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A Method for Continuous Kinetic Evaluation of Osmotic Dehydration
Article No. fs970364
Lebensm.-Wiss. u.-Technol., 31, 317–321 (1998)
A Method for Continuous Kinetic Evaluation of Osmotic Dehydration Ebner Azuara*, Cesar I. Beristain and Gustavo F. Gutierrez ´ E. Azuara, C. I. Beristain: Instituto de Ciencias Basicas, ´ Universidad Veracruzana, Apdo. Postal 575, Xalapa, Ver. (Mexico) ´ G. F. Gutierrez: ´ Departamento de Graduados en Alimentos, ENCB-IPN, Apdo. Postal 42186, CP 06470 Mexico, ´ D.F. (Mexico) ´ G. F. Gutierrez: ´ Food Science and Technology Department, University of Reading, Reading RG6 6AP (UK) (Received June 9, 1997; accepted December 1, 1997)
A method to determine the kinetics of osmotic dehydration was developed. The method involves the measurement of weight loss in a single sample and its final moisture at the end of the process. The method was tested in the osmotic dehydration of apple disks of 4.6 cm diameter and 0.4 cm thickness. Temperature was set at 30 °C and a drying solution containing 500 g sucrose/kg was used. The results showed that dispersion of the experimental points was worse when several samples were used to measure average weight loss (14.8%) than when the continuous method was employed (3.9%). The continuous method is useful to predict variations of the final moisture with respect to drying time. ©1998 Academic Press Keywords: mass transfer; continuous method; apple
of the predictions made by the traditional discontinuous method.
Introduction Traditionally, experimental data for mass transfer studies during osmotic dehydration are obtained from a number of foodstuff samples having the same geometry and dimensions, assuming that all the pieces have equal weight, volume and moisture content (discontinuous method). Many authors have used the discontinuous method to follow the kinetics of osmodehydration (1–7). When a number of samples with an average geometry and size are used to determine the kinetics of osmotic dehydration, it becomes necessary to keep a tight control on the degree of ripening of the product. This precaution reduced excessive dispersion of experimental data (8) which could lead to erroneous interpretations. It is difficult to obtain samples with the same degree of ripening; therefore the same sample was used throughout the osmotic dehydration process to prevent the data scattering (9, 10). We have called this approach the continuous method. The purpose of the present work was to study the continuous method to obtain kinetic data for osmodehydration at different times and evaluate the dispersion *To whom correspondence should be addressed.
Materials and Methods Materials Golden apples and refined sugar were purchased in a local market and used in all experiments. The apples were cut into disks of 4.6 cm diameter and 0.4 cm thickness. Immediately after being cut, the apple pieces were immersed in a 2 g/L solution of ascorbic acid. Procedure Apple pieces were osmotically dehydrated at 30 °C in a solution containing 500 g of sucrose/kg. The ratio of foodstuff to solution was greater than 1:20. Samples were withdrawn at periodic intervals during 5 h. Excess solution from the surface was blot dried using paper towels. In the continuous method, each sample was weighed and returned to the osmotic solution to continue the drying process. After 5 h, the moisture content of the sample was determined in a vacuum oven at 70 °C for 24 h (11). Alternatively, for the traditional discontinuous method 17 samples from the same batch were used, and weight loss and moisture in different samples of the same dimensions and geometry
