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Effect of osmotic dehydration on the dielectric properties of carrots and strawberries
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Journal of Food Engineering 88 (2008) 280–286 www.elsevier.com/locate/jfoodeng
Effect of osmotic dehydration on the dielectric properties of carrots and strawberries V. Changrue, V. Orsat *, G.S.V. Raghavan, D. Lyew Department of Bioresource Engineering, Macdonald Campus, McGill University, 21 111 Lakeshore Road, Ste-Anne-de-Bellevue, QC, Canada H9X 3V9 Received 5 December 2006; received in revised form 18 January 2008; accepted 11 February 2008 Available online 19 February 2008
Abstract Osmotic dehydration can be used as a pretreatment for microwave drying. Since microwave drying is dependent on the dielectric properties of the material to be dried, it is important to know if osmotic dehydration has any effect on these properties. Strawberries and Carrots were used in this study. Two osmotic agents, sucrose and salt, were used for carrots but only sucrose was used for strawberries. The effects of variations in sucrose and salt concentrations, solution temperature, and length of immersion time on the dielectric constant (e0 ) and the loss factor (e00 ) were measured. A predictive model was established for the range of variation used for each of the conditions studied. In general, the e0 decreased with an increase in osmotic conditions. The e00 of strawberries was not affected by osmotic dehydrations. The use of salt as the osmotic agent did have a significant effect on the e00 of carrots. Predictive models of dielectric properties of strawberries and carrots were developed using response surface methodology. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Osmotic dehydration; Dielectric properties; Carrots; Strawberries
1. Introduction The application of microwave heating to drying processes has been investigated primarily because of its capacity at improving drying rates. The better quality of microwave dried products and significant energy savings have also been noted (Prabhanjan et al., 1995; Sanga et al., 2000). The dielectric properties of materials are the key governing factors in microwave-assisted drying processes. The dielectric constant (e0 ) is a measure of the ability of a material to couple with microwave energy. The dielectric loss factor (e00 ) is a measure of the ability of the material to heat by absorbing energy (Datta et al., 2005). Experimental data on the dielectric properties of various foods are available in the literature (Tinga and Nelson, 1973; Venkatesh et al., 1998; Liao, 2002; Venkatesh and Raghavan, 2004). To improve microwave-assisted drying, there
*
Corresponding author. Tel.: +1 514 398 7680; fax: +1 514 398 8387. E-mail address: [email protected] (V. Orsat).
0260-8774/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2008.02.012
are many combinations to be considered for study. Among these combinations, researchers have shown that osmotic drying prior to microwave-assisted drying leads to lower energy consumption and better qualities of dried product (Venkatachalapathy and Raghavan, 1999; Beaudry et al., 2003). Osmotic dehydration is the simultaneous process of water and solute diffusion (Ponting et al., 1966; Lerici et al., 1985; Krokida and Marinos-Kouris, 2003). Moisture is known to be the main factor affecting dielectric properties, lower moisture content tends to provide a lower e0 (Rajnish et al., 1995). Solute diffusion could be the transfer of either osmotic agents i.e., sugar and salt, or solid components of produce which also will affect the dielectric properties. To enhance performance of osmotic dehydration, increasing temperature resulted in greater water removal (Ravindra and Chattapadhyay, 2000). Changes in water removal rates due to temperature also influence dielectric properties (Venkatesh and Raghavan, 2004). Thus, it could be assumed that the osmotic process will affect the dielectric properties of the product for its subsequent microwave
V. Changrue et al. / Journal of Food Engineering 88 (2008) 280–286
drying. Since the dielectric properties are an important factor of any microwave study, data on the dielectric properties of produce after osmotic processing will be helpful to researchers to find the proper conditions for this process prior to microwave-assisted dehydration. Most applications of microwaves in food processing are designed for operation at the frequency of 2450 MHz (Venkatesh and Raghavan, 2004). Free water has a relaxation frequency around 26 GHz at 37 °C, whereas for bound water, microwave energy below 5 GHz will be absorbed more readily (Dawkins et al., 1979). Biological materials have large quantities of bound water, hence absorb microwave energy well at 2450 MHz frequency. The present study will focus on dielectric properties at this frequency. Strawberries and carrots were selected for the study as representatives of fruits and vegetables. 2. Materials and methods 2.1. Materials The cultivar of strawberries and carrots used in this study were not known. Carrots were obtained from a local market and were cut into 10 mm sized cubes with a mechanical cutting assembly. Strawberries were cut into halves with a stainless steel knife. Strawberries and carrots were stored at 4 °C and were allowed to sit at room temperature (22 ± 1 °C) one hour before the tests were started. 2.2. Osmotic treatments Carrots and strawberries were treated by placement in an osmotic solution. Since the results of the study by Singh et al. (1999) showed no significant difference among the ratios of sample to solution at 1:4, 1:7 and 1:10 for the osmotic dehydration of carrot, all samples in this study were kept with a ratio of sample to solution of 1:5 (w/w). Three sugar concentrations (30, 40 and 50% w/w) and three salt concentrations (5, 10, and 15% w/w) were mixed to obtain nine different osmotic solutions for carrots while three sugar concentrations (40, 50 and 60% w/w) were used for strawberries. These concentrations were selected as being representative of osmotic ranges recommended in published research (Singh et al., 1999 and Lerici et al., 1985). The temperature of the osmotic solutions was set to 20, 30 and 40 °C for both strawberries and carrots to represent room temperature and moderately enhanced diffusion with increased temperature. There is a limit to how much the temperature can be raised. For product quality maintenance, the temperature should be kept as low as possible. For improved mass diffusion, the temperature should be raised. To satisfy both conditions, the three temperatures of 20, 30 and 40 °C were selected. To investigate the effect of treatment time, carrots were placed in an osmotic solution for 2, 5 and 8 h and 12, 18 and 24 h for strawberries. After osmotic treatment the samples were dipped in ambient temperature water (20 °C) in order to remove
