current trends in drying and dehydration of foods-academia-libre

In: Food Engineering, pp 311-352 ISBN 978-1-61728-913-2 Editor: Brendan C. Siegler 2011 Nova Science Publishers, Inc.

Submitted: July, 2010 Accepted: December, 2010 Published: March, 2011

Chapter 6

CURRENT TRENDS IN DRYING AND DEHYDRATION OF FOODS

Debabandya Mohapatra1 and Sabyasachi Mishra2 1College of Food Processing Technology & Bio-Energy,

Anand Agricultural University, Anand-388110, Gujarat, India 2College of Agricultural Engineering & Post Harvest Technology,

Central Agricultural University, Ranipool-737135, Gangtok, Sikkim, India Emails: [email protected]; [email protected]

ABSTRACT

Drying and dehydration techniques have constantly been evolving since ancient time; from sun drying to solar drying, from convective air drying to impingement drying. The heating medium has changed from sunlight to dielectric and electromagnetic radiation, from hot air to jet impingement, from steam to superheated steam etc. Drying essentially is a simultaneous heat and mass transfer process, wherein heating medium or internal heat generation helps in evaporation of free water molecules from the product. Mass transfer rate, during the drying/dehydration process, can be enhanced by different pretreatments, apart from using enhanced temperature, optimum air flow rate in case of convective drying or using high intensity electric field as in case of dielectric and other electromagnetic drying systems however, opting for extreme conditions, product quality may be compromised. To suit the consumer demand for quality product, current technologies are aiming at integrating different pre-treatments like blanching, chemical treatment, physical modification, application of thermal and non-thermal processes, for inactivation of enzymes, reduction in microbial load and structural modification with an aim to enhance mass transfer rate. Enhanced mass transfer rate eventually overcomes the drying cost and deterioration associated with longer drying time. Innovative drying technologies such as refractance window drying, corona air or electrohydrodynamic drying, super-critical CO2 drying and bio-film drying are trying to address some problems associated with drying. Various hybrid drying technologies that manifest judicious integration of several dehydration techniques such as osmosis, convective, vacuum, microwave, radiofrequency, infrared and ohmic heating and freeze drying with non-thermal processing like high pressure, ultrasound, pulse electric field and irradiation are cost effective, as these methods reduce drying time considerably at the same time maintaining the product quality.

Keywords: pre-treatment, hybrid drying, non-thermal processing, electromagnetic heating.

Debabandya Mohapatra and Sabyasachi Mishra 312

1. INTRODUCTION Drying or dehydrations is one of the ancient methods and important unit operations

for preserving food. Considering its importance, it is imperative to pay attention to the end product quality and cost associated with it, as more and more health conscious consumers are demanding for better quality product. Sun drying, a renewable source of energy has been used since time immemorial, but the product quality and storability is not satisfactory owing to its longer drying time. Moreover, this process is weather dependent and possible in places where, abundant sunshine is available. Alternatively, freeze drying (FD) provides an excellent means of food preservation with maximum retention of nutrients, flavor and quality; it has its limitations, however. Freeze-dried product often tastes spongy as the ice crystals formed during freezing damage the tissue structure. Besides, it is very expensive and only applicable to high end product. On the other hand, convective air drying (AD), though economical, takes excess time and does not contribute to uniform product quality. Therefore, researchers all over the world are coming up with new drying technologies that cater for the product quality in terms of color, flavor, texture and microbial safe product, at the same time minimize drying time and cost.

The term drying may be loosely referred to as the removal of free moisture through addition of heat. This phenomenon is dependent on heat and mass transfer rate, which in turn is dependent on the shape, size, composition, and structure of the product as well as mode of drying. Simultaneous heat and mass transfers largely depend on moisture movement in the capillary zone, liquid and molecular diffusion, vapour diffusion through the pore spaces, hydrodynamic flow; all of them occurring either in parallel or series throughout or in some phases of the drying process. Considering the fact that all biological materials have different composition and structure, providing altogether different cellular barrier to the moisture migration, adds to the complexity (Nieto et al., 2001) to mass transfer rate during drying or dehydration process. Plant tissues, in particular, comprised of cells with vacuoles, cytoplasm, tonoplast, plasmalemma, cellwall, and intercellular spaces. Moisture can migrate out through (i) trans-membrane transport via tonoplast and plasmalemma, (ii) symplastic transport via cytoplasm, and (iii) cellwall (Molz & Ikenberry, 1974 and Tyree, 1970 cited by Nieto et al., 2001). During the first falling rate period of drying, moisture diffusion governs the process. According to Ficks second law of diffusion, the moisture transfer during the dehydration process depends on the intercellular space, tortuosity, deformation of the vegetable tissue, chemical composition, and structure of the food. Looking at the diverse nature of the food products undergoing drying/dehydration process, it is essential to consider the nature of inherent barriers and the means to modify it, to gain maximum advantage for the drying/ dehydration process.

