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Drying and quality characteristics of tilapia fish fillets dried with hot air-microwave heating
food and bioproducts processing 8 9 ( 2 0 1 1 ) 472–476
Contents lists available at ScienceDirect
Food and Bioproducts Processing journal homepage: www.elsevier.com/locate/fbp
Drying and quality characteristics of tilapia fish fillets dried with hot air-microwave heating Zhen-hua Duan a,b,∗ , Li-na Jiang b , Ju-lan Wang a , Xiao-yang Yu b , Tao Wang b a b
Key Laboratory of Tropical Biological Resources of MOE, Hainan University, Haikou 570228, China College of Food Science and Technology, Hainan University, No. 58, Renmin Road, Haikou 570228, China
a b s t r a c t The aim of this work was to study the effect of hot air-microwave heating on the drying and quality characteristics of fresh tilapia fish fillets. Experimental drying curves were obtained at three microwave powers (200, 400 and 600 W) after hot air drying at two air temperatures (40 and 50 ◦ C) and a constant air velocity of 1.5 m/s. Some quality indicators such as shrinkage, rehydration and recovery properties were investigated. Results showed that an increase in microwave power resulted in a decrease in final moisture content when drying for the same period of time. The higher the hot air temperature, the higher the dehydration rate was. The shrinkage ratio and rehydration ratio increased as the microwave power and air temperature increased. However, the recovery ratio decreased as the microwave power and air temperature increased. Lower hot air temperature and microwave power are beneficial to keep the quality of tilapia fillets. © 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Hot air-microwave drying; Tilapia; Shrinkage; Rehydration; Recovery
1.
Introduction
Tilapia is the common name now applied to three genera and species of fish in the family Cichlidae: Oreochromis, Sarotherodon, and Tilapia. Native to Africa and the Middle East, these species have been distributed throughout the world and have become the second most important food fishes in the world (Watanabe et al., 2002). Tilapias possess an impressive range of attributes that make them ideal for aquaculture. They grow rapidly, reproduce easily, adapt to a wide range of environmental conditions and accept artificial feeds readily. They have good-tasting flesh with a mild flavor, are widely accepted as food fish, are used in many cuisines, and their consumption is not restricted by religious observances. Tilapia culture has expanded rapidly during the last decade as a result of technological advances associated with the intensification of culture practices. Global production was influenced by rapid expansion of tilapia culture in China, the Philippines, Thailand, Indonesia, and Egypt. China is now the leading producer having increased production from 18,000 tonnes in 1984 to 1,210,000 tonnes in 2007, more than 45% of
the total world production (Li et al., 2009). However, the quantity of the tilapia used for processing is too small to meet the requirement of increased production, and it is highly perishable product as it consists of up to 80% of water (Duan et al., 2005a; Li et al., 2009). Therefore, to solve the problems of high enzymatic and bacterial activity in fresh fish, the use of processing and preservation technology is necessary. Drying of fish is important (Bellagha et al., 2002; Bala and Mondol, 2001), because it preserves fish by inactivating enzymes and removing the moisture necessary for bacterial and mold growth (Duan et al., 2004). Drying processes can be broadly classified as thermal drying, osmotic dehydration, and mechanical dewatering. Many researchers and applications focus on air drying (Aktas et al., 2007; Desmorieux and Decaen, 2005; Duan et al., 2004; Gogus and Maskan, 2006). But air drying has drawbacks of both long drying time required and poor quality (Chou and Chua, 2001). Microwave energy is widely used in agricultural products processing because it heats quickly, is energy efficient, safe and harmless, and easy to control (Duan et al., 2003; Zhang and Datta, 2005). Microwave drying technology has become one of emerging
∗ Corresponding author at: College of Food Science and Technology, Hainan University, No. 58, Renmin Road, Haikou, China. Tel.: +86 898 66289662; fax: +86 898 66279215. E-mail addresses: [email protected], [email protected] (Z.-h. Duan). Received 15 November 2009; Received in revised form 9 October 2010; Accepted 9 November 2010 0960-3085/$ – see front matter © 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.fbp.2010.11.005
food and bioproducts processing 8 9 ( 2 0 1 1 ) 472–476
areas in food drying (McLoughlin et al., 2003; Orsat et al., 2007), especially hybrid drying technologies such as air-microwave drying (Datta and Ni, 2002; Duan et al., 2005b). Microwave heating of temperature sensitive materials offers a more rapid method of moisture removal (McLoughlin et al., 2003). Microwaves heat material simultaneously inside and outside, and can produce high rates of evaporation. If heating is too fast, fresh tilapia fillets, which have high water content, may be burst into pieces. However, few reports were focused on investigating the air-microwave drying of tilapia fillets. The aim of this work was to study the effect of hot air-microwave drying process on the drying and quality characteristics of tilapia fillets. Evaluation of quality characteristics was studied by determining the changes of the shrinkage, rehydration and recovery ratio. The results obtained may provide the important theory basis for developing the application technology of hot air-microwave drying in tilapia processing.
2.
Materials and methods
2.1.
Sample preparation
2.4.
2.2.
