- Email: [email protected]
Spray drying of tomato pulp in dehumidified air: II. The effect on powder properties
Journal of Food Engineering 66 (2005) 35–42 www.elsevier.com/locate/jfoodeng
Spray drying of tomato pulp in dehumidified air: II. The effect on powder properties Athanasia M. Goula, Konstantinos G. Adamopoulos
*
Department of Chemical Engineering, School of Engineering, Laboratory of Food Process Engineering, Aristotle University of Thessaloniki, 541 24 University Campus, Thessaloniki, Greece Received 14 October 2003; accepted 23 February 2004
Abstract Tomato powders were produced by spray drying tomato pulp using a modified spray drying system. Modifications to the original dryer design consisted of connecting the spray dryer inlet air intake to an air dehumidifier. Samples of tomato pulp with a 14% constant total solids concentration were used. Sixty-four different experiments were conducted keeping constant the feed rate, the feed temperature and the atomizer pressure, and varying the compressed air flow rate, the flow rate of drying rate, and the air inlet temperature. In all experiments, the atomizer pressure, the feed rate and the feed temperature were kept at 5 ± 0.1 bar, 1.75 ± 0.05 g/ min, and 32.0 ± 0.5 C respectively. The variable operating conditions were within the following ranges: inlet air temperatures 110– 140 (±1) C; drying air flow rate 17.50–22.75 (±0.18) m3 /h, and compressed air flow rate 500–800 (±20) l/h. The tomato powders were analyzed for moisture content, bulk density and solubility. Analysis of experimental data yielded correlations between the powder properties and the variable operating conditions. Regression analysis was used to fit mathematical models to the data of each of the powder properties evaluated. Comparisons between the moisture content, the bulk density, and the solubility of powders produced by the two drying systems proved that the use of dehumidified air, promoting rapid particulate skin formation, decreased powder moisture content and increased powder bulk density and solubility. The modified spray drying system proved advantageous over the standard laboratory spray dryer. Preliminary air dehumidification improved not only product recovery, but, also product properties. 2004 Elsevier Ltd. All rights reserved. Keywords: Bulk density; Dehumidified air; Moisture; Powder properties; Solubility; Spray drying; Stickiness; Tomato pulp
1. Introduction Spray drying is the transformation of feed from a fluid state into a dried particulate form by spraying the feed into a hot drying medium. The production of dry particles from a liquid feed in a single processing step makes spray drying a unique and important unit operation. The design of a spray drying process includes establishment of the operating conditions that increase product recovery and produce an end product of a precise quality specification. Product recovery is mainly determined by powder collection efficiency. Material loss in a spray drying system is due mostly to the attachment of sprayed droplets and dry powder to the wall of the dryer. *
Corresponding author. Tel.: +30-2310-996205/995903; fax: +302310-996259. E-mail address: [email protected] (K.G. Adamopoulos). 0260-8774/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2004.02.031
Particle adhesion to the wall is affected by the nature of the spray-dried material and spray drying conditions, and is a commonly recognized effect in spray drying solutions containing sugars, such as fruit juices and tomato products (Bhandari, Datta, & Howes, 1997; Bhandari, Datta, Crooks, Howes, & Rigby, 1997). During the drying process they may either remain as syrup or stick on the dryer chamber wall. Various ways of coping with such products have been researched for many years (Goose & Binsted, 1964; Gransmith, 1971; Jayaraman & Das Gupta, 1995; Karatas, 1989; Ponting, Stanley, & Copley, 1973; Spicer, 1974). The most commonly quoted specifications of a powder involve moisture content, bulk density and solubility. The temperatures and drying conditions experienced by a droplet during drying have an important influence on the above powder properties (Masters, 1997; Oakley, 1997). However, the effect of process variables (e.g. the
36
A.M. Goula, K.G. Adamopoulos / Journal of Food Engineering 66 (2005) 35–42
residence time of particles within the drying chamber, the conditions of atomization, the drying air temperature) upon powder properties are difficult to assess in general terms (Walton & Mumford, 1999a). This is due to the lack of information within the literature and to the specific drying nature of most materials. Generally, in a spray drying system, the moisture content is controlled by the temperature of the exhaust air leaving the drying chamber (Cook, 1991; Masters, 1979a). The humidity of the air may also be a factor. High ambient air humidity may require an increase in outlet air temperature in order to maintain the desired powder moisture content (Masters, 1997). The residual moisture in the powder influences many other powder properties such as bulk density and solubility. Bulk density is also affected by particle size and density, occluded and interstitial air content, which are related to the feed properties, drying air temperature, drying time and powder handling procedures e.g. crushing and grinding. Generally, the effect of the drying conditions on bulk density is highly product dependent (Krokida & Maroulis, 1997; Krokida, Zogzas, & Maroulis, 1998; Masters, 1979b). However, in spray drying there are some general trends. Higher atomizer wheel speeds and nozzle pressures decrease droplet and therefore particle size, thus increasing bulk density (Al-Kahtani & Hassan, 1990; Masters, 1979a). Spherical shaped particles can result in a low degree of interstitial air, as the small particles in the size distribution fill the void spaces between the large particles. Irregular shaped particles and agglomerates can lead to a lower bulk density (Walton & Mumford, 1999b). Bulk density tends to increase with a decrease in air outlet temperature, and with an increase in feed solids (Nath & Satpathy, 1998; Walton, 2000). In Part I of the study (Goula & Adamopoulos, submitted for publication), an experimental spray dryer was modified for drying tomato concentrate. The modification made to the original dryer design consisted of connecting the spray dryer inlet air intake to an absorption air dryer. The performance of the modified dryer and the effect of preliminary air dehumidification on product recovery were studied. The modified drying system proved advantageous over the standard laboratory spray dryer, since the much lower outlet temperatures and humidities of the drying air resulted in the formation of a solid particle surface, which decreased residue accumulation or dryer fouling, and minimized the number of thermoplastic particles sticking to the dryer wall. The rapid particulate skin formation may also affect the powder properties. The most famous effect of the formation of a dry surface layer on product properties is the diminishment to 0 of the volatile components loss, in accordance with the selective-diffusion theory (Hecht & King, 2000). In addition, according to Oakley (1997), the speed of crust forma-
tion, highly dependent on drying rates, can affect the particle size and density. When designing a spray drying process, required powder properties are a significant consideration. The objective of this work was to study the effect of spray drying conditions and preliminary air dehumidification on tomato powder moisture, bulk density and solubility.
2. Materials and methods 2.1. Production of tomato powders Tomato powders were prepared as described in Part I of this study (Goula & Adamopoulos, submitted for publication). A modified laboratory spray dryer, again described in Part I, was employed for the spray drying process. Sixty-four different experiments were conducted in triplicate. In all experiments, the atomizer pressure, the feed rate, and the feed temperature were kept at 5 ± 0.1 bar, 1.75 ± 0.05 g/min, and 32.0 ± 0.5 C respectively. The variable operating conditions were within the following ranges: inlet air temperatures (Tinlet ) 110–140 (±1) C; drying air flow rates (Qa ) 17.50–22.75 (±0.18) m3 /h, and compressed air flow rates (Qc ) 500–800 (±20) l/h. Inlet and outlet drying air temperatures were read and manually logged from the digital displays on the dryer’s control panel with an accuracy of ±1 C. 2.2. Analysis of powders Moisture: The moisture content was determined by drying the powders at 70 C to a constant weight (Goose & Binsted, 1964), and expressing the moisture loss in terms of percent wet basis (wb), i.e. 100 · kg water/kg wet material. Solubility: The solubility of the spray-dried powder was carried out by adding 2 g of the material to 50 ml of distilled water at 26 C (El-Tinay & Ismail, 1985). The mixture was agitated in a low form glass beaker 100 ml with a magnetic stirrer (Falc, 50–60 Hz, 0.2 A) at 892 rpm, using a stirring bar with a size of 2 mm · 7 mm. The time required for the material to dissolve completely was recorded. Density: 2 g of powder was transferred to a 50 ml graduated cylinder. The bulk density was calculated by dividing the mass of the powder by the volume occupied in the cylinder. All analyses were done in triplicate and the averages of these triplicate measurements recorded. Additional determinations were carried out if the single values from the triplicates deviated by more than ±0.6% from the triplicate mean.
