Physical Properties and Fluidization Behaviour of Fresh Green Bean Particulates During Fluidized Bed Drying

0960±3085/00/$10.00+0.00 # Institution of Chemical Engineers Trans IChemE, Vol 78, Part C, March 2000

PHYSICAL PROPERTIES AND FLUIDIZATION BEHAVIOUR OF FRESH GREEN BEAN PARTICULATES DURING FLUIDIZED BED DRYING W. SENADEERA, B. BHANDARI, G. YOUNG and B. WIJESINGHE* Food Science and Technology, School of Land and Food Sciences, The University of Queensland, Australia *Centre for Food Technology, Department of Primary Industries, Australia

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hanges in the physical properties (such as particle density, bulk density of the bed, shrinkage and bed porosity) of fresh green bean particulates were investigated during drying. Three length:diameter ratios (1:1, 2:1 and 3:1) were considered, using drying conditions of 50 2 C and 13 2% relative humidity in a heat pump dehumidi®er system. The ¯uidization behaviour was also evaluated at 10 levels of moisture content. The ¯uidization experiments demonstrated that the minimum ¯uidization velocity decreases as the drying proceeds due to the reduced moisture content and changes in the physical properties of the bean particulates. Empirical relationships of the following nature were developed for the change in shrinkage ‰VR ˆ 1 ¡ Be¡kMR Š, particle density ‰rp ˆ A ‡ BMR ‡ C exp…¡D MR†Š; bulk density ‰rb ˆ a1 ‡ b1 MR ‡ c1 MR2 Š and bed porosity ‰e ˆ a2 ‡ b2 MR ‡ c2 MR2 Š with the moisture content during ¯uidized bed drying. Keywords: green beans; drying; ¯uidization; shrinkage; physical properties

through beds, storage and packing volume, and heat transfer. These properties vary during drying due to moisture removal, structural shrinkage and internal collapse. Bed porosity changes as a result of deformations of the overall dimensions, as well as change in particle density7a,b. By selecting a suitable drying method, ®nal product porosity can be controlled8. To predict the changes in physical properties of various fruits and vegetables during drying, many researchers have published empirical models based on experimental observations5,7a,±13. There are also models based on mass and volume relations7a,b,12,14,15. It is evident in these papers that the change in physical properties is a function of the type of produce. Different products have speci®c cellular structure and surface (skin) characteristics. This investigation of green beans provides additional information in this context.

INTRODUCTION Drying is a major operation in the food industry, consuming large quantities of energy, and the use of dried foods is expanding rapidly. Dried foods are stable under ambient conditions, easy to handle, and can be easily incorporated during food formulation and preparation. The drying operation is used either as a primary process for preservation, or as a secondary process in certain product manufacturing operations. It is a complex process and involves simultaneous mass and heat transfer, accompanied by physical and structural changes1. Quality of food materials undergoing drying depends on their initial quality and changes occurring during drying2. Shape and size of the products change appreciably, in¯uencing their physical properties such as bulk density, particle density and porosity, which in turn modify ®nal texture and transport properties of the dry foods3. Shrinkage is one of the major changes which takes place during the drying process. This change in volume of the food particulate is mainly as a result of removal of water4. Shrinkage is important to consider as it in¯uences several other physical properties, such as bulk density and particle shape and density, and can also cause stresses, which may result in cracks in the internal cellular structure5. Shrinkage also in¯uences moisture removal rate during the drying process and rehydration properties6. Bulk density, particle density and bed porosity are important parameters in designing equipment pertaining to the food industry, affecting areas such as ¯ow resistance

EXPERIMENTAL Preparation of Cut Green Beans Fresh green beans, Phaseolus vulgaris, of the variety Labrador, were used. Beans of similar maturity were purchased from the same supplier to maximize reproducibility of results. Care was taken, when selecting the size of beans, to obtain a consistent diameter of 10 1 mm. Size (diameter and length) was measured using vernier callipers, to an accuracy of 0.05 mm. Both ends of the beans were trimmed off and only the middle portion, which resembled a 43

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SENADEERA et al.

