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Oil encapsulation by spray drying and fluidised bed agglomeration
Innovative Food Science and Emerging Technologies 6 (2005) 29 – 35 www.elsevier.com/locate/ifset
Oil encapsulation by spray drying and fluidised bed agglomeration C. Turchiulia,*, M. Fuchsa, M. Bohina, M. E. Cuvelierb, C. Ordonnaudb, M. N. Peyrat-Maillardb, E. Dumoulina a
ENSIA-UMR GENIAL, 1 avenue des Olympiades, F-91744 Massy Cedex, France b UMR SCALE, 1 avenue des Olympiades, F-91744 Massy Cedex, France Received 25 June 2004; accepted 25 November 2004
Abstract Many active components (anti-oxidants, aromas) are lipophilic substances, available in liquid form and have to be protected from the environment. Encapsulation of oil drops into a solid matrix is regarded as an efficient protection method and a means of formulating liquid compounds in a solid dosed form. The aim of this study is to investigate the feasibility of encapsulation of a vegetable oil (ISIO4R, 5% w/w dry matter) used as a model into a mixture of maltodextrin and acacia gum. Encapsulation was completed in three stages, i.e. emulsification, spray drying and fluid bed agglomeration. Optimal operating conditions for spray drying and agglomeration were identified. Powders were characterized before and after agglomeration in terms of oil content and protection (dispersion into the matrix, surface oil content, oxidation) and powder handling properties (flowability, wettability, friability). The proposed encapsulation method provided powders where oil droplets were well dispersed and protected (oil droplets diameter lower than 1 Am in reconstituted emulsions, less than 2% of the total oil content at the particle surface, oil oxidation lowered compared to unprotected oil). Agglomeration did not change oil encapsulation properties of the spray-dried powder but considerably improved its wettability. D 2004 Elsevier Ltd. All rights reserved. Keywords: Spray drying; Fluidised bed; Encapsulation; Agglomeration; Vegetable oil Industrial relevance: Encapsulation of active components finds many applications in food industry for aromas, vitamins, anti-oxidants, etc. used in powdered foods (soups, instant drinks and sauces), prepared meals and food complements with a market in constant increase. It is also widely used in pharmaceutical, chemical and cosmetics industries where products are different but techniques are similar. This study aimed to test the combination of spray drying and fluidised bed agglomeration to encapsulate sensitive lipid-based compounds into dosed powders to provide protection and storage stability and render their use easier.
1. Introduction The encapsulation of active components in powders has become a very attractive process in the last decades, not only in food but also for pharmaceutical industry. The main objective is to entrap a sensitive ingredient in a capsule or bwallQ, physically isolating the ingredient from the environment. This barrier may confer protection * Corresponding author. Tel.: +33 1 69 93 50 93; fax: +33 1 69 93 50 44. E-mail address: [email protected] (C. Turchiuli). 1466-8564/$ - see front matter D 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ifset.2004.11.005
against oxygen, water and/or light; allow a controlled release of the entrapped ingredient and/or prevent contact with other constituents in a mixture for example. The wall material is generally made of compounds that create a network like structure. These compounds usually harbour hydrophilic and/or hydrophobic groups (starches, gums, gelatines, polymers, etc.). Many sensitive ingredients such as anti-oxidants, nutraceutics or flavours are lipid-based compounds, which exist in liquid form at room temperature. For these lipophilic substances the simplest means of encapsulation consists in emulsifying/dispersing the component in an
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aqueous solution. For use in a conventional tablet or capsule form for oral absorption for example, the emulsion may be incorporated into a solid form (i.e. powder). This can be performed by drying the aqueous emulsion such that, when solidifying the wall materials contained in the aqueous phase entrap dispersed oil drops. Among several encapsulation techniques (Shahidi & Han, 1993), spray drying is the most common one. Dry solid particles are obtained by hot air drying of liquid droplets (solution, emulsion or suspension) produced at the top of the drying chamber. Drying occurs during their fall to the bottom of the chamber due to contact with drying air flowing either co- or counter- currently. Residence times of particles in the chamber are very short ensuring low particle temperature despite their contact with hot air. The quality of the microcapsules produced is often estimated from the fraction of non-encapsulated lipid corresponding to the oil exposed at the surface (Imagi, Muraya, Yamashita, Adachi, & Matsuno, 1992). Another quality index may be the amount of internal porosity in the solid particles representing the amount of air entrapped and therefore subjected to be in contact with oil droplets causing oil