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Drying rate control in the middle stage of microwave drying
Journal of Food Engineering 104 (2011) 234–238
Contents lists available at ScienceDirect
Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Drying rate control in the middle stage of microwave drying Zhenfeng Li a,b,⇑, G.S.V. Raghavan b, Ning Wang c, Clément Vigneault d a
College of Mechanical and Electronic Engineering, Northwest A&F University, Yangling, Shaanxi 712100, China Department of Bioresource Engineering, McGill University, 21111 Lakeshore Road, Ste-Anne-de-Bellevue, QC, Canada H9X 3V9 c Biosystems and Agricultural Engineering, Division of Agricultural Sciences and Natural Resources, Oklahoma State University, 111 Ag Hall, Stillwater, OK 74078, USA d Horticulture Research and Development Center, Agriculture and Agri-food Canada, 430 Gouin Blvd., Saint-Jean-sur-Richelieu, QC, Canada J3B 3E6 b
a r t i c l e
i n f o
Article history: Received 4 August 2010 Received in revised form 15 December 2010 Accepted 18 December 2010 Available online 23 December 2010 Keywords: Microwave drying Drying curve Drying rate control Apple
a b s t r a c t Microwave drying of apples at constant temperatures follows typical drying curves. The middle stage of the drying process shows a faster drying rate and accelerated moisture evaporation. Meanwhile, more flavors are lost, surface color is degraded, and charring often occurs in this stage. To improve the drying effects, drying curves were controlled and changed in this study. The drying curve was linearized by automatically varying drying temperatures in the middle stages. The controlled drying process therefore led to an optimized temperature profile. To simplify the drying methods, apples were further dried with the obtained temperature profile, while drying curves were online monitored but not controlled. It was proved that slowing down the drying rate in the middle stage could improve the product quality in terms of color, flavor, and overall appearance, while the drying time and energy consumption were still acceptable (within 200 min and 22 kJ/g, respectively). Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Microwave drying of fruits is a time and energy efficient method and can improve product quality in some cases (Garcia et al., 1988; Prabhanjan et al., 1995). This method has also been combined with hot air, vacuum, freeze drying and applied in numerous drying practices (Zhang et al., 2006). In microwave drying, electromagnetic waves interact directly with ionic particles and dipolar molecules, causing excitation and friction among them, generating heat and raising temperature rapidly (Buffler, 1993). When the microwave power is switched off, these activities all stop and the temperature falls immediately. These characteristics distinguish microwave drying itself from all other conventional drying methods, where heat is usually transported from the surface into the core and temperature increases and decreases slowly (Feng et al., 2001). Therefore, a rapid temperature control is possible only in microwave drying if the power can be instantly and properly controlled during drying process. A microwave drying of fruit at a constant temperature usually follows a typical drying curve. In the middle drying stage, moisture is rapidly removed, large amount of aroma is lost, and charring often occurs (Li et al., 2009, 2010b). Also, ‘Puffing’ usually happens in this stage, causing quality damage and undesirable changes in the food texture (Zhang et al., 2006). A number of studies have
been conducted to improve microwave drying (Andres et al., 2004; Cui et al., 2005; Clary et al., 2005; Lu et al., 1999; Zhang et al., 2006), yet no study was reported on attempts to control the drying rate during a drying process, especially in the middle stage. Chua et al. (2001) reported a step-wise air temperature change in the drying of banana pieces, where drying kinetics was changed and better color effect was achieved. However, no continuous and rapid temperature adjustment of the fruit itself was reported in drying process. With the implementation of an instant power control, a new microwave drying system was developed in this study. The developed system can automatically and continuously adjust the power levels, control the product temperature, and measure the samples’ mass online. Hence it becomes possible for the first time to change the typical drying curves according to one’s expectation by adaptively adjusting drying temperatures. The specific objectives were: (1) to investigate the characteristics of drying curve variations with respect to drying temperatures; (2) to change the drying curve in the middle stage to improve drying effects; and (3) to develop a simplified temperature control method where mass measurement can be omitted. 2. Materials and methods 2.1. Microwave drying system
⇑ Corresponding author at: Department of Bioresource Engineering, McGill University, 21111 Lakeshore Road, Ste-Anne-de-Bellevue, QC, Canada H9X 3V9. Tel.: +1 514 3984400x7632; fax: +1 514 398 8387. E-mail addresses: [email protected], [email protected] (Z. Li). 0260-8774/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2010.12.014
A domestic microwave oven with a maximum power output of 600 W (Beaumark 02314, Matsushita Electric Ind. Co. Ltd., Yamatokoriyama, Japan) was used in this study. The microwave
Z. Li et al. / Journal of Food Engineering 104 (2011) 234–238
235
oven had a rotatable antenna installed under the cavity to evenly distribute the microwave power. The control circuits were modified to allow the power to be controlled with a phase controller, which could continuously and automatically adjust the microwave power output within a range of 0–500 W. An extra fan was installed on the back of the cavity for fast moisture removal. The speed of the fan was kept constant throughout the experiment (Fig. 1). The details of the system can be found from a previous work (Li et al., 2009). Samples were placed on a porous Teflon plate in one layer for dehydration. The plate with samples was suspended under an electric balance (P-2002, Denver Instrument, Denver, CO, USA) which located on the top of the oven for mass measurement. A fiber optical sensor (T2TM and T/Guard Link, Neoptix, Inc., Quebec, Canada) was inserted in the center of one of the cubic apple samples for temperature measurement. The temperature and mass of the sample were collected and sent to a PC for control and record purpose through a data acquisition card (PCI 6014, National Instruments, TX, USA). A LabView program (National Instrument, TX, USA) was developed to realize power control, mass reading, temperature monitoring and control (Li et al., 2009).