cylindrical shape, was used for the required samples. Samples were prepared for three length:diameter ratios of 1:1, 2:1 and 3:1. After the beans were prepared, they were stored in a sealed container for more than 12 hours in a cold room at 4 C to stabilize moisture content before experimentation. Drying Drying experiments were carried out in a ¯uidized bed dryer connected to a heat pump dehumidi®er system using a drying air temperature of 50 2 C and relative humidity of 13 2%. During drying, samples were taken from the dryer at several arbitrary moisture levels for physical property measurements. Each experiment was repeated three times. Separate samples were dried using the same drying conditions to evaluate ¯uidization behaviour. Materials were placed inside the drying system on mesh trays and dried using a cross-¯ow air ¯ow at 50 C. During drying, samples were taken from the dryer at ten arbitrary moisture levels and used for ¯uidizing experiments. Physical Property Measurements Bulk density …rb † was calculated by measuring volume and weight of representative bean samples. Volume was determined by loosely ®lling a 250 ml measuring cylinder up to the 100 ml mark. Weight of the material was measured using a Sartorious electronic balance with an accuracy of 0.001 g. From each sample, an average of three readings was recorded. For the determination of particle density …rp †, a certain number of particles were weighed by a Sartorious electronic balance. Volume was measured by the difference in meniscus levels before and after immersion of particles in liquid paraf®n in a measuring cylinder. Meniscus difference was measured using vernier callipers (accuracy 0.05 mm). All the measurements were taken quickly to minimize any absorption of paraf®n into the product. To observe volume changes, volume ratio was calculated by dividing volume by initial volume. The bed porosity …e† was calculated based on the measured bulk and particle density values, by applying the following equation16: r ˆ1¡ b …1† rp

Figure 1. Schematic diagram of batch ¯uidized bed system used in this study.

This also helped to minimize edge effects inside the ¯uidizing column. Air¯ow entering the bed was controlled using a valve. Velocity of the incoming air in the duct was read from a digital manometer (EMA 84, range 0±10 kPa) connected to a pitot tube (Dwyer DS-300). Air velocity in the chamber was calculated from the measured velocity in the duct. Pressure drop across the bed was measured using a U-tube manometer connected to the drying chamber below the air distributor plate and above the material. The used samples were collected and dried further for experimentation at other moisture contents. The experimental procedure is given in Figure 2. Model accuracy was tested by calculating mean absolute error percentage (MAE%) given by the equation (2) according to the methods by Mayer and Butler18 for shrinkage, bulk density and bed porosity.

MAE% ˆ

n jy 100 P obs ¡ yprej j yobs j n i

…2†

Moisture content was determined by measuring the loss in weight of the ®nely chopped sample desiccated in a vacuum oven at 70 C and ¡13.3 kPa vacuum for 24 h17. Fluidization All ¯uidization trials were conducted in a batch-type plexi-glass ¯uidizing column of 185 mm inside diameter and 1 m height. Air was supplied from a heat pump dehumidi®er coupled to the dryer (see Figure 1). Fluidizing air (50 C) entered the material bed through a perforated plate having 18 holes cm ¡2 . Each hole was 1 mm in diameter. An even air distribution across the material was achieved by placing another perforated plate with 10 mm diameter holes, 10 mm vertically below the perforated plate.

Figure 2. Schematic diagram of volume shrinkage behaviour of fresh cut green beans during drying at three length diameter ratios.

Trans IChemE, Vol 78, Part C, March 2000

PHYSICAL PROPERTIES AND FLUIDIZATION BEHAVIOUR OF GREEN BEAN PARTICULATES

EXPERIMENTAL RESULTS AND DISCUSSION Shrinkage Behaviour Figure 3 is a diagrammatic representation of the changes at various moisture ratio values for different L:D ratios. It was observed that the shrinkage was not a perfectly homogenous phenomenon, which was also observed in potatoes4. Changes in volume and shape were different for different L:D ratios. The two ends closed as the beans shrunk due to dehydration. This closure of ends was more evident as the L:D ratio increased. The skin surface of the beans was wrinkled, which suggests that the internal tissue structure of the skin has some plasticity to resist cracking and that the skin surface remains relatively unaffected as compared to the total volume shrinkage of the particulate. The combination of shrinkage and wrinkled skin surface is expected to affect the bulk properties (such as bulk density, porosity) as well as the ¯uidization properties of the dried beans. At the initial stage of drying, beans retained their smooth surface, though some reduction in volume was observed (see Figure 3). The shrinkage was more evident below approximately 80% (wet basis) moisture. The shrinkage behaviour (volume ratio) of the beans was correlated to moisture ratio using an equation of the form: VR ˆ 1 ¡ Be¡kMR