2. PRE-TREATMENTS Enhanced heat and mass transfer rate during drying or dehydration can be achieved

by disintegrating the cellular matrix through various pre-treatments such as physical modification, chemical treatment, enzymatic treatment, blanching, and non-thermal treatments like ultrasound, high pressure processing (HPP), irradiation, pulse electric field (PEF) etc. Some of these pre-treatment are discussed briefly in the following sections.

Current Trends in Drying and Dehydration of Foods 313

2.1. Physical Modification Moisture diffusion can be enhanced by cutting the fruits and vegetable surface by

rupturing the cellwall or removing the resinous cellwall that deters the moisture transfer, as in case of raisins, plums or prunes. Enhanced diffusion can also be achieved by sizing or cutting into pieces, as it would increase exposed surface area to drying environment (Doymaz, 2008). Since drying behaviour depends on the thermal and mass diffusivity, it is imperative to consider size and shape of the food material prior to drying to improve the drying efficiency. Piercing or abrasion of skin can also increase the diffusion, as this process will aid in diffusion through the breakage of relatively impervious intact cellwall and chemical pre-treatment can be avoided (Di Matteo et al., 2000). Jazini and Hatamipour (2009) have tried to enhance the mass transfer rate during air-drying by piercing the plums. As compared to the chemical dip of the plums in NaOH solution, the pierced plums dried faster. This provides an insight how the chemical treatment can be avoided by opting for simpler physical methods. Many fruits and vegetables are frozen prior to drying. On freezing the samples, the water molecules form icicles, disrupt the cellular structure; on drying the larger icicles collapse. Since the cellular matrix do not regain their original shape, water diffusion become faster during dehydration process (Arvalo-Pinedo & Murr, 2007).

2.2. Chemical Treatment The cellwall, which is the characteristic feature of higher plant tissues, is very

complex in nature, comprising of fibrous cellulosic matrix, in which pectin, hemicellulose, proteins, pigments and some phenolic compounds are embedded (Sila et al., 2008). In case of fruits and vegetables, cellulosic matrix and pectin that turn into protopectins during ripening process, retard the moisture movement. Fruits can be treated with enzymes like pectinase, pectin methyl esterase, polygalactouronase for acting on pectin and converting them into protopectins, which then disassociated itself from the inner core (Baker & Wicker, 1996), making way for moisture diffusion and subsequent evaporation from the surface during drying/dehydration process. Grapes had long been treated with alkali to dissolve the resinous cellwall that prevents water diffusion across the impermeable membrane for production of raisin. Ethyl or methyl oleate and water solution in potassium carbonate emulsion are known to dissolve the wax cuticle, wherein potassium carbonate serves as an emulsifier to maintain the ethyl or methyl oleate in suspension. These chemical treatments reduce the drying time by physically cracking the grape skin (Petrucci et al., 1974; Gabas et al., 1999). Fruits and vegetables loose their texture and color during dehydration process; treating the samples with calcium chloride prior to drying can impart firmness to the dried food sample. Ca+2 facilitate cellwall cross-linking, thereby preventing shrinkage and textural collapse during dehydration. Citric acid, sodium chloride, sodium meta bisulphite, sodium bicarbonate, sodium hydroxide, potassium meta bisulphite (KMS), and magnesium oxide have been used as chemical pre-treatment prior to drying of various fruits and vegetables (Saravacos et al., 1988; Sian & Ishak, 1991; Rocha et al., 1993; Mahmutolu et al., 1996; Pangavhane et al., 1999; Negi & Roy, 2000; Doymaz, 2004a; Kadam et al., 2006) for reducing drying time. Some of the works are listed in table 1.


Table 1. Chemicals used in pre-treatments to reduce drying time

Chemical Product References Citric acid Apple Doymaz, 2009

Paw Scallop meat Marquez-Rios et al., 2009 Ethyl oleate solution Mulberry fruits Doymaz, 2004c

Seedless grape Esmaiili et al., 2007 Sour cherry Doymaz, 2007

Ethyl oleate and NaOH Grape Dev et al., 2008 Ethyl oleate and K2CO3 Black grape Doymaz, 2006

Seedless grapes Doymaz & Pala, 2002 Ethyl oleate and KMS Apricot Doymaz, 2004b Fermented whey Mushroom Walde et al., 2006 KMS Mushroom Walde et al., 2006 KOH Plum Tarhan, 2007 K2CO3 with olive oil Seedless grapes Caglar et al., 2009 MgCO3 Dasheen leaves Maharaj & Sankat, 1996 NaOH Plum Tarhan, 2007

