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Infrared drying of apple slices
Innovative Food Science and Emerging Technologies 5 (2004) 353 – 360 www.elsevier.com/locate/ifset
Infrared drying of apple slices Dorota Nowak, Piotr P. Lewicki * Department of Food Engineering and Process Management, Warsaw Agricultural University (SGGW), Nowoursynowska 159c, 02-766 Warsaw, Poland Received 7 October 2003; accepted 12 March 2004
Abstract Laboratory dryer was designed in such a way that drying could be done either with infrared energy or by convection. It was equipped with near-infrared radiators with peak wavelength at 1200 nm. The energy efficiency of the infrared dryer was between 35% and 45%. Apple slices were dried with infrared energy and by convection under equivalent conditions. Kinetics of infrared drying was dependent on the distance between emitters and the heat-irradiated surface and air velocity. Drying kinetics was inversely proportional to both the distance and the air velocity. It was found that both surfaces of apple slice participate in water evaporation. However, the heat-irradiated surface evaporates much more water than that not heated by infrared energy until 80% of water is removed from the material. At the final stages of drying, there is no difference between upper and bottom surfaces of the apple slice as far as the flux of evaporated water is concerned. Comparison of infrared drying with convective drying done at equivalent parameters showed that time of the process can be shortened by up to 50% when heating is done with infrared energy. D 2004 Elsevier Ltd. All rights reserved. Keywords: Infrared drying; Evaporation; Apple slices Industrial relevance: Advantages of infrared radiation over convective heating include high heat transfer coefficients, short process times and low energy costs. Since heat and mass transfer during drying of food with infrared energy is not well described in the literature it is of importance to collect such data and compare them with those from convective drying. The data clearly showed that in addition to short drying times the material temperature can easily be controlled and thus monitoring of food quality and functionality during drying can be achieved.
1. Introduction Drying is one of the oldest methods of food preservation. The most common is drying, in which heat is transferred from the hot air to the product by convection, and evaporated water is transported to the air also by convection. In convective drying, resistances to the heat and mass transfer are in the boundary layer and their magnitude is dependent on air velocity, or more generally on the Reynold’s number. On the other hand, resistances to heat and mass transfer in the material undergoing drying are large and strongly affect kinetics of the water evaporation. Convective drying usually is long and causes many undesirable changes in the material (Lewicki, 1998). One of the ways to shorten the drying time is to supply heat by infrared radiation. This method of heating is especially suitable to dry thin layers of material with large * Corresponding author. E-mail address: [email protected] (P.P. Lewicki). 1466-8564/$ - see front matter D 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ifset.2004.03.003
surface exposed to radiation. Application of infrared heating to food drying is recently of special interest because of the progress in radiator construction. Their efficiency is between 80% and 90%, the emitted radiation is in narrow wavelength range and they are miniaturized (Sandu, 1986). Food products subjected to drying usually contain large amounts of water. Hence, absorption of infrared energy by water is an important variable, which affects drying kinetics. Generally, solid materials absorb infrared radiation in a thin surface layer. However, moist porous materials are penetrated by radiation to some depth and their transmissivity depends on the moisture content (Lampinen, Ojala, & Koski, 1991). During drying, radiation properties of the material are changing due to decreasing water content. As a consequence, its reflectivity increases and the absorptivity decreases. Infrared radiation is transmitted through water at short wavelength, while at long wavelength, it is absorbed on the surface (Sakai & Hanzawa, 1994). Hence, drying of thin layers seems to be more efficient at far-infrared
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radiation—FIR (25 – 100 Am), while drying of thicker bodies should give better results at near-infrared radiation—NIR (0.75 – 3.00 Am). Infrared radiation has some advantages over convective heating. Heat transfer coefficients are high, the process time is short and the cost of energy is low. Since air is transparent to infrared radiation, the process can be done at ambient air temperature. Equipment can be compact and automated with high degree of control of process parameters. This is especially important because of fast heating and possibilities of overheating of the material (Sakai & Hanzawa, 1994). In spite of these advantages, application of infrared energy in food processing is rather scarce. It is used for heating and cooking soybeans, cereal grains, cocoa beans and nuts, ‘‘ready-to-eat’’ products, braising meat and frying (Ratti & Mujumdar, 1995). Drying of seaweed, vegetables, fish flakes and pasta is also done in tunnel infrared dryers. Infrared drying found also application in food analysis to measure water content in food products (Anon., 1995; Hagen & Drawert, 1986). Heat and mass transfer during drying of food material with infrared energy is not well described in literature. The heat transfer differs substantially with respect to convective heating. The radiation energy is absorbed by the surface layers and converted to heat. In wet bodies, the highest temperature occurs under the irradiated surface layer and depends on the extinction coefficient. The smaller the extinction coefficient, the larger the distance from the surface at which maximum temperature occurs (Ginzburg, 1969). Hence, heat generated in a layer under the surface is conducted towards the center of the body as well as to its surface. Heat from the surface to the surrounding air is transferred by convection. On the other hand, water flux is transported all the time from the center of the material to its surface. As a consequence, in the part of the material, fluxes of heat and mass are countercurrent and in layers close to the surface are cocurrent. At the surface, both fluxes are cocurrent and the concentration and temperature profiles in
the air should be different than those occurring during convective drying. A model of drying by infrared energy was developed by Hasatani, Itaya, and Miura (1988). The model assumes that the energy is absorbed on the surface and divides drying into three parts. The first part accounts for heating up the material and constant drying rate period. It is assumed that internal mass transfer resistance is negligible and water is evaporated from the surface. Water vapor pressure on the surface is equal to the saturated vapor pressure at the surface temperature. The second part occurs at the beginning of the falling rate –drying period. Dry patches occur on the surface and drying rate begins to decrease. Further drying leads to a dry surface layer and the zone of water evaporation retreats towards the center of the body. Water is transported as vapor through the dry layer and this period of drying is treated as the third part. In the energy balance, heat absorbed by the material is taken into account, but the external heat transfer resistance is omitted. Energy and mass balance developed by Ratti and Mujumdar (1995) accounts for the shrinkage of the heated particle and absorption of infrared energy. Theoretical calculations showed that intermittent infrared drying with energy input 10 kW/m2 becomes equivalent to convective drying in which heat transfer coefficient would be as high as 200 W/(m2 K). The aim of this work is to investigate heat and mass transfer during infrared drying of apple slices and compare it with convective drying.
