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New Hybrid Drying Technologies
New Hybrid Drying Technologies Kian Jon Chua and Siaw Kiang Chou Department of Mechanical Engineering , National University of Singapore, Singapore
It is well known that the dehydration process influences the quality of the bioproducts. The current drive towards improving drying technologies is motivated by the economic incentive to produce better quality products at a faster rate and lower operating costs. In recent years, the engineering focus has been to improve the design and operation of dryers to achieve the dried food product with desired characteristics. Even though much has been done in the development of improving individual drying technology, much remains to be achieved in the study of new hybrid systems whereby drying technologies can be combined to evolve new age drying systems. This chapter summarizes some recent developments in hybrid drying technologies of interest to the bioproduct industry. New emerging hybrid drying technologies are listed and discussed. The potential roles of these hybrid technologies in product quality enhancement are also identified.
1 Introduction In many agricultural countries, large quantities of bioproducts are dried to enhance shelflife, reduce packaging costs, lower shipping weights, enhance appearance, encapsulate original flavour and maintain nutritional value. According to Okos et al. (1992), the goals of drying process research in the bioproduct industry are summarized by three issues:
• Economic considerations: to reduce operating costs and improve capacity per unit amount of drying equipment; to develop simple drying equipment that is reliable and requires minimal labour; to minimize off-specification product; and to develop a stable process that is capable of continuous operation. • Environmental concerns: to minimize energy consumption during the drying operation and to reduce environmental impact by reducing product loss in waste streams, i.e. to incorporate the possibility of waste heat recovery systems. • Product quality aspects: to have precise control of the product moisture content at the end of the drying process; to minimize chemical degradation reactions; to reduce change in product structure and texture; to obtain the desired product colour; to control the product density; and to develop a versatile drying process that can produce products of different physical structures for various end-users. Emerging technologies for food processing ISBN: 0-12-676757-2
Copyright © 2005 Elsevier Ltd All rights of reproduction in any form reserved
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Though the primary objective of food drying is preservation, depending on the drying technology employed, the raw material may end up as a completely different material with significant variation in product quality. Therefore, great care must be taken in choosing a suitable dryer considering the closely knitted relationship between the bioproduct and dryer. A mismatch between the product and drying technology would result in dire consequences, often resulting in great financial losses. As new discovery of hybrid bioproducts is constantly being made, the spectrum of dried bioproduct gets wider. When a single drying system cannot handle the stringent quality requirements of these new bioproducts, the need for new hybrid drying systems becomes essential. Therefore, the principal motivation in developing hybrid drying technologies is to meet consumer expectations of quality and yet produce a product with the desired moisture content. Figure 20.1 shows a general classification scheme of hybrid drying technologies. Under the umbrella of hybrid drying are drying techniques that employ multiple modes of heat transfer as well as those that use two or more stages of drying to achieve the desired dryness, product quality, drying time and manufacturing throughout. A more common definition of hybrid drying would consist of a successful integration or intelligent combination of two or more conventional drying know-hows. This alone can constitute the emergence of a wide-range of new hybrid drying technologies.
Hybrid drying technologies
Combined drying technology
Infrared-heat pump drying Infrared-convective drying Microwaveconvective drying Microwave-vacuum drying
Multiple-stage drying
Same drier type at each stage
Drying and cooling
– Two-stage fluid beds – Two-stage vibrofluid beds
Drying and coating
Different drier type at each stage – Flash/fluid bed – Spray/fluid bed – Fluid bed/packed bed Different drying technologies per stage – Superheated steam drying followed by convective air drying – IR/microwave drying followed by convective air drying
Figure 20.1
Multiple-process drying
A general classification scheme of hybrid drying technologies.
Drying and granulation Drying and filtration
Hybrid drying systems 537
This chapter serves to provide an overview of some newly developed hybrid drying technologies applicable for bioproducts that are particularly sensitive to thermal treatment. Drying technologies incorporating convective, radiation and electromagnetic heat transfer modes will be presented along with novel technologies such as pressureswing, vacuum-superheated steam and rotating spouted jet drying. The impact of each hybrid drying technology on various quality parameters will also be briefly discussed in the light of growing interest and demand for high quality dried products.
2 Product quality degradation during dehydration To understand how the employment of hybrid drying technologies can improve product quality, it will be useful first to understand the degradation process of foodstuffs. The quality of many food products degrades during dehydration above room temperature. The added heat and exposure time of the product at elevated temperature affects the rate of nutrient quality degradation. The types of food degradation during drying are listed in Table 20.1. Dried foods are known to be high in fibre and carbohydrates and low in fat, making them healthy food choices. Food scientists have found that by reducing the moisture content of foodstuffs to between 10 and 20 per cent, bacteria, yeasts, moulds and enzymes are all prevented from spoiling it. The flavour and aroma are well preserved and concentrated. The loss of nutrient can be viewed as the decomposition of a particular chemical compound. This decomposition of a single monomolecular reaction may be described using zero or first-order kinetics equations (Chua et al., 2000a). As the temperature of the product increases, the reaction rate constant is increased. The dependence of the reaction constant on temperature implies that a low temperature drying process results in less nutrient degradation. A longer constant rate drying period increases the nutrient retention because, owing to evaporative cooling, the product is at a lower temperature.
3 Hybrid drying systems The diversity of food products has introduced many types of dryers to the food industry. Often the selection of the appropriate dryer is based on the drying characteristics Table 20.1 Change in food quality parameters during dehydration Chemical
Physical
Nutritional
Browning reaction Lipid oxidation Colour loss Gelatinization
Rehydration Solubility Texture and shrinkage Aroma loss
Vitamin loss Protein loss Microbial survival
538 New Hybrid Drying Technologies
of the food product. For heat-sensitive bioproducts, the methods of supplying heat to the product and transporting the moisture from the product become the critical considerations for selecting the right drier to achieve the desired product moisture content. In the following sections, the potential of employing new hybrid driers for drying of bioproducts is presented.
3.1 Heat pump drying There has been a growing interest in recent years to apply heat pump drying (HPD) technology to foods and biomaterials where low-temperature drying and well-controlled drying conditions are required to enhance the quality of food products. High-valued products, which are extremely heat-sensitive, are often freeze-dried. This is an extremely expensive drying process (Baker, 1997). Therefore, there has been great interest in looking at the heat pump drying system as a replacement system for freeze-dried products. Table 20.2 presents a summary of some hybrid heat pump drying systems for selected bioproducts.
Table 20.2 Selected works on hybrid heat pump drying of selected food products Researchers
Hybrid system
Application(s)
Conclusions
Chua et al. (2000b) Chou et al. (2001) (Singapore)
Heat pump/IR system
Agricultural and marine products (mushrooms, fruits, sea-cucumber and oysters)
The quality of the agricultural and marine products can be improved with scheduled drying conditions
O’Neill et al. (1998) (New Zealand)
Modified atmosphere heat pump system
Agricultural food drying (apples)
Modified atmosphere heat pump drying (MAHPD) produces products with a high level of open pore structure, contributing to the unique physical properties, such as floatability and rehydration capabilities. MAHPD drying system affords the possibility of drying with colour qualities comparable to sulphured products
Best et al. (1994) (Mexico)
Solar-assisted heat pump drier
Rice
Advantage of low temperature and better control in the drier results in good quality rice
Rossi et al. (1992) (Brazil)
Heat pump assisted heating drier
Vegetable (onion)
Drying of sliced onions confirmed energy saving of the order of 30% and better product quality due to shorter processing time
Alves-Filho and Strømmen (1996) (Norway)
Heat pump assisted fluidized bed drier (HPFBD)
Marine products (fish)
The high quality of the dried products was highlighted as the major advantage of HPFBD and introducing a temperature controllable programme to HPFBD makes it possible to regulate the product properties such as porosity, rehydration rates, strength, texture and colour
Hybrid drying systems 539
Some of the advantages of the heat pump drier are as follows:
• Higher energy efficiency with improved heat recovery results in lower energy consumed for each unit of water removed.