2.3. Blanching

2.3.1. Conventional Hot Water or Steam Blanching Fruits, vegetables and other biological materials are considered living even after

harvesting. They undergo various biochemical and physiological changes, unless until the changes causing substrates are inactivated by different means, such as blanching. Blanching is a pre-processing step where fruits and vegetables are subjected to high temperature, generally either in the form of hot water or steam. Lately, microwave (MW), radiofrequency (RF), infrared (IR) and ohmic heating (OH) are also used to reduce the blanching time and overcome nutrient losses. Blanching causes inactivation of enzymes responsible for biochemical changes such as browning, chlorophyll, lycopene, and carotene degradation, off-flavor development, reduction in microbial load and escape of entrapped gas in the intracellular spaces. At the same time, it leads to structural and ultra-structural alteration of tonoplast and plasmalemma associated with cellwall, gelatinization of starch, thermal denaturation of mucilage, and increase in intercellular spaces. The disintegration of the cellwall, which otherwise prevents moisture transfer, aids to the diffusion of moisture from centre to the surface of the product, resulting in reduction of drying time and better quality product (Doymaz, 2008).

Hot water is one of the common methods of blanching fruits and vegetables; the temperature ranging between 70-100 C. It results in more uniform heating and allows processing at lower temperature (De Corcuera et al., 2004). In hot water blanching however, there are more leaching and nutrient losses (Abdel-Kader, 1991) and requires higher blanching time and water as compared to steam blanching (Ogunmoyela, 1989). On the other hand, steam blanching though more efficient in restricting leaching and nutrient losses, may cause over blanching at the surface and under blanching at the centre of larger food product. Another limitation is the use of food grade steam for blanching purpose (De Corcuera et al., 2004). Sotomea et al. (2009) has used super-heated steam at 115 C and hot water micro droplets (18 %) to achieve blanching in potato. Color and


texture degradation were prevented with super-heated steam and water micro droplet blanching as compared to hot water blanching.

Blanching when done in sugar or brine solution prevents browning, pigment deterioration, leaching loss, and firming up the cellular structure. Though, the cellular disruption enhances the drying rate but for osmotically treated samples; due to glucose uptake and shrinkage, the effective diffusivity decreases (Nieto et al., 2001).

2.3.2. Microwave Heating

Use of MW energy started during the World War II as by product of radar technology. These electromagnetic waves lie in the frequency ranges of 300 MHz to 300 GHz with corresponding wavelengths from 1 to 0.001 m. Worldwide, MW frequency of 915 and 2450 MHz are used for domestic and industrial purpose (Vadivambal & Jayas, 2008). MW heating system generally consists of MW generator or magnetron, wave guide and applicator. MW heating, however depends on the dielectric property of the food material, which in turn depends on the frequency of the microwaves, food temperature, moisture content, salt content or ionic conductivity, and other constituents. Food materials are poor insulator, have the capability of store and dissipate electrical energy when subjected to rapidly alternating electromagnetic field. The electromagnetic field polarizes the bound water molecules and causes ionization. As the electromagnetic field polarity changes very fast (at frequency of 2450 MHz), the polarized molecules and ions oscillate between the electromagnetic field. Collision between the polarized molecules, ionized atoms and polar molecules thus generate enough heat energy to evaporate the water molecule, by freeing it from its present state. The volumetric heat generation is instantaneous, throughout the food material, compared to conventional conduction or convection heating process (Tang et al., 2002). In MW heating system, both the dielectric constants and the loss factor due to polarization of bound water in foods would increase with temperature. On the other hand, these two properties of free water would decrease when temperature increases (Tang et al., 2002). There are certain limitations with MW heating though i.e. hot spot generation due to non-uniform heating and limitation of penetration depth (Vadivambal & Jayas, 2007). Microwaves at 915 MHz have more penetration depth as compared to 2450 MHz but heating rate is slower in the latter case. Literatures suggests that MW pretreatment prior to osmotic dehydration and convective air drying, increases mass transfer rate, reduces drying time considerably by increasing the effective diffusivity (Moreno et al., 2000; Severini, et al., 2005; Contreras et al., 2008).

2.3.3. Radiofrequency Heating

RF waves basically falls under the 10-300 MHz category of the electromagnetic radiation range. The commercial and domestic operating frequencies are 13.56, 27.12 or 40.68 MHz. A RF heating system generally consists of two main components (i) a generator and (ii) an applicator. The generator section is where the RF power is generated and the applicator section is where the material is placed and heated. RF has the advantage over MW that it can penetrate to a greater depth (22 m at 13.56 MHz) as compared to MW (0.3 and 7 cm at 2450 MHz to 915 MHz), simpler in construction and higher electric to electromagnetic power conversion (Tang et al., 2005). Since it can penetrate to greater depth larger fruits and vegetables can be blanched using RF energy prior to processing and drying. Some of the limitations of the industrial scaling up of RF system for blanching are lack of sufficient data on dielectric properties of food material in the RF range as compared to MW systems. But the loss factor of most moist foods,


especially those with high salt content, increases with product temperature in the RF frequency range. This often leads to significant non-uniform heating. A major challenge in RF heating research and development is to design RF applicators that provide uniform field patterns in foods and to overcome possible thermal runaway in moist foods containing dissolved salts (Tang et al., 2005).