2. Materials and methods Apples v. Idared were washed and sliced into 5.5 F 0.1 mm thick slices. The central part of the slice was removed with cork borer. Slices of uniform diameter were soaked in 0.1% citric acid to avoid enzymatic browning, blotted with filter paper and spread on a wire tray in a single layer. The load was 2 kg/m2 on an average.
Fig. 1. Laboratory infrared-convective dryer. 1—glass lamps, 2—wire tray, 3—distance adjustment screws, 4—drying chamber, 5—fan, 6—fan output control, 7—electric heater, 8—electric heater control, 9—air baffles, 10—anemometer, 11—quartz glass, 12—balance, 13—computers and 14—thermocouples.
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Drying was done in a prototype laboratory infrared dryer presented on Fig. 1. The dryer was designed in such a way that convective or infrared drying could be done individually or in combination. The dryer was equipped with nine red glass lamps (Philips) with power 175 W, each emitting radiation with a peak wavelength 1200 nm. Radiators were arranged in three rows, with three lamps in each row. Lamps were mounted on a construction that could be moved up and down and the distance between radiators and apple slices could be adjusted between 100 and 400 mm. Emitted radiation reached a wire tray 500 400 mm placed on a balance, hence changes in mass could be recorded continuously. Infrared radiation chamber was connected to a duct 120 430 mm in cross-section in which electric heater with a power 5.8 kW and air baffles were installed. A part of the duct in which a wire tray with apple slices was placed was separated from the infrared chamber with a quartz glass. This arrangement assured uniform flow of air over apple slices. Moreover, either infrared lamps or hot air could supply heat to material undergoing drying. Dryer was equipped with measuring and recording devices, which made it possible to control air parameters and to design drying process in different arrangements. Temperature of the material undergoing drying was measured with thermocouples type K (NiCr– NiAl) with diameter 0.5 mm. The tip of the thermocouples were inserted into apple slices from that side which was not exposed to infrared radiation. Temperature of three apple slices was measured in each experiment. To measure power efficiency of the dryer instead of apple slices, Petri dishes filled with water were placed on a wire tray. Evaporation of water was done at the same parameters as drying of apple slices. Petri dishes with a diameter of 87 mm were filled with 50 g of water each and arranged on a wire tray in the same way as apple slices. Shrinkage of apple slices was measured at prescribed drying times. Each slice was numbered and its diameter and thickness were measured with calipers. Convection drying was done in the same dryer at hot air temperature, 65 and 75 jC, and at an air velocity of 1.5 m/s.
3. Results and discussion 3.1. Water evaporation The flux of evaporated water from Petri dishes was dependent on the distance of infrared emitters from the surface of water and the ambient air velocity. Further was the emitter from the surface of water, the lower was the flux of water (Fig. 2). The time to reach constant evaporation rate was little influenced by air velocity and pronouncedly affected by the distance of emitters from the surface of water. At the distance of 10 cm, the time to reach constant evaporation rate was close to 20 min, and at 30 cm, the time was between 30
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Fig. 2. Influence of the distance between infrared emitters and heated surface on the flux of evaporated water at the air velocity of 0.5 m/s. Parameter is the distance.
and 35 min. On the other hand, at an air velocity of 0.5 m/s, the difference between the largest and smallest constant rates of evaporation is close to 0.25 g/(m2 s) and at 1.5 m/s, it is about 0.17 g/(m2 s). The air velocity affects constant rate of evaporation at the given distance between emitters and the surface of water (Fig. 3). The influence is inversely proportional, hence the higher is the air velocity, the slower is the evaporation of water. The results obtained in this experiment are contrary to those which could be expected. Classic theory of heat and mass transfer predicts the higher the air velocity, the higher is the flux of evaporated water. The relationship is not linear since Reynolds number is taken to the power 0.5 for the laminar flow or 0.8 for the turbulent flow (Baehr & Stephan, 1998). Results of this experiment show that water absorbs infrared radiation and loses heat to the air stream. The higher is the air velocity, the larger is the heat transfer coefficient from the surface of water to the air stream. Hence, air is heated by hot water and its temperature in the outlet of the dryer should be higher than that in the inlet. Increase of air temperature during the flow through the drying chamber is substantial, little dependent on the distance between emitters and surface of water and strongly affected by air velocity. At an air velocity of 0.5 m/s, the increase in air temperature is between 13 and 17 jC, at 1.0 m/s, it is from 8.5 to 13 jC and at 1.5 m/s, it ranges from 8 to 11 jC. It can be easily calculated that at air velocities of 0.5, 1 and 1.5 m/s, energy used to heat the air amounts from 340 to 450 W, from 450 to 680 Wand from 630 to 870 W, respectively. The higher is the air velocity, the more is the energy used to increase its temperature, hence less energy is used to evaporate water. The evaporation temperature pronounces the cooling effect of air flowing over the surface of water. The effect of air velocity is substantial, while the influence of the distance between emitters and surface of water is evident, but of lesser importance (Fig. 4). Increase of air velocity from 0.5 to 1.5 m/s causes drop of evaporation temperature by 11– 13 jC, while increase of the distance from 10 to 30 cm lowers the evaporation temperature by 4.5– 6.5 jC.
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Fig. 3. Relationship between air velocity and the flux of evaporated water. Parameter is the distance between infrared radiators and surface of water.
