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Modeling of dewatering and impregnation soaking process (osmotic dehydration)
Food Research International 27 (1994) 207-209
Modeling of dewatering and impregnation soaking process (osmotic dehydration) A. L. Raoult-Wack,” G. Rios,b R. Saurel,” F. Giroux~ & S. Guilberf’ “Food Engineering Research Unit, CIRAD-SAR,
34032 Montpellier, France
bUSTL, Place E. Bataillon, 34095 Montpellier, France
Partial dewatering and simultaneous solute impregnation can be obtained by immersion of food pieces in concentrated solutions. Recently, further understanding of cross mass transfer was achieved by experimental study and modelling on agar model gel foods. Keywords: modeling, gel, water, sucrose, mass transfer.
INTRODUCTION
developed: diffusive modeling of interfacial fluxes and interactions involving IPT concepts (RaoultWack, 1991), description of the boundary layer dilution for natural convection (Raoult-Wack, 1991) and inverse fluidization (Raoult-Wack et al., 199 14, and a compartmental model intended for automated control (Raoult-Wack et al., 1991b). Some of the results are reported in the present paper.
The osmotic dehydration process, more recently referred to as dewatering and impregnation soaking process (DIS), consists in placing water-rich food (mainly fruit and vegetables) in concentrated solutions (mainly sugar or salt). Soaking induces two major simultaneous cross flows: a water outflow, from the product into the solution, and a solute entrance from the solution into the product. These cross mass transfer are isothermal, without phase change, and are accompanied by shrinkage. Research works in the field of DIS processes were recently reviewed by Le Maguer (1988) and Raoult-Wack et al. (1992). DIS processes may provide, aside from improved organoleptic qualities, an energy-efficient system for water removal before any further water-activity lowering treatment. Up to now, the poor understanding of the mechanisms involved in the counter-current water and solute transfer inside the complex natural tissue structure has hindered the development of industrial applications of the process. Recently, further understanding of mass transfer phenomena was achieved through experimental study on agar model food (Raoult-Wack, 1991; RaoultWack, 1991a, b, c, d) and various models were
MATERIALS AND METHODS Agar model gel cubes (initial side dimension = a(O)), with initial low solute content, were soaked in higher concentration solutions. The influence of the concentration (denoted WJ and molecular weight of the solute in the soaking solution (glycerol, dextrose, sucrose, PEG 200-200 000), the temperature (30-7O”C), the agar and solute content of the model food (~~(0) and B(O), respectively) on water loss WL and solute gain SG rates (inferred from refractometry, dry matter and weight analysis), as well as the overall volume reduction (pycnometric analysis) and solute distribution in the gel (obtained by coring, slicing and HPLC analysis), were observed. Based on these results, a simple compartmental model of simultaneous water loss (WL) and solute gain (SG), with three adjustable parameters was studied. The model was based on the physical
Food Research International 0963~9969/94l$O7.00 0 1994 Canadian Institute of Food Science and Technology
201
208
A. L. Raoult- Wack et al.
water solute
Fig. 1. Schematic representation of water and solute transfer in the compartmental model. (Raoult-Wack et al., 19926).
representation of the gel cube with two concentric cubic compartments, as shown in Figure 1.
0
I
I
100
200 Time
1
300
(min)
Fig. 3. Experimental (0, WL exp.; 0, SG exp.) and simulated kinetics curves (Raoult-Wack et al., 19926).
RESULTS AND DISCUSSION Performances for WL and SG observed at t = 180 min, i.e. when phase 1 is over in all cases, as a function of initial interface concentration difference, denoted AC, are shown in Figure 2. WL increases markedly when the concentration of the solution increases. At low concentrations, SG is greater than water loss (impregnation situation), but it then reaches a maximum, before decreasing and becoming much lower than water loss (dewatering situation), for higher concentrations. High WL proves thus to be detrimental to SG, which was related to the formation of a sucrose surface layer (Raoult-Wack et al., 1991a, c), at the very beginning of the process. In fact, such a behaviour with clear breaking point between dewatering and impregnation was encountered whatever the studied variable (temperature, molecular weight, agar or solute content) (Raoult-Wack et al., 1991a)
which shows that a good choice of operating parameters may result in good control of mass transport phenomena. Figure 3 shows the compartmental model simulation of mass transfer kinetics for a dewatering situation. Figure 4 shows the corresponding simulated average water and solute concentration levels of each compartment as a function of time, which proved to be consistent with experimental measurements (Raoult-Wack et al., 1991~).