tionally according to the difference of preset and actual moisture contents. For example, at 0th min, the preset temperature was 50 °C, the preset moisture content was 7.0 (d.b.), while the actual moisture content might be 7.5 (d.b.). The difference between the moisture contents was +0.5 (Actual moisture content–preset moisture content). This difference multiplied by a coefficient of 50 (determined by trial and error) equaled to +25; hence the drying temperature was set to 50 + 25 = 75 °C. Similar control process lasted for 120 min. After the 120 min, the drying temperature was kept at a constant value of 50 °C. The 50 °C was selected because all quality aspects achieved good results at this temperature and the consumed energy and time were all in accepted middle values, as indicated by the drying result of mode one. The maximum temperature was limited to 75 °C and the minimum was limited to 25 °C during drying process to protect the products and to speed up the drying process respectfully, because a temperature over 75 °C would burn the product and a temperature below 25 °C was near the ambient temperature. Power on–off operations was also controlled with a LabView program: when the actual temperature was below the preset temperature, the power was turned on; otherwise it was turned off.
2.2. Samples preparation
2.3.3. Drying mode 3 A temperature profile was obtained in the mode two where linear drying curve can be achieved approximately. In the current mode, a similar but simpler temperature curve was designed to simulate the previously obtained temperature profile and apple was dried following this temperature curve, i.e., temperature was increased linearly from 25 to 50 °C in 120 min, and kept at 50 °C thereafter. The temperature, drying curve and power were also recorded as before.
Apple samples (cv. Granny Smith) were used as test materials. The initial moisture content was around 87% (w.b.). Samples were cut into 10 10 10 mm cubes with a cutting machine and blanched in hot water (80 °C) for 1 min before drying to suppress enzymatic reaction. Cubic samples of 40 g were used in each experiment and were dried to around 10% moisture content (w.b.). All experiments were performed in triplicates. 2.3. Experimental procedures Four different drying modes were attempted for apples drying. The output power of the microwave oven was 300 W with on–off operations to maintain the preset temperatures for all experiments (Li et al., 2009; Raghavan et al., 2010). 2.3.1. Drying mode 1 Apple samples were dried at fixed temperatures of 30, 40, 50, 60, 70, 80 °C. The temperature of the samples and power used were recorded each second. By summing up the power used in every second, the total energy consumptions were obtained (Li et al., 2009, 2010c). 2.3.2. Drying mode 2 The apple samples were dried according to an expected drying curve, where moisture content of the sample was linearly decreased from 7.0 to 1.0 (d.b.) in 120 min. The moisture content change was controlled by adjusting drying temperature propor-
Fig. 1. Schematic diagram of the microwave drying system.
2.3.4. Drying mode 4 To further simplify the drying method, a 3-step temperature mode was finally developed. In this mode, apple was dried first at 30 °C for 60 min, then the temperature was set to 40 °C for another 60 min, and finally the temperature was kept at 50 °C for the rest of the drying stage until the 10% moisture content (w.b.) was reached. Using this method, the continuous temperature control was unnecessary and temperature can be adjusted manually, which was much convenient for an industrial application. Also, online mass measurement and moisture content calculation were all omitted. 2.4. Quality assessment A chromameter (CR-300X, Minolta Camera Co. Ltd., Japan) with a 5 mm diameter measuring area was used for surface color measurements. The L⁄ coordinate ranged from 0 (black) to 100 (white), the a⁄ coordinate indicated red-green color, and the b⁄ coordinate indicated yellow-blue color. Measurements were conducted in triplicates and mean values were reported. Color parameters were subjected to analysis of variance (ANOVA). Differences were estimated by Duncan’s multiple range test and identified as significant or non-significant at a = 0.05 level. Sensory evaluation in terms of overall appearance, taste and textural consistency was conducted by a taste panel of ten untrained judges (five males and five females, age 20–50, students and staffs in food engineering) to acquire the preliminary information of consumers’ preference and acceptance. Five gram samples were given to each panelist after color measurements on the same day that the samples were dehydrated. Assessments were conducted at room temperature (20–22 °C) under fluorescence light. The judges were asked to indicate their preference for each sample based on the overall appearance, taste and textural consistency.
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The results were expressed in the categories of unsatisfactory, poor, neutral, good, and satisfactory.