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the osmotic agents at the surface of samples and gently wiped with a soft tissue paper and left for 15 min in ambient air to remove the surface moisture. The osmotic concentration took place over long time periods, 2–24 h, where the diffusion concentration gradients took place. Equilibrium was assumed when dielectric property measurements were made on the dry surface of rinsed-off samples. 2.3. Dielectric properties measurement After osmotic dehydration, the dielectric properties of samples were measured by an open-ended coaxial probe (Agilent-85070D, California) at a frequency of 2450 MHz. An Agilent network analyzer (Agilent8722ES, California) was used to analyze the dielectric properties signal. The instrument was first calibrated using three different loads: (i) solid metal, (ii) air and (iii) distilled water at 20 °C. The measurement were conducted by touching the samples, as uniformly as possible without applying significant pressure, against the flat face of an open-ended probe (19 mm diameter). The measurement accuracy of the probe lies within 5% and all measurements were made in triplicate. The sample size of carrots failed to reach the recommended minimum sample diameter of greater than 20 mm. To validate the accuracy of measurements with sample size, additional large carrots having a diameter greater than 20 mm were also tested. Eight sections of carrots, 25 mm in diameter, were tested against measurements made on eight sections of carrots having 10 mm in diameter. Measurements were made in triplicates for e0 and e00 . No significant difference was obtained for e0 and e00 between the two carrot sample sizes. 2.4. Experimental designs Response surface methodology (RSM) was used to estimate the main treatment effects. A second-order central composite design (CCD) in the form of a face-centered cube (FCC) with four factors (sucrose concentration, salt concentration, temperature and immersion time) at three levels each was used for carrots. Only three factors, sucrose concentration, temperature and immersion time at three levels each were applied for strawberries. All experiments were conducted in triplicate. The actual factor values and corresponding coded values (1, 0, 1) for carrots and strawberries are given in Tables 1 and 2, respectively. 2.5. Statistical analysis and model development Data was analyzed by using the software package STATGRAPHIC Plus 5.1 (Manugistics, Inc., Rockwille, MD). The model was developed from regression coefficients under a range of experimental factors. The coefficient of determination (r2) was used to indicate how the model fits the variability of the results. The terms of second-order polynomial model consist of linear, quadratic
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Table 1 Second-order central composite design (CCD) for carrots Experiment number
Sucrose concentration (%w/w)
Salt concentration (%w/w)
Temperature (°C)
Time (h)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
30 50 30 50 30 50 30 50 30 50 30 50 30 50 30 50 30 50 40 40 40 40 40 40 40 40
5 (1) 5 (1) 15 (+1) 15 (+1) 5 (1) 5 (1) 15 (+1) 15 (+1) 5 (1) 5 (1) 15 (+1) 15 (+1) 5 (1) 5 (1) 15 (+1) 15 (+1) 10 (0) 10 (0) 5 (1) 15 (+1) 10 (0) 10 (0) 10 (0) 10 (0) 10 (0) 10 (0)
20 20 20 20 40 40 40 40 20 20 20 20 40 40 40 40 30 30 30 30 20 40 30 30 30 30
2 2 2 2 2 2 2 2 8 8 8 8 8 8 8 8 5 5 5 5 5 5 2 8 5 5
(1) (+1) (1) (+1) (1) (+1) (1) (+1) (1) (+1) (1) (+1) (1) (+1) (1) (+1) (1) (+1) (0) (0) (0) (0) (0) (0) (0) (0)
(1) (1) (1) (1) (+1) (+1) (+1) (+1) (1) (1) (1) (1) (+1) (+1) (+1) (+1) (0) (0) (0) (0) (1) (+1) (0) (0) (0) (0)
(1) (1) (1) (1) (1) (1) (1) (1) (+1) (+1) (+1) (+1) (+1) (+1) (+1) (+1) (0) (0) (0) (0) (0) (0) (1) (+1) (0) (0)
Table 2 Second-order central composite design (CCD) for strawberries Experiment number
Sucrose concentration (%w/w)
Temperature (°C)
Time (h)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
40 60 40 60 40 60 40 60 40 60 50 50 50 50 50 50
20 20 40 40 20 20 40 40 30 30 20 40 30 30 30 30
12 (1) 12 (1) 12 (1) 12 (1) 24 (+1) 24 (+1) 24 (+1) 24 (+1) 18 (0) 18 (0) 18 (0) 18 (0) 12 (1) 24 (+1) 18 (0) 18 (0)
(1) (+1) (1) (+1) (1) (+1) (1) (+1) (1) (+1) (0) (0) (0) (0) (0) (0)
(1) (1) (+1) (+1) (1) (1) (+1) (+1) (0) (0) (1) (+1) (0) (0) (0) (0)
where bn are the regression coefficients; Y1 is the response either e0 or e00 of carrots; X1, X2, X3 and X4 in Eq. (1) are sucrose concentration (% w/w), salt concentration (% w/ w), temperature (°C) and immersion time (h), respectively; Y2 is the response either e0 or e00 of strawberries; X1, X2 and X3 in Eq. (2) are sucrose concentration (% w/w), temperature (°C) and time (h), respectively. Also note that the values of Xn correspond to the real values (uncoded values) of the variable. The response surface was developed by the same software. 3. Results and discussions The initial (pre-osmotic concentration) e0 and e00 were measured in triplicate and their average was 66.1 and 16.3 for the carrots, and 69.1 and 18 for the strawberries tested in this experiment. Since the dielectric properties are significantly influenced by the presence of moisture, the higher e0 of strawberries over the e0 of carrots would be expected due to the higher initial moisture content, 93.5% and 87.7% (wet basis), respectively. 3.1. The influence of osmotic dehydration on dielectric constant of carrots The results show that a decrease of e0 is attributable to an increase in immersion time, temperature and concentration of osmotic agents as shown in Fig. 1. The main effects plots of osmotic dehydration on dielectric properties are a result of removed moisture and gained solid (sucrose and salt) of the end product. Main effect plots, generated by the statistical software STATGRAPHIC, are plots of means at the various levels of each factor compared to the overall mean. Similar result was observed by Tulasidas et al. (1995) where decreasing moisture decreased e0 and increasing sugar concentration decreased e0 . However, the influence of salt concentration on e0 in this study was contrary to the result of Goedeken et al. (1997) and Bengtsson and Risman (1971) which reported the insignificant influence of salt content on e0 . Since the amount of removed
(squared) and interaction terms as shown by the following equations Y 1 ¼ b0 þ b1 X 1 þ b2 X 2 þ b3 X 3 þ b4 X 4 þ b11 X 21 þ b22 X 22 þ b33 X 23 þ b44 X 24 þ b12 X 1 X 2 þ b13 X 1 X 3 þ b14 X 1 X 4 þ b23 X 2 X 3 þ b24 X 2 X 4 þ b34 X 3 X 4
ð1Þ
Y 2 ¼ b0 þ b1 X 1 þ b2 X 2 þ b3 X 3 þ b11 X 21 þ b22 X 22 þ b33 X 23 þ b12 X 1 X 2 þ b13 X 1 X 3 þ b23 X 2 X 3
ð2Þ
Fig. 1. Main effect plots of sucrose, salt, temperature and immersion time on e0 of carrots.