A.M. Goula, K.G. Adamopoulos / Journal of Food Engineering 66 (2005) 35–42
2.3. Statistical analysis
12
7 6 5 4
400
500
Q a : 17.50 m 3 /h
Tinlet (° C) 110 120 modified 130 system 140 110 120 standard 130 system 140 fitted model
10 9 8 7 6 5 4 3 2 600 700 Q c (L/h)
600 700 Q c (L/h)
800
900
Fig. 2. Powder moisture content as a function of inlet air temperature and compressed air flow rate. Drying air flow rate, 19.25 m3 /h. Symbols are the same as in Fig. 1.
Q a : 21.00 m3 /h
11
11 powder moisture content (% wb)
8
12
800
900
Fig. 1. Powder moisture content as a function of inlet air temperature and compressed air flow rate. Drying air flow rate, 17.50 m3 /h.
powder moisture content (% wb)
The moisture content of the tomato powders varied from 3.11% to 9.43% wb. Figs. 1–4 show the achieved values against inlet air temperature and compressed air flow rate for different drying air flow rates. Each data point in the figures represents the averaged values of three determinations. The repeatability for moisture content, expressed as the average standard deviation of the three determinations, was 0.05%. As it can be drawn from Figs. 1–4, powder moisture content increases with an increase in drying air flow rate. Generally, the energy available for evaporation varies
500
9
2
3.1. Powder moisture content
400
10
3
3. Results and discussion
12
Q a : 19.25 m3 /h
11 powder moisture content (% wb)
The data were analyzed using the statistical software MINITAB (Release 13.32). Regression analysis was used to fit full second order polynomials, reduced second order polynomials containing the three linear terms, and linear models to the data of each of the variables evaluated (response variables). F values for all reduced and linear models with a coefficient of determination (R2 ) greater than 0.70 were calculated to determine if the models could be used in place of full second order polynomials to predict the response of a variable to compressed air flow rate, drying rate flow rate and air inlet temperature (independent variables). The best fitting models were determined on the basis of a high R2 , a low square root of mean square error (S), and a Mallows’ Cp statistic close to the number of predictors contained in the model. The statistical analysis is described analytically in Part I of the study.
37
10 9 8 7 6 5 4 3 2 400
500
600 700 Q c (L/h)
800
900
Fig. 3. Powder moisture content as a function of inlet air temperature and compressed air flow rate. Drying air flow rate, 21.00 m3 /h. Symbols are the same as in Fig. 1.
according to the amount of drying air. This could give the impression that the drying air flow rate must be at a maximum in all cases. However, the movement of air predetermines the rate and degree of droplet evaporation by influencing (a) the passage of spray through the drying zone, (b) the concentration of product in the region of the dryer walls, and (c) the extent to which semi-dried droplets re-enter the hot areas around the air disperser. A lower drying air flow rate, causes an increase in product sojourn time in the drying chamber (Masters, 1979a) and enforces circulation effects (Goula & Adamopoulos, 2004; Oakley & Bahu, 1991). Increased residence times lead to a greater degree of
38
A.M. Goula, K.G. Adamopoulos / Journal of Food Engineering 66 (2005) 35–42
9
moisture being driven from the food to a greater extent than with cooler air. The best model predictions of moisture content to process variables such as, air inlet temperature, drying rate flow rate and compressed air flow rate was as follows:
8
moisture ¼ 124 1:86 Tinlet þ 0:681 Qa þ 0:00115 Qc
12
Q a : 22.75 m 3 /h
powder moisture content (% wb)
11 10
7
2 þ 0:00692 Tinlet 0:00735 Q2a
6
0:000003 Q2c 0:00213 Tinlet Qa
5 4 3 2 400
500
600
700
800
900
Q c (L/h) Fig. 4. Powder moisture content as a function of inlet air temperature and compressed air flow rate. Drying air flow rate, 22.75 m3 /h. Symbols are the same as in Fig. 1.
moisture removal. As a result, an increase in drying air flow rate, decreasing the residence time of the product in the drying chamber, led to higher moisture contents. Moisture content shows a decrease with an increase in compressed air flow rate due to the effect of this flow rate on mean particle size. Increase in air–liquid flow ratio in a two-fluid nozzle atomizer decreases the mean size of the spray droplets (Nath & Satpathy, 1998). With smaller particles, narrow spray cones are formed and air may not penetrate the centre of the spray pattern until droplets have travelled quite some distance from the nozzle (Liang & King, 1991). This reduced mixing of hot air should make the drying rates decrease. However, drying is facilitated by smaller particle sizes for two reasons. First, larger surface area provides more surface in contact with the heating medium and more surface from which the moisture can escape. Second, smaller particles reduce the distance heat must travel to the centre of the particles and reduce the distance through which moisture in the centre of the particles must travel to reach the surface and escape. As shown in Figs. 1–4, an increase in air inlet temperature leads to a decrease in moisture content. The greater the temperature difference between the drying medium and the particles, the greater will be the rate of heat transfer into the particles, which provides the driving force for moisture removal. When the drying medium is air, temperature plays a second important role. As water is driven from the particles in the form of water vapor, it must be carried away, or the moisture will create a saturated atmosphere at the particle surface. This will slow down the rate of subsequent water removal. The hotter the air, the more moisture it will hold before becoming saturated. Thus, high temperature air in the vicinity of the drying particles will take up the
ð1Þ
Eq. (1) has an R2 value of 0.997, and F , Cp and S equal to 0.117, 6.2 and 0.128, respectively. In experiments conducted under the same operating conditions using a standard laboratory spray drying system powder moisture content, varying from 4.16% to 11.27%, was higher (Goula & Adamopoulos, 2003; Goula, Adamopoulos, & Kazakis, 2004). Comparison between the moisture content of powders produced by the two drying systems is shown in Figs. 1–4. Generally, in a spray drying system, the temperature of the exhaust air leaving the drying chamber controls the residual moisture in the powder. A lower moisture content can be reached by higher temperatures at the outlet. In the modified system, the outlet air temperature was 19–24 C lower than in the standard spray drying system. However, the modified system decreased residual moisture content due to the preliminary air dehumidification. The drier the air, the more rapid is the rate of drying. Experimentally determined phase and state transition temperatures of food components and food solids suggest that, in dehydration of liquids during the evaporation of droplets with dissolved substances, the rapid removal of water results in vitrification of the droplets within a short time, and the formation of a solid particle surface (Roos, 2003). This does not allow formation of liquid bridges between contacting particles or particle adhesion to the dryer walls. Diffusion of water vapor however, occurs rapidly within a porous, glassy surface allowing further dehydration of the particle core as the particle travels through the drying chamber. If dehydration conditions do not permit surface solidification before particles collide with each other, as in a standard spray drying system, stickiness results in agglomeration and so in a smaller surface area, which provides less surface in contact with the heating medium and less surface from which the moisture can escape. According to Downton, Flores-Luna, and King (1982), during spray drying the main requirement is that a solid particle surface is formed rapidly, increasing surface viscosity and decreasing sticking of particles with each other or on dryer surfaces. 3.2. Powder bulk density Bulk density results, ranging from 0.199 to 0.365 g/ ml, are given in Figs. 5–8. Data represent the average
A.M. Goula, K.G. Adamopoulos / Journal of Food Engineering 66 (2005) 35–42 Q a : 17.50 m 3 /h
0.40
Tinlet (°C) 110 120 modified 130 system 140 110 120 standard 130 system 140 fitted model
0.35 0.30
0.25 0.20
0.15 0.10 400
0.30
0.25
0.20
0.15 500
600
700
800
900
Qc (L/h)
0.10
400
Fig. 5. Powder bulk density as a function of inlet air temperature and compressed air flow rate. Drying air flow rate, 17.50 m3 /h.