Table 2 Color parameters of dried apple samples. L⁄
Drying modes
3. Results and discussion
Mode 1
80 °C 70 °C 60 °C 50 °C 40 °C 30 °C Real time Linear 3-step
3.1. Drying mode 1 The recorded temperatures in apples drying at constant temperatures are shown in Fig. 2a and the corresponding drying curves are shown in Fig. 2b. It is clear that drying apples at lower temperatures required longer time and more energy (Table 1). High temperature drying was preferred in terms of time and energy saving. However, quality evaluation showed that middle and low temperature drying resulted in better quality (Tables 2 and 3), and high temperature drying resulted in worse quality, probably because of the charring effects. Although the medium drying temperature resulted in acceptable time, energy, and quality, it could still be improved. In our previous study it was found that during the beginning and middle drying stages (Li et al., 2010a, 2010b), the emanation of the aroma from food samples was fast, as long as the moisture removal rate. The reason might be the fast loss of moisture in these stages created greater pores in the apples, where more and bigger aroma molecules were able to escape from the apples; or the fast moving moisture brought more aroma molecules out of the samples. A large microwave power requirement at these stages also caused product charring. If the drying rate in this stage could be
Mode 2 Mode 3 Mode 4 a–f
a⁄ a,b
b⁄ a,b
65.60 58.73b,c,d 62.02a,b 53.94c,d 69.50a 65.06a,b 61.22b,c 52.83d 61.84a,b
0.59 1.57b,c 0.88a,b,c 2.19c,d 3.42d,e 3.67e 1.1a,b,c 0.18a 3.21d,e
16.29b,c,d,e 17.94b,c,d 15.15d,e 15.42c,d,e 19.08b 22.37a 14.58e 10.98f 18.16b,c
Values in the same column with the same letters are not significantly different.
Table 3 Sensory evaluation of dried apple samples. Drying modes
Visual appearance
Taste
Textural consistency
Mode 1
Neutral Neutral Good Good Satisfactory Satisfactory Good Good Good
Poor Neutral Neutral Good Good Neutral Good Neutral Neutral
Poor Poor Neutral Neutral Good Good Good Neutral Neutral
Mode 2 Mode 3 Mode 3
80 °C 70 °C 60 °C 50 °C 40 °C 30 °C Real time Linear 3-step
slowed down, the pores might be smaller and less aroma would be lost, hence better product quality might be achieved. 3.2. Drying mode 2
Fig. 2. Apple drying curves at constant temperatures. (a) temperatures in apple drying at 30–80 °C (from up to down: 80, 70, 60, 50, 40, 30 °C) (b) moisture contents in apple drying at 30–80 °C (from left to right: 80, 70, 60, 50, 40, 30 °C) (representative curves of three replicates).
From Tables 1–3 it can be observed that drying apple at 40 °C resulted in a good product quality, and the drying time and consumed energy were also acceptable. To mimic and improve this drying curve in the beginning and middle drying stages, a linear line was developed where moisture content is decreased from 7.0 to 1.0 (d.b.) in exactly 120 min, after that the drying temperature was maintained at 50 °C until the end to reduce drying time and energy consumption. By applying the linear line in drying process, apple drying rate was controlled through temperature adjustment (Fig. 3b). It can be noticed that although the actual drying curve didn’t follow the designed drying line exactly, the difference was small (less than ± 0.5). This difference was hard to reduce because the drying temperature was limited to 25–75 °C and this range cannot be expanded for the quality requirements and ambient temperature limits. The adjusted temperature profile is shown in Fig. 3a. It can be observed that the temperature almost increased from 30 to 50 °C in 120 min, and then kept at 50 °C until the end. The high temperature at the very beginning was because the designed initial moisture content was 7.0 (d.b.) and the actual moisture content is different from this value. However, this would not affect the drying effects much because it did decrease fast and did not last long. The temperature fluctuations became larger in the final drying stage when the mass became smaller and power density became greater, as discussed in our previous study (Li et al., 2010d). This was also one of the reasons why the final temperature was chosen at 50 °C,
Table 1 Energy and time consumptions in different drying modes. Drying modes
80 °C
70 °C
60 °C
50 °C
40 °C
30 °C
Real time
Linear
Three step
Energy consumed (kJ/g) Time consumed (min)
17.92 ± 1.2 50 ± 3
17.34 ± 1.1 70 ± 5
16.44 ± 1.0 90 ± 5
17.52 ± 1.1 110 ± 8
17.93 ± 1.2 185 ± 15
21.22 ± 2.3 480 ± 30
16.96 ± 1.0 180 ± 15
19.83 ± 1.8 180 ± 15
21.15 ± 2.3 200 ± 16
Z. Li et al. / Journal of Food Engineering 104 (2011) 234–238
Fig. 3. Apple drying curves with real time moisture content control. (a) temperature record (b) moisture content control (up: designed line, down: recorded actual line) (representative curves of three replicates).
as the highest temperature almost reached 75 °C at certain moments. 3.3. Drying mode 3 Although the previous drying mode did achieve a desired drying curve, the system requirement was complex. It needed the data of the initial moisture content, an accurate mass measurement, an online calculation of the moisture content, and a calculation and control of the real time drying temperature. This might be difficult for some industrial applications. To simplify the system, a linear temperature control method was developed. In this method, drying temperature was increased from 25 to 50 °C in exactly 120 min and then kept at 50 °C until the end of drying. The moisture content was not controlled but recorded for analysis purpose. The temperature control is shown in Fig. 4a and the drying curve is shown in Fig. 4b. This method avoided the acquirement of the initial moisture content of the sample, and avoided the temperature fluctuation at the beginning. Also the calculation of the online moisture content was not necessary. The resulted drying curve is almost a linear line in the first 120 min, very similar to the drying curve in Fig. 3b. Hence, a linear temperature profile resulted in a near linear drying curve, and the system was greatly simplified. In a previous study of carrot drying (Li et al., 2010a), a similar linear temperature line at the beginning and middle stages was achieved, although it was through a fuzzy logic control based on aroma emanation. These results indicate that a gradually increasing temperature at the drying beginning can achieve good control effects and product quality, as analyzed later in this study and the results reported in our previous paper. 3.4. Drying mode 4 As a continuous temperature change still need complex software control and may not be applicable in some industrial prac-
237
Fig. 4. Apple drying curves with linear temperature control. (a) temperature control (b) moisture content record (representative curves of three replicates).
