Heat Pump Dehumudifier Drying of Food

March 18, 2018 | Author: Yoong Kit | Category: Clothes Dryer, Humidity, Heat Pump, Latent Heat, Hazard Analysis And Critical Control Points


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ReviewHeat pump dehumidifier Although heat pumps have been used extensively in industry for many years, their use for drying, especially foods, has been limited. This article reviews the potential of heat pump dehumidifier (HPD) dryers for use in food drying. HPD dryers offer several advantages over conventional hot-air dryers for the drying of food products, including higher energy efficiency, better product quality, and the ability to operate independently of outside ambient weather conditions. In addition, this technology is environmentally friendly in that gases and fumes are not given off into the atmosphere. The condensate can be recovered and disposed of in an appropriate manner, and there is also the potential to recover valuable volatiles from the condensate. A large proportion of the energy required within the food industry is used to remove water from food products. Because drying processes are energy intensive, knowledge about their efficiency and optimum operating conditions is vital for the economical operation of dryers. Drying uses one or a combination of convection, conduction or radiation to conduct heat to the product that is to be dried. Much work has been done to increase the drying efficiency of convection drying, particularly by the application of heat pump dehumidifiers (HPDs). HPD dryers are finding increasing applications in the food industry for the drying of nuts, fruit, vegetables, herbs and fish products in several countries including Australia, New Zealand and Norway. drying of food Conrad O. Pereraand M. ShafiurRahman air within the dryer ~. The system is entirely recirculatory, leading to a thermal efficiency approaching 100% 1. Removal of water in its liquid state rather than its vapour state allows the latent heat of vaporization to be captured and only a small amount of sensible heat is lostL In practice, design modifications such as partial evaporator bypass systems and additional heat exchangers are used to maximize efficiency 2. Although most of the HPD dryers currently available recirculate all the air, a few non-recirculatory units are also now available 3'4. The three major advantages of HPD dryers are: • high energy efficiencies are achievable because both the sensible and the latent heat of vaporization are recovered; • drying can be carried out at relatively low temperatures; • drying can be conducted independent of the ambient weather conditions. Drying efficiency Drying efficiency is a measure of the quantity of energy used in removing a unit mass of water from a product. Normally, it is measured in terms of kJ/kg, although when considering electrically operated HPD dryers, units of kW, h/kg are used. The efficiency of a heat pump can be expressed as its coefficient of performance (COP)~: COP = Qh/W where Qh is the energy absorbed at the evaporator plus the thermal equivalent of the total electrical energy input, W (in kW), required to bring about the compression: Q,= electrical input + water extraction rate × latent heat of vaporization Although it is usual to determine the efficiency of a heat pump by its COP value, in the case of a dehumidifier a more useful measure is the amount of water condensed per unit of electricity consumed. This is termed the specific moisture extraction rate or SMER: COP = 1 + SMER × hlg where SMER is given in kg/kW, h and hfg is the latent heat of vaporization. The SMER for a well-designed dehumidifier is in the range of l - 4 k g / k W . h , with an average value of -2.5 kg/kW- h. It is useful to compare this figure with the latent heat of vaporization of water, which is 2255 kJ/kg at 100°C or 1.596kg/kW.h. This demonstrates that a Mode of operation of HPD dryers The dehumidifier is an air-to-air heat pump that functions in a manner similar to the domestic refrigerator; it consists of a condenser (hot heat exchanger), a compressor, an evaporator (cold heat exchanger) and a fan to provide air movement. The heat pump is located along with the product in an enclosed chamber that has insulated walls. Dry, heated air is supplied continuously to the product to pick up moisture and is recirculated. Some of this humid air passes through the evaporator of the heat pump where it condenses, giving up its latent heat of vaporization, which is taken up by the refrigerant in the evaporator. This heat is used to reheat the cool dry air passing over the hot condenser