0.40
500
600
700
800
900
Qc (L/h) Fig. 7. Powder bulk density as a function of inlet air temperature and compressed air flow rate. Drying air flow rate, 21.00 m3 /h. Symbols are the same as in Fig. 5.
Q a : 19.25 m 3 /h
0.35
0.40
0.30
Q a : 22.75 m 3 /h
0.35
powder bulk density (g/mL)
powder bulk density (g/mL)
Q a : 21.00 m 3 /h
0.35
powder bulk density (g/mL)
powder bulk density (g/mL)
0.40
39
0.25
0.20
0.15
0.10
400
500
600 700 Q c (L/h)
800
900
0.30
0.25
0.20
0.15
0.10
Fig. 6. Powder bulk density as a function of inlet air temperature and compressed air flow rate. Drying air flow rate, 19.25 m3 /h. Symbols are the same as in Fig. 5.
values of the three determinations. The repeatability for bulk density, expressed as the average standard deviation of the three determinations, was 0.004 g/ml. The effect of drying air flow rate on powder bulk density depends on its effect on moisture content due to the sticky nature of the product. The higher the powder moisture content, the more particles tend to stick together, leaving more interspaces between them and consequently resulting in a larger bulk volume. As a result, air flow rate increases lead to an increase in powder moisture content and a decrease in powder bulk density. Masters (1979b) reported that increasing residual moisture content increases bulk density of a dry product. However, this trend was not observed here, due to the thermoplastic nature of the product.
400
500
600
700
800
900
Qc (L/h) Fig. 8. Powder bulk density as a function of inlet air temperature and compressed air flow rate. Drying air flow rate, 22.75 m3 /h. Symbols are the same as in Fig. 5.
Higher compressed air flow rate causes an increase of powder bulk density due to its effect on mean particle size. Smaller particles produced with higher compressed air rates are also more dense and so further increase bulk density. Increased air inlet temperature causes a reduction in bulk density, as evaporation rates are faster and products dry to a more porous or fragmented structure. According to Walton (2000), increasing the drying air temperature generally produces a decrease in bulk and particle density, and there is a greater tendency for the particles to be hollow. The former can be caused by
A.M. Goula, K.G. Adamopoulos / Journal of Food Engineering 66 (2005) 35–42
bulkdensity ¼ 0:405 þ 0:0132 Tinlet þ 0:00514 Qa 2 þ 0:000239 Qc 0:000044 Tinlet
0:000001 Q2c 0:000188 Tinlet Qa ð2Þ Eq. (2) has an R2 value of 0.974, and F , Cp and S equal to 0.326, 5.0 and 7.295 · 103 , respectively. In experiments conducted under the same operating conditions using a standard laboratory spray drying system, powder bulk density was lower, varying from 0.100 to 0.258 g/ml (Goula et al., 2004). Comparisons between the bulk densities of the powders produced by the two drying systems are shown in Figs. 5–8. Since a low moisture content of tomato powder is associated with a high bulk density, the preliminary air dehumidification increases powder bulk density due to its effect on powder moisture content. In addition, in a standard spray drying system, the particles tend to collide with each other formatting agglomerates with more interspaces between the particles and consequently resulting in lower bulk density. 3.3. Powder solubility Figs. 9–12 show powder solubility in relation to inlet air temperature, compressed air flow rate and drying air flow rate. Solubility of the tomato powders varied from 121 to 245 s. The repeatability expressed as the average standard deviation of the three determinations was 5 s. The effect of drying air flow rate on powder solubility depends on its effect on powder moisture content, as a
Q a : 19.25 m3/h
450 400
powder solubility (sec)
particle inflation-ballooning or puffing, and is particularly common in skin-forming materials. The best model predictions for bulk density in relation to the process variables, air inlet temperature, drying air flow rate and compressed air flow rate, was as follows:
350 300 250 200 150 100 400
Q a : 17.50 m 3 /h
Tinlet (° C) 110 120 modified 130 system 140 110 120 standard 130 system 140 fitted model
powder solubility (sec)
350 300 250 200 150
500
600 700 Q c (L/h)
700
800
900
450
Q a : 21.00 m 3 /h
400 350 300 250 200 150 100 500
600
700
800
900
Q c (L/h)
400
100 400
600
Fig. 10. Powder solubility as a function of inlet air temperature and compressed air flow rate. Drying air flow rate, 19.25 m3 /h. Symbols are the same as in Fig. 9.
400 450
500
Q c (L/h)
powder solubility (sec)
40
800
900
Fig. 9. Powder solubility as a function of inlet air temperature and compressed air flow rate. Drying air flow rate, 17.50 m3 /h.
Fig. 11. Powder solubility as a function of inlet air temperature and compressed air flow rate. Drying air flow rate, 21.00 m3 /h. Symbols are the same as in Fig. 9.
low moisture content seems to be associated with fast dissolution. Air flow rate increases lead to an increase in powder moisture content and a decrease in powder solubility. This trend is similar to that reported by other researchers (Papadakis, Gardeli, & Tzia, 1998). The time required for the powder to dissolve was found to increase with an increase in compressed air flow rate, since particle size affects solubility rate. Large particles may sink, whereas small ones are more dusty and generally float on water making for uneven wetting and reconstitution (Potter, 1968).
A.M. Goula, K.G. Adamopoulos / Journal of Food Engineering 66 (2005) 35–42 450
4. Conclusions
Q a : 22.75 m 3 /h
An experimental spray dryer was modified for drying tomato concentrate. The modification made on the original design consisted of connecting the spray dryer inlet air intake to an absorption air dryer. The effect of spray drying conditions, i.e. air inlet temperature, drying air flow rate, and compressed air flow rate on tomato powder moisture content, bulk density and solubility, was studied. It was observed that:
400
powder solubility (sec)
41
350 300 250 200 150 100 400
500
600
700
800
900
Q c (L/h) Fig. 12. Powder solubility as a function of inlet air temperature and compressed air flow rate. Drying air flow rate, 22.75 m3 /h. Symbols are the same as in Fig. 9.