• Better product quality with well-controlled temperature schedules to meet specific production requirements. • A wide range of drying conditions typically 20–100°C (with auxiliary heating) and relative humidity 15–80 per cent (with humidification system) can be generated. • Excellent control of drying environment for high-value products and reduced electrical energy consumption for low-value products. Along with the advantages are limitations given as follows:
• Requires regular maintenance of components (compressor, refrigerant filters, etc.) and charging of refrigerant. • Increased capital costs. For many of the research studies conducted in Table 20.2, the common conclusion was that the heat pump drier offers products of better quality with reduced energy consumption. This is particularly true of food products that require a closed-control drying environment such as temperature, humidity and/or even a special drying medium (O’Neill et al., 1998). Heat-sensitive food products, requiring low-temperature drying, can take advantage of HPD technology since the drying temperature of the HPD system can be adjusted from 20 to 100°C. With proper control, it is also possible for HPD to produce freeze-drying conditions at atmospheric pressure (Prasertsan and Saen-saby, 1998). So far as food drying is concerned, HPD offers an alternative to improve product quality through proper regulation of the drying conditions. Chua et al. (2000b) have demonstrated that HPD can produce pre-selected cyclic temperature schedules to improve the quality of various agricultural products dried in their two-stage HPD. They have shown that with appropriate choice of temperature-time variation, it is possible to reduce the overall colour change and ascorbic acid degradation by up to 87 and 20 per cent, respectively. Besides yielding better food quality, Rossi et al. (1992) have reported that onion slices dried by HPD used less energy in comparison to a conventional hot air system. Food products with a high water content can be dried efficiently using HPD. As the drying air absorbs more of the latent energy due to vaporization, this energy can be transferred at the evaporators for higher heat recovery. Lower energy input is then required at the compressor to enable sensible heating of the air when it passes through the condenser. Ginger dried in a heat pump drier was found to retain over 26 per cent of gingerol, the principal volatile flavour component responsible for its pungency, compared to only about 20 per cent in rotary dried commercial samples (Mason et al., 1994). The higher volatile retention in heat pump dried samples is probably due to the reduced degradation of gingerol when lower drying temperatures are employed. The loss of volatiles varies with concentration, with the greatest loss occurring during the early stages of drying when the initial concentration of the volatile components is low (Saravacos et al., 1988). Since heat pump drying is conducted in a closed chamber, any compound that volatilizes will remain within the drying chamber and the
540 New Hybrid Drying Technologies
partial pressure for that compound will gradually build up within the chamber, retarding further volatilization from the product (Perera and Rahman, 1990). A recent hybrid system combines radio frequency (RF) and heat pump drying (Marshall and Metaxas, 1998). Such a system has potential application in the food industry. The limitation of relative low heat transfer rates in convective air drying, particularly towards the falling rate period, can be overcome by introducing volumetric heat generation such as the RF technology. The RF field generates heat volumetrically within the material wetted with polar molecules, such as water, by the combined mechanisms of dipole rotation and ionic conduction. The internal heat generation speeds up the drying process because of unidirectional temperature and moisture gradient and internal pressure build-up. Figure 20.2 shows a schematic of a radiofrequency-assisted heat pump drier (RF-assisted HPD). Such a hybrid drier is suitable for food materials that are difficult to dry with convective heating, especially food products that have a film of wax on the surface such as chillies, cherries and tomatoes. With regards to product quality, it appears that RF-assisted HPD reduces colour degradation, surface cracking and differential shrinkage of the product. In terms of energy consumption, RF-assisted drying has been observed to improve the specific moisture extraction rate (SMER) and coefficient of performance (COP) of the heat pump system. Furthermore, the potential for increasing the product throughput is good. For example, in the bakery industry, the throughput for crackers and cookies can be improved by as much as 30 and 40 per cent, respectively (Clark, 1997).
Evaporator
Compressor Expansion valve
Condenser Axial fan
Drying product
Metallic perforated plate
Heat pump drying chamber Figure 20.2
RF generator
Schematic diagram of RF-assisted heat pump drier (Marshall and Metaxas, 1998).
Hybrid drying systems 541
Alternatively, to dry heat-sensitive materials, a combined radiant-convective drying method may be applied. An infrared-augmented HPD drying system could be used for fast removal of surface moisture during the initial stages of drying, followed by intermittent drying over the rest of the drying process. This mode of operation ensures a faster initial drying rate. Therefore, an infrared (IR) assisted HPD would offer the advantage of compactness, simplicity, ease of control and low equipment costs (Mujumdar, 2000). Also, there are the possibilities of significant energy savings and enhanced product quality due to the reduced residence time in the drying chamber. For such an IR-assisted HPD system, it is essential to implement a good control strategy for IR operation in order to achieve the desired results in terms of drying kinetics and product quality, as well as to ensure safe operation. A typical example of a good feedback control is one that enables the IR power source to be cut off if excessively high temperatures are measured in the chamber, which may lead to overheating of the product. Chou et al. (2001) designed a feedback system by coupling a PID controller to the IR lamps and by inserting a type ‘T’ thermocouple needle to the bioproduct, feedback signals were sent to the controller. In comparison to the IR operating in intermittent mode, they found that by pre-programming different food sample temperatures, greater reduction in drying time and improvement in product colour could be achieved.
3.2 Fluidized bed drying Fluidized bed drying (FBD) has found many applications for drying granular solids in the food, pharmaceutical and agriculture industries. For drying of powders in the 50–2000 m range, FBD competes successfully with other more traditional drier types, e.g. rotary, tunnel, conveyor, continuous tray, etc. The advantages of FBD include: 1 high heat and mass transfer phenomena between the particles and the gas 2 closed control product temperature making FBD ideal for processing temperaturesensitive solids 3 highest thermal efficiency of any gas-suspension drying system. The disadvantages of FBD include: 1 it is able to dry only a limited range of materials 2 the size of the product particles is relatively large 3 difficulty involved in processing needle or platelet-shaped particles. Recent hybrid fluidized bed driers incorporating a heat pump drying mechanism have been developed at the Norwegian Institute of Technology (Alves-Filho and Strømmen, 1996) as shown in Figure 20.3. The drying chamber receives wet material and discharges dried product through the product inlet and outlet ducts. The desired operating temperature is obtained by adjusting the condenser capacity, while the required air humidity is maintained by regulating the compressor capacity via frequency control of the motor speed. According to Alves-Filho and Strømmen (1996), this set-up can produce drying temperatures from 20 to 60°C and air humidity spanning 20 to 90 per cent. With these features, heat-sensitive food materials can be dried
542 New Hybrid Drying Technologies
Fluidized bed Wet material
Dry material
Condenser Centrifugal fan
Evaporator
3-way valve
Expansion valve
External condenser Liquid receiver
Compressor
Figure 20.3 Fluidized bed driers incorporating heat pump drying technology (Alves-Filho and Strømmen, 1996).
under convective air or freeze drying conditions. It is also possible to sequence these two operations (convective and freeze drying). This will be advantageous for drying of food and bioproducts since freeze drying causes minimal shrinkage but produces low drying rates while convective air drying can be applied to enhance drying rates. Therefore, a combination of drying processes, e.g. freeze drying at 5°C followed by convective drying of 20–30°C, enables the control of quality parameters such as porosity, rehydration rates, strength, texture, colour, taste, etc. Experiments performed at the department of mechanical engineering, Norwegian University of Science and Technology, on various heat-sensitive materials such as pharmaceutical products, fruits and vegetables have shown that this new hybrid fluidized bed drying offers a better product quality but at higher cost. Even as experimental work is still being conducted with this hybrid drier, a two-stage fluidized bed heat pump dryer has already interested some food industries in Norway (Strømmen and Jonassen, 1996). The two-stage system simply comprises two fluidized beds connected in series. Two heat pumps supply conditioned air independently to each drying chamber. The drying chambers are connected in series so that one receives wet product and discharges the semi-dried product to the next, which produces the final dried product. Such a hybrid two-stage drier is versatile as it allows independent freezing and convective drying to be carried out. The advantages of multiple-stage fluidized bed drying over single fluidized bed heat pump drying include improved product quality and enhanced energy efficiency (Alves-Filho and Strømmen, 1996) but at higher capital cost.
Hybrid drying systems 543
Recently, Taechapairoj et al. (2003) employed a fluidized bed system incorporating superheated steam as the drying medium for paddy drying. They observed that when using superheated steam drying the head rice yield is more sustainable and has higher values than those obtained from hot air drying. However, there was some colour depreciation in terms of the rice whiteness. The main cause of the rapid change in the colour is partly due to the steam condensation on the paddy surface.
3.3 Radio-frequency drying Radio-frequency drying (RFD) is a simple precise process and is common place in the food industry with appropriate and proven processes available for a wide range of applications, such as pre-heating, pre-cooking, sterilization, tempering, post-baking and moisture control. A limitation of heat transfer in conventional drying with hot air alone, particularly in the falling rate period, can be overcome by combining RF heating with conventional convective drying (Thomas, 1996). RF generates heat volumetrically within the wet material by the combined mechanisms of dipole rotation and conduction effects which speed up the drying process (Marshall and Metaxas, 1998). A typical RF-convective drier comprises a convective drying system retro-fitted with an RF generating system capable of imparting radio-frequency energy to the drying material at various stages of the drying process. Biomaterials that are difficult to dry with convection heating alone are good candidates for RF-assisted drying. Food materials with poor heat transfer characteristics have traditionally been problem materials when it comes to heating and drying. Radio frequency heats all parts of the product mass simultaneously and evaporates the water in situ at relatively low temperatures, usually not exceeding 82°C (Thomas, 1996). Since water moves through the product in the form of a gas rather than by capillary action, migration of solids is avoided. Warping, surface discoloration and cracking associated with conventional drying methods are also avoided. The potential for application of RF drying in the food industries can be appreciated for the following reasons:
• RFD prevents over-drying because radio waves concentrate in the wettest and densest areas of the biomaterial. It improves the colour of products, especially those that are highly susceptible to surface colour change, since RF drying starts from the centre and moves to the product surface, minimizing any surface effect. • Cracking, caused by the stresses of uneven shrinkage in drying, can be eliminated by RF-assisted drying. This is achieved in the drier by even heating throughout the product maintaining moisture uniformity from the centre to the surface during the drying process. • Simultaneous external and internal drying significantly reduces the drying time to reach the desired moisture content. The potential for improving the throughput of product is good. • Closer tolerance of the dielectric heating frequency significantly improves the level of control for internal drying and thus has potential in industry that produces food products that require precision moisture removal (Clark, 1997).