On the basis of the amount of evaporated water and temperature of evaporation, heat efficiency of the infrared dryer was calculated. It varied from 35.2% to 45.7% depending on the air velocity and the distance between emitters and surface of water (Fig. 5). The efficiency can be considered as high since only a part of electric energy is converted to infrared radiation. According to Ginzburg (1969), the conversion amounts to 65 –70% at wavelengths shorter than 2.6 Am. Hence, during water evaporation, up to 70% of infrared energy was used efficiently. Zbicinski, Jakobsen, and Driscoll (1992) investigating water evaporation from bed of porous mineral with the use of infrared heating obtained heat efficiency not exceeding 50%. In a laboratory dryer heated by short wavelength infrared radiation, heat efficiency was 49% (Evin, 1992). Nowak and Lewicki (1998) using infrared dryer described in this paper, but equipped with 250 W emitters obtained heat efficiency of about 30% at the distance between lamps and evaporating surface equal to 30 cm. Concluding this part of the experiment, it can be stated that the distance of emitters from the heated surface and velocity of air flowing over that surface can easily control infrared
Fig. 4. Relationship between air velocity and air temperature at the outlet of the dryer. Parameter is the distance between infrared emitters and surface of water.
Fig. 5. Influence of air velocity and distance between emitters and surface of water on heat efficiency of infrared dryer.
heating. The shorter is the distance between emitter and the surface, the more energy is absorbed by the heated material. However, passing air over the surface can effectively lower its temperature. Hot surface transfers heat to air stream, its temperature decreases, and less energy is available for water evaporation. This effect is opposite to that taking place during convective heating of material undergoing drying. Heat efficiency of infrared heating of evaporating water is high. Temperature of evaporation, one of the most important parameters in food dehydration, can be controlled by the choice of appropriate distance between emitters and surface of the material and air velocity in the dryer chamber. Due to high thermal efficiency and ease of control, infrared heating seems to be a good choice for food dehydration. 3.2. Drying of apple slices Kinetics of drying of apple slices with infrared energy was dependent on both the distance between emitters and surface of slices, and air velocity. The influence of air velocity is presented in Fig. 6 and is in very good agreement
Fig. 6. Influence of air velocity on the course of drying of apple slices heated by infrared radiation.
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with results obtained during water evaporation. The higher was the air velocity, the longer was the drying time. Increase of air velocity from 0.5 to 1.5 m/s extends the drying time by some 45– 55%, while increase of the distance between emitters and surface of apple slices extends the drying time by 20– 30%. These results show that the influence of air velocity on the kinetics of drying of apple slices by infrared energy is more pronounced than that of distance between emitters and the surface of dried material. Combined effect of air velocity and distance between emitters and surface of material undergoing drying on the time needed to reach prescribed water content is presented in Fig. 7 and is described by the following equation: s99 ¼ 27:2 þ 0:00712:2 þ 102:9v0:5 where s99 is the time needed to evaporate 99% of water (min), l, the distance between emitters and evaporating surface (cm) and v the air velocity (m/s). Apple slices heated by infrared energy dry in most cases in the falling rate period. Only at the lowest air velocity (0.5 m/s), independently of the distance between emitters and surface of apple slices, a constant drying rate period was observed. It was short and some 20– 25% of water was evaporated during that period. The influence of distance between emitters and evaporating surface on the constant drying rate period, at air velocity 0.5 m/s, is evident. The ratio between the drying rates is 1.25:1.10:1.00 for distances 10, 20 and 30 cm, respectively. The course of the falling rate – drying period is strongly affected by air velocity and the effect is equivalent to that arising from the distance between emitters and surface of apple slices (Fig. 8). At the distance of 10 cm, the rates for 0.5 and 1.5 m/s differ by as much as 20% when 50% of water is evaporated from the apple slices. When the distance is increased to 30 cm, the difference is equal to 28%. On the other hand, increase of the distance between emitters and surface of apple slices from 10 to 30 cm at air velocity 0.5 m/s causes decrease of drying rate by about 26% when 50%
Fig. 7. Influence of air velocity and distance between emitters and surface of apple slices on time to evaporate 99% of water.
Fig. 8. Influence of air velocity on relative drying rate of apple slices.
of water is evaporated. At an air velocity of 1.5 m/s, the increased distance lowers drying rate by 34%. Apple slices undergoing drying do not shrink isometrically. The diameter changes much less than the thickness. The diameter did not change significantly until 20% water was evaporated. Then the diameter decreased linearly in relation to water content. Shrinkage of the surface was from 15% to 18% when 90% of water was evaporated. On the other hand, the thickness of apple slices decreased almost fourfold during infrared drying. The effect of drying variables, that is air velocity and the distance between emitters and surface of slices, on diameter shrinkage was statistically significant, while for the changes of thickness was insignificant. Decrease of the thickness of the apple slices (h) during infrared drying was described by the following equation: hs ¼ 0:146 þ 0:404
us ; u0
r2 ¼ 0:9796
where u is water content (g/g d.m.) and subscripts 0 and s denote initial and prescribed time. Shrinkage of the surface during drying affected the amount of evaporated water. Hence, the drying rate discussed above is not equivalent to the water flux removed from apple slices during infrared drying. However, to calculate the flux of evaporated water, the share of upper and bottom surfaces must be known. The upper surface is absorbing infrared energy, while the bottom surface exchanges heat with surrounding air by convection. In order to estimate the share of upper and bottom surfaces in the drying process, the lower surface was smeared with edible oil and covered with aluminum foil. Apple slices prepared this way were dried under conditions identical with those in which both surface were available to water evaporation. Drying of apple slices in which only upper surface was available to water evaporation was much longer than that in which both surfaces evaporated water (Fig. 9). To remove 90% of water, time of drying was extended by 56% and 70% when 98% of water was evaporated from apple slices.
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Fig. 9. Drying curves of apple slices heated by infrared energy.
Fig. 11. Influence of the distance between emitters and surface of apple slices on the course of rate of drying curves.