oxidation. Besides providing a good protection of the active ingredient, the bdry emulsionQ must also have a given homogeneous composition regarding this component and a good stability during storage (no wall collapse, sticking) ensured by a low water content and a low water activity. It must be able to reform the original emulsion by reconstitution, in water for example (Christensen, Pedersen, & Kristensen, 2001). And, the ease of use of the powder during dosage, mixing with other powders, etc. is another critical index of its quality (Barker et al., 2003). Spray drying yields a fine powder with generally poor handling properties. It is therefore often followed by an agglomeration stage where the physical properties of the particles are modified to change handling properties of the powder (flowability, compressibility, mechanical resistance) (Buffo, Probst, Zehentbauer, Luo, & Reineccius, 2002). Agglomerates are obtained by creating solid bridges between individual particles. The surface of particles is made bstickyQ by wetting with water (solvent) or with a binder solution (or by melting). Liquid bridges are formed when particles come into contact and consolidate during drying leading to solid links after solvent evaporation. Agglomerates have larger diameters and more porous structures than the initial particles they are made of, resulting in a different behaviour during flowing, compression, mixing, etc. Spray-dried particles agglomeration can be performed in a separate equipment or, as in many industrial plants, in a fluidised bed at the bottom of the spray drier. The oil droplets dispersion within the solid matrix and the efficiency of protection and further controlled release of the active ingredient depend not only on the process used and on the operating conditions, but also on the composition of the established wall (Christensen et al., 2001;
Desobry, Netto, & Labuza, 1997; Minemoto, Adachi, & Matsuno, 1997). In the case of lipid-based ingredient encapsulation in powder, the aqueous phase of the initial emulsion generally contains different compounds with specific properties fulfilling different roles. Some of the components must have surface-active and film-forming properties to allow a stable oil-in-water emulsion to be prepared, with very low oil droplets diameters (below 1 Am) and to protect the active component during the spray drying operation. Other compounds used as fillers must form a dry matrix around the oil droplets. This function requires materials capable of forming low-viscosity concentrates, which dry readily and yield powders with specific properties depending on the further uses (non-hygroscopic, nonporous, soluble, stable, etc.). Acacia gum, a hydrocolloid produced by natural exudation of acacia trees, is ideally suited to the encapsulation of lipids. It is composed of a highly branched arrangement of the simple sugars (galactose, arabinose, rhamnose) and glucuronic acids and contains a protein component (~2% w/w) covalently bound within its molecular arrangement. This protein fraction plays a significant role in determining the functional properties of acacia gum: high water solubility, low viscosity of concentrated solutions compared to other hydrocolloid gums, bland flavour, very good oil-in-water emulsifier. To produce stable acacia gum emulsions, oil droplets must have diameters below 1 Am (Thevenet, 1988). In the case of soy oil in 10% w/w acacia gum solutions, such oil droplet diameters are obtained for gum/oil weight ratios higher than 1.0 (oil contents inferior to 50% w/w of dry matter in the emulsion). Below this value, the quantity of acacia gum available to act as an emulsifier becomes limiting, resulting in the production of larger oil droplets and the decrease of the encapsulation efficiency in spraydried powders (McNamee, O’Riordan, & O’Sullivan, 1998). However, acacia gum is an expensive ingredient, its production and costs are susceptible of climatic and political turbulence and the viscosity of concentrated solutions is sometimes troublesome for spraying; hence, there is a need to find total or partial substitutes. In most of the oil encapsulation applications, oil contents are greatly below 50% w/w of the total dry matter corresponding to amounts of wall material far in excess of what is required for the emulsification role of acacia gum (gum/ oilz1). Consequently, the large proportion of the gum fulfilling the matrix forming function is thought to be effectively replaceable by a less expensive and less viscous ingredient. Hydrolysed and chemically modified starches have been proposed to replace acacia gum. Hydrolysed starches (called maltodextrin when the dextrose equivalent DE is less than 20) are often used as wall material due to their low cost, bland flavour, high water solubility (up to 75%) and the low viscosity of solutions. But their major shortcoming is a virtual lack of emulsifying capacity. Chemically modified starches have been partially hydrolysed and derivatized to impart lipophilic properties and
