Drying 2004 – Proceedings of the 14th International Drying Symposium (IDS 2004) São Paulo, Brazil, 22-25 August 2004, vol. C, pp. 1604-1611
SHRINKAGE, DENSITY AND POROSITY VARIATIONS DURING THE CONVECTIVE DRYING OF POTATO STARCH GEL
A.H. Al-Muhtaseb, W.A.M McMinn. and T.R.A. Magee Food Process Engineering Research Group, School of Chemical Engineering, Queen’s University Belfast, BT9 5AG, UK, e-mail:
[email protected] Keywords: convective drying, density, porosity, potato starch gel, shrinkage ABSTRACT Quality factors are those that determine the worth, or value, of a food product to the consumer. A thorough understanding of the factors affecting the quality of the product is thus of major relevance. With this in mind, the macro-structural properties (porosity, bulk and particle density) and physical changes (shrinkage) of cylindrical potato starch gel samples during convective drying were examined. Moisture removal was accompanied by the formation of a dense structure, with air temperature imparting a negligible influence. The system also exhibited a non-linear increase in particle density with decreasing moisture content. Drying, however, induced limited internal porosity development, i.e. ε < 10%. Volumetric shrinkage of the potato starch gel samples decreased almost linearly with moisture content. INTRODUCTION Drying of food is a heat and mass transfer operation in a multiphase system and takes place simultaneously with physical and microstructural modifications (Nieto et al., 2003), which affect the quality of the dehydrated product. Quality factors are those that determine the worth, or value, of a food product to the consumer (Aguilera & Stanley, 1999). This broad definition encompasses many different factors, including colour, texture, shape, size, porosity, density and shrinkage (Krokida & Maroulis, 2001). Therefore, examination of the physical properties of foods, and their responses to process conditions, is crucial for analysis of the drying process (Tsen & King, 2002). Bulk and particle densities are vital parameters in the design, modelling and optimisation of food processing operations because they have a direct affect on the thermo-physical properties of food materials (Rahman, 1995). Density characteristics are influenced by the method and rate of drying, where slow drying yields a fully consolidated product and accelerated drying results in a solid comprised of cracks and holes with a lower density (Brennan, 1990). 1604
Wang and Brennan (1995) studied the effect of moisture content and temperature on the bulk density of potato. In the early stages of drying the density increased as the moisture content decreased and, after reaching a maximum, decreased with further decrease in moisture content. The density, at a given moisture content, was also found to decrease with increasing drying temperature. Similar results were reported by McMinn and Magee (1997) and Hatamipour and Mowla (2003) in the drying of potato starch gel and maize starch, respectively. However, Marousis and Saravacos (1990) observed different trends during the drying of granular and gelatinized starch materials. The bulk densities were found to decrease linearly with decreasing moisture content, while the particle densities increased as the moisture content decreased and, after reaching a maximum, decreased with further decrease in moisture content. Aydin (2003) proposed a first order correlation to evaluate the experimental bulk and particle densities of nut and kernel. Marousis and Saravacos (1990) proposed a fourth degree polynomial relationship to predict the particle density of corn starch as a function of moisture content. A power law relationship was also established to characterize the bulk density data of maize starch (Hatamipour and Mowla, 2003). A further property assuming fundamental importance in food processing operations is internal porosity (McMinn, 1996). It plays a major role in the reconstitution of the dried products, effectively controlling the speed of rewetting as well as taste and appearance (Krokida et al., 1997). The development of porosity during drying depends on the initial moisture content, composition and size (i.e. thickness or diameter) of the food material, as well as the type of drying and drying conditions (Madamba et al., 1994). In general, internal porosity increases as drying proceeds, with each material presenting a distinctive functional relation with moisture content (Madamba et al., 1994). Internal porosity also plays an important role in the prediction of the diffusional properties of food materials (Rotstein, 1987). Saravacos et al. (1990) and Leslie et al. (1991) studied the effect of porosity on the effective diffusivity of granular and gelatinized starch. The three-dimensional network, or close-knit gel matrix, was found to trap the water