found in plant. It can catalyze a large number of reactions that are closely associated with quality deterioration in raw and un-blanched products [3]. POD enzyme can be combined with endogenous hydrogen peroxide to produce free radicals that react with a wide range of food constituents including ascorbic acid, carotenoids and fatty acids. This can cause undesirable changes in products, such as color and flavor loss, as well as nutrients degradation

[33-35]. POD is the most heat stable enzyme within the enzyme group responsible for quality deterioration during processing and storage of fruits and vegetables [2,7]. It is well documented that the destruction of POD assures the inactivation of other enzymes responsible for the deterioration of food quality [36]. Polyphenol oxidase (PPO) is another enzyme commonly used as an indicator for effectiveness of blanching process. PPO is present in nearly all plant tissues, and can also be fungi, bacteria, and insects [37, 38]. Containing four atoms of copper per molecule and binding site for two aromatic compounds and oxygen, PPO can catalyze the O-hydroxylation of O-monophenols to O-diphenols and produce O-quinones (a kind of substance with black, brown, or red). The latter is responsible for fruit and vegetable browning reactions that causes undesirable quality changes [1, 39]. POD is the most heat-resistant enzyme and requires a long-time blanching for complete inactivation (i.e., over blanching). This could cause heavy loss of nutrients and increase the cost of energy [40]. A

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On the other hand, research demonstrated that the quality of blanched and frozen product is better if

there is some POD activity left after the blanching [41]. It was suggested that optimal blanching should there is some POD activity left after the blanching [41]. It was suggested that optimal blanching should

attain 3-10% as a residual of peroxidase activity. These activity residuals were sufficient to prevent any deterioration in fruits and vegetables [41-43].

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

Thermal blanching has negative effects on heat sensitive nutrient contents, texture, and color of products. Therefore, it is essential to correlate the adequate enzymatic inactivation by the thermal blanching and nutrients loss, undesirable color changes, and texture degradation of the products. Ascorbic acid is an important substance found in almost all fruits and vegetables. It does not only prevent diseases such as scurvy, lung, bladder, and prostate cancers, but can also be used as a biological

antioxidant to delay the aging process [44, 45]. In addition, ascorbic acid can combine with other antioxidants, including vitamin E, ^-carotene, and selenium, to provide a synergistic antihypertensive effect [46]. Ascorbic acid is water soluble that makes it prone for leaching from cells. It is thermally labile, pH-, metal- ion-, and light-sensitive, and can be degraded by ascorbic acid oxidase [3,47]. Therefore, ascorbic acid is usually selected as the most frequently measured nutrient to evaluate the nutrients loss during blanching process. The preservation of ascorbic acid after blanching is a good indicator for the preservation of other nutrients [48, 49].

While the main mechanisms of ascorbic acid loss during steam, infrared, or microwave blanching could be enzymatic oxidation and thermal degradation, the main mechanism of ascorbic acid losses during hot water blanching is leaching or diffusion from the plant to the blanching water [6, 50, 51]. The loss of ascorbic acid during hot water blanching strongly depends on the blanching temperature

acid retention. Ramesh et al. [53] found that the vitamin C retention was significantly higher in microwave-blanched spinach, bell pepper, and carrots than those blanched with hot water. This was due to the low leaching losses of vitamin C in the microwave blanching.

3.3 Color as an indicator of product quality change during blanching

Color is one of the most important appearance attributes. Undesirable changes in color of food may

lead to a decrease in consumer's acceptance and market value [54, 55]. The color of raw materials or

final products can be associated with other quality attributes, such as freshness, sensory, nutritional, visual, and non-visual defects. It also has a good correlation with the antioxidant abilities, oxidation and Maillard reactions, and controls them indirectly [56-59]. The color intensity was considered as a reliable indicator of high nutritional value of carrots during hot water blanching [42]. Color is often

used as an indicator to evaluate severity of the heat treatment and to predict the corresponding quality degradation caused by blanching process.

Krokida et al. [32] studied the effect of sulfite pretreatment, water and steam blanching pretreatment on the color of dehydrated apples, bananas, potatoes, and carrots. Non-pretreated dried materials showed extensive browning, indicated by a significant drop in the lightness of color and an increase in redness and yellowness. Sulfite pretreatment prevented significant color deterioration, while water and steam blanching prevented enzymatic browning during convective drying. Color can be used as a critical parameter to optimize carrot quality attributes during hot water blanching [42]. Xiao et al. [60] studied the effect of different superheated steam blanching timeon color preservation of yam slices. When the blanching time was increased from 0 to 9 min, the whiteness index of dried yam slices increased from about 44 to 71, then dropped to 61 as the blanching time was prolonged to 11 min.

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

Product texture is a primary indicator of product quality for consumers [54, 61]. The texture of food determines the physic-chemical characteristics of the cell wall, and it indicates how they change during

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processing [42]. In general, thermal blanching significantly reduces the final textural properties of the cell structure of fruits and vegetables. The softening of the final textural properties of the product is due

to both turgor loss caused by cell membrane disruption and changes in cell wall polymers, especially the pectic substances [62].

Greve et al. [62] observed that the tissue firmness of carrot was quickly lost during the first few minutes when carrot was blanched at 90 oC, it mainly due to the loss of cellular turgor and cell wall integrity during hot water blanching. Song et al. [63] investigated the effect of three hot water blanching conditions (80 oC for 30 min, 90 oC for 20 min, and 100 oC for 10 min) on the color, texture,

nutrient content, and sensory value of soybeans. It was found that the hardness of the sample was decreased from 468.9 to 283.8 g (breaking force) as the blanching temperature increased from 80 to 100 oC. The increase in softness of soybeans during blanching was probably due to the gelatinization of

starch granules and the formation of soluble pectic substances. Sila et al. [65] found that increasi b'"d,'n8t,meenh"ed,hesoft"n8of"°,s.

solubility properties and the accompanying depolymerisation mechanisms. Fraeye et al. [66] found that thermal blanching caused a strong decrease in firmness and major tissue disruption of strawberries. Gongalves et al. [42] examined the texture change kinetics of carrot slices during hot water blanching. They found that the firmness of carrot rapidly decreased with the increase in blanching time (10-15 min) and temperature (75-90 oC) until it was at the residual texture level.

4. The traditional hot water blanching technology

4.1 Hot water blanching processing and its application

Hot water blanching is the most popular and commercially adopted blanching method, as it is simple to establish and easy to operate [4]. In a typical hot water blanching, products are immersed in hot water (70 to 100 oC) for several minutes. Then blanched samples are drained and cooled before being sent to the next processing operation. In general, after a certain amount of blanching time, the blanching water s to be replenished as it becomes saturated with nutrients leached from the products. This step does not only consume high amounts of water and energy [67]. In order to preserve the color of product and inactivate microbial activity, sodium sulfite and sodium metabisulfite are often added to the blanching water. This makes it more difficult to deal with the wastewater generated from the blanching operation.


lS1ng pectin

In order to produce high quality paprika and chili powders, immediate and complete inactivation of endogenous enzymes is a necessary prerequisite. Under humid conditions, the deteriorative enzymes such as POD, PPO, and lipoxygenase (LOX) can negatively affect taste, pungency, color intensity, and color stability during long-term storage. Schweiggert et al. [68] determined residual activities of POD, PPO, and LOX in paprika and chili powder after immediate hot water and steam blanching ml_„s,„8hot^atet,ndste„lt,l80oCfoгl0mn,90oC1„5,nd ^ ^^ min. Paprika pods were blanched at 90 0C for 1 and 5 min in water and at 100 0C for 5 min in water and steam, respectively. It was found that POD activities decreased by approximately 98% in chili and paprika powder, while PPO showed the lowest heat stability and was completely inactivated by heating at 80 oC for 10 min. It was observed that LOX inactivation was also largely accomplished by blanching at 90 oC for 5 min and 100 oC for 5 min [68].


ing. Chili was

• 100 oC for 5 and ] 0 C for 5 min in

Brussels sprouts

Lisiewska et al. [69] evaluated the hot water blanching of Brussels sprouts at 96-98 oC hot water for 5 min. After blanching, samples were cooled in cold water and left to drip on sieves for 30 min. The total

amino acids decreased from 2783 mg/100g in fresh samples to 2345 mg/100g in the blanched samples.

There was less of a decrease in amino acids caused by hot water blanching of Brussels sprouts when There was less of a decrease in amino acids caused by hot water blanching of Brussels sprouts when

compared t compared t

blanching c

mpared to the cassava leaves and broccoli [70]. The differences can be attributed to hot water anching conditions such as the ratio of material to water, blanching time, temperature, and product properties [69]. Almond

Harris et al. [71] studied the effect of hot water blanching for 12 min at different temperatures (60, 70, 80, and 88 oC) on the removal of the pellicle from the almond kernels. They also evaluated the survival

of Salmonella Enteritidis PT 30, Salmonella Senftenberg 775 W and Enterococcus faecalis on whole almond kernels before and after hot water blanching. The initial microorganism load on almonds was5 log CFU/g. It was observed that neither Salmonella serovar could be recovered after blanching at 88 oC for 2 min. Currently, in almond industry, the almonds are submerged in hot water (85-100 oC) for 2 to 5 min [72]. Therefore, these findings provided more data and information to validate almond industry

lmond indust

blanching processes. Potato chips

Potato chips are often produced by deep-frying that resulted in final products with an oil content of up to 45% (w.b.) [73]. A high fat and caloric diet can cause serious health diseases, especially cardiovascular disease. In addition, a high oil content not only increases the production cost, but also often makes the chips greasy or oily. Therefore, alternative technologies are needed to produce potato

Pimpaporn et al. [23] studied the influence of various pretreatment methods on the low-pressure superheated steam drying kinetics and quality of dried potato chips. It was observed that combining hot water blanching with freezing was the most suitable methods of pretreatment for producing good quality potato chips. Furthermore, Kingcam et al. [74] studied the effect of three pretreatments (hot water blanching and then freezing for 24 h, hot water blanching and then repeated freezing/thawing either for 3 or 5 cycles) on the degree of starch retrogradation. The pre-treated samples were then dried through low-pressure superheated steam drying, and the effects of three pretreatments on the degree of crystallinity of dried potato chips were studied. This investigation found that an increase in the degree of starch retrogradation led to higher degree of crystallinity of dried potato chips.