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therefore improved emulsification properties. But their cost is high and there is potential off-flavour. The use of mixtures of acacia gum and maltodextrin appears therefore to offer a good compromise between cost and effectiveness. It has been shown that the replacement of 50% of acacia gum with maltodextrins of various DE values (between 5.5 and 38) in emulsions with wall material/oil weight ratios of 2.0 (oil content 33% w/w of total dry matter) had no significant effect on the diameter of soy oil droplets and very little influence on the spray drying encapsulation efficiency (McNamee, O’Riordan, & O’Sullivan, 2001). However, higher DE maltodextrins with lower molecular weight are known to form a denser more oxygen impermeable matrix providing longer shelf life for orange oil and h-carotene for example. But, at the same time, because of the lowering of the glass transition temperature as the molecular weight decreases, the material has higher hygroscopicity and can cake destabilizing the product (Desobry et al., 1997). In the case of flavour encapsulation by spray drying, it has been shown that flavour retention depends on the ratio between maltodextrin and acacia gum with an optimal retention for weight ratios between 2/3 and 3/2 (Bhandari, Dumoulin, Richard, Noleau, & Lebert, 1992). The objective of this work was first to investigate the feasibility of encapsulation in powder of a vegetable oil used as a model for oil-based compounds. Acacia gum was used as emulsifier and protective support in association with maltodextrin. Solid oil dosed particles were produced by spray drying oil-in-water/gum/maltodextrin emulsions. The spray-dried powders produced had to be stable, dosed in the active component (5% w/w of dry matter as the initial emulsion) and exhibit good handling properties. Spray-dried particles therefore had to be as large as possible with a regular shape and with a low water content and a low water activity. The first part of the work dealt with the evaluation of the influence of the operating conditions during spray drying on these properties. In a second part, the quality of encapsulation and the impact of further fluid bed agglomeration were estimated from the oil content and oil protection (surface oil, lipid oxidation) and the handling properties of the powders.
3. Materials and methods 3.1. Materials Acacia gum (IRX 40693) was provided by CNI (France). Maltodextrin Glucidex with a dextrose equivalent of 12 was purchased from Roquette (France). The vegetable oil (ISIO 4R, Lesieur, France) was a market oil commonly used in cooking. It is composed of 35% sunflower oil, 25% sunflower oil enriched in oleic acid, 7% rapeseed oil and 3% grapes pip oil and contains 45 to 60 mg/100 g atocopherol. Some trials were also performed with pure a-
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tocopherol (Fluka, Switzerland) instead of vegetable oil to check that for the same operating conditions, powders obtained with both oils had similar properties confirming that vegetable oil can be used as a model for other lipophilic compounds encapsulation. 3.2. Emulsification Acacia gum (AG) was first dissolved in demineralised water at room temperature under slow mixing then maltodextrin (MD) was slowly added. When total dissolution was obtained oil was added to the solution and the mixture was homogenized for 10 min (Polytron Pt 3100, Kinematica, Switzerland). Emulsions were prepared with different dry matter concentrations (DM) comprised between 30% and 50% w/w and two different weight ratios MD/AG (3/2 and 2/3) (Bhandari et al., 1992). In all the emulsions, the oil represented 5% w/w of the dry matter (DM) corresponding to gum/oil weight ratio of 7.6 and 11.4 for MD/AG ratios of 3/2 and 2/3 respectively. Before use, emulsions were stored at 4 8C. 3.3. Spray drying Emulsions were spray-dried in a Niro Minor dryer (A/S Niro Atomizer) equipped with a rotating disc for the atomization of the emulsion into small droplets at the top of the chamber. The dryer was operated at air temperatures of 200 and 220 8C (F2 8C) for inlet and 100–130 8C for outlet. Emulsions were fed by means of a peristaltic pump with flow rates between 22 and 68 ml min1. 3.4. Fluid bed agglomeration Agglomeration of the spray-dried powders was performed in a pilot scale batch fluid bed Uni Glatt (Glatt, Germany). 300 g of powder was agglomerated by top spraying water at room temperature by means of a two fluid nozzle fed by a peristaltic pump. The sprayed water flow rate was adapted to keep constant the air temperature in the fluid bed of particles (45F2 8C). The fluidising air flow rate was regularly increased to ensure a good fluidisation of the particles. In order to assess powder properties along the trial, samples (i1g) were regularly picked up in the fluid bed of particles. Agglomeration trials were stopped when no more size increase was observed. Three trials were performed to test the repeatability. 3.5. Characterization methods 3.5.1. Emulsions The size of the oil droplets in the initial emulsions and in the reconstituted emulsions obtained by dissolving the final powder in water was measured under a light microscope (Olympus BX60) equipped with a CCD camera (Sony, XC-711P).