Shrinkage ratio
The volume of the test sample was measured using an excluding method, and the clean sea sand was selected as filling material (Shang et al., 2007). The mean particle diameter of sand is about 0.6 mm. Five replicates were performed for each sample and mean value was calculated. The shrinkage ratio was calculated as: Rs =
V0 − Vt V0
(1)
where Rs is the shrinkage ratio of the sample, V0 is the initial volume of the sample before drying, Vt is the volume of the sample during drying at time t.
Rrec =
(3)
2.5.
Moisture content
The moisture content of test sample was determined according to the vacuum oven method (AOAC, 2005). At regular time intervals during the drying period, samples were taken out and dried in a vacuum oven (Model ZKF030, Shanghai Experiment Instrument Co. Ltd., China) until constant weight.
Drying procedure
The following procedure was chosen according the previous results (Duan et al., 2005b, 2006). The fresh fish fillets were pre-dried using a digital constant temperature blast drying oven (Model 101-2, Changzhou Huapuda Instrument Co. Ltd., China) for 4 h at 40 ◦ C, and 50 ◦ C with a constant air velocity of 1.5 m/s. The pre-dehydrated fish fillets were immediately put into a laboratory microwave drying oven (Model NJL07-3, Nanjing Microwave Equipment Co. Ltd., China) to continue to dry for different times (2, 4, 6, 8, 10 min) at different microwave powers (200, 400, 600 W). For every batch of dried sample, the moisture content, shrinkage ratio, rehydration ratio and recovery ratio were determined. At least three replicates were performed for each batch of test samples and the results were averaged.
2.7.
Statistical analysis
Analysis of variance was performed by the ANOVA procedures (SPSS 10.0 for Windows). Differences among the mean values of the various treatments were determined by the least significant difference test, and the significance was defined at p < 0.05.
Results and discussion
Rehydration ratio
Dry test samples were weighed with an electronic balance (Model BS124S, Sartorius Company, Germany). Dried samples were then put into the 40 ◦ C of clean warm water to rehydrate for 30 min (Shang et al., 2007), and they are moved from the water for weighing. The temperature of the water was maintained to 40 ± 0.5 ◦ C by temperature controller (Model 501, Shanghai Experiment Instrument Factory, China). Three replicates were performed for each sample and mean value was calculated. The rehydration ratio was calculated as: Rreh =
m mo
where Rrec is the recovery ratio of the sample, and mo is the weight of the fresh sample before drying.
3. 2.3.
Recovery ratio
The recovery ratio was calculated using Eq. (3) (Duan et al., 2006). Three replicates were performed for each sample and mean value was calculated.
2.6.
Fresh tilapias (Oreochromis niloticus) were purchased from a fish market in Haikou, China. They were quickly transported to the laboratory in sealed polystyrene boxes containing ice. Fish were cleaned, gutted, skinned and headed, then cut into fillets with the size of 30 mm × 20 mm × 3 mm. The fish fillets were rinsed by tap water, and then placed in single layer on a stainless steel wire mesh for drying experiments.
473
m − mt mt
(2)
where Rreh is the rehydration ratio of the sample, m is the weight of the sample after rehydration, mt is the weight of the sample before rehydration.
3.1. Effect of pre-drying on the drying curves of fish fillets during microwave drying The drying curves of tilapia fillets are shown in Figs. 1 and 2. The moisture contents decreased with the increasing in microwave power in the same conditions (air temperature, air velocity and drying time) of pre-drying using hot air. The decreased moisture content could be attributed to increased evaporation of water both on the surface of and in the fish fillets due to increasing temperatures by microwave heating (Duan et al., 2005b). However, the drying curves were different significantly under the different conditions of microwave powers. Effect of the microwave power on the moisture content of pre-dehydrated fish fillets was significant (p < 0.05). Comparing the curves between Figs. 1 and 2, the drying rates in Fig. 2 can be seen to be greater than those of the curves in Fig. 1. This is because the initial moisture contents are dif-
food and bioproducts processing 8 9 ( 2 0 1 1 ) 472–476
1.6
90
1.4
80
1.2
shrinkage ratio (%)
moisture content (d.b)
474
1 0.8 0.6 200W 0.4
60 200W 50
400W 600W
40
400W
0.2
70
600W
30 0
0 0
2
4
6
8
2
4
time (min) Fig. 1 – The microwave drying curves of tilapia fillets after pre-drying at 40 ◦ C.
6
8
10
time (min)
10
Fig. 3 – Effect of microwave drying on the shrinkage ratio of tilapia fillets after pre-drying at 40 ◦ C.
90
3.2. Effect of pre-drying on the shrinkage ratio of fish fillets during microwave drying Figs. 3 and 4 showed effects of temperature of pre-drying with hot air on the shrinkage ratio of samples during microwave drying period. First, as can be seen, shrinkage increased with
0.6 200W
0.5
moisture content (d.b)
400W 600W
0.4 0.3 0.2 0.1 0 0
2
4
6
8
10
time (min) Fig. 2 – The microwave drying curves of tilapia fillets after pre-drying at 50 ◦ C.
