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Agglomeration Tendency during Top-Spray Fluidized Bed Coating with Gums
Article No. fs980421
Lebensm.-Wiss. u.-Technol., 31, 576]584 Ž1998 .
Agglomeration Tendency during Top-Spray Fluidized Bed Coating with Gums Koen Dewettinck, Lidewij Deroo, Winy Messens and Andre ´ Huyghebaert University of Ghent, Faculty of Agricultural and Applied Biological Sciences, Department of Food Technology and Nutrition, Coupure Links 653, B-9000 Ghent ŽBelgium. (Recei¨ ed May 13, 1998; accepted July 8 1998) Top-spray fluidized bed coating of glass beads with gums leads to an ‘all or nothing’ side-effect agglomeration in the case of locust bean gum, carboxymethylcellulose (CMC), and sodium alginate, meaning that it appeared ¨ ery drastically when a critical spray rate was exceeded. By contrast, a continuous sticking of the particles onto the reactor wall was obser¨ ed for k-carrageenan. It was illustrated that wet film properties instead of solution properties determine agglomeration tendency since locust bean gum, high and low ¨ iscosity CMC show a comparable agglomeration tendency. The glass transition concept offers some rele¨ ant explanation as to why k-carrageenan permits working at considerably higher spray rates than sodium alginate, although both biopolymers show similar hygroscopical and rheological characteristics. Glass transition phenomena also seem to influence the magnitude of coating imperfections: coatings consisting of locust bean gum and k-carrageenan appear to ha¨ e a surprisingly uniform structure. q 1998 Academic Press Keywords: fluid-bed; coating; microencapsulation; agglomeration; gums
Introduction A variety of encapsulation techniques are used in the pharmaceutical industry. The applicability and utility of microencapsulation in the food industry has become generally recognized Ž1]9.. Virtually any material that needs to be protected, isolated or slowly released can be encapsulated Ž10.. The main benefits of such miniature packages, known as microcapsules, include an increased shelf-life, taste masking, ease of handling, controlled release, and improved aesthetics, taste and colour Ž4, 5.. Fluidized bed drier systems are currently being used to microencapsulate many food ingredients Ž4, 5. and coatings can be applied to fluidized particles by a variety of techniques, including spraying from the top, bottom, or tangentially Ž11.. However, top-spray fluid-bed processing has a greater possibility of being applied in food technology when compared to the other configurations due to its high versatility, relatively large batch size, and relative simplicity. The main constraints of fluid-bed coating as a microencapsulation process are the type of core material Ži.e. free flowing solid particles with a size varying between 100 mm and several millimeters., and the type of coating material Ži.e. mainly water-soluble biopolymers.. The choice of the appropriate film-forming material or edible polymer film is mainly influenced by the specified core and barrier characteristics. The properties have been reviewed for coating materials such as edible films and coatings in general Ž12., edible and biodegradable polymer films Ž13., lipid coatings Ž14.,
milk-protein-based edible films and coatings Ž15., and wheat and corn-protein-based edible films and coatings Ž16.. However, the agglomeration tendency Žundesired side-effect agglomeration. during the coating of the particles is also influenced by the type of film. The objective of this paper was to investigate the agglomeration tendency of several gums Ži.e. locust bean gum, carboxymethylcellulose, sodium alginate, and k-carrageenan . and to explain their variations in behaviour. Glass beads were chosen as a model system for food ingredients because they are perfectly spherically shaped with a smooth, shiny surface and chemically inert. This should guarantee that the experimental observations may be primarily attributed to the type of coating applied.
Materials and Methods Core material Micropearl W glass beads Žarticle code: C. purchased from Sovitec s.a. ŽFleurus, Belgium. with a volume weighed average size of 365 mm and a density of 2496 kgrm3 were used as a core material.
Coating materials Locust bean gum ŽLBG. Žarticle code: Viscogum BCR 20., sodium alginate ŽALG. Žarticle code: Satialgine SG 300., and k-carrageenan ŽCAR. Žarticle code: Satiagel AMP 35. were provided by SBI ŽBrussels, Belgium..
0023-6438r98r060576 q 09 $30.00r0 q 1998 Academic Press
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Table 1 Viscosity and density data of locust bean gum ŽLBG., high viscosity CMC ŽCMCHV., low viscosity CMC ŽCMCLV., sodium alginate ŽALG., and k-carrageenan ŽCAR. m ŽmPa ? s r c Žkgrm3 . b .a
LBG
CMCHV
CMCLV
ALG
CAR
548 1491
933 1594
8 1536
85 1661
79 1648
a
viscosity determined in duplicate at 20 8C with a capillary viscometer at a concentration of 10 grkg. b coating density determined in duplicate using a density flask.
Two carboxymethylcelluloses with a high ŽCMCHV. Žarticle code: Blanose 7LXF. and low ŽCMCLV. Žarticle code: Blanose 7H3SXF. solution viscosity were provided by Hercules Belgium n.v. ŽBeringen, Belgium.. The degree of substitution of both gums was 0.7. The viscosity and density data of the coatings applied are given in Table 1.
Fluidized bed equipment All coating investigations were performed in the topspray reactor of the Glatt GPCG-1 ŽGlatt GmbH, Binzen, Germany.. The pneumatic nozzle was placed in the lower position and the screw position was kept constant so that the spray cone did not undergo excessive contact with the reactor wall; the filter system was not shaken. After coating, the particles were dried for 10 min to a bed temperature of approximately 60 8C.