CONCLUSION Studies carried out on model gel made it possible to obtain further understanding of mass transfer on real food, e.g. product’s own solute leaching study (Saurel et al., 1993), equipment design and process control for continuous systems (Giroux, 0.61
0
10
20
30
40
50
I 60
AC(%) Fig. 2. Evolution of WL (0) and SG (Cl) obtained at t = 180 min as a function of AC. Treatment of model food cubes (u(0) = 0.9 cm; B(O) = 10%; ~~(0) = 4%) in sucrose solution (50°C) (Raoult-Wack et al., 1991b).
Time Fig.
(min)
4. Evolution of the sucrose mass fraction in both compartments (Raoult-Wack et al., 19923).
Recent advances in dewatering
1992) or modeling of mass transfer in mixed blends (Bohuon, 1992; Collignan & Raoult-Wack, 1992).
209
Raoult-Wack, A. L., Guilbert, S., Le Maguer, M. & Rios, G. (1991~). Simultaneous water and solute transport in shrinking media-Part 1: Application to dewatering and impregnation soaking process analysis (osmotic dehydration). Drying Technology, 9(3), 589412.
REFERENCES Bohuon, P. (1992). Etude experimentale et modelisation des transferts de mat&e en deshydratation impregnation par immersion-Cas des solutes ioniques. DEA University of Montpellier, Montpellier, France. Collignan, A. & Raoult-Wack, A. L. (1992). Dewatering through immersion in sugar/salt concentrated solutions at low temperature. An interesting alternative for animal foodstuffs stabilisation. In Drying 92, ed. A. S. Mujumdar. Elsevier, London, pp. 1887-97. Giroux, F. (1992). Conception et realisation d’un pro&de automatise de deshydratation impregnation par immersion. PhD thesis ENSIA, Massy, France. Le Maguer, M. (1988). Osmotic dehydration: Review and future directions. In Proc. Sym. on Progress in Food Preservation Processes, Brussels, Vol. 1, pp. 283-309. Raoult-Wack, A. L. (1991). Les proddes de D&hydra&&ion Impregnation par immersion dans des solutions concentrees. Etudes experimentale et modelisation des transferts d’eau et de solute sur aliment modele. PhD thesis, University of Montpellier, Montpellier, France.
Raoult-Wack, A. L., Petitdemange, F., Giroux, F., Rios, G., Guilbert, S. & Lebert, A. (1991b). Simultaneous water and solute transport in shrinking media-Part 2: A compartmental model for the control of dewatering and impregnation soaking processes. Drying Technology, 9(3), 613-30. Raoult-Wack, A. L., Botz, O., Guilbert, S. & Rios, G. (1991~). Simultaneous water and solute transport in shrinking media-Part 3: A tentative analysis of the spatial distribution of the impregnating solute in the model gel. Drying Technology, 9(3), 63Ck42.
Raoult-Wack, A. L., Jourdain, P., Guilbert, S. & Rios, G. (19914. Technique de fluidisation inverse appliqde aux proddes de Deshydratation Impregnation par Immersion (DII) In Rkcents progrks en gknie des pro&d&. Lavoisier Tee. et Dot., Coordonateurs G. Antonini & R. Ben Aim, Paris, France. 5( 14), 329-34. Raoult-Wack, A. L., Guilbert, S. & Lenart, A. (1992). Recent advances in drying through immersion in concentrated solutions. In Drying of Soli&, 4. A. S. Mujumdar. International Science Publishers, New York, pp. 21-51. Saurel, S., Raoult-Wack, A. L., Rios, G., Guilbert, S. Osmotic dehydration of apple. Part 1: Use of response surface methodology to study mass transfer phenomena in fresh plant tissue. Znt. J. Food. Sci. Tech. (submitted).