of the heat pump. A schematic diagram of the operation of a typical HPD dryer is shown in Fig. I. In an HPD dryer, the source of the heat that is absorbed at the evaporator is the humid air that is drawn from a product during the drying process. As this moist air passes through the evaporator, it is rapidly cooled to a temperature below its dew point, resulting in water condensing out. The latent heat recovered in the process (-2255 kJ/kg of water condensed) is released at the condenser of the refrigeration circuit and used to reheat the Conrad O. Perera and M. Shafiur Rahmanare with the FoodScienceGroup, HortResearch,PrivateBag 92 169, Auckland, New Zealandifax: +64-9-815 4243; [email protected]~z). Trends in Food Science & Technology March 1997 [Vol. 8] Capvrigh~ ©19,!7. Elsevier S~ien~e All rightsreserved. 2244/'97/$17.00 Ltd 0924 Pll: s0q24 2244(97)01013 3 75 .. The comparative efficiencies and advantages of HPD.. Thus.h at 50°C and 80% relative humidity.Specificmoistureextractionrate the drying of foods and other products in those instances where heat-and-venttype dryers are inefficient because of 76 Trends in Food Science & Technology March 1997 [Vol. h. Therefore. the drying process but improves the energy efficiency ~2. which translates to a SMER of only 0. energy is also . external condenser. The motors driving the fan Fig. such Operating % RH range Variable Low 10-65 as the drying of macadamia nuts. 1 and the compressor can be located within the chamber A schematic diagram of the operation of a typical heat pump dehumidifierdryer..h) 0... energy required to drive the compressor.0-4.. meat. within the drying chamber instead of being lost to the 4. especially fruit. primaryair circulation fan: 9. Increasing the humidity in the drying air slows down 8. . 2. heat pump dehumidifier. the dehumidifier efficiency and capacity are proportional to increases in temperature and humidity. and to replace any heat loss through conduction and air leakages. chestCapital cost Low High Moderate nuts. are well suited to the totally enclosed HPD sysRunning cost High Very high Low tem.3. Vapour-sealedand insulated structure.12-1.. /" / z/ Process efficiency od ertmsmoreutedtobtchopeato than to continuous operation because batch systems 4[ ~ allow total recirculation with a very low air leakage rate.0 3.2 1... However.. Generalcomparisonof heat pumpdehumidifier(HPD) with vacuumand hot-airdrying finished product. The pilot-plant electric HPD dryer v developed by the Electricity Corporation of New Zealand (ECNZ) has a peak SMER of 7. atmosphere.. required to pre-heat the product and chamber structure. herbs. 2). condensate. fish and several biologically active products. The efficiency of hot-air dryers is generally <60% of the efficiency of evaporation. counter-current HPD fluidized-bed dryers with high SMERs for the drying of heat-sensitive 7 £ products. air distributor.. . 5. etc. the prototype developed by ECNZ represents a significant breakthrough in the performance of HPD dryers. 8] . 5 such systems is that they can be operated Drying efficiency(%) 35 40 _<70 95 6 independently of the ambient weather conditions.. Most of these products... Thus. product tray. it is important to match the heat transfer area of the evaporator to the moisture load in the air. 60°C). This is over twice the SMER reported by Hesse 3 for his HPD dryer operating with recuperator coils and air returning to the evaporator coils at 45°C RH.28 0. another major advantage of SMER(kg H20/kW.. In addition to the electrical i . which yield a sticky Table 1.. However. Strommen and Jonassen 9 5 J and Alves-Filho and Strommen ~° described the development of novel. Ideally. con~ tinuous HPD dryer systems have been built for the dryi~ ~ ing of vegetables and gelatin 8.. vacuum and hot-air dryers are shown in Table 1. HPD drying should be carried out at temperatures <60°C and average relative humidities >30%.. at high temperatures (e. The SMER also decreases as the relative humidity of the air in the chamber decreases. HPD dryers are more energy efficient than conventional hot-air dryers. HPD dryers are ideally suited for dehumidifier is able to remove water from a process at a SMER that is greater than that necessary for 100% efficiency of evaporation.. humidifier.. Thus..72-1. processes that require Operating temperaturerange (°C) 40 90 30-60 10-65 close control of drying conditions.. to drive the fan for primary air flow over the product 6 that is to be dried. In general. Some of the earlier failures of HPD dryers l~ t were due to inadequate thermal insulation and gasI tightness of the seals of the chamber structure.g. These are ideal conditions Ior the drying of many food products including fruit and vegetables. 