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©1998 Academic Press
lwt/vol. 31 (1998) No. 4
were measured. In both methods, each experimental treatment was performed in triplicate runs. S1 =
SG =
s1tWFL °
8
1 + s1t
SG¥ =
Eqn [1]
[(
WFL –1 SG m
[(
(1/b) WFL –1 SG m
s2tSG ° 1 + s2t
ML = WFL – SG
S2 = SG¥
WFL M0
Eqn [3]
SG WFL
[ (
8
s1tWFL¥ 1 –
)]
Eqn [4]
1 + s1t
Eqn [4] can be written in the following form: –1 [(WFL SG ) ]
8
ML =
SG M0
=
[ (
8
8
)]
]
=
M0C0 – MtCf M0
Eqn [12]
M0
Eqn [13]
where M0 = initial weight of the foodstuff (t = 0), Mt = weight of the food at time t, C0 = initial moisture of the foodstuff (wet basis), and Cf = final moisture of the foodstuff (wet basis) at time t. If s1, s2, WFL ° and SG ° are known, it is possible to calculate WFL and SG at any time. Cf can also be predicted using the following equation: Cf =
Linearizing Eqn [4] and Eqn [5]: 1 t t = + SG ML s WFL 1 – SG WFL¥ 1 – 1 ¥ WFL WFL
)
Eqn [11]
M0 (C0 – 1) – Mt (Cf – 1)
Eqn [5]
1 + s2t
[ (
]
Subscript m means that WFL and SG are determined at the last point of the experiment, using the equations of Beristain et al. (13),
Subtracting Eqn [1] from Eqn [2] and rearranging, we obtain:
s2tSG¥
)
Eqn [10]
Eqn [2]
where t = time, s1 = a constant related to water loss, s2 = a constant related to solids gain, WFL = amount of water lost by the foodstuff at time t (fraction, percent, g, or kg), SG = amount of solids gained by the foodstuff at time t (fraction, percent, g or kg), WFL ° = amount of water lost at equilibrium, and SG ° = amount of solids gained at equilibrium. Mass loss (ML) during osmotic dehydration is equal to water lost (WFL) minus solids gained (SG) at the same time.
ML =
Eqn [9]
) ]
(1/p)
8
WFL =
[ (
8
Mathematical modelling Azuara et al. (12) calculated water loss and solid gain during osmotic dehydration using equations with two parameters obtained from mass balances.
(1/b) WFL¥ 1 – SG WFL m
M0C0 – (WFL)M0 Mt
Eqn [14]
Moisture loss (Cf L) was obtained according to the equation:
)] Eqn [6]
700 600
[(
) ]
[(
t WFL –1 SG
) ]
500
Eqn [7]
If we plot t/ML vs. t, we can define b to be the intercept and p the slope of the resulting straight line, then the following equations arise:
t/ML (min/g)
¥
8
2
1 + WFL –1 SG¥ SG
8
t = ML s SG
400 300 200 100 0 –50
8
WFL¥ =
(1/p) SG 1– WFL m
[ ( ) ]
Eqn [8]
0
50
100 150 200 Time (min)
250
300
350
Fig. 1 Plot of (t/ML) vs. t for the continuous method, during osmotic dehydration of apple disks. (s), Sample 1 (r2 = 0.994); (e), sample 2 (r2 = 0.998); (n), sample 3 (r2 = 0.998); (d), average (r2 = 0.998)
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lwt/vol. 31 (1998) No. 4
CfL = 1 – Cf/C0
Eqn [15]
The criterion used to evaluate goodness of fit was the modulus of the mean relative deviation (P%) between the experimental values and those predicted by equations [6] and [7] as proposed by Lomauro et al. (14): P(%) =
100 n
m
|Mi – Mpi|
i = 1
Mi
Σ
Results and Discussion Eqn [16]
where Mi and Mpi are respectively the experimental and predicted values and n is the number of experimental data points. For each experimental point, the relative % deviation (D) between the mean and the individual values was determined by the method described by Yanniotis and Zarmboutis (15): D =
100 3
3
| xi – x| ¯
i = 1
x¯
Σ
Statistical analysis A two-sample t-test was performed between the means of the mass loss, moisture loss, water loss and solids gain obtained by both methods.
Eqn [17]
where xi is the normalized individual value of mass transfer parameter (mass loss, water loss, solids gain or moisture loss) of the three samples in each time and x¯ is their arithmetic mean.