The food-borne illness outbreaks have increased in recent years due to the consumption of raw or processed products polluted by microorganisms, such as Salmonella in fresh vegetables and fruits, and Listeria monocytogenesin ready to eat meat [75]. No detection strategy can guarantee food safety, so in

order to best protect consumers multiple prevention efforts should be enhanced [76]. Reducing


evaluated the effect of different blanching methods on the inactivating of Salmonella during preparation, home-type (60 °C, 6 h) dehydration and storage of carrot slices. The studied methods were namely steam blanching (88 oC, 10 min), hot water blanching (88 oC, 4 min), hot water blanching (88 oC, 4 min) combined with 0.105% or 0.21% citric acid solution. It was observed that bacterial populations were reduced by 3.8-4.1, 4.6-5.1 and 4.2-4.6 log cfu/g immediately following steam, hot


h, the total reductions were 4.0-5.0 log cfu/g after steam blanching, 4.1-4.6 log cfu/g after hot water blanching, and 4.9-5.4 log cfu/g after hot water combined with citric acid blanching [77]. Hot water blanching at 88 oC for 4 min combined with 0.21% citric acid blanching was proposed as the best pretreatment method for inactivating Salmonella.


Bureau et al. [78] explored the effects of boiling water, steaming, high pressure, and microwave pretreatment on quality of 13 vegetables including green bean, pea, brussels sprout, leek (slices), broccoli, zucchini (slices), spinach branch, hashed spinach, yellow French bean, cauliflower, mushroom, carrot (slices). It was found that boiling water cooking resulted a higher loss of total ascorbic acid loss (average of -51% on fresh matter) than other three.

4.2 Limitations of hot water blanching

• Losses of nutrients during blanching

The loss of nutrients during hot water blanching is caused mainly by leaching or diffusion [4]. All

water-soluble nutrients, such as vitamins, flavors, minerals, carbohydrates, sugars, and proteins, can

leach out from plant tissues to the blanching water. In addition, hot water blanching can also lead to

degradation of some thermal sensitive substances such as ascorbic acid, aroma and flavor compounds.

It was found that about 8% of tissues and 3% of total solids were lost after hot water blanching of

carrots for 10 min at 70oC[79]. Mukherjee and Chattopadhyay [4] observed that more than 10% of

solid was lost after 129 seconds of hot water blanching of potato at 100oC. Haase and Weber [80]

investigated the effect of cutting, hot water blanching, par-frying and freezing, and final frying step, on

the loss of ascorbic acid in French fries. Ascorbic acid content decreased from 94.6 to 69.7 mg/100 g

dry matter. The reduction of ascorbic acid during blanching was also reported in broccoli and

cauliflower [81]. Ismail et al. [82] observed 20% loss of total phenolic content in cabbage after 1 min

of blanching in boiling water. The degradation of total phenolic compounds during blanching (100 oC,

1 min) of swap cabbage, spinach, shallots and kale was 26%, 14%, 13% and 12%, respectively [83].

Gawlik-Dziki [84] demonstrated that boiling water treatment significantly reduced the polyphenol t

fresh broccoli. Similarly, Sikora et al. [85] reported a significant decrease in total polyphenol antioxidant components in thermal water processed broccoli. Garrote et al. [50] reported that the loss of ascorbic acid during hot water blanching was entirely a diffusion-controlled phenomenon. The apparent diffusion coefficient of ascorbic acid in potato tissues increases as the blanching temperature increased. Lin et al. [48] performed hot water blanching (90 oC, 7 min) prior to the drying of carrot slices to inactivate ascorbic acid oxidase and prevent its enzymatic


and and

degradation in the subsequent processes. The results indicated that a substantial loss of vitamin C content from 770 to 443 ^g/g solid occurred, probably due to leaching, during the blanching. The leaching or diffusion of ascorbic acid in hot water blanching process can be positively influenced by the solid content of the water; therefore, the recycled water with a high content will lead to less [6]. This assertion has been confirmed by Arroqui et al. [85], who observed that the r< ascorbic acid was higher when potatoes were blanched in recycled hot water than when they were blanched in distilled water. •Wastewater from blanching The discharged wastewater from hot water blanching contain high concentrations of biochemical, soluble solids, and chemical oxygen demand due to leaching and dissolution of sugars, proteins, carbohydrates and water-soluble minerals. This wastewater can cause environmental pollution, e.g. eutrophication [86], if not well treated before discharge. Hot water blanching is a water-intensive industry. To alleviate the problems of the traditional hot water blanching method, new energy efficient blanching technologies are being developed and applied.

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5. The emerging and innovative blanching technologies

New blanching technologies with higher energy efficiency, less nutrient loss and less environmental

impacts are being developed and applied. The principles, characteristics, the current status of

application, and the challenges or limitations of several emerging blanching technologies are identified

and discussed in the following sections. The emerging and innovative blanching technologies include high-humidity hot air impingement blanching (HHAIB), microwave, ohmic, and infrared combined with hot air blanching.The principles, characteristics, current status of application, and challenges or limitations of these emerging blanching technologies are identified and discussed in the following

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5.1 Steam blanching and high-humidity hot air impingement blanching (HHAIB) 5.1.1The principle of steam blanching

Superheated steam is commonly used as a heating media for blanching due to its high enthalpy contents. During the early stage of steam blanching, it condenses on the surface of the products and a large amount of latent heat transfers to the material because product temperature is lower than that of steam. The temperature of the products gradually increases until reaching the critical temperature of enzymes or organisms activity, after which they are inactivated.

It is believed that the steam blanching is relatively inexpensive and retains most minerals and

water-soluble components when compared with water blanching due to the negligible leaching effects [87]. On the other hand, during the steam blanching process, softening of the tissue and undesirable quality changes often resulted a long heating time due to the lower heat transfer in steam blanching than hot water blanching, especially when the velocity of the steam is very low.

5.1.2 Applications of steam blanching

Spinach leaves

Teng and Chen [88] found that the application of boiling water and microwave blanching on spinach, ich is then followed by steam and baking blanching, resulted in the highest degradation rate of both chlorophylls a and b. In addition, pyrochlorophylls a and b were detected in spinach leaves after being steam blanched for 30 min or microwave (at 700 W and 2450 MHz) blanched for 1min. However, the authors found that pyropheophytins a and b were not formed until steam or microwave blanching took place for 30 or 5 min, respectively. The authors concluded that steam blanching favors the formation of

pyropheophytins, whereas microwave blanching favors the formation of pyrochlorophylls. Kiwifruit

To optimize blanching process, Llano et al. [89] studied the effect of steam blanching on mechanical and biochemical properties of kiwifruit. The changes in the microstructure during the blanching were determined by transmission electron microscopy (TEM) and fluorescence microscopy (FM). It was found that after 5 min of blanching, tissue became yellow-brown probably due to chlorophyll

degradation and loss in vitality of the cell membrane in the outer pericarp tissue. TEM analysis h a s also degradation and loss in vitality of the cell membrane in the outer pericarp tissue. TEM analysis has also

indicated that plasmodesmatal areas have lost the stain intensity when compared with raw tissue. The firmness of kiwifruit decreased with the increase ofblanching time and decrease in residual tissue elasticity, which coincided with membrane damage. Potato

Sotome et al. [90] compared the effects of hot water blanching (HWB), superheated steam blanching

ffects of h combined

hing time a

(SHS), and superheated steam combined with spraying of hot water microdroplets blanching

hindi almo

(SHS+WMD) on the color, texture, and microstructure of potato. The potato blanched in hot water became soft and brittle, and its brightness and chromatic quality decreased due to the absorption of water and dissolution of solid content to the water. On the contrary, these quality degradation was hindered using SHS and SHS+WMD blanching. Furthermore, while the weight of potato was kept ost constant during the SHS+WMD blanching, it was 3.3% after SHS blanching for 16 min. SHS+WMD blanching also significantly reduced water loss during blanching when compared to SHS blanching.