544 New Hybrid Drying Technologies
Another potential hybrid system worth mentioning is the combination of RF and vacuum drying. Even though current application of RF-vacuum drying is concentrated mainly in the wood industry (Rasev, 1999; Saito and Sulaiman, 1999), there is great potential for applying it to the food industry. RFD under vacuum condition allows for moisture to be removed at temperatures as low as 30°C. With the ability to improve product quality and nutrient retention, low temperature bulk drying with RF-vacuum technology is ideal for functional food manufacturing processes. Some of the advantages of such a hybrid system would include: 1 lower drying temperature resulting from a reduced boiling point due to a lowering of chamber pressure 2 better improvement of quality parameters such as product colour and shrinkage compared to RF-convection or RFD alone 3 chemical oxidation due to contact with drying air can be eliminated.
3.4 Microwave drying The physical mechanisms involved in heating and drying with microwaves are distinctly different from those of conventional means. Microwaves (MW) can penetrate into dielectric materials and generate internal heat (Jia et al., 1993). The internal heat generated establishes a vapour pressure within the product and gently ‘pumps’ the moisture to the surface (Turner and Jolly, 1991). This moisture pumping effect results in moisture being forced to the surface and preventing case hardening from occurring. Drying rates and product quality are subsequently enhanced. Because of this unique advantage, microwave drying has been used in a number of industries, e.g. timber, paper, textile, food and ceramic industries (Schiffmann, 1987). However, the progress of microwave drying at the industrial level has been relatively slow because of its high initial capital investment and low energy efficiency when compared with conventional drying technologies. To improve on the economic aspects of microwave drying, it is necessary to incorporate energy conservation features. Funebo and Ohlsson (1998) and Prabhanjan et al. (1995) have demonstrated that employing microwave-assisted air dehydration, the drying time for apple and mushroom can be significantly shortened and the products have better quality. The incorporation of MW technology with conventional driers can, perhaps, produce a more commercially viable drying technology. The advantages of microwave drying can be summarized as:
• • • •
Enhancement of heat and mass transfer processes Development of internal moisture gradients which enhance drying rates Increased drying rates without increased surface temperatures Improved product quality.
Currently, industrial microwave driers can be commercially viable for applications in food industries that require short drying time and higher product throughput at the expense of higher energy input. Also, food industries dealing with products that are susceptible to case hardening may consider microwave drying to be a good alternative in
Hybrid drying systems 545
quality enhancement. Recently, several technical staffs at the Dried Foods Technology Laboratory at Washington State University developed a state-of-the-art microwave vacuum drier (Clary, 1999). Figure 20.4 shows a simplified schematic of the drier. The microwave/vacuum process occurs inside large stainless steel vessels under vacuum conditions. Inside, the vessel contains a conveyor, a microwave unit and a radiant heat source. There are three zones in the vessel. As the food product is transported via the conveyor, it enters each zone with different microwave power. In the first zone, the product is subjected to a high level of microwave energy of either 12 kW at 2450 Hz or 30 kW at 915 Hz under vacuum conditions of about 1.3–4.0 kPa. In this zone, the product undergoes rapid dehydration because of the high level of microwave energy. In the second zone, the product is subjected to a moderate level of microwave energy of 6 kW at 2450 Hz. The final zone may or may not be accompanied by even lower microwave energy to ensure equalization of the moisture content. In this zone, the product is cooled and finally transported by the conveyor system for packaging. The incorporation of vacuum to the microwave system minimizes product oxidation and lowers the boiling point of the water in the food making it possible for drying to occur rapidly at temperatures below 55°C. Lower temperature drying allows food to minimize the degradation of quality parameters such as colour, flavour and nutritional value. The subtle raising of the microwave energy in different zones is another distinct feature of this hybrid drier. When the product possesses high moisture during the early stages of drying, it is able to undergo high thermal impact without significant quality degradation. As the moisture is removed, it is more susceptible to thermal-related quality change. Therefore, the microwave energy scheduling ensures rapid drying while minimizing quality degradation. It is also noteworthy that the drier heats food uniformly and thus preserves its
Product infeed Microwave power supplies
ZONE 1 – 12 kW at 2450 MHz or 30 kW at 915 MHz ZONE 2 – 6 kW at 2450 MHz or 30 kW at 915 MHz ZONE 3 – Product rest and equalization
Condenser and vacuum pump Product out-feed Figure 20.4
Combined microwave-vacuum drying system with different zones.
546 New Hybrid Drying Technologies
original shape. Moreover, it was found that microwave vacuum dehydration technology produces food quality superior to that of freeze-dried products and only at a fraction of the cost (Clary, 1999). It has huge potential in the fruit drying industry. Nindo et al. (2003) have recently evaluated several drying technologies namely, tray drying, spouted bed drying, combined microwave and spouted bed drying (MWSB) and freeze drying. Figure 20.5 shows a schematic of the MWSB experimental set-up. From their experiments, they observed that combined microwave and spouted bed drying of asparagus slices at a power level of 4 W/g was at least 5 times faster than tray drying when air temperatures between 50 and 70°C were used. At a drying air temperature of 60°C, the MWSB, operating with a power level of 4 W/g, was observed to be 6.0 and 2.8 times faster than tray and spouted bed drying, respectively. MWSB-dried asparagus had the highest rehydration. Microwave spouted bed drying at 60°C resulted in the highest retention of total antioxidant activity in asparagus.
3.5 Novel drying technologies The following sections contain some novel drying technologies being recently developed that are suitable for bioproducts. Most research and development works conducted for these technologies are still at their infancy stage. Nevertheless, they are worth mentioning in order to present more available options for consideration in choosing the most practical and efficient drying technology for the wide range of bioproducts.
Circulator Wave-guide Microwave magnetron Tub tuners
Directional coupler Microwave power controller Multi-mode microwave cavity
Spouted bed
Sample
Temperature controller
Hot air
Ambient air Heater Valve Blower
Figure 20.5
A schematic layout of combined microwave-spouted bed drying system (Nindo et al., 2003).
Hybrid drying systems 547
3.5.1 Combined microwave and superheated steam drying
Microwave (MW) and superheated steam (SHS) are both well-established drying technologies. The advantages of MW drying have been previously mentioned. The advantages of superheated steam drying are: 1 it is a non-polluting and safe drying method requiring low energy consumption 2 it can improve drying efficiency, sometimes as much as 50 per cent greater than a conventional drying system 3 steam is known to be a better agent compared to dry air in destroying all stages of insects, moulds and microorgansims found in foodstuffs. In general, both are known to be more expensive than traditional driers and hence they are only considered in some niche industries where the bioproducts are considered to be of high-value. Shibata et al. (2000) have studied the combined MW-SHS drying technology using sintered glass as a model material. From their experiments, they found that, in comparison to MW-nitrogen drying, the drying rates under MW-SHS drying were higher than those at less than the critical moisture content. The result is a reduction in drying time to achieve the desired moisture content. According to Kudra and Mujumdar (2001), there is good potential for employing such a hybrid system to produce products with low apparent density due to puffing, which may or may not be a desirable attribute. For food products such as cereals and selective snack-food that require both drying and puffing processes, MW-SHS would then present itself as an attractive option. 3.5.2 Pressure regulating drying
A very useful way to enhance the quality of heat-sensitive food products and yet achieve the desired product dryness is through the use of a pressure-regulatory system. The operating pressure range is usually from vacuum to close to one atmosphere. A total vacuum system may be costly to build because of the need for stronger materials and better leakage-prevention. Therefore, the system that is proposed here is recommended to operate above vacuum conditions. The period of operating at lower pressure may be continuous at a fixed level, intermittent or a prescribed cyclic pattern. The suitability of employing the appropriate type of pressure-swing pattern depends chiefly on the drying kinetics of the product and its thermal properties. Maache-Rezzoug et al. (2002) have recommended a pressure-swing drying mechanism for food products requiring the production of homogeneous thin sheets. Their experiments in drying a collagen gel in order to obtain a homogeneous film were recently carried out using a new process: dehydration by successive decompression. This process involves a series of cycles during which the collagen gel is placed in desiccated air at a given pressure then subjected to an instantaneous (200 ms) pressure drop to a vacuum (7–90 kPa). This procedure is repeated until the desired moisture content is obtained. A comparative study between this new pressure-swing drying process and conventional methods indicated that the respective saving in drying time could be as high as 480 and 700 minutes in comparison to vacuum and hot air drying systems. Chua and Chou (2003) studied a successive pressure drops method on the drying kinetics as well as the colour degradation of two bioproducts, namely potato and carrot. The parameters under investigation include the cycle duration, pressure
548 New Hybrid Drying Technologies
levels and chamber temperature on the drying kinetics, product colour and porosity changes. Their experimental results showed that the pressure level has a positive impact on the drying kinetics of heat-sensitive bioproducts. Also, for a given drying period, a shorter drying cycle time was observed to result in better drying kinetics for the agro-samples. Finally, drying conditions employing lower chamber pressure were shown to have a significant impact in reducing the colour change of bioproducts. Based on the studies presented here, the general conclusion is that integrating a pressure-swing system to any convective drier would significantly improve product quality and, at the same time, reduce the drying time which would result in a smaller drying chamber to obtain similar product throughput. 3.5.3 Rotating jet spouted bed
Jumah et al. (1996) implemented the principle of intermittent drying in a novel spouted bed system. They studied the drying kinetics of corns using a rotating jet spouted bed (RJSB). A schematic of their set-up is shown in Figure 20.6. Briefly, the rotating jet spouted bed is formed when the air jet moves circumferentially in the annular region between the chamber wall and the central spout. One distinct advantage of the rotating jet configuration is the prescription of an intermittent spout due to the continuous movement of the air jet. Intermittent drying due to periodic heat supply is then possible. By varying the rotational speed of the spouting jet of heated air, the intermittency frequency and hence the intermittent drying schedule can be varied. Jumah et al. (1996) performed experiments to test the hypothesis that corn, as a slow drying material, could be dried to produce high quality grain with lower energy consumption via prescribing an intermittent air schedule. The intermittent scheduling was achieved by using various drying periods alternated by long tempering periods.