Experimental design Each coating experiment was performed in duplicate. The coating solution spray rate R sol , and hence the evaporation efficiency E, were increased stepwise and agglomeration was detected visually. The process variables held constant were; mass of core material, Wp s 0.75 kg; inlet air velocity, Vi s 3 mrs; inlet air temperature, T1 s 70 8C; mass of coating solution, Wcs s 1.5 kg, and coating solution dry matter content, DM s 10 grkg. The coating solution was held at 70 8C for LBG, at 50 8C for CMCHV, ALG, and CAR, and at ambient temperature for CMCLV. Calculation of the e¨ aporation efficiency In convective drying, the processing air has a certain drying or Žwater. evaporation capacity Ce Žkgrkg dry air., depending on its temperature T1 and humidity x 1. Ce can be calculated using Ce s x s y x 1
Eqn w 1 x
with: x 1: inlet air absolute humidity Žkgrkg. x s : saturated air absolute humidity Žkgrkg. The saturated air absolute humidity x s is determined from the Mollier-diagram at the intersection of a t-line, representing air conditions with the same wet-bulb temperature, constructed from the point representing
the condition of the inlet drying air and the curve of constant relative humidity, w s 1. At higher inlet air temperatures the drying capacity of the air increases. However, in spray drying processes the full evaporation capacity is never used due to the technical limitations of the equipment, the droplet drying time, and the product characteristics. Consequently the evaporation efficiency, E of a convective drying process can be calculated using: Es
x 2 y x1 xs y x1
Eqn w 2 x
x 1: inlet air absolute humidity Žkgrkg. x 2 : outlet air absolute humidity Žkgrkg. x s : saturated air absolute humidity Žkgrkg. Reiland et al. Ž17. defined a similar vaporization efficiency by comparing the spray rate to a maximum possible application rate. It should be noted that the lower the inlet air temperature the larger the weather effect will be on the evaporation efficiency. In traditional industrial spray drying relatively high inlet air temperatures are applied ranging from 150 to 270 8C Ž18.. In this case changes in ambient air condition will have a negligible effect on the drying air evaporation capacity and efficiency. In air suspension coating however, lower inlet air temperatures are applied, ranging from 50 to 90 8C when using aqueous spraying solutions. This results in very significant batch variations due to the effects of the weather when using unconditioned air. Under these conditions it can be more interesting to compare the processes on the basis of evaporation efficiency instead of spray rate. Ambient air relative humidity wr and temperature Tr were recorded by means of a Testo 610 Žn.v. Testoterm, Ternat, Belgium. during the coating experiments. Since it was difficult to control the peristaltic pump exactly, the spray rate R sol was derived by carefully monitoring the mass of coating solution as a function of time using an electronic balance Sartorius 2472 ŽSartorius AG, Goettingen, Germany.. The evaporation efficiency, E was calculated using the computer program, Topsim ŽThermodynamic Operation Point Simulation. Ž19..
Determination of coating adsorption isotherm, coating solution ¨ iscosity, and deposited coating mass The adsorption isotherms of the several biopolymers were determined at 25 8C using the Novasina Thermoconstanter TH 200 ŽAxair Ltd., Pfaffikon SZ, The ¨ Netherlands.. The viscosity at increasing stress was determined in five-fold at 35 and 50 8C with a controlled stress Bohlin CVO rheometer ŽBohlin Instruments, Cirencester, Gloucestershire, U.K.. to monitor for pseudoplastic nonNewtonian behaviour. The C25 cup and bob geometry was used. At high stress values, the measurement can become unstable due to Taylor vortex flow Ž20, 21. resulting in an apparent viscosity rise. Consequently, the viscosity at high shear stress Žm` , in Pa ? s. was
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estimated by omitting the unstable measurements and extrapolation using the exponential eqn w3x Eqn w 3 x
m s a ? e Žybt. q m` m: t: m` : a: b:
viscosity ŽPa ? s. ishear stress ŽPa. estimated viscosity at high stress ŽPa ? s. constant ŽPa ? s. constant ŽPay1 .
Microcapsule coating content Žgrkg. was determined in three-fold by accurately weighing 20 g of coated beads before and after thoroughly washing with boiling water. The total deposited coating mass Wc Žg. was then calculated, based on the coating content.
Calculation of coating solution droplet size and coating thickness The droplet size produced from pneumatic nozzles may be predicted using the following correlation, which yields the Sauter mean droplet diameter Dd in mm Ž22. 3
Dd s
585 = 10 3 's n REL 's
q 597
m
0.45
ž' / ž sr
1000Usol Uat
1.5
/
Eqn w 4 x where: s: m: Usol : Usol : Uat : n RE L :
fluid surface tension ŽNrm. fluid density Žkgrm3 . fluid viscosity ŽmPa s. fluid volumetric flow rate Žm3rs. air volumetric flow rate Žm3rs. relative velocity outlet air velocity Žmrs.
The surface tension effect of the dissolved biopolymer was not taken into account because of the extremely short droplet formation time in the pneumatic nozzle. Therefore, the surface tension of water at 40 8C was used as a value for s Ži.e. 0.06956 Nrm. Ž23.. The coating thickness d c Žmm. of spherical microcapsules can be calculated using Ž19.: dc s
rp
žžž / ž rc
?
Wp q Wc Wp
1r3
/ /
y1 q1
/ž
y1 ?
Dp 2
/
Eqn w 5 x Dp : diameter of the core material Žmm. r p :density of core material Žkgrm3 . r c :density of coating material Žkgrm3 . Wp :mass of core material Žg. Wc :mass of coating Žg. Microcapsule surface examination The coated beads were placed on double-sided adhesive carbon tabs mounted on SEM-stubs and coated with gold in a sputter coater. Using a Philips SEM 505
scanning electron microscope ŽPhilips, Eindhoven, The Netherlands., the surface structure of the different films attached to the glass beads was examined at 15 keV. At least 10 capsules were examined for each treatment.
Statistical analysis The mean values were compared using the ‘one way ANOVA’ procedure in combination with the Duncan multiple comparison test included in the SPSS 7.0 software.
Results and Discussion Detection of the agglomeration The agglomeration of glass beads during coating with locust bean gum, carboxymethylcellulose ŽCMC., and sodium alginate was observed to be an ‘all or nothing’ phenomenon that remained completely absent at a particular evaporation efficiency and appeared very drastically when a critical evaporation efficiency was exceeded. This resulted in a complete collapse of the fluid-bed, accompanied by a coverage of reactor walls and nozzle with agglomerated particles. The actual downfall of the fluid-bed was generally preceded by temperature fluctuations of about 1.5 8C in the fluid-bed and about 1 8C at the air outlet. These fluctuations are most likely caused by irregular fluidization due to the formation and break up of agglomerates accompanied by strong humidity fluctuations in the fluid-bed. In contrast, coating with k-carrageenan at an evaporation efficiency of about 0.85, resulted in a continuous adhesion of the particles onto the reactor wall, resulting in a reduction of the fluid-bed density, and finally a fluid-bed disappearance. This phenomenon could also be observed to a lesser extent at lower spray rates. These successfully terminated processes exhibited a critical phase during which the particles temporarily adhered onto the reactor wall, followed by a uniform and gentle fluidization. This critical phase was always accompanied by bed temperature fluctuations of around 1 8C and appeared earlier in the process at higher evaporation efficiencies.