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3.5.2. Powders The particle size distribution and median diameter were measured by laser diffraction (Mastersizer, Malvern, France) for the spray-dried powders and by sieving for the larger agglomerated particles. Internal porosity (e int) of particles was calculated from the apparent (q app) and true (q true) densities of dried powder samples, measured with an air pycnometer (ACCUPYC 1330, Micrometric, France) using a pressure of 1.34 bar, respectively, before and after grinding in a mortar: eint ¼ 1
qapp qtrue
The wettability was estimated by the time required for 1 g of powder to disappear from the surface of water (100 ml, 20 8C). The flowability was estimated by the time required for 100 g of powder to flow out of a calibrated glass funnel (AFNOR NF B 35032). The Carr index (Table 1) was calculated from the bulk and tapped densities of the powder estimated from the volume occupied by a given amount of powder in a 25 cm3 test tube, respectively, before and after ~1000 knocks (AFNOR NF T 51042). The water content was measured by drying a powder sample (1 g) in an oven at 102 8C during 24 h to 48 h. The water activity at 20 8C was measured using a Novasina Thermoconstanter (RTD-32/TH2, Defensor, Switzerland). 3.5.3. Oil Powder samples for oil characterization were stored at 20 8C in sealed plastic bags under vacuum (850 mbar). The total oil content of the powder (2.5 g) was measured after dissolution in water and oil extraction with a methanol/chloroform mixture (30/60 v/v) (adapted from Imagi et al., 1992). The amount of non-encapsulated oil was estimated as the surface oil content. It was measured, after washing the surface of the solids particles (20 g) with petroleum ether (Bolton reflux extraction). The solvent was evaporated to dryness and the extracted oil was determined gravimetrically. The amount of oxidized oil was estimated from the quantity of conjugated dienes contained in oil (AFNOR NF T 60-223). In order to check the efficiency of protection, an accelerated oxidation test was performed storing powder samples (25 g) in air at 60 8C for 8 weeks. For conjugated Table 1 Estimation of powder flowability from the Carr index value (Carr index=(q tappedq bulk)/q tapped) Carr index
Flowability
0.05 to 0.15 to 0.18 to 0.22 to 0.35 to N 0.40
Very good Good Poor Bad Very bad Awful
0.15 0.18 0.22 0.35 0.40
dienes dosage, oil in powders had to be separated by dissolving the powder in water and extracting oil as described before.