V. Changrue et al. / Journal of Food Engineering 88 (2008) 280–286
water in the osmotic process increases with an increase in immersion time, temperature and concentration, decreasing of e0 in this study could imply that the influence of water removal overcame the influence of solid gain. This confirmed that moisture plays an important role in e0 (Venkatesh and Raghavan, 2004). Table 3 presents F-values of e0 and e00 . The F-value expresses the similarity of the data with respect to the reference values. The lower the F-value, the more similar is the data with the reference. The highest F-value was obtained for the time factor in Table 3 which indicates, with the conditions tested, that in osmotic dehydration of carrot, the time factor is the dominant factor affecting e0 . It was found that the lowest value of e0 , affected by time occurred within the time range of this study, 2–8 h. It is assumed that e0 will not be lower even with process times longer than 8 h. According to the Table 3 the results show a significant interaction correlation between sucrose and time. The response surface plot of these factors was selected in STATGRAPHIC and presented in Fig. 2. The figure shows that the combination increase in time and sucrose concentration caused a significant decrease of e0 .
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Fig. 2. Response surface plot of the effect of sucrose and time on the e0 of carrots.
3.2. The influence of osmotic dehydration on the loss factor of carrots The concentrations of sucrose and salt were the more dominant factors affecting the change in e00 when compared with temperature and time as shown in Fig. 3. The F-value of e00 presented in Table 3 confirms that temperature and time did not have a significant effect. The osmotic dehydraTable 3 Regression equation coefficients for e0 and e00 of osmotically dehydrated carrotsa Coefficients
e0
e00
b0
42.5479
9.0091
Linear b1 (Sucrose) b2 (Salt) b3 (Temperature) b4 (Time)
1.7708 (15.17)** 2.5678 (82.03)** .1807 (23.19)** 2.1675 (171.18)**
1.5686 (51.25)** 3.8418 (159.44)** .8315 (0.88) 4.2985 (0)
Quadratic b11 b22 b33 b44
.0202 (2.70) .0590 (1.43) .0017 (0.02) .4140 (9.14)*
.0209 (1.49) .0195 (0.08) .0126 (0.55) .1430 (0.57)
Interaction b12 b13 b14 b23 b24 b34 r2
.0143 (2.09) .005 (1.03) .065 (15.63)** .00625 (0.40) .0425 (1.67) .0325 (3.91) 0.97
.0454 (11.03)** .0052 (0.58) .0123 (0.29) .0099 (0.52) .0613 (1.81) .0585 (6.61)* 0.96
Standard error
1.97
2.73
*, ** a
: F-value significant at level 0.05 and 0.01, respectively. Value in the parenthesis show F-values.
Fig. 3. Main effect plots of sucrose, salt, temperature and immersion time on e00 of carrots.
tion had a simultaneous effect of decreasing moisture and increasing sucrose or salt contents of the end product. The effect of changing moisture, sucrose and salt on the loss factor e00 was reported in the literature and there was no significant effect on e00 for moisture levels between 40– 80% (wet basis) and sucrose content (Tulasidas et al., 1995), however, increased salt contents increased the loss factor (Goedeken et al., 1997). In this study, the effect of salt was found dominant. This means that salt uptake significantly affected the e00 for the osmotically dehydrated carrots. The effect from salt alone was not determined. Only the combined effect of adding salt to sugar was analyzed. The strength of the influence of salt is shown by the F-value in Table 3. The high e00 value ranging between 29 and 46, of the end product (Fig. 3) shows the strong influence of salt on the loss factor since the initial e00 was only 16.3. The interaction response surface plot in Fig. 4 shows that an increase in sucrose concentration had a reduced effect on increasing e00 when compared to salt concentrations. 3.3. Predictive model for dielectric constant and loss factor of carrots The coefficient of determination (r2 value) of e0 and e00 of carrots were 0.97 and 0.96, respectively. Regression coefficients of Eq. (1) for the predictive models e0 and e00 of
284
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Fig. 4. Response surface plot of the effect of sucrose and salt on the e00 of carrots.
Fig. 5. Main effect plots of sucrose, temperature and immersion time on e0 of strawberries.
carrots as shown in Table 3 provided the predictive equations in actual terms (uncoded) as the following: e0 ¼ 42:5479 þ 1:7708ðSuÞ 2:5678ðSaÞ þ :1807ðT Þ 2:1675ðtÞ :2024ðSuÞ2 þ :0143ðSuÞðSaÞ :005ðSuÞðT Þ :065ðSuÞðtÞ þ :0590ðSaÞ
2
þ :0063ðSaÞðT Þ :0425ðSaÞðtÞ :0017ðT Þ
2
:0325ðT ÞðtÞ þ :4140ðtÞ2
ð3Þ
00
e ¼ 9:0091 þ 1:5686ðSuÞ þ 3:8418ðSaÞ :8315ðT Þ 2
þ 4:2985ðtÞ :0209ðSuÞ :0454ðSuÞðSaÞ þ :0052ðSuÞðT Þ :0123ðSuÞðtÞ :0195ðSaÞ
2
Fig. 6. Response surface plot of the effect of sucrose and time on the e0 of strawberries.
þ :0098ðSaÞðT Þ :0613ðSaÞðtÞ þ :0126ðT Þ2 :0585ðT ÞðtÞ :1431ðtÞ2
ð4Þ
where, Su is the sugar (%w/w), 30 < Su < 50, Sa is the salt concentration (%w/w), 5 < Sa < 15, T is the temperature (°C), 20 < T < 40 andt is the immersion time (hour), 2 < t < 8. 3.4. The influence of osmotic dehydration on dielectric constant of strawberries The sucrose concentration, temperature and immersion time, had strong influences on the e0 of strawberries (Fig. 5). The e0 decreased as the values of all experimental factors increased. Changes in e0 of strawberries were in agreement with the results for carrots. This means that water removal in the osmotic processing of strawberries and carrots was the dominant effect affecting the changes in the dielectric constant e0 . A significant interaction was found between temperature and time. The response surface plot presented in Fig. 6 shows that increasing temperature causes a large decrease of e0 which is further affected by time. This means that the osmotic processing of strawberries at a high temperature tends to provide a product having a lower dielectric constant e0 which may reduce dielectric heating performance. Hence high temperature during osmotic dehydration should be avoided while it will help also to prevent quality deterioration.