High-¨ iscous gums as coating agents Two gums with a high solution viscosity, locust bean gum ŽLBG. and high viscosity carboxymethylcellulose ŽCMCHV., were studied at an atomization pressure of 3.5 bar. The highly viscous CMCHV was also compared with a low viscosity carboxymethylcellulose ŽCMCLV. at the same atomization pressure. The evaporation efficiency E, total process time t, and coating mass Wc for the gums are given in Table 2; when a collapse of the fluid-bed occurred, the time for agglomeration ta is also indicated. The Wc values of the highly viscous gums LBG and CMCHV are not statistically different Ž P ) 0.05. and their agglomeration tendency is comparable. The characteristics of high and low viscosity
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Table 2 Evaporation efficiency E, total process time t, deposited coating mass Wc , and agglomeration time ta for locust bean gum ŽLBG., high viscosity CMC ŽCMCHV., and low viscosity CMC ŽCMCLV. Coating LBG LBG CMCHV CMCHV CMCLV CMCLV
E 0.51 0.56 0.47 0.52 0.47 0.52
t Žmin. 213 Ž197. a 247 Ž221. a 260 Ž237. a
Wc Žg.
ta Žmin. .b
12.83 Ž0.44 n.d. 13.22 Ž0.23. b n.d. 13.05 Ž0.26. b n.d.
n.ag. 27 n.ag. 195 n.ag. 35
a
the expected total process time. the standard deviation Žs x .. n.d.s not determined. n.ag.s not agglomerated. b
Fig. 1 Adsorption isotherms of locust bean gum ŽLBG., high viscosity CMC ŽCMCHV., and low viscosity CMC ŽCMCLV. at 25 8C Ž a w s water activity Ž } ... Ž }=} . s LBG; Ž }v} . s CMCHV; Ž }`} . s CMCLV
CMC are practically identical; their Wc values are not statistically different Ž P ) 0.05. and both polymers agglomerate the beads at the same evaporation efficiency. The fact that all three coatings showed a comparable agglomeration tendency was quite unexpected because they have diverse hygroscopical ŽFig. 1. and rheological properties. This illustrates that wet film properties rather than solution properties determine the agglomeration tendency. Both high-viscous polymers show pseudoplastic behaviour ŽFig. 2. so the droplets produced by the high-shear pneumatic nozzle may be much smaller than expected, a phenomenon that delays agglomeration. Scanning electron micrographs of glass beads coated with the different biopolymers reveal drastic differences in coating quality ŽFigs 3–5.. In the case of the glass beads coated with CMCHV ŽFig. 3., it appears that the settled droplets experienced difficulties in spreading out, resulting in a very rough structure. By
Fig. 2 Pseudoplastic behaviour of a 10 grkg solution of locust bean gum ŽLBG. and high viscosity CMC ŽCMCHV. at 35 and 50 8C. The 95% confidence limits are indicated. Ž }l} . s MCHV, 35 8C; Ž }e} . s CMCHV, 50 8C; Ž }'} . s LBG, 35 8C; Ž }^} . s LBG, 50 8C
Fig. 3 Scanning electron micrograph Ž=131. of glass beads coated with high viscosity CMC
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Fig. 4 Scanning electron micrograph Ž=131. of glass beads coated locust bean gum
contrast, the glass beads coated with LBG show a very smooth structure and coating imperfections are practically absent ŽFig. 4.. The higher hygroscopicity of CMC ŽFig. 1. is thought to contribute to the poor coating quality compared to LBG. The micrograph in Fig. 5 confirms this assumption because the low viscosity type is seen to have a similar coating structure. The high viscosity of the 10 grkg CMCHV solution is therefore not necessarily the reason for its poor coating quality.
Moderate-¨ iscous gums as coating agents The agglomeration tendency of two gums with a moderate solution viscosity, namely sodium alginate ŽALG. and k-carrageenan ŽCAR., was studied.