4. Results and discussion 4.1. Emulsification Whatever the dry matter content in the emulsion (30% to 50%) and the weight ratio between maltodextrin and acacia gum (3/2 or 2/3), oil droplets in the emulsions had diameters inferior to 1 Am ensuring emulsion stability and a good dispersion of the active component in the wall material. This result indicates that conditions chosen for the emulsification (homogenizer, rotation speed, duration) were satisfying and it is in agreement with the high gum/oil weight ratio in the emulsions (7.6 and 11.4), greatly above 1.0, corresponding to an excess of acacia gum for emulsification. 4.2. Powders production 4.2.1. Spray drying Different spray drying trials were performed with different inlet air temperatures, emulsion flow rates, dry matter contents and two weight ratios between maltodextrin and acacia gum (Table 2). The objectives were to get a stable free flowing powder with low water content, low water activity and particles as large as possible with a regular shape. In most of the trials the vegetable oil ISIO 4R was used as the active component. For some of them (quoted (1) in Table 2) it was replaced by a-tocopherol in the same proportion (5% w/w DM). Two trials (quoted (2) in Table 2) were performed with both vegetable oil ISIO 4R and atocopherol and it was checked that in both cases the powders obtained were similar (size, shape) confirming that the vegetable oil can be used as a model for such lipid-based products. For all the trials, the temperature of the powder collected at the bottom of the spray dryer chamber was lower than 70 8C indicating that the very short residence time of particles in the chamber allowed to keep their temperature below 70 8C during spray drying. Whatever the conditions chosen, the water activity of the spray-dried powder obtained was close to 0.2 and the water content lower than 5 g/100 g DM. As a result, particles were not sticky and a good bacteriological stability during storage could be ensured. The powder water content was found to increase with the emulsion flow rate. And, whatever the emulsion flow rate and dry matter content, it was linearly correlated to the temperature of outlet air (Fig. 1) known to be a good indicator of the final powder water content. The emulsion dry matter content (DM) was found to influence the size and shape of the spray-dried particles obtained. When it was increased from 30% to 50%, the
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Table 2 Operating conditions for the different spray drying tests Parameter studied
Inlet air temperature (8C)
Emulsion flow rate (ml min1)
Dry matter (%)
Weight ratio MD/AG
Oil
Inlet air temperature (8C) Emulsion flow rate (ml min1) Dry matter (%) Weight ratio MD/AG Oil
200–220 220 220 220 220
24 24–64–68 24 24 24
40 40 30(1)–35(2)–40(2)–50 40 35(2)–40(2)
3/2 3/2 3/2 3/2(2)–2/3(1) 3/2
IS IS TC TC TC–IS
Each line corresponds to one parameter studied. Values for this parameter are underlined (TC: a-tocopherol, IS: ISIO4 ; MD: maltodextrin, AG: acacia gum). Trials were performed with IS except those with (1) performed with TC only and (2)performed with TC and IS.
particles median diameter increased from 18 to 85 Am. A regular spherical shape can only be achieved for dry matter contents below 40%. Above this value some threads appeared as well as when the inlet air temperature was lowered to 200 8C or when the weight ratio between maltodextrin and acacia gum was set to 2/3 instead of 3/2. This is probably due to the fact that, high dry matter contents (N40%) as well as the increase of the amount of acacia gum cause the increase of the viscosity of the emulsion leading to inhomogeneities in the sprayed droplets size and shape. The operating conditions finally chosen for the spray drying of the emulsion were: 40% DM in the emulsion with a weight ratio MD/AG of 3/2, an emulsion flow rate of 68 ml min1, and inlet and outlet air temperatures of 220 and 100 8C, respectively. Three production trials were performed with these conditions with a mean powder flow rate of 1150 g h1. The whole product yield still had to be improved since only 65F4% of the dry matter sprayed was recovered as a powder. The other part was stuck on the dryer walls probably due to a too large angle of the spray that
could be changed by modifying the spraying conditions (rotation speed or type of the atomizer). 4.2.2. Fluid bed agglomeration For the fluid bed agglomeration of the fine spray-dried powders (b100 Am), the difficulty was to find operating conditions allowing agglomerates to grow without uncontrolled agglomeration. Actually, if the spraying flow rate is too high, or if the sprayed liquid drops are too large compared to the solids particles and drying is not sufficient, particles stick together or to the agglomerator walls and the fluid bed collapses. On the other hand, if the spraying flow rate is too low and drying is too important, liquid droplets and wet particles dry before colliding with another particle. No liquid bridge between particles can be built and no size increase is possible. It was found here that the best way to control the process was to keep a constant air temperature of 45F2 8C in the fluid bed by varying the flow rate of sprayed water. Three agglomeration trials were performed with each 300 g of the spray-dried powder. Trials were stopped when no more size increase was observed (samples taken all along the trial). At the end of the three trials, agglomerates had reached a diameter of about 200 Am. Due to the way of operating chosen, the duration and the amount of water sprayed were not exactly the same for each trial. For the three trials performed, duration was comprised between 65 and 115 min and the amount of sprayed water between 500 and 780 g. In the three cases, more than 87% of the powder was recovered at the end of the agglomeration process. The losses corresponded to the fine particles (from the initial powder or formed by attrition during the trial) dragged to the filters. 4.3. Powders properties
Fig. 1. Relation between outlet air temperature and spray dried powder water content for emulsion flow rates between 22.0 and 68.2 ml min1 and dry matter contents in emulsions between 30% and 50% (weight ratio maltodextrin/acacia gum=3/2, oil 5% w/w of dry matter, inlet air temperature 220 8C, mean drying air flow rate=105 kg h1).