3.5. The influence of osmotic dehydration on the loss factor of strawberries Although Fig. 7 shows that the experimental factors had some effects on the e00 , only the quadratic term of immersion time shows a significant influence on e00 by statistic proof as seen in Table 4. There were no significant effects in linear term. This result was in accordance with previous studies on grape (Tulasidas et al., 1995) where changes in
Fig. 7. Main effect plots of sucrose, temperature and immersion time on e00 of strawberries.
V. Changrue et al. / Journal of Food Engineering 88 (2008) 280–286 Table 4 Regression equation coefficients for e0 and e00 during osmotic dehydration of strawberriesa Coefficients
e0
e00
b0
77.6979
1.1906
Linear b1 (Sucrose) b2 (Temperature) b3 (Time)
1.47789 (16.74)** 2.76354 (42.12)** 0.51794 (26.56)**
0.18744 (3.09) 0.39626 (1.75) 1.0577 (0.01)
Quadratic b11 b22 b33
0.01671 (0.83) 0.02679 (2.14) 0.05682 (1.25)
0.00088 (0.1) 0.00263 (0.87) 0.0219 (7.78)*
Interaction b12 b13 b23 r2
0.01327 (1.59) 0.00995 (0.32) 0.06125 (12.22)* 0.94
0.00266 (2.69) 0.0024 (0.84) 0.00477 (3.11) 0.83
Standard error
2.97
0.46
*, ** a
: F-value significant at level 0.05 and 0.01, respectively. Value in the parenthesis show F-values.
moisture content did not affect e00 when the moisture content was over 40% (wet basis) and the different sugar solutions did not cause significant changes to e00 . It can be concluded that there was no effect on the loss factor e00 of strawberries following the removal of moisture and sugar gain during the osmotic dehydration process. 3.6. Predictive model for dielectric constant and loss factor of strawberries The coefficient of determination (r2 value) of e0 and e00 of strawberry were 0.94 and 0.83, respectively. Regression coefficients of Eq. (2) for the predictive models e0 and e00 of carrots as shown in Table 4 provided the predictive equations in actual terms (uncoded) as the following: e0 ¼ 77:6979 1:4779ðSuÞ þ 2:7635ðT Þ :51794ðtÞ þ :01671ðSuÞ2 :0133ðSuÞðT Þ :0010ðSuÞðtÞ 2
:0268ðT Þ :0613ðT ÞðtÞ þ :05683ðtÞ
2
ð5Þ
00
e ¼ 1:1906 þ :1875ðSuÞ þ :3963ðT Þ þ 1:0577ðtÞ 2
:0009ðSuÞ :0027ðSuÞðT Þ :0025ðSuÞðtÞ :0026ðT Þ2 :0048ðT ÞðtÞ :0219ðtÞ2
ð6Þ
where, Su is the sugar concentration (%w/w), 40 < Su < 60, T is the temperature (°C), 20 < T < 40 and t is the immersion time (hour), 12 < T < 24. 4. Conclusions It can be concluded from the results that the e0 of carrots and strawberries decreased with an increase in the concentration of the osmotic agents, temperature and immersion time. The immersion time was the most signifi-
285
cant factor affecting the e0 of carrots and strawberries. The addition of salt to the osmotic sucrose solution had the most significant effect on the loss factor of carrots when compared with sucrose alone, temperature and time effects for the conditions tested. There was no factor (time, temperature, or sucrose) which preferentially influenced the loss factor during osmotic dehydration of strawberries. Overall osmotic dehydration permits to reduce the moisture content at a low energy intake. It does not represent a saving in total drying time but can permit a significant energy saving per kg of water removed. Furthermore, the change in the dielectric properties of osmotically dehydrated products is reasonable as to not affect the adequate energy coupling during a subsequent microwave-assisted drying process. Acknowledgements The authors would like to acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada and the Postgraduate Education and Research Development Project in Postharvest Technology, Chiangmai University, Thailand. References Beaudry, C., Raghavan, G.S.V., Rennie, T.J., 2003. Microwave finish drying of osmotically dehydrated cranberries. Drying Technology 21 (9), 1797–1810. Bengtsson, N., Risman, P., 1971. Dielectric properties of food at 3 GHz as determined by cavity perturbation technique, measurement on food materials. Journal of Microwave Power 6, 107–123. Datta, A., Sumnu, G., Raghavan, G.S.V., 2005. Dielectric properties of foods. In: Rao, M.A., Rizvi, Syed S.H., Datta, A.K. (Eds.), Engineering Properties of Food. CRC Press, Florida, pp. 501– 566. Dawkins, A.W.J., Nightingale, N.R.V., South, G.P., Sheppard, R.J., Grant, E.H., 1979. The role of water in microwave absorption by biological material with particular reference to microwave hazards. Physics in Medicine and Biology 24 (6), 1168–1176. Goedeken, D.L., Tong, C.H., Virtanen, A.J., 1997. Dielectric properties of a pregelatinized bread system at 2450 MHz as a function of temperature, moisture, salt and specific volume. Journal of Food Science 62 (1), 145–149. Krokida, M.K., Marinos-Kouris, D., 2003. Rehydration kinetics of dehydrated products. Journal of Food Engineering 57, 1–7. Lerici, C.R., Pinnavaia, G., Rosa, M., Bartolucci, L., 1985. Osmotic dehydration of fruits; influence of osmotic agents on drying behavior and product quality. Journal of Food Science 50, 1217– 1226. Liao, X., 2002. Dielectric properties and their application in microwaveassisted organic chemical reactions. Ph.D. Thesis , McGill University, Montreal, Canada, p. 160. Ponting, J.D., Walters, G.G., Forrey, R.R., Jackson, R., Stanley, E.L., 1966. Osmotic dehydration of fruits. Food Technology 20, 125– 128. 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. Rajnish, K.C., Marcus, N., Douglas, P., Pargat, S.C., 1995. Predictive equations for the dielectric properties of foods. International Journal of Food Science and Technology 29, 699–713.
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Ravindra, M.R., Chattapadhyay, P.K., 2000. Optimisation of osmotic preconcentration and fluidized bed drying to produce dehydrated quick-cooking potato cubes. Journal of Food Engineering 44, 5– 11. Sanga, E., Mujumdar, A.S., Raghavan, G.S.V., 2000. Principles and applications of microwave drying. In: Mujumdar, A.S. (Ed.), Drying Technology in Agricultural and Food Sciences. Science Publishers, Enfield, pp. 283–289. Singh, S., Shivhare, U.S., Ahmed, J., Raghavan, G.S.V., 1999. Osmotic concentration kinetics and quality of carrot preserve. Food Research International 32, 509–514. Tinga, W.R., Nelson, S.O., 1973. Dielectric properties of materials for microwave processing-tabulated. The journal of Microwave Power 8 (1), 23–65.