Spouting particles
Centrifugal fan
PID controller Gate valve
Electric heaters Figure 20.6
Fabric filter
Rotating air distributor
Motorized drive
Experimental rotating jet spouted bed with central and peripheral air jets (Jumah, 1995).
References 549
During the active periods, the corn particles were subjected to very intense mixing and circulation due to the hydrodynamics of the rotating spouts. The resulting effect was a period of high intensity heat and mass transfer. During the no-flow periods, the temperature and moisture gradients were effectively relaxed and favourable moisture re-distribution inside the particle occurred. Minimal mechanical damage to the kernels due to reduced attrition caused by inter-particle collisions during spouting was also observed. It was further demonstrated that moisture levelling occurred during the tempering periods with moisture migration to the corn kernel surface. In terms of energy saving, intermittent drying conducted with the RJSB resulted in substantial energy saving of up to 37 per cent when compared to a continuous spouting bed dryer (Jumah et al., 1996).
4 Conclusions One of the main contributions of the twentieth century has been to lay the fundamental platform for new technology to emerge. This review chapter has summarized some of the recent developments of hybrid-drying technologies. As technology advances to new frontiers, the method to dehydrate food is constantly evolving to produce new hybrid drying systems. Some of the hybrid drying techniques, if combined in an intelligent fashion, would promote efficient drying in terms of enhanced product quality and reduction in energy consumption. However, R&D effort is still required to study system scale-up, optimization and control of these hybrid systems. This chapter may not have covered all novel drying technologies available, but it is hoped that those hybrid technologies presented would give dried food manufacturers a better understanding of the technologies available to improve their drying processes. Since product quality and energy consumption are usually the primary concerns during food dehydration, there is still scope for discovering new drying technologies. It is hoped that in the coming years more hybrid systems can be developed to handle even the most complex bioproduct drying problems.
References Alves-Filho O, Strømmen I (1996) Performance and improvements in heat pump dryers. In Drying ’96 (Strumillo C, Pakowski Z, eds). Krakow: Elsevier, pp. 405–415. Baker GJ (1997) Industrial Drying of Foods. London: Chapman and Hall. Best R, Soto W, Pilatowsky I, Gutierrez LJ (1994) Evaluation of a rice drying system using a solar assisted heat pump. Renewable Energy, 5 (1–4), 465–468. Chou SK, Chua KJ, Mujumdar AS, Tan M, Tan SL (2001) Study on the osmotic pre-treatment and infrared radiation on drying kinetics and colour changes during drying of agricultural products. ASEAN Journal on Science and Technology for Development, 18 (1), 11–23.
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Chou SK, Chua KJ (2001) New hybrid drying technologies for heat sensitive foodstuffs. Trends in Food Science and Technology, 12 (10), 359–369. Chua KJ, Chou SK (2003) Evaluating a pressure regulatory system for drying of heat-sensitive bioproducts. Journal of Food Engineering, 63 (3), 151–158. Chua KJ, Chou SK, Ho JC, Mujumdar AS, Hawlader MNA (2000a) Cyclic air temperature drying of guava pieces: Effects on moisture and ascorbic acid contents. Transactions of IChemE, Part C, 78, 72–78. Chua KJ, Mujumdar AS, Chou SK, Hawlader MNA, Ho JC (2000b) Convective drying of banana, guava and potato pieces: Effect of cyclical variations of air temperature on convective drying kinetics and colour change. Drying Technology, 18 (5), 907–936. Clark TD (1997) The current status of radio frequency post-baking drying. In Proceedings of the 72nd Annual Technical Conference of the Biscuit and Cracker Manufacturers’ Association, USA: Forth Worth, Texas, pp. 104–107. Clary C (1999) Microwave vacuum drying. Washington State University Today, 15 (9), 3–4. Funebo T, Ohlsson T (1998) Microwave-assisted air dehydration of apple and mushroom. Journal of Food Engineering, 38, 353–367. Jia X, Clements S, Jolly P (1993) Study of heat pump assisted microwave drying. Drying Technology, 11 (7), 1583–1616. Jumah RY, Mujumdar AS, Raghavan GSV (1996) A mathematical model for constant and intermittent batch drying of grains in a novel rotating jet spouted bed. In Mathematical Modelling and Numerical Techniques in Drying Technology (Turner I, Mujumdar AS, eds). New York: Marcel Dekker, pp. 339–380. Kudra T, Mujumdar AS (2001) Advanced Drying Technologies. New York: Marcel Dekker Inc. Maache-Rezzoug Z, Rezzoug SA, Allaf K (2002) Development of a new drying process – Dehydration by successive pressure drops: Application to the drying of collagen gel. Drying Technology, 20 (1), 109–129. Marshall MG, Metaxas AC (1998) Modelling the radio frequency electric field strength developed during the radio frequency assisted heat pump drying of particulates. International Microwave Power Institute, 33 (3), 167–177. Mason R, Britnell YG, Birchall S, Fitz-Payne S, Hesse BJ (1994) Development and application of heat pump dryers to the Australian food industry. Food Australia, 46, 319–320. Mujumdar AS (2000) Dryers for particulate solids, slurries and sheet-form materials. In Mujumdar’s Practical Guide to Industrial Drying: Principles, Equipment and New Developments (Devahastin S, ed.). Thailand: Thananuch Business Ltd Publication, pp. 37–61. Nindo CI, Sun T, Wang SW, Tang J, Powers JR (2003) Evaluation of drying technologies for retention of physical quality and antioxidants in asparagus (Asparagus officinalis L). Lebensmittel Wissenschaft und Technologie, 36, 507–516. O’Neill MB, Rahman MS, Perera CO, Smith B, Melton LD (1998) Colour and density of apple cubes dried in air and modified atmosphere. International Journal of Food Properties, 1 (3), 197–205.
References 551
Okos MR, Narsimhan G, Singh RK, Weitnauer AC (1992) Food dehydration. In Handbook of Food Engineering (Heldman DR, Lund DB, eds). New York: Marcel Dekker, pp. 437–562. Perera CO, Rahman MS (1990) Heat pump drying. Trends in Food Science and Technology, 8 (3), 75–78. Prabhanjan DG, Ramaswamy HS, Saravacos GSV (1995) Microwave-assisted convective air drying of thin layer carrots. Journal of Food Engineering, 25, 283–293. Prasertsan S, Saen-saby P (1998) Heat pump drying of agricultural materials. Drying Technology, 16 (1&2), 235–250. Rasev AI (1999) Particular features of dielectric and vacuum wood drying. In Proceedings of the 6th International Wood Drying Conference (Vermaas HF, ed.). Stellenbosch, South Africa: Stellenbosch Publisher, pp. 37–41. Rossi SJ, Neues C, Kicokbusch TG (1992) Thermodynamics and energetic evaluation of a heat pump applied to drying of vegetables. In Drying ’92 (Mujumdar AS, ed.). Amsterdam: Elsevier Science, pp. 1475–1483. Saito S, Sulaiman IB (1999) Radio frequency vacuum (RF/V) drying of small diameter Keruing. In Proceedings of the 6th International Wood Drying Conference (Vermaas HF, ed.). Stellenbosch, South Africa: Stellenbosch Publisher, pp. 234–238. Saravacos GD, Marousis SN, Raouzeos GS (1988) Effect of ethyloleate on the rate of airdrying of foods. Journal of Food Engineering, 7, 263–267. Schiffmann RF (1987) Microwave and dielectric drying. In Handbook of Industrial Drying (Mujumdar AS, ed.). New York: Marcel Dekker Inc, pp. 345–372. Shibata H, Iwao Y, Ide M (2000) Combined superheated steam and microwave drying of sintered glass beads. In Proceedings of 12th International Drying Symposium (Coumans WJ, Kerkhop PJAM, eds). Noordwijkerhout, The Netherlands: Elsevier Publication, Paper no. 363. Strømmen I, Jonassen O (1996) Performance tests of a new 2-stage counter-current heat pump fluidised bed dryer. In Drying ’96 (Strumillo C, Pakowski Z, eds). Krakow: Elsevier, pp. 563–568. Taechapairoj C, Dhuchakallaya I, Soponronnarit S, Wetchacama S, Prachayawarakorn S (2003) Superheated steam fluidised bed paddy drying. Journal of Food Engineering, 58, 67–73. Thomas WJ (1996) Radio frequency drying provides process savings: New systems optimize radio frequency drying for the ceramic and glass fibre industries. Ceramic Industry Magazine, April, 30–34. Turner IW, Jolly P (1991) Combined microwave and convective drying of a porous material. Drying Technology, 9 (5), 1209–1270.