6.. require drying at comparatively high relative humidities to prevent rapid drying resulting in case-hardening and sugar sweating. 1 ~ giving rise to high thermal efficiencies.. 7.94kg/kW..95 kg/kW. fish. overheatvent.3 4 and 80% relative humidity. resulting in loss of energy efficiency u.Relativehumidity SMER. In addition to the relatively low drying Hot-air drying Vacuum drying HPD drying Refs temperatures (10-45°C) used in HPD systems. ginger. so that the residual heat produced by them is absorbed 1. the increase in SMER with increase in relative humidity is compromised (Fig. Ginger dried in an HPD dryer was found to retain over 26% of its gingerol. 2 tent.the high humidity of the ambient air. It is by the characteristic drying curve for a set of operating recognized that the rate of reaction for non-enzymatic conditions of the dryer and physical characteristics of browning in dried products is highest at moisture levels the material. Jolly e t a l . 8] 77 . The rate of loss of provides a tool to enable drying rate and efficiency to volatiles varies with the concentration. Trends in Food Science & Technology March 1997 [Vol. even though they were dried at • COP of the HPD drying system. During drying.rate is defined and correlated with the air-to-product out the drying cycle in HPD dryers reduce the extent of moisture ratio for the constant and falling-rate periods. and this is currently considered quires exhaustive testing and monitoring of both the to be the general form representing air-drying c u r v e s 22. Use of such occur during the early stages of drying. the relative drying rate method.terms has been used. 0 30 40 50 60 70 80 90 Major advantages of using an HPD 20 dryer for drying food products are the Relative humidity (%) potential improvements in the quality of resultant products. 7). The moisture loss from the product can be simulated flavour and nutritive value associated with dried products are attributed to non-enzymatic browning. flavour and to some extent texture. suffer loss of heat-labile vitamins Specific moisture extraction rate (SMER)as a function of humidity and dry-bulb temperature (data taken and have a high incidence of colour from Ref. dried products have a low aroma volatile con. This temperature on food products. resuiting in longer drying times and higher energy requirements. for development of the model. Because HPD drying is conducted in mine their effects on product quality. • air conditions in the system. when the initial models to identify appropriate drying conditions should concentration of the volatile component in the drying limit the number of trials that may be necessary to determedium is low ~4. compared with only ~20% products. any compound that volatilizes will oped a model for the continuous operation of an HPD remain within the chamber. 50°C. Because of the deleterious effects of high retention for rotary-dried commercial samples ~3. 5 devela sealed chamber. ated with the HPD drying process. degradation. v 45°C 60°C c" 5 30°C Quality improvements 3 The quality of dried food products can be assessed by the degree of deg2 radation of their colour. a normalized drying However. flow The development of a brown centre occurs in macrate and humidity. Mathematical modelling temperature used in the HPD system. and the partial pressure for dryer that predicts the following under given system that compound will gradually build up. such as temperature. adamia nuts if high-moisture nuts are dried at elevated temperatures ~5. Usually. condenser and external waste system. the principal volatile flavour component re.ture to model drying curves: the relative drying rate ing cycle. such as in the tropics and on islands where high humidity prevails during most of the year. that HPD drying of macadamia nuts did not result in the above defect. the lower drying temperatures used through. when the drying rate is low and the product method ~2° and the constant drying rate method 2j-23. the 1 major factors that affect these attributes are the drying times and temperatures. They are ideal for drying in weather conditions where the relative humidity of the outside air is very high. retarding further specifications and operating conditions: volatilization from the product. Two approaches are available in the literathat are commonly attained towards the end of the dry. a two-component model with two-exponent HPD dryer design The determination of optimum drying conditions re. This may be due to the faster drying rates