Figure 1 shows the data obtained using the continuous method for apple disks. It is evident that Eqn [6] and Eqn [7] apply and a good fit is obtained with a r2 of 0.999 and a P value of 2.915%. Calculated average values for WFL ° , SG ° and s1 from the slope and intercept of the straight lines were 0.721 g of water/g of fresh apple, 0.197 g of sucrose/g of fresh apple and 0.032 min–1, respectively. Figures 2–5 depict the kinetics of normalized values of mass loss, water loss, solids gain and moisture loss. These normalized mass transport data are defined as the ratio of the mass loss, water loss, solids gain and moisture loss at t time to the maximum value obtained with each method during osmotic dehydration. The two-sample t-test did not reveal differences between discontinuous and continuous methods at the 5% significance level. On the other hand, Table 1 shows the average relative % deviation
1
1
Normalized moisture loss
(a) 1.2
Normalized mass loss
(a) 1.2
0.8 0.6 0.4 0.2 0 –0.2 –50
0
50
100 150 200 Time (min)
250
300
350
(b) 1.2
0.4 0.2
0
Normalized moisture loss
Normalized mass loss
0.6
50
100
150 200 Time (min)
250
300
350
50
100
150 200 Time (min)
250
300
350
(b) 1.2
1 0.8 0.6 0.4 0.2 0 –0.2 –50
0.8
0
50
100 150 200 Time (min)
250
300
350
Fig. 2 Comparison of the normalized mass loss (NML) obtained from (a) the discontinuous method with NML obtained from (b) the continuous method. (F), Experiment or sample 1; (e), experiment or sample 2; (n), experiment or sample 3; (d), average
1 0.8 0.6 0.4 0.2
0
Fig. 3 Comparison of the normalized moisture loss (NHL) obtained from (a) the discontinuous method with NHL obtained from (b) the continuous method. (F), Experiment or sample 1; (e), experiment or sample 2; (n), experiment or sample 3; (d), average
319
lwt/vol. 31 (1998) No. 4
1
1 Normalized solids gain
(a) 1.2
Normalized water loss
(a) 1.2
0.8 0.6 0.4 0.2
0
0.8 0.6 0.4 0.2
50
100
150 200 Time (min)
250
300
350
0
1
1
Normalized solids gain
(b) 1.2
Normalized water loss
(b) 1.2
0.8 0.6 0.4 0.2
0
50
100
150 200 Time (min)
250
300
350
50
100
200 150 Time (min)
250
300
350
50
100
150 200 Time (min)
250
300
350
0.8 0.6 0.4 0.2
0
Fig. 4 Comparison of the normalized water loss (NWFL) obtained from (a) the discontinuous method with NWFL obtained from (b) the continuous method. For (a), (e), experiment 1; (n), experiment 2; (F), experiment 3. For (b), (F), sample 1; (e), sample 2; (n), sample 3. (d), average
Fig. 5 Comparison of the normalized solids gain (NSG) obtained from (a) the discontinuous method with NSG obtained from (b) the continuous method. For (a), (e), experiment 1; (n), experiment 2; (F), experiment 3. For (b), (F), sample 1; (e), sample 2; (n), sample 3. (d), average
¯ calculated of the arithmetic mean of D. Greater (D) deviation is evident for samples taken with the discontinuous method. The highest deviation values were found at first experimental points. Even though the pieces have the same dimensions and geometry, there may be differences in weight, moisture and initial soluble solids. Practically no dispersion can be observed in the measurements made on the sample used for the continuous method (see Figs 2b, 3b, 4b, 5b and Table 1). The single-sample approach may have additional advantages of time, reagent and labour savings. It is important to notice that, in both methods, water loss and solid gain increased linearly with each other throughout the process. Water loss proceeds parallel to solid uptake, which is in agreement with the results of other workers (16).
Table 1 Average D values calculated for the discontinuous and continuous methods
Conclusions The proposed continuous method to monitor the osmotic dehydration process offers simplified data collection and less scatter. This method may facilitate the interpretation and modelling of the process and makes it easier to predict moisture changes as a function of drying time.