80
shrinkage ratio (%)
ferent for the two different air temperatures. The temperature of pre-drying using hot air had evident impact on the moisture content of fish fillets during the microwave drying period (p < 0.05). It seemed to be concluded that the moisture content at the change-over between air drying and microwave drying should be about 0.5 g H2 O/g. The higher hot air temperatures led to the higher drying rates of samples. Similar results have been reported for drying of vegetables materials (Mwithiga and Olwal, 2005). Therefore, both hot air temperature and microwave power are considered to be beneficial to improve the drying rate of fish fillets. The required drying time was less than 6 min for obtaining the final products of 12% of moisture content (wet basis) when fish fillets dried by microwave drying at 400 W after pre-dried at 50 ◦ C for 4 h (Fig. 2). However, it at least required 19 h to dry the fresh fish fillet to final products of the same moisture content with 50 ◦ C of hot air (Duan et al., 2006). Therefore, the hot air-microwave drying may shorten greatly the required drying time compared with the hot air drying of tilapia fillets.
70 60 200W 50
400W 600W
40 30 0
2
4
6
8
10
time (min) Fig. 4 – Effect of microwave drying on the shrinkage ratio of tilapia fillets after pre-drying at 50 ◦ C.
the drying time. Shrinkage is highest during the first 2 min and then seems to be relatively constant. The shrinkage ratio of samples dried at 200 W, 400 W and 600 W of microwave power for 2 min after pre-drying at 40 ◦ C, were 54%, 62% and 65%, respectively (Fig. 3). When microwave drying time was constant, the shrinkage ratio increased slightly with the microwave power increasing, but the influence of microwave power on the shrinkage ratio of pre-dried fillets was not evident (p > 0.05). Second, the higher hot air temperature led to the higher shrinkage ratio of fish fillets (p < 0.05). The shrinkage ratios of the fish fillets pre-dried with hot air between 40 ◦ C and 50 ◦ C were 42% and 58%, respectively. In addition, the influence of pre-drying at 40 ◦ C was more than that at 50 ◦ C on the shrinkage ratio of samples during microwave drying period by comparing the differences of Figs. 3 and 4. This is because moisture content of samples pre-dried for 4 h at 40 ◦ C was higher than that at 50 ◦ C. Much more moisture was removed rapidly during microwave drying due to the instantaneous heating of microwaves (Duan et al., 2005b), which resulted in greater changes of shrinkage ratio under the same drying condition. With the microwave drying continued, the moisture content of samples decreased, the difference of moisture content between two temperatures was reduced, so the changes in shrinkage ratio decreased accordingly. Therefore, temperature of pre-drying before microwave drying was the main factor affecting the shrinkage ratio of dried fillets.
475
food and bioproducts processing 8 9 ( 2 0 1 1 ) 472–476
120
70 200W
rehydration ratio (%)
100
60
400W 600W
50
recovery ratio (%)
80 60 40 20
40 30 200W 20
400W
10
0 0
2
4
6
8
10
600W
0
time (min)
0
2
4
6
8
10
time (min) Fig. 5 – Effect of microwave drying on the rehydration ratio of tilapia fillets after pre-drying at 40 ◦ C.
Fig. 7 – Effect of microwave drying on the recovery ratio of tilapia fillets after pre-drying at 40 ◦ C.
3.3. Effect of pre-drying on the rehydration ratio of tilapia fillets during microwave drying
60
100
rehydration ratio(%)
80
60 200W
40
400W 600W
20
0 0
2
4
6
8
10
time (min) Fig. 6 – Effect of microwave drying on the rehydration ratio of tilapia fillets after pre-drying at 50 ◦ C.
recovery ratio (%)
50
Normally rehydration curves are plotted for a dry sample (a final product) immersed in water for different lengths of time (Giri and Prasad, 2007). However, the final moisture content of a dry sample depends on the specific requirement of product, such as semi-dried products. Therefore, the rehydration ratios of samples of varying moisture content were compared, here. Influences of pre-drying with different air temperature (40 ◦ C, 50 ◦ C) on the rehydration ratio of samples during microwave drying were showed in Figs. 5 and 6. As can be seen from Fig. 5, the rehydration ratio increased with the extension of microwave drying when the microwave power was constant. The similar result can also be derived from Fig. 6. On the other hand, the rehydration ratio increased with the increase in microwave power when microwave drying time was unchanged. As can be seen from Fig. 5, the rehydration ratio of samples dried at 200 W, 400 W and 600 W of microwave power for 4 min after pre-drying at 40 ◦ C, were 40%, 53% and 73%, respectively. However, the increase in the rehydration ratios was significantly different between values shown in Figs. 5 and 6 (p < 0.05). Different temperatures of pre-drying brought different changes of the rehydration ratio of fish fillets during microwave drying period. When the fresh tilapia fillets were dried through microwave drying after pre-dried at 50 ◦ C, the increase values of the rehydration ratio at 50 ◦ C were much less than those at 40 ◦ C, resulted from microwave power increased.