Modeling of dewatering and impregnation soaking process (osmotic dehydration) A. L. Raoult-Wack,” G. Rios,b R. Saurel,” F. Giroux~ & S. Guilberf’ “Food Engineering Research Unit, CIRAD-SAR,
34032 Montpellier, France
bUSTL, Place E. Bataillon, 34095 Montpellier, France
Partial dewatering and simultaneous solute impregnation can be obtained by immersion of food pieces in concentrated solutions. Recently, further understanding of cross mass transfer was achieved by experimental study and modelling on agar model gel foods. Keywords: modeling, gel, water, sucrose, mass transfer.
INTRODUCTION
developed: diffusive modeling of interfacial fluxes and interactions involving IPT concepts (RaoultWack, 1991), description of the boundary layer dilution for natural convection (Raoult-Wack, 1991) and inverse fluidization (Raoult-Wack et al., 199 14, and a compartmental model intended for automated control (Raoult-Wack et al., 1991b). Some of the results are reported in the present paper.
The osmotic dehydration process, more recently referred to as dewatering and impregnation soaking process (DIS), consists in placing water-rich food (mainly fruit and vegetables) in concentrated solutions (mainly sugar or salt). Soaking induces two major simultaneous cross flows: a water outflow, from the product into the solution, and a solute entrance from the solution into the product. These cross mass transfer are isothermal, without phase change, and are accompanied by shrinkage. Research works in the field of DIS processes were recently reviewed by Le Maguer (1988) and Raoult-Wack et al. (1992). DIS processes may provide, aside from improved organoleptic qualities, an energy-efficient system for water removal before any further water-activity lowering treatment. Up to now, the poor understanding of the mechanisms involved in the counter-current water and solute transfer inside the complex natural tissue structure has hindered the development of industrial applications of the process. Recently, further understanding of mass transfer phenomena was achieved through experimental study on agar model food (Raoult-Wack, 1991; RaoultWack, 1991a, b, c, d) and various models were
MATERIALS AND METHODS Agar model gel cubes (initial side dimension = a(O)), with initial low solute content, were soaked in higher concentration solutions. The influence of the concentration (denoted WJ and molecular weight of the solute in the soaking solution (glycerol, dextrose, sucrose, PEG 200-200 000), the temperature (30-7O”C), the agar and solute content of the model food (~~(0) and B(O), respectively) on water loss WL and solute gain SG rates (inferred from refractometry, dry matter and weight analysis), as well as the overall volume reduction (pycnometric analysis) and solute distribution in the gel (obtained by coring, slicing and HPLC analysis), were observed. Based on these results, a simple compartmental model of simultaneous water loss (WL) and solute gain (SG), with three adjustable parameters was studied. The model was based on the physical
Food Research International 0963~9969/94l$O7.00 0 1994 Canadian Institute of Food Science and Technology
201
208
A. L. Raoult- Wack et al.
water solute
Fig. 1. Schematic representation of water and solute transfer in the compartmental model. (Raoult-Wack et al., 19926).
representation of the gel cube with two concentric cubic compartments, as shown in Figure 1.
0
I
I
100
200 Time
1
300
(min)
Fig. 3. Experimental (0, WL exp.; 0, SG exp.) and simulated kinetics curves (Raoult-Wack et al., 19926).
RESULTS AND DISCUSSION Performances for WL and SG observed at t = 180 min, i.e. when phase 1 is over in all cases, as a function of initial interface concentration difference, denoted AC, are shown in Figure 2. WL increases markedly when the concentration of the solution increases. At low concentrations, SG is greater than water loss (impregnation situation), but it then reaches a maximum, before decreasing and becoming much lower than water loss (dewatering situation), for higher concentrations. High WL proves thus to be detrimental to SG, which was related to the formation of a sucrose surface layer (Raoult-Wack et al., 1991a, c), at the very beginning of the process. In fact, such a behaviour with clear breaking point between dewatering and impregnation was encountered whatever the studied variable (temperature, molecular weight, agar or solute content) (Raoult-Wack et al., 1991a)
which shows that a good choice of operating parameters may result in good control of mass transport phenomena. Figure 3 shows the compartmental model simulation of mass transfer kinetics for a dewatering situation. Figure 4 shows the corresponding simulated average water and solute concentration levels of each compartment as a function of time, which proved to be consistent with experimental measurements (Raoult-Wack et al., 1991~).