Calculated flux of water evaporated from apple slices (Fig. 10) shows that the share of upper and bottom surfaces in drying is related to water content in the material. The share of the upper surface increases from the very beginning of the drying and reaches maximum when some 10% of water is evaporated. At this moment, the flux of water evaporated from the upper surface was three times larger than that evaporated from the bottom surface. Then the difference between fluxes gradually decreased and when 80% of water was evaporated, both surfaces participated in drying equally. From the above experiment, it is evident that both upper and bottom surfaces participate in evaporation of water during infrared drying of apple slices. Hence, it would be inappropriate to calculate water flux based only on the surface absorbing infrared energy. In this work, water flux was calculated on the basis of both surfaces, treating it as a technical measure of dryer efficiency. Water flux reaches maximum when about 10% of water is evaporated from the apple slices (Fig. 11). The maximum flux depends upon the distance between emitters and the surface of slices and practically is not affected by the air velocity. At the distance of 10 cm, the flux is between 0.43
and 0.47 g/(m2 s), while increase of the distance to 30 cm decreases the flux to 0.38– 0.39 g/(m2 s). Increase of the distance decreases the water flux by 13– 20% while increase of air velocity decreases flux of evaporated water by 2– 9%. Decreasing water content in the material causes the effect of air velocity to be more evident. At 50% of evaporated water, the increase in distance between emitters and surface of slices decreases water flux by 20– 26% and larger the decrease, lower is the air velocity. On the other hand, increase of air velocity from 0.5 to 1.5 m/s decreases water flux by 29– 34%. When 90% of water is evaporated from apple slices, the influence of the distance on the water flux becomes small, while the effect of air velocity approaches 50%. Temperature of apple slices was affected by the distance between emitters and the absorbing surface, air velocity and the location of the slice on the wire tray. The infrared energy field was not even and there were places that received more energy and places on the wire tray, which received less energy than the average. The unevenness of the energy field was the more evident when larger was the distance between
Fig. 10. Relationship between relative water content and flux of water evaporated from apple slices.
Fig. 12. Relationship between relative water content and temperature of apple slices during infrared drying.
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Fig. 13. Influence of air velocity and distance between emitters and irradiated surface on temperature of infrared dried apple slices.
emitters and irradiated surface. At the distance of 10 cm, the temperature of apple slices differed by F 1.1– 2.4 jC from the average, at 20 cm, the difference was equal to F 2.0 to F 2.5 jC and at 30 cm, standard deviation was from F 5.6 to F 6.2 jC. Temperature of apple slices, at prescribed parameters of drying, was dependent on water content of the material (Fig. 12). At the beginning of the drying process, temperature increased fast and then leveled off. Heating up of the material takes 11 – 13 min, and less than 10% of water is evaporated during this time. Further increase of temperature is slow and does not exceed 65 jC, regardless of the applied drying parameters until 80% of water is evaporated from the material. Removal of the final 20% of water is accompanied by fast increase of temperature. Temperature of the material at the end of drying is dependent on the distance between emitters and the surface of slices and air velocity. It is between 53.2 F 6.2 jC and 95.7 F 2.4 jC. The relationship between final material temperature and air velocity, and the distance between emitters and surface of apple slices is presented in Fig. 13. It is evident that the influence of the distance on the final slice temperature is equivalent to that caused by the increase of air velocity. Increase of the
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Fig. 15. Comparison of infrared and convective drying of apple slices.
distance from 10 to 30 cm decreases the final material temperature by 28 –35%, while increase of air velocity from 0.5 to 1.5 m/s lowers the temperature by 33– 45%. Combination of the distance between infrared radiators and surface of apple slices and air velocity made it possible to dry the material at prescribed temperature. Applying a distance of 30 cm and an air velocity of 1 m/s, the final temperature of apple slices was 63.0 F 5.7 jC. The distance 30 cm and air velocity 0.5 m/s, or 20 cm and 1 m/s, or 10 cm and 1.5 m/s yielded final material temperatures of 74.7 F 5.6 jC, 72.7 F 0.4 jC and 72.0 F 1.7 jC, respectively. All these infrared drying processes could be compared with convective drying done either at 65 or 75 jC. Convective drying was done at an air velocity of 1.5 m/s and was much longer than that with infrared heating (Fig. 14). At 65 jC, time of convective drying to 10% moisture content was 250 min and was by 50% longer than that done with infrared energy. At 75 jC, the drying time was 180 min and was longer from 13% to 40% from that observed for infrared drying. The effect of infrared energy on drying of apple slices is clearly seen when amount of water evaporated during that process is compared with mass of water evaporated during convective drying (Fig. 15). Practically from the very beginning, the mass of evaporated water during infrared drying is larger than that measured during convective drying. The longer is the drying, the larger is the observed difference.
4. Conclusions
Fig. 14. Influence of infrared and convection heating on the course of drying curves of apple slices.
Infrared drying of apple slices is an effective method of water removal. Drying with application of infrared energy is much faster than convective drying done under equivalent parameters. Drying kinetics depends on the distance between infrared energy emitters and the heat-irradiated surface, and the air velocity as well. The air velocity is an important variable, since air flowing over the surface cools it down and lowers its temperature. Hence, the effect of air
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velocity on kinetics of infrared drying of apple slices is opposite to that observed during convective drying. Adjusting distance between infrared emitters and the slice surface as well as air velocity, a temperature of the material undergoing drying can be easily controlled. Both short time of drying and ease of control of material temperature are advantages of the use of infrared energy in food dehydration.