40 30 200W
20
400W 10
600W
0 0
2
4
6
8
10
time (min) Fig. 8 – Effect of microwave drying on the recovery ratio of tilapia fillets after pre-drying at 50 ◦ C. As can be seen from Fig. 6, the rehydration ratio of samples dried at 200 W, 400 W and 600 W of microwave power for 4 min after pre-drying at 50 ◦ C, were 72%, 75% and 82%, respectively. So, the effect of the microwave power on the rehydration ratio was closely related to the temperature of pre-drying. The lower the temperature of pre-drying, the more rapid the increase in rehydration ratio.
3.4. Effect of pre-drying on the recovery ratio of tilapia fillets during microwave drying In general, recovery curves are plotted for a dry sample (a final product) immersed in water for different lengths of time. However, the final moisture content of a dry sample depends on certain specific requirement of products. Therefore, the recovery ratios of samples of varying moisture content were compared. Figs. 7 and 8 showed the effect of different predrying temperature (40 ◦ C, 50 ◦ C) on the recovery ratio of samples. The recovery ratio decreased slightly with the extension of microwave drying time when the microwave power was constant, and the change trends were very flat, which showed that the microwave drying time was not the main factor affecting the recovery ratio. On the other hand, the recovery ratio reduced with the increase in microwave power when microwave drying time was fixed. As can be seen from Fig. 7, the recovery ratio of samples dried at 200 W, 400 W and 600 W for 8 min after pre-drying at 40 ◦ C were 52%, 45% and 42%, respectively. The recovery ratio-drying time curve at
476
food and bioproducts processing 8 9 ( 2 0 1 1 ) 472–476
200 W was higher than the other two curves; the latter two were relatively closer. Therefore, decreasing the microwave power resulted in increase in the recovery ratio. Comparing Figs. 7 and 8, all the recovery ratios of fish fillets pre-dried at 40 ◦ C were higher than those at 50 ◦ C when the microwave drying time and power was fixed (p < 0.05). So, the temperature of pre-drying was important factors affecting the recovery ratio.
4.
Conclusion
Hot air drying followed by microwave drying can decrease remarkably drying time for drying fresh tilapia fillets compared with hot air drying. The moisture content at the change-over between air drying and microwave drying should be about 0.5 g H2 O/g. Hot air-microwave drying technology can be used for dehydration of fresh tilapia fillets due to decrease drying time and improve quality (rehydration ratio). Effects of microwave power and hot air temperature on drying characteristic of fish fillets during the hot air-microwave drying were significant. An increase in microwave power resulted in a decrease in moisture content of fillets; the difference is obvious in starting point for the microwave drying regarding moisture content due to the two different air temperatures. The influence of the hot air temperature on the shrinkage ratio and the rehydration ratio was significant. The shrinkage ratio and the rehydration ratio increased with increase of air temperature. In addition, the lower the temperature of predrying, the more rapid the increase in rehydration ratio. These may have resulted from the difference in the initial moisture content for the microwave drying due to the two different air temperatures. However, the change of the recovery ratio was contrary to those of the shrinkage and rehydration ratio, the recovery ratio decreased with increase of microwave power and air temperature.
Acknowledgements The authors wish to thank the financial support provided by the National Natural Science Foundation of China (Grant Nos. 30660145 and 31060218), the National Key Technologies R&D Program (Grant No. 2007BAD76B06), and the Natural Science Foundation of Hainan Province, China (Grant No. 309006) for the research work.
References Aktas, T., Fujii, S., Kawano, Y., Yamamoto, S., 2007. Effects of pretreatments of sliced vegetables with trehalose on drying
characteristics and quality of dried products. Food Bioprod. Process. 85 (3), 178–183. AOAC, 2005. Official Methods of Analysis. AOAC International, Maryland, USA. Bala, B.K., Mondol, M.R.A., 2001. Experimental investigation on solar drying of fish using solar tunnel dryer. Dry. Technol. 19 (2), 427–436. Bellagha, S., Amami, E., Farhat, A., Kechaou, N., 2002. Drying kinetics and characteristic drying curve of lightly salted Sardine (Sardinella aurita). Dry. Technol. 20 (7), 1527–1538. Chou, S.K., Chua, K.J., 2001. New hybrid drying technologies for heat sensitive foodstuffs. Trends Food Sci. Technol. 12 (10), 359–369. Datta, A.K., Ni, H., 2002. Infrared and hot-air-assisted microwave heating of foods for control of surface moisture. J. Food Eng. 51 (4), 355–364. Desmorieux, H., Decaen, N., 2005. Convective drying of spirulina in thin layer. J. Food Eng. 66, 497–503. Duan, Z.H., Shang, J., Xu, S., 2006. Study on characteristics of hot-air drying of tilapia and changes of its main components. Food Sci. 27 (12), 479–482. Duan, Z.H., Yi, M.H., Wang, Z.G., 2005a. Processing technique of tilapia. Fish Sci. Technol. Inform. 