associ• maximum efficiency achievable. In the constant drying rate method. Knowledge of the critical moisture content is necessary the non-enzymatic browning reactions ~6. Van Blarcom and Mason ~6 have shown • heat transfer rate in the evaporator. optimization of product higher volatile retention in HPD-dried samples may be due quality is often at odds with the aims of maximizing to reduced degradation of gingerol at the lower drying efficiency and throughput.Fig. In temperature approaches that of the drying medium Iv. the greatest losses be predicted under a range of conditions ~8. The losses in colour.efficiency of the process and the quality of the dried sponsible for its pungency. giving rise to poor flavour. N. O. such as the viability of microbial cell cultures and enzyme activity. too low a temperature will limit drying rates. P. B. continuous HPD dryers have been developed for gelatin and vegetable drying s. Some of the newer refrigerants now available will allow the use of higher drying temperatures. 771 782 3 Hesse.Proceedings of the 70th International Drying Symposium (Strumil]o. The construction of continuous drying systems may require high engineering. 68 71. 14. BHRA Fluid Engineering. and Pakowski. The use of modified atmospheres for drying sensitive materials including food products is another important aspect of' HPD drying technology. and Carrington. and Jonassen. Institute of Professional Engineers of New Zealand 8 Anon. ~6d2 Technical University. pp.Proceedings of the 9th International Drying Symposium (Rudolph. Marcel Dekker 7 Barneveld. Z. (1990) 'Heat Pump Assisted Continuous Drying. References 1 Oliver.. thus avoiding microbial growth.S. 563-568. Paris. J.O. The use of modified atmospheres to replace air should permit new dry products to be developed without oxidative reactions occurring. Poland 5 Jolly. (19901 'Heat Pump Assisted Continuous Drying. (19961 'Effects of Drying Rate and Ambient Conditions on the Operating Modes of Heat Pump Dryer' in Drying '96 . Part l.). HPD dryers are more suited to batch drying because drying takes place in a hermetically sealed container. Bannister. benefits need to be evaluated on the basis of cost rather than on energy efficiency alone. Prateepchaikul.. edsi. and Pakowski. Z. Keey. 14. International Union for Electroheat. Part 2. A. L6d2 Technical University.. ed. eds).P. Energy Res. Negligible information is available in the literature on the model constants for food materials in the case of the two methods discussed above. such high drying temperatures may compromise the benefits of low-temperature drying. Saen-Saby. if the refrigeration capacity of the refrigeration circuit is below that required to condense the moisture in the air. However. pp. V. 529-534.B. An HPD dryer is basically a convective dryer. UK 2 Jia. Jia. Cranfield.S. and Clements.N. C. yeasts and moulds 2s. (1982I 'Process Drying with Dehumidifying Heat Pump' in International Symposium on the Industrial Application of Heat Pumps. C. S. P. which has implications on throughput. For example. Simulation Results' in Int. Such information is scarce and is needed not only for improving the design and control of dryers but also for setting standards for different food products. however. P.26 29. it will lead to high humidity in the chamber. C. 73-88. its application at this stage is limited mainly to solid products..S. Poland 10 Alves-Filho. except a few species of heat-resistant bacteria. and Clements. S. where heat is transferred by the convection of warm air. Oxygen-sensitive materials such as flavour compounds and fatty acids can undergo oxidation during drying. 2.Such models can be used to simulate drying behaviour over a range of conditions.. pp. Equilibrium moisture isotherms are also necessary for both of the methods. slower drying rates at low temperatures may give rise to microbial growth problems. and Strommen. the rate of vaporization of moisture from the food and the rate of condensation of moisture from the air will be such that the water activity at the surface is maintained below the critical value of 0. Bedford. X. 591-598. There is also a need to obtain data on the physical properties of foods to provide the model constants required to develop suitable drying models for HPD dryers. pp.6. and Lopez-Cacicedo. Energy Res. (1994) 'Energy Efficient Electric Drying Systemsfor Industry' in Drying '94 . and Mujumdar.G. Nesvadba 3° recently highlighted the initiatives that have been undertaken in this area in Europe. pp. especially for heat-labile materials. especially in the case of biologically active material9. modelling and design