Solubility showed an increase with an increase in inlet air temperature. This is due to the effect of inlet air temperature on residual moisture content. The lower the powder moisture content, the more soluble is the powder. In addition, increasing the drying air temperature generally produces an increase in particle size, and so a decrease in time required for the powder to dissolve. The best model predictions for powder solubility in relation to the process variables, air inlet temperature, drying air flow rate, and compressed air flow rate, was as follows: solubility ¼ 352 2:22 Tinlet 10:5 Qa þ 0:186 Qc
Moisture content, bulk density and solubility are the most commonly quoted specifications of a powder product. The lower the moisture content and the higher the bulk density and the solubility, the better will be considered the product. In experiments conducted under the same operating conditions using the standard laboratory spray drying system, powder moisture content was higher and powder bulk density and solubility were lower. It appears that the preliminary air dehumidification promoting rapid particulate skin formation improved not only product recovery, but, also product properties.
References
þ 0:393 Q2a 0:000083 Q2c þ 0:0301 Tinlet Qa
• powder moisture content decreases with an increase in air inlet temperature and compressed air flow rate, and with a decrease in drying air flow rate, • bulk density increases with a decrease in drying air flow rate and air inlet temperature, and with an increase in compressed air flow rate, • solubility increases with a decrease in drying and compressed air flow rate, and with an increase in air inlet temperature.
ð3Þ
Eq. (3) has an R2 value of 0.979, and F , Cp and S equal to 0.205, 4.6 and 4.122, respectively. In experiments conducted under the same operating conditions using a standard laboratory spray drying system powder solubility was lower, varying from 188 to 435 s (Goula et al., 2004). Comparisons between the solubilities of the powders produced with the two drying systems in relation to the air inlet temperature are shown in Figs. 9–12. The modified system increased powder solubility due to its effect on powder moisture content, since a low moisture content seems to be associated with fast dissolution. Furthermore, the much higher air temperatures in the standard system may have resulted in denaturing more protein and hence affected solubility.
Al-Kahtani, H. A., & Hassan, B. H. (1990). Spray drying of Roselle (Hibiscus sabdariffa L.) extract. Journal of Food Science, 55(4), 1073–1076. Bhandari, B. R., Datta, N., & Howes, T. (1997). Problems associated with spray drying of sugar-rich foods. Drying Technology, 15(2), 671–685. Bhandari, B. R., Datta, N., Crooks, R., Howes, T., & Rigby, S. (1997). A semi-empirical approach to optimize the quantity of drying aids required to spray dry sugar-rich foods. Drying Technology, 15(10), 2509–2525. Cook, E. M. (1991). Introduction. In Process drying practice (pp. 3– 12). New York: McGraw-Hill. Downton, D. P., Flores-Luna, J. L., & King, C. J. (1982). Mechanism of stickiness in hygroscopic, amorphous powders. Industrial & Engineering Chemistry Fundamentals, 21, 447–451. El-Tinay, A. H., & Ismail, I. A. (1985). Effect of some additives and processes on the characteristics of agglomerated and granulated spray-dried Roselle powder. Acta Alimentaria Hungaricae, 14(3), 283–295. Goose, P. G., & Binsted, R. (1964). Allied tomato products. In Tomato paste, puree, juice and powder (1st ed., pp. 124–126). UK, London: Food Trade Press.
42
A.M. Goula, K.G. Adamopoulos / Journal of Food Engineering 66 (2005) 35–42
Goula, A. M., & Adamopoulos, K. G. (2003). Influence of spray drying conditions on tomato powder moisture. In Proceedings of the third international symposium of food rheology and structure, Zurich, Switzerland. Goula, A. M., & Adamopoulos, K. G. (2004). Influence of spray drying conditions on residue accumulation––Simulation using CFD. Drying Technology (5). Goula, A. M., & Adamopoulos, K. G. (in press). Spray drying of tomato pulp in dehumidified air. I. The effect on the product recovery. Journal of Food Engineering. doi:10.1016/j.jfoodeng.2004.02.029. Goula, A. M., Adamopoulos, K. G., & Kazakis, N. A. (2004). Influence of spray drying conditions on tomato powder properties. Drying Technology (5). Gransmith, M. (1971). Dehydration of foods. In Practical dehydration (pp. 125–133). UK, London: Food Trade Press. Hecht, J. P., & King, C. J. (2000). Spray drying: Influence of developing drop morphology on drying rates and retention of volatile substances. 1. Single-drop experiments. Industrial & Engineering Chemistry Research, 39(6), 1756–1765. Jayaraman, K. S., & Das Gupta, D. K. (1995). Drying of fruits and vegetables. In Handbook of industrial drying (2nd ed., pp. 661–662). New York: Marcel Dekker. Karatas, S. (1989). A laboratory scraped surface drying chamber for spray drying of tomato paste. Lebensmittel-Wissenschaft und Technologie, 23(4), 354–357. Krokida, M. K., & Maroulis, Z. B. (1997). Effect of drying method on shrinkage and porosity. Drying Technology, 15(10), 2441–2458. Krokida, M. K., Zogzas, N. P., & Maroulis, Z. B. (1998). Shrinkage and porosity during drying. Modeling the effect of vacuum. International Journal of Food Science and Technology, 32, 445–458. Liang, B., & King, C. J. (1991). Factors influencing flow patterns, temperature fields and consequent drying rates in spray drying. Drying Technology, 9(1), 1–25. Masters, K. (1979a). Spray drying fundamentals: Process stages and layouts. In Spray drying handbook (pp. 21–53). New York: Halsted Press.
Masters, K. (1979b). Drying of droplets/sprays. In Spray drying handbook (pp. 291–335). New York: Halsted Press. Masters, K. (1997). Spray dryers. In C. G. J. Baker (Ed.), Industrial drying of foods (pp. 90–114). New York: Blackie Academic & Professional. Nath, S., & Satpathy, G. R. (1998). A systematic approach for investigation of spray drying processes. Drying Technology, 16(6), 1173–1193. Oakley, D. E. (1997). Produce uniform particles by spray drying. Chemical Engineering Progress, 93(10), 48–54. Oakley, D. E., & Bahu, R. E. (1991). Spray/gas mixing behaviour within spray dryers. In A. S. Mujumdar & I. Filkova (Eds.), Drying (pp. 303–313). Amsterdam: Elsevier. Papadakis, S. E., Gardeli, C., & Tzia, C. (1998). Raisin extract powder: Production, physical and sensory properties. In Proceedings of the 11th international drying symposium IDS ’98, Halkidiki, Greece. Ponting, J. D., Stanley, W. L., & Copley, M. J. (1973). Fruit and vegetables juices. In B. S. Van Arsdel, M. J. Copley, & A. I. Morgan (Eds.), Food dehydration (2nd ed., pp. 211–215). Westport, Connecticut: AVI Publishing Company. Potter, N. N. (1968). Food science (pp. 227–257). Westport, Connecticut: AVI Publishing Company. Roos, Y. (2003). Phase and state transitions in food dehydration. In Proceedings of the symposium EUDrying ’03, Heraklion, Crete, Greece. Spicer, A. (1974). Effects of latest developments of spray drying. In Advances in preconcentration and dehydration of foods (pp. 337– 340). New York: John Wiley and Sons. Walton, D. E. (2000). The morphology of spray-dried particles––A qualitative view. Drying Technology, 18(9), 1943–1986. Walton, D. E., & Mumford, C. J. (1999a). Spray dried products–– Characterization of particle morphology. Transactions of the Institution of Chemical Engineers––London, 77(A), 21–38. Walton, D. E., & Mumford, C. J. (1999b). The morphology of spraydried particles––The effect of process variables upon the morphology of spray-dried particles. Transactions of the Institution of Chemical Engineers––London, 77(A), 442–460.