Liu and Scanlon [91] blanched potato strips in a steam-heated kettle at temperatures ranging from 62.8 to 90.6 oC for 2-20 min. Results showed that at low temperatures (<74 oC), blanching time had little

effect on the texture of blanched strips, while at high temperatures (^74 oC), the texture softened as blanching time increased [92]. In order to provide information to operators to manipulate the blanching process, a quantitative description model of the texture changes during steam blanching operation was developed. Mango slices

Ndiaye et al. [92] studied the effect of saturated steam blanching of mango slices (1 cm thick) at 94 ± 1 oC for 0, 1, 3, 5, and 7 min on the color and the activation of PPO and POD enzymes. They found that PPO and POD were completely inactivated after 5 and 7 min of steam blanching, respectively. If the blanching time exceeded 5 min, color loss became more serio Garlic slices

Peeled garlic suffers undesirable changes in quality, such as rapid browning, due to PPO and POD, which can be inactivated using thermal blanching. For garlic slices, FanteandNorena [93] investigated

at 80 and

the inactivation kinetics of PPO, POD, and inulinase, as well as the color. Results showed that blanching in steam for 4 min was the best treatment that achieved no changes in texture and reduced the enzymatic activities of POD, PPO, and inulinase by 93.53%, 92.15% and 81.96%, respectively. Prolonged hot water blanching could lead to serious undesirable changes in the products such as color degradation, nutrients and texture loss. Fresh broccoli

Roy et al. [87] investigated the effect of steam blanching on the total antioxidant activity of fresh broccoli by determining oxygen radical capacity (ORAC) and the reactive oxygen species (ROS). It was found that the steam blanching increased the total ORAC value by 2.3 fold. The hydrophilic part of

the effects of hot water blanching at 80 and 90 oC and steam blanching at a temperature of 100 oC on


a steam blanched broccoli had a significant reduction of 2,2-azobis [2-amidinopropane] dihydrochloride (AAPH) induced intracellular ROS level when compared to that of the fresh samples. Furthermore, the total phenolic content and total flavonoid content increased after steam blanching.


Rossi et al. [94] evaluated the effect of steam blanching on the inactivation of PPO before millir

blueberry fruits, as well as the recovery of total and individual anthocyanins and total cinnamates that are important radical scavengers of blueberry juices. It was observed that the steam blanching resulted in a significant increase in the recovery of anthocyanins in blueberry juice. Additionally, the juice produced from blanched fruits was bluer and less red than that obtained from un-blanched fruits. The authors attributed this phenomenon to the positive effect of thermal blanching on the extraction of the most soluble anthocyanin pigments, which are the most intense blue.


omeric anthoc

purees in terms of color, monomeric anthocyanin pigments (MAP), and total phenolic compounds (TPC). The steam blanching increased MAP and TPC contents by 11.3% and 51.6%), respectively as compared to the un-blanched samples.

Sal< see

Vegetable soybean

ldivar et al. [96] compared steam hot water blanching at 100 oC for 10 min of shelled green soybean t

eeds in order to identify the proper technology that preserves its sugar content. Steam blanching preserved soluble sugars in both green pods and seeds. Soluble sugars decreased in soybean seeds during water blanching due to leaching. The presence of pods effectively prevented the leaching of sugars in water blanching.

bbage samp

ater for 4 m ater for 4 m


Drying does not completely destroy microorganisms contaminating vegetables and fruits. A pre-treatmentis always needed prior to drying to ensure the deactivation of microorganisms, especially pathogens such as Salmonella [97]. Phungamngoen et al. [97] pre-treated cabbage before hot air drying, vacuum drying (10 kPa) or low-pressure superheated steam drying (10 kPa) at 60 oC. Cabba; were pre-treated either by soaking in 0.5% (v/v) acetic acid for 5 min, blanching in hot water for 4 min, or blanching in saturated steam for 2 min. They found that the Salmonella load decreased from the initial level of 6.4 logcfu/g to 1.6, 3.8, and 3.6logcfu/g after being pre-treated by soaking in acetic acid, hot water blanching, and steam blanching, respectively. It was postulated that heat accumulated during thermal blanching might damage cell membranes and cause protein denaturation of bacterial cells. The production of value-added functional dietary fibre (DF) from white cabbage was proposed because the high concentration of dietary fibre and glucosinolates. The production of DF from cabbage leaves involves thermal blanching and drying processes.Tanongkankit et al. [98] found that steam blanching better preserved glucosinolates than hot water blanching. Steam blanching of the outer cabbage leaves prior to slicing in combination with vacuum drying at 80 oC was the most favourite processing step for the production of DF.


1.3 Limitations of steam blanching

Steam blanching carried out in thick layers on moving belts, often resulted in non-uniform blanching effects [4, 99]. It needs longer blanching time than hot water and therefore it affect the capacity and the economics of processing. Selman [79] found that hot water blanching of carrots achieved higher degree of POD inactivation than steam blanching. Although steam blanching avoids the leaching of nutrients in the blanching medium, it sometimes could cause weight loss and the formation of a dried layer on

technology a

100]. educe loss of wa

product surface due to evaporation of water. Sotome et al. [90] found that employing a hot water spray systemon blanching of potato achieved a constant weight as compared to superheated steam blanching, while dipping the sample in water before steam blanching reduced water loss during steam blanching. Recently, new blanching techniques such as high-humidity hot air impingement blanching (HHAIB) technology have been developed. In the HHAIB, advantages of steam and impingement technology are combined, resulting in a uniform, rapid, wastewater free, and efficient processing [10 Compared to traditional hot water blanching, HHAIB can extensively reduce loss of water-soluble nutrients. Moreover, HHAIB is more efficient than traditional superheated steam blanching because it has high heat transfer rates [100]. For these advantages, HHAIB was used to blanch yam slices to prevent browning and to maintain color [61]. HHAIB was applied to increase drying rates of grape

[101], obtain desired color and texture in sweet potato bar [44], denature the autolyze enzyme in sea

and inactivate polyphenol oxidase in apple quarters [105]. HHAIB was appliedto blanch of red peppers, the results showed that HHAIB pre-treatment effectively denatured PPO and increased drying rate of pepper [106]. Xiao et al. [100] also presented a comprehensive review on HHAIB.

5.2 Microwave blanching

5.2.1 The operation principle and advantages of microwave heating

Microwaves are electromagnetic waves with wavelengths ranging from 1 mm to 1 m that have

corresponding frequencies ranging from 300 MHz to 300 GHz [107]. Microwaves have many uses in

modern society including communication, radar, radio astronomy, navigation, and food processing. For

industrial, scientific and medical (ISM) heating applications, only 915 MHz and 2450 MHz

microwaves are allowed because the Federal Communications Commission (FCC) of USA wants to

prevent those devices from interfering with communication signals.

In microwave heating, heated materials absorb microwave energy and convert it into heat by dielectric heating effect caused by molecular dipole rotation and agitation of charged ions within a high-frequency alternating electric field [108]. Specifically, when the oscillating electric field interacts with high water content materials, the permanently polarized-dipolar molecules particularly water molecules will align themselves in the direction of the electromagnetic field alternates at 915 or 2450 MHz [107]. The internal resistance due rotating molecules that push, pull, and collide with other adjacent molecules or atoms, produces volumetric heating [109]. Agitation of charged ions in the alternating electrical field also contributes to microwave heating, more so at 915 MHz than 2450 MHz. Microwave heating not only takes place on the surface of wet biological materials, but also within them. In conventional thermal processing, energy is transferred by conduction from the product surface to the inner part. This depends mainly on temperature gradient and the thermal conductivity of the product. Compared to conventional heating methods applied in the food industry, microwave heating has several advantages such as volumetric heating, high heating rates and short processing times. Therefore, it has been successfully used in drying, pasteurization, blanching, thawing, tempering, baking, etc. [10, 101, 110]. One of the most important features of microwave blanching is that it involves direct interaction between the electromagnetic field and food materials for heating generation. Thus, compared to that in ventional hot water blanching, the amount of nutrients loss by leaching is significantly reduced [8, 53, 111]. For example, the ascorbic acid retention was found to be higher in green beans, peas, and carrots blanched by microwave than those blanched by hot water [41]. In addition, microwave heating is rapid, very energy efficient, easy to install and clean-up, and requires a short start-up time, etc. [112].

betw conv

prod Mild

5.2.2 Applications of microwave blanching

Carrot slices

Kidmose and Martens [8] compared the influence of microwave blanching with that of steam and hot water blanching on dry matter losses and quality attributes of carrot slices in terms of texture, microstructure, sugars and carotene contents and drip losses. The microwave blanching was performed using a continuous conveyer microwave oven with 4 magnetrons (power of 1.25 kW for each magnetron) at 2450 MHz and a conveyer speed of 0.5 m/min. Steam blanching was carried out by a steamer at 90 oC for 3 min; the water blanching was performed in a jacket vessel at 90 oC for 4 min. No significant difference was found in the carrots texture after the three blanching methods. However, the microwave-blanched sample had a significantly different appearance from those blanched by steam or hot water. The microwave-blanched samples had a texture composed of a patchwork of groups of well-preserved cells, layers of collapsed and sunken cells. This was believed to be caused by high internal vapor pressure when water was converted into steam during the microwave blanching process. The dry matter, sucrose, and carotene of the samples blanched by microwave were significantly higher than those of the steam blanched samples. Hot water blanching yielded samples with the least amount of these substances. In conclusion, although the microwave blanching did not improve the texture of oduct when compared to steam and hot water blanching method, it enhanced the nutritional quality. ild blanching conditions are more appropriate in mitigating the negative effect of microwave on the texture and microstructure of the products. Lemmens et al. [113], confirmed the findings with the blanching of carrots using strong microwave blanching (90 oC, 1 min) and mild microwave blanching (60 oC, 40 min).They found that the microstructure of the samples before and after blanching (as shown in Fig.3) illustrated that the raw carrots have an intact cell structure with well-defined and

well-organized individual cells. The microstructure of samples blanched in mild microwave was more similar to the fresh ones as compared to the samples blanched in strong microwave, which caused the cell wells to disappear and different cells to melt together [113]. Mushroom

The shelf life of minimally processed mushroom is limited to a few days due to the browning during storage. Inactivation of enzymes that cause browning such as PPO through thermal

blanching, application of antioxidants, or enzyme inhibitors is essential to prevent enzymatic browning. blanching, application of antioxidants, or enzyme inhibitors is essential to prevent enzymatic browning.