and slow down the diffusion process. The granular starch, however, incurred enhanced pore formation, thus allowing water molecules to migrate more easily (Vagenas & Karathanos, 1993). One of the most important physical changes that the food undergoes during drying is a reduction in external volume. Loss of water and heating cause stresses in the cellular structure of the food, leading to a change in shape and a decrease in dimensions, which adversely affect the quality of the dehydrated product (Mayor & Sereno, 2003). Shrinkage is inter-related to the internal porosity characteristics and takes place simultaneously with moisture diffusion. It affects the rate of drying as well as the physical and functional properties of the product (Aguilera & Stanley, 1999). It is evident from the foregoing that a thorough understanding of the factors affecting the quality of the product is thus of major relevance. With this in mind, the macro-structural properties (porosity, bulk and particle density) and physical changes (shrinkage) of potato starch gel were examined. MATERIALS AND METHODS Cylindrical potato starch gel samples (initial moisture content 2.0 kgkg-1, dry solid; radius 13.5mm and length-to-radial ratio 3:1) were dried in an experimental air tunnel dryer according to the procedure outlined in Al-Muhtaseb (2003). The experimental conditions used were (air temperatures of 30 – 60°C, and air velocity of 3ms-1). The samples were dried, under the required conditions, for a specified time period in order to reach the desired moisture content. Each experimental run was performed in triplicate. The change in sample dimensions was determined by measuring the sample diameter and length at several positions, around the sample circumference and along the length, respectively, using sliding vernier callipers. An average of ten measurements was taken. The change in radial and longitudinal dimensions were evaluated using: % change in radial dimension =
R0 − R f R0
× 100
(1)
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% change in longitudinal dimension =
l0 − l f l0
× 100
(2)
A mathematical model proposed by Zogzas et al. (1994) was used to determine the bulk (ρb) and particle (ρp) densities as a function of moisture content:
ρb =
1+ X 1 / ρ bo + β ' X / ρ w
(3)
ρp =
1+ X 1 / ρs + X / ρw
(4)
The dry solid bulk density (ρbo) and bone dry solid density (ρs) of potato starch are 1500 kgm-3 and 1610 kgm-3, respectively, and the volume shrinkage coefficient (β’) of potato starch gel (including air) is 1.03 for convective air drying (Saravacos, 1998). The internal porosity (ε) and volumetric shrinkage (Sb) were determined using equation (5) and equation (6), respectively (Wang & Brennan, 1995).
ρb ρp
(5)
Vt ρ (1 + X ) = b Vo ρ o (1 + X o )
(6)
ε =1−
Sb =
RESULTS AND DISCUSSION
Bulk Density The experimental bulk density data for potato starch gel, determined using equation 3 at various air drying temperatures, is shown as a function of dimensionless moisture content in Figure 1. An increase in bulk density with decreasing moisture content can be observed. The negative relationship of bulk density with moisture content can be explained based on the density of the sorbed water. Bound water interacting with the solid matrix exhibits a higher density than free water at the surface of the solid, due to the higher solid bonding energy. As drying proceeds, the free water evaporates and the proportion of bound water within the solid matrix increases, inducing an increase in bulk density (Marousis & Saravacos, 1990). Similar observations were reported by Wang and Brennan (1995), McMinn (1996) and Aydin (2003) for the drying of potato, potato starch gel and kernel samples, respectively. As shown in Figure 1, the bulk density of potato starch gel exhibits a negligible dependence on air temperature. The experimental bulk density data were correlated with the dimensionless moisture content according to a power law correlation (Figure 1) (Table Curve 2D, Jandel Scientific).
ρ b = 455 + 644 ( X / X o )
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−0.168
(7)
This functional form was selected on the basis of the coefficient of determination (r2). Comparison between the calculated and experimental values gives a maximum relative percentage deviation, E, of 0.595%. A power law correlation was also reported by Hatamipour and Mowla (2003) for maize.
1400 -3
Particle Density (kgm )
-3
Bulk Density (kgm )
1350 1300 1250 1200 1150 1100
1300 1200 1100 1000
1050 0.0
0.2 30°C
0.4
X/Xo
45°C
0.6 60°C
0.8
0.0
1.0
0.4
0.6
0.8
1.0
X/Xo 30°C
Pred.
Figure 1. Variation of bulk density of potato starc gel with dimensionless moisture content at various air temperatures.
0.2
45°C
60°C
Pred.
Figure 2. Variation in particle density of potato starch gel with dimensionless moisture content at various air temperatures.