2.3.4. Infrared Heating

When radiant electromagnetic energy impinges upon a food surface, it may induce changes in the electronic, vibrational, and rotational states of atoms and molecules. As food is exposed to IR radiation, it is absorbed, reflected, or scattered. IR waves are the part of electromagnetic spectrum with wavelengths varying form 300 GHz (1 mm) to 400 THz (750 nm). It can be divided into three parts (i) near IR 120 to 400 THz (2,500 to 750 nm), (ii) Mid-IR, 30 to 120 THz (10 to 2.5 m) and (iii) far IR 300 GHz to 30 THz (1 mm to 10 m) (Krishnamurthy et al., 2008). Radiant energy belonging to IR region on impingement on the biomaterials is absorbed on the surface and increases the heat flux (Parroufe et al., 1992), but Dutta and Ni (2002), in their work, postulated that IR waves are also capable of internal heating, depending on their penetrating depth. The food substances absorb FIR energy most efficiently through the mechanism of changes in the molecular vibrational state, which can lead to radiative heating. Water and organic compounds such as proteins and starches, which are the main components of food, absorb FIR energy at wavelengths greater than 2.5 m (Sakai & Hanzawa 1994), with most food falling in the high transmissivities (low absorptivities) 2.5-3.0 m (Sandu, 1986) region. IR energy is more effective in surface moisture removal as compared to convective hot air, as the radiant energy has higher heat flux. The waves directly heat the product without heating the ambient air (Jones, 1992), through internal heat generation as well as absorbance of radiant energy, which in turn converts to heat energy (Ginzburg, 1969 cited by Sharma et al., 2005a). IR has excellent radiation characteristics and high energy conversion rates can be achieved. Since radiant energy very quickly generates heat within the material, lowering the temperature gradient quickly, energy consumption reduces considerably. IR energy can be achieved by using ceramic coated radiators (Mongpraneet et al., 2002). Since the penetration depth is not much as compared to other electromagnetic waves such as MW, RF; IR can only be employed in cases where the sample thickness is very small (about 1 mm) and surface heating governing the process as it is the case of conductive or convective heating. This mode of energy transfer is quite energy efficient and not environmentally hazardous (van der Drift et al., 1997). Because of its low penetration depth, IR energy has not been used extensively as a thermal processing mode for blanching fruits and vegetables. Research findings suggests that IR heating causes surface cell damage limited to less than a millimeter distance and has better texture compared to hot water blanching process (Galindo et al., 2005). Simultaneous blanching and partial dehydration of apple slices has been attempted by Zhu et al. (2010) and it was found that PPO and POD inactivation was better in thinner slices and with good visual quality.

2.3.5. Ohmic Heating

Ohmic heating or Joules heating or resistance heating is the heating of food materials by passage of alternating current, through two oppositely charged electrodes, where the food material itself serves as the resistance. Applied alternating electrical field ionizes the molecules which collide with each other to dissipate heat energy. The dissipated heat energy is proportional to the square of the electric field strength and


electrical conductivity (Sastry & Li, 1996). Electrical conductivity increases with temperature and field strength as a result of reduction in drag movement by the ionized particles. Solid content and particle distribution reduce the electrical conductivity, as the larger and solid particles resist ionic movement (Castro et al., 2003). Linear distribution of electrical conductivity () with respect to temperature and voltage strength has been proposed (Eq.1) for semi-liquid materials like apricot and peach puree (Icier & Ilicali, 2005).

CBTD n (1) where, is voltage strength (V/cm), T is temperature (C), and D, B and C are

constants. Since OH depends on the flow of current, distribution of heat is uniform and rapid. It

does not involve any moving part, utilizes clean electrical energy, and poses no environmental hazard. However, there is irreversible electroporation in the food samples when high intensity electric field is applied across the food sample (Lebovka et al., 2005b). This subsequently brings about changes in the cellular matrix. The membrane destruction changes the mobile water and the voltage gradient induces fluid motion through the capillary. Thus, heating can cause enhanced mobile moisture movement and ionic mobility which is then reflected as increased thermal conductivity and OH rate (Wang & Sastry, 1997). OH has been successfully utilized for the purpose of blanching, fermentation, extraction of juice, evaporation, and dehydration (Eliot-Godereaux et al., 2001; Sensoy & Sastry, 2004; Lakkakula et al., 2004). OH in blanching reduces leaching losses in vegetables and takes lesser time as compared to hot water blanching (Mizrahi, 1996; Icier et al., 2006). The efficiency of OH is dependent on the conductive nature of the food to be processed (Zoltai & Swearingen, 1996). Conductivity of few food materials is presented in table 2, which signifies that the electrical conductivity has higher for animal product with higher protein content compared to that of fruits and vegetables and starch solutions. These properties can be used to the advantage of using OH for processing animal products. As OH causes electroporation in the food materials, its use as blanching process prior to dehydration has been worked out for various food materials. Some selected examples of fruits and vegetables pre-treated with OH prior to drying for enhancing drying rate delineated in table 3.