The adsorption isotherms ŽFig. 6. imply that both gums have a similar hygroscopicity. However, the equilibration of the sorption measurements took approximately half the time for ALG as compared to CAR. It can thus by concluded that, from the dynamic point of view, ALG adsorbs water twice as fast as CAR. The pseudoplastic behaviour of both gums was determined at two temperatures ŽFig. 7.. If the high shear forces present in a pneumatic nozzle are taken into account, it is important to have an accurate estimate of the viscosity of both gums at a high shear stress. Equation w3x was fitted on the viscosity data using CurveExpert 1.2. Using the m` values ŽTable 3., the viscosity at a high shear stress and temperature of 40 8C Žwhich approximates the average nozzle tempera-
Fig. 5 Scanning electron micrograph Ž=131. of glass beads coated with low viscosity CMC
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Fig. 6 Adsorption isotherms of sodium alginate ŽALG. and k-carrageenan ŽCAR. at 25 8C Ž a w : water activity Ž-... Ž }^} . s ALG; Ž }I} . s CAR
Fig. 8 Sauter average droplet size, Dd Žmm. of sodium alginate ŽALG. and k-carrageenan ŽCAR. as a function of evaporation efficiency, E at different atomization pressures. Ž }B} . s ALG, 1.5 bar; Ž }l} . s ALG, 2.5 bar; Ž }'} . s ALG, 3.5 bar; Ž }I} . s CAR, 1.5 bar; Ž }e} . s CAR, 2.5 bar; Ž }^} . s CAR, 3.5 bar; Ž }=} . s ALG, 3 bar Fig. 7 Pseudoplastic behaviour of a 10 grkg solution of sodium alginate ŽALG. and k-carrageenan ŽCAR. at 35 and 50 8C. The 95% confidence limits are indicated. Ž }'} . s ALG, 35 8C; Ž }^} . s ALG;, 50 8C; Ž }B} . s CAR, 35 8C; Ž }I} . s CAR, 50 8C
ture. can be approximated by interpolation. This gives viscosities of 33 and 22 mPa.s for ALG and CAR, respectively. These values were used to calculate the average droplet size as a function of the evaporation efficiency at different atomization pressures using Eqn w4x in which the fluid viscosity m is an important parameter. At an atomization pressure of 3 bar, the size of droplets produced from a 10 grkg ALG solution becomes practically equal to those produced from a 10 grkg CAR solution at 2.5 bar ŽFig. 8.. A series of investigations on ALG were also performed at 3 bar in
an attempt to compare both gums at a similar droplet size, Dd . At an atomization pressure of 2.5 bar and at similar droplet size the gums, ALG and CAR differ surprisingly in agglomeration tendency ŽFig. 9. although both appear to have comparable hygroscopical and rheological properties ŽFigs 6 and 7.. However, during the evaporation and hence concentration of the settling
Table 3 Calculating the viscosity at high shear stress of sodium alginate ŽALG. and k-carrageenan ŽCAR. at 35 and 50 8C by extrapolation Coating ALG ALG CAR CAR
T Ž8C. 35 50 35 50
m s a ? e Žyb t . q m` a ŽPa ? s. b ŽPay1 . m` ŽPa ? s. 0.0494 0.0325 0.0199 0.0145
0.0272 0.0317 0.0583 0.0547
0.0340 0.0309 0.0234 0.0200
R2 0.9988 0.9990 0.9990 0.9993
Fig. 9 Deposited coating mass Wc and agglomeration point Ž=. as a function of evaporation efficiency E for sodium alginate ŽALG. at 2.5 and 3 bar atomization pressures, and k-carrageenan ŽCAR. at 2.5 bar atomization pressure. The agglomeration time Žmin. and 95% confidence limits are indicated. Ž }B} . s CAR, 2.5 bar; Ž }'} . s ALG, 2.5 bar; Ž }^} . s ALG, 3 bar
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Fig. 10 Scanning electron micrograph Ž=625. of a glass bead coated with sodium alginate
droplets, the rheological properties can alter very drastically due to the glass transition phenomena. Some hypotheses based on the glass transition theory Ž24. can be proposed to explain the difference in agglomeration tendency of ALG and CAR; Ži. the glass transition temperature ŽTg . of ALG is generally lower compared to CAR, resulting in stickiness at lower temperatures; Žii. both coatings have a similar Tg without the presence of water but the plasticizing effect of water is more pronounced in ALG; Žiii. both coatings have a similar Tg and plasticizing effect of water but a coating of ALG has a higher moisture content at the same bed relative humidity compared to CAR. The third hypothesis is countered by the fact that both coatings appear to have very similar adsorption isotherms at 25 8C ŽFig. 6.. However, as discussed ear-
lier it was clearly observed that the equilibration of the measurements of the adsorption isotherms took about half the time for ALG compared to CAR. As a consequence, a higher moisture content of ALG at a certain relative humidity could be possible. Scanning electron micrographs of glass beads coated with both gums ŽFigs 10 and 11. clearly demonstrate that the surface structure of the film is different for the two gums used. CAR droplets appear to have spread out almost perfectly leaving a film with few imperfections. Conversely, the dried ALG droplets experienced more difficulties in spreading out. This further illustrates that a totally different phenomenon occurs during the water evaporation from the ALG coating, and the possible existence of a rubbery, sticky state with a high liquid viscosity.
Fig. 11 Scanning electron micrograph Ž=625. of a glass bead coated with k-carrageenan
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Table 4 Agglomeration time ta and estimated coating thickness d c at higher spray rates R sol using sodium alginate ŽALG. as a coating agent R sol Žgrmin. 5.48 6.40 7.20 7.80 8.26
Ea
T2, exp Ž8C. b
T2, sim Ž8C. a
w 2 Ž%. a
ta Žmin.
d c Žmm.
0.42 0.47 0.51 0.57 0.61
44.2 42.1 39.2 38.5 37.4
44.9 42.6 40.4 38.8 37.8
20.4 20.2 21.1 27.7 32.2
227 205 114 70 55
1.15 1.21 0.76 0.51 0.42
a
evaporation efficiency E, outlet air temperature T2, sim , and outlet air relative humidity w 2 were calculated by using the program Topsim Ž19.. b experimental outlet air temperature.
The fact that at a higher atomization pressure, the agglomeration is introduced at higher evaporation efficiencies when using ALG as coating agent ŽFig. 9. can be explained by extra fluid-bed turbulence resulting in a better particle separation and a smaller droplet size. At an atomization pressure of 2.5 bar a significant Ž*P - 0.05. decrease of Wc can also be observed for ALG compared to the results obtained at 3 bar. It is probable that the higher speed of the droplets produced at 3 bar compared to 2.5 bar positively influences the coating yield. Table 4 displays the evolution of the agglomeration time ta and the estimated coating thickness at the point of agglomeration when using ALG as coating agent at higher values of the spray rate R sol . At higher spray rates, and hence higher bed relative humidities, the agglomeration phenomenon occurs at a lower coating thickness. The agglomeration is therefore determined by the actual coating thickness and by the thermodynamic operation point, which influences the coating properties such as the stickiness Žviscosity. and water evaporation rate.
Conclusions Despite high viscosities, locust bean gum and high viscosity CMC enable fluid-bed coating to occur at moderately elevated spray rates. The fact that the low viscosity type CMC shows similar characteristics to the high viscosity type, further illustrates that the wet film and not the solution characteristics mainly determine the agglomeration tendency. Sodium alginate exhibits similar hygroscopical and rheological characteristics to k-carrageenan, but k-carrageenan permits the use of significantly higher spray rates before fluid-bed collapse occurs, for which the glass transition concept can offer some relevant explanation. The glass transition phenomena also appear to influence the magnitude of coating imperfections; coatings consisting of locust bean gum and k-carrageenan appear to have a surprisingly uniform structure.
