Food Bioprocess Technol (2010) 3:843–852 DOI 10.1007/s11947-010-0353-1 ORIGINAL PAPER Drying Technology: Trends and Applications in Postharvest Processing Arun S. Mujumdar & Chung Lim Law Received: 17 November 2009 / Accepted: 26 March 2010 / Published online: 23 April 2010 # Springer Science+Business Media, LLC 2010 Abstract Thermal drying technologies have attracted significant R&D efforts owing to the rising demand for improved product quality and reduced operating cost as well as diminished environmental impact. Drying materials may appear in the form of wet solid, liquid, suspension, or paste, which require drying to extend the period of storage, ease of transportation, and for downstream processing to produce value added products. Most of these materials are heat-sensitive and require careful drying; conventional hot air drying can be detrimental to the retention of bioactive ingredients. High temperature tends to damage and denature the product, destroy active ingredients, cause case hardening and discoloration, etc. This article briefly summarizes some of the emerging drying methods and selected recent developments applicable to postharvest processing. These include: heat pump-assisted drying with multimode and time-varying heat input, low and atmospheric pressure superheated steam drying, modified atmosphere drying, intermittent batch drying, osmotic pretreatments, microwave-vacuum drying, etc. Keywords Dehydration . Bioactive ingredients . Preservation . Energy savings . Quality A. S. Mujumdar Department of Mechanical Engineering, Faculty of Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore e-mail: mpeasm@nus.edu.sg C. L. Law (*) Department of Chemical and Environmental Engineering, Faculty of Engineering, The University of Nottingham, Malaysia Campus, Broga Road, Semenyih, Selangor 43500, Malaysia e-mail: chung-lim.law@nottingham.edu.my Introduction Drying is one of the most energy-intensive unit operations in postharvest processing. This unit operation is applied to reduce the water content of products such as various fruits, vegetables, agricultural and herbal products, etc. after harvest. The purpose of reducing the water content is to prolong the shelf-life of the products of bio-origin by reducing the water activity to a level low enough where growth of microorganisms, enzymatic reactions, and other deteriorative reactions are inhibited. Some bio-origin products such as herbs have to be dried before the active ingredients can be extracted. Furthermore, the products in the dry form weigh less and thus reduce transportation costs. The harvested bio-origin products are diverse in physical, chemical, and biochemical properties. A large assortment of dryers has been developed to dehydrate and preserve these products to meet different quality and cost requirements. Over 500 dryer types have been reported in the technical literature, and about 100 types are commercially available. Differences in dryer design are due to different physical attributes of the product, different modes of heat input, different operating temperature and pressure, different quality specifications on the dried product, etc. Most conventional dryers use hot air as the drying medium, convection as the single mode of heat transfer, and are operated at atmospheric pressure under steady drying conditions. For smaller capacities and long drying times (e.g., solar dryers), batch operation is preferred for obvious reasons. Conventional dryers have several limitations, e.g., nonuniform product quality due to over-drying/under-drying caused by long or inadequate or non-uniform exposure to the drying medium; long drying times due to low contacting efficiency between the drying medium and solids being 844 Food Bioprocess Technol (2010) 3:843–852 dried; harder texture owing to case hardening of product surface which is caused by over-drying (López et al. 2010); significant color change from the original product which is caused by browning reaction, redox reaction, etc. (Arabhosseini et al. 2010); and change in physical, chemical, rheological, and sensory attributes (Falade and Omojola 2010; Aversa et al. 2010). This gives rise to low drying performance and high operating costs. Many studies have been carried out over the years to overcome the operational problems or difficulties of conventional dryers as well as to improve the quality of the dried products (Fernandes et al. 2010; Law et al. 2008). New developments of dryers and emerging drying technologies can be classified into the following categories. The categories indeed represent areas in drying technology that require improvement. However, all new developments and emerging drying technologies must be cost-effective. This is needed to ensure market acceptance (Mujumdar 2007b). New technologies are needed for: developments in drying that are briefly discussed in the following sections. Some general attributes of the recently