Table 2. Electrical conductivity of some food materials at 25 C

Commodity Conductivity, S/cm Commodity Conductivity, S/cm Apple golden delicious 0.067 0.020a Peach 0.170 0.018a Apple Red delicious 0.075 0.016a Pear 0.084 0.019a Beef 0.44b Pineapple 0.037 0.014a Chicken Breast Tender Thigh Drumstick

0.37b

0.665 0.048a 0.549 0.023a 0.348 0.040a 0.444 0.038a

Pork Top loin Shoulder Tender loin

0.560 0.051a 0.532 0.031a 0.584 0.033a

Carrot 0.13b Potato 0.32b Wheat starch 13.30.210-4 c Potato starch 1.140.210-4 c Corn starch 2.30.210-4 c Rice starch 0.650.210-4 c Strawberry 0.186 0.047a Yam 0.11b a Sarang et al., 2008 b Palaniappan and Sastry, 1991a,b c Morales-Sanchez et al., 2009, starches of water suspension of 30:70 (w/w)


Table 3. Ohmic heating as pretreatment for drying of fruits and vegetables

Product Ohmic heating Salient features References Apple cubes 60V/cm Enhanced electrical conductivity for higher electric strength and

time, higher water and sugar transfer rates at a moderate temperature of 37 C

Allali et al., 2009

Red delicious apple cylinders

4 Hz saw tooth (20 & 40V/cm), 60Hz sine wave (40,60,70V /cm)

4 Hz saw tooth pre-treatment yielded higher air drying rate, advantageous for intermediate foods

Lima & Sastry, 1999

Grapes 14 V/cm, 30 Hz, 60 Hz and7.5 kHz

Increased drying rate dependence on frequency, higher at lower frequency (30 and 60 Hz)

Salengke & Sastry, 2005

Potato cubes 30-100V/cm Enhanced diffusivity in convective drying Lebovka et al., 2006 Strawberry Enhanced mass transfer in osmotic dehydration Allali et al., 2008 Sweet potato Higher hot-air and vacuum drying rates Wang & Sastry, 2000

Zhong & Lima, 2003 Yam 60 Hz sine wave and

4 Hz saw tooth wave 4 Hz saw tooth pre-treatment yielded higher air drying rate Lima & Sastry, 1999


2.4. Non-Thermal Process

2.4.1. Pulse Electric Field Biological tissues react differently when external electrical field is applied across

them. The electric potential, if high enough (above 10 kV/cm), when applied even for less duration (micro or nano seconds) can alter the tissue structure, resulting in permeabilization of cellwall and pore formation (Knorr et al., 1994; Ho & Mittal, 1996; Knorr & Angersbach, 1998; Jeyamkondan et al., 1999; Ade-Omowaye et al., 2000). Since the pore formation is limited to certain area, not the whole membrane, the cell matrix remains intact. Although various food products have different morphological structure and electro physical properties, most of them respond to voltage application in the similar fashion. Membrane breakage is linked to electric field strength and pulse, suggesting that when electric field strength is within the limit of cell resistance, there can be reversible electroporation, but once this exceeds, the cells undergoes permanent damage (Angersbach et al., 1999, 2000, 2002). Additionally, the magnitude of the electric field necessary for reversible membrane permeabilization by electric pulses decreases when the cell radius increases (Zimmermann, 1986 as cited by Ade-Omowaye et al., 2001a). This characteristic feature of PEF, especially in food system has great potential for its use as pre-treatment to drying, as it would enhance the mass transfer rate (Kemp & Fryer, 2007). PEF application deviates from OH by the high magnitude and very less duration of electric field. PEF coupled with temperature can decrease the damage time of the cellular structure (Lebovka et al., 2005a) and invariably cut down the total drying time, significantly (Ade-Omowaye et al., 2001a). Compared to other thermal treatments, this process does not increase the temperature of the product, thus reducing nutrition loss and heat sensitive volatiles (Butz & Tauscher, 2002). Moreover it take very less time to achieve same degree of structural disintegration required for enhancing mass transfer rate, compared to other thermal processes. PEF has been applied to increase the mass transfer rate during osmotic dehydration of apples (Taiwo et al., 2002, 2003a), strawberries (Taiwo et al., 2003b), bell peppers (Ade-Omowaye et al., 2002), carrots (Rastogi et al., 1999; Mishra et al., 2006), drying of red beet roots (Shynkaryk et al., 2008), potato (Lebovka et al., 2005), red paprika (Ade-Omowaye et al., 2001b). The success of PEF treatment, however, lies on the design of electrodes preventing electrolysis and rise in temperature for heat sensitive food product.