Tulasidas, T.N., Raghavan, G.S.V., Van de Voort, Girard, R., 1995. Dielectric properties of grapes and sugar solutions at 245 GHz. Journal of Microwave Power and Electromagnetic Energy 30 (2), 117– 123. Venkatachalapathy, K., Raghavan, G.S.V., 1999. Combined osmotic and microwave drying of strawberries. Drying Technology 17 (4&5), 837– 853. Venkatesh, M.S., St-Denis, E., Raghavan, G.S.V., Alvo, P., Akyel, C., 1998. Dielectric properties of whole, chopped and powder grain at various bulk densities. Canadian Agricultural Engineering 40 (3), 191– 200. Venkatesh, M.S., Raghavan, G.S.V., 2004. An overview of microwave processing and dielectric properties of agri-food materials. Biosystems Engineering 88 (1), 1–18.
Journal of Food Engineering 88 (2008) 280–286 www.elsevier.com/locate/jfoodeng
Effect of osmotic dehydration on the dielectric properties of carrots and strawberries V. Changrue, V. Orsat *, G.S.V. Raghavan, D. Lyew Department of Bioresource Engineering, Macdonald Campus, McGill University, 21 111 Lakeshore Road, Ste-Anne-de-Bellevue, QC, Canada H9X 3V9 Received 5 December 2006; received in revised form 18 January 2008; accepted 11 February 2008 Available online 19 February 2008
Abstract Osmotic dehydration can be used as a pretreatment for microwave drying. Since microwave drying is dependent on the dielectric properties of the material to be dried, it is important to know if osmotic dehydration has any effect on these properties. Strawberries and Carrots were used in this study. Two osmotic agents, sucrose and salt, were used for carrots but only sucrose was used for strawberries. The effects of variations in sucrose and salt concentrations, solution temperature, and length of immersion time on the dielectric constant (e0 ) and the loss factor (e00 ) were measured. A predictive model was established for the range of variation used for each of the conditions studied. In general, the e0 decreased with an increase in osmotic conditions. The e00 of strawberries was not affected by osmotic dehydrations. The use of salt as the osmotic agent did have a significant effect on the e00 of carrots. Predictive models of dielectric properties of strawberries and carrots were developed using response surface methodology. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Osmotic dehydration; Dielectric properties; Carrots; Strawberries
1. Introduction The application of microwave heating to drying processes has been investigated primarily because of its capacity at improving drying rates. The better quality of microwave dried products and significant energy savings have also been noted (Prabhanjan et al., 1995; Sanga et al., 2000). The dielectric properties of materials are the key governing factors in microwave-assisted drying processes. The dielectric constant (e0 ) is a measure of the ability of a material to couple with microwave energy. The dielectric loss factor (e00 ) is a measure of the ability of the material to heat by absorbing energy (Datta et al., 2005). Experimental data on the dielectric properties of various foods are available in the literature (Tinga and Nelson, 1973; Venkatesh et al., 1998; Liao, 2002; Venkatesh and Raghavan, 2004). To improve microwave-assisted drying, there
*
Corresponding author. Tel.: +1 514 398 7680; fax: +1 514 398 8387. E-mail address: [email protected] (V. Orsat).
0260-8774/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2008.02.012
are many combinations to be considered for study. Among these combinations, researchers have shown that osmotic drying prior to microwave-assisted drying leads to lower energy consumption and better qualities of dried product (Venkatachalapathy and Raghavan, 1999; Beaudry et al., 2003). Osmotic dehydration is the simultaneous process of water and solute diffusion (Ponting et al., 1966; Lerici et al., 1985; Krokida and Marinos-Kouris, 2003). Moisture is known to be the main factor affecting dielectric properties, lower moisture content tends to provide a lower e0 (Rajnish et al., 1995). Solute diffusion could be the transfer of either osmotic agents i.e., sugar and salt, or solid components of produce which also will affect the dielectric properties. To enhance performance of osmotic dehydration, increasing temperature resulted in greater water removal (Ravindra and Chattapadhyay, 2000). Changes in water removal rates due to temperature also influence dielectric properties (Venkatesh and Raghavan, 2004). Thus, it could be assumed that the osmotic process will affect the dielectric properties of the product for its subsequent microwave
V. Changrue et al. / Journal of Food Engineering 88 (2008) 280–286
drying. Since the dielectric properties are an important factor of any microwave study, data on the dielectric properties of produce after osmotic processing will be helpful to researchers to find the proper conditions for this process prior to microwave-assisted dehydration. Most applications of microwaves in food processing are designed for operation at the frequency of 2450 MHz (Venkatesh and Raghavan, 2004). Free water has a relaxation frequency around 26 GHz at 37 °C, whereas for bound water, microwave energy below 5 GHz will be absorbed more readily (Dawkins et al., 1979). Biological materials have large quantities of bound water, hence absorb microwave energy well at 2450 MHz frequency. The present study will focus on dielectric properties at this frequency. Strawberries and carrots were selected for the study as representatives of fruits and vegetables. 2. Materials and methods 2.1. Materials The cultivar of strawberries and carrots used in this study were not known. Carrots were obtained from a local market and were cut into 10 mm sized cubes with a mechanical cutting assembly. Strawberries were cut into halves with a stainless steel knife. Strawberries and carrots were stored at 4 °C and were allowed to sit at room temperature (22 ± 1 °C) one hour before the tests were started. 2.2. Osmotic treatments Carrots and strawberries were treated by placement in an osmotic solution. Since the results of the study by Singh et al. (1999) showed no significant difference among the ratios of sample to solution at 1:4, 1:7 and 1:10 for the osmotic dehydration of carrot, all samples in this study were kept with a ratio of sample to solution of 1:5 (w/w). Three sugar concentrations (30, 40 and 50% w/w) and three salt concentrations (5, 10, and 15% w/w) were mixed to obtain nine different osmotic solutions for carrots while three sugar concentrations (40, 50 and 60% w/w) were used for strawberries. These concentrations were selected as being representative of osmotic ranges recommended in published research (Singh et al., 1999 and Lerici et al., 1985). The temperature of the osmotic solutions was set to 20, 30 and 40 °C for both strawberries and carrots to represent room temperature and moderately enhanced diffusion with increased temperature. There is a limit to how much the temperature can be raised. For product quality maintenance, the temperature should be kept as low as possible. For improved mass diffusion, the temperature should be raised. To satisfy both conditions, the three temperatures of 20, 30 and 40 °C were selected. To investigate the effect of treatment time, carrots were placed in an osmotic solution for 2, 5 and 8 h and 12, 18 and 24 h for strawberries. After osmotic treatment the samples were dipped in ambient temperature water (20 °C) in order to remove