tices, it is desirable to develop a drying strategy where temperature control can be further simplified. To address this problem, a 3-step temperature drying was developed, i.e., drying apples at 30 °C for 60 min, at 40 °C for another 60 min, and then kept at 50 °C until the end of drying. This method is a simulation of the linear temperature controls in drying mode two and three, but the temperature adjustment could be implemented manually (Fig. 5a). Moisture content was also recorded but not controlled (Fig. 5b). It can be observed that the drying curve is still a nearly linear line, although not so smooth as in the previous modes. Two turning points in the drying curve were corresponding to the two temperature changes. In general, the drying curve is similar to that in drying mode two and three with little changes.
3.5. Quality assessment The overall results of color measurements are shown in Table 2. In drying mode 1, 40 °C drying resulted in the highest L⁄ value, indicating a very white product. In other modes, real time and 3step all got the high L⁄ value. The linear control did not achieve a high L⁄ value, but had low (absolute) a⁄ and b⁄ values, indicating a none-burned good color. Sensory evaluation is presented in Table 3. In drying mode one, middle drying temperature resulted in good taste, but low temperature resulted in good visual appearance. Among all, 40 °C drying has the best drying effects. Real time, linear, and 3-step temperature control all resulted in good visual appearance. However, only real time control achieved the best taste and textural consistency. Hence, a linear drying curve is recommended in microwave drying where drying rate can be controlled and less aroma is lost, by which the final product quality can be improved. Similar effects can also be found from our previous study (Li et al., 2010a, 2010b), where real-time control always achieved the best product quality. The reason is that the real-time control can best adapt to the actual product’s characteristic and achieve the
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and energy, and achieved median product quality, they still have the potential to be improved. Drying apples with a drying curve which followed a pre-designed linear line could improve product quality, save time and energy in a single process. A gradually increasing temperature (from 25 to 50 °C) in the beginning and middle drying stages could achieve a nearly linear drying curve, while real time curve control was omitted. A 3-step temperature control (30, 40, and 50 °C) could also improve the drying effects and greatly simplify the control system. Hence, a controlled drying curve and drying temperature are always recommended in microwave drying practices. Further study is recommended to investigate the microstructure change during different drying methods and in different stages during microwave drying. Acknowledgement The authors thank the financial support by NSERC (Natural Science and Engineering Research Council of Canada). References
Fig. 5. Apple drying curves with 3-step temperature control. (a) temperature control (b) moisture content record (representative curves of three replicates).
best control effects, while the other fixed methods are only universal control modes and cannot vary according to individual samples. The novel control methods found in this study have the potential to be used in industry. The principle is just to reduce the drying rate in the middle drying stage. With this treatment, more preferred aroma can be retained, charring can be avoided, drying time can be reduced and energy can be saved. In an industrial microwave drying system where the products are moving, infrared sensor can be used to replace the optic fiber sensor. All other parameters can also be optimized by specifically designed experiments conducted in advance. 4. Conclusions Microwave drying of apples at any constant temperature cannot balance the time, energy, and quality requirements. A high temperature consumed short time and less energy, but resulted in poor product quality. A low temperature resulted in good product quality, but consumed long time and more energy. Although drying apples at a middle drying temperature consumed intermediate time
Andres, A., Bilbao, C., Fito, P., 2004. Drying kinetics of apple cylinders under combined hot air-microwave dehydration. Journal of Food Engineering 63, 71– 78. Buffler, C.R., 1993. Microwave Cooking and Processing. Van Nostrand, Reinhold, New York. Chua, K.J., Mujumdar, A.S., Hawlader, M.N.A., Chou, S.K., Ho, J.C., 2001. Batch drying of banana pieces — effect of stepwise change in drying air temperature on drying kinetics and product color. Food Research International 34, 721–731. Clary, C.D., Wang, S., Petrucci, V.E., 2005. Fixed and incremental levels of microwave power application on drying grapes under vacuum. Journal of Food Science 70 (5), 344–349. Cui, Z.W., Xu, S.Y., Sun, D.W., Chen, W., 2005. Temperature changes during microwave-vacuum drying of sliced carrots. Drying Technology 23, 1057–1074. Feng, H., Tang, J., Cavalirie, R.P., Plumb, O.A., 2001. Heat and mass transport in microwave drying of porous materials in a spouted bed. American Institute of Chemical Engineers Journal 47, 1499–1512. Garcia, R., Leal, F., Rolz, C., 1988. Drying of bananas using microwave and air ovens. International Journal of Food Science and Technology 23, 81–90. Li, Z., Raghavan, G.S.V., Wang, N., Gariepy, Y., 2009. Real-time, volatile-detectionassisted control for microwave drying. Computers and Electronics in Agriculture 69, 177–184. Li, Z., Raghavan, G.S.V., Wang, N., 2010a. Carrot volatiles monitoring and control in microwave drying. LWT-Food Science and Technology 43, 291–297. Li, Z., Raghavan, G.S.V., Wang, N., 2010b. Apple volatiles monitoring and control in microwave drying. LWT-Food Science and Technology 43, 684–689. Li, Z., Raghavan, G.S.V., Orsat, V., 2010c. Temperature and power control in microwave drying. Journal of Food Engineering 97, 478–483. Li, Z., Raghavan, G.S.V., Orsat, V., 2010d. Optimal power control strategies in microwave drying. Journal of Food Engineering 99, 263–268. Lu, L., Tang, J., Ran, X., 1999. Temperature and moisture changes during microwave drying of sliced food. Drying Technology 17, 413–432. Prabhanjan, D.G., Ramaswamy, H.S., Raghavan, G.S.V., 1995. Microwave-assisted convective air drying of thin layer carrots. Journal of Food Engineering 25, 283– 293. Raghavan, G.S.V., Li, Z., Wang, N., Gariepy, Y., 2010. Control of microwave drying process through aroma monitoring. Drying Technology 28, 591–599. Zhang, M., Tang, J., Mujumdar, A.S., Wang, S., 2006. Trends in microwave-related drying of fruits and vegetables. Trends in Food and Technology 17, 524–534.