2.4.2. Power Ultrasound

Sound waves carry acoustic energy and can be transmitted though pressure fluctuations in air, water or any other elastic media. These acoustic waves when encounter any deviation of particles from their mean position, they try to level it off; thereby passing some amount of energy to the next particle. So the disturbances go on in a cyclic manner, forming compression, through increase in pressure and rarefaction, though decrease in pressure, in the medium (Mulet et al., 2003). Sound waves can be classified into three categories i.e. supersonic (frequency < 20 Hz), audible (20 Hz < frequency >20 kHz), or ultrasound (frequency >20 kHz). Ultrasound waves can again be classified into two categories, high frequency-low energy waves that are used for non-destructive quality measurement and analysis and low frequencyhigh energy waves or power ultrasound, which is of importance in drying and dewatering process. Power ultrasound usually refers to the frequency range between 20-40 kHz. Power ultrasound has been used to accelerate processes such as dehydration, drying, freezing and thawing, tenderization of meat, crystallization of lactose and fat and to improve processes such as


cutting, extraction, emulsification, ageing of wines and esterification (Gallego-Juarez, 2006; Bhaskaracharya et al., 2009). Its use as dewatering or drying of biomaterials has gained momentum just few years ago. Power ultrasound waves, when propagated through any food medium, result in cavitation and bubble formation, compression and rarefaction, as well as cavitation heating caused due to absorbance of acoustic energy on the product interfaces and streaming. Cavitation occurs when acoustic waves propagate through liquid media, forming very small bubbles. If the bubbles are of critical size, as determined by the wave frequency; then they explode releasing energy in the form of impulses which can have local point temperature about 1000 K and pressure of 1000 atm. But it does not raise the temperature of the liquid food material; if the bubbles explode near the boundary then it gets evaporated. These phenomena result in structural disintegration and energy dissipation to the medium (Mason et al., 1996). The high intensity acoustic waves (20-40 kHz) disintegrate the cellular structure, leading to cell permeabilization; opening up the pores leading to higher mass transfer during drying/ dehydration process, almost doubling the moisture diffusion coefficient (Simal et al., 1998; Gallego-Juarez et al., 1999; Fuente-Blanco et al., 2006; Garcia-Perez et al., 2006). Power ultrasound application as pre-treatment prior to drying has been applied to different fruits and vegetables such as melon (Fernandes et al., 2008), apple (Carcel, et al., 2007, Deng & Zhao, 2008), banana slices (Fernandes & Rodrigues, 2007; Mohapatra et al., 2009; Azoubel et al., 2010; Mohapatra et al., 2010 a,b,c), button mushrooms, brussels sprouts, cauliflower (Jambrak et al., 2007). Reports claim that the rehydrated product has better quality compared to product treated with conventional blanching process and untreated samples.

2.4.3. High Pressure Processing

HPP is essentially a non-thermal processing in which the food products are subjected to high pressure (100 to 600 MPa). High pressure equipments include high pressure generating piston and pressure vessels. In the inlet end, oil of relatively low pressure and smaller area on displacement, enables to generate high pressure (to the tune of 700 MPa) on the outlet end, which then is applied to process the food product. HPP have advantage over the thermal processing, being capable to affect the product instantaneously and uniformly, without affected by the shape and size of the product (Torres & Velazquez, 2005). Since this process involves minimal heating, the organoleptic and nutritional properties are similar to unprocessed foods, with better stability (Butz et al., 2003) and has been accepted by consumers for its naturalness and improved taste (Nielsen et al., 2009). In addition to cause enzymatic inactivation (Montero et al., 2001; Phunchaisri & Apichartsrangkoon, 2005; Niu et al., 2009; Terefe et al., 2010), microbial inactivation (Buzrul et al., 2008; Castro et al., 2008; Shao & Ramaswamy, 2008; Jofre et al., 2009) and retention of antioxidative capacity (Garca et al., 2001; Butz et al., 2002), this process also brings about textural changes in the food (Butz et al., 2002, De Roeck et al., 2010), which could be of importance considering mass transfer phenomena in drying and dehydration process. Since there are structural changes associated with HPP, as it exerts non-hydrostatic stress on the cellwall, modifying it (Hartmann et al., 2006); thermal conductivity increases, confirming the uniform structural disintegration during the processing (Zhu et al., 2008). Ultra-high pressure can affect the hydrophobic and ionic bonds within or between macromolecules, but does not influence other stronger bonds, like hydrogen bonds in hemicellulose, cellulose and lignin (Yang et al., 2009). Some reports also claim protein (Tedford et al.,1998) and carbohydrate structural modification, thus loss of allerginity and change in functional properties in case of cereals and legumes (Estrada-Giron et al., 2005). The textural properties of plant origin depend on the


enzymatic and non-enzymatic reactions; choosing proper temperature and pressure combination, some of the enzymes can be activated and some can be inactivated to aid in processing, depending on the food product and the nature of enzymes (Oeya et al., 2008).