It is well known that the dehydration process influences the quality of the bioproducts. The current drive towards improving drying technologies is motivated by the economic incentive to produce better quality products at a faster rate and lower operating costs. In recent years, the engineering focus has been to improve the design and operation of dryers to achieve the dried food product with desired characteristics. Even though much has been done in the development of improving individual drying technology, much remains to be achieved in the study of new hybrid systems whereby drying technologies can be combined to evolve new age drying systems. This chapter summarizes some recent developments in hybrid drying technologies of interest to the bioproduct industry. New emerging hybrid drying technologies are listed and discussed. The potential roles of these hybrid technologies in product quality enhancement are also identified.
1 Introduction In many agricultural countries, large quantities of bioproducts are dried to enhance shelflife, reduce packaging costs, lower shipping weights, enhance appearance, encapsulate original flavour and maintain nutritional value. According to Okos et al. (1992), the goals of drying process research in the bioproduct industry are summarized by three issues:
• Economic considerations: to reduce operating costs and improve capacity per unit amount of drying equipment; to develop simple drying equipment that is reliable and requires minimal labour; to minimize off-specification product; and to develop a stable process that is capable of continuous operation. • Environmental concerns: to minimize energy consumption during the drying operation and to reduce environmental impact by reducing product loss in waste streams, i.e. to incorporate the possibility of waste heat recovery systems. • Product quality aspects: to have precise control of the product moisture content at the end of the drying process; to minimize chemical degradation reactions; to reduce change in product structure and texture; to obtain the desired product colour; to control the product density; and to develop a versatile drying process that can produce products of different physical structures for various end-users. Emerging technologies for food processing ISBN: 0-12-676757-2
Copyright © 2005 Elsevier Ltd All rights of reproduction in any form reserved
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536 New Hybrid Drying Technologies
Though the primary objective of food drying is preservation, depending on the drying technology employed, the raw material may end up as a completely different material with significant variation in product quality. Therefore, great care must be taken in choosing a suitable dryer considering the closely knitted relationship between the bioproduct and dryer. A mismatch between the product and drying technology would result in dire consequences, often resulting in great financial losses. As new discovery of hybrid bioproducts is constantly being made, the spectrum of dried bioproduct gets wider. When a single drying system cannot handle the stringent quality requirements of these new bioproducts, the need for new hybrid drying systems becomes essential. Therefore, the principal motivation in developing hybrid drying technologies is to meet consumer expectations of quality and yet produce a product with the desired moisture content. Figure 20.1 shows a general classification scheme of hybrid drying technologies. Under the umbrella of hybrid drying are drying techniques that employ multiple modes of heat transfer as well as those that use two or more stages of drying to achieve the desired dryness, product quality, drying time and manufacturing throughout. A more common definition of hybrid drying would consist of a successful integration or intelligent combination of two or more conventional drying know-hows. This alone can constitute the emergence of a wide-range of new hybrid drying technologies.
Hybrid drying technologies
Combined drying technology
Infrared-heat pump drying Infrared-convective drying Microwaveconvective drying Microwave-vacuum drying
Multiple-stage drying
Same drier type at each stage
Drying and cooling
– Two-stage fluid beds – Two-stage vibrofluid beds
Drying and coating
Different drier type at each stage – Flash/fluid bed – Spray/fluid bed – Fluid bed/packed bed Different drying technologies per stage – Superheated steam drying followed by convective air drying – IR/microwave drying followed by convective air drying
Figure 20.1
Multiple-process drying
A general classification scheme of hybrid drying technologies.
Drying and granulation Drying and filtration
Hybrid drying systems 537
This chapter serves to provide an overview of some newly developed hybrid drying technologies applicable for bioproducts that are particularly sensitive to thermal treatment. Drying technologies incorporating convective, radiation and electromagnetic heat transfer modes will be presented along with novel technologies such as pressureswing, vacuum-superheated steam and rotating spouted jet drying. The impact of each hybrid drying technology on various quality parameters will also be briefly discussed in the light of growing interest and demand for high quality dried products.
2 Product quality degradation during dehydration To understand how the employment of hybrid drying technologies can improve product quality, it will be useful first to understand the degradation process of foodstuffs. The quality of many food products degrades during dehydration above room temperature. The added heat and exposure time of the product at elevated temperature affects the rate of nutrient quality degradation. The types of food degradation during drying are listed in Table 20.1. Dried foods are known to be high in fibre and carbohydrates and low in fat, making them healthy food choices. Food scientists have found that by reducing the moisture content of foodstuffs to between 10 and 20 per cent, bacteria, yeasts, moulds and enzymes are all prevented from spoiling it. The flavour and aroma are well preserved and concentrated. The loss of nutrient can be viewed as the decomposition of a particular chemical compound. This decomposition of a single monomolecular reaction may be described using zero or first-order kinetics equations (Chua et al., 2000a). As the temperature of the product increases, the reaction rate constant is increased. The dependence of the reaction constant on temperature implies that a low temperature drying process results in less nutrient degradation. A longer constant rate drying period increases the nutrient retention because, owing to evaporative cooling, the product is at a lower temperature.
3 Hybrid drying systems The diversity of food products has introduced many types of dryers to the food industry. Often the selection of the appropriate dryer is based on the drying characteristics Table 20.1 Change in food quality parameters during dehydration Chemical
Physical
Nutritional
Browning reaction Lipid oxidation Colour loss Gelatinization
Rehydration Solubility Texture and shrinkage Aroma loss
Vitamin loss Protein loss Microbial survival
538 New Hybrid Drying Technologies
of the food product. For heat-sensitive bioproducts, the methods of supplying heat to the product and transporting the moisture from the product become the critical considerations for selecting the right drier to achieve the desired product moisture content. In the following sections, the potential of employing new hybrid driers for drying of bioproducts is presented.
3.1 Heat pump drying There has been a growing interest in recent years to apply heat pump drying (HPD) technology to foods and biomaterials where low-temperature drying and well-controlled drying conditions are required to enhance the quality of food products. High-valued products, which are extremely heat-sensitive, are often freeze-dried. This is an extremely expensive drying process (Baker, 1997). Therefore, there has been great interest in looking at the heat pump drying system as a replacement system for freeze-dried products. Table 20.2 presents a summary of some hybrid heat pump drying systems for selected bioproducts.
Table 20.2 Selected works on hybrid heat pump drying of selected food products Researchers
Hybrid system
Application(s)
Conclusions
Chua et al. (2000b) Chou et al. (2001) (Singapore)
Heat pump/IR system
Agricultural and marine products (mushrooms, fruits, sea-cucumber and oysters)
The quality of the agricultural and marine products can be improved with scheduled drying conditions
O’Neill et al. (1998) (New Zealand)
Modified atmosphere heat pump system
Agricultural food drying (apples)
Modified atmosphere heat pump drying (MAHPD) produces products with a high level of open pore structure, contributing to the unique physical properties, such as floatability and rehydration capabilities. MAHPD drying system affords the possibility of drying with colour qualities comparable to sulphured products
Best et al. (1994) (Mexico)
Solar-assisted heat pump drier
Rice
Advantage of low temperature and better control in the drier results in good quality rice
Rossi et al. (1992) (Brazil)
Heat pump assisted heating drier
Vegetable (onion)
Drying of sliced onions confirmed energy saving of the order of 30% and better product quality due to shorter processing time
Alves-Filho and Strømmen (1996) (Norway)
Heat pump assisted fluidized bed drier (HPFBD)
Marine products (fish)
The high quality of the dried products was highlighted as the major advantage of HPFBD and introducing a temperature controllable programme to HPFBD makes it possible to regulate the product properties such as porosity, rehydration rates, strength, texture and colour
Hybrid drying systems 539
Some of the advantages of the heat pump drier are as follows:
• Higher energy efficiency with improved heat recovery results in lower energy consumed for each unit of water removed.