Acknowledgement The authors acknowledge with gratitude the technical support given by M. Jooris.
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Lebensm.-Wiss. u.-Technol., 31, 576]584 Ž1998 .
Agglomeration Tendency during Top-Spray Fluidized Bed Coating with Gums Koen Dewettinck, Lidewij Deroo, Winy Messens and Andre ´ Huyghebaert University of Ghent, Faculty of Agricultural and Applied Biological Sciences, Department of Food Technology and Nutrition, Coupure Links 653, B-9000 Ghent ŽBelgium. (Recei¨ ed May 13, 1998; accepted July 8 1998) Top-spray fluidized bed coating of glass beads with gums leads to an ‘all or nothing’ side-effect agglomeration in the case of locust bean gum, carboxymethylcellulose (CMC), and sodium alginate, meaning that it appeared ¨ ery drastically when a critical spray rate was exceeded. By contrast, a continuous sticking of the particles onto the reactor wall was obser¨ ed for k-carrageenan. It was illustrated that wet film properties instead of solution properties determine agglomeration tendency since locust bean gum, high and low ¨ iscosity CMC show a comparable agglomeration tendency. The glass transition concept offers some rele¨ ant explanation as to why k-carrageenan permits working at considerably higher spray rates than sodium alginate, although both biopolymers show similar hygroscopical and rheological characteristics. Glass transition phenomena also seem to influence the magnitude of coating imperfections: coatings consisting of locust bean gum and k-carrageenan appear to ha¨ e a surprisingly uniform structure. q 1998 Academic Press Keywords: fluid-bed; coating; microencapsulation; agglomeration; gums
Introduction A variety of encapsulation techniques are used in the pharmaceutical industry. The applicability and utility of microencapsulation in the food industry has become generally recognized Ž1]9.. Virtually any material that needs to be protected, isolated or slowly released can be encapsulated Ž10.. The main benefits of such miniature packages, known as microcapsules, include an increased shelf-life, taste masking, ease of handling, controlled release, and improved aesthetics, taste and colour Ž4, 5.. Fluidized bed drier systems are currently being used to microencapsulate many food ingredients Ž4, 5. and coatings can be applied to fluidized particles by a variety of techniques, including spraying from the top, bottom, or tangentially Ž11.. However, top-spray fluid-bed processing has a greater possibility of being applied in food technology when compared to the other configurations due to its high versatility, relatively large batch size, and relative simplicity. The main constraints of fluid-bed coating as a microencapsulation process are the type of core material Ži.e. free flowing solid particles with a size varying between 100 mm and several millimeters., and the type of coating material Ži.e. mainly water-soluble biopolymers.. The choice of the appropriate film-forming material or edible polymer film is mainly influenced by the specified core and barrier characteristics. The properties have been reviewed for coating materials such as edible films and coatings in general Ž12., edible and biodegradable polymer films Ž13., lipid coatings Ž14.,
milk-protein-based edible films and coatings Ž15., and wheat and corn-protein-based edible films and coatings Ž16.. However, the agglomeration tendency Žundesired side-effect agglomeration. during the coating of the particles is also influenced by the type of film. The objective of this paper was to investigate the agglomeration tendency of several gums Ži.e. locust bean gum, carboxymethylcellulose, sodium alginate, and k-carrageenan . and to explain their variations in behaviour. Glass beads were chosen as a model system for food ingredients because they are perfectly spherically shaped with a smooth, shiny surface and chemically inert. This should guarantee that the experimental observations may be primarily attributed to the type of coating applied.
Materials and Methods Core material Micropearl W glass beads Žarticle code: C. purchased from Sovitec s.a. ŽFleurus, Belgium. with a volume weighed average size of 365 mm and a density of 2496 kgrm3 were used as a core material.
Coating materials Locust bean gum ŽLBG. Žarticle code: Viscogum BCR 20., sodium alginate ŽALG. Žarticle code: Satialgine SG 300., and k-carrageenan ŽCAR. Žarticle code: Satiagel AMP 35. were provided by SBI ŽBrussels, Belgium..
0023-6438r98r060576 q 09 $30.00r0 q 1998 Academic Press
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Table 1 Viscosity and density data of locust bean gum ŽLBG., high viscosity CMC ŽCMCHV., low viscosity CMC ŽCMCLV., sodium alginate ŽALG., and k-carrageenan ŽCAR. m ŽmPa ? s r c Žkgrm3 . b .a
LBG
CMCHV
CMCLV
ALG
CAR
548 1491
933 1594
8 1536
85 1661
79 1648
a
viscosity determined in duplicate at 20 8C with a capillary viscometer at a concentration of 10 grkg. b coating density determined in duplicate using a density flask.
Two carboxymethylcelluloses with a high ŽCMCHV. Žarticle code: Blanose 7LXF. and low ŽCMCLV. Žarticle code: Blanose 7H3SXF. solution viscosity were provided by Hercules Belgium n.v. ŽBeringen, Belgium.. The degree of substitution of both gums was 0.7. The viscosity and density data of the coatings applied are given in Table 1.
Fluidized bed equipment All coating investigations were performed in the topspray reactor of the Glatt GPCG-1 ŽGlatt GmbH, Binzen, Germany.. The pneumatic nozzle was placed in the lower position and the screw position was kept constant so that the spray cone did not undergo excessive contact with the reactor wall; the filter system was not shaken. After coating, the particles were dried for 10 min to a bed temperature of approximately 60 8C.