Particle Density The potato starch particle density characteristics were analysed by adopting the model proposed by Zogzas et al. (1994) (equation 4). The non-linear particle density variation with moisture content, at various air temperatures, is illustrated in Figure 2. The particle density shows a progressive increase in the early stages of drying, followed by a more significant increase in the latter stages. At higher moisture contents, the density of the sorbed water approaches that of liquid water, causing an expansion (swelling) of the starch granules and a decrease in the particle density. Similar particle density values for starch materials were reported by Marousis & Saravacos (1990), McMinn (1996), Donsi et al. (1996) and Krokida et al. (2001). It can also be seen that air temperature has a negligible affect on the particle density of potato starch gel. The experimental particle density data were correlated with the dimensionless moisture content according to the power law correlation:
ρ p = 488 + 655( X / X o )
−0.187
(8)
A maximum relative percentage deviation, E, of 0.662% was calculated.
Internal Porosity The internal porosity characteristics of potato starch gel as a function of dimensionless moisture content are illustrated in Figure 3. The internal porosity exhibits a non-linear variation with moisture content, as manifested in the form of an exponential function. The moisture removal facilitates pore formation and thus, an open structure. However, the porosity development during drying is relatively insignificant in magnitude, i.e. ε < 10%. This behaviour is characteristic of a gelatinized system, when most of the existing pores disappear during gelatinisation (Karathanos & Saravacos, 1993). Marousis and Saravacos (1990) stated that high amylopectin starch gels, such as potato, have a high molecular weight with a high degree of branching and thus behave like viscoelastic solids. The viscoelastic nature of the solid tends to reduce the sample rigidity and restrict the formation of cracks.
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1.0
ε = 0.038+0.03 exp (-X/0.328)
0.8
r2 = 0.998, E % = 0.492
0.05
0.6 Sb
Internal-Pore Porosity
0.06
0.4
0.04
Sb = 0.734(X/Xo) + 0.267
0.2
0.03 0.0
0.2
0.4
0.6
0.8
0.0
1.0
0.0
X/Xo 30°C
45°C
60°C
r2 = 1, E% = 0.002
Pred.
0.2 30°C
Figure 3. Internal porosity development during potato starch gel drying at various air temperatures.
0.4
X/Xo
45°C
0.6
0.8 60°C
1.0 Pred.
Figure 4. Volumetric shrinkage of potato starch gel at various air temperatures.
Volumetric Shrinkage Shrinkage of food materials during air drying adversely affects the quality of the dried products, and despite its technological and economic importance, it is not well understood (Aguilera & Stanley, 1999). There is a strong relation between moisture content and shrinkage, however, no standard methodology has been established for theoretical determination of this physical property (Zogzas et al., 1994). On observation of the potato starch gel samples throughout the drying process, the shrinkage was found to be non-homogeneous. In the early stages of drying the sample edges dry and hence, establish a degree of rigidity. Subsequently, as drying proceeds, the cells shrink in size and the circumferential surfaces are drawn in, as the interior section looses moisture. Hence, the sample exhibits a concave appearance. This behaviour reflects the elasticity properties of the cellular structure (McMinn, 1996). A graphical representation of the shrinkage behaviour of potato starch gel is shown in Figure 4. The experimental shrinkage data show a linear behaviour with moisture content, with a reduction in sample volume with decreasing moisture content. The linear behaviour of shrinkage with respect to moisture content was also reported by a number of researchers (Lozano et al., 1980, 1983; Madamba et al., 1994; Wang and Brennan, 1995; McMinn and Magee, 1997). It can be seen from Figure 4 that air temperature has a negligible affect on the bulk shrinkage of potato starch gel. Similar observations were found by Tong and Lund (1990), who reported that air temperature variation has a negligible affect on the shrinkage of biscuit and bread, and by Ratti (1994), who stated that the negligible temperature affects on the bulk shrinkage of potato, apple and carrot could be attributed to the temperature dependence of elastic and mechanical properties. CONCLUSIONS
• • • •