• Better product quality with well-controlled temperature schedules to meet specific production requirements. • A wide range of drying conditions typically 20–100°C (with auxiliary heating) and relative humidity 15–80 per cent (with humidification system) can be generated. • Excellent control of drying environment for high-value products and reduced electrical energy consumption for low-value products. Along with the advantages are limitations given as follows:
• Requires regular maintenance of components (compressor, refrigerant filters, etc.) and charging of refrigerant. • Increased capital costs. For many of the research studies conducted in Table 20.2, the common conclusion was that the heat pump drier offers products of better quality with reduced energy consumption. This is particularly true of food products that require a closed-control drying environment such as temperature, humidity and/or even a special drying medium (O’Neill et al., 1998). Heat-sensitive food products, requiring low-temperature drying, can take advantage of HPD technology since the drying temperature of the HPD system can be adjusted from 20 to 100°C. With proper control, it is also possible for HPD to produce freeze-drying conditions at atmospheric pressure (Prasertsan and Saen-saby, 1998). So far as food drying is concerned, HPD offers an alternative to improve product quality through proper regulation of the drying conditions. Chua et al. (2000b) have demonstrated that HPD can produce pre-selected cyclic temperature schedules to improve the quality of various agricultural products dried in their two-stage HPD. They have shown that with appropriate choice of temperature-time variation, it is possible to reduce the overall colour change and ascorbic acid degradation by up to 87 and 20 per cent, respectively. Besides yielding better food quality, Rossi et al. (1992) have reported that onion slices dried by HPD used less energy in comparison to a conventional hot air system. Food products with a high water content can be dried efficiently using HPD. As the drying air absorbs more of the latent energy due to vaporization, this energy can be transferred at the evaporators for higher heat recovery. Lower energy input is then required at the compressor to enable sensible heating of the air when it passes through the condenser. Ginger dried in a heat pump drier was found to retain over 26 per cent of gingerol, the principal volatile flavour component responsible for its pungency, compared to only about 20 per cent in rotary dried commercial samples (Mason et al., 1994). The higher volatile retention in heat pump dried samples is probably due to the reduced degradation of gingerol when lower drying temperatures are employed. The loss of volatiles varies with concentration, with the greatest loss occurring during the early stages of drying when the initial concentration of the volatile components is low (Saravacos et al., 1988). Since heat pump drying is conducted in a closed chamber, any compound that volatilizes will remain within the drying chamber and the
540 New Hybrid Drying Technologies
partial pressure for that compound will gradually build up within the chamber, retarding further volatilization from the product (Perera and Rahman, 1990). A recent hybrid system combines radio frequency (RF) and heat pump drying (Marshall and Metaxas, 1998). Such a system has potential application in the food industry. The limitation of relative low heat transfer rates in convective air drying, particularly towards the falling rate period, can be overcome by introducing volumetric heat generation such as the RF technology. The RF field generates heat volumetrically within the material wetted with polar molecules, such as water, by the combined mechanisms of dipole rotation and ionic conduction. The internal heat generation speeds up the drying process because of unidirectional temperature and moisture gradient and internal pressure build-up. Figure 20.2 shows a schematic of a radiofrequency-assisted heat pump drier (RF-assisted HPD). Such a hybrid drier is suitable for food materials that are difficult to dry with convective heating, especially food products that have a film of wax on the surface such as chillies, cherries and tomatoes. With regards to product quality, it appears that RF-assisted HPD reduces colour degradation, surface cracking and differential shrinkage of the product. In terms of energy consumption, RF-assisted drying has been observed to improve the specific moisture extraction rate (SMER) and coefficient of performance (COP) of the heat pump system. Furthermore, the potential for increasing the product throughput is good. For example, in the bakery industry, the throughput for crackers and cookies can be improved by as much as 30 and 40 per cent, respectively (Clark, 1997).
Evaporator
Compressor Expansion valve
Condenser Axial fan
Drying product
Metallic perforated plate
Heat pump drying chamber Figure 20.2
RF generator
Schematic diagram of RF-assisted heat pump drier (Marshall and Metaxas, 1998).
Hybrid drying systems 541
Alternatively, to dry heat-sensitive materials, a combined radiant-convective drying method may be applied. An infrared-augmented HPD drying system could be used for fast removal of surface moisture during the initial stages of drying, followed by intermittent drying over the rest of the drying process. This mode of operation ensures a faster initial drying rate. Therefore, an infrared (IR) assisted HPD would offer the advantage of compactness, simplicity, ease of control and low equipment costs (Mujumdar, 2000). Also, there are the possibilities of significant energy savings and enhanced product quality due to the reduced residence time in the drying chamber. For such an IR-assisted HPD system, it is essential to implement a good control strategy for IR operation in order to achieve the desired results in terms of drying kinetics and product quality, as well as to ensure safe operation. A typical example of a good feedback control is one that enables the IR power source to be cut off if excessively high temperatures are measured in the chamber, which may lead to overheating of the product. Chou et al. (2001) designed a feedback system by coupling a PID controller to the IR lamps and by inserting a type ‘T’ thermocouple needle to the bioproduct, feedback signals were sent to the controller. In comparison to the IR operating in intermittent mode, they found that by pre-programming different food sample temperatures, greater reduction in drying time and improvement in product colour could be achieved.
3.2 Fluidized bed drying Fluidized bed drying (FBD) has found many applications for drying granular solids in the food, pharmaceutical and agriculture industries. For drying of powders in the 50–2000 m range, FBD competes successfully with other more traditional drier types, e.g. rotary, tunnel, conveyor, continuous tray, etc. The advantages of FBD include: 1 high heat and mass transfer phenomena between the particles and the gas 2 closed control product temperature making FBD ideal for processing temperaturesensitive solids 3 highest thermal efficiency of any gas-suspension drying system. The disadvantages of FBD include: 1 it is able to dry only a limited range of materials 2 the size of the product particles is relatively large 3 difficulty involved in processing needle or platelet-shaped particles. Recent hybrid fluidized bed driers incorporating a heat pump drying mechanism have been developed at the Norwegian Institute of Technology (Alves-Filho and Strømmen, 1996) as shown in Figure 20.3. The drying chamber receives wet material and discharges dried product through the product inlet and outlet ducts. The desired operating temperature is obtained by adjusting the condenser capacity, while the required air humidity is maintained by regulating the compressor capacity via frequency control of the motor speed. According to Alves-Filho and Strømmen (1996), this set-up can produce drying temperatures from 20 to 60°C and air humidity spanning 20 to 90 per cent. With these features, heat-sensitive food materials can be dried
542 New Hybrid Drying Technologies
Fluidized bed Wet material
Dry material
Condenser Centrifugal fan
Evaporator
3-way valve
Expansion valve
External condenser Liquid receiver
Compressor
Figure 20.3 Fluidized bed driers incorporating heat pump drying technology (Alves-Filho and Strømmen, 1996).
under convective air or freeze drying conditions. It is also possible to sequence these two operations (convective and freeze drying). This will be advantageous for drying of food and bioproducts since freeze drying causes minimal shrinkage but produces low drying rates while convective air drying can be applied to enhance drying rates. Therefore, a combination of drying processes, e.g. freeze drying at 5°C followed by convective drying of 20–30°C, enables the control of quality parameters such as porosity, rehydration rates, strength, texture, colour, taste, etc. Experiments performed at the department of mechanical engineering, Norwegian University of Science and Technology, on various heat-sensitive materials such as pharmaceutical products, fruits and vegetables have shown that this new hybrid fluidized bed drying offers a better product quality but at higher cost. Even as experimental work is still being conducted with this hybrid drier, a two-stage fluidized bed heat pump dryer has already interested some food industries in Norway (Strømmen and Jonassen, 1996). The two-stage system simply comprises two fluidized beds connected in series. Two heat pumps supply conditioned air independently to each drying chamber. The drying chambers are connected in series so that one receives wet product and discharges the semi-dried product to the next, which produces the final dried product. Such a hybrid two-stage drier is versatile as it allows independent freezing and convective drying to be carried out. The advantages of multiple-stage fluidized bed drying over single fluidized bed heat pump drying include improved product quality and enhanced energy efficiency (Alves-Filho and Strømmen, 1996) but at higher capital cost.
Hybrid drying systems 543
Recently, Taechapairoj et al. (2003) employed a fluidized bed system incorporating superheated steam as the drying medium for paddy drying. They observed that when using superheated steam drying the head rice yield is more sustainable and has higher values than those obtained from hot air drying. However, there was some colour depreciation in terms of the rice whiteness. The main cause of the rapid change in the colour is partly due to the steam condensation on the paddy surface.
3.3 Radio-frequency drying Radio-frequency drying (RFD) is a simple precise process and is common place in the food industry with appropriate and proven processes available for a wide range of applications, such as pre-heating, pre-cooking, sterilization, tempering, post-baking and moisture control. A limitation of heat transfer in conventional drying with hot air alone, particularly in the falling rate period, can be overcome by combining RF heating with conventional convective drying (Thomas, 1996). RF generates heat volumetrically within the wet material by the combined mechanisms of dipole rotation and conduction effects which speed up the drying process (Marshall and Metaxas, 1998). A typical RF-convective drier comprises a convective drying system retro-fitted with an RF generating system capable of imparting radio-frequency energy to the drying material at various stages of the drying process. Biomaterials that are difficult to dry with convection heating alone are good candidates for RF-assisted drying. Food materials with poor heat transfer characteristics have traditionally been problem materials when it comes to heating and drying. Radio frequency heats all parts of the product mass simultaneously and evaporates the water in situ at relatively low temperatures, usually not exceeding 82°C (Thomas, 1996). Since water moves through the product in the form of a gas rather than by capillary action, migration of solids is avoided. Warping, surface discoloration and cracking associated with conventional drying methods are also avoided. The potential for application of RF drying in the food industries can be appreciated for the following reasons:
• RFD prevents over-drying because radio waves concentrate in the wettest and densest areas of the biomaterial. It improves the colour of products, especially those that are highly susceptible to surface colour change, since RF drying starts from the centre and moves to the product surface, minimizing any surface effect. • Cracking, caused by the stresses of uneven shrinkage in drying, can be eliminated by RF-assisted drying. This is achieved in the drier by even heating throughout the product maintaining moisture uniformity from the centre to the surface during the drying process. • Simultaneous external and internal drying significantly reduces the drying time to reach the desired moisture content. The potential for improving the throughput of product is good. • Closer tolerance of the dielectric heating frequency significantly improves the level of control for internal drying and thus has potential in industry that produces food products that require precision moisture removal (Clark, 1997).