Spray drying of tomato pulp in dehumidified air: II. The effect on powder properties Athanasia M. Goula, Konstantinos G. Adamopoulos
*
Department of Chemical Engineering, School of Engineering, Laboratory of Food Process Engineering, Aristotle University of Thessaloniki, 541 24 University Campus, Thessaloniki, Greece Received 14 October 2003; accepted 23 February 2004
Abstract Tomato powders were produced by spray drying tomato pulp using a modified spray drying system. Modifications to the original dryer design consisted of connecting the spray dryer inlet air intake to an air dehumidifier. Samples of tomato pulp with a 14% constant total solids concentration were used. Sixty-four different experiments were conducted keeping constant the feed rate, the feed temperature and the atomizer pressure, and varying the compressed air flow rate, the flow rate of drying rate, and the air inlet temperature. In all experiments, the atomizer pressure, the feed rate and the feed temperature were kept at 5 ± 0.1 bar, 1.75 ± 0.05 g/ min, and 32.0 ± 0.5 C respectively. The variable operating conditions were within the following ranges: inlet air temperatures 110– 140 (±1) C; drying air flow rate 17.50–22.75 (±0.18) m3 /h, and compressed air flow rate 500–800 (±20) l/h. The tomato powders were analyzed for moisture content, bulk density and solubility. Analysis of experimental data yielded correlations between the powder properties and the variable operating conditions. Regression analysis was used to fit mathematical models to the data of each of the powder properties evaluated. Comparisons between the moisture content, the bulk density, and the solubility of powders produced by the two drying systems proved that the use of dehumidified air, promoting rapid particulate skin formation, decreased powder moisture content and increased powder bulk density and solubility. The modified spray drying system proved advantageous over the standard laboratory spray dryer. Preliminary air dehumidification improved not only product recovery, but, also product properties. 2004 Elsevier Ltd. All rights reserved. Keywords: Bulk density; Dehumidified air; Moisture; Powder properties; Solubility; Spray drying; Stickiness; Tomato pulp
1. Introduction Spray drying is the transformation of feed from a fluid state into a dried particulate form by spraying the feed into a hot drying medium. The production of dry particles from a liquid feed in a single processing step makes spray drying a unique and important unit operation. The design of a spray drying process includes establishment of the operating conditions that increase product recovery and produce an end product of a precise quality specification. Product recovery is mainly determined by powder collection efficiency. Material loss in a spray drying system is due mostly to the attachment of sprayed droplets and dry powder to the wall of the dryer. *
Corresponding author. Tel.: +30-2310-996205/995903; fax: +302310-996259. E-mail address: [email protected] (K.G. Adamopoulos). 0260-8774/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2004.02.031
Particle adhesion to the wall is affected by the nature of the spray-dried material and spray drying conditions, and is a commonly recognized effect in spray drying solutions containing sugars, such as fruit juices and tomato products (Bhandari, Datta, & Howes, 1997; Bhandari, Datta, Crooks, Howes, & Rigby, 1997). During the drying process they may either remain as syrup or stick on the dryer chamber wall. Various ways of coping with such products have been researched for many years (Goose & Binsted, 1964; Gransmith, 1971; Jayaraman & Das Gupta, 1995; Karatas, 1989; Ponting, Stanley, & Copley, 1973; Spicer, 1974). The most commonly quoted specifications of a powder involve moisture content, bulk density and solubility. The temperatures and drying conditions experienced by a droplet during drying have an important influence on the above powder properties (Masters, 1997; Oakley, 1997). However, the effect of process variables (e.g. the
36
A.M. Goula, K.G. Adamopoulos / Journal of Food Engineering 66 (2005) 35–42
residence time of particles within the drying chamber, the conditions of atomization, the drying air temperature) upon powder properties are difficult to assess in general terms (Walton & Mumford, 1999a). This is due to the lack of information within the literature and to the specific drying nature of most materials. Generally, in a spray drying system, the moisture content is controlled by the temperature of the exhaust air leaving the drying chamber (Cook, 1991; Masters, 1979a). The humidity of the air may also be a factor. High ambient air humidity may require an increase in outlet air temperature in order to maintain the desired powder moisture content (Masters, 1997). The residual moisture in the powder influences many other powder properties such as bulk density and solubility. Bulk density is also affected by particle size and density, occluded and interstitial air content, which are related to the feed properties, drying air temperature, drying time and powder handling procedures e.g. crushing and grinding. Generally, the effect of the drying conditions on bulk density is highly product dependent (Krokida & Maroulis, 1997; Krokida, Zogzas, & Maroulis, 1998; Masters, 1979b). However, in spray drying there are some general trends. Higher atomizer wheel speeds and nozzle pressures decrease droplet and therefore particle size, thus increasing bulk density (Al-Kahtani & Hassan, 1990; Masters, 1979a). Spherical shaped particles can result in a low degree of interstitial air, as the small particles in the size distribution fill the void spaces between the large particles. Irregular shaped particles and agglomerates can lead to a lower bulk density (Walton & Mumford, 1999b). Bulk density tends to increase with a decrease in air outlet temperature, and with an increase in feed solids (Nath & Satpathy, 1998; Walton, 2000). In Part I of the study (Goula & Adamopoulos, submitted for publication), an experimental spray dryer was modified for drying tomato concentrate. The modification made to the original dryer design consisted of connecting the spray dryer inlet air intake to an absorption air dryer. The performance of the modified dryer and the effect of preliminary air dehumidification on product recovery were studied. The modified drying system proved advantageous over the standard laboratory spray dryer, since the much lower outlet temperatures and humidities of the drying air resulted in the formation of a solid particle surface, which decreased residue accumulation or dryer fouling, and minimized the number of thermoplastic particles sticking to the dryer wall. The rapid particulate skin formation may also affect the powder properties. The most famous effect of the formation of a dry surface layer on product properties is the diminishment to 0 of the volatile components loss, in accordance with the selective-diffusion theory (Hecht & King, 2000). In addition, according to Oakley (1997), the speed of crust forma-
tion, highly dependent on drying rates, can affect the particle size and density. When designing a spray drying process, required powder properties are a significant consideration. The objective of this work was to study the effect of spray drying conditions and preliminary air dehumidification on tomato powder moisture, bulk density and solubility.
2. Materials and methods 2.1. Production of tomato powders Tomato powders were prepared as described in Part I of this study (Goula & Adamopoulos, submitted for publication). A modified laboratory spray dryer, again described in Part I, was employed for the spray drying process. Sixty-four different experiments were conducted in triplicate. In all experiments, the atomizer pressure, the feed rate, and the feed temperature were kept at 5 ± 0.1 bar, 1.75 ± 0.05 g/min, and 32.0 ± 0.5 C respectively. The variable operating conditions were within the following ranges: inlet air temperatures (Tinlet ) 110–140 (±1) C; drying air flow rates (Qa ) 17.50–22.75 (±0.18) m3 /h, and compressed air flow rates (Qc ) 500–800 (±20) l/h. Inlet and outlet drying air temperatures were read and manually logged from the digital displays on the dryer’s control panel with an accuracy of ±1 C. 2.2. Analysis of powders Moisture: The moisture content was determined by drying the powders at 70 C to a constant weight (Goose & Binsted, 1964), and expressing the moisture loss in terms of percent wet basis (wb), i.e. 100 · kg water/kg wet material. Solubility: The solubility of the spray-dried powder was carried out by adding 2 g of the material to 50 ml of distilled water at 26 C (El-Tinay & Ismail, 1985). The mixture was agitated in a low form glass beaker 100 ml with a magnetic stirrer (Falc, 50–60 Hz, 0.2 A) at 892 rpm, using a stirring bar with a size of 2 mm · 7 mm. The time required for the material to dissolve completely was recorded. Density: 2 g of powder was transferred to a 50 ml graduated cylinder. The bulk density was calculated by dividing the mass of the powder by the volume occupied in the cylinder. All analyses were done in triplicate and the averages of these triplicate measurements recorded. Additional determinations were carried out if the single values from the triplicates deviated by more than ±0.6% from the triplicate mean.