Microwave blanching has been explored as an alternative method for industrial blanching of mushrooms. Direct application of microwave energy to an entire mushroom was found to unsuitable because the large temperature gradients generated within the samples during heating can result in

internal water vaporization, which is associated with damage in the texture of mushrooms [114]. In ™ethe " [1I5] explored the effea ot ~

microwave heating at 850C for different times and then immediately immersed in a 92 0C water bath for 20 s. Results clearly showed that this new blanching method completely inactivated PPO in 2 min. However, conventional hot water blanching needed more than 6 min. Product browning and the loss of antioxidant contents were significantly lower in the samples blanched by the combined microwave and water method than microwave or hot water blanching [115]. Asparagus

Kidmose and Kaack [116] compared the effects of microwave, hot water and steam blanching on the toughness and vitamin C content of asparagus. Similar or greater toughness and lower vitamin C were obtained by microwave than by steam and hot water blanching. Sun et al. [117] studied the effect of microwave-circulated water blanching on the antioxidant content and color of asparagus, while

ater and stea

comparing it to hot water blanching and steam blanching. It was observed that there was no significant difference in texture and rutin content of asparagus blanched by these methods. In addition, compared to steam blanching and hot water blanching, the microwave-circulated water blanching obtained higher antioxidant activity and better retention of green color [117]. This work confirmed that microwave-circulated water blanching has better advantages over conventional hot water and steam blanching. It is shown to be a potential alternative blanching method for asparagus. Artichokes

Ihl et al. [118] evaluated the effect of microwave-, steam-, and boiling water blanching on chlorophyllase inactivation, color changes, and loss of ascorbic acid in artichokes. It took 2, 6, and 8 min for microwave, steam, and boiling water blanching, respectively, to completely inactivate chlorophyllase. Microwave and boiling water blanching were best in preserving the original perceptual

~ whue he steamiT sample showed ,i8h,ness "

angle and chroma. Microwave blanching did not cause a significant loss in ascorbic acid when compared to the 16.7% decrease in ascorbic acid with boiling water blanching. In view of chlorophyllase inactivation, color changes, and ascorbic acid loss, this investigation clearly showed that microwave blanching is a more suitable method for blanching artichokes when compared with boiling water and/or steam blanching.

Lin and Brewer [112] evaluated the effects of direct and indirect (i.e., product in bags) microwave-, steam-, and boiling water blanching prior to freezing of manually shelled peas. Direct microwave blanching was conducted by immersing peas in water for 4 min; indirect microwave blanching was conducted by immersing packed peas in plastic bags in water for 4 min; steam and boiling water

blanching was conducted for 4 min. After blanching, the samples were frozen. The quality attributes of frozen products include peroxidase activity, ascorbic acid content, visual appearance, color, aroma, flavor, and texture were determined after storage for 0, 6, 12 weeks at -18 oC.The results showed that no significant differences were found among the studied blanching methods in the reduction peroxidase activity that was determined to be 97%. After the storage for 6 or 12 weeks, steam blanched peas


blanched by both microwave methods had more breakage and splitting appearance compared to boiling water and steam blanched ones. The authors attributed this phenomenon to the non-uniform heating characteristics of microwave, especially for the round shape materials [112]. In terms of color, both microwave blanching methods had equivalent lightness and were darker compared to the other blanched ones. There was no significant difference among blanching methods on greenness/redness (a*), blueness/yellowness (b*), grassy, grainy or earthy aromas, and sweet, fruity, or buttery flavors. Unblanched peas had the most umami flavor, while the microwave blanched ones had the least. With respect to texture, steam blanched peas were not as tough as the unblanched control samples. However, they were tougher than the samples blanched by steam or boiling water. Although the better chemical and sensory attributes (color, aroma, and flavor) obtained with microwave than conventional blanching methods, microwave blanching produces poor visual appearance and loss of physical integrity. More investigations are needed to overcome these shortcoming on peas and other products. Herbs and spices

Drying of herbs and spices is essential to extend their shelf life. This is because low moisture contents prevent the growth and reproduction of microorganisms that cause decay. Blanching is a crucial step before drying to inactivate enzymes. Application of a suitable blanching technology with a selection of

appropriate conditions are of great importance, since blanching directly affects the quality of the dried product in terms of its physical and nutritional property.

Singh et al. [17] evaluated the effects microwave (2450 MHz, 800W), boiling water, and steam blanching methods on the quality attributes of marjoram and rosemary in terms of volatile oil, color, texture, and chlorophyll and ascorbic acid contents. Immediately after blanching, the samples were dried by microwave (2450 MHz, 800W). Marjoram, cut into segments of 5cm in length, and rosemary leaves were blanched for 1 min. Prior to microwave radiation the herbs were wetted with minimum amount of water. Volatile oil content in marjoram was almost lost in all studied methods. The authors attributed the losses to the delicate nature of the herb with soft stem and flower buds and also to the presence of low boiling and more volatile non-oxygenated terpene hydrocarbons. Microwave and hot water blanching of rosemary had a 47.5% reduction in volatile oil and the steam blanching resulted in a

green color of the fresh herbs than with direct drying. Forboth herbs, hot water blanched samples had the best color (i.e., Chlorophyll retain), followed by microwave blanched samples, and lastly, the steam blanched samples. Steam blanching resulted in softer products than the other two blanching methods. Maximum retention of ascorbic acids in both herbs was obtained with microwave, followed by steam, and hot water. The latter methods caused had lower retention of ascorbic acid due to leaching in surrounding water and thermal breakdown during blanching [17].

Dorantes-Alvarez et al. [5] used microwave blanching without water for 10, 15, 20, 25, and 30 s to evaluate the changes in antioxidant activity of pepper, when treating with microwaves to inactivate PPO enzymes. After microwave blanching, the phenolic compounds of the products were reduced by 20.8% (from 9.6 to 7.6 mg/g peppers in dry weight basis), whereas the antioxidant activity was

increased by 44.8% (from 29 to 42 ^M de trolox/g peppers in dry weight basis). It is likely that microwave blanching not only inactivates enzymes, but also induces the formation of derivatives of phenolics, which enhances the antioxidant activity of the products after being blanched [5].

5.2.3 Limitations of microwave blanching

irawbacks tl

. „r____e. High ini

;tructure [8

Despite being energy efficient and requiring less time, microwave blanching has some drawbacks that could limit its application.

• Loss water during blanching

During microwave blanching, moisture in vegetables may evaporate. High intensity microwave power may cause cells folding and destruction of product microstructure [8]. To reduce water loss during blanching and increase heat absorption, vegetables may be heated while immersed water. However, water-soluble nutrients can be lost through leaching or diffusion to blanching water.

• Penetration depth of microwave is limited

The penetration depth of microwave in a sample is a function of its dielectric properties, which determines the temperature distribution within the material [119]. The dielectric properties (e) of a

product is strongly dependent on the dielectric constant ( £ ), which is a measure of the ability food material to store electromagnetic energy, and the dielectric loss factor ( £ ), which determines the ability of the material to dissipate electromagnetic energy after being heated [120]. The penetration depth (dP) of microwave into material can be determined using the following equation [121]:

X 1 -" 1

where, dP is the penetration depth, X is the wavelength of microwaves, e' is the dielectric constant, e" is the loss factor.

During microwave heating the loss factors decreased with moisture reduction, so the conversion of

Furt of th

microwave energy into heat is reduced at lower moisture contents. It has been determined that the microwave penetration depth for whey protein is about 12 mm at 915 MHz at 20 oC [122], for mashed potato sample (82.7% moisture content) is 1.6 cm [119], for sweet potato, red bell pepper, and broccolior is about 1.5-3.5 cm [121]. In addition, the penetration depth of dielectric heating decreases as frequency increases. It was observed that penetration depths in radio frequency range (27 and 40 MHz) are several times as that in microwave frequencies (915 and 2450 MHz) at each corresponding temperature [122]. Therefore, it is recommended that for larger or thick product radio frequency technology is suitable while for the small or thin samples microwave heating is better. • Non-uniform heating

Microwave heating mainly depends on the conversion of electromagnetic energy into heat via friction

of dipolar molecules, especially water molecules, and ions that follow the oscillating electrical field at of dipolar molecules, especially water molecules, and ions that follow the oscillating electrical field at

very high frequencies [107]. However, since there is an uneven distribution of moisture and ions in different parts of the samples, the microwave heating also ends up being non-uniform. With the microwave applicator producing a non-uniform microwave field, the uneven energy distribution caused hot and cold points in the sample. Furthermore, the limited penetration depth made the heating with microwave more inhomogeneous. All

hese factors cause large temperature variations when it comes to processing large and bulky materials. Koskiniemi et al. [121] used a 915 MHz and 4 kW continuous microwave system with a residence time of 4 min to pasteurize packaged acidified vegetables. It was found that the heating of the package was non-uniform. There was a hot spot of about 95 oC and cold area of approximately 80 oC, as shown in Fig.4. Walde et al. [123] also found that microwave drying of mushroom resulted in the

charring of edges due to non-uniform heating. •Difficulties to precisely control blanching temperature

The effective conversion of electrical energy in a microwave applicator to thermal energy depends

largely on the dielectric properties of products, especially the dielectric loss. The dielectric properties

are mainly determined by the chemical composition, structure and density of the products [108]. Water content and its state (free or bound water), along with ionic contents, play important roles in determining the dielectric properties of the products. Often water distribution in a product and ionic concentration in vegetables may not be uniform; this will cause non-uniform heating. This complication is confounded with standing waves in microwave heating cavities, as common design for domestic and industrial microwave heating system, causing unpredictable hot and cold spots in the

material during microwave heating. In addition, the energy decreases rapidly as the microwave _ - d _ Due w _

distribution of water in the product, standing wave effect, and rapid decay of microwave within heated foods, it hard to predict and precisely control the temperature; this results in overheating or inadequate heating during blanching. These challenges can be mitigated with a proper microwave system design for vegetables that have consistent compositions and are packed in well-defined geometries (e.g., diced carrots in vacuum sealed bags).