Trans IChemE, Vol 78, Part C, March 2000

Table 1. Estimated parameters of equation (3), the relationship of volume ratio to moisture content. L:D

B

k

Correlation coef®cient (r2)

MAE%

1:1 2:1 3:1

0.9318 0.9304 0.9168

1.039 1.2797 1.3321

0.98 0.95 0.94

9.18 7.88 9.80

due to the skin effect and heterogenous internal structure of the beans as compared to other materials described in the literature. In equation (3), at zero moisture content, volume ratio (VR) would equal (1-B). The constant B is directly related to the maximum shrinkability of the product. In Table 1, it can be seen that the variability of parameter B is relatively low, indicating that the total shrinkability of the product at very low moisture value tends to be independent of lengthdiameter ratios. Therefore, for general prediction purposes, an average of the B value can be considered. The value k is a measure of the rate of change of shrinkage with moisture reduction. Increased k value means lower rate of shrinkage. As demonstrated in Table 1, the k value decreased as the L:D ratio increased, which shows that the rate of shrinkage is lower as the length of the beans is decreased.

…3†

where VR ˆ volume ratio, B ˆ constant, k ˆ constant, MR ˆ moisture ratio (db). A non-linear regression procedure was used to estimate the parameters in equation (3) for different L:D ratios19. The estimated parameters are presented in Table 1. The shrinkage measured in this research followed an exponential curve. To the contrary, most of the models reported by other researchers for various other food materials were linearly correlated with the moisture ratio3,10,11. This means that in their experiments the rate of shrinkage was constant with moisture loss, which is in contradiction to the results obtained in the case of beans. This is probably

Figure 3. Experimental ¯ow chart.

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Particle Density (rp ) Figure 4 demonstrates the variation of particle density with moisture ratio. The particle density of beans increased

Figure 4. Particle density variation of green beans for different L:D ratios ( experimental Ð model), [ (a) 1:1 (b) 2:1 (c) 3:1].

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SENADEERA et al. Table 2. Prediction equations for changes in particle density of green beans during ¯uid bed drying. L:D

Equation

1:1 2:1 3:1

rp ˆ 0:9955 ¡ 0:0391 MR ‡ 1:0499 exp …¡408:49 MR† rp ˆ 0:9649 ¡ 0:2321 MR ‡ 0:1449 exp …¡26:6705 MR† rp ˆ 1:0189 ¡ 0:09275 MR ¡ 0:245 exp …¡27:43685 MR†

with decreasing moisture content. However, in the case of L:D ratio of 3, this value decreased at low moisture levels (see Figure 4(c)). The constant C of the exponential part of the equation ‰r ˆ A ‡ BMR ‡ C exp…¡DMR†Š is decreasing as the L:D ratio increased (see Table 2). This indicates that towards the end of the drying, L:D ratio affects the particulate density. This trend could not be explained and may be due to an increase in internal porosity. This behaviour was similar to that in other studies of fruits and vegetables7b. Particle density of the initial fresh material ranged from 943 kgm¡3 (965% db moisture) to 964 kgm¡3 (1035% db moisture) and ®nal dry material density ranged from 779 kgm¡3 (3.5% db moisture) to 1193 kgm¡3 (4.1% db moisture). McMinn and Magee10 also reported a decrease in particle density (increase in internal porosity) of potato in the ®nal dried product, though there was an increase of density during the early stages of drying. On the other hand, Medeiros and Sereno20 ®tted a nonlinear model characterizing the change in particle density with change in moisture during drying for peas. The change of particle density were correlated with the moisture ratio with the non-linear model ‰r ˆ A ‡ BMR ‡ C exp…¡DMR†Š (see Table 2). A non-linear regression procedure NLIN was used to estimate the parameters in Table 2 for different L:D ratios19. A similar model was found to describe the change in particle density with moisture content of sweet potato, potato, carrot, pears and garlic7b. Comparing the results from the published reports, it can be assumed that the beans also tend to behave similarly with respect to the relationship between particle density and moisture content.