Spray-dried powders were made of spherical particles (Fig. 2a) with a median diameter of 32 Am (Fig. 3). After fluid bed agglomeration, larger irregular agglomerates were obtained (Fig. 2b) with a median diameter of about 200 Am. As can be seen in Fig. 3, the particle size distributions of the
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Fig. 2. Photos of spray dried particles (a) before and (b) after fluid bed agglomeration.
three agglomerated powders were very close despite the difficulty to reproduce exactly the operating conditions from one trial to another. The properties of the spray-dried powders, before and after fluid bed agglomeration are summarised in Table 3. 4.3.1. Oil protection The total oil content was 4.7 g/100 g DM in the initial spray-dried powder and reduced to 4.4 g/100 g DM after agglomeration to compare with 5 g/100 g DM in the initial emulsion. Oil losses during spray drying were therefore low (i6%) and increased during further agglomeration (i13%). After reconstitution of the emulsion by dissolving the powder in water, oil drops with diameters comparable to those in the initial emulsion (b1 Am) were obtained confirming a good dispersion in the solids matrix and that no coalescence of oil drops had occurred either during spray drying or agglomeration. Oil dispersion within the solid matrix is very important to avoid any propagation of oxidation within the assembly. The amount of non-encapsulated oil at the surface of the solids particles was low (less than 2% of the total oil) and was not changed after agglomeration. The internal porosity of spray-dried particles was inferior to 3% and slightly increased due to agglomeration (5%). In both powders, it corresponds to very low amounts of air entrapped and
Fig. 3. Particle size distribution of spray dried powders (- - - -) before and (—) after fluid bed agglomeration.
subjected to cause oil oxidation. Results of conjugated dienes dosage after powder production by spray drying or direct agglomeration (Initial), and during the accelerated oxidation test are given in Fig. 4. Conjugated dienes concentration in encapsulated oil just after powders production is slightly higher than in initial oil (unprotected) used to produce the emulsion. Oil is therefore slightly oxidized during processing. After 8 weeks, the conjugated dienes concentration in unprotected oil has considerably increased compared to that in oil encapsulated in powder by spray drying with and without agglomeration.
Table 3 Properties of the spray dried powders before and after fluid bed agglomeration Properties Particles Median diameter [Am] Shape Density Apparent [g cm3] True [g cm3] Internal porosity [%] Oil Total oil content [% w/w DM] Surface oil content [% w/w total oil] Oil drop size Initial emulsion Reconstituted emulsion (in water) Powder Water content [g/100 g DM] Water activity Flowability [s] Carr index [–] Wettability [min] Friability [%] Density Bulk [g cm3] Tapped [g cm3]
Spray dried powder
Agglomerated spray dried powder
32F8 Spherical
190F40 Irregular
1.35F0.02 1.39F0.01 2.6F1.2
1.35F0.02 1.42F0.01 5F1.8
4.7F0.45
4.4F0.7
1.2F0.5
1.7F0.5
b1 Am b1 Am+some larger drops (b5 Am)
b1 Am b1 Am+some larger drops (b3 Am)
3.8F0.9
6.0F2
0.1F0.03 l 0.44F0.01 28F9 n.d.
0.16F0.06 l 0.27F0.01 3F1 89F17
0.33F0.01 0.59F0.03
0.27F0.08 0.38F0.12
n.d.: not determined; l: infinite time (no powder flow).
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Fig. 4. Oil oxidation measured by conjugated dienes dosage during accelerated oxidation test (air, 60 8C).