Contents lists available at ScienceDirect
Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Drying rate control in the middle stage of microwave drying Zhenfeng Li a,b,⇑, G.S.V. Raghavan b, Ning Wang c, Clément Vigneault d a
College of Mechanical and Electronic Engineering, Northwest A&F University, Yangling, Shaanxi 712100, China Department of Bioresource Engineering, McGill University, 21111 Lakeshore Road, Ste-Anne-de-Bellevue, QC, Canada H9X 3V9 c Biosystems and Agricultural Engineering, Division of Agricultural Sciences and Natural Resources, Oklahoma State University, 111 Ag Hall, Stillwater, OK 74078, USA d Horticulture Research and Development Center, Agriculture and Agri-food Canada, 430 Gouin Blvd., Saint-Jean-sur-Richelieu, QC, Canada J3B 3E6 b
a r t i c l e
i n f o
Article history: Received 4 August 2010 Received in revised form 15 December 2010 Accepted 18 December 2010 Available online 23 December 2010 Keywords: Microwave drying Drying curve Drying rate control Apple
a b s t r a c t Microwave drying of apples at constant temperatures follows typical drying curves. The middle stage of the drying process shows a faster drying rate and accelerated moisture evaporation. Meanwhile, more flavors are lost, surface color is degraded, and charring often occurs in this stage. To improve the drying effects, drying curves were controlled and changed in this study. The drying curve was linearized by automatically varying drying temperatures in the middle stages. The controlled drying process therefore led to an optimized temperature profile. To simplify the drying methods, apples were further dried with the obtained temperature profile, while drying curves were online monitored but not controlled. It was proved that slowing down the drying rate in the middle stage could improve the product quality in terms of color, flavor, and overall appearance, while the drying time and energy consumption were still acceptable (within 200 min and 22 kJ/g, respectively). Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Microwave drying of fruits is a time and energy efficient method and can improve product quality in some cases (Garcia et al., 1988; Prabhanjan et al., 1995). This method has also been combined with hot air, vacuum, freeze drying and applied in numerous drying practices (Zhang et al., 2006). In microwave drying, electromagnetic waves interact directly with ionic particles and dipolar molecules, causing excitation and friction among them, generating heat and raising temperature rapidly (Buffler, 1993). When the microwave power is switched off, these activities all stop and the temperature falls immediately. These characteristics distinguish microwave drying itself from all other conventional drying methods, where heat is usually transported from the surface into the core and temperature increases and decreases slowly (Feng et al., 2001). Therefore, a rapid temperature control is possible only in microwave drying if the power can be instantly and properly controlled during drying process. A microwave drying of fruit at a constant temperature usually follows a typical drying curve. In the middle drying stage, moisture is rapidly removed, large amount of aroma is lost, and charring often occurs (Li et al., 2009, 2010b). Also, ‘Puffing’ usually happens in this stage, causing quality damage and undesirable changes in the food texture (Zhang et al., 2006). A number of studies have
been conducted to improve microwave drying (Andres et al., 2004; Cui et al., 2005; Clary et al., 2005; Lu et al., 1999; Zhang et al., 2006), yet no study was reported on attempts to control the drying rate during a drying process, especially in the middle stage. Chua et al. (2001) reported a step-wise air temperature change in the drying of banana pieces, where drying kinetics was changed and better color effect was achieved. However, no continuous and rapid temperature adjustment of the fruit itself was reported in drying process. With the implementation of an instant power control, a new microwave drying system was developed in this study. The developed system can automatically and continuously adjust the power levels, control the product temperature, and measure the samples’ mass online. Hence it becomes possible for the first time to change the typical drying curves according to one’s expectation by adaptively adjusting drying temperatures. The specific objectives were: (1) to investigate the characteristics of drying curve variations with respect to drying temperatures; (2) to change the drying curve in the middle stage to improve drying effects; and (3) to develop a simplified temperature control method where mass measurement can be omitted. 2. Materials and methods 2.1. Microwave drying system