32 (6), 250–252. Duan, Z.H., Zhang, M., Hao, J., 2003. Preparation of crispy bighead carp slices. Sci. Technol. Food Ind. 2, 44–47. Duan, Z.H., Zhang, M., Hu, Q.G., 2005b. Characteristics of microwave drying of bighead carp. Dry. Technol. 23 (3), 637–643. Duan, Z.H., Zhang, M., Tang, J., 2004. Thin layer hot-air drying of bighead carp. Fisheries Sci. 23 (3), 29–32. Giri, S.K., Prasad, S., 2007. Drying kinetics and rehydration characteristics of microwave-vacuum and convection hot-air dried mushrooms. J. Food Eng. 78, 512–521. Gogus, F., Maskan, M., 2006. Air-drying characteristics of solid waste (pomace) of olive oil processing. J. Food Eng. 72, 378–382. Li, J., Li, B.S., Li, W., 2009. Study on tilapia pickling technique. Modern Food Sci. Technol. 25 (6), 646–649. McLoughlin, C.M., McMinn, W.A.M., Magee, T.R.A., 2003. Microwave vacuum drying of pharmaceutical powders. Dry. Technol. 21 (9), 1719–1733. Mwithiga, G., Olwal, J.O., 2005. The drying kinetics of kale (Brassica oleracea) in a convective hot air dryer. J. Food Eng. 71, 373–378. Orsat, V., Yang, W., Changrue, V., Raghavan, G.S.V., 2007. Microwave-assisted drying of biomaterials. Food Bioprod. Process. 85 (3), 255–263. Shang, J., Duan, Z.H., Feng, A.G., 2007. Effect of pre-treatment of low-concentration salt on the drying of tilapia fillets with hot air. Food Sci. Technol. 32 (4), 111–114. Watanabe, W.O., Losordo, T.M., Fitzsimmons, K., Hanley, F., 2002. Tilapia production systems in the Americas: technological advances, trends, and challenges. Rev. Fish Sci. 10 (3), 465–498. Zhang, H., Datta, A.K., 2005. Heating concentrations of microwaves in spherical and cylindrical foods. Part two: in a cavity. Food Bioprod. Process. 83 (1), 14–24.
Contents lists available at ScienceDirect
Food and Bioproducts Processing journal homepage: www.elsevier.com/locate/fbp
Drying and quality characteristics of tilapia fish fillets dried with hot air-microwave heating Zhen-hua Duan a,b,∗ , Li-na Jiang b , Ju-lan Wang a , Xiao-yang Yu b , Tao Wang b a b
Key Laboratory of Tropical Biological Resources of MOE, Hainan University, Haikou 570228, China College of Food Science and Technology, Hainan University, No. 58, Renmin Road, Haikou 570228, China
a b s t r a c t The aim of this work was to study the effect of hot air-microwave heating on the drying and quality characteristics of fresh tilapia fish fillets. Experimental drying curves were obtained at three microwave powers (200, 400 and 600 W) after hot air drying at two air temperatures (40 and 50 ◦ C) and a constant air velocity of 1.5 m/s. Some quality indicators such as shrinkage, rehydration and recovery properties were investigated. Results showed that an increase in microwave power resulted in a decrease in final moisture content when drying for the same period of time. The higher the hot air temperature, the higher the dehydration rate was. The shrinkage ratio and rehydration ratio increased as the microwave power and air temperature increased. However, the recovery ratio decreased as the microwave power and air temperature increased. Lower hot air temperature and microwave power are beneficial to keep the quality of tilapia fillets. © 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Hot air-microwave drying; Tilapia; Shrinkage; Rehydration; Recovery
1.
Introduction
Tilapia is the common name now applied to three genera and species of fish in the family Cichlidae: Oreochromis, Sarotherodon, and Tilapia. Native to Africa and the Middle East, these species have been distributed throughout the world and have become the second most important food fishes in the world (Watanabe et al., 2002). Tilapias possess an impressive range of attributes that make them ideal for aquaculture. They grow rapidly, reproduce easily, adapt to a wide range of environmental conditions and accept artificial feeds readily. They have good-tasting flesh with a mild flavor, are widely accepted as food fish, are used in many cuisines, and their consumption is not restricted by religious observances. Tilapia culture has expanded rapidly during the last decade as a result of technological advances associated with the intensification of culture practices. Global production was influenced by rapid expansion of tilapia culture in China, the Philippines, Thailand, Indonesia, and Egypt. China is now the leading producer having increased production from 18,000 tonnes in 1984 to 1,210,000 tonnes in 2007, more than 45% of
the total world production (Li et al., 2009). However, the quantity of the tilapia used for processing is too small to meet the requirement of increased production, and it is highly perishable product as it consists of up to 80% of water (Duan et al., 2005a; Li et al., 2009). Therefore, to solve the problems of high enzymatic and bacterial activity in fresh fish, the use of processing and preservation technology is necessary. Drying of fish is important (Bellagha et al., 2002; Bala and Mondol, 2001), because it preserves fish by inactivating enzymes and removing the moisture necessary for bacterial and mold growth (Duan et al., 2004). Drying processes can be broadly classified as thermal drying, osmotic dehydration, and mechanical dewatering. Many researchers and applications focus on air drying (Aktas et al., 2007; Desmorieux and Decaen, 2005; Duan et al., 2004; Gogus and Maskan, 2006). But air drying has drawbacks of both long drying time required and poor quality (Chou and Chua, 2001). Microwave energy is widely used in agricultural products processing because it heats quickly, is energy efficient, safe and harmless, and easy to control (Duan et al., 2003; Zhang and Datta, 2005). Microwave drying technology has become one of emerging