The variations in bulk density were correlated to moisture ratio using quadratic equations ‰rb ˆ a1 ‡ b1 MR ‡ c1 MR2 Š, and are presented in Table 3. GLM regression procedure was used to estimate the parameters of the equations in Table 3 for different L:D ratios using SAS19. The bulk density of the fresh beans for all L:D ratios reached a maximum early in the drying period, and then decreased as the material dried further. The bulk density of dried material during the later stages of drying was much lower than that of fresh material. A higher density for fresh material was also reported for garlic slices13a. There was an increase in bulk density early

MAE

0.93 0.99 0.98

0.06 1.12 2.13

in the drying process for beans. At the earlier stages of drying, there is a uniform shrinkage without any irregularities on the surface, which contributed to the increase in the bulk density as the particles could more easily pack together. However, further drying resulted in a wrinkled surface and twisting and deformation of the shape, increasing the interparticulate air space (see Figure 2). This reduced the density of the bulk material.

Bed Porosity (ee) The variations in the bed porosity were correlated to moisture ratio with quadratic equations for different L:D ratios, these equations ‰e ˆ a2 ‡ b2 MR ‡ c2 MR2 Š are shown in Table 4. GLM regression procedure was used to estimate the parameters of the equations using SAS19. Bed porosity attained a minimum value early in the drying period for each L:D ratio. Bed porosity is related to the particle density and bulk density of the material (see equation (1)), which means that at a given particle density the increase in bulk density will inversely affect the porosity and vice-versa. The trends of bulk density of the bed and bed porosity are opposite. Table 4. Prediction equations for the change in bed porosity of fresh green beans during drying. L:D

Bulk Density of the Bed (rb )

Correlation coef®cient (r2)

1:1 1:1 3:1

Equation 2

e ˆ 0:77 ¡ 0:86 MR ‡ 0:58 MR e ˆ 0:75 ¡ 0:71 MR ‡ 0:49 MR2 e ˆ 0:77 ¡ 0:68 MR ‡ 0:46 MR2

Correlation coef®cient (r2)

MAE%

0.95 0.98 0.95

7.37 1.58 2.60

Table 3. Prediction equations for changes in bulk density of green beans during ¯uid bed drying.

L:D 1:1 2:1 3:1

Equation 2

rb ˆ 251:0 ‡ 682:8 MR ¡ 426:2 MR rb ˆ 217:1 ‡ 686:2 MR ¡ 452:2 MR2 rb ˆ 198:1 ‡ 712:0 MR ¡ 473:5 MR2

Correlation coef®cient (r2)

MAE%

0.97 0.91 0.95

3.34 9.06 6.53

Figure 5. Effect of moisture content on the minimum ¯uidization velocity …Umf † of green beans for different L:D ratios at the constant bed height of 100 mm.

Trans IChemE, Vol 78, Part C, March 2000

PHYSICAL PROPERTIES AND FLUIDIZATION BEHAVIOUR OF GREEN BEAN PARTICULATES

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Minimum Fluidization Velocity (Umf )

REFERENCES

The minimum ¯uidization point was identi®ed by visual observation of the bed, determined as when the bed started expanding followed by instantaneous ¯uidization. This value was con®rmed by noting the variation in bed pressure drop with velocity. The variation of the minimum ¯uidization velocity with moisture content is shown in Figure 5 for all L:D ratios, and for a constant bed height of 100 mm. Slugging and channelling were common phenomena for the beans at higher moisture levels. Also, as L:D ratio increased, the effect of channelling and slugging became more prominent. Relatively good quality ¯uidization occurred below the critical moisture values; 32% (wb) for L:D ratio 3:1, 52% (wb) for L:D ratio 2:1 and 60% (wb) for L:D ratio 1:1. In Figure 5, it can be seen that for L:D ratio 3:1, no ¯uidization was observed until the moisture level was reduced down to about 500% dry basis (83% wb). The reduction in the minimum ¯uidization velocity during drying might not only be due to the reduced moisture content. Other physical changes (such as geometrical shape and dimension) could also have contributed to this effect.