.3 Ohmic blanching 5.3.1 The principle of ohmic blanching

The ohmic heating is also known as Joule heating, electrical resistance heating, or electro-heating.

During ohmic heating, food products are placed between two electrodes. Food productsbehave as an

electrical resistance,in which heat is generated and product temperature rapidly increases [124, 125].

The principle of ohmic heating is shown in Fig. 5. The heat generated inside the food depends mainly on the current induced and the electrical conductivity of the product [126]. Ohmic heating has several advantages, such as fast and uniform heating. Therefore, ohmic heating systems can achieve a mild thermal treatment, instant shutdown and no residual heat transfer after shut off of the current, operation costs, high energy conversion efficiencies, and less problems of surface fouling [1

uling [12 7]. Ohn hing, evaporatic

heating has extensive potential applications in food industry, such as blanching, evaporation, dehydration, fermentation, extraction, sterilization, and pasteurization [128, 129]. The frequency of applied voltage strongly influences the performance of ohmic heating. It was found that the heating rate decreased with increasing of the frequency [130], so low frequency is frequently used. Compared to conventional hot water blanching, ohmic blanching requires a shorter time due to it volumetric heating characteristics. In addition, it yields better product quality as it reduces solids and nutrients leaching and preserves color and texture [131,132]. Furthermore, it can be used for blanching vegetables and fruits with alarger volume, which are difficult to be blanched using conventional hot water blanching that could cause quality degradation due to its low conduction and convention heat

transfer rate.

5.3.2 Applications of ohmic blanching


iehoke heads

Guida et al. [133] compared the effects of ohmic blanching with hot water blanching of artichoke heads on the inactivation of POD and PPO enzymes, total protein and bioactive compounds, and texture and color degradations. Results showed that compared with hot water blanching, ohmic blanching inactivated both enzymes at a lower blanching time and preserved the texture and color. In addition, total protein and polyphenolic contents, immediately after blanching as well as after three months of

canning storage, were higher than those of the hot water blanched ones. Carrot, red beet and golden carrot

The effects of ohmic blanching on kinetics of textural softening of cylindrical pieces of carrot roots, red beet and golden carrots were compared with that of hot water and microwave blanching [128]. It was found that ohmic heating resulted in greater softening rates and weight losses and significantly less

—-——if-indicated ohmic blanching may be not a suitable technology for blanching of the selected vegetables.

Acerola fruit (Malpighiaemarginata D.C.), a tropical fruit, is a rich source of health-promoting compounds, such as vitamin C, anthocyanins, carotenoids, and elements. Mercali et al. [134] performed

an investigation to explore the effect of pulp'ssolids content (2-8g/100g) and heating voltage -.............-........—

that the vitamin C degradation ranged from 3.08 to 10.63%, which was significantly influenced by the applied voltage and the solids content of the pulp during ohmic heating. In the case of voltage gradient, it was observed that an increase in the voltage gradient from 120 to 200 V led to an increase in the vitamin C degradation from 2.0 to 5.1% [134]. Ohmic heatingat low voltage gradients exhibited vitamin C degradation similar to that with conventional heating, while higher voltage gradients accelerated vitamin C degradation. The latter was attributed to the occurrence of electrochemical reactions that yielded oxygen, which enhanced vitamin C deterioration. The effect of electric field frequency on ascorbic acid degradation and color changes in acerola pulp during ohmic heating was explored and compared with the conventional thermostatic water heating [135]. It was found that greater ascorbic acid degradation and more color changesoccurred when the samples were blanched at

egradation ra

degradation occurre

low electric field frequency (10 Hz). Ohmic and conventional heating processes at 100 Hz demonstrated similar degradation rates of ascorbic acid and similar color changes [135]. Mercali et al. [136] experimentally compared the effect of ohmic heating and conventional hot water heating on the degradation kinetics of anthocyanins in acerola pulp at temperatures ranging from 75 to 90 oC. It was found that there was no significant difference between both heating methods on the degradation rate constants at the same temperature. This may indicate that similar mechanisms of degradation occurred with both ohmic and conventional heating. Strawberries

The effects of ohmic heating and vacuum impregnation on the osmotic dehydration kinetics and microstructure of strawberries were investigated by determining water loss, solid gain, color, and firmness of the products [137]. It was found that the greatest amount of solute gain occurred with the treatment that combines osmotic with ohmic heating, along with vacuum impregnation. This indicates that ohmic heating and vacuum impregnation can enhance mass transfer during osmotic dehydration of strawberry [137]. However, a loss of firmness was found in the samples pretreated with ohmic heating and vacuum impregnation at a higher temperature of 50oC. This was mainly because the destruction ofthe microstructure. These findings indicated that the application of ohmic heating and vacuum impregnation can enhance mass transfer and improve quality attributes when performed at a lower temperature.

In addition, to evaluate the influence of different electric field strengths (9.2, 13, 17 v/cm) on the effect of osmotic dehydration combined with ohmic heating and vacuum impregnation combined with ohmic heating on physiochemical and quality attributes of strawberry as well as on microbial stability of starage samples at 5 and 10 oC, another investigation was carried out with a 65% (w/w) sucrose

solution at 30 oC [138]. It was found that the vacuum impregnation combined with ohmic heating at 13 V/cm produced products with the greatest solute gain, least loss in firmness and least color degradation. Furthermore, the shelf life of products processed under this condition and stored at 5oC was extended from 12 d (control samples) to 25 d [138]. Blueberry pulp

Blueberry is becoming more and more popular, as it has health benefits because it is high in anthocyanins, which are potent antioxidants that have high radical-scavenging activities. Blanching is an essential step for blueberry processing to extend its shelf life through the inactivation of primary enzymes that contribute to quality deterioration and anthocyanin degradation. Ohmic heating was used to blanch blueberry pulp, and the optimal processing conditions were identified [139]. It was observed

that the degradation of anthocyanins increased with the increase of voltage and solids content. In

^^^^ ——

was similar or even lower than that obtained with conventional hot water blanching. The authors attributed the higher degradation of anthocyanins under high voltage gradients to the electrochemical reactions catalyzed by the oxygen that was generated by water electrolysis [139]. The findings in this work highlighted the parameters for the optimization of ohmic heating and the need to use inert material in electrode and electrode coating to limit water electrolysis and mitigate nutrients degradation.

Milk, fruit and vegetable juices

In order to inactivation of alkaline phosphatase, pectin methylesterase and peroxidase, ohmic heating of milk, fruits and vegetable juices was performed at several incubation temperatrues compared with conventional indirect heating [140]. It was found that compared with inactivation by conventional

indirect heating, ohmic heating enhanced the rate of enzymes inactivation in food materials. Furthermore, the kinetic parameters had changed, while inactivation mechanisms remained the same. In addition, the peroxidase in vegetable juices was more prone to destabilization with ohmic heating [140]. It was also found that only the activation entropy, not the activation enthalpy, is different ohmic heating, indicated that a cause of its decreased stability was not due to the modi enzyme tertiary structure by the electric field. Apples

The enzymatic browning and spoilage caused by polyphenoloxidase (PPO) activity in fruits and vegetables during processing and storage is a great problem for the food industry. Moreno et al. [141] investigated the effects of combining ohmic heating and osmotic dehydration with vacuum impregnation on PPO inactivation, physical properties and microbial stability of apples stored at 5 or 10 oC. It was found that there was a complete inactivation of PPO, and the least change in firmness and color was obtained with the vacuum impregnation combined with ohmic heating treatment at 50 oC.In


PPO) acti

tion of

e of the pro

addition, the shelf life of the productswas extended by more than 4 weeks when stored at 5oC.

5.3.3 Limitations of ohmic blanching

5.3.3 Limitations

•Difficulty ii con Elect ■'■ ■■■

'Difficulty in controlling the blanching temperature Electric conductivity is a crucial factor that affect the performance of ohmic heating. However, the electrical conductivity is a temperature dependent [142]. Therefore, in order to control the blanching temperature precisely, it is necessary to develop a real-time temperature monitoring system and a reliable feedback control technology to adjust the supply power according to the temperature change of the processed products. To improve the performance of ohmic heating, Zell et al. [143] designed a triple-point probe to monitor temperature changes during the blanching.

• Generating oxygen and hydrogen

The frequency of applied voltage strongly influences the performance of ohmic heating. It was found that the heating rate decreased with increasing of the frequency [130]. Conventional ohmic heating underlow frequency alternative current ranging between 50 to 60 Hz, could generate oxygen and hydrogen from the electrolyzation of water [145]. Degradation of nutrients in ohmic heating was attributed to the generation of oxygen and the anode and hydrogen at the cathode during the

ic heating w — ,rin8.

electrolysis of water. Sarkis et al. [139] found that the molecular oxygen produced through water

Hc action of oxy

electrolysis enhances oxidation of the anthocyanin. Mercali et al. [135] also observed that the use of low electric field frequency (10 Hz) led to greater ascorbic acid degradation and more color changes in acerola pulp, which may be due to the catalytic action of oxygen released by the electrolysis of water. • Corrosion and erosion of electrodes As for cellular foodstuffs such as vegetables, the cell membrane is an electrical insulator, sopure water is not a good conductor of electricity. As a result, metal ions or acidic solutions are often used to increase the electric conductivity [129]. However, the added ionic substances such as acids and salts accelerate corrosion and erosion of electrodes. It was found that the electrode materials suffered intense electrode corrosion at pH 3.5 [145]. In addition, the added salts and acids can influence the quality attributes, especially the flavor of products. Unfortunately, it is difficult toalleviate the problems associated with solutions containing salts and acids.