4.3.2. Handling properties Spray-dried powders had poor handling properties. Their wettability (28 min) was poor, probably due to the small particle size (32 Am). And the Carr index of 0.44 confirmed a bad flowability. Fluid bed agglomeration was expected to improve these properties by increasing the particle size and modifying the structure of particles. The wettability was actually improved and decreased to a satisfying mean value lower than 3 min. But, despite a small decrease of the Carr index (0.27), no real improvement of the powder flowability was obtained after agglomeration. This may be due to the irregular shape of the agglomerates with many asperities (Fig. 2b). The friability test showed that the mechanical resistance of these agglomerates was low (about 90% were destroyed). This may be linked to their very open structure. It has to be improved to keep agglomerates properties during powder handling. This may be done modifying agglomeration conditions. For example, instead of spraying pure water, a solution of maltodextrin and/or acacia gum or other binder could be used to create stronger solid bridges between particles.
5. Conclusion Operating conditions were found for the spray drying of a formulated emulsion allowing encapsulation of liquid oil in a powder (5% w/w of dry matter). Oil drops were well dispersed within the solid matrix and only 1.2% of the total oil content was found unprotected at the surface of the solid particles. During accelerated oxidation test, oil oxidation was reduced when the oil was protected, compared to unprotected one. However, handling properties of the spraydried powder (e.g. flowability and wettability) were very poor probably due to their small size (32 Am).
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In a next step, fluid bed agglomeration of these particles with help of spraying of water was performed. Optimal operating conditions to agglomerate fine particles were found to consist in controlling the air temperature within the fluid bed of particles by varying the flow rate of sprayed water. Whilst the powder wettability was actually increased, its flowability was not sufficiently improved. It has to be further improved as well as the mechanical resistance of agglomerates that were found to be very fragile. In this study, a vegetable oil was used as a model for other lipid-based ingredients. The behaviour of vegetable oil during spray drying was similar to that of a-tocopherol. This proposed encapsulation method could therefore be used for other oils in which it is also possible to incorporate oilsoluble substances such as aromas, antioxidants, etc. Instead of pure acacia gum or maltodextrin, a 3/2 mixture of maltodextrin DE12 and acacia gum was used as the solid wall material. Further trials could be performed with higher oil contents and other wall materials (maltodextrins with different DE, modified starches, proteins, etc.).
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Oil encapsulation by spray drying and fluidised bed agglomeration C. Turchiulia,*, M. Fuchsa, M. Bohina, M. E. Cuvelierb, C. Ordonnaudb, M. N. Peyrat-Maillardb, E. Dumoulina a
ENSIA-UMR GENIAL, 1 avenue des Olympiades, F-91744 Massy Cedex, France b UMR SCALE, 1 avenue des Olympiades, F-91744 Massy Cedex, France Received 25 June 2004; accepted 25 November 2004
Abstract Many active components (anti-oxidants, aromas) are lipophilic substances, available in liquid form and have to be protected from the environment. Encapsulation of oil drops into a solid matrix is regarded as an efficient protection method and a means of formulating liquid compounds in a solid dosed form. The aim of this study is to investigate the feasibility of encapsulation of a vegetable oil (ISIO4R, 5% w/w dry matter) used as a model into a mixture of maltodextrin and acacia gum. Encapsulation was completed in three stages, i.e. emulsification, spray drying and fluid bed agglomeration. Optimal operating conditions for spray drying and agglomeration were identified. Powders were characterized before and after agglomeration in terms of oil content and protection (dispersion into the matrix, surface oil content, oxidation) and powder handling properties (flowability, wettability, friability). The proposed encapsulation method provided powders where oil droplets were well dispersed and protected (oil droplets diameter lower than 1 Am in reconstituted emulsions, less than 2% of the total oil content at the particle surface, oil oxidation lowered compared to unprotected oil). Agglomeration did not change oil encapsulation properties of the spray-dried powder but considerably improved its wettability. D 2004 Elsevier Ltd. All rights reserved. Keywords: Spray drying; Fluidised bed; Encapsulation; Agglomeration; Vegetable oil Industrial relevance: Encapsulation of active components finds many applications in food industry for aromas, vitamins, anti-oxidants, etc. used in powdered foods (soups, instant drinks and sauces), prepared meals and food complements with a market in constant increase. It is also widely used in pharmaceutical, chemical and cosmetics industries where products are different but techniques are similar. This study aimed to test the combination of spray drying and fluidised bed agglomeration to encapsulate sensitive lipid-based compounds into dosed powders to provide protection and storage stability and render their use easier.