A.M. Goula, K.G. Adamopoulos / Journal of Food Engineering 66 (2005) 35–42
2.3. Statistical analysis
12
7 6 5 4
400
500
Q a : 17.50 m 3 /h
Tinlet (° C) 110 120 modified 130 system 140 110 120 standard 130 system 140 fitted model
10 9 8 7 6 5 4 3 2 600 700 Q c (L/h)
600 700 Q c (L/h)
800
900
Fig. 2. Powder moisture content as a function of inlet air temperature and compressed air flow rate. Drying air flow rate, 19.25 m3 /h. Symbols are the same as in Fig. 1.
Q a : 21.00 m3 /h
11
11 powder moisture content (% wb)
8
12
800
900
Fig. 1. Powder moisture content as a function of inlet air temperature and compressed air flow rate. Drying air flow rate, 17.50 m3 /h.
powder moisture content (% wb)
The moisture content of the tomato powders varied from 3.11% to 9.43% wb. Figs. 1–4 show the achieved values against inlet air temperature and compressed air flow rate for different drying air flow rates. Each data point in the figures represents the averaged values of three determinations. The repeatability for moisture content, expressed as the average standard deviation of the three determinations, was 0.05%. As it can be drawn from Figs. 1–4, powder moisture content increases with an increase in drying air flow rate. Generally, the energy available for evaporation varies
500
9
2
3.1. Powder moisture content
400
10
3
3. Results and discussion
12
Q a : 19.25 m3 /h
11 powder moisture content (% wb)
The data were analyzed using the statistical software MINITAB (Release 13.32). Regression analysis was used to fit full second order polynomials, reduced second order polynomials containing the three linear terms, and linear models to the data of each of the variables evaluated (response variables). F values for all reduced and linear models with a coefficient of determination (R2 ) greater than 0.70 were calculated to determine if the models could be used in place of full second order polynomials to predict the response of a variable to compressed air flow rate, drying rate flow rate and air inlet temperature (independent variables). The best fitting models were determined on the basis of a high R2 , a low square root of mean square error (S), and a Mallows’ Cp statistic close to the number of predictors contained in the model. The statistical analysis is described analytically in Part I of the study.
37
10 9 8 7 6 5 4 3 2 400
500
600 700 Q c (L/h)
800
900
Fig. 3. Powder moisture content as a function of inlet air temperature and compressed air flow rate. Drying air flow rate, 21.00 m3 /h. Symbols are the same as in Fig. 1.
according to the amount of drying air. This could give the impression that the drying air flow rate must be at a maximum in all cases. However, the movement of air predetermines the rate and degree of droplet evaporation by influencing (a) the passage of spray through the drying zone, (b) the concentration of product in the region of the dryer walls, and (c) the extent to which semi-dried droplets re-enter the hot areas around the air disperser. A lower drying air flow rate, causes an increase in product sojourn time in the drying chamber (Masters, 1979a) and enforces circulation effects (Goula & Adamopoulos, 2004; Oakley & Bahu, 1991). Increased residence times lead to a greater degree of
38
A.M. Goula, K.G. Adamopoulos / Journal of Food Engineering 66 (2005) 35–42
9
moisture being driven from the food to a greater extent than with cooler air. The best model predictions of moisture content to process variables such as, air inlet temperature, drying rate flow rate and compressed air flow rate was as follows:
8
moisture ¼ 124 1:86 Tinlet þ 0:681 Qa þ 0:00115 Qc
12
Q a : 22.75 m 3 /h
powder moisture content (% wb)
11 10
7
2 þ 0:00692 Tinlet 0:00735 Q2a
6
0:000003 Q2c 0:00213 Tinlet Qa
5 4 3 2 400
500
600
700
800
900
Q c (L/h) Fig. 4. Powder moisture content as a function of inlet air temperature and compressed air flow rate. Drying air flow rate, 22.75 m3 /h. Symbols are the same as in Fig. 1.
moisture removal. As a result, an increase in drying air flow rate, decreasing the residence time of the product in the drying chamber, led to higher moisture contents. Moisture content shows a decrease with an increase in compressed air flow rate due to the effect of this flow rate on mean particle size. Increase in air–liquid flow ratio in a two-fluid nozzle atomizer decreases the mean size of the spray droplets (Nath & Satpathy, 1998). With smaller particles, narrow spray cones are formed and air may not penetrate the centre of the spray pattern until droplets have travelled quite some distance from the nozzle (Liang & King, 1991). This reduced mixing of hot air should make the drying rates decrease. However, drying is facilitated by smaller particle sizes for two reasons. First, larger surface area provides more surface in contact with the heating medium and more surface from which the moisture can escape. Second, smaller particles reduce the distance heat must travel to the centre of the particles and reduce the distance through which moisture in the centre of the particles must travel to reach the surface and escape. As shown in Figs. 1–4, an increase in air inlet temperature leads to a decrease in moisture content. The greater the temperature difference between the drying medium and the particles, the greater will be the rate of heat transfer into the particles, which provides the driving force for moisture removal. When the drying medium is air, temperature plays a second important role. As water is driven from the particles in the form of water vapor, it must be carried away, or the moisture will create a saturated atmosphere at the particle surface. This will slow down the rate of subsequent water removal. The hotter the air, the more moisture it will hold before becoming saturated. Thus, high temperature air in the vicinity of the drying particles will take up the
ð1Þ
Eq. (1) has an R2 value of 0.997, and F , Cp and S equal to 0.117, 6.2 and 0.128, respectively. In experiments conducted under the same operating conditions using a standard laboratory spray drying system powder moisture content, varying from 4.16% to 11.27%, was higher (Goula & Adamopoulos, 2003; Goula, Adamopoulos, & Kazakis, 2004). Comparison between the moisture content of powders produced by the two drying systems is shown in Figs. 1–4. Generally, in a spray drying system, the temperature of the exhaust air leaving the drying chamber controls the residual moisture in the powder. A lower moisture content can be reached by higher temperatures at the outlet. In the modified system, the outlet air temperature was 19–24 C lower than in the standard spray drying system. However, the modified system decreased residual moisture content due to the preliminary air dehumidification. The drier the air, the more rapid is the rate of drying. Experimentally determined phase and state transition temperatures of food components and food solids suggest that, in dehydration of liquids during the evaporation of droplets with dissolved substances, the rapid removal of water results in vitrification of the droplets within a short time, and the formation of a solid particle surface (Roos, 2003). This does not allow formation of liquid bridges between contacting particles or particle adhesion to the dryer walls. Diffusion of water vapor however, occurs rapidly within a porous, glassy surface allowing further dehydration of the particle core as the particle travels through the drying chamber. If dehydration conditions do not permit surface solidification before particles collide with each other, as in a standard spray drying system, stickiness results in agglomeration and so in a smaller surface area, which provides less surface in contact with the heating medium and less surface from which the moisture can escape. According to Downton, Flores-Luna, and King (1982), during spray drying the main requirement is that a solid particle surface is formed rapidly, increasing surface viscosity and decreasing sticking of particles with each other or on dryer surfaces. 3.2. Powder bulk density Bulk density results, ranging from 0.199 to 0.365 g/ ml, are given in Figs. 5–8. Data represent the average
A.M. Goula, K.G. Adamopoulos / Journal of Food Engineering 66 (2005) 35–42 Q a : 17.50 m 3 /h
0.40
Tinlet (°C) 110 120 modified 130 system 140 110 120 standard 130 system 140 fitted model
0.35 0.30
0.25 0.20
0.15 0.10 400
0.30
0.25
0.20
0.15 500
600
700
800
900
Qc (L/h)
0.10
400
Fig. 5. Powder bulk density as a function of inlet air temperature and compressed air flow rate. Drying air flow rate, 17.50 m3 /h.