⇑ Corresponding author at: Department of Bioresource Engineering, McGill University, 21111 Lakeshore Road, Ste-Anne-de-Bellevue, QC, Canada H9X 3V9. Tel.: +1 514 3984400x7632; fax: +1 514 398 8387. E-mail addresses: [email protected], [email protected] (Z. Li). 0260-8774/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2010.12.014
A domestic microwave oven with a maximum power output of 600 W (Beaumark 02314, Matsushita Electric Ind. Co. Ltd., Yamatokoriyama, Japan) was used in this study. The microwave
Z. Li et al. / Journal of Food Engineering 104 (2011) 234–238
235
oven had a rotatable antenna installed under the cavity to evenly distribute the microwave power. The control circuits were modified to allow the power to be controlled with a phase controller, which could continuously and automatically adjust the microwave power output within a range of 0–500 W. An extra fan was installed on the back of the cavity for fast moisture removal. The speed of the fan was kept constant throughout the experiment (Fig. 1). The details of the system can be found from a previous work (Li et al., 2009). Samples were placed on a porous Teflon plate in one layer for dehydration. The plate with samples was suspended under an electric balance (P-2002, Denver Instrument, Denver, CO, USA) which located on the top of the oven for mass measurement. A fiber optical sensor (T2TM and T/Guard Link, Neoptix, Inc., Quebec, Canada) was inserted in the center of one of the cubic apple samples for temperature measurement. The temperature and mass of the sample were collected and sent to a PC for control and record purpose through a data acquisition card (PCI 6014, National Instruments, TX, USA). A LabView program (National Instrument, TX, USA) was developed to realize power control, mass reading, temperature monitoring and control (Li et al., 2009).
tionally according to the difference of preset and actual moisture contents. For example, at 0th min, the preset temperature was 50 °C, the preset moisture content was 7.0 (d.b.), while the actual moisture content might be 7.5 (d.b.). The difference between the moisture contents was +0.5 (Actual moisture content–preset moisture content). This difference multiplied by a coefficient of 50 (determined by trial and error) equaled to +25; hence the drying temperature was set to 50 + 25 = 75 °C. Similar control process lasted for 120 min. After the 120 min, the drying temperature was kept at a constant value of 50 °C. The 50 °C was selected because all quality aspects achieved good results at this temperature and the consumed energy and time were all in accepted middle values, as indicated by the drying result of mode one. The maximum temperature was limited to 75 °C and the minimum was limited to 25 °C during drying process to protect the products and to speed up the drying process respectfully, because a temperature over 75 °C would burn the product and a temperature below 25 °C was near the ambient temperature. Power on–off operations was also controlled with a LabView program: when the actual temperature was below the preset temperature, the power was turned on; otherwise it was turned off.
2.2. Samples preparation
2.3.3. Drying mode 3 A temperature profile was obtained in the mode two where linear drying curve can be achieved approximately. In the current mode, a similar but simpler temperature curve was designed to simulate the previously obtained temperature profile and apple was dried following this temperature curve, i.e., temperature was increased linearly from 25 to 50 °C in 120 min, and kept at 50 °C thereafter. The temperature, drying curve and power were also recorded as before.
Apple samples (cv. Granny Smith) were used as test materials. The initial moisture content was around 87% (w.b.). Samples were cut into 10 10 10 mm cubes with a cutting machine and blanched in hot water (80 °C) for 1 min before drying to suppress enzymatic reaction. Cubic samples of 40 g were used in each experiment and were dried to around 10% moisture content (w.b.). All experiments were performed in triplicates. 2.3. Experimental procedures Four different drying modes were attempted for apples drying. The output power of the microwave oven was 300 W with on–off operations to maintain the preset temperatures for all experiments (Li et al., 2009; Raghavan et al., 2010). 2.3.1. Drying mode 1 Apple samples were dried at fixed temperatures of 30, 40, 50, 60, 70, 80 °C. The temperature of the samples and power used were recorded each second. By summing up the power used in every second, the total energy consumptions were obtained (Li et al., 2009, 2010c). 2.3.2. Drying mode 2 The apple samples were dried according to an expected drying curve, where moisture content of the sample was linearly decreased from 7.0 to 1.0 (d.b.) in 120 min. The moisture content change was controlled by adjusting drying temperature propor-
Fig. 1. Schematic diagram of the microwave drying system.