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the osmotic agents at the surface of samples and gently wiped with a soft tissue paper and left for 15 min in ambient air to remove the surface moisture. The osmotic concentration took place over long time periods, 2–24 h, where the diffusion concentration gradients took place. Equilibrium was assumed when dielectric property measurements were made on the dry surface of rinsed-off samples. 2.3. Dielectric properties measurement After osmotic dehydration, the dielectric properties of samples were measured by an open-ended coaxial probe (Agilent-85070D, California) at a frequency of 2450 MHz. An Agilent network analyzer (Agilent8722ES, California) was used to analyze the dielectric properties signal. The instrument was first calibrated using three different loads: (i) solid metal, (ii) air and (iii) distilled water at 20 °C. The measurement were conducted by touching the samples, as uniformly as possible without applying significant pressure, against the flat face of an open-ended probe (19 mm diameter). The measurement accuracy of the probe lies within 5% and all measurements were made in triplicate. The sample size of carrots failed to reach the recommended minimum sample diameter of greater than 20 mm. To validate the accuracy of measurements with sample size, additional large carrots having a diameter greater than 20 mm were also tested. Eight sections of carrots, 25 mm in diameter, were tested against measurements made on eight sections of carrots having 10 mm in diameter. Measurements were made in triplicates for e0 and e00 . No significant difference was obtained for e0 and e00 between the two carrot sample sizes. 2.4. Experimental designs Response surface methodology (RSM) was used to estimate the main treatment effects. A second-order central composite design (CCD) in the form of a face-centered cube (FCC) with four factors (sucrose concentration, salt concentration, temperature and immersion time) at three levels each was used for carrots. Only three factors, sucrose concentration, temperature and immersion time at three levels each were applied for strawberries. All experiments were conducted in triplicate. The actual factor values and corresponding coded values (1, 0, 1) for carrots and strawberries are given in Tables 1 and 2, respectively. 2.5. Statistical analysis and model development Data was analyzed by using the software package STATGRAPHIC Plus 5.1 (Manugistics, Inc., Rockwille, MD). The model was developed from regression coefficients under a range of experimental factors. The coefficient of determination (r2) was used to indicate how the model fits the variability of the results. The terms of second-order polynomial model consist of linear, quadratic
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Table 1 Second-order central composite design (CCD) for carrots Experiment number
Sucrose concentration (%w/w)
Salt concentration (%w/w)
Temperature (°C)
Time (h)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
30 50 30 50 30 50 30 50 30 50 30 50 30 50 30 50 30 50 40 40 40 40 40 40 40 40
5 (1) 5 (1) 15 (+1) 15 (+1) 5 (1) 5 (1) 15 (+1) 15 (+1) 5 (1) 5 (1) 15 (+1) 15 (+1) 5 (1) 5 (1) 15 (+1) 15 (+1) 10 (0) 10 (0) 5 (1) 15 (+1) 10 (0) 10 (0) 10 (0) 10 (0) 10 (0) 10 (0)
20 20 20 20 40 40 40 40 20 20 20 20 40 40 40 40 30 30 30 30 20 40 30 30 30 30
2 2 2 2 2 2 2 2 8 8 8 8 8 8 8 8 5 5 5 5 5 5 2 8 5 5
(1) (+1) (1) (+1) (1) (+1) (1) (+1) (1) (+1) (1) (+1) (1) (+1) (1) (+1) (1) (+1) (0) (0) (0) (0) (0) (0) (0) (0)
(1) (1) (1) (1) (+1) (+1) (+1) (+1) (1) (1) (1) (1) (+1) (+1) (+1) (+1) (0) (0) (0) (0) (1) (+1) (0) (0) (0) (0)
(1) (1) (1) (1) (1) (1) (1) (1) (+1) (+1) (+1) (+1) (+1) (+1) (+1) (+1) (0) (0) (0) (0) (0) (0) (1) (+1) (0) (0)
Table 2 Second-order central composite design (CCD) for strawberries Experiment number
Sucrose concentration (%w/w)
Temperature (°C)
Time (h)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
40 60 40 60 40 60 40 60 40 60 50 50 50 50 50 50
20 20 40 40 20 20 40 40 30 30 20 40 30 30 30 30
12 (1) 12 (1) 12 (1) 12 (1) 24 (+1) 24 (+1) 24 (+1) 24 (+1) 18 (0) 18 (0) 18 (0) 18 (0) 12 (1) 24 (+1) 18 (0) 18 (0)
(1) (+1) (1) (+1) (1) (+1) (1) (+1) (1) (+1) (0) (0) (0) (0) (0) (0)
(1) (1) (+1) (+1) (1) (1) (+1) (+1) (0) (0) (1) (+1) (0) (0) (0) (0)
where bn are the regression coefficients; Y1 is the response either e0 or e00 of carrots; X1, X2, X3 and X4 in Eq. (1) are sucrose concentration (% w/w), salt concentration (% w/ w), temperature (°C) and immersion time (h), respectively; Y2 is the response either e0 or e00 of strawberries; X1, X2 and X3 in Eq. (2) are sucrose concentration (% w/w), temperature (°C) and time (h), respectively. Also note that the values of Xn correspond to the real values (uncoded values) of the variable. The response surface was developed by the same software. 3. Results and discussions The initial (pre-osmotic concentration) e0 and e00 were measured in triplicate and their average was 66.1 and 16.3 for the carrots, and 69.1 and 18 for the strawberries tested in this experiment. Since the dielectric properties are significantly influenced by the presence of moisture, the higher e0 of strawberries over the e0 of carrots would be expected due to the higher initial moisture content, 93.5% and 87.7% (wet basis), respectively. 3.1. The influence of osmotic dehydration on dielectric constant of carrots The results show that a decrease of e0 is attributable to an increase in immersion time, temperature and concentration of osmotic agents as shown in Fig. 1. The main effects plots of osmotic dehydration on dielectric properties are a result of removed moisture and gained solid (sucrose and salt) of the end product. Main effect plots, generated by the statistical software STATGRAPHIC, are plots of means at the various levels of each factor compared to the overall mean. Similar result was observed by Tulasidas et al. (1995) where decreasing moisture decreased e0 and increasing sugar concentration decreased e0 . However, the influence of salt concentration on e0 in this study was contrary to the result of Goedeken et al. (1997) and Bengtsson and Risman (1971) which reported the insignificant influence of salt content on e0 . Since the amount of removed
(squared) and interaction terms as shown by the following equations Y 1 ¼ b0 þ b1 X 1 þ b2 X 2 þ b3 X 3 þ b4 X 4 þ b11 X 21 þ b22 X 22 þ b33 X 23 þ b44 X 24 þ b12 X 1 X 2 þ b13 X 1 X 3 þ b14 X 1 X 4 þ b23 X 2 X 3 þ b24 X 2 X 4 þ b34 X 3 X 4
ð1Þ
Y 2 ¼ b0 þ b1 X 1 þ b2 X 2 þ b3 X 3 þ b11 X 21 þ b22 X 22 þ b33 X 23 þ b12 X 1 X 2 þ b13 X 1 X 3 þ b23 X 2 X 3
ð2Þ
Fig. 1. Main effect plots of sucrose, salt, temperature and immersion time on e0 of carrots.