Variable
D 95% confidence interval
Discontinuous
Mass loss Moisture loss Water loss Solids gain
14.8 ±6.5 9.1 ±4.0 8.1 ±4.1 17.8 ±7.1
Continuous
Mass loss Moisture loss Water loss Solids gain
3.9 ±3.5 8.9 ±7.7 4.7 ±2.1 4.7 ±2.1
Method
References 1 PONTING, J. D., WATTERS, G. G., FORREY, R. R., JACKSON, R. AND STANLEY, W. L. Osmotic dehydration of fruits. Food Technology, 20, 125–128 (1966) 2 FARKAS, D. F. AND LAZAR, M. E. Osmotic dehydration of apple pieces: effect of temperature and syrup concentration rates. Food Technology, 23, 688–690 (1969) 3 LENART, A. AND FLINK, J. M. Osmotic concentration of potato I. Criteria for the end point of the osmosis process. Journal of Food Technology, 19, 45–63 (1984) 4 HENG, W., GULBERT, S. AND CUQ, J. L. Osmotic dehydration of papaya: influence of process variables on the quality. Sciences des Aliments, 10, 831–848 (1990)
320
lwt/vol. 31 (1998) No. 4 ´ 5 ARGAIZ, A., LOPEZ -MALO, A., PALOU, E. AND WELTI, J. Osmotic dehydration of papaya with corn syrup solids. Drying Technology, 12, 1709–1725 (1994) 6 SAUREL, R., RAOULT-WACK, A. L., RIOS, G. AND GUILBERT, S. Mass transfer phenomena during osmotic dehydration of apple I. Fresh plant tissue. International Journal of Food Science and Technology, 29, 531–542 (1994) 7 AZUARA, E., GARCIA, H. S. AND BERISTAIN, C. I. Effect of the centrifugal force on osmotic dehydration of potatoes and apples. Food Research International, 29, 195–199 (1996) 8 KARATHANOS, V. T., KOSTAROPOULOS, A. E. AND SARAVACOS, G. D. Air-drying kinetics of osmotically dehydrated fruits. Drying Technology, 13, 1503–1521 (1995) 9 NEUBER, E. E. Evaluation of critical parameters for developing moisture sorption isotherms of cereal grains. In: ROCKLAND, L. B. AND STEWART, G. F. (Eds), Water Activity: Influences on Food Quality. New York: Academic Press, pp. 199–222 (1981) 10 RIZVI, S. H. AND BENADO, A. L. Thermodynamic properties of dehydrated foods. Food Technology, 38, 83–92 (1984)
11 AOAC. Official Methods of Analysis 14th edn. Washington, D.C.: Association of Official Analytical Chemists (1984) ´ , R., GARCIA, H. S. AND BERISTAIN, 12 AZUARA, E., CORTES C. I. Kinetic model for osmotic dehydration and its relationship with Fick’s Second Law. International Journal of Food Science and Technology, 27, 409–418 (1992) ´ , R. AND GARCIA, 13 BERISTAIN, C. I., AZUARA, E., CORTES H. S. Mass transfer during osmotic dehydration of pineapple rings. International Journal of Food Science and Technology, 25, 576–582 (1990) 14 LOMAURO, C. J., BAKSHI, A. S. AND LABUZA, T. P. Evaluation of food moisture sorption isotherm equations. Part I: fruit, vegetable and meat products. Lebensmittel-Wissenschaft und-Technologie, 18, 111–117 (1985) 15 YANNIOTIS, S. AND ZARMBOUTIS, I. Water sorption isotherms of pistachio nuts. Lebensmittel-Wissenschaft undTechnologie, 29, 372–375 (1996) 16 LAZARIDES, H. N. AND MAVROUDIS, N. E. Kinetics of osmotic dehydration of a highly shrinking vegetable tissue in a salt-free medium. Journal of Food Engineering, 30, 61–74 (1996)
321
Lebensm.-Wiss. u.-Technol., 31, 317–321 (1998)
A Method for Continuous Kinetic Evaluation of Osmotic Dehydration Ebner Azuara*, Cesar I. Beristain and Gustavo F. Gutierrez ´ E. Azuara, C. I. Beristain: Instituto de Ciencias Basicas, ´ Universidad Veracruzana, Apdo. Postal 575, Xalapa, Ver. (Mexico) ´ G. F. Gutierrez: ´ Departamento de Graduados en Alimentos, ENCB-IPN, Apdo. Postal 42186, CP 06470 Mexico, ´ D.F. (Mexico) ´ G. F. Gutierrez: ´ Food Science and Technology Department, University of Reading, Reading RG6 6AP (UK) (Received June 9, 1997; accepted December 1, 1997)
A method to determine the kinetics of osmotic dehydration was developed. The method involves the measurement of weight loss in a single sample and its final moisture at the end of the process. The method was tested in the osmotic dehydration of apple disks of 4.6 cm diameter and 0.4 cm thickness. Temperature was set at 30 °C and a drying solution containing 500 g sucrose/kg was used. The results showed that dispersion of the experimental points was worse when several samples were used to measure average weight loss (14.8%) than when the continuous method was employed (3.9%). The continuous method is useful to predict variations of the final moisture with respect to drying time. ©1998 Academic Press Keywords: mass transfer; continuous method; apple
of the predictions made by the traditional discontinuous method.