Some of the advantages of HPP can be summarized as follows (i) better quality retention compared to thermal process, (ii) in some case cheaper alternative to thermal processing, (iii) uniform and homogenous effect on the food regardless of its shape and size, (iv) suitable combination with temperature, can activate or inactivate enzymes, as per the requirement (v) cold pasteurization can be achieved for heat sensitive material, but it is more effective with high temperature combination, (vi) requisite pressure can be achieved with minimum time, thus lowering the processing time, (vii) the structural changes in proteins, carbohydrates can be effectively used as different functional food and dehydration system using proper temperature and pressure limit, (viii) change in cellwall and pectin structure with chemical impregnation can retain better firmness of the product, (ix) structural modification in the cellwall can aid in osmotic dehydration and drying process (Butz et al., 2002; Ibarz et al., 2004; Wennberg & Nyman, 2004).

Some research work indicated the benefits of HP treatment prior to or during osmotic dehydration of pineapple (Rastogi & Niranjan, 1998), potato (Rastogi et al., 2000), mango (Tedjo et al., 2002) and found the mass transfer rate to be higher compared to the conventional blanching process for red paprika (Ade-Omowaye et al., 2001b), high drying rate due to cell permeabilization for Amasya and red delicious apples, green beans and carrots (Yucel et al., 2010). Effect of HPP on the structural modification of various food products are compiled in table 4.

2.4.4. Irradiation

Ionizing radiation has been used for pasteurization and sterilization process for fresh and processed fruits and vegetables as high ionizing radiation dose (above 1 kGy) destroys the bacterial spore and vegetative cell. Its use as pre-treatment to drying or dehydration process is gaining acceptance as exposure to gamma irradiation causes micro structural breakage in the cellwall or cellwall permeabilization, facilitating mass transfer. Thus, the irradiated food tissues with structural damage assist the movement of mobile water molecules, acting as capillaries. Research findings on dried potato, beetroot, carrot, apple slices subjected to gamma-irradiation, doses varying from 2.5 to 12 kGy, resulted in histological changes and enhanced mass transfer rate with the increase in doses (Wang & Chao, 2002, 2003a,b; Wang & Du, 2005; Rastogi, 2005; Rastogi, et al., 2006; Mishra et al., 2006; Nayak et al., 2006a,b, 2007). Some report, however, claims that irradiation in low doses (1-2 kGy) causes higher cellcell adhesion through increasing of calcium-cross linking at the middle lamellae regions, in addition to an increment of cross-links of polymers into the cellwall (Latorre et al., 2010), this may adversely affect the moisture migration. Thorough research is needed before recommending the doses and duration of irradiation for drying pre-treatment. Though consumer have some apprehension about the irradiated food, in long run, irradiation seems to be a better option for disinfection of food, adhering to the safety norms as well as an energy saving process.

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Table 4. Effect of HPP on structural changes in foods

Commodity Operating conditions HP effect References

Broccoli 210 MPa Less cell damage, higher electrical conductivity and drip losses compared to conventionally frozen at atmospheric pressure

Fernandez et al., 2006

Carrot 100 to 550 MPa for 2, 10 or 30 min in polyethylene packed condition

100400 MPa, 5070 C, CaCl2 (0.51.5 % w/v)

80 C -0.1 MPa, 100 C-0.1 MPa and 80 C & 600 MPa

Pressure above 200 MPa resulted in loss of firmness, turgor pressure, disruption in cellwall, thickening of cellwall above 300 Mpa pressure

HP treated samples better texture than that of thermally processed

limited pectin solubilization and cell breakage indicating strong intercellular adhesion

Araya et al., 2007

Rastogi et al., 2008

De Roeck et al., 2008

Cowpea seeds

300500MPa for 15 min Protein aggregation in cell protoplasm, swelling and breakage of the cellwalls and starch granules at higher pressure

Biaszczak et al., 2007

Lychee 200600 MPa, 2060 C, & 10 or 20 min)

Marked increase in soluble solids Phunchaisri & Apichartsrangkoon, 2005

Oysters 260 MPa for 3 min Change in protein structure and higher yield

Cruz-Romero et al., 2007

Potato 700 MPa, -50 to 150 C, up to 24 h

HP combined with temperatures increased cell permeabilization

Luscher et al., 2005

Thai glutinous Rice

100600 MPa, 2070 C , 5120 min

Pressures < 300 MPa, temperatures < 60 C gave high effective diffusion coefficients but temperature > 60 C resulted in starch gelatinization

Ahromrit et al., 2006

Turkey breast 50300 MPa, 25 C, 1-15 min

NaCl diffusion increased up to 150 MPa, complete protein denaturation above 300 MPa

Villacs et al., 2008

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3. NOVEL DRYING AND DEHYDRATION TECHNOLOGIES

3.1. Corona Wind/ Electro-hydrodynamic / High Electric Field Drying Existence of electrostatic force of repulsion has been first observed by Niccolo Cabeo in