∗ Corresponding author at: College of Food Science and Technology, Hainan University, No. 58, Renmin Road, Haikou, China. Tel.: +86 898 66289662; fax: +86 898 66279215. E-mail addresses: [email protected], [email protected] (Z.-h. Duan). Received 15 November 2009; Received in revised form 9 October 2010; Accepted 9 November 2010 0960-3085/$ – see front matter © 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.fbp.2010.11.005
food and bioproducts processing 8 9 ( 2 0 1 1 ) 472–476
areas in food drying (McLoughlin et al., 2003; Orsat et al., 2007), especially hybrid drying technologies such as air-microwave drying (Datta and Ni, 2002; Duan et al., 2005b). Microwave heating of temperature sensitive materials offers a more rapid method of moisture removal (McLoughlin et al., 2003). Microwaves heat material simultaneously inside and outside, and can produce high rates of evaporation. If heating is too fast, fresh tilapia fillets, which have high water content, may be burst into pieces. However, few reports were focused on investigating the air-microwave drying of tilapia fillets. The aim of this work was to study the effect of hot air-microwave drying process on the drying and quality characteristics of tilapia fillets. Evaluation of quality characteristics was studied by determining the changes of the shrinkage, rehydration and recovery ratio. The results obtained may provide the important theory basis for developing the application technology of hot air-microwave drying in tilapia processing.
2.
Materials and methods
2.1.
Sample preparation
2.4.
2.2.
Shrinkage ratio
The volume of the test sample was measured using an excluding method, and the clean sea sand was selected as filling material (Shang et al., 2007). The mean particle diameter of sand is about 0.6 mm. Five replicates were performed for each sample and mean value was calculated. The shrinkage ratio was calculated as: Rs =
V0 − Vt V0
(1)
where Rs is the shrinkage ratio of the sample, V0 is the initial volume of the sample before drying, Vt is the volume of the sample during drying at time t.
Rrec =
(3)
2.5.
Moisture content
The moisture content of test sample was determined according to the vacuum oven method (AOAC, 2005). At regular time intervals during the drying period, samples were taken out and dried in a vacuum oven (Model ZKF030, Shanghai Experiment Instrument Co. Ltd., China) until constant weight.
Drying procedure
The following procedure was chosen according the previous results (Duan et al., 2005b, 2006). The fresh fish fillets were pre-dried using a digital constant temperature blast drying oven (Model 101-2, Changzhou Huapuda Instrument Co. Ltd., China) for 4 h at 40 ◦ C, and 50 ◦ C with a constant air velocity of 1.5 m/s. The pre-dehydrated fish fillets were immediately put into a laboratory microwave drying oven (Model NJL07-3, Nanjing Microwave Equipment Co. Ltd., China) to continue to dry for different times (2, 4, 6, 8, 10 min) at different microwave powers (200, 400, 600 W). For every batch of dried sample, the moisture content, shrinkage ratio, rehydration ratio and recovery ratio were determined. At least three replicates were performed for each batch of test samples and the results were averaged.
2.7.
Statistical analysis
Analysis of variance was performed by the ANOVA procedures (SPSS 10.0 for Windows). Differences among the mean values of the various treatments were determined by the least significant difference test, and the significance was defined at p < 0.05.
Results and discussion
Rehydration ratio
Dry test samples were weighed with an electronic balance (Model BS124S, Sartorius Company, Germany). Dried samples were then put into the 40 ◦ C of clean warm water to rehydrate for 30 min (Shang et al., 2007), and they are moved from the water for weighing. The temperature of the water was maintained to 40 ± 0.5 ◦ C by temperature controller (Model 501, Shanghai Experiment Instrument Factory, China). Three replicates were performed for each sample and mean value was calculated. The rehydration ratio was calculated as: Rreh =
m mo
where Rrec is the recovery ratio of the sample, and mo is the weight of the fresh sample before drying.
3. 2.3.
Recovery ratio
The recovery ratio was calculated using Eq. (3) (Duan et al., 2006). Three replicates were performed for each sample and mean value was calculated.
2.6.
Fresh tilapias (Oreochromis niloticus) were purchased from a fish market in Haikou, China. They were quickly transported to the laboratory in sealed polystyrene boxes containing ice. Fish were cleaned, gutted, skinned and headed, then cut into fillets with the size of 30 mm × 20 mm × 3 mm. The fish fillets were rinsed by tap water, and then placed in single layer on a stainless steel wire mesh for drying experiments.
473
m − mt mt
(2)
where Rreh is the rehydration ratio of the sample, m is the weight of the sample after rehydration, mt is the weight of the sample before rehydration.