544 New Hybrid Drying Technologies
Another potential hybrid system worth mentioning is the combination of RF and vacuum drying. Even though current application of RF-vacuum drying is concentrated mainly in the wood industry (Rasev, 1999; Saito and Sulaiman, 1999), there is great potential for applying it to the food industry. RFD under vacuum condition allows for moisture to be removed at temperatures as low as 30°C. With the ability to improve product quality and nutrient retention, low temperature bulk drying with RF-vacuum technology is ideal for functional food manufacturing processes. Some of the advantages of such a hybrid system would include: 1 lower drying temperature resulting from a reduced boiling point due to a lowering of chamber pressure 2 better improvement of quality parameters such as product colour and shrinkage compared to RF-convection or RFD alone 3 chemical oxidation due to contact with drying air can be eliminated.
3.4 Microwave drying The physical mechanisms involved in heating and drying with microwaves are distinctly different from those of conventional means. Microwaves (MW) can penetrate into dielectric materials and generate internal heat (Jia et al., 1993). The internal heat generated establishes a vapour pressure within the product and gently ‘pumps’ the moisture to the surface (Turner and Jolly, 1991). This moisture pumping effect results in moisture being forced to the surface and preventing case hardening from occurring. Drying rates and product quality are subsequently enhanced. Because of this unique advantage, microwave drying has been used in a number of industries, e.g. timber, paper, textile, food and ceramic industries (Schiffmann, 1987). However, the progress of microwave drying at the industrial level has been relatively slow because of its high initial capital investment and low energy efficiency when compared with conventional drying technologies. To improve on the economic aspects of microwave drying, it is necessary to incorporate energy conservation features. Funebo and Ohlsson (1998) and Prabhanjan et al. (1995) have demonstrated that employing microwave-assisted air dehydration, the drying time for apple and mushroom can be significantly shortened and the products have better quality. The incorporation of MW technology with conventional driers can, perhaps, produce a more commercially viable drying technology. The advantages of microwave drying can be summarized as:
• • • •
Enhancement of heat and mass transfer processes Development of internal moisture gradients which enhance drying rates Increased drying rates without increased surface temperatures Improved product quality.
Currently, industrial microwave driers can be commercially viable for applications in food industries that require short drying time and higher product throughput at the expense of higher energy input. Also, food industries dealing with products that are susceptible to case hardening may consider microwave drying to be a good alternative in
Hybrid drying systems 545
quality enhancement. Recently, several technical staffs at the Dried Foods Technology Laboratory at Washington State University developed a state-of-the-art microwave vacuum drier (Clary, 1999). Figure 20.4 shows a simplified schematic of the drier. The microwave/vacuum process occurs inside large stainless steel vessels under vacuum conditions. Inside, the vessel contains a conveyor, a microwave unit and a radiant heat source. There are three zones in the vessel. As the food product is transported via the conveyor, it enters each zone with different microwave power. In the first zone, the product is subjected to a high level of microwave energy of either 12 kW at 2450 Hz or 30 kW at 915 Hz under vacuum conditions of about 1.3–4.0 kPa. In this zone, the product undergoes rapid dehydration because of the high level of microwave energy. In the second zone, the product is subjected to a moderate level of microwave energy of 6 kW at 2450 Hz. The final zone may or may not be accompanied by even lower microwave energy to ensure equalization of the moisture content. In this zone, the product is cooled and finally transported by the conveyor system for packaging. The incorporation of vacuum to the microwave system minimizes product oxidation and lowers the boiling point of the water in the food making it possible for drying to occur rapidly at temperatures below 55°C. Lower temperature drying allows food to minimize the degradation of quality parameters such as colour, flavour and nutritional value. The subtle raising of the microwave energy in different zones is another distinct feature of this hybrid drier. When the product possesses high moisture during the early stages of drying, it is able to undergo high thermal impact without significant quality degradation. As the moisture is removed, it is more susceptible to thermal-related quality change. Therefore, the microwave energy scheduling ensures rapid drying while minimizing quality degradation. It is also noteworthy that the drier heats food uniformly and thus preserves its
Product infeed Microwave power supplies
ZONE 1 – 12 kW at 2450 MHz or 30 kW at 915 MHz ZONE 2 – 6 kW at 2450 MHz or 30 kW at 915 MHz ZONE 3 – Product rest and equalization
Condenser and vacuum pump Product out-feed Figure 20.4
Combined microwave-vacuum drying system with different zones.
546 New Hybrid Drying Technologies
original shape. Moreover, it was found that microwave vacuum dehydration technology produces food quality superior to that of freeze-dried products and only at a fraction of the cost (Clary, 1999). It has huge potential in the fruit drying industry. Nindo et al. (2003) have recently evaluated several drying technologies namely, tray drying, spouted bed drying, combined microwave and spouted bed drying (MWSB) and freeze drying. Figure 20.5 shows a schematic of the MWSB experimental set-up. From their experiments, they observed that combined microwave and spouted bed drying of asparagus slices at a power level of 4 W/g was at least 5 times faster than tray drying when air temperatures between 50 and 70°C were used. At a drying air temperature of 60°C, the MWSB, operating with a power level of 4 W/g, was observed to be 6.0 and 2.8 times faster than tray and spouted bed drying, respectively. MWSB-dried asparagus had the highest rehydration. Microwave spouted bed drying at 60°C resulted in the highest retention of total antioxidant activity in asparagus.
3.5 Novel drying technologies The following sections contain some novel drying technologies being recently developed that are suitable for bioproducts. Most research and development works conducted for these technologies are still at their infancy stage. Nevertheless, they are worth mentioning in order to present more available options for consideration in choosing the most practical and efficient drying technology for the wide range of bioproducts.
Circulator Wave-guide Microwave magnetron Tub tuners
Directional coupler Microwave power controller Multi-mode microwave cavity
Spouted bed
Sample
Temperature controller
Hot air
Ambient air Heater Valve Blower
Figure 20.5
A schematic layout of combined microwave-spouted bed drying system (Nindo et al., 2003).
Hybrid drying systems 547
3.5.1 Combined microwave and superheated steam drying
Microwave (MW) and superheated steam (SHS) are both well-established drying technologies. The advantages of MW drying have been previously mentioned. The advantages of superheated steam drying are: 1 it is a non-polluting and safe drying method requiring low energy consumption 2 it can improve drying efficiency, sometimes as much as 50 per cent greater than a conventional drying system 3 steam is known to be a better agent compared to dry air in destroying all stages of insects, moulds and microorgansims found in foodstuffs. In general, both are known to be more expensive than traditional driers and hence they are only considered in some niche industries where the bioproducts are considered to be of high-value. Shibata et al. (2000) have studied the combined MW-SHS drying technology using sintered glass as a model material. From their experiments, they found that, in comparison to MW-nitrogen drying, the drying rates under MW-SHS drying were higher than those at less than the critical moisture content. The result is a reduction in drying time to achieve the desired moisture content. According to Kudra and Mujumdar (2001), there is good potential for employing such a hybrid system to produce products with low apparent density due to puffing, which may or may not be a desirable attribute. For food products such as cereals and selective snack-food that require both drying and puffing processes, MW-SHS would then present itself as an attractive option. 3.5.2 Pressure regulating drying
A very useful way to enhance the quality of heat-sensitive food products and yet achieve the desired product dryness is through the use of a pressure-regulatory system. The operating pressure range is usually from vacuum to close to one atmosphere. A total vacuum system may be costly to build because of the need for stronger materials and better leakage-prevention. Therefore, the system that is proposed here is recommended to operate above vacuum conditions. The period of operating at lower pressure may be continuous at a fixed level, intermittent or a prescribed cyclic pattern. The suitability of employing the appropriate type of pressure-swing pattern depends chiefly on the drying kinetics of the product and its thermal properties. Maache-Rezzoug et al. (2002) have recommended a pressure-swing drying mechanism for food products requiring the production of homogeneous thin sheets. Their experiments in drying a collagen gel in order to obtain a homogeneous film were recently carried out using a new process: dehydration by successive decompression. This process involves a series of cycles during which the collagen gel is placed in desiccated air at a given pressure then subjected to an instantaneous (200 ms) pressure drop to a vacuum (7–90 kPa). This procedure is repeated until the desired moisture content is obtained. A comparative study between this new pressure-swing drying process and conventional methods indicated that the respective saving in drying time could be as high as 480 and 700 minutes in comparison to vacuum and hot air drying systems. Chua and Chou (2003) studied a successive pressure drops method on the drying kinetics as well as the colour degradation of two bioproducts, namely potato and carrot. The parameters under investigation include the cycle duration, pressure
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levels and chamber temperature on the drying kinetics, product colour and porosity changes. Their experimental results showed that the pressure level has a positive impact on the drying kinetics of heat-sensitive bioproducts. Also, for a given drying period, a shorter drying cycle time was observed to result in better drying kinetics for the agro-samples. Finally, drying conditions employing lower chamber pressure were shown to have a significant impact in reducing the colour change of bioproducts. Based on the studies presented here, the general conclusion is that integrating a pressure-swing system to any convective drier would significantly improve product quality and, at the same time, reduce the drying time which would result in a smaller drying chamber to obtain similar product throughput. 3.5.3 Rotating jet spouted bed
Jumah et al. (1996) implemented the principle of intermittent drying in a novel spouted bed system. They studied the drying kinetics of corns using a rotating jet spouted bed (RJSB). A schematic of their set-up is shown in Figure 20.6. Briefly, the rotating jet spouted bed is formed when the air jet moves circumferentially in the annular region between the chamber wall and the central spout. One distinct advantage of the rotating jet configuration is the prescription of an intermittent spout due to the continuous movement of the air jet. Intermittent drying due to periodic heat supply is then possible. By varying the rotational speed of the spouting jet of heated air, the intermittency frequency and hence the intermittent drying schedule can be varied. Jumah et al. (1996) performed experiments to test the hypothesis that corn, as a slow drying material, could be dried to produce high quality grain with lower energy consumption via prescribing an intermittent air schedule. The intermittent scheduling was achieved by using various drying periods alternated by long tempering periods.