1. Introduction The encapsulation of active components in powders has become a very attractive process in the last decades, not only in food but also for pharmaceutical industry. The main objective is to entrap a sensitive ingredient in a capsule or bwallQ, physically isolating the ingredient from the environment. This barrier may confer protection * Corresponding author. Tel.: +33 1 69 93 50 93; fax: +33 1 69 93 50 44. E-mail address: [email protected] (C. Turchiuli). 1466-8564/$ - see front matter D 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ifset.2004.11.005
against oxygen, water and/or light; allow a controlled release of the entrapped ingredient and/or prevent contact with other constituents in a mixture for example. The wall material is generally made of compounds that create a network like structure. These compounds usually harbour hydrophilic and/or hydrophobic groups (starches, gums, gelatines, polymers, etc.). Many sensitive ingredients such as anti-oxidants, nutraceutics or flavours are lipid-based compounds, which exist in liquid form at room temperature. For these lipophilic substances the simplest means of encapsulation consists in emulsifying/dispersing the component in an
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aqueous solution. For use in a conventional tablet or capsule form for oral absorption for example, the emulsion may be incorporated into a solid form (i.e. powder). This can be performed by drying the aqueous emulsion such that, when solidifying the wall materials contained in the aqueous phase entrap dispersed oil drops. Among several encapsulation techniques (Shahidi & Han, 1993), spray drying is the most common one. Dry solid particles are obtained by hot air drying of liquid droplets (solution, emulsion or suspension) produced at the top of the drying chamber. Drying occurs during their fall to the bottom of the chamber due to contact with drying air flowing either co- or counter- currently. Residence times of particles in the chamber are very short ensuring low particle temperature despite their contact with hot air. The quality of the microcapsules produced is often estimated from the fraction of non-encapsulated lipid corresponding to the oil exposed at the surface (Imagi, Muraya, Yamashita, Adachi, & Matsuno, 1992). Another quality index may be the amount of internal porosity in the solid particles representing the amount of air entrapped and therefore subjected to be in contact with oil droplets causing oil oxidation. Besides providing a good protection of the active ingredient, the bdry emulsionQ must also have a given homogeneous composition regarding this component and a good stability during storage (no wall collapse, sticking) ensured by a low water content and a low water activity. It must be able to reform the original emulsion by reconstitution, in water for example (Christensen, Pedersen, & Kristensen, 2001). And, the ease of use of the powder during dosage, mixing with other powders, etc. is another critical index of its quality (Barker et al., 2003). Spray drying yields a fine powder with generally poor handling properties. It is therefore often followed by an agglomeration stage where the physical properties of the particles are modified to change handling properties of the powder (flowability, compressibility, mechanical resistance) (Buffo, Probst, Zehentbauer, Luo, & Reineccius, 2002). Agglomerates are obtained by creating solid bridges between individual particles. The surface of particles is made bstickyQ by wetting with water (solvent) or with a binder solution (or by melting). Liquid bridges are formed when particles come into contact and consolidate during drying leading to solid links after solvent evaporation. Agglomerates have larger diameters and more porous structures than the initial particles they are made of, resulting in a different behaviour during flowing, compression, mixing, etc. Spray-dried particles agglomeration can be performed in a separate equipment or, as in many industrial plants, in a fluidised bed at the bottom of the spray drier. The oil droplets dispersion within the solid matrix and the efficiency of protection and further controlled release of the active ingredient depend not only on the process used and on the operating conditions, but also on the composition of the established wall (Christensen et al., 2001;
Desobry, Netto, & Labuza, 1997; Minemoto, Adachi, & Matsuno, 1997). In the case of lipid-based ingredient encapsulation in powder, the aqueous phase of the initial emulsion generally contains different compounds with specific properties fulfilling different roles. Some of the components must have surface-active and film-forming properties to allow