1629. It is only in 1709, the mechanism of air movement through corona discharge was discovered (Robinson, 1962). When sufficiently high electric field (AC or DC), in the order of kilovolts, in the domestic or industrial frequency range (50 or 60 Hz) is applied across the food, which is composed of complex molecules like carbohydrate, protein, fat, vitamins, polyglycerides and water, it polarizes the bipolar molecules. The movement and collision of the polarized molecules thus ionizes the neutral air particles (Bajgai & Hashinaga, 2001). The collision and movement of the clustered ionized particles hold by Coulombs force, and uncharged air particles produces an appreciable ionic wind like that of forced air convection. This enhances the heat and mass transfer rate (Wong & Lai, 2004; Lai, 2009). At the same time, polarization and evaporation of water molecules lower the entropy of the drying material. This results in giving up of heat by the drying material and subsequent lowering of temperature. The evaporative cooling effect and the exothermic heat dissipation by the food thus relates to the low temperature drying (Isobe et al., 1999). For electro-hydrodynamic drying (EHD) system, however, the Coulombs force, which results from the interaction of free charges and imposed electric field, is the major driving force, whilst the polarization forces are negligible (Lai et al., 2004).

Corona wind/ electrohydrodynamic (EHD)/high electric field (HEF) principle has been in use for quite some time as a mean to enhance heat transfer (Laohalertdech et al., 2007), for baking (Kulacki & Daubenmier, 1978), disinfestations (Shayesteh & Barthakur, 1996, 1997), enhancement of shelf life (Bajgai et al., 2006a) and increase water evaporation (Lai & Lai, 2002; Lai & Wong, 2003; Lai & Sharma, 2005; Alem-Rajabif & Lai, 2005; Jung et al., 2009) through use of various electrodes. It has been noticed that this system enhances mass transfer rate and eliminates the use of high temperature. Though many advantages of the corona wind or ionic wind drying has been reported by many researchers, it is still in nascent stage to have an industrial application. Some of the advantages of the corona wind drying over the conventional air drying as reported (Lai et al., 2004; Bajgai et al., 2006b; Brown & Lai, 2009) are (i) lower power consumption, (ii) increased heat and mass transfer rate from the body, (iii) vibration less action, (iv) absence of hygroscopic effects, (v) no moving part involved and, (vi) instantaneous control over velocity. This type of drying can be applied to food with complex geometries and has the ability to work in absence of gravity. Drying can be achieved at sub-ambient temperatures and augment osmotic dehydration (Jumah et al., 2005). Some of the characteristics of the EHD dried food materials are summarized in table 5. Corona wind/EHD/High electric field drying as a non-thermal drying method has some potential over the conventional drying process. The electrical power consumption is reported to be lower compared to oven and freeze drying techniques and multiple point system works better than the single point system (Bajgai et al., 2006b). Extensive research is needed for process optimization, before it can be adopted for large scale industrial application.

325

Table 5. List of EHD drying of food materials and their advantages over conventional

air/oven drying

Commodity Salient features References Apple slices Needle electrode had better drying rate, color of

dried product did not deviate from the pre-dried sample, no extraneous material formed in the dried product

Hashinaga et al., 1999

Baking (augmented)

Augmented Kulacki & Daubenmier, 1978

Biscuits High drying rate, more energy effective compared to fluidized bed and agitated contact drying

Goodenough et al., 2007

Japanese white radish

Reduction in shrinkage, high water absorption ratio, reduced solid loss during soaking, better color compared to oven-dried samples

Bajgai & Hashinaga, 2001a

Okara Higher drying rate, but darker color Li et al., 2006 Potato slabs Drying achieved at low temperature; Chen & Barthakur,

1991; 1994 Rough rice No significant difference in germination rate, and

percent fissured grain at lower temperature compared to control

Cao et al., 2004b

Spinach Better chlorophyll, ascorbic acid retention, no browning, and extraneous chemical formed, better storability

Bajgai & Hashinaga, 2001b

Wheat High drying rate for multiple electrodes, Low power consumption,

Cao et al., 2004a

Whey protein No significant color change, no alteration in protein structure compared to original

Xue et al., 1999

3.2. Refractance Window Drying Refractance Window (RW) drying system is a relatively new concept in food drying and

dehydration and mostly applied to liquid, and heat sensitive food. The drying system utilizes the IR energy of the heated water. In this method water heated below boiling temperature under atmospheric pressure, is circulated under a transparent plastic sheet, with the food materials flowing concurrently over it. In normal case when heated water is exposed to atmosphere, it would radiate IR energy through evaporation, but in case of RW heating system, the transparent plastic film overlaying it, acts as a barrier to evaporation and the associated heat loss. The IR energy is refracted back and the heat loss occurs only through conduction. Furthermore, when some wet material is placed over the plastic material, the IR energy is conducted and radiated through the plastic film, which acts like a window of heat transfer to the moist food material, causing the material to loose moisture in the process. As long as there is moisture in the food material, the window is open; it slowly closes as the material dries. As the IR energy is refracted back to the hot water, conductive heat is used to

current trends in drying and dehydration of foods-academia-libre

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