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water in the osmotic process increases with an increase in immersion time, temperature and concentration, decreasing of e0 in this study could imply that the influence of water removal overcame the influence of solid gain. This confirmed that moisture plays an important role in e0 (Venkatesh and Raghavan, 2004). Table 3 presents F-values of e0 and e00 . The F-value expresses the similarity of the data with respect to the reference values. The lower the F-value, the more similar is the data with the reference. The highest F-value was obtained for the time factor in Table 3 which indicates, with the conditions tested, that in osmotic dehydration of carrot, the time factor is the dominant factor affecting e0 . It was found that the lowest value of e0 , affected by time occurred within the time range of this study, 2–8 h. It is assumed that e0 will not be lower even with process times longer than 8 h. According to the Table 3 the results show a significant interaction correlation between sucrose and time. The response surface plot of these factors was selected in STATGRAPHIC and presented in Fig. 2. The figure shows that the combination increase in time and sucrose concentration caused a significant decrease of e0 .
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Fig. 2. Response surface plot of the effect of sucrose and time on the e0 of carrots.
3.2. The influence of osmotic dehydration on the loss factor of carrots The concentrations of sucrose and salt were the more dominant factors affecting the change in e00 when compared with temperature and time as shown in Fig. 3. The F-value of e00 presented in Table 3 confirms that temperature and time did not have a significant effect. The osmotic dehydraTable 3 Regression equation coefficients for e0 and e00 of osmotically dehydrated carrotsa Coefficients
e0
e00
b0
42.5479
9.0091
Linear b1 (Sucrose) b2 (Salt) b3 (Temperature) b4 (Time)
1.7708 (15.17)** 2.5678 (82.03)** .1807 (23.19)** 2.1675 (171.18)**
1.5686 (51.25)** 3.8418 (159.44)** .8315 (0.88) 4.2985 (0)
Quadratic b11 b22 b33 b44
.0202 (2.70) .0590 (1.43) .0017 (0.02) .4140 (9.14)*
.0209 (1.49) .0195 (0.08) .0126 (0.55) .1430 (0.57)
Interaction b12 b13 b14 b23 b24 b34 r2
.0143 (2.09) .005 (1.03) .065 (15.63)** .00625 (0.40) .0425 (1.67) .0325 (3.91) 0.97
.0454 (11.03)** .0052 (0.58) .0123 (0.29) .0099 (0.52) .0613 (1.81) .0585 (6.61)* 0.96
Standard error
1.97
2.73
*, ** a
: F-value significant at level 0.05 and 0.01, respectively. Value in the parenthesis show F-values.
Fig. 3. Main effect plots of sucrose, salt, temperature and immersion time on e00 of carrots.
tion had a simultaneous effect of decreasing moisture and increasing sucrose or salt contents of the end product. The effect of changing moisture, sucrose and salt on the loss factor e00 was reported in the literature and there was no significant effect on e00 for moisture levels between 40– 80% (wet basis) and sucrose content (Tulasidas et al., 1995), however, increased salt contents increased the loss factor (Goedeken et al., 1997). In this study, the effect of salt was found dominant. This means that salt uptake significantly affected the e00 for the osmotically dehydrated carrots. The effect from salt alone was not determined. Only the combined effect of adding salt to sugar was analyzed. The strength of the influence of salt is shown by the F-value in Table 3. The high e00 value ranging between 29 and 46, of the end product (Fig. 3) shows the strong influence of salt on the loss factor since the initial e00 was only 16.3. The interaction response surface plot in Fig. 4 shows that an increase in sucrose concentration had a reduced effect on increasing e00 when compared to salt concentrations. 3.3. Predictive model for dielectric constant and loss factor of carrots The coefficient of determination (r2 value) of e0 and e00 of carrots were 0.97 and 0.96, respectively. Regression coefficients of Eq. (1) for the predictive models e0 and e00 of
284
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Fig. 4. Response surface plot of the effect of sucrose and salt on the e00 of carrots.
Fig. 5. Main effect plots of sucrose, temperature and immersion time on e0 of strawberries.
carrots as shown in Table 3 provided the predictive equations in actual terms (uncoded) as the following: e0 ¼ 42:5479 þ 1:7708ðSuÞ 2:5678ðSaÞ þ :1807ðT Þ 2:1675ðtÞ :2024ðSuÞ2 þ :0143ðSuÞðSaÞ :005ðSuÞðT Þ :065ðSuÞðtÞ þ :0590ðSaÞ
2
þ :0063ðSaÞðT Þ :0425ðSaÞðtÞ :0017ðT Þ
2
:0325ðT ÞðtÞ þ :4140ðtÞ2
ð3Þ
00
e ¼ 9:0091 þ 1:5686ðSuÞ þ 3:8418ðSaÞ :8315ðT Þ 2
þ 4:2985ðtÞ :0209ðSuÞ :0454ðSuÞðSaÞ þ :0052ðSuÞðT Þ :0123ðSuÞðtÞ :0195ðSaÞ
2
Fig. 6. Response surface plot of the effect of sucrose and time on the e0 of strawberries.
þ :0098ðSaÞðT Þ :0613ðSaÞðtÞ þ :0126ðT Þ2 :0585ðT ÞðtÞ :1431ðtÞ2
ð4Þ
where, Su is the sugar (%w/w), 30 < Su < 50, Sa is the salt concentration (%w/w), 5 < Sa < 15, T is the temperature (°C), 20 < T < 40 andt is the immersion time (hour), 2 < t < 8. 3.4. The influence of osmotic dehydration on dielectric constant of strawberries The sucrose concentration, temperature and immersion time, had strong influences on the e0 of strawberries (Fig. 5). The e0 decreased as the values of all experimental factors increased. Changes in e0 of strawberries were in agreement with the results for carrots. This means that water removal in the osmotic processing of strawberries and carrots was the dominant effect affecting the changes in the dielectric constant e0 . A significant interaction was found between temperature and time. The response surface plot presented in Fig. 6 shows that increasing temperature causes a large decrease of e0 which is further affected by time. This means that the osmotic processing of strawberries at a high temperature tends to provide a product having a lower dielectric constant e0 which may reduce dielectric heating performance. Hence high temperature during osmotic dehydration should be avoided while it will help also to prevent quality deterioration.