Introduction Traditionally, experimental data for mass transfer studies during osmotic dehydration are obtained from a number of foodstuff samples having the same geometry and dimensions, assuming that all the pieces have equal weight, volume and moisture content (discontinuous method). Many authors have used the discontinuous method to follow the kinetics of osmodehydration (1–7). When a number of samples with an average geometry and size are used to determine the kinetics of osmotic dehydration, it becomes necessary to keep a tight control on the degree of ripening of the product. This precaution reduced excessive dispersion of experimental data (8) which could lead to erroneous interpretations. It is difficult to obtain samples with the same degree of ripening; therefore the same sample was used throughout the osmotic dehydration process to prevent the data scattering (9, 10). We have called this approach the continuous method. The purpose of the present work was to study the continuous method to obtain kinetic data for osmodehydration at different times and evaluate the dispersion *To whom correspondence should be addressed.
Materials and Methods Materials Golden apples and refined sugar were purchased in a local market and used in all experiments. The apples were cut into disks of 4.6 cm diameter and 0.4 cm thickness. Immediately after being cut, the apple pieces were immersed in a 2 g/L solution of ascorbic acid. Procedure Apple pieces were osmotically dehydrated at 30 °C in a solution containing 500 g of sucrose/kg. The ratio of foodstuff to solution was greater than 1:20. Samples were withdrawn at periodic intervals during 5 h. Excess solution from the surface was blot dried using paper towels. In the continuous method, each sample was weighed and returned to the osmotic solution to continue the drying process. After 5 h, the moisture content of the sample was determined in a vacuum oven at 70 °C for 24 h (11). Alternatively, for the traditional discontinuous method 17 samples from the same batch were used, and weight loss and moisture in different samples of the same dimensions and geometry
0023-6438/98/040317 + 05 $30.00/0
317
©1998 Academic Press
lwt/vol. 31 (1998) No. 4
were measured. In both methods, each experimental treatment was performed in triplicate runs. S1 =
SG =
s1tWFL °
8
1 + s1t
SG¥ =
Eqn [1]
[(
WFL –1 SG m
[(
(1/b) WFL –1 SG m
s2tSG ° 1 + s2t
ML = WFL – SG
S2 = SG¥
WFL M0
Eqn [3]
SG WFL
[ (
8
s1tWFL¥ 1 –
)]
Eqn [4]
1 + s1t
Eqn [4] can be written in the following form: –1 [(WFL SG ) ]
8
ML =
SG M0
=
[ (
8
8
)]
]
=
M0C0 – MtCf M0
Eqn [12]
M0
Eqn [13]
where M0 = initial weight of the foodstuff (t = 0), Mt = weight of the food at time t, C0 = initial moisture of the foodstuff (wet basis), and Cf = final moisture of the foodstuff (wet basis) at time t. If s1, s2, WFL ° and SG ° are known, it is possible to calculate WFL and SG at any time. Cf can also be predicted using the following equation: Cf =
Linearizing Eqn [4] and Eqn [5]: 1 t t = + SG ML s WFL 1 – SG WFL¥ 1 – 1 ¥ WFL WFL
)
Eqn [11]
M0 (C0 – 1) – Mt (Cf – 1)
Eqn [5]
1 + s2t
[ (
]
Subscript m means that WFL and SG are determined at the last point of the experiment, using the equations of Beristain et al. (13),
Subtracting Eqn [1] from Eqn [2] and rearranging, we obtain:
s2tSG¥
)
Eqn [10]
Eqn [2]
where t = time, s1 = a constant related to water loss, s2 = a constant related to solids gain, WFL = amount of water lost by the foodstuff at time t (fraction, percent, g, or kg), SG = amount of solids gained by the foodstuff at time t (fraction, percent, g or kg), WFL ° = amount of water lost at equilibrium, and SG ° = amount of solids gained at equilibrium. Mass loss (ML) during osmotic dehydration is equal to water lost (WFL) minus solids gained (SG) at the same time.