5.4 Infrared blanching

5.4.1 The principle of infrared heating

Infrared heating is generated by the electromagnetic radiation that falls between the regions of visible

light waves (0.38-0.78 ^m) and microwaves (1-1000 mm) [110]. Unlike thermal conduction or

convection, infrared radiation heat can propagate through both vacuum and atmosphere. It is absorbed by molecules of food components through the mechanism of rotational-vibrational movements that produces heat [110].

(ISO 20473:2007, ISO), infrared heaters can be classified into three regions: near infrared (NIR) with

Infrared heating is dependent on the wavelength of the radiation. According to the ISO 20473 scheme

red ( NIR) with

wavelengths between 0.78 to 3 ^m, mid infrared (MIR) with wavelengths between 3 to 50 ^m, and far

infrared (FIR) with wavelengths between 50 to 1000 ^m [146]. The wavelength of infrared is determined by the temperature of the radiation body; the higher the temperature, the shorter the wavelength. Water, proteins, starches, and fats, which are the main components of food, absorb far infrared energy better than near infrared energy [147]. In addition, the penetration depth of infrared radiation strongly depends on the composition and structure of the food, and the radiation wavelengths. The longer the wavelength of radiation, the deeper its penetration depth. Therefore, in food processing far infrared heating is frequently used.

The heat transfer rate and efficiency are higher for infrared than conventional heating under similar conditions. This implies that infrared heating can shorten the heating time and save energy. The intermittent infrared drying with an energy input of 10 W/m2 is equivalent to convective drying with a heat transfer coefficient of 200 W/ (m2 K) [148]. Infrared predominantly heats opaque, absorbent

ects, rather than the air around them. Therefore, in infrared heating, the ambient temperature can be kept at normal levels, which reduces energy consumption. In addition, infrared heating is multifunctional and can be used in drying, baking, roasting, pasteurization, thawing and blanching. It is a space-saving, environmentally friendly, easy to operate, simple to construct, and a contactless heating method.

g and dryi &

1 intermittei

5.4.2 Applications of infrared blanching

Infrared (IR) blanching is a new blanching technology that is applied to inactivate enzymes and simultaneously removes a certain amount of moisture in fruits and vegetables [149, 150]. Compared to conventional heating systems, infrared blanchinghas higher energy efficiency, shorter process time, larger heat transfer coefficient, and also, it accommodates convective, conductive, and heating.Infrared blanching can achieve the purposes of conventional blanching and drying in one simple step.

Infrared blanching can work in two heating modes: continuous and intermittent heating. In the case of continuous mode, the infrared radiation intensity is kept constant. The continuous infrared heating mode is suitable for quick enzyme inactivation, as it delivers a high constant energy to products [151]. For example, Zhu and Pan [152] found that it took 2-15 min to achieve 90% inactivation of POD in apple slices with thicknesses of 5-13 mm using the continuous heating mode. Intermittent heating can be performed by operating the infrared radiation using on and off modes during the blanching process. This saves energy and yields good quality products, since the desired processing temperature can be maintained [153].With these advantages, infrared blanching has been applied to several fruits and vegetables. Apple

Zhu et al.[154] evaluated the effectiveness of dipping treatments on reducing enzymatic browning of apple cubes before the infrared blanching process. It was found that the combination of any two chemicals among the three chemicals (ascorbic acid, citric acid, and calcium chloride) tested could effectively reduce browning rate. Results showed that dipping apple cubes in 0.5% ascorbic acid and 0.5% citric acid for 5 minwas the most favorable pretreatment that maximally preserved product color

and texture and avoided excessive solid loss conditions for blanching apple cubes. In addition, Lin et al. [155] developed an infrared blanching and drying process to improve the quality of apple slices. Apple slices were blanched under infrared radiation for 10 min with an intensity of 4000 W/m2. Heat and mass transfer models were developed to predict temperature and moisture profiles and enzyme inactivation rate during blanching and dehydration. It was found that thinner apple slices were more suited to the models than the thicker slices, which might be due to ununiformed

et al. [

temperature distributions within the thicker slices [155]. Furthermore, Zhu et al. [151] applied intermittent infrared heating for blanching and drying of apple slices. A three-factor factorial experimental design was performed to evaluate the effects of processing parameters including apple slice surface temperature (70, 75, and 80 oC), slice thickness (5, 9, and 13 mm) and processing time (2, 5, 7, 10, 15, 20, 30, and 40 min) on drying rate, drying kinetics, andfinal product quality in terms of surface color, moisture reduction, and PPO and POD activities. It was found that the intermittent

rate, dryir on, and PPO han continuou

heating was generally slower than continuous heating, resulting in greater moisture reduction, but a

similar overall surface color change [151].

surface color

Carrot slices

Vishwanathan et al. [156] employed intermittent infrared blanching and a combination of infrared and air drying to dry carrot slices. The performance was compared with conventional water or steam blanching and hot air drying in terms of POD inactivation kinetics, vitamin C retention and rehydration characteristics. A maximum vitamin C retention of 62% was observed in the samples blanched by infrared blanching, while vitamin C retention was 49% and 43% for steam and hot water blanching, respectively. However, the blanching time required to inactivate the POD enzyme to the desired level was 3, 5, and 15 min for steam, hot water, and infrared blanching, respectively. In the case of drying

time, it was found that infrared blanched samples dried by the combined drying mode took approximately 45% less time than that of the hot water blanched and hot air dried samples. It was also found that the samples blanched by infrared heating and then dried by the combined method had a rehydration rate of about 5% higher than the ones blanched by hot water and then dried by hot air. This investigation illustrated that infrared blanching and infrared combined with hot air drying not only

reduced the processing time, but also obtained better product quality when compared with the traditional method involving hot water blanching followed by hot air drying Although the benefits of IR blanching in destroying enzymes that deteriorate quality, however, it can also negatively affect tissue cell membrane disruption, protein denaturation, turgor loss, deteriorated firmness and crispness. Galindo et al. [157] compared the blanching of carrot slices for 7 s with radiant energy in the far infrared region at a radiant surface of 810K, for 5 to 30 s in boiling water. It was observed that cell damage was superficial and less pronounced in infrared blanching than that in boiling water blanching for 5 s. The carrot slices blanched by infrared heating maintained higher

quality in terms of tissue strength than that blanched in boiling water. quality in terms of tissue strength than that blanched in boiling water.

5.4.3 Limitations of infrared blanching

Although the aforementioned advantages of infrared blanching, it does have some limitations such as surface deterioration due to overheating, non-uniform heating due to the poor penetration, oxidation, charring due to the surface temperature of food products increasing rapidly and overheating with time, and low yields due to loss water. •Poor heat penetration

Infrared radiation cannot penetrate deep in product with only a few millimeters below the surface of the sample [110]. This makes the IR blanching not suitable for the thick samples such as potato cubes and

apple quarters. Therefore, in order to alleviate this problem and enlarge the application of infrared blanching technology, the combination of infrared technology with microwave and other traditional conductive and convective modes of heating were proposed. For example, Hebbar and Ramesh [158] designed a combined system of continuous infrared and hot air (shown as Fig. 6) that can be used different operations, with minor changes in design, such as drying, blanching, roasting, and food materials. It was found that the combination of infrared and hot air might be a better al IR processing as it provides the synergistic effect, resulting in an efficient thermal process and giving the synergistic effect [159]. Furthermore, the combined infrared with hot air technology reduced the drying time of products by 48% and saved more energy (63%) when compared with hot air drying [158].


In addition to combining IR with other heating methods, intermittent heating was also suggested to

temperature can be obtained [160]. • Un-uniform heating

According to the black body radiation law, the radiation energy that the material can absorb is negatively associated with the square of the distance between the sample surface and the radiator. The wide variation of energy absorbed in different parts of food means the penetration capacity is poor. These factors cause the un-uniform heating of infrared radiation. Generally, the surface temperature of food products increases rapidly and heat is transferred to the inner part by conduction. However, due to the poor penetration, the sample temperature decreases as the sample depth increases. Therefore, this may cause overheating and even charring on the surface of produce, while the inner part is insufficiently heated to inactivate PPO and POD. Hence, infrared blanching is suitable for the leafy

iroblems occr-iter loss durir

vegetables with thin thickness. It should be noted that even for the thin food samples, external agitation or moved belt is needed to expose all parts of the food to uniform radiation so that it could be possible to alleviate the un-uniform heating problem. • Serious water loss and surface color degradation Infrared blanching technology is still not widely used in industry, since many technical problems occur during the process. One of them is that it can cause a large percentage (up to 49%) of water loss during blanching and severe surface color degradation, especially when it comes to thick samples and requiring a high inactivation rate (over 90%) of POD [152]. Water loss is a serious problem for certain fruits and vegetables. It can negatively affect product quality such as poor texture, reduced size and undesirable color. In this case, alternative means of infrared blanching with high efficient blanching and minimum moisture dehydration are very tempting for the food industry.

For the convenience of comparation, different blanching technologies and their applications are summaryied in Table 1.