3.1. Effect of pre-drying on the drying curves of fish fillets during microwave drying The drying curves of tilapia fillets are shown in Figs. 1 and 2. The moisture contents decreased with the increasing in microwave power in the same conditions (air temperature, air velocity and drying time) of pre-drying using hot air. The decreased moisture content could be attributed to increased evaporation of water both on the surface of and in the fish fillets due to increasing temperatures by microwave heating (Duan et al., 2005b). However, the drying curves were different significantly under the different conditions of microwave powers. Effect of the microwave power on the moisture content of pre-dehydrated fish fillets was significant (p < 0.05). Comparing the curves between Figs. 1 and 2, the drying rates in Fig. 2 can be seen to be greater than those of the curves in Fig. 1. This is because the initial moisture contents are dif-
food and bioproducts processing 8 9 ( 2 0 1 1 ) 472–476
1.6
90
1.4
80
1.2
shrinkage ratio (%)
moisture content (d.b)
474
1 0.8 0.6 200W 0.4
60 200W 50
400W 600W
40
400W
0.2
70
600W
30 0
0 0
2
4
6
8
2
4
time (min) Fig. 1 – The microwave drying curves of tilapia fillets after pre-drying at 40 ◦ C.
6
8
10
time (min)
10
Fig. 3 – Effect of microwave drying on the shrinkage ratio of tilapia fillets after pre-drying at 40 ◦ C.
90
3.2. Effect of pre-drying on the shrinkage ratio of fish fillets during microwave drying Figs. 3 and 4 showed effects of temperature of pre-drying with hot air on the shrinkage ratio of samples during microwave drying period. First, as can be seen, shrinkage increased with
0.6 200W
0.5
moisture content (d.b)
400W 600W
0.4 0.3 0.2 0.1 0 0
2
4
6
8
10
time (min) Fig. 2 – The microwave drying curves of tilapia fillets after pre-drying at 50 ◦ C.
80
shrinkage ratio (%)
ferent for the two different air temperatures. The temperature of pre-drying using hot air had evident impact on the moisture content of fish fillets during the microwave drying period (p < 0.05). It seemed to be concluded that the moisture content at the change-over between air drying and microwave drying should be about 0.5 g H2 O/g. The higher hot air temperatures led to the higher drying rates of samples. Similar results have been reported for drying of vegetables materials (Mwithiga and Olwal, 2005). Therefore, both hot air temperature and microwave power are considered to be beneficial to improve the drying rate of fish fillets. The required drying time was less than 6 min for obtaining the final products of 12% of moisture content (wet basis) when fish fillets dried by microwave drying at 400 W after pre-dried at 50 ◦ C for 4 h (Fig. 2). However, it at least required 19 h to dry the fresh fish fillet to final products of the same moisture content with 50 ◦ C of hot air (Duan et al., 2006). Therefore, the hot air-microwave drying may shorten greatly the required drying time compared with the hot air drying of tilapia fillets.
70 60 200W 50
400W 600W
40 30 0
2
4
6
8
10
time (min) Fig. 4 – Effect of microwave drying on the shrinkage ratio of tilapia fillets after pre-drying at 50 ◦ C.
the drying time. Shrinkage is highest during the first 2 min and then seems to be relatively constant. The shrinkage ratio of samples dried at 200 W, 400 W and 600 W of microwave power for 2 min after pre-drying at 40 ◦ C, were 54%, 62% and 65%, respectively (Fig. 3). When microwave drying time was constant, the shrinkage ratio increased slightly with the microwave power increasing, but the influence of microwave power on the shrinkage ratio of pre-dried fillets was not evident (p > 0.05). Second, the higher hot air temperature led to the higher shrinkage ratio of fish fillets (p < 0.05). The shrinkage ratios of the fish fillets pre-dried with hot air between 40 ◦ C and 50 ◦ C were 42% and 58%, respectively. In addition, the influence of pre-drying at 40 ◦ C was more than that at 50 ◦ C on the shrinkage ratio of samples during microwave drying period by comparing the differences of Figs. 3 and 4. This is because moisture content of samples pre-dried for 4 h at 40 ◦ C was higher than that at 50 ◦ C. Much more moisture was removed rapidly during microwave drying due to the instantaneous heating of microwaves (Duan et al., 2005b), which resulted in greater changes of shrinkage ratio under the same drying condition. With the microwave drying continued, the moisture content of samples decreased, the difference of moisture content between two temperatures was reduced, so the changes in shrinkage ratio decreased accordingly. Therefore, temperature of pre-drying before microwave drying was the main factor affecting the shrinkage ratio of dried fillets.
475
food and bioproducts processing 8 9 ( 2 0 1 1 ) 472–476
120
70 200W
rehydration ratio (%)
100
60
400W 600W
50
recovery ratio (%)
80 60 40 20
40 30 200W 20
400W
10
0 0
2
4
6
8
10
600W
0
time (min)
0
2
4
6
8
10
time (min) Fig. 5 – Effect of microwave drying on the rehydration ratio of tilapia fillets after pre-drying at 40 ◦ C.
Fig. 7 – Effect of microwave drying on the recovery ratio of tilapia fillets after pre-drying at 40 ◦ C.