ML =
Eqn [9]
) ]
(1/p)
8
WFL =
[ (
8
Mathematical modelling Azuara et al. (12) calculated water loss and solid gain during osmotic dehydration using equations with two parameters obtained from mass balances.
(1/b) WFL¥ 1 – SG WFL m
M0C0 – (WFL)M0 Mt
Eqn [14]
Moisture loss (Cf L) was obtained according to the equation:
)] Eqn [6]
700 600
[(
) ]
[(
t WFL –1 SG
) ]
500
Eqn [7]
If we plot t/ML vs. t, we can define b to be the intercept and p the slope of the resulting straight line, then the following equations arise:
t/ML (min/g)
¥
8
2
1 + WFL –1 SG¥ SG
8
t = ML s SG
400 300 200 100 0 –50
8
WFL¥ =
(1/p) SG 1– WFL m
[ ( ) ]
Eqn [8]
0
50
100 150 200 Time (min)
250
300
350
Fig. 1 Plot of (t/ML) vs. t for the continuous method, during osmotic dehydration of apple disks. (s), Sample 1 (r2 = 0.994); (e), sample 2 (r2 = 0.998); (n), sample 3 (r2 = 0.998); (d), average (r2 = 0.998)
318
lwt/vol. 31 (1998) No. 4
CfL = 1 – Cf/C0
Eqn [15]
The criterion used to evaluate goodness of fit was the modulus of the mean relative deviation (P%) between the experimental values and those predicted by equations [6] and [7] as proposed by Lomauro et al. (14): P(%) =
100 n
m
|Mi – Mpi|
i = 1
Mi
Σ
Results and Discussion Eqn [16]
where Mi and Mpi are respectively the experimental and predicted values and n is the number of experimental data points. For each experimental point, the relative % deviation (D) between the mean and the individual values was determined by the method described by Yanniotis and Zarmboutis (15): D =
100 3
3
| xi – x| ¯
i = 1
x¯
Σ
Statistical analysis A two-sample t-test was performed between the means of the mass loss, moisture loss, water loss and solids gain obtained by both methods.
Eqn [17]
where xi is the normalized individual value of mass transfer parameter (mass loss, water loss, solids gain or moisture loss) of the three samples in each time and x¯ is their arithmetic mean.