References Anon. (1995). Determination of moisture content in Finnish honey using an infrared dryer. Food Marketing and Technology, 9(1), 40 – 41. Baehr, H. S., & Stephan, K. (1998). Heat and mass transfer. Berlin: Springer Verlag. Evin, F. (1992). Optimizing efficiency of short wave infrared dryer. In A. S. Mujumdar (Ed.), Proceedings of the international drying symposium IDS ’92 (pp. 924 – 930). New York: Elsevier, part A. Ginzburg, A. S. (1969). Infrared radiation in food industry. Warszawa: Wydawnictwo Naukowo-Techniczne, in Polish. Hagen, W., & Drawert, F. (1986). Determination of water content by infrared. Monatsschrift fu¨r Brauwissenschaft, 40(6), 240 – 246. Hasatani, M., Itaya, Y., & Miura, K. (1988). Hybrid drying of granular
materials by combined radiative and convective heating. Drying Technology, 6(1), 43 – 68. Lampinen, M. J., Ojala, K. T., & Koski, E. (1991). Modeling and measurements of dryers for coated paper. Drying Technology, 9(4), 973 – 1017. Lewicki, P. P. (1998). Effect of pre-drying, drying and rehydration on plant tissue properties. A review. International Journal of Food Properties, 1, 1 – 22. Nowak, D., & Lewicki, P. P. (1998). Characteristics of infrared-convective dryer. Proceedings of the XVIth National Conference of Chemical and Process Engineering ( pp. 194 – 199). Krakow: Zaklad Graficzny Politechniki Krakowskiej, in Polish. Ratti, C., & Mujumdar, A. S. (1995). Infrared drying. In A. S. Mujumdar (Ed.), Handbook of industrial drying, vol. 1 (pp. 567 – 588). New York: Marcel Dekker. Sakai, N., & Hanzawa, T. (1994). Application and advances in farinfrared heating in Japan. Trends in Food Science and Technology, 5(11), 357 – 362. Sandu, C. (1986). Infrared radiative drying in food engineering: A process analysis. Biotechnology Progress, 2(3), 109 – 119. Zbicinski, I., Jakobsen, A., & Driscoll, J. L. (1992). Application of infrared radiation for drying of particular materials. In A. S. Mujumdar (Ed.), Proceedings of the international drying symposium IDS ’92 (pp. 704 – 711). New York: Elsevier, part A.
Infrared drying of apple slices Dorota Nowak, Piotr P. Lewicki * Department of Food Engineering and Process Management, Warsaw Agricultural University (SGGW), Nowoursynowska 159c, 02-766 Warsaw, Poland Received 7 October 2003; accepted 12 March 2004
Abstract Laboratory dryer was designed in such a way that drying could be done either with infrared energy or by convection. It was equipped with near-infrared radiators with peak wavelength at 1200 nm. The energy efficiency of the infrared dryer was between 35% and 45%. Apple slices were dried with infrared energy and by convection under equivalent conditions. Kinetics of infrared drying was dependent on the distance between emitters and the heat-irradiated surface and air velocity. Drying kinetics was inversely proportional to both the distance and the air velocity. It was found that both surfaces of apple slice participate in water evaporation. However, the heat-irradiated surface evaporates much more water than that not heated by infrared energy until 80% of water is removed from the material. At the final stages of drying, there is no difference between upper and bottom surfaces of the apple slice as far as the flux of evaporated water is concerned. Comparison of infrared drying with convective drying done at equivalent parameters showed that time of the process can be shortened by up to 50% when heating is done with infrared energy. D 2004 Elsevier Ltd. All rights reserved. Keywords: Infrared drying; Evaporation; Apple slices Industrial relevance: Advantages of infrared radiation over convective heating include high heat transfer coefficients, short process times and low energy costs. Since heat and mass transfer during drying of food with infrared energy is not well described in the literature it is of importance to collect such data and compare them with those from convective drying. The data clearly showed that in addition to short drying times the material temperature can easily be controlled and thus monitoring of food quality and functionality during drying can be achieved.
1. Introduction Drying is one of the oldest methods of food preservation. The most common is drying, in which heat is transferred from the hot air to the product by convection, and evaporated water is transported to the air also by convection. In convective drying, resistances to the heat and mass transfer are in the boundary layer and their magnitude is dependent on air velocity, or more generally on the Reynold’s number. On the other hand, resistances to heat and mass transfer in the material undergoing drying are large and strongly affect kinetics of the water evaporation. Convective drying usually is long and causes many undesirable changes in the material (Lewicki, 1998). One of the ways to shorten the drying time is to supply heat by infrared radiation. This method of heating is especially suitable to dry thin layers of material with large * Corresponding author. E-mail address: [email protected] (P.P. Lewicki). 1466-8564/$ - see front matter D 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ifset.2004.03.003
surface exposed to radiation. Application of infrared heating to food drying is recently of special interest because of the progress in radiator construction. Their efficiency is between 80% and 90%, the emitted radiation is in narrow wavelength range and they are miniaturized (Sandu, 1986). Food products subjected to drying usually contain large amounts of water. Hence, absorption of infrared energy by water is an important variable, which affects drying kinetics. Generally, solid materials absorb infrared radiation in a thin surface layer. However, moist porous materials are penetrated by radiation to some depth and their transmissivity depends on the moisture content (Lampinen, Ojala, & Koski, 1991). During drying, radiation properties of the material are changing due to decreasing water content. As a consequence, its reflectivity increases and the absorptivity decreases. Infrared radiation is transmitted through water at short wavelength, while at long wavelength, it is absorbed on the surface (Sakai & Hanzawa, 1994). Hence, drying of thin layers seems to be more efficient at far-infrared
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radiation—FIR (25 – 100 Am), while drying of thicker bodies should give better results at near-infrared radiation—NIR (0.75 – 3.00 Am). Infrared radiation has some advantages over convective heating. Heat transfer coefficients are high, the process time is short and the cost of energy is low. Since air is transparent to infrared radiation, the process can be done at ambient air temperature. Equipment can be compact and automated with high degree of control of process parameters. This is especially important because of fast heating and possibilities of overheating of the material (Sakai & Hanzawa, 1994). In spite of these advantages, application of infrared energy in food processing is rather scarce. It is used for heating and cooking soybeans, cereal grains, cocoa beans and nuts, ‘‘ready-to-eat’’ products, braising meat and frying (Ratti & Mujumdar, 1995). Drying of seaweed, vegetables, fish flakes and pasta is also done in tunnel infrared dryers. Infrared drying found also application in food analysis to measure water content in food products (Anon., 1995; Hagen & Drawert, 1986). Heat and mass transfer during drying of food material with infrared energy is not well described in literature. The heat transfer differs substantially with respect to convective heating. The radiation energy is absorbed by the surface layers and converted to heat. In wet bodies, the highest temperature occurs under the irradiated surface layer and depends on the extinction coefficient. The smaller the extinction coefficient, the larger the distance from the surface at which maximum temperature occurs (Ginzburg, 1969). Hence, heat generated in a layer under the surface is conducted towards the center of the body as well as to its surface. Heat from the surface to the surrounding air is transferred by convection. On the other hand, water flux is transported all the time from the center of the material to its surface. As a consequence, in the part of the material, fluxes of heat and mass are countercurrent and in layers close to the surface are cocurrent. At the surface, both fluxes are cocurrent and the concentration and temperature profiles in
the air should be different than those occurring during convective drying. A model of drying by infrared energy was developed by Hasatani, Itaya, and Miura (1988). The model assumes that the energy is absorbed on the surface and divides drying into three parts. The first part accounts for heating up the material and constant drying rate period. It is assumed that internal mass transfer resistance is negligible and water is evaporated from the surface. Water vapor pressure on the surface is equal to the saturated vapor pressure at the surface temperature. The second part occurs at the beginning of the falling rate –drying period. Dry patches occur on the surface and drying rate begins to decrease. Further drying leads to a dry surface layer and the zone of water evaporation retreats towards the center of the body. Water is transported as vapor through the dry layer and this period of drying is treated as the third part. In the energy balance, heat absorbed by the material is taken into account, but the external heat transfer resistance is omitted. Energy and mass balance developed by Ratti and Mujumdar (1995) accounts for the shrinkage of the heated particle and absorption of infrared energy. Theoretical calculations showed that intermittent infrared drying with energy input 10 kW/m2 becomes equivalent to convective drying in which heat transfer coefficient would be as high as 200 W/(m2 K). The aim of this work is to investigate heat and mass transfer during infrared drying of apple slices and compare it with convective drying.