6. Future trends


tion, diffei

6.1 Investigations on products microstructure change during thermal blanching

Microstructure of a material determines its macroscopic properties [161,162]. The change of product microstructure is needed to enhance the understanding of the mechanisms of the changes in food texture,and mechanical performance of the products. In addition, the information on change in microstructure is essential for better process control and improving product appearance by optimizing the blanching parameters. Research is needed in the following areas:

-Determine the kinetics of the ultra-structure change during the blanching of fruits and vegetables using

atomic force microscope (AFM), environmental scanning electron microscopy (ESEM), or transmission electron microscope (TEM)

- Explore the relationship between microstructure change and macro-properties such as texture, the mechanical performance of the products

- Elucidate the relationship between microstructure changes, extraction of bioactive compounds, moisture transfer, air elimination, and microbial inactivation

- Determine the degradation kinetics of cell wall polysaccharides such as pectin, cellulose and hemicelluloses under different thermal blanching conditions

■ Develop quantitative methodologies to evaluate and describe microstructure change 6.2 Development of new hybrid technologies for blanching

e compound

Jh as pectin, cel

Application of only one thermal blanching method is sometimes not very effective in inactivating enzymes while maintaining product quality. Hybrid technologies that combine two of the blanching technologies could alleviate the shortcomings of using single technology. There is a need to develop new hybrid technologies for blanching in order to achieve uniform heating, minimizing loss of nutrients, increasing energy efficiency, and reducing pollution. Several hybrid technologies have been prop

bining traditional thermal blanching with ultrasound can significantly accelerate the heat transfer rate, reducing the blanching time accordingly, reducing nutrition loss, and increasing energy efficiency. Combining these two technologies can successfully accelerate the inactivation of PPO and POD enzymes by cavitation phenomena and enhance mass transfer [163]. Combining hot water and ultrasound blanching could significantly improve the quality of the color pigments, contents, chlorogenic acid and mineral elements, as well as reduced the microbial population [25].

-Steam blanching combined with vacuum could increase the penetration of the superheated steam in the product that could reduce the blanching time due to the improvement of heat transfer coefficient between product and steam.The vacuum could be provided by a vacuum pump that removes the air in the blanching chamber.

- Radio-frequency heating with lower frequencies (13.56, 27.12, and 40.68) and longer wavelengths,

could penetrates into products deeper than microwave heating. Therefore, radio frequency heating is more suitable for thick fruits and vegetables. Research is needed to evaluate the combination of radio frequency heating other blanching methods to replace microwave blanching.

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

house g

The consumption of energy, the emissions of the greenhouse gas, and the environmentally safe disposal methods of wastes are among the challenges for all people all over the world. Thermal blanching is an energy intensive process with the production of wastewater from steam and hot water methods. Reducing the energy consumption in blanching and cost effective wastewater treatment of wastewater areimportant to increase the profits of food processing, reduce CO2 emission, and promotes sustainable

development of the industry.

Life cycle assessment (LCA) is a powerful tool to evaluate the sustainability of product or a process. It

compiles and evaluates inputs and outputs and the potential environmental impacts of a product, or a

process throughout its life cycle [164, 165]. LCA can help in identifying the most environmentally

friendly blanching process, enhancing energy efficiency, andidentifying the best wastewater management method. There is a need to evaluate the LCA for different blanching technologies "from cradle to grave".

7. Conclusions

Blanching is a very important unit operation in fruits and vegetables processing. It not only affects the inactivation of PPO, POD, but also affects other quality attributes of products. Thermal blanching can inactivate enzymes present in products, enhance dehydrationrate, removepesticide residue, and reduce microbial load. The indicators that are frequently used to assess include POD and PPO enzymes, ascorbic acid and nutrient contents, color and texture. The conventional water and steam blanching methods are mature technologies that are being applied in many food processors. However they need a lot of energy, negatively affect the nutrient contents, and produce highly polluted wastewater. There are other emerging thermal blanching technologies, including HHAIB-, microwave-, ohmic-, and infrared blanching that have also advantages and shortcomings. Several future research trendsneeds are discussed and identified.

In today's advanced food processing technologies, the trend is to minimize nutrients loss, environment load and cost of production; and to maximize nutrients retention, sustainability of the process, and energy efficiency to produce better quality products. Therefore, selecting a suitable blanching technology that could achieve the desire product quality and reduce the negative environmental foot iff

print is crucial for food products. To select a suitable blanching technology, it is crucial to understand

nisms of different blanching technologies; physical and chemical properties of the products; the effect of different technologies on the quality attributes of final products, and the environment. Due to the variations of the properties among different fruits and vegetables no single blanching technology can be effectively applied for all products.


ncy to _ a for ,

the mechani the mechani


This research is supported by the National Natural Science Foundation of China (No. 31360399), the

National Key Technology R&D Program of China during the Twelfth Five-year Plan Period (2015BAD19B010201) and the Chinese Universities Scientific Fund (No.2011JS018). We thank Prof. Tang Juming (Washington State University) for his useful discussion and comments on the current work.


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ifferent co

Figure Captions

Fig. 1 The purposes of blanching. Fig.2. The comparation of inactivation kinetics of POD and PPO in potato during blanching (from Sotome et al. [137] with some changes).

Fig.3. The microstructure of the fresh and blanched carrot samples under di fferent co nditions [75]. Fig.4. The time-temperature profile and temperature distribution in different part during 915 MHz continuous microwave processing of packages of red bell peppers [71]. Fig.5. A schematic diagram of the principle of ohmic heating [48]. Fig.6. Infrared combined with hot air system-front view (A) and side view (B) [57].

Fig. 1 The purposes of blanching

Fig. 2. The comparation of inactivation kinetics of POD and PPO in potato during blanching (from Sotome et al. [137] with some changes).

(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 [75].

ions [/5

Fig. 4. The time-temperature profile and temperature distribution in different part during 915 MHz

continuous microwave processing of packages of red bell peppers [71]. A, B, C, D, E represent different locations of the package.

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

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

Table 1. Application of emerging and innovative blanching technologies

Blanching technology

Steam blanching


Spinach leaves




Mango slices

processing conditions

Steamed over boiling water for 7.5, 15, 30, 45, or 60 min.

Water vapor at atmospheric pressure (99.8 °C) and hold on a predetermined heating time.

main findings


Steaming favored the formation of

pheophytins a and b, while microwave

cooking favored that of pyrochlorophylls a and b.

dec incr

Superheat» (SHS), water

heated s , spray of micro-drc

— steam

of hot -droplets

(WMD): steam was set to 115 °C, total water supply rate was set to 3.0 kg/h

Steam temperatures from 62.8 °C to 90.6 °C and periods of time from 2 to 20 min were usedd.

Steamed at 94 ± 1 oC for 0, 1, 3, 5, and 7 min


yellow-brown atte: min of blanching, plasmodesmatal areas have lost the stain intensity, firmness reased with the crease of blanching time

The soft and brittle, and brightness and

chromatic quality

decreased of potato were hindered by SHS+WMD and SHS, when compared to blanching in hot water; SHS+WMD also

significantly reduced water loss.

Low temperatures (<74 oC), blanching time had little effect on the texture product, while at high temperatures (>74 oC), the texture softened as blanching time increased

PPO and POD were completely inactivated after 5 and 7 min of steam blanching,


Garlic slices


Steam temperature at 100 °C, for 1, 2, 4, 6, 8 and 10 min.

steamed for 5 and 10 min, respectively

Compared blanching 90 °C) duration,

to water (80 and and other steam

blanching for 4 min was the best treatment that achieved no changes in texture and reduced the enzymatic activities of POD, PPO, and inulinase by 93.53%, 92.15% and 81.96%, respectively.


blueberry purées



steamed for 3 min

steamed for 3min

steamed at 100 °C for 10 min

blanching in saturated

When compared to fresh samples, the total ORAC value increased by 2.3 fold, the 2, 2-azobis dinopropane] hydrochloride induced intracellular ROS level reduced, the total phenolic content and total flavonoid content increased after steam blanching

The recovery of anthocyanins increased, the juice was bluer and less red than that obtained from

un-blanched fruits.

The steam blanching increased MAP and TPC contents by 11.3% and 51.6%), respectively as compared to the un-blanched samples.

Soluble sugars in both green pods and seeds after steam blanching were significantly higher (16~333%) than water blanched samples.

The Salmonella load

high-humidity hot

impinge: blanching (HHAIB)



* R ment

Red pepper

Yam slice

Sweet potato

steam for 2 min

suspended over boiling water for 1 min.

air velocity of 14.0 ± 0.5 m/s, temperature of 110 °C, and hot air relative humidity of 35-40%, for 30, 60, 90, 120, 150, 180, 210, and 240 s.

air velocity of 14.0 ± .5 m/s, temperature of 110 °C for 1, 2 and 3

Superheated steam at 120 °C and 35% relative humidity, and air outlet velocity at 10.0 m/s, for 3, 6, 9 and 12 min, respectively.

Superheated steam at 120 °C and 35%

decreased from the initial level of 6.4 log cfu/g to 3.6 log cfu/g after being steam blanching, dried

blanched samples

exhibited greener and darker color than untreated samples.

Total glucosinolates retention was up to 92.40% of blanched

much higher than hot water blanched

(95 ± 2 °C for 2 min) with 52.92.

PPO residual activity was decreased to 7% after 120 s, drying time was reduced up to 7 h when blanching for 120 s, created superficial micro-cracks.

HHAIB maintained higher red pigments, ascorbic acid retention, total antioxidant activity and DPPH values, as compared with the samples blanched by hot water blanching,

reduced drying time for 4.0 h (blanching for 2 and 3 min) compared to untreated ones.

When blanching for 6 min, drying time was reduced by 35%, whiteness index was increased by 50%.