3.5. The influence of osmotic dehydration on the loss factor of strawberries Although Fig. 7 shows that the experimental factors had some effects on the e00 , only the quadratic term of immersion time shows a significant influence on e00 by statistic proof as seen in Table 4. There were no significant effects in linear term. This result was in accordance with previous studies on grape (Tulasidas et al., 1995) where changes in
Fig. 7. Main effect plots of sucrose, temperature and immersion time on e00 of strawberries.
V. Changrue et al. / Journal of Food Engineering 88 (2008) 280–286 Table 4 Regression equation coefficients for e0 and e00 during osmotic dehydration of strawberriesa Coefficients
e0
e00
b0
77.6979
1.1906
Linear b1 (Sucrose) b2 (Temperature) b3 (Time)
1.47789 (16.74)** 2.76354 (42.12)** 0.51794 (26.56)**
0.18744 (3.09) 0.39626 (1.75) 1.0577 (0.01)
Quadratic b11 b22 b33
0.01671 (0.83) 0.02679 (2.14) 0.05682 (1.25)
0.00088 (0.1) 0.00263 (0.87) 0.0219 (7.78)*
Interaction b12 b13 b23 r2
0.01327 (1.59) 0.00995 (0.32) 0.06125 (12.22)* 0.94
0.00266 (2.69) 0.0024 (0.84) 0.00477 (3.11) 0.83
Standard error
2.97
0.46
*, ** a
: F-value significant at level 0.05 and 0.01, respectively. Value in the parenthesis show F-values.
moisture content did not affect e00 when the moisture content was over 40% (wet basis) and the different sugar solutions did not cause significant changes to e00 . It can be concluded that there was no effect on the loss factor e00 of strawberries following the removal of moisture and sugar gain during the osmotic dehydration process. 3.6. Predictive model for dielectric constant and loss factor of strawberries The coefficient of determination (r2 value) of e0 and e00 of strawberry were 0.94 and 0.83, respectively. Regression coefficients of Eq. (2) for the predictive models e0 and e00 of carrots as shown in Table 4 provided the predictive equations in actual terms (uncoded) as the following: e0 ¼ 77:6979 1:4779ðSuÞ þ 2:7635ðT Þ :51794ðtÞ þ :01671ðSuÞ2 :0133ðSuÞðT Þ :0010ðSuÞðtÞ 2
:0268ðT Þ :0613ðT ÞðtÞ þ :05683ðtÞ
2
ð5Þ
00
e ¼ 1:1906 þ :1875ðSuÞ þ :3963ðT Þ þ 1:0577ðtÞ 2
:0009ðSuÞ :0027ðSuÞðT Þ :0025ðSuÞðtÞ :0026ðT Þ2 :0048ðT ÞðtÞ :0219ðtÞ2
ð6Þ
where, Su is the sugar concentration (%w/w), 40 < Su < 60, T is the temperature (°C), 20 < T < 40 and t is the immersion time (hour), 12 < T < 24. 4. Conclusions It can be concluded from the results that the e0 of carrots and strawberries decreased with an increase in the concentration of the osmotic agents, temperature and immersion time. The immersion time was the most signifi-
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cant factor affecting the e0 of carrots and strawberries. The addition of salt to the osmotic sucrose solution had the most significant effect on the loss factor of carrots when compared with sucrose alone, temperature and time effects for the conditions tested. There was no factor (time, temperature, or sucrose) which preferentially influenced the loss factor during osmotic dehydration of strawberries. Overall osmotic dehydration permits to reduce the moisture content at a low energy intake. It does not represent a saving in total drying time but can permit a significant energy saving per kg of water removed. Furthermore, the change in the dielectric properties of osmotically dehydrated products is reasonable as to not affect the adequate energy coupling during a subsequent microwave-assisted drying process. Acknowledgements The authors would like to acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada and the Postgraduate Education and Research Development Project in Postharvest Technology, Chiangmai University, Thailand. References Beaudry, C., Raghavan, G.S.V., Rennie, T.J., 2003. Microwave finish drying of osmotically dehydrated cranberries. Drying Technology 21 (9), 1797–1810. Bengtsson, N., Risman, P., 1971. Dielectric properties of food at 3 GHz as determined by cavity perturbation technique, measurement on food materials. Journal of Microwave Power 6, 107–123. Datta, A., Sumnu, G., Raghavan, G.S.V., 2005. Dielectric properties of foods. In: Rao, M.A., Rizvi, Syed S.H., Datta, A.K. (Eds.), Engineering Properties of Food. CRC Press, Florida, pp. 501– 566. Dawkins, A.W.J., Nightingale, N.R.V., South, G.P., Sheppard, R.J., Grant, E.H., 1979. The role of water in microwave absorption by biological material with particular reference to microwave hazards. Physics in Medicine and Biology 24 (6), 1168–1176. Goedeken, D.L., Tong, C.H., Virtanen, A.J., 1997. Dielectric properties of a pregelatinized bread system at 2450 MHz as a function of temperature, moisture, salt and specific volume. Journal of Food Science 62 (1), 145–149. Krokida, M.K., Marinos-Kouris, D., 2003. Rehydration kinetics of dehydrated products. Journal of Food Engineering 57, 1–7. Lerici, C.R., Pinnavaia, G., Rosa, M., Bartolucci, L., 1985. Osmotic dehydration of fruits; influence of osmotic agents on drying behavior and product quality. Journal of Food Science 50, 1217– 1226. Liao, X., 2002. Dielectric properties and their application in microwaveassisted organic chemical reactions. Ph.D. Thesis , McGill University, Montreal, Canada, p. 160. Ponting, J.D., Walters, G.G., Forrey, R.R., Jackson, R., Stanley, E.L., 1966. Osmotic dehydration of fruits. Food Technology 20, 125– 128. 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. Rajnish, K.C., Marcus, N., Douglas, P., Pargat, S.C., 1995. Predictive equations for the dielectric properties of foods. International Journal of Food Science and Technology 29, 699–713.
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