costs. Therefore.. in practice there have not been any reports of increased numbers of microorganisms in HPD-dried foods compared with those in foods dried by conventional means ~2. (1996) 'Performance Tests of a New 2-stage Counter-current Heat Pump Fluidized Bed Dryer' in Drying '96 . Future trends An application of HPD dryers in combination with fluidized-bed dryers has been investigated with respect to drying efficiency and improvements in product quality. I. T. The Guggenheim-Anderson-de Boer (GAB) isotherm is one of the semi-theoretical multi-layer sorption models and has been considered to be the best-fit model for most foods over a wide range of temperatures and water activities. should also be possible to maintain without the use of expensive freezedrying techniques. Rahman 24 has discussed the GAB model and reviewed the model constants for a wide range of food products. and is more suitable for drying solid products than liquid or semi-solid products. P. however. The ability to simulate freeze drying at atmospheric pressures and then carry out drying at temperatures above 0°C in the same dryer is an interesting concept. J. For example. Such a process should offer the ability to regulate the physical properties such as apparent density and rehydration rate by increasing the porosity of the product owing to the formation of ice crystals in the frozen product. 58-59. Like vacuum or freeze dryers. C. and one that needs further study. (1996) 'Development of the ECNZ Electric Heat Pump Dehumidifier Drier Pi[ot-plant' in Proceedings of the Annual Conference ot the Institute of Professional Engineers of New Zealandl Vol. and Ngamsritrakul. 823-862. use of the refrigerant 'R134a' should enable the operation of an HPD dryer up to -75°C without supplementary heating ~3. (1988) 'Drying of Vegetables' in Industrial Drying by Electricity. R. (1987) 'Energy Aspects in Drying' in Handbook of Industrial Drying (Mujumdar. 757-770 6 Strumil[o. (1996} 'Performance and Improvements in Microbial safety Most vegetative cells of microorganisms will be destroyed by normal hot-air drying at 60-80°C. Serious microbiological problems may arise if the dryer is poorly designed. G.. C. University of Queensland. colour and rehydration properties. Under normal circumstances. Australia 4 Prasertsan. pp. Part 1. eds).. Constraints on HPD drying Although low-temperature drying is a potential advantage for drying foods. 78 Trends in Food Science & Technology March 1997 [Vol. pp. Simulation Model' in Int. Biological activity. 52-53. 8] . France 9 Strommen. Working Group 'Heat Recovery'.. Jolly.Proceedings of the 10th International Drying Symposium (Strumillo. Brisbane. A. I.. Furthermore. Although there is some concern about the potential for growth of microorganisms at the temperatures used in HPD dryers. Thus. X. R. M.D. Jane-Marie Hawronskyjand John Holah are at Campden & Chorleywood abbreviated to HACCP. 405 416.B. (1978) Introduction to Industrial Drying Operations. V. and Marines-Knurls.. international body responsible for the execution of the of doubtful significance. and Weitnauer. A. VoL 1. C.D. it was in the volved in the production. R. a.P. A. adverse publicity or even food aimed at protecting the health of consumers and facilitating international trade in foods) provides internaand/or profits. University of Queensland. 129-144 22 Lebert. Instituteof Professional Engineersof New Zealand 12 Britnell.Y. V. the identification of specific hazards and the necessUK GL55 6LD Ifax: +44-1386-842100. 123-130. Bauman introduced the concept of hazard analysis and critical control points.S. co. Singh.K. Technol.. Keey. Elsevier 23 Rahman. 511-517. R. Ngamsritrakul. {1996) 'Heat Pump Dryer Part 1: Simulation of the Model' in Int. Brisbane. pp. the Codex Alimentarius Commission (the Instead of relying on postmortem analysis inspections. R.Proceedings of the 9th International Drying Symposium (Rudolph. NSW. thus avoiding recall campaigns. and Eikova. PergamonPress 20 Zhang. pp. Exotic Fruit Growers Association. 897-904.. 8] Cop'~right. 23. and Vagle."L6d. R. In order to evaluate a sanitation programme effectively. all of which w o u l d ultimately end in reduced sales Trends in Food Science & Technology March 1997 [Vol.Z.food quality and safety is not a new one. The definition of a scares. and Kramer. eds).uk). Brisbane. e924-2244/97/$17. Energy Res. R..B. he advocated the introduction Joint FAOAVHO Food Standards Programme.00 PILS09242244(97)O1009-1 79 ... and Keey. R.R. ed.. and Yamamoto.G.W. J.Proceedings of the lOth International Drying Symposium (Strumillo. 