Figure 1 shows the data obtained using the continuous method for apple disks. It is evident that Eqn [6] and Eqn [7] apply and a good fit is obtained with a r2 of 0.999 and a P value of 2.915%. Calculated average values for WFL ° , SG ° and s1 from the slope and intercept of the straight lines were 0.721 g of water/g of fresh apple, 0.197 g of sucrose/g of fresh apple and 0.032 min–1, respectively. Figures 2–5 depict the kinetics of normalized values of mass loss, water loss, solids gain and moisture loss. These normalized mass transport data are defined as the ratio of the mass loss, water loss, solids gain and moisture loss at t time to the maximum value obtained with each method during osmotic dehydration. The two-sample t-test did not reveal differences between discontinuous and continuous methods at the 5% significance level. On the other hand, Table 1 shows the average relative % deviation
1
1
Normalized moisture loss
(a) 1.2
Normalized mass loss
(a) 1.2
0.8 0.6 0.4 0.2 0 –0.2 –50
0
50
100 150 200 Time (min)
250
300
350
(b) 1.2
0.4 0.2
0
Normalized moisture loss
Normalized mass loss
0.6
50
100
150 200 Time (min)
250
300
350
50
100
150 200 Time (min)
250
300
350
(b) 1.2
1 0.8 0.6 0.4 0.2 0 –0.2 –50
0.8
0
50
100 150 200 Time (min)
250
300
350
Fig. 2 Comparison of the normalized mass loss (NML) obtained from (a) the discontinuous method with NML obtained from (b) the continuous method. (F), Experiment or sample 1; (e), experiment or sample 2; (n), experiment or sample 3; (d), average
1 0.8 0.6 0.4 0.2
0
Fig. 3 Comparison of the normalized moisture loss (NHL) obtained from (a) the discontinuous method with NHL obtained from (b) the continuous method. (F), Experiment or sample 1; (e), experiment or sample 2; (n), experiment or sample 3; (d), average
319
lwt/vol. 31 (1998) No. 4
1
1 Normalized solids gain
(a) 1.2
Normalized water loss
(a) 1.2
0.8 0.6 0.4 0.2
0
0.8 0.6 0.4 0.2
50
100
150 200 Time (min)
250
300
350
0
1
1
Normalized solids gain
(b) 1.2
Normalized water loss
(b) 1.2
0.8 0.6 0.4 0.2
0
50
100
150 200 Time (min)
250
300
350
50
100
200 150 Time (min)
250
300
350
50
100
150 200 Time (min)
250
300
350
0.8 0.6 0.4 0.2
0
Fig. 4 Comparison of the normalized water loss (NWFL) obtained from (a) the discontinuous method with NWFL obtained from (b) the continuous method. For (a), (e), experiment 1; (n), experiment 2; (F), experiment 3. For (b), (F), sample 1; (e), sample 2; (n), sample 3. (d), average
Fig. 5 Comparison of the normalized solids gain (NSG) obtained from (a) the discontinuous method with NSG obtained from (b) the continuous method. For (a), (e), experiment 1; (n), experiment 2; (F), experiment 3. For (b), (F), sample 1; (e), sample 2; (n), sample 3. (d), average
¯ calculated of the arithmetic mean of D. Greater (D) deviation is evident for samples taken with the discontinuous method. The highest deviation values were found at first experimental points. Even though the pieces have the same dimensions and geometry, there may be differences in weight, moisture and initial soluble solids. Practically no dispersion can be observed in the measurements made on the sample used for the continuous method (see Figs 2b, 3b, 4b, 5b and Table 1). The single-sample approach may have additional advantages of time, reagent and labour savings. It is important to notice that, in both methods, water loss and solid gain increased linearly with each other throughout the process. Water loss proceeds parallel to solid uptake, which is in agreement with the results of other workers (16).
Table 1 Average D values calculated for the discontinuous and continuous methods
Conclusions The proposed continuous method to monitor the osmotic dehydration process offers simplified data collection and less scatter. This method may facilitate the interpretation and modelling of the process and makes it easier to predict moisture changes as a function of drying time.
Variable
D 95% confidence interval
Discontinuous
Mass loss Moisture loss Water loss Solids gain
14.8 ±6.5 9.1 ±4.0 8.1 ±4.1 17.8 ±7.1
Continuous
Mass loss Moisture loss Water loss Solids gain
3.9 ±3.5 8.9 ±7.7 4.7 ±2.1 4.7 ±2.1
Method
References 1 PONTING, J. D., WATTERS, G. G., FORREY, R. R., JACKSON, R. AND STANLEY, W. L. Osmotic dehydration of fruits. Food Technology, 20, 125–128 (1966) 2 FARKAS, D. F. AND LAZAR, M. E. Osmotic dehydration of apple pieces: effect of temperature and syrup concentration rates. Food Technology, 23, 688–690 (1969) 3 LENART, A. AND FLINK, J. M. Osmotic concentration of potato I. Criteria for the end point of the osmosis process. Journal of Food Technology, 19, 45–63 (1984) 4 HENG, W., GULBERT, S. AND CUQ, J. L. Osmotic dehydration of papaya: influence of process variables on the quality. Sciences des Aliments, 10, 831–848 (1990)
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