2.3.4. Drying mode 4 To further simplify the drying method, a 3-step temperature mode was finally developed. In this mode, apple was dried first at 30 °C for 60 min, then the temperature was set to 40 °C for another 60 min, and finally the temperature was kept at 50 °C for the rest of the drying stage until the 10% moisture content (w.b.) was reached. Using this method, the continuous temperature control was unnecessary and temperature can be adjusted manually, which was much convenient for an industrial application. Also, online mass measurement and moisture content calculation were all omitted. 2.4. Quality assessment A chromameter (CR-300X, Minolta Camera Co. Ltd., Japan) with a 5 mm diameter measuring area was used for surface color measurements. The L⁄ coordinate ranged from 0 (black) to 100 (white), the a⁄ coordinate indicated red-green color, and the b⁄ coordinate indicated yellow-blue color. Measurements were conducted in triplicates and mean values were reported. Color parameters were subjected to analysis of variance (ANOVA). Differences were estimated by Duncan’s multiple range test and identified as significant or non-significant at a = 0.05 level. Sensory evaluation in terms of overall appearance, taste and textural consistency was conducted by a taste panel of ten untrained judges (five males and five females, age 20–50, students and staffs in food engineering) to acquire the preliminary information of consumers’ preference and acceptance. Five gram samples were given to each panelist after color measurements on the same day that the samples were dehydrated. Assessments were conducted at room temperature (20–22 °C) under fluorescence light. The judges were asked to indicate their preference for each sample based on the overall appearance, taste and textural consistency.
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The results were expressed in the categories of unsatisfactory, poor, neutral, good, and satisfactory.
Table 2 Color parameters of dried apple samples. L⁄
Drying modes
3. Results and discussion
Mode 1
80 °C 70 °C 60 °C 50 °C 40 °C 30 °C Real time Linear 3-step
3.1. Drying mode 1 The recorded temperatures in apples drying at constant temperatures are shown in Fig. 2a and the corresponding drying curves are shown in Fig. 2b. It is clear that drying apples at lower temperatures required longer time and more energy (Table 1). High temperature drying was preferred in terms of time and energy saving. However, quality evaluation showed that middle and low temperature drying resulted in better quality (Tables 2 and 3), and high temperature drying resulted in worse quality, probably because of the charring effects. Although the medium drying temperature resulted in acceptable time, energy, and quality, it could still be improved. In our previous study it was found that during the beginning and middle drying stages (Li et al., 2010a, 2010b), the emanation of the aroma from food samples was fast, as long as the moisture removal rate. The reason might be the fast loss of moisture in these stages created greater pores in the apples, where more and bigger aroma molecules were able to escape from the apples; or the fast moving moisture brought more aroma molecules out of the samples. A large microwave power requirement at these stages also caused product charring. If the drying rate in this stage could be
Mode 2 Mode 3 Mode 4 a–f
a⁄ a,b
b⁄ a,b
65.60 58.73b,c,d 62.02a,b 53.94c,d 69.50a 65.06a,b 61.22b,c 52.83d 61.84a,b
0.59 1.57b,c 0.88a,b,c 2.19c,d 3.42d,e 3.67e 1.1a,b,c 0.18a 3.21d,e
16.29b,c,d,e 17.94b,c,d 15.15d,e 15.42c,d,e 19.08b 22.37a 14.58e 10.98f 18.16b,c
Values in the same column with the same letters are not significantly different.
Table 3 Sensory evaluation of dried apple samples. Drying modes
Visual appearance
Taste
Textural consistency
Mode 1
Neutral Neutral Good Good Satisfactory Satisfactory Good Good Good
Poor Neutral Neutral Good Good Neutral Good Neutral Neutral
Poor Poor Neutral Neutral Good Good Good Neutral Neutral
Mode 2 Mode 3 Mode 3
80 °C 70 °C 60 °C 50 °C 40 °C 30 °C Real time Linear 3-step
slowed down, the pores might be smaller and less aroma would be lost, hence better product quality might be achieved. 3.2. Drying mode 2
Fig. 2. Apple drying curves at constant temperatures. (a) temperatures in apple drying at 30–80 °C (from up to down: 80, 70, 60, 50, 40, 30 °C) (b) moisture contents in apple drying at 30–80 °C (from left to right: 80, 70, 60, 50, 40, 30 °C) (representative curves of three replicates).
From Tables 1–3 it can be observed that drying apple at 40 °C resulted in a good product quality, and the drying time and consumed energy were also acceptable. To mimic and improve this drying curve in the beginning and middle drying stages, a linear line was developed where moisture content is decreased from 7.0 to 1.0 (d.b.) in exactly 120 min, after that the drying temperature was maintained at 50 °C until the end to reduce drying time and energy consumption. By applying the linear line in drying process, apple drying rate was controlled through temperature adjustment (Fig. 3b). It can be noticed that although the actual drying curve didn’t follow the designed drying line exactly, the difference was small (less than ± 0.5). This difference was hard to reduce because the drying temperature was limited to 25–75 °C and this range cannot be expanded for the quality requirements and ambient temperature limits. The adjusted temperature profile is shown in Fig. 3a. It can be observed that the temperature almost increased from 30 to 50 °C in 120 min, and then kept at 50 °C until the end. The high temperature at the very beginning was because the designed initial moisture content was 7.0 (d.b.) and the actual moisture content is different from this value. However, this would not affect the drying effects much because it did decrease fast and did not last long. The temperature fluctuations became larger in the final drying stage when the mass became smaller and power density became greater, as discussed in our previous study (Li et al., 2010d). This was also one of the reasons why the final temperature was chosen at 50 °C,
Table 1 Energy and time consumptions in different drying modes. Drying modes
80 °C
70 °C
60 °C
50 °C
40 °C
30 °C
Real time
Linear
Three step
Energy consumed (kJ/g) Time consumed (min)
17.92 ± 1.2 50 ± 3
17.34 ± 1.1 70 ± 5
16.44 ± 1.0 90 ± 5
17.52 ± 1.1 110 ± 8
17.93 ± 1.2 185 ± 15
21.22 ± 2.3 480 ± 30
16.96 ± 1.0 180 ± 15
19.83 ± 1.8 180 ± 15
21.15 ± 2.3 200 ± 16
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Fig. 3. Apple drying curves with real time moisture content control. (a) temperature record (b) moisture content control (up: designed line, down: recorded actual line) (representative curves of three replicates).