developed dryers include: & & & & & & & & & & Drying of new products and/or processes Higher capacities than current technology permits Better quality and quality control than currently feasible Reduced environmental impact, use of renewable energy Higher energy efficiency, use of heat recovery system (resulting in lower energy cost) Reduced fire, explosion, toxic hazards, safer operation Better efficiency (resulting in lower cost) Lower cost (operating, maintenance cost, and capital) Shorter processing time while maintaining high product quality The conventional dryers found in the food industry are spray dryers, freeze dryers, vacuum dryers, tray dryers, rotary dryers, fixed bed dryers, fluidized bed dryers, etc. Readers may refer to the Handbook of Postharvest Technology (Chakraverty et al. 2003), Handbook of Industrial Drying (Mujumdar 2007a), and Guide to Industrial Drying (Mujumdar 2008) for detailed information about most of the common dryer types as well as many new designs. In addition to this, readers may also refer to Fernandes et al. (2010) for drying of exotic fruit. Table 1 summarizes the general characteristics of these dryers. Although these dryers are classified as conventional, there is great number of areas suitable for further improvement. Mujumdar (2007b) has identified various areas and aspects that need further R&D efforts. Several books and texts published recently have also covered the topic in detail (Kudra and Mujumdar 2009; Chen and Mujumdaer 2008). Some of the areas (including limitations) stated in Table 1 have been addressed by researchers in recent years. Significant improvements as well as advancement have been made over the past few years which lead to the new & & & & & Multimode heat input concurrently or sequentially to match instantaneous (for batch) or local (for continuous) drying kinetics without adverse effect on product quality, e.g., convection followed by or simultaneously with conduction, radiation, or microwave heat input Time-dependent heat input for batch drying to match drying kinetics with heat input Superheated steam as drying medium at high, atmospheric, or sub-atmospheric pressure Low-temperature dehumidified air as drying medium at modified atmosphere which eliminates existence of oxygen Multistage drying and hybrid drying that combine different types of dryer or different types of heat input, e.g., microwave–vacuum drying, spray drying followed by fluid bed/vibrated bed as second and/or third stage Use of absorbent to remove water vapor as it can maintain high driving force for mass transfer even at lower temperatures Heat Pump-Assisted Drying with Multimode, Time-Varying Heat Input Heat pump dryers use low-temperature dehumidified air as the convective drying medium. Hence, drying in heat pump dryer can be carried out at relative low temperature as compared to conventional hot air dryers. This drying system incorporates a dehumidification cycle where condensation of dew allows the removal of water from the closed system of drying air circulation. A heat pump is used to perform condensation and heating of the dehumidified air. The heat pump recovers the sensible and latent heats by condensing moisture from the drying air. The recovered heat is recycled back to the dryer through heating of the dehumidified drying air. Heat pump drying of sapota pulp to produce sapota powder was reported to be faster than hot air drying (Jangam et al 2008). Drying of Australian nectarine slices in a heat pump dryer was found to produce the dried product that was better than that from cabinet and tunnel dryers in terms of lactone and terpenoid retention (Sunthonvit et al 2007). Mujumdar (1991) proposed the application of intermittent drying to heat pump drying some 20 years ago to save operating cost and also capital cost since it allows the use of a lower capacity heat pump or use of a single heat pump to service several drying chambers. This strategy is currently under active research and development at several laboratories. Islam and Mujumdar (2008) presented various Food Bioprocess Technol (2010) 3:843–852 845 Table 1 General characteristics of some conventional dryers and suggested areas for further improvement Dryer type General characteristics Areas requiring further R&D Tray •Materials are placed on trays and directly make contact with drying medium (typically hot air) •Heat transfer mode is typically convective. Conductive is possible by heating the trays •Uniformity of air flow distribution Rotary •Uniformity of final product quality and moisture content •Hybrid mode by combining with microwave heat input •A cylindrical drying chamber rotates while material tumble in the chamber •Precise prediction of particle motion, particle residence time distribution and uniformity of final moisture content •Drying medium (typically hot air) is charged into the chamber contacts the •Effect of polydispersity and cohesiveness of solids