2. Materials and methods Apples v. Idared were washed and sliced into 5.5 F 0.1 mm thick slices. The central part of the slice was removed with cork borer. Slices of uniform diameter were soaked in 0.1% citric acid to avoid enzymatic browning, blotted with filter paper and spread on a wire tray in a single layer. The load was 2 kg/m2 on an average.
Fig. 1. Laboratory infrared-convective dryer. 1—glass lamps, 2—wire tray, 3—distance adjustment screws, 4—drying chamber, 5—fan, 6—fan output control, 7—electric heater, 8—electric heater control, 9—air baffles, 10—anemometer, 11—quartz glass, 12—balance, 13—computers and 14—thermocouples.
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Drying was done in a prototype laboratory infrared dryer presented on Fig. 1. The dryer was designed in such a way that convective or infrared drying could be done individually or in combination. The dryer was equipped with nine red glass lamps (Philips) with power 175 W, each emitting radiation with a peak wavelength 1200 nm. Radiators were arranged in three rows, with three lamps in each row. Lamps were mounted on a construction that could be moved up and down and the distance between radiators and apple slices could be adjusted between 100 and 400 mm. Emitted radiation reached a wire tray 500 400 mm placed on a balance, hence changes in mass could be recorded continuously. Infrared radiation chamber was connected to a duct 120 430 mm in cross-section in which electric heater with a power 5.8 kW and air baffles were installed. A part of the duct in which a wire tray with apple slices was placed was separated from the infrared chamber with a quartz glass. This arrangement assured uniform flow of air over apple slices. Moreover, either infrared lamps or hot air could supply heat to material undergoing drying. Dryer was equipped with measuring and recording devices, which made it possible to control air parameters and to design drying process in different arrangements. Temperature of the material undergoing drying was measured with thermocouples type K (NiCr– NiAl) with diameter 0.5 mm. The tip of the thermocouples were inserted into apple slices from that side which was not exposed to infrared radiation. Temperature of three apple slices was measured in each experiment. To measure power efficiency of the dryer instead of apple slices, Petri dishes filled with water were placed on a wire tray. Evaporation of water was done at the same parameters as drying of apple slices. Petri dishes with a diameter of 87 mm were filled with 50 g of water each and arranged on a wire tray in the same way as apple slices. Shrinkage of apple slices was measured at prescribed drying times. Each slice was numbered and its diameter and thickness were measured with calipers. Convection drying was done in the same dryer at hot air temperature, 65 and 75 jC, and at an air velocity of 1.5 m/s.
3. Results and discussion 3.1. Water evaporation The flux of evaporated water from Petri dishes was dependent on the distance of infrared emitters from the surface of water and the ambient air velocity. Further was the emitter from the surface of water, the lower was the flux of water (Fig. 2). The time to reach constant evaporation rate was little influenced by air velocity and pronouncedly affected by the distance of emitters from the surface of water. At the distance of 10 cm, the time to reach constant evaporation rate was close to 20 min, and at 30 cm, the time was between 30
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Fig. 2. Influence of the distance between infrared emitters and heated surface on the flux of evaporated water at the air velocity of 0.5 m/s. Parameter is the distance.
and 35 min. On the other hand, at an air velocity of 0.5 m/s, the difference between the largest and smallest constant rates of evaporation is close to 0.25 g/(m2 s) and at 1.5 m/s, it is about 0.17 g/(m2 s). The air velocity affects constant rate of evaporation at the given distance between emitters and the surface of water (Fig. 3). The influence is inversely proportional, hence the higher is the air velocity, the slower is the evaporation of water. The results obtained in this experiment are contrary to those which could be expected. Classic theory of heat and mass transfer predicts the higher the air velocity, the higher is the flux of evaporated water. The relationship is not linear since Reynolds number is taken to the power 0.5 for the laminar flow or 0.8 for the turbulent flow (Baehr & Stephan, 1998). Results of this experiment show that water absorbs infrared radiation and loses heat to the air stream. The higher is the air velocity, the larger is the heat transfer coefficient from the surface of water to the air stream. Hence, air is heated by hot water and its temperature in the outlet of the dryer should be higher than that in the inlet. Increase of air temperature during the flow through the drying chamber is substantial, little dependent on the distance between emitters and surface of water and strongly affected by air velocity. At an air velocity of 0.5 m/s, the increase in air temperature is between 13 and 17 jC, at 1.0 m/s, it is from 8.5 to 13 jC and at 1.5 m/s, it ranges from 8 to 11 jC. It can be easily calculated that at air velocities of 0.5, 1 and 1.5 m/s, energy used to heat the air amounts from 340 to 450 W, from 450 to 680 Wand from 630 to 870 W, respectively. The higher is the air velocity, the more is the energy used to increase its temperature, hence less energy is used to evaporate water. The evaporation temperature pronounces the cooling effect of air flowing over the surface of water. The effect of air velocity is substantial, while the influence of the distance between emitters and surface of water is evident, but of lesser importance (Fig. 4). Increase of air velocity from 0.5 to 1.5 m/s causes drop of evaporation temperature by 11– 13 jC, while increase of the distance from 10 to 30 cm lowers the evaporation temperature by 4.5– 6.5 jC.