CONCLUSION Studies carried out on model gel made it possible to obtain further understanding of mass transfer on real food, e.g. product’s own solute leaching study (Saurel et al., 1993), equipment design and process control for continuous systems (Giroux, 0.61
0
10
20
30
40
50
I 60
AC(%) Fig. 2. Evolution of WL (0) and SG (Cl) obtained at t = 180 min as a function of AC. Treatment of model food cubes (u(0) = 0.9 cm; B(O) = 10%; ~~(0) = 4%) in sucrose solution (50°C) (Raoult-Wack et al., 1991b).
Time Fig.
(min)
4. Evolution of the sucrose mass fraction in both compartments (Raoult-Wack et al., 19923).
Recent advances in dewatering
1992) or modeling of mass transfer in mixed blends (Bohuon, 1992; Collignan & Raoult-Wack, 1992).
209
Raoult-Wack, A. L., Guilbert, S., Le Maguer, M. & Rios, G. (1991~). Simultaneous water and solute transport in shrinking media-Part 1: Application to dewatering and impregnation soaking process analysis (osmotic dehydration). Drying Technology, 9(3), 589412.
REFERENCES Bohuon, P. (1992). Etude experimentale et modelisation des transferts de mat&e en deshydratation impregnation par immersion-Cas des solutes ioniques. DEA University of Montpellier, Montpellier, France. Collignan, A. & Raoult-Wack, A. L. (1992). Dewatering through immersion in sugar/salt concentrated solutions at low temperature. An interesting alternative for animal foodstuffs stabilisation. In Drying 92, ed. A. S. Mujumdar. Elsevier, London, pp. 1887-97. Giroux, F. (1992). Conception et realisation d’un pro&de automatise de deshydratation impregnation par immersion. PhD thesis ENSIA, Massy, France. Le Maguer, M. (1988). Osmotic dehydration: Review and future directions. In Proc. Sym. on Progress in Food Preservation Processes, Brussels, Vol. 1, pp. 283-309. Raoult-Wack, A. L. (1991). Les proddes de D&hydra&&ion Impregnation par immersion dans des solutions concentrees. Etudes experimentale et modelisation des transferts d’eau et de solute sur aliment modele. PhD thesis, University of Montpellier, Montpellier, France.
Raoult-Wack, A. L., Petitdemange, F., Giroux, F., Rios, G., Guilbert, S. & Lebert, A. (1991b). Simultaneous water and solute transport in shrinking media-Part 2: A compartmental model for the control of dewatering and impregnation soaking processes. Drying Technology, 9(3), 613-30. Raoult-Wack, A. L., Botz, O., Guilbert, S. & Rios, G. (1991~). Simultaneous water and solute transport in shrinking media-Part 3: A tentative analysis of the spatial distribution of the impregnating solute in the model gel. Drying Technology, 9(3), 63Ck42.
Raoult-Wack, A. L., Jourdain, P., Guilbert, S. & Rios, G. (19914. Technique de fluidisation inverse appliqde aux proddes de Deshydratation Impregnation par Immersion (DII) In Rkcents progrks en gknie des pro&d&. Lavoisier Tee. et Dot., Coordonateurs G. Antonini & R. Ben Aim, Paris, France. 5( 14), 329-34. Raoult-Wack, A. L., Guilbert, S. & Lenart, A. (1992). Recent advances in drying through immersion in concentrated solutions. In Drying of Soli&, 4. A. S. Mujumdar. International Science Publishers, New York, pp. 21-51. Saurel, S., Raoult-Wack, A. L., Rios, G., Guilbert, S. Osmotic dehydration of apple. Part 1: Use of response surface methodology to study mass transfer phenomena in fresh plant tissue. Znt. J. Food. Sci. Tech. (submitted).