Experimental design Each coating experiment was performed in duplicate. The coating solution spray rate R sol , and hence the evaporation efficiency E, were increased stepwise and agglomeration was detected visually. The process variables held constant were; mass of core material, Wp s 0.75 kg; inlet air velocity, Vi s 3 mrs; inlet air temperature, T1 s 70 8C; mass of coating solution, Wcs s 1.5 kg, and coating solution dry matter content, DM s 10 grkg. The coating solution was held at 70 8C for LBG, at 50 8C for CMCHV, ALG, and CAR, and at ambient temperature for CMCLV. Calculation of the e¨ aporation efficiency In convective drying, the processing air has a certain drying or Žwater. evaporation capacity Ce Žkgrkg dry air., depending on its temperature T1 and humidity x 1. Ce can be calculated using Ce s x s y x 1
Eqn w 1 x
with: x 1: inlet air absolute humidity Žkgrkg. x s : saturated air absolute humidity Žkgrkg. The saturated air absolute humidity x s is determined from the Mollier-diagram at the intersection of a t-line, representing air conditions with the same wet-bulb temperature, constructed from the point representing
the condition of the inlet drying air and the curve of constant relative humidity, w s 1. At higher inlet air temperatures the drying capacity of the air increases. However, in spray drying processes the full evaporation capacity is never used due to the technical limitations of the equipment, the droplet drying time, and the product characteristics. Consequently the evaporation efficiency, E of a convective drying process can be calculated using: Es
x 2 y x1 xs y x1
Eqn w 2 x
x 1: inlet air absolute humidity Žkgrkg. x 2 : outlet air absolute humidity Žkgrkg. x s : saturated air absolute humidity Žkgrkg. Reiland et al. Ž17. defined a similar vaporization efficiency by comparing the spray rate to a maximum possible application rate. It should be noted that the lower the inlet air temperature the larger the weather effect will be on the evaporation efficiency. In traditional industrial spray drying relatively high inlet air temperatures are applied ranging from 150 to 270 8C Ž18.. In this case changes in ambient air condition will have a negligible effect on the drying air evaporation capacity and efficiency. In air suspension coating however, lower inlet air temperatures are applied, ranging from 50 to 90 8C when using aqueous spraying solutions. This results in very significant batch variations due to the effects of the weather when using unconditioned air. Under these conditions it can be more interesting to compare the processes on the basis of evaporation efficiency instead of spray rate. Ambient air relative humidity wr and temperature Tr were recorded by means of a Testo 610 Žn.v. Testoterm, Ternat, Belgium. during the coating experiments. Since it was difficult to control the peristaltic pump exactly, the spray rate R sol was derived by carefully monitoring the mass of coating solution as a function of time using an electronic balance Sartorius 2472 ŽSartorius AG, Goettingen, Germany.. The evaporation efficiency, E was calculated using the computer program, Topsim ŽThermodynamic Operation Point Simulation. Ž19..
Determination of coating adsorption isotherm, coating solution ¨ iscosity, and deposited coating mass The adsorption isotherms of the several biopolymers were determined at 25 8C using the Novasina Thermoconstanter TH 200 ŽAxair Ltd., Pfaffikon SZ, The ¨ Netherlands.. The viscosity at increasing stress was determined in five-fold at 35 and 50 8C with a controlled stress Bohlin CVO rheometer ŽBohlin Instruments, Cirencester, Gloucestershire, U.K.. to monitor for pseudoplastic nonNewtonian behaviour. The C25 cup and bob geometry was used. At high stress values, the measurement can become unstable due to Taylor vortex flow Ž20, 21. resulting in an apparent viscosity rise. Consequently, the viscosity at high shear stress Žm` , in Pa ? s. was
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estimated by omitting the unstable measurements and extrapolation using the exponential eqn w3x Eqn w 3 x
m s a ? e Žybt. q m` m: t: m` : a: b:
viscosity ŽPa ? s. ishear stress ŽPa. estimated viscosity at high stress ŽPa ? s. constant ŽPa ? s. constant ŽPay1 .
Microcapsule coating content Žgrkg. was determined in three-fold by accurately weighing 20 g of coated beads before and after thoroughly washing with boiling water. The total deposited coating mass Wc Žg. was then calculated, based on the coating content.
Calculation of coating solution droplet size and coating thickness The droplet size produced from pneumatic nozzles may be predicted using the following correlation, which yields the Sauter mean droplet diameter Dd in mm Ž22. 3
Dd s
585 = 10 3 's n REL 's
q 597
m
0.45
ž' / ž sr
1000Usol Uat
1.5
/
Eqn w 4 x where: s: m: Usol : Usol : Uat : n RE L :
fluid surface tension ŽNrm. fluid density Žkgrm3 . fluid viscosity ŽmPa s. fluid volumetric flow rate Žm3rs. air volumetric flow rate Žm3rs. relative velocity outlet air velocity Žmrs.
The surface tension effect of the dissolved biopolymer was not taken into account because of the extremely short droplet formation time in the pneumatic nozzle. Therefore, the surface tension of water at 40 8C was used as a value for s Ži.e. 0.06956 Nrm. Ž23.. The coating thickness d c Žmm. of spherical microcapsules can be calculated using Ž19.: dc s
rp
žžž / ž rc
?
Wp q Wc Wp
1r3
/ /
y1 q1
/ž
y1 ?
Dp 2
/
Eqn w 5 x Dp : diameter of the core material Žmm. r p :density of core material Žkgrm3 . r c :density of coating material Žkgrm3 . Wp :mass of core material Žg. Wc :mass of coating Žg. Microcapsule surface examination The coated beads were placed on double-sided adhesive carbon tabs mounted on SEM-stubs and coated with gold in a sputter coater. Using a Philips SEM 505
scanning electron microscope ŽPhilips, Eindhoven, The Netherlands., the surface structure of the different films attached to the glass beads was examined at 15 keV. At least 10 capsules were examined for each treatment.
Statistical analysis The mean values were compared using the ‘one way ANOVA’ procedure in combination with the Duncan multiple comparison test included in the SPSS 7.0 software.
Results and Discussion Detection of the agglomeration The agglomeration of glass beads during coating with locust bean gum, carboxymethylcellulose ŽCMC., and sodium alginate was observed to be an ‘all or nothing’ phenomenon that remained completely absent at a particular evaporation efficiency and appeared very drastically when a critical evaporation efficiency was exceeded. This resulted in a complete collapse of the fluid-bed, accompanied by a coverage of reactor walls and nozzle with agglomerated particles. The actual downfall of the fluid-bed was generally preceded by temperature fluctuations of about 1.5 8C in the fluid-bed and about 1 8C at the air outlet. These fluctuations are most likely caused by irregular fluidization due to the formation and break up of agglomerates accompanied by strong humidity fluctuations in the fluid-bed. In contrast, coating with k-carrageenan at an evaporation efficiency of about 0.85, resulted in a continuous adhesion of the particles onto the reactor wall, resulting in a reduction of the fluid-bed density, and finally a fluid-bed disappearance. This phenomenon could also be observed to a lesser extent at lower spray rates. These successfully terminated processes exhibited a critical phase during which the particles temporarily adhered onto the reactor wall, followed by a uniform and gentle fluidization. This critical phase was always accompanied by bed temperature fluctuations of around 1 8C and appeared earlier in the process at higher evaporation efficiencies.