Increased drying time by 11% and 44%, but

Sea cucumber

Seedless grape

Microwave blanching

Carrot slices


relative humidity, and

air velocity was 10.0

m/s for 3 and 5 min separately.

superheated steam at relative humidity 10~50%, temperature 90~200°C, air velocity 3~20m/s. for 5~40 min.

four different

temperatures: 90, 100, 110 and 120 °C, air velocity at 15.0 m/s and relative humidity was 40~45%

obtained a homogeneous compact structure, softer texture, and desirable color.

Autolytic enzyme was completely inactivated, the color and shape of the products were improved.

(38cm - 7 min,

different temperatures (90, 100, 110, and 120 °C) and several durations (30, 60, 90, and 120 s).

a continuous conveyer microwave oven with 4 magnetrons (power of 1.25kW for each magnetron) and a conveyer speed of 0.5m/min.

microwave temperature of 60, 90 oC, for 1 or

The time PPO totally inactivated (38'

quarters, 90 °C -100 °C - 6 min, 110 °C -5 min, 120 °C - 5 min) was shorter than infrared (7min), the percentage retention of vitamin C was (11.29%, 10.79%, 7.78% and 4.48%, respectively) higher than water blanching of asparagus (3~8%).

PPO residual activity lower than 10%, the drying times reduced 12~25 h at drying temperatures of

55~75 °C, yielded desirable green-yellow or green raisins when blanching of 110 °C for 90s.

Microwaveblanching resulted in enhanced nutritional quality as compared to water and steam blanching, for instance, the content of dry matter, sucrose and carotene were 11% ~39% higher than steam and water blanching.

Mildmicrowave blanching (60 oC)is




Sweet Potato

40 min, respectively

a frequency of 2450 Hz, at low power of 450 W, for 10 min.

microwaveheating at 85oC for different times and then immediate immersed in a water bath

recommended, it

decreases the degree of methoxylation, and maintained the

microstructure well.

Microwaving led to a good texture with losing only 39.8 % of the initial energy required to cut raw samples, while boiling water and steam blanching exhibited higher percentage values (91.1 and 92.5 %, respectively).

■ [.,] m

in a 92 oC for 20 s

Microwave oven

working at

2450 MHz-900 W, for 40, 50, 60, 70 and 80s, respectively.

microwave oven with input power: 1200W, output 700W, at maximum power

The PPO completely inactivated in 2 min, lowed than nventional hot water blanching, which needed more than 6 min.

ly much C conve

The total inactivation of POD was achieved in 80 s used the microwave treatment, shorter than steam and boiling water (needed 90 and 120 s, respectively); increased of ascorbic acid content, contrary to water and steam, which decrease the ascorbic acid by 47% and 25%, respectively.

Microwave required the least of time (60 s) to inactive POD, compared to steam (110 s) or boiling water (130 s); reduced drying time by 44%, more efficient than steam or boiling water (22%); reduced the reduction of anthocyanin



Red pepper


Useda rotating plate in a stationary cavity microwave oven with a microwave power output of 556 W, blanched for 3 and 0.5

microwave-blanched in a Pyrex beaker (80 mL of water; 4 min; 800 W);

microwave-blanched a bag (80 mL of 4 min).

(59.34%), compared to steam (53.55%) and boiling water (40.37%) samples.

Spears with similar or higher shear force values and lower vitamin C were obtained by microwave blanching than by steam (at 90°C for 6 or 1 min) and water (at 90°C for 7.5 or 1 min).

Both microwave

treatments reduced peroxidase activity by compared with ls.

or 97% in contro

microwave (2450


MHz, 8i

00 W).

650, 750 and 900 W for 100 s

used a microwave oven, duration for 10, 15, 20, 25 and 30 s

Microwave and boiling water blanching of rosemary had a 47.5% reduction in volatile oil and the steam blanching resulted in a 62.5% reduction in volatile oil; microwaveblanching retained the original green color of the fresh sample than with direct drying.

The residual activity values of PPO and POD are 9.80% and 16.43%, respectively; reduced drying time for 3.5 h, at power of 900 Wfor 100

PPO was inactivated, the phenolic compounds of the products were reduced by 20.8%, and the antioxidant activity

Ohmic blanching

Artichoke heads

Acerola pulp



Useda constant

gradient voltage of 24 V/cm, once the temperature of

80 ± 2 °C was reached in the core of the artichokes, the samples were maintained at this temperature for the following holding times: 0, 60, 120, 180, 240, and 300 s.

the solids content of the pulp (2-8 g/100 g) and the heating voltage (120~200 V), samples were heated to 85 °C for 3 min

sed ohmic heating at 60 Hz, manual

transform 0~240 V, temperature at 30 °C, 40 °C and 50 °C for 300 min, as the ratio of solution to fruit was 3:1 (w/w).

Used 24 V/cm electric field strength, held at 90 oC (the center of the sample) for 15, 30, 45, and 60 s.

was increased by 44.8% after blanched for 20s.

The ohmic blanching inactivated POD and PPO at a higher rate than conventional boiling blanching, with a total inactivation time of 360 s and 480 s, respectively, higher firmness, 24% and 53% higher of proteins and polyphenol content than conventional ones, respectively.

The ohmic heating experiments carried out at low voltages (<140 V) exhibited ascorbic acid degradation similar to the conventional heating (0 V), high voltage gradients induced

greater ascorbic acid degradation.

Ohmic heating enhanced mass transference

kinetics of sample during osmotic

dehydration, larger amount of solute gain (0.181~0.262) than unheated (0.149~0.223), induced changes in the shape and thickness of the middle lamellae and increased cellular


PG (Polygalacturonase) and PME (Pectin methyl esterase) enzyme

inactivation achieved by the OH (1 min.) was similar as compared to

Vegetable baby

purees (40%

carrots, 20%

peas, 15%

zucchini, 0.1%

salt and 24.9% water)


Blueberry pulp

Used 25 kHz high voltage from the regular 50 Hz network, temperature at 129 °C for 11 min

CT (hot water bath of 90 oC) of 5 min; the ascorbic acid of OH was 29~51% higher than CT; OH Paste was more viscous and bright red than CT treatment.

Used 25 electro

field intensity 3.57 4.39 V/cm),

temperatures of 60, 70 and 80 °C for 2.0, 3.0, 3.5 and 4.0 min.

Used 30 V (60 Hz of frequency),

temperature at 80, 85, 90 and 95 °C for 0, 10, 20, 30, 40, 50 and 60 min, respectively.

Applied the voltage (160, 172, 200, 228 and 240V),maintain sample at temperature of 90 °C for 2 min.

Heating temperature at 30 °C, 40 °C and 50 °C

Ohmic heating did not have effect on the total amino acid content; contrarily, fo

conventional retor sterilization (spraying hot water on the jars at 129 °C for 10 min), the content of total essential and non-essential amino s significantly creased in 35% and 9%, respectively.

PPO was almost totally inactivated, the

degradation of total phenolics was between 11 and 23% and the degradation of total flavonoids varied from 20 to 38%, both for 6 and 12 min, and similar to hot water blanching

Ascorbic acid and carotenoid degradation was similar between IR with and without the application of the electric field.

Thedegradation of anthocyanin ohmic heating (5.7 ~ 14.7%) was lower or similar to conventional heating (7.2%).

Combined with ohmic heating at 50 °C was the

Infrared blanching



: arrot slices

Carrots slices

for 90 min, as the ratio of solution to fruit was 3:1 (w/w), with an alternating current at 60 Hz and 100 V, generating an electric field of 13 V/cm

Radiationintensity (3000, 4000 and 5000 W/m2), slice thickness (5, 9 and 13 mm) and processing time (2, 5, 7, 10, 15 and 20 min).

Appleslices with three different thicknesses, 5, 9, and 13mm, were heated used infrared for

to 10min

4000W/m2 IR intensity.

Infrared radiation chamber maintained at

180-240 °C) 8~15 min

Carrotslice surface temperature (85, 90 and 95 °C), carrot slice thickness (3, 5 and 7 mm) and processing time (2, 4, 7, 10, 15, 20 and 30 min)

best process for dehydrating apples: PPO was completely

inactivated; the water loss and the solid gain were 48% and 37% greater than untreated, respectively;the firmness was 68% higher than untreated.

It took 2-15 achieve

inactivation of P apple slices with thicknesses of 5-13 mm used the continuous heating mode.

Theprocess of

simultaneous dry

blanching and

dehydration of apple slices under IR heating can be predicted with the first-order kinetics

IR blanching reduced the moisture content by 13~23%; increased the retention of vitamin C (62%), water (43%) and steam (49%) blanching; took about 45% lesser drying time and possessed ~5% higher rehydration moisture, compared to water blanched-hot air dried samples.

IR blanching process which produced 1 log reduction in POD activity has resulted in moisture reduction from 40.2 to 88.8 g/100 g, overall color change

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

(AE) from 3.17 to 5.13 and retention of vitamin C from 56.92 to 77.34 g/100 g compared to control.

PPO was completely inactivated, AAO had remained 30% and 9%~15% after LTST and HTST; IR blanching had higher VC retention o 88.3+1.0% (HTS 69.2 + 2.9% ( compared with water blanching 61.4 + 5.3% (HTST) and 50.7 + 9.6% LT); reduced by 23%~28% drying

compared untreated.

Theresidual activity of PPO and POD are 12.18% and 16.75%, respectively, reduced drying time for 4.0 h, when treated for 3 min.


• Examined the purposes of thermal blanching.

• Summerized the indicators for assessment of blanching process.

• Outlined the principles, applications and limitations of thermal blanching technologies.

• Identified and discussed the future trends of thermal blanching.