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Fig. 3. Relationship between air velocity and the flux of evaporated water. Parameter is the distance between infrared radiators and surface of water.
On the basis of the amount of evaporated water and temperature of evaporation, heat efficiency of the infrared dryer was calculated. It varied from 35.2% to 45.7% depending on the air velocity and the distance between emitters and surface of water (Fig. 5). The efficiency can be considered as high since only a part of electric energy is converted to infrared radiation. According to Ginzburg (1969), the conversion amounts to 65 –70% at wavelengths shorter than 2.6 Am. Hence, during water evaporation, up to 70% of infrared energy was used efficiently. Zbicinski, Jakobsen, and Driscoll (1992) investigating water evaporation from bed of porous mineral with the use of infrared heating obtained heat efficiency not exceeding 50%. In a laboratory dryer heated by short wavelength infrared radiation, heat efficiency was 49% (Evin, 1992). Nowak and Lewicki (1998) using infrared dryer described in this paper, but equipped with 250 W emitters obtained heat efficiency of about 30% at the distance between lamps and evaporating surface equal to 30 cm. Concluding this part of the experiment, it can be stated that the distance of emitters from the heated surface and velocity of air flowing over that surface can easily control infrared
Fig. 4. Relationship between air velocity and air temperature at the outlet of the dryer. Parameter is the distance between infrared emitters and surface of water.
Fig. 5. Influence of air velocity and distance between emitters and surface of water on heat efficiency of infrared dryer.
heating. The shorter is the distance between emitter and the surface, the more energy is absorbed by the heated material. However, passing air over the surface can effectively lower its temperature. Hot surface transfers heat to air stream, its temperature decreases, and less energy is available for water evaporation. This effect is opposite to that taking place during convective heating of material undergoing drying. Heat efficiency of infrared heating of evaporating water is high. Temperature of evaporation, one of the most important parameters in food dehydration, can be controlled by the choice of appropriate distance between emitters and surface of the material and air velocity in the dryer chamber. Due to high thermal efficiency and ease of control, infrared heating seems to be a good choice for food dehydration. 3.2. Drying of apple slices Kinetics of drying of apple slices with infrared energy was dependent on both the distance between emitters and surface of slices, and air velocity. The influence of air velocity is presented in Fig. 6 and is in very good agreement
Fig. 6. Influence of air velocity on the course of drying of apple slices heated by infrared radiation.
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with results obtained during water evaporation. The higher was the air velocity, the longer was the drying time. Increase of air velocity from 0.5 to 1.5 m/s extends the drying time by some 45– 55%, while increase of the distance between emitters and surface of apple slices extends the drying time by 20– 30%. These results show that the influence of air velocity on the kinetics of drying of apple slices by infrared energy is more pronounced than that of distance between emitters and the surface of dried material. Combined effect of air velocity and distance between emitters and surface of material undergoing drying on the time needed to reach prescribed water content is presented in Fig. 7 and is described by the following equation: s99 ¼ 27:2 þ 0:00712:2 þ 102:9v0:5 where s99 is the time needed to evaporate 99% of water (min), l, the distance between emitters and evaporating surface (cm) and v the air velocity (m/s). Apple slices heated by infrared energy dry in most cases in the falling rate period. Only at the lowest air velocity (0.5 m/s), independently of the distance between emitters and surface of apple slices, a constant drying rate period was observed. It was short and some 20– 25% of water was evaporated during that period. The influence of distance between emitters and evaporating surface on the constant drying rate period, at air velocity 0.5 m/s, is evident. The ratio between the drying rates is 1.25:1.10:1.00 for distances 10, 20 and 30 cm, respectively. The course of the falling rate – drying period is strongly affected by air velocity and the effect is equivalent to that arising from the distance between emitters and surface of apple slices (Fig. 8). At the distance of 10 cm, the rates for 0.5 and 1.5 m/s differ by as much as 20% when 50% of water is evaporated from the apple slices. When the distance is increased to 30 cm, the difference is equal to 28%. On the other hand, increase of the distance between emitters and surface of apple slices from 10 to 30 cm at air velocity 0.5 m/s causes decrease of drying rate by about 26% when 50%
Fig. 7. Influence of air velocity and distance between emitters and surface of apple slices on time to evaporate 99% of water.
Fig. 8. Influence of air velocity on relative drying rate of apple slices.
of water is evaporated. At an air velocity of 1.5 m/s, the increased distance lowers drying rate by 34%. Apple slices undergoing drying do not shrink isometrically. The diameter changes much less than the thickness. The diameter did not change significantly until 20% water was evaporated. Then the diameter decreased linearly in relation to water content. Shrinkage of the surface was from 15% to 18% when 90% of water was evaporated. On the other hand, the thickness of apple slices decreased almost fourfold during infrared drying. The effect of drying variables, that is air velocity and the distance between emitters and surface of slices, on diameter shrinkage was statistically significant, while for the changes of thickness was insignificant. Decrease of the thickness of the apple slices (h) during infrared drying was described by the following equation: hs ¼ 0:146 þ 0:404
us ; u0
r2 ¼ 0:9796
where u is water content (g/g d.m.) and subscripts 0 and s denote initial and prescribed time. Shrinkage of the surface during drying affected the amount of evaporated water. Hence, the drying rate discussed above is not equivalent to the water flux removed from apple slices during infrared drying. However, to calculate the flux of evaporated water, the share of upper and bottom surfaces must be known. The upper surface is absorbing infrared energy, while the bottom surface exchanges heat with surrounding air by convection. In order to estimate the share of upper and bottom surfaces in the drying process, the lower surface was smeared with edible oil and covered with aluminum foil. Apple slices prepared this way were dried under conditions identical with those in which both surface were available to water evaporation. Drying of apple slices in which only upper surface was available to water evaporation was much longer than that in which both surfaces evaporated water (Fig. 9). To remove 90% of water, time of drying was extended by 56% and 70% when 98% of water was evaporated from apple slices.