on drying kinetics material in cross flow •Flights are used to lift the material •Design of flights, internal heat exchangers, delumpers •Internal heat exchangers installed to allow conductive heat transfer •Effect of solid holdup and hot air injection on drying kinetics •Model-based control Flash •Flash dryer is used to remove surface moisture. Material is charged into a •Modeling of particle motion including effects of agglomeration, attrition fast moving drying medium stream, drying occurs while the drying and geometry of dryer medium conveys the material pneumatically •Cyclone is normally used to separate the drying medium and the material •Use of pulse combustion exhaust, superheated steam, internal heat exchangers, variable cross-section ducts, hot air injection along length of dryer duct Spray •Atomizer mounted on top of a drying chamber sprays liquid/suspension and forms droplets •Effects atomizer design on droplet trajectories, product properties, agglomeration, size reduction •Drying medium (typically hot air) is supplied into the chamber concurrently or counter currently •Hot air exits the chamber at the chamber outlet and carries dried powder •Effect of chamber geometry •Injection of supplementary air •Separation of hot air and powder takes place in cyclone •Use of superheated steam •Uniformity of product quality and final moisture content Fluidized •Similar to fixed bed dryer but operating hot air velocity is higher to ensure •Effect of particle moisture content/polydispersity on fluidization bed the particles are suspended in the sir stream hydrodynamics, agglomeration, heat and mass transfer •Large contacting surface areas between the drying medium and the material if compare with fixed bed dryer •Effect of agitation, vibration, pulsation, acoustic, radiation on drying kinetics and characteristics •Conventional fluidized bed is not suitable for drying fine powders (due to •Design of internal heat exchangers channeling and slugging) and coarse particles (due to formation of big bubbles) •However, modified FBD such as vibrating FBD, agitating FBD, etc. can be •Classification of particle type based on fluidization quality at varying used to dry difficult-to-fluidized particles particle moisture content and stickiness •If the materials are polydispersed, the hot air stream may carries over some •Mathematical modeling of fluidization hydrodynamics, heat and mass fine particles transfer by taking into account agitation, vibration, pulsation, internal heat exchanger, varying particle moisture content, etc. Vacuum Freeze •A cyclone is used to separate the fine particles from the gas stream •Over 30 variants possible •Need to maintain high vacuum; expensive •Combined mode of heat transfer, e.g., MW vacuum drying •Drying chamber is operated at reduced pressure or vacuum •Hybrid drying, e.g., vacuum superheated steam drying, etc. •Boiling point of water/solvent is reduced thus reducing the operating •Use of internal heating media temperature •However, absence of drying medium in the vacuum drying chamber •Enhancement in drying kinetics by incorporating radiant heat input, disables convective heat transfer but enhances mass transfer at low internal heating media, etc. temperatures •Vacuum freeze drying is expensive in terms of capital costs and operating •Use of magnetic/electric/acoustic fields to control nucleation and crystal costs due to very low vacuum required at very low temperature size of ice during freezing; permits better quality product •Drying times are long; most operated batchwise •Suitable only for very high value products like pharmaceutical products Batch dryer •Not all dryers can operate in batch mode •Effects of intermittent/cyclic/variable heat inputs and variable operating profiles on drying kinetics and characteristics as well as product quality •Good for low capacity needs •Use of heat pump including chemical heat pump •Tray, rotary, drum, fixed bed, fluidized bed vacuum dryers etc. can be operated batchwise •Reduction in labor costs •Model-based control •Intermittent drying 846 innovative heat pump drying systems such as multistage compression heat pump drying, cascade heat pump drying, heat pump drying system with multiple evaporators in series and in parallel, and vapor adsorption heat pump dryer. Furthermore, it is possible to use a smaller heat pump to service two or more drying chambers in cyclical mode, which may dry the same or different products in different chambers (Mujumdar 2006). Chua et al. (2002) have presented the effect of different temperature–time profiles on the quality of agricultural products in a tunnel heat pump dryer. Various profiles are possible, e.g., cyclic temperature, step-down temperature, cyclic pressure, variable gas flow, etc. (Law et al. 2008). In addition, heat pump drying can be operated at vacuum condition (Artnaseaw et al. 2010). Furthermore, heat pump drying can be incorporated with other