High-¨ iscous gums as coating agents Two gums with a high solution viscosity, locust bean gum ŽLBG. and high viscosity carboxymethylcellulose ŽCMCHV., were studied at an atomization pressure of 3.5 bar. The highly viscous CMCHV was also compared with a low viscosity carboxymethylcellulose ŽCMCLV. at the same atomization pressure. The evaporation efficiency E, total process time t, and coating mass Wc for the gums are given in Table 2; when a collapse of the fluid-bed occurred, the time for agglomeration ta is also indicated. The Wc values of the highly viscous gums LBG and CMCHV are not statistically different Ž P ) 0.05. and their agglomeration tendency is comparable. The characteristics of high and low viscosity
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Table 2 Evaporation efficiency E, total process time t, deposited coating mass Wc , and agglomeration time ta for locust bean gum ŽLBG., high viscosity CMC ŽCMCHV., and low viscosity CMC ŽCMCLV. Coating LBG LBG CMCHV CMCHV CMCLV CMCLV
E 0.51 0.56 0.47 0.52 0.47 0.52
t Žmin. 213 Ž197. a 247 Ž221. a 260 Ž237. a
Wc Žg.
ta Žmin. .b
12.83 Ž0.44 n.d. 13.22 Ž0.23. b n.d. 13.05 Ž0.26. b n.d.
n.ag. 27 n.ag. 195 n.ag. 35
a
the expected total process time. the standard deviation Žs x .. n.d.s not determined. n.ag.s not agglomerated. b
Fig. 1 Adsorption isotherms of locust bean gum ŽLBG., high viscosity CMC ŽCMCHV., and low viscosity CMC ŽCMCLV. at 25 8C Ž a w s water activity Ž } ... Ž }=} . s LBG; Ž }v} . s CMCHV; Ž }`} . s CMCLV
CMC are practically identical; their Wc values are not statistically different Ž P ) 0.05. and both polymers agglomerate the beads at the same evaporation efficiency. The fact that all three coatings showed a comparable agglomeration tendency was quite unexpected because they have diverse hygroscopical ŽFig. 1. and rheological properties. This illustrates that wet film properties rather than solution properties determine the agglomeration tendency. Both high-viscous polymers show pseudoplastic behaviour ŽFig. 2. so the droplets produced by the high-shear pneumatic nozzle may be much smaller than expected, a phenomenon that delays agglomeration. Scanning electron micrographs of glass beads coated with the different biopolymers reveal drastic differences in coating quality ŽFigs 3–5.. In the case of the glass beads coated with CMCHV ŽFig. 3., it appears that the settled droplets experienced difficulties in spreading out, resulting in a very rough structure. By
Fig. 2 Pseudoplastic behaviour of a 10 grkg solution of locust bean gum ŽLBG. and high viscosity CMC ŽCMCHV. at 35 and 50 8C. The 95% confidence limits are indicated. Ž }l} . s MCHV, 35 8C; Ž }e} . s CMCHV, 50 8C; Ž }'} . s LBG, 35 8C; Ž }^} . s LBG, 50 8C
Fig. 3 Scanning electron micrograph Ž=131. of glass beads coated with high viscosity CMC
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Fig. 4 Scanning electron micrograph Ž=131. of glass beads coated locust bean gum
contrast, the glass beads coated with LBG show a very smooth structure and coating imperfections are practically absent ŽFig. 4.. The higher hygroscopicity of CMC ŽFig. 1. is thought to contribute to the poor coating quality compared to LBG. The micrograph in Fig. 5 confirms this assumption because the low viscosity type is seen to have a similar coating structure. The high viscosity of the 10 grkg CMCHV solution is therefore not necessarily the reason for its poor coating quality.
Moderate-¨ iscous gums as coating agents The agglomeration tendency of two gums with a moderate solution viscosity, namely sodium alginate ŽALG. and k-carrageenan ŽCAR., was studied.