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Fig. 9. Drying curves of apple slices heated by infrared energy.
Fig. 11. Influence of the distance between emitters and surface of apple slices on the course of rate of drying curves.
Calculated flux of water evaporated from apple slices (Fig. 10) shows that the share of upper and bottom surfaces in drying is related to water content in the material. The share of the upper surface increases from the very beginning of the drying and reaches maximum when some 10% of water is evaporated. At this moment, the flux of water evaporated from the upper surface was three times larger than that evaporated from the bottom surface. Then the difference between fluxes gradually decreased and when 80% of water was evaporated, both surfaces participated in drying equally. From the above experiment, it is evident that both upper and bottom surfaces participate in evaporation of water during infrared drying of apple slices. Hence, it would be inappropriate to calculate water flux based only on the surface absorbing infrared energy. In this work, water flux was calculated on the basis of both surfaces, treating it as a technical measure of dryer efficiency. Water flux reaches maximum when about 10% of water is evaporated from the apple slices (Fig. 11). The maximum flux depends upon the distance between emitters and the surface of slices and practically is not affected by the air velocity. At the distance of 10 cm, the flux is between 0.43
and 0.47 g/(m2 s), while increase of the distance to 30 cm decreases the flux to 0.38– 0.39 g/(m2 s). Increase of the distance decreases the water flux by 13– 20% while increase of air velocity decreases flux of evaporated water by 2– 9%. Decreasing water content in the material causes the effect of air velocity to be more evident. At 50% of evaporated water, the increase in distance between emitters and surface of slices decreases water flux by 20– 26% and larger the decrease, lower is the air velocity. On the other hand, increase of air velocity from 0.5 to 1.5 m/s decreases water flux by 29– 34%. When 90% of water is evaporated from apple slices, the influence of the distance on the water flux becomes small, while the effect of air velocity approaches 50%. Temperature of apple slices was affected by the distance between emitters and the absorbing surface, air velocity and the location of the slice on the wire tray. The infrared energy field was not even and there were places that received more energy and places on the wire tray, which received less energy than the average. The unevenness of the energy field was the more evident when larger was the distance between
Fig. 10. Relationship between relative water content and flux of water evaporated from apple slices.
Fig. 12. Relationship between relative water content and temperature of apple slices during infrared drying.
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Fig. 13. Influence of air velocity and distance between emitters and irradiated surface on temperature of infrared dried apple slices.
emitters and irradiated surface. At the distance of 10 cm, the temperature of apple slices differed by F 1.1– 2.4 jC from the average, at 20 cm, the difference was equal to F 2.0 to F 2.5 jC and at 30 cm, standard deviation was from F 5.6 to F 6.2 jC. Temperature of apple slices, at prescribed parameters of drying, was dependent on water content of the material (Fig. 12). At the beginning of the drying process, temperature increased fast and then leveled off. Heating up of the material takes 11 – 13 min, and less than 10% of water is evaporated during this time. Further increase of temperature is slow and does not exceed 65 jC, regardless of the applied drying parameters until 80% of water is evaporated from the material. Removal of the final 20% of water is accompanied by fast increase of temperature. Temperature of the material at the end of drying is dependent on the distance between emitters and the surface of slices and air velocity. It is between 53.2 F 6.2 jC and 95.7 F 2.4 jC. The relationship between final material temperature and air velocity, and the distance between emitters and surface of apple slices is presented in Fig. 13. It is evident that the influence of the distance on the final slice temperature is equivalent to that caused by the increase of air velocity. Increase of the
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Fig. 15. Comparison of infrared and convective drying of apple slices.
distance from 10 to 30 cm decreases the final material temperature by 28 –35%, while increase of air velocity from 0.5 to 1.5 m/s lowers the temperature by 33– 45%. Combination of the distance between infrared radiators and surface of apple slices and air velocity made it possible to dry the material at prescribed temperature. Applying a distance of 30 cm and an air velocity of 1 m/s, the final temperature of apple slices was 63.0 F 5.7 jC. The distance 30 cm and air velocity 0.5 m/s, or 20 cm and 1 m/s, or 10 cm and 1.5 m/s yielded final material temperatures of 74.7 F 5.6 jC, 72.7 F 0.4 jC and 72.0 F 1.7 jC, respectively. All these infrared drying processes could be compared with convective drying done either at 65 or 75 jC. Convective drying was done at an air velocity of 1.5 m/s and was much longer than that with infrared heating (Fig. 14). At 65 jC, time of convective drying to 10% moisture content was 250 min and was by 50% longer than that done with infrared energy. At 75 jC, the drying time was 180 min and was longer from 13% to 40% from that observed for infrared drying. The effect of infrared energy on drying of apple slices is clearly seen when amount of water evaporated during that process is compared with mass of water evaporated during convective drying (Fig. 15). Practically from the very beginning, the mass of evaporated water during infrared drying is larger than that measured during convective drying. The longer is the drying, the larger is the observed difference.
4. Conclusions
Fig. 14. Influence of infrared and convection heating on the course of drying curves of apple slices.
Infrared drying of apple slices is an effective method of water removal. Drying with application of infrared energy is much faster than convective drying done under equivalent parameters. Drying kinetics depends on the distance between infrared energy emitters and the heat-irradiated surface, and the air velocity as well. The air velocity is an important variable, since air flowing over the surface cools it down and lowers its temperature. Hence, the effect of air
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velocity on kinetics of infrared drying of apple slices is opposite to that observed during convective drying. Adjusting distance between infrared emitters and the slice surface as well as air velocity, a temperature of the material undergoing drying can be easily controlled. Both short time of drying and ease of control of material temperature are advantages of the use of infrared energy in food dehydration.
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