3.3. Effect of pre-drying on the rehydration ratio of tilapia fillets during microwave drying
60
100
rehydration ratio(%)
80
60 200W
40
400W 600W
20
0 0
2
4
6
8
10
time (min) Fig. 6 – Effect of microwave drying on the rehydration ratio of tilapia fillets after pre-drying at 50 ◦ C.
recovery ratio (%)
50
Normally rehydration curves are plotted for a dry sample (a final product) immersed in water for different lengths of time (Giri and Prasad, 2007). However, the final moisture content of a dry sample depends on the specific requirement of product, such as semi-dried products. Therefore, the rehydration ratios of samples of varying moisture content were compared, here. Influences of pre-drying with different air temperature (40 ◦ C, 50 ◦ C) on the rehydration ratio of samples during microwave drying were showed in Figs. 5 and 6. As can be seen from Fig. 5, the rehydration ratio increased with the extension of microwave drying when the microwave power was constant. The similar result can also be derived from Fig. 6. On the other hand, the rehydration ratio increased with the increase in microwave power when microwave drying time was unchanged. As can be seen from Fig. 5, the rehydration ratio of samples dried at 200 W, 400 W and 600 W of microwave power for 4 min after pre-drying at 40 ◦ C, were 40%, 53% and 73%, respectively. However, the increase in the rehydration ratios was significantly different between values shown in Figs. 5 and 6 (p < 0.05). Different temperatures of pre-drying brought different changes of the rehydration ratio of fish fillets during microwave drying period. When the fresh tilapia fillets were dried through microwave drying after pre-dried at 50 ◦ C, the increase values of the rehydration ratio at 50 ◦ C were much less than those at 40 ◦ C, resulted from microwave power increased.
40 30 200W
20
400W 10
600W
0 0
2
4
6
8
10
time (min) Fig. 8 – Effect of microwave drying on the recovery ratio of tilapia fillets after pre-drying at 50 ◦ C. As can be seen from Fig. 6, the rehydration ratio of samples dried at 200 W, 400 W and 600 W of microwave power for 4 min after pre-drying at 50 ◦ C, were 72%, 75% and 82%, respectively. So, the effect of the microwave power on the rehydration ratio was closely related to the temperature of pre-drying. The lower the temperature of pre-drying, the more rapid the increase in rehydration ratio.
3.4. Effect of pre-drying on the recovery ratio of tilapia fillets during microwave drying In general, recovery curves are plotted for a dry sample (a final product) immersed in water for different lengths of time. However, the final moisture content of a dry sample depends on certain specific requirement of products. Therefore, the recovery ratios of samples of varying moisture content were compared. Figs. 7 and 8 showed the effect of different predrying temperature (40 ◦ C, 50 ◦ C) on the recovery ratio of samples. The recovery ratio decreased slightly with the extension of microwave drying time when the microwave power was constant, and the change trends were very flat, which showed that the microwave drying time was not the main factor affecting the recovery ratio. On the other hand, the recovery ratio reduced with the increase in microwave power when microwave drying time was fixed. As can be seen from Fig. 7, the recovery ratio of samples dried at 200 W, 400 W and 600 W for 8 min after pre-drying at 40 ◦ C were 52%, 45% and 42%, respectively. The recovery ratio-drying time curve at
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200 W was higher than the other two curves; the latter two were relatively closer. Therefore, decreasing the microwave power resulted in increase in the recovery ratio. Comparing Figs. 7 and 8, all the recovery ratios of fish fillets pre-dried at 40 ◦ C were higher than those at 50 ◦ C when the microwave drying time and power was fixed (p < 0.05). So, the temperature of pre-drying was important factors affecting the recovery ratio.
4.
Conclusion
Hot air drying followed by microwave drying can decrease remarkably drying time for drying fresh tilapia fillets compared with hot air drying. The moisture content at the change-over between air drying and microwave drying should be about 0.5 g H2 O/g. Hot air-microwave drying technology can be used for dehydration of fresh tilapia fillets due to decrease drying time and improve quality (rehydration ratio). Effects of microwave power and hot air temperature on drying characteristic of fish fillets during the hot air-microwave drying were significant. An increase in microwave power resulted in a decrease in moisture content of fillets; the difference is obvious in starting point for the microwave drying regarding moisture content due to the two different air temperatures. The influence of the hot air temperature on the shrinkage ratio and the rehydration ratio was significant. The shrinkage ratio and the rehydration ratio increased with increase of air temperature. In addition, the lower the temperature of predrying, the more rapid the increase in rehydration ratio. These may have resulted from the difference in the initial moisture content for the microwave drying due to the two different air temperatures. However, the change of the recovery ratio was contrary to those of the shrinkage and rehydration ratio, the recovery ratio decreased with increase of microwave power and air temperature.
Acknowledgements The authors wish to thank the financial support provided by the National Natural Science Foundation of China (Grant Nos. 30660145 and 31060218), the National Key Technologies R&D Program (Grant No. 2007BAD76B06), and the Natural Science Foundation of Hainan Province, China (Grant No. 309006) for the research work.
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