The adsorption isotherms ŽFig. 6. imply that both gums have a similar hygroscopicity. However, the equilibration of the sorption measurements took approximately half the time for ALG as compared to CAR. It can thus by concluded that, from the dynamic point of view, ALG adsorbs water twice as fast as CAR. The pseudoplastic behaviour of both gums was determined at two temperatures ŽFig. 7.. If the high shear forces present in a pneumatic nozzle are taken into account, it is important to have an accurate estimate of the viscosity of both gums at a high shear stress. Equation w3x was fitted on the viscosity data using CurveExpert 1.2. Using the m` values ŽTable 3., the viscosity at a high shear stress and temperature of 40 8C Žwhich approximates the average nozzle tempera-
Fig. 5 Scanning electron micrograph Ž=131. of glass beads coated with low viscosity CMC
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Fig. 6 Adsorption isotherms of sodium alginate ŽALG. and k-carrageenan ŽCAR. at 25 8C Ž a w : water activity Ž-... Ž }^} . s ALG; Ž }I} . s CAR
Fig. 8 Sauter average droplet size, Dd Žmm. of sodium alginate ŽALG. and k-carrageenan ŽCAR. as a function of evaporation efficiency, E at different atomization pressures. Ž }B} . s ALG, 1.5 bar; Ž }l} . s ALG, 2.5 bar; Ž }'} . s ALG, 3.5 bar; Ž }I} . s CAR, 1.5 bar; Ž }e} . s CAR, 2.5 bar; Ž }^} . s CAR, 3.5 bar; Ž }=} . s ALG, 3 bar Fig. 7 Pseudoplastic behaviour of a 10 grkg solution of sodium alginate ŽALG. and k-carrageenan ŽCAR. at 35 and 50 8C. The 95% confidence limits are indicated. Ž }'} . s ALG, 35 8C; Ž }^} . s ALG;, 50 8C; Ž }B} . s CAR, 35 8C; Ž }I} . s CAR, 50 8C
ture. can be approximated by interpolation. This gives viscosities of 33 and 22 mPa.s for ALG and CAR, respectively. These values were used to calculate the average droplet size as a function of the evaporation efficiency at different atomization pressures using Eqn w4x in which the fluid viscosity m is an important parameter. At an atomization pressure of 3 bar, the size of droplets produced from a 10 grkg ALG solution becomes practically equal to those produced from a 10 grkg CAR solution at 2.5 bar ŽFig. 8.. A series of investigations on ALG were also performed at 3 bar in
an attempt to compare both gums at a similar droplet size, Dd . At an atomization pressure of 2.5 bar and at similar droplet size the gums, ALG and CAR differ surprisingly in agglomeration tendency ŽFig. 9. although both appear to have comparable hygroscopical and rheological properties ŽFigs 6 and 7.. However, during the evaporation and hence concentration of the settling
Table 3 Calculating the viscosity at high shear stress of sodium alginate ŽALG. and k-carrageenan ŽCAR. at 35 and 50 8C by extrapolation Coating ALG ALG CAR CAR
T Ž8C. 35 50 35 50
m s a ? e Žyb t . q m` a ŽPa ? s. b ŽPay1 . m` ŽPa ? s. 0.0494 0.0325 0.0199 0.0145
0.0272 0.0317 0.0583 0.0547
0.0340 0.0309 0.0234 0.0200
R2 0.9988 0.9990 0.9990 0.9993
Fig. 9 Deposited coating mass Wc and agglomeration point Ž=. as a function of evaporation efficiency E for sodium alginate ŽALG. at 2.5 and 3 bar atomization pressures, and k-carrageenan ŽCAR. at 2.5 bar atomization pressure. The agglomeration time Žmin. and 95% confidence limits are indicated. Ž }B} . s CAR, 2.5 bar; Ž }'} . s ALG, 2.5 bar; Ž }^} . s ALG, 3 bar
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Fig. 10 Scanning electron micrograph Ž=625. of a glass bead coated with sodium alginate
droplets, the rheological properties can alter very drastically due to the glass transition phenomena. Some hypotheses based on the glass transition theory Ž24. can be proposed to explain the difference in agglomeration tendency of ALG and CAR; Ži. the glass transition temperature ŽTg . of ALG is generally lower compared to CAR, resulting in stickiness at lower temperatures; Žii. both coatings have a similar Tg without the presence of water but the plasticizing effect of water is more pronounced in ALG; Žiii. both coatings have a similar Tg and plasticizing effect of water but a coating of ALG has a higher moisture content at the same bed relative humidity compared to CAR. The third hypothesis is countered by the fact that both coatings appear to have very similar adsorption isotherms at 25 8C ŽFig. 6.. However, as discussed ear-
lier it was clearly observed that the equilibration of the measurements of the adsorption isotherms took about half the time for ALG compared to CAR. As a consequence, a higher moisture content of ALG at a certain relative humidity could be possible. Scanning electron micrographs of glass beads coated with both gums ŽFigs 10 and 11. clearly demonstrate that the surface structure of the film is different for the two gums used. CAR droplets appear to have spread out almost perfectly leaving a film with few imperfections. Conversely, the dried ALG droplets experienced more difficulties in spreading out. This further illustrates that a totally different phenomenon occurs during the water evaporation from the ALG coating, and the possible existence of a rubbery, sticky state with a high liquid viscosity.
Fig. 11 Scanning electron micrograph Ž=625. of a glass bead coated with k-carrageenan
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Table 4 Agglomeration time ta and estimated coating thickness d c at higher spray rates R sol using sodium alginate ŽALG. as a coating agent R sol Žgrmin. 5.48 6.40 7.20 7.80 8.26
Ea
T2, exp Ž8C. b
T2, sim Ž8C. a
w 2 Ž%. a
ta Žmin.
d c Žmm.
0.42 0.47 0.51 0.57 0.61
44.2 42.1 39.2 38.5 37.4
44.9 42.6 40.4 38.8 37.8
20.4 20.2 21.1 27.7 32.2
227 205 114 70 55
1.15 1.21 0.76 0.51 0.42
a
evaporation efficiency E, outlet air temperature T2, sim , and outlet air relative humidity w 2 were calculated by using the program Topsim Ž19.. b experimental outlet air temperature.
The fact that at a higher atomization pressure, the agglomeration is introduced at higher evaporation efficiencies when using ALG as coating agent ŽFig. 9. can be explained by extra fluid-bed turbulence resulting in a better particle separation and a smaller droplet size. At an atomization pressure of 2.5 bar a significant Ž*P - 0.05. decrease of Wc can also be observed for ALG compared to the results obtained at 3 bar. It is probable that the higher speed of the droplets produced at 3 bar compared to 2.5 bar positively influences the coating yield. Table 4 displays the evolution of the agglomeration time ta and the estimated coating thickness at the point of agglomeration when using ALG as coating agent at higher values of the spray rate R sol . At higher spray rates, and hence higher bed relative humidities, the agglomeration phenomenon occurs at a lower coating thickness. The agglomeration is therefore determined by the actual coating thickness and by the thermodynamic operation point, which influences the coating properties such as the stickiness Žviscosity. and water evaporation rate.
Conclusions Despite high viscosities, locust bean gum and high viscosity CMC enable fluid-bed coating to occur at moderately elevated spray rates. The fact that the low viscosity type CMC shows similar characteristics to the high viscosity type, further illustrates that the wet film and not the solution characteristics mainly determine the agglomeration tendency. Sodium alginate exhibits similar hygroscopical and rheological characteristics to k-carrageenan, but k-carrageenan permits the use of significantly higher spray rates before fluid-bed collapse occurs, for which the glass transition concept can offer some relevant explanation. The glass transition phenomena also appear to influence the magnitude of coating imperfections; coatings consisting of locust bean gum and k-carrageenan appear to have a surprisingly uniform structure.
Acknowledgement The authors acknowledge with gratitude the technical support given by M. Jooris.
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