drying methods such as spray drying (Alves-Filho et al. 2009), atmospheric freeze drying (Alves-Filho and Eikevik 2009; Bantle et al. 2009), and solar drying (Chen et al. 2008). Intermittent Batch Drying By varying the airflow rate, temperature, humidity, or operating pressure individually or in tandem, the operating condition of a drying process can be monitored in order to reduce the operating cost, e.g., thermal input and power input. The objective is to obtain high energy efficiency without subjecting the product beyond its permissible temperature limit and stress limit while maintaining high moisture removal rate. There are two ways in applying intermittent heat input profiles. The first one is to subject the drying materials to intermittent heat input, time-varying flow of drying medium or use of cyclically varying operating pressure in the drying chamber. The main purpose is to allow internal moisture to migrate to the material surface during non-active phase of drying, often termed the tempering period. Intermittent drying consists of two distinctive drying periods, namely, active drying and non-active drying. During active drying, heat input is applied by the drying medium, while during the non-active drying period, heat input or flow of the drying medium is stopped. The two distinctive periods are carried out in an alternating mode. Since water content on the surface is increased during the tempering period, the drying rate during the subsequent active drying is increased noticeably, which helps enhance the drying kinetics. However, since the rate of drying is finite during the passive period, the overall drying time is increased somewhat, but it is offset by the reduction in energy consumed and the better product quality due to lower product temperature. In this regard, it is important to identify the intermittency (the ratio of intermittent time and active drying time). Bon and Kudra Food Bioprocess Technol (2010) 3:843–852 (2007) performed an optimization in terms of energy performance by taking into account the enthalpy gain. Thomkapanich et al. (2007) compared the performance of intermittent temperature, low-pressure superheated steam drying at 90 °C with continuous low-pressure superheated steam drying and vacuum drying. They reported that intermittent mode of low-pressure superheated steam drying could reduce energy consumption by up to 65% and steam savings up to 58%. Since the effective drying in intermittent mode was reduced noticeably, higher ascorbic acid retention (11–25% with reference to vacuum drying sample) was found in the dried product. Kowalski and Pawłowski (2009) applied intermittent drying for wood by changing the temperature and humidity of drying air periodically; it was found that the quality of dried wood was better than that dried in constant operating profiles. Tuyen et al. (2009) reported the use of tempering between initial stage fluidized bed drying and final stage thin-layer drying of rice could reduce kernel fissuring and improve head rice yield. Thakur and Gupta (2006) also reported that tempering sandwiched between two fluidized bed drying periods as well as two packed bed drying periods could reduce energy consumption and improve head rice yield. The second intermittent drying strategy is to apply stepwise change of operating conditions in order to minimize energy requirement. This is due to the fact that drying toward the end of the process is controlled by internal diffusion where the external factors have limited effect on the drying kinetics. As such, one possible way to reduce energy loss is to gradually reduce the heat input to the materials along the drying process. However, it should be noted that drying temperature at the final stage of drying cannot be too low as the equilibrium moisture content is dependent on temperature (Chong and Law 2009). One can also vary the mode of heat input (e.g., convection, conduction, radiation, infrared (Afzal 2003; King and Lin 2009), or microwave (Soysal et al. 2009)/radio frequency heating). Multiple heat inputs can be used to remove both surface and internal moisture simultaneously. In this regard, Kowalski and Rajewska (2009) reported the use of microwave, infrared, and microwave–infrared coupled with convective drying which gave higher drying rates and reduced temperature gradient. Intermittent drying can be applied to any direct dryer and batch dryer such as tray dryer, convective dryer, conveyor dryer, fluidized bed dryer, spouted bed dryer, etc. Pulsating fluidized bed dryer can be considered as an intermittent fluidized bed dryer as the bed of material is fluidized intermittently. Nitz and Taranto (2009) reported that pulsating fluidized bed did not significantly improve the water removal. However, with the application of microwave, Reyes et al. (2006) reported that the effective diffusivity was increased four folds. Food Bioprocess Technol (2010) 3:843–852 Mujumdar (1991) has identified and proposed for the first time the use of multiple modes of variable levels of heat input, simultaneous or consecutive, as well as cyclical variations in velocity or operating pressure as technologies of the future for batch and continuous heat pump drying processes. Using multiple modes of heat input, it is possible to speed up drying kinetics without adversely affecting the quality of dried products. Dryers such as rotary, spouted bed, or the multi-cylinder paper dryer are all inherently intermittent since heat is supplied intermittently due to the inherent operational mode of the dryer, although none of the operating variable such as flow rate, temperature, or pressure is altered with time. They are still not termed intermittent since the on and off times of heat input cannot be altered independently of the other operating variables. Modified Atmosphere Drying To avoid oxidation of the drying material and destruction of its bioactive ingredients, hot drying air, which contains 21% of oxygen, can be replaced with nitrogen or carbon dioxide. By eliminating oxygen, oxidation and some undesirable reactions which require oxygen are thus avoided. This in turn reduces/eliminates browning of products and improves retention of bioactive ingredients. In addition, modified atmosphere heat pump drying reportedly increases the effective diffusivities of some food products. O’Neill et al. (1998) and Perera (2001) have discussed the application of modified atmosphere drying for some food products using a heat pump. Hawlader et al. (2006a, b, c) carried out a number of experimental investigations of modified heat pump drying on various types of food products and have shown great enhancement of product quality which is in term of the retention of 6-gingerol in dehydrated sliced West Indian ginger. On the other hand, the drying atmosphere can be modified by adding volatile chemical compounds such as ethanol to reduce the volatilization of volatile compounds in the food samples. Braga et al. (2009) added 0.5% (v/v) ethanol to the air stream and found that it would retain important volatile compounds of pineapple aroma (which are found in fresh pineapple) in dried samples, besides promoting rapid water evaporation. Santos and Silva (2009) attributed the higher retention of L-ascorbic acid in dried pineapple to the presence of ethanol in the drying atmosphere which promoted intense water evaporation, thus reducing the drying time which in turn shortened the degradation of L-ascorbic acid. Kudra and Poirier (2007) carried out a comparison of drying kinetics and energy consumption of fluidized bed drying of wheat kernels with air and with CO2. It was found that fluidized bed drying using CO2 as the drying medium could shorten the drying time by about 20%, which in turn offered energy savings of about 3% of 847 the heat input. It is noteworthy that additional energy savings of 4% of the heat load can be obtained for drying in CO2 atmosphere at temperatures below 100 °C owing to the lower wet bulb temperature of CO2. Superheated Steam Drying Superheated steam is an attractive drying medium for some processes since the net energy consumption can be minimized if the exhaust (also superheated steam) can be utilized elsewhere in the plant and hence is not charged to the dryer. Superheated steam does not contain oxygen; hence, oxidative or combustion reactions are avoided. In addition, it also eliminates the risk of fire and explosion hazard. The quality of superheated steam-dried products tends to be better than that from conventional hot air dryer. Superheated steam also allows pasteurization, sterilization, and deodorization of food products. This is particularly important for food and pharmaceutical products that require a high standard of hygienic processing. In addition, superheated steam drying can also give higher drying rates in both constant and falling rate periods under certain conditions. Closed system superheated steam drying enables emitted odors, dust, or other hazardous components to be contained and thus mitigate the risks of these hazards. The pollutants are concentrated in the condensate of the effluent steam. On the other hand, desirable organic compounds can also be captured using the superheated steam drying method. Mujumdar has discussed the principles, advantages and limitations, as well as diverse applications of superheated steam drying technologies in a number of papers and books (Kumar and Mujumdar 1990; Mujumdar 1992; Kudra and Mujumdar 2001, 2009), including the Handbook of Industrial Drying (Mujumdar 2007c). This drying technique has been applied to the drying of foods (Rahse and Fues 1995; van Deventer and Heijmans 2001; Elustondo et al. 2002; Pronyk et al. 2004), sugar beet pulp (Tang et al. 2000), spent grain (Tang and Cenkowski 2001), noodles (Markowski et al. 2003; Pronyk et al. 2008a, b), soybeans (Prachayawarakorn et al. 