Spouting particles
Centrifugal fan
PID controller Gate valve
Electric heaters Figure 20.6
Fabric filter
Rotating air distributor
Motorized drive
Experimental rotating jet spouted bed with central and peripheral air jets (Jumah, 1995).
References 549
During the active periods, the corn particles were subjected to very intense mixing and circulation due to the hydrodynamics of the rotating spouts. The resulting effect was a period of high intensity heat and mass transfer. During the no-flow periods, the temperature and moisture gradients were effectively relaxed and favourable moisture re-distribution inside the particle occurred. Minimal mechanical damage to the kernels due to reduced attrition caused by inter-particle collisions during spouting was also observed. It was further demonstrated that moisture levelling occurred during the tempering periods with moisture migration to the corn kernel surface. In terms of energy saving, intermittent drying conducted with the RJSB resulted in substantial energy saving of up to 37 per cent when compared to a continuous spouting bed dryer (Jumah et al., 1996).
4 Conclusions One of the main contributions of the twentieth century has been to lay the fundamental platform for new technology to emerge. This review chapter has summarized some of the recent developments of hybrid-drying technologies. As technology advances to new frontiers, the method to dehydrate food is constantly evolving to produce new hybrid drying systems. Some of the hybrid drying techniques, if combined in an intelligent fashion, would promote efficient drying in terms of enhanced product quality and reduction in energy consumption. However, R&D effort is still required to study system scale-up, optimization and control of these hybrid systems. This chapter may not have covered all novel drying technologies available, but it is hoped that those hybrid technologies presented would give dried food manufacturers a better understanding of the technologies available to improve their drying processes. Since product quality and energy consumption are usually the primary concerns during food dehydration, there is still scope for discovering new drying technologies. It is hoped that in the coming years more hybrid systems can be developed to handle even the most complex bioproduct drying problems.
References Alves-Filho O, Strømmen I (1996) Performance and improvements in heat pump dryers. In Drying ’96 (Strumillo C, Pakowski Z, eds). Krakow: Elsevier, pp. 405–415. Baker GJ (1997) Industrial Drying of Foods. London: Chapman and Hall. Best R, Soto W, Pilatowsky I, Gutierrez LJ (1994) Evaluation of a rice drying system using a solar assisted heat pump. Renewable Energy, 5 (1–4), 465–468. Chou SK, Chua KJ, Mujumdar AS, Tan M, Tan SL (2001) Study on the osmotic pre-treatment and infrared radiation on drying kinetics and colour changes during drying of agricultural products. ASEAN Journal on Science and Technology for Development, 18 (1), 11–23.
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Chou SK, Chua KJ (2001) New hybrid drying technologies for heat sensitive foodstuffs. Trends in Food Science and Technology, 12 (10), 359–369. Chua KJ, Chou SK (2003) Evaluating a pressure regulatory system for drying of heat-sensitive bioproducts. Journal of Food Engineering, 63 (3), 151–158. Chua KJ, Chou SK, Ho JC, Mujumdar AS, Hawlader MNA (2000a) Cyclic air temperature drying of guava pieces: Effects on moisture and ascorbic acid contents. Transactions of IChemE, Part C, 78, 72–78. Chua KJ, Mujumdar AS, Chou SK, Hawlader MNA, Ho JC (2000b) Convective drying of banana, guava and potato pieces: Effect of cyclical variations of air temperature on convective drying kinetics and colour change. Drying Technology, 18 (5), 907–936. Clark TD (1997) The current status of radio frequency post-baking drying. In Proceedings of the 72nd Annual Technical Conference of the Biscuit and Cracker Manufacturers’ Association, USA: Forth Worth, Texas, pp. 104–107. Clary C (1999) Microwave vacuum drying. Washington State University Today, 15 (9), 3–4. Funebo T, Ohlsson T (1998) Microwave-assisted air dehydration of apple and mushroom. Journal of Food Engineering, 38, 353–367. Jia X, Clements S, Jolly P (1993) Study of heat pump assisted microwave drying. Drying Technology, 11 (7), 1583–1616. Jumah RY, Mujumdar AS, Raghavan GSV (1996) A mathematical model for constant and intermittent batch drying of grains in a novel rotating jet spouted bed. In Mathematical Modelling and Numerical Techniques in Drying Technology (Turner I, Mujumdar AS, eds). New York: Marcel Dekker, pp. 339–380. Kudra T, Mujumdar AS (2001) Advanced Drying Technologies. New York: Marcel Dekker Inc. Maache-Rezzoug Z, Rezzoug SA, Allaf K (2002) Development of a new drying process – Dehydration by successive pressure drops: Application to the drying of collagen gel. Drying Technology, 20 (1), 109–129. Marshall MG, Metaxas AC (1998) Modelling the radio frequency electric field strength developed during the radio frequency assisted heat pump drying of particulates. International Microwave Power Institute, 33 (3), 167–177. Mason R, Britnell YG, Birchall S, Fitz-Payne S, Hesse BJ (1994) Development and application of heat pump dryers to the Australian food industry. Food Australia, 46, 319–320. Mujumdar AS (2000) Dryers for particulate solids, slurries and sheet-form materials. In Mujumdar’s Practical Guide to Industrial Drying: Principles, Equipment and New Developments (Devahastin S, ed.). Thailand: Thananuch Business Ltd Publication, pp. 37–61. Nindo CI, Sun T, Wang SW, Tang J, Powers JR (2003) Evaluation of drying technologies for retention of physical quality and antioxidants in asparagus (Asparagus officinalis L). Lebensmittel Wissenschaft und Technologie, 36, 507–516. O’Neill MB, Rahman MS, Perera CO, Smith B, Melton LD (1998) Colour and density of apple cubes dried in air and modified atmosphere. International Journal of Food Properties, 1 (3), 197–205.
References 551
Okos MR, Narsimhan G, Singh RK, Weitnauer AC (1992) Food dehydration. In Handbook of Food Engineering (Heldman DR, Lund DB, eds). New York: Marcel Dekker, pp. 437–562. Perera CO, Rahman MS (1990) Heat pump drying. Trends in Food Science and Technology, 8 (3), 75–78. Prabhanjan DG, Ramaswamy HS, Saravacos GSV (1995) Microwave-assisted convective air drying of thin layer carrots. Journal of Food Engineering, 25, 283–293. Prasertsan S, Saen-saby P (1998) Heat pump drying of agricultural materials. Drying Technology, 16 (1&2), 235–250. Rasev AI (1999) Particular features of dielectric and vacuum wood drying. In Proceedings of the 6th International Wood Drying Conference (Vermaas HF, ed.). Stellenbosch, South Africa: Stellenbosch Publisher, pp. 37–41. Rossi SJ, Neues C, Kicokbusch TG (1992) Thermodynamics and energetic evaluation of a heat pump applied to drying of vegetables. In Drying ’92 (Mujumdar AS, ed.). Amsterdam: Elsevier Science, pp. 1475–1483. Saito S, Sulaiman IB (1999) Radio frequency vacuum (RF/V) drying of small diameter Keruing. In Proceedings of the 6th International Wood Drying Conference (Vermaas HF, ed.). Stellenbosch, South Africa: Stellenbosch Publisher, pp. 234–238. Saravacos GD, Marousis SN, Raouzeos GS (1988) Effect of ethyloleate on the rate of airdrying of foods. Journal of Food Engineering, 7, 263–267. Schiffmann RF (1987) Microwave and dielectric drying. In Handbook of Industrial Drying (Mujumdar AS, ed.). New York: Marcel Dekker Inc, pp. 345–372. Shibata H, Iwao Y, Ide M (2000) Combined superheated steam and microwave drying of sintered glass beads. In Proceedings of 12th International Drying Symposium (Coumans WJ, Kerkhop PJAM, eds). Noordwijkerhout, The Netherlands: Elsevier Publication, Paper no. 363. Strømmen I, Jonassen O (1996) Performance tests of a new 2-stage counter-current heat pump fluidised bed dryer. In Drying ’96 (Strumillo C, Pakowski Z, eds). Krakow: Elsevier, pp. 563–568. Taechapairoj C, Dhuchakallaya I, Soponronnarit S, Wetchacama S, Prachayawarakorn S (2003) Superheated steam fluidised bed paddy drying. Journal of Food Engineering, 58, 67–73. Thomas WJ (1996) Radio frequency drying provides process savings: New systems optimize radio frequency drying for the ceramic and glass fibre industries. Ceramic Industry Magazine, April, 30–34. Turner IW, Jolly P (1991) Combined microwave and convective drying of a porous material. Drying Technology, 9 (5), 1209–1270.