1. Fusco, A.J., Avanza, J.R., Aguerre, R.J. and Gabritto, J.F., 1991, A diffusion model for drying with volume change, Drying Technology, 9(2): 397±417. 2. Karel, M., 1991, Physical structure and quality of dehydrated food, in Drying ’91, Mujumdar, A.S. and Filkova, I. (eds) (Elsevier Science Publishers, Amsterdam) pp. 26±35. 3. Ratti, C., 1994, Shrinkage during drying of foodstuffs, J Food Eng, 23: 91±105. 4. Khraisheh, M.A.M., Cooper, T.J.R. and Magee, T.R.A., 1997, Shrinkage characteristics of potato dehydrated under combined microwave and convective drying conditions, Drying Technology, 18(31022. 5. Balaban, M., 1989, Effect of volume change in foods on the temperature and moisture content predictions of simultaneous heat and moisture transfer models, J Food Process Eng, 12: 67±88. 6. Suarez, C. and Viollaz, P.E., 1991, Shrinkage effect on drying behaviour of potato slabs, J Food Engineering, 13: 103±114. 7a. Lozano, J.E., Rotstein, E. and Urbicain, M.J., 1980, Total porosity and open pore porosity in the drying of fruits, J Food Science, 45: 1403± 1407. 7b Lozano, J.E., Rotstein, E. and Urbicain, M.J., 1983, Shrinkage, porosity and bulk density of foodstuffs at changing moisture content, J Food Science, 48:1497±1553. 8. Krokida, M.K. and Maroulis, Z.B., 1997, Effect of drying method on 7. shrinkage and porosity, Drying Technology, 15(10): 2441±2458. 9. Chou, S.K., Hawlader, M.N.A. and Chua, K.J., 1997, On the drying of food products in a tunnel dryer, Drying Technology, 15(3880. 10. McMinn, W.A.M. and Magee, T.R.A., 1997, Physical characteristics of dehydrated potatoes ± Part I, J Food Eng, 33: 37±48. 11. Wang, N. and Brennan, J.G., 1995, Changes in structure, density and porosity of potato during dehydration, J Food Eng, 24: 61±76. 12. Zogas, N.P., Maroulis, Z.B. and Marinos-Kouris, D., 1994, Densities, shrinkage and porosity of some vegetables during air drying, Drying Technology, 12(7): 1653±1666. 13a. Madamba, P.S., Driscoll, R.H. and Buckle, K.A., 1994a, Bulk density, porosity and resistance to air ¯ow of garlic slices, Drying Technology, 12(4): 937±954. 13b. Madamba, P.S., Driscoll, R.H. and Buckle, K.A., 1994b, Shrinkage, density and porosity of garlic during drying, J Food Eng, 23: 309±319. 14. Suzuki, K., Kubota, K., Kasegawa, T. and Hosaka, H., 1976, Shrinkage 13. in dehydration of root vegetables, J Food Science, 41: 1189±1193. 15. Rahman, M.D.S. and Driscoll, R.H., 1994, Density of fresh and frozen seafood, J Food Proc Eng, 17: 121±140. 16. Magdalani, K. K., Nikolaos, P. Z. and Zacharias, B. M., 1997, Modelling shrinkage and porosity during vacuum dehydration, Int J Food Sci Tech, 32: 4445±458. 17. AOAC, 1995, Of®cial Methods of Analysis, 16th edn (Association of Of®cial Analytical Chemists, Washington, DC). 18. Mayer, D.G. and Butler, D.G., 1993, Statistical validation, Ecological Modelling, 68: 21±32. 19. SAS, 1985, User’s Guide: Statistics, 5th edn (SAS Institute Inc, Cary, NC). 20. Medeiros, G.L. and Sereno, A.M., 1994, Physical and transfer properties of peas during warm air drying, J Food Eng, 21: 355±363.

CONCLUSION During the drying process, the bulk and apparent densities, shrinkage and porosity of green beans vary progressively due to the reduction in moisture, and structural and geometrical changes in the bean paticulates. It was possible to predict these changes using some empirical models, except in the case of apparent density. The ¯uidization behaviour of the bean particulates also changes progressively as the drying proceeds. NOMENCLATURE A B C D i k L MR n U VR y e r

constant constant constant diameter, m integer rate constant length, m moisture ratio, kg kg db¡1 number of experimental observations velocity, ms¡1 volume ratio value bed porosity density, kg m¡3

Subscripts p b obs pre mf

particle bulk observed predicted minimum ¯uidization

Trans IChemE, Vol 78, Part C, March 2000

ADDRESS Correspondence concerning this paper should be addressed to Mr W. Senadeera, Food Science and Technology, School of Land and Food Science, The University of Queensland, Gatton College 4345, Australia. The manuscript was communicated via our International Editor for Sri Lanka, Dr A. de Alwis. It was received 17 August 1999 and accepted for publication after revision 24 November 1999.