Moisture removal was accompanied by the formation of a dense structure. Air temperature imparted a negligible influence. Potato starch gel samples exhibited a non-linear increase in particle density with decreasing moisture content. Drying induced limited internal porosity development, i.e. ε < 10%. The volumetric shrinkage of potato starch gel decreased almost linearly with moisture content. NOTATION
l0 lf E 1608
initial sample half-length (m) final sample half-length (m) mean relative percentage deviation modulus (%)
R0 Rf r2 Sb X Xo Va Vt
initial sample radius (m) final sample radius (m) coefficient of determination shrinkage coefficient moisture content at a given time t (kgkg-1, dry solid) initial moisture content (kgkg-1, dry solid) initial sample volume (m3) volume of sample at a given time t (m3)
Greek Symbols
ε ρo ρb ρbo ρp ρs ρw β’
internal porosity initial bulk density (kgm-3) bulk density (kgm-3) bulk density of the sample at moisture content X = 0 (kgm-3) particle density (kgm-3) bone dry solid density (kgm-3) water density (kgm-3) volume shrinkage coefficient LITERATURE
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Leslie, R.B., Carillo, P.J., Chung, T.Y., Gilbert, S.G., Hayakawa, K., Marousis, S., Saravacos, G.D. and Solberg, M. (1991), Water diffusivity in starch-based systems, In: Water Relationships in Food, Levine, H. and Slade, L. (ed), Plenum Press, New York. pp. 365-390 Lozano, J.E., Urbicain, M.J. and Rotstein, E. (1980), Total porosity and open-pore porosity in the drying of fruits, Journal of Food Science, 45, pp. 1403-1407 Lozano, J.E., Rotstein, E. and Urbicain, M.J. (1983), Shrinkage, porosity and bulk density of foodstuffs at changing moisture contents, Journal of Food Science, 48, pp. 1497-1502 Madamba, P.S., Driscoll, R.H. and Buckle, K.A. (1994), Shrinkage, density and porosity of garlic during drying, Journal of Food Engineering, 23, pp. 309-319 McMinn, W.A.M. (1996), Transport and thermophysical property variations during the convective drying of starch materials, Doctoral thesis, Queen’s University Belfast, UK. McMinn, W.A.M. and Magee, T.R.A. (1997), Physical characteristics of dehydrated potatoes-Part I, Journal of Food Engineering, 33, pp. 37-48 Marousis, S.N. and Saravacos, G.D. (1990), Density and porosity in drying starch materials, Journal of Food Science, 55, pp. 1367-1372 Mayor, L. and Sereno, A.M. (2003), Modelling shrinkage during convective drying of food materials: a review, Journal of Food Engineering, In Press. Nieto, A.B., Salvatori, D.M., Castro, M.A. and Alzamora, S.M.,(2003), Structural changes in apple tissue during glucose and sucrose osmotic dehydration: shrinkage, porosity, density and microscopic features, Journal of Food Engineering, In Press. Rahman, M.S. (1995), Food Properties Handbook, CRC Press, Inc. New York. Ratti, C. (1994), Shrinkage during drying of foodstuffs, Journal of Food Engineering, 23, pp. 91-105 Rotstein, E. (1987), The prediction of diffusivities and diffusion-related properties in the drying of cellular foods, In: Physical Properties of Foods, Jowitt, R., Escher, F., Kent, M., McKenna, B.M. and Roques, M. (ed), Elsevier Applied Science Publ., London. pp. 131-145 Saravacos, G.D., Karathanos, V.T., Marousis, S.N., Drouzas, A.E. and Maroulis, Z.B. (1990), Effect of gelatinisation on the heat and mass transport properties of starch materials, In: Engineering and Food, Vol. 1, Spiess, W.E.L. and Schubert, H. (ed), Elsevier Applied Science, London, pp. 390-398 Saravacos, G.D. (1998), Physical aspects of food dehydration, In: Drying 98, Vol. 1, Akritidis, A., Marinos-Kouris, C.A. and Saravacos, G.D. (ed), ziti Publ., Greece. pp. 35-46 Tong, C.H. and Lund, D.B. (1990), Effective moisture diffusivity in porous materials as a function of temperature and moisture content, Biotechnology Progress, 6, pp. 67-75 Tsen, J.H. and King, V.A.E. (2002), Density of banana puree as a function of soluble solids concentration and temperature, Journal of Food Engineering, 55, pp. 305-308 1610
Vagenas, G.K. and Karathanos, V.T. (1993), Prediction of the effective moisture diffusivity in gelatinized food systems, Journal of Food Engineering, 18, pp. 159-179 Wang, N. and Brennan, J.G. (1995), Changes in structure, density and porosity of potato during dehydration, Journal of Food Engineering, 24, pp. 61-76 Zogzas, N.P., Maroulis, Z.B. and Marinos-Kouris, D. (1994), Densities, shrinkage and porosity of some vegetables during air drying, Drying Technology, 12, pp. 1653-1666
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