2006), and shrimp (Prachayawarakorn et al. 2008) Recently, this drying technique has been tested on food and non-food materials such as fish meal (Nygaard and Hostmark 2008; Høstmark et al. 2009), pork (Uengkimbuan et al. 2006), oil palm empty fruit bunches (Hasibuan and Wan Daud 2009), noodles (Pronyk et al. 2008a, b), paper (McCall and Douglas 2006), raw starch sphere (Iyota et al. 2008), wood (Yamsaengsung and Sattho 2008), and saw dust (Borquez et al. 2008). Some products are not stable at 100 °C if the dryer operates at atmospheric pressure; one option to overcome this problem is to lower the operating pressure. Indeed, silk cocoons and many fruit and vegetable products have been successfully dried in low-pressure superheated steam dryers (Kongsoontornkijkul et al. 2006; Thomkapanich et al. 2007; 848 Devahastin and Suvarnakuta 2008). Recently, this technique has been tested on other products such as chitosan film (Mayachiew and Devahastin 2008) porous media (Tatemoto et al. 2009). Since the heat transfer for drying is still by convection, the drying rates are very low at low steam pressures. Although one can obtain good quality at low pressures, the process is still not popular due to large equipment size caused by low drying rates. Perhaps it is necessary to include supplementary heat sources, e.g., microwave, radiation, or conduction, to speed up drying rates at low steam pressures. Much R&D is needed in this area. Hybrid Drying—Microwave Vacuum Drying Microwave drying offers advantages in enhancing drying kinetics, precise control, fast start-up and shutdown times, quality of dried product, smaller footprint of equipment, etc. (Gunasekaran 1999; Puschner 2005; Stanisławski 2005; Cui et al. 2006; Schiffmann 2007; Vadivambala and Jayas 2007; Setiady et al. 2009a). Microwave drying is typically combined with other drying methods to overcome the limitations of uneven heating resulted from focusing, corner and edge heating, inhomogeneous electromagnetic field, and irregular shape and non-uniform composition of material, for instance microwave freeze drying (Wang and Chen 2005; Duan et al. 2007; Cui et al. 2008a; Huang et al. 2009; Wang et al. 2009), convective drying (Reyes et al. 2007; WitrowaRajchert and Rzaca 2009; Askari et al. 2009), and vacuum drying (Scaman and Durance 2005; Stepien 2007; Setiady et al. 2009b). Vacuum microwave has been tested as a predrying method in the frying of food materials (Song et al. 2007a, b). Furthermore, its start-up costs are relatively high and it requires sophisticated mechanical and electronic components (Zhang et al. 2006). Microwave vacuum drying has been shown to produce dried products with improved texture and color. Microwave field allows volumetric heating whereby heat is transferred to the inner core of material without the need of a temperature gradient even in the initial stage of drying. Combination of microwave and vacuum drying results in improved color and texture of dried products over air-dried products. Reduction of drying times in microwave is beneficial for color, porosity, aroma, shrinkage, and rehydration (Yonsawatdigul and Gunasekaran 1996; QingGuo et al. 2006; Giri and Prasad 2006, 2007a, b; Sundaram and Durance 2007; Tsuruta and Hayashi 2007; Cui et al. 2008b; Mitra and Meda 2009; Maddikeri et al. 2009; Markowski et al. 2009). Osmotic Dehydration and Pretreatments Thermal drying is an energy-intensive operation because it involves evaporation of water that requires vast amount of Food Bioprocess Technol (2010) 3:843–852 latent heat. Osmotic dehydration may be applied to partially remove liquid in the material before the material is subjected to thermal drying and water removal by phase change. The high latent heat of vaporization implies a large amount of energy consumed for the phase change. However, in osmotic treatment, some of the nutrients and color components may be lost to the osmotic agent. To recover the osmotic solution or to recycle it, one must concentrate the solution. This can be done by applying evaporation which is much less energy-intensive than drying. Note that mechanical pretreatments coupled with chemical pretreatment can also be applied prior to or during thermal drying to enhance the efficiency of moisture transport. Recycling and reconditioning of osmotic solution has been reported by Germera et al. (2009) that it did not affect the water loss and solid gain of peach drying. By applying osmotic pretreatment, the moisture of materials to be subjected to subsequent thermal drying is lowered (Sutar and Prasad 2007; Duan et al. 2008; LemusMondaca et al. 2009). Thus, the drying load is reduced. However, extensive experiments have shown that almost always, the drying kinetics is lowered as well once the solute starts to precipitate out and crystallize in the pores. Thus, the total drying time may not be reduced by as much as one would expect from the reduced drying load. During