as the highest temperature almost reached 75 °C at certain moments. 3.3. Drying mode 3 Although the previous drying mode did achieve a desired drying curve, the system requirement was complex. It needed the data of the initial moisture content, an accurate mass measurement, an online calculation of the moisture content, and a calculation and control of the real time drying temperature. This might be difficult for some industrial applications. To simplify the system, a linear temperature control method was developed. In this method, drying temperature was increased from 25 to 50 °C in exactly 120 min and then kept at 50 °C until the end of drying. The moisture content was not controlled but recorded for analysis purpose. The temperature control is shown in Fig. 4a and the drying curve is shown in Fig. 4b. This method avoided the acquirement of the initial moisture content of the sample, and avoided the temperature fluctuation at the beginning. Also the calculation of the online moisture content was not necessary. The resulted drying curve is almost a linear line in the first 120 min, very similar to the drying curve in Fig. 3b. Hence, a linear temperature profile resulted in a near linear drying curve, and the system was greatly simplified. In a previous study of carrot drying (Li et al., 2010a), a similar linear temperature line at the beginning and middle stages was achieved, although it was through a fuzzy logic control based on aroma emanation. These results indicate that a gradually increasing temperature at the drying beginning can achieve good control effects and product quality, as analyzed later in this study and the results reported in our previous paper. 3.4. Drying mode 4 As a continuous temperature change still need complex software control and may not be applicable in some industrial prac-
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Fig. 4. Apple drying curves with linear temperature control. (a) temperature control (b) moisture content record (representative curves of three replicates).
tices, it is desirable to develop a drying strategy where temperature control can be further simplified. To address this problem, a 3-step temperature drying was developed, i.e., drying apples at 30 °C for 60 min, at 40 °C for another 60 min, and then kept at 50 °C until the end of drying. This method is a simulation of the linear temperature controls in drying mode two and three, but the temperature adjustment could be implemented manually (Fig. 5a). Moisture content was also recorded but not controlled (Fig. 5b). It can be observed that the drying curve is still a nearly linear line, although not so smooth as in the previous modes. Two turning points in the drying curve were corresponding to the two temperature changes. In general, the drying curve is similar to that in drying mode two and three with little changes.
3.5. Quality assessment The overall results of color measurements are shown in Table 2. In drying mode 1, 40 °C drying resulted in the highest L⁄ value, indicating a very white product. In other modes, real time and 3step all got the high L⁄ value. The linear control did not achieve a high L⁄ value, but had low (absolute) a⁄ and b⁄ values, indicating a none-burned good color. Sensory evaluation is presented in Table 3. In drying mode one, middle drying temperature resulted in good taste, but low temperature resulted in good visual appearance. Among all, 40 °C drying has the best drying effects. Real time, linear, and 3-step temperature control all resulted in good visual appearance. However, only real time control achieved the best taste and textural consistency. Hence, a linear drying curve is recommended in microwave drying where drying rate can be controlled and less aroma is lost, by which the final product quality can be improved. Similar effects can also be found from our previous study (Li et al., 2010a, 2010b), where real-time control always achieved the best product quality. The reason is that the real-time control can best adapt to the actual product’s characteristic and achieve the
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and energy, and achieved median product quality, they still have the potential to be improved. Drying apples with a drying curve which followed a pre-designed linear line could improve product quality, save time and energy in a single process. A gradually increasing temperature (from 25 to 50 °C) in the beginning and middle drying stages could achieve a nearly linear drying curve, while real time curve control was omitted. A 3-step temperature control (30, 40, and 50 °C) could also improve the drying effects and greatly simplify the control system. Hence, a controlled drying curve and drying temperature are always recommended in microwave drying practices. Further study is recommended to investigate the microstructure change during different drying methods and in different stages during microwave drying. Acknowledgement The authors thank the financial support by NSERC (Natural Science and Engineering Research Council of Canada). References
Fig. 5. Apple drying curves with 3-step temperature control. (a) temperature control (b) moisture content record (representative curves of three replicates).
best control effects, while the other fixed methods are only universal control modes and cannot vary according to individual samples. The novel control methods found in this study have the potential to be used in industry. The principle is just to reduce the drying rate in the middle drying stage. With this treatment, more preferred aroma can be retained, charring can be avoided, drying time can be reduced and energy can be saved. In an industrial microwave drying system where the products are moving, infrared sensor can be used to replace the optic fiber sensor. All other parameters can also be optimized by specifically designed experiments conducted in advance. 4. Conclusions Microwave drying of apples at any constant temperature cannot balance the time, energy, and quality requirements. A high temperature consumed short time and less energy, but resulted in poor product quality. A low temperature resulted in good product quality, but consumed long time and more energy. Although drying apples at a middle drying temperature consumed intermediate time
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