0.40
500
600
700
800
900
Qc (L/h) Fig. 7. Powder bulk density as a function of inlet air temperature and compressed air flow rate. Drying air flow rate, 21.00 m3 /h. Symbols are the same as in Fig. 5.
Q a : 19.25 m 3 /h
0.35
0.40
0.30
Q a : 22.75 m 3 /h
0.35
powder bulk density (g/mL)
powder bulk density (g/mL)
Q a : 21.00 m 3 /h
0.35
powder bulk density (g/mL)
powder bulk density (g/mL)
0.40
39
0.25
0.20
0.15
0.10
400
500
600 700 Q c (L/h)
800
900
0.30
0.25
0.20
0.15
0.10
Fig. 6. Powder bulk density as a function of inlet air temperature and compressed air flow rate. Drying air flow rate, 19.25 m3 /h. Symbols are the same as in Fig. 5.
values of the three determinations. The repeatability for bulk density, expressed as the average standard deviation of the three determinations, was 0.004 g/ml. The effect of drying air flow rate on powder bulk density depends on its effect on moisture content due to the sticky nature of the product. The higher the powder moisture content, the more particles tend to stick together, leaving more interspaces between them and consequently resulting in a larger bulk volume. As a result, air flow rate increases lead to an increase in powder moisture content and a decrease in powder bulk density. Masters (1979b) reported that increasing residual moisture content increases bulk density of a dry product. However, this trend was not observed here, due to the thermoplastic nature of the product.
400
500
600
700
800
900
Qc (L/h) Fig. 8. Powder bulk density as a function of inlet air temperature and compressed air flow rate. Drying air flow rate, 22.75 m3 /h. Symbols are the same as in Fig. 5.
Higher compressed air flow rate causes an increase of powder bulk density due to its effect on mean particle size. Smaller particles produced with higher compressed air rates are also more dense and so further increase bulk density. Increased air inlet temperature causes a reduction in bulk density, as evaporation rates are faster and products dry to a more porous or fragmented structure. According to Walton (2000), increasing the drying air temperature generally produces a decrease in bulk and particle density, and there is a greater tendency for the particles to be hollow. The former can be caused by
A.M. Goula, K.G. Adamopoulos / Journal of Food Engineering 66 (2005) 35–42
bulkdensity ¼ 0:405 þ 0:0132 Tinlet þ 0:00514 Qa 2 þ 0:000239 Qc 0:000044 Tinlet
0:000001 Q2c 0:000188 Tinlet Qa ð2Þ Eq. (2) has an R2 value of 0.974, and F , Cp and S equal to 0.326, 5.0 and 7.295 · 103 , respectively. In experiments conducted under the same operating conditions using a standard laboratory spray drying system, powder bulk density was lower, varying from 0.100 to 0.258 g/ml (Goula et al., 2004). Comparisons between the bulk densities of the powders produced by the two drying systems are shown in Figs. 5–8. Since a low moisture content of tomato powder is associated with a high bulk density, the preliminary air dehumidification increases powder bulk density due to its effect on powder moisture content. In addition, in a standard spray drying system, the particles tend to collide with each other formatting agglomerates with more interspaces between the particles and consequently resulting in lower bulk density. 3.3. Powder solubility Figs. 9–12 show powder solubility in relation to inlet air temperature, compressed air flow rate and drying air flow rate. Solubility of the tomato powders varied from 121 to 245 s. The repeatability expressed as the average standard deviation of the three determinations was 5 s. The effect of drying air flow rate on powder solubility depends on its effect on powder moisture content, as a
Q a : 19.25 m3/h
450 400
powder solubility (sec)
particle inflation-ballooning or puffing, and is particularly common in skin-forming materials. The best model predictions for bulk density in relation to the process variables, air inlet temperature, drying air flow rate and compressed air flow rate, was as follows:
350 300 250 200 150 100 400
Q a : 17.50 m 3 /h
Tinlet (° C) 110 120 modified 130 system 140 110 120 standard 130 system 140 fitted model
powder solubility (sec)
350 300 250 200 150
500
600 700 Q c (L/h)
700
800
900
450
Q a : 21.00 m 3 /h
400 350 300 250 200 150 100 500
600
700
800
900
Q c (L/h)
400
100 400
600
Fig. 10. Powder solubility as a function of inlet air temperature and compressed air flow rate. Drying air flow rate, 19.25 m3 /h. Symbols are the same as in Fig. 9.
400 450
500
Q c (L/h)
powder solubility (sec)
40
800
900
Fig. 9. Powder solubility as a function of inlet air temperature and compressed air flow rate. Drying air flow rate, 17.50 m3 /h.
Fig. 11. Powder solubility as a function of inlet air temperature and compressed air flow rate. Drying air flow rate, 21.00 m3 /h. Symbols are the same as in Fig. 9.
low moisture content seems to be associated with fast dissolution. Air flow rate increases lead to an increase in powder moisture content and a decrease in powder solubility. This trend is similar to that reported by other researchers (Papadakis, Gardeli, & Tzia, 1998). The time required for the powder to dissolve was found to increase with an increase in compressed air flow rate, since particle size affects solubility rate. Large particles may sink, whereas small ones are more dusty and generally float on water making for uneven wetting and reconstitution (Potter, 1968).
A.M. Goula, K.G. Adamopoulos / Journal of Food Engineering 66 (2005) 35–42 450
4. Conclusions
Q a : 22.75 m 3 /h
An experimental spray dryer was modified for drying tomato concentrate. The modification made on the original design consisted of connecting the spray dryer inlet air intake to an absorption air dryer. The effect of spray drying conditions, i.e. air inlet temperature, drying air flow rate, and compressed air flow rate on tomato powder moisture content, bulk density and solubility, was studied. It was observed that:
400
powder solubility (sec)
41
350 300 250 200 150 100 400
500
600
700
800
900
Q c (L/h) Fig. 12. Powder solubility as a function of inlet air temperature and compressed air flow rate. Drying air flow rate, 22.75 m3 /h. Symbols are the same as in Fig. 9.