31. Poland 11 Bannister. This maintains control of the process.A. 475-477.. ed.ed.C. (1965) 'Effectof Drying Temperatureon Chemical Composition and Quality of Macadamia Nuts' in Food TechnoL 19. and Mujumdar.M. 1.295-296 Review ATP: A universal hygiene All food producers have a direct responsibility to ensure the safety and quality of their products. Q. R. and Kramer. McGraw-Hill 26 Strommen.L. 239 248. Keey. and Mu)umdar. (19961 'Thermal and Other Physical Propertiesof Foods:Needs for Data and for Standards'in Int. (1994) 'The Application of Heat Pump Dryers in the Australian Food Industry' in Drying '94 . Brisbane. I. 1007-1014. 46(7). Strommen. Kramer. Bauman suggested a complete change in to ensure the safety and quality of their products.O.H. and Mujumdar. which is of a forward control strategy. C. University of Queensland. Australia 30 Nesvadba. pp. Saen-Saby. Brisbane. and Bimbenet. pp.R. eds). eds). A.K.S. and Strommen. eds).S.. D..S. and Perera. A. E.. Marcel Dekker 18 Prasertsan. and Driscoll.I.W. The use of this technique for monitoring allows information to be provided in time for corrective monitor Jane-Marie Hawronskyjand JohnHolah action to be taken. E. (1994) 'Non-adiabatic Heat Pump Fluidised Bed Drying of a Sticky.B.Z.Australia 13 Mason. et al. CRC Press 25 Frazier. {1994) 'Development and Application of Heat Pump Dryers to the Australian Food Industry' in FoodAust. Lismore. (1988) 'Low Humidity Drying of Macadamia Nuts' in Proceedings of the Fourth Australasian Conference on Tree and Nut Crops (Batten. loseffsen.I.B. Food Sci.B. K. A. and Lund. (1994)'New Applications of Heat Pump Drying Process' in Drying TechnoL 12. (1995) 'Dehumidification Using Heat Pump Dryers' in Proceedings of the Annual Conference of the Institute ot Professional Engineers of New Zealand..P. Elsevier 15 Priehavudhi. and Prateepchaikul.889-901 27 Strommen. course with respect to ensuring microbiological safety 3.Australia 28 Jonassen. and Carrington.B. A. eds). However. Chipping Campden. University of Queensland. D.O. Australia 29 Jonassen. and Muiumdar. V.P. P. G.S. The HACCP philosophy involves Food Research Association. (1994)'Heat Pump Eluidised Bed Drying of Biologically Active Solutions' in Drying '94 .B. Narsimhan.B. 327-333.G. (1994) 'An ExperimentalTest of the Concept of the Characteristic Drying Curve Using the Thin-layer Method' in Drying '94 Proceedings of the 9th International Drying Symposium (Rudolph. tional rules and guidelines: of particular relevance to the food industry are the General Principles of Food The introduction of the Food Safety Act I in the UK Hygiene guidelines 2. 20.Proceedings of the 9th International Drying Symposium {Rudolph. 347-356.K.. R. (19941 'The Thin-layer Drying Characteristics of Macadamia In-shell Nuts and Kernels' in L Food Eng. 1053-1060. pp.Proceedings of the 9th International Drying Symposium (Rudolph. and Pakowski. (1994)'Non-adiabatic Two-stageCounter-currentFluidised Bed Drier with Heat Pump' in Drying "94 . University of Queensland. (1978) Food Microbiology (3rd edn).ll rightsreser'.). eds).J. 319-322 14 Saravacos. pp. I. pp.G. Technical University.. and Pakowski... pp.. Australia 17 Okos.G.P. (19951 Food Properties Handbook. pp.©1997 ElsevierS(ienceLtd. distribution and sale of food 1970s that H. k6d2 Technical Universib. I.S. 181-190. A. Keey. etal. Poland 24 Rahman.K. Keey. pp.C. (1991) 'Drying Curves A General Processfor Their Representation'in Drying 91 (Mujumdar. 1153-1156 16 Van Blarcom. Keey.C (1992)'Food Dehydration' in Handbook of Food Engineering (Heldman. D.Heat Pump Dryers' in Drying '96 .). and Mason.H. pp. eds). e-mail: hawronskyj@campden. Food Hygiene Department. ATP (adenosine triphosphate) bioluminescence provides a reliable and rapid alternative to traditional microbiological techniques.M..L. R..S. and Mujumdar. pp..O. 1067-1079 19 Keey. 147-187.K. it is important to obtain results rapidly. (1988)'Effect of Water Activity on the Volatile Componentsof Dried Fruits' in Mechanisms of Action offend Preservation Procedures (Gould. eds).. and Westhoff. Brisbane. Tsami.. V. V. K. R.. D.. 1015-1022..Proceedings of the 9th International Drying Symposium (Rudolph. pp.I.. A. K. ary measures for their prevention.S. University of Queensland.B. (1996) 'Effectof Pre-treatmenton Air Drying Rareand Thin LayerDrying Kineticsof FreshCherry' in Drying '96 Proceedings of the 10th International Drying Symposium iStrumillo. Biological Material' in Drying "94 .S.C. eds). C. Furthermore. Australia 21 Palipane. the concept of ensuring in 1990 placed a direct responsibility on all those in.. 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