osmotic dehydration, moisture diffusion that governs the mass transfer of water is dependent on various factors such as the operating conditions (pressure, concentration of the osmotic medium, treatment time, size and geometry, specific surface area of the material, and temperature), mode of phase contacting (solid–liquid phases), sample-to-solution ratio, composition of the solute, and agitation level of the solution. Some products have a layer of membrane or wax on the surface, limiting the rate of moisture transport at the surface. Often, pretreatments are required to enhance the rate of diffusion. Pretreatment methods such as freezing/thawing, vacuum treatment (Corzo and Bracho 2007), exposure to ultrahigh hydrostatic pressure, high-intensity electrical field pulses (Ade-Omowaye et al. 2001; Amami et al. 2007), microwave, ultrasound (Fernandes et al. 2008; Shemaei and Moeini 2009), application of centrifugal force (Amami et al. 2007), use of supercritical carbon dioxide, coating of edible layer, etc. have been proven to enhance mass transfer of solvent for osmotic dehydration (Rastogi et al. 2002). It has been reported that if osmotic dehydration is applied as a pretreatment method, it would preserve higher content of polyphenols, procyanidins in the case of cider apple drying (Emilie et al. 2009), better aroma retention in dehydrated cherry tomato (Heredia et al. 2009), higher carbohydrate content in the case of lychee drying (Carvalho et al. 2009), better color attributes, and higher antioxidant Food Bioprocess Technol (2010) 3:843–852 activity due to better retention of β-carotene and lycopene (Shadan et al. 2009) if compared to untreated samples. Monitoring of Quality Attributes and Drying Parameters Conventional drying system does not apply process control; thus, the system tends to encounter problems such as uneven product quality, over-dry, low energy efficiency, etc. In this regard, monitoring of product properties such as moisture content, product appearance such as color, and operating parameters such as temperature are some of the aspects one may consider to monitor during a drying process in order to enhance product quality or improve operating efficiency. Research and development in this aspect is rather scarce, and it remains a challenge to researchers in this area. It has been reported that: & & & monitoring of product moisture content can be carried out using laser light backscattering imaging (Romano et al. 2008) or triboelectric probes (Portoghese et al. 2007). monitoring of surface water activity by controlling air relative humidity (Stawczyk et al. 2009) and monitoring of solvent residue in solvent drying (Tewari et al. 2009) can be achieved using near-infrared sensor. monitoring of the development of fracture in food material can be conducted using acoustic sensor (Kowalski and Mielniczuk 2006). Use of Renewable Energy It must be pointed out that the use of renewable energies, e.g., solar and wind, should be looked at seriously as current concerns over potential energy shortage and global climate change will likely result in legislative actions to minimize fossil fuel usage. A solar drying system, particularly for agroproducts and marine products, is viable already, particularly in developing countries where labor costs are low and cost of fossil fuel energy is very high. In future, larger systems could be designed utilizing solar thermal, photovoltaic panels combined with wind power. As solar and wind energy is necessarily intermittent, advances in thermal and electrical energy storage are needed to make use of renewable energy viable in drying. To minimize use of oil or gas, one could use biomass to provide backup heating in the absence of insolation and wind (Augustus 2009). Use of thermal energy storage in water pools, pebble beds, and/or in phase change materials can be coupled with the use of intermittent energy sources like solar and wind energy. Much R&D is needed at the systems level to make this concept commercially viable. In addition to this, application of geothermal energy in drying of fish products has been practiced in Iceland (Arason 2009). 849 Closing Remarks A brief overview of the application of drying in postharvest processing is presented for both conventional and emerging drying technologies. These include: heat pump-assisted drying with multimode and time-varying heat input, low and atmospheric pressure superheated steam drying, modified atmosphere drying, intermittent batch drying, osmotic pretreatments and their influence on drying kinetics and product quality, microwave–vacuum drying, etc. As energy costs soar, energy efficiency will be a key criterion in the selection, design, and operation of dryers. Use of renewable energy, e.g. solar, wind, geothermal, etc., needs to be examined in depth in order to cut down the use of fossil fuels for postharvest drying. 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