Solubility showed an increase with an increase in inlet air temperature. This is due to the effect of inlet air temperature on residual moisture content. The lower the powder moisture content, the more soluble is the powder. In addition, increasing the drying air temperature generally produces an increase in particle size, and so a decrease in time required for the powder to dissolve. The best model predictions for powder solubility in relation to the process variables, air inlet temperature, drying air flow rate, and compressed air flow rate, was as follows: solubility ¼ 352 2:22 Tinlet 10:5 Qa þ 0:186 Qc
Moisture content, bulk density and solubility are the most commonly quoted specifications of a powder product. The lower the moisture content and the higher the bulk density and the solubility, the better will be considered the product. In experiments conducted under the same operating conditions using the standard laboratory spray drying system, powder moisture content was higher and powder bulk density and solubility were lower. It appears that the preliminary air dehumidification promoting rapid particulate skin formation improved not only product recovery, but, also product properties.
References
þ 0:393 Q2a 0:000083 Q2c þ 0:0301 Tinlet Qa
• powder moisture content decreases with an increase in air inlet temperature and compressed air flow rate, and with a decrease in drying air flow rate, • bulk density increases with a decrease in drying air flow rate and air inlet temperature, and with an increase in compressed air flow rate, • solubility increases with a decrease in drying and compressed air flow rate, and with an increase in air inlet temperature.
ð3Þ
Eq. (3) has an R2 value of 0.979, and F , Cp and S equal to 0.205, 4.6 and 4.122, respectively. In experiments conducted under the same operating conditions using a standard laboratory spray drying system powder solubility was lower, varying from 188 to 435 s (Goula et al., 2004). Comparisons between the solubilities of the powders produced with the two drying systems in relation to the air inlet temperature are shown in Figs. 9–12. The modified system increased powder solubility due to its effect on powder moisture content, since a low moisture content seems to be associated with fast dissolution. Furthermore, the much higher air temperatures in the standard system may have resulted in denaturing more protein and hence affected solubility.
Al-Kahtani, H. A., & Hassan, B. H. (1990). Spray drying of Roselle (Hibiscus sabdariffa L.) extract. Journal of Food Science, 55(4), 1073–1076. Bhandari, B. R., Datta, N., & Howes, T. (1997). Problems associated with spray drying of sugar-rich foods. Drying Technology, 15(2), 671–685. Bhandari, B. R., Datta, N., Crooks, R., Howes, T., & Rigby, S. (1997). A semi-empirical approach to optimize the quantity of drying aids required to spray dry sugar-rich foods. Drying Technology, 15(10), 2509–2525. Cook, E. M. (1991). Introduction. In Process drying practice (pp. 3– 12). New York: McGraw-Hill. Downton, D. P., Flores-Luna, J. L., & King, C. J. (1982). Mechanism of stickiness in hygroscopic, amorphous powders. Industrial & Engineering Chemistry Fundamentals, 21, 447–451. El-Tinay, A. H., & Ismail, I. A. (1985). Effect of some additives and processes on the characteristics of agglomerated and granulated spray-dried Roselle powder. Acta Alimentaria Hungaricae, 14(3), 283–295. Goose, P. G., & Binsted, R. (1964). Allied tomato products. In Tomato paste, puree, juice and powder (1st ed., pp. 124–126). UK, London: Food Trade Press.
42
A.M. Goula, K.G. Adamopoulos / Journal of Food Engineering 66 (2005) 35–42
Goula, A. M., & Adamopoulos, K. G. (2003). Influence of spray drying conditions on tomato powder moisture. In Proceedings of the third international symposium of food rheology and structure, Zurich, Switzerland. Goula, A. M., & Adamopoulos, K. G. (2004). Influence of spray drying conditions on residue accumulation––Simulation using CFD. Drying Technology (5). Goula, A. M., & Adamopoulos, K. G. (in press). Spray drying of tomato pulp in dehumidified air. I. The effect on the product recovery. Journal of Food Engineering. doi:10.1016/j.jfoodeng.2004.02.029. Goula, A. M., Adamopoulos, K. G., & Kazakis, N. A. (2004). Influence of spray drying conditions on tomato powder properties. Drying Technology (5). Gransmith, M. (1971). Dehydration of foods. In Practical dehydration (pp. 125–133). UK, London: Food Trade Press. Hecht, J. P., & King, C. J. (2000). Spray drying: Influence of developing drop morphology on drying rates and retention of volatile substances. 1. Single-drop experiments. Industrial & Engineering Chemistry Research, 39(6), 1756–1765. Jayaraman, K. S., & Das Gupta, D. K. (1995). Drying of fruits and vegetables. In Handbook of industrial drying (2nd ed., pp. 661–662). New York: Marcel Dekker. Karatas, S. (1989). A laboratory scraped surface drying chamber for spray drying of tomato paste. Lebensmittel-Wissenschaft und Technologie, 23(4), 354–357. Krokida, M. K., & Maroulis, Z. B. (1997). Effect of drying method on shrinkage and porosity. Drying Technology, 15(10), 2441–2458. Krokida, M. K., Zogzas, N. P., & Maroulis, Z. B. (1998). Shrinkage and porosity during drying. Modeling the effect of vacuum. International Journal of Food Science and Technology, 32, 445–458. Liang, B., & King, C. J. (1991). Factors influencing flow patterns, temperature fields and consequent drying rates in spray drying. Drying Technology, 9(1), 1–25. Masters, K. (1979a). Spray drying fundamentals: Process stages and layouts. In Spray drying handbook (pp. 21–53). New York: Halsted Press.
Masters, K. (1979b). Drying of droplets/sprays. In Spray drying handbook (pp. 291–335). New York: Halsted Press. Masters, K. (1997). Spray dryers. In C. G. J. Baker (Ed.), Industrial drying of foods (pp. 90–114). New York: Blackie Academic & Professional. Nath, S., & Satpathy, G. R. (1998). A systematic approach for investigation of spray drying processes. Drying Technology, 16(6), 1173–1193. Oakley, D. E. (1997). Produce uniform particles by spray drying. Chemical Engineering Progress, 93(10), 48–54. Oakley, D. E., & Bahu, R. E. (1991). Spray/gas mixing behaviour within spray dryers. In A. S. Mujumdar & I. Filkova (Eds.), Drying (pp. 303–313). Amsterdam: Elsevier. Papadakis, S. E., Gardeli, C., & Tzia, C. (1998). Raisin extract powder: Production, physical and sensory properties. In Proceedings of the 11th international drying symposium IDS ’98, Halkidiki, Greece. Ponting, J. D., Stanley, W. L., & Copley, M. J. (1973). Fruit and vegetables juices. In B. S. Van Arsdel, M. J. Copley, & A. I. Morgan (Eds.), Food dehydration (2nd ed., pp. 211–215). Westport, Connecticut: AVI Publishing Company. Potter, N. N. (1968). Food science (pp. 227–257). Westport, Connecticut: AVI Publishing Company. Roos, Y. (2003). Phase and state transitions in food dehydration. In Proceedings of the symposium EUDrying ’03, Heraklion, Crete, Greece. Spicer, A. (1974). Effects of latest developments of spray drying. In Advances in preconcentration and dehydration of foods (pp. 337– 340). New York: John Wiley and Sons. Walton, D. E. (2000). The morphology of spray-dried particles––A qualitative view. Drying Technology, 18(9), 1943–1986. Walton, D. E., & Mumford, C. J. (1999a). Spray dried products–– Characterization of particle morphology. Transactions of the Institution of Chemical Engineers––London, 77(A), 21–38. Walton, D. E., & Mumford, C. J. (1999b). The morphology of spraydried particles––The effect of process variables upon the morphology of spray-dried particles. Transactions of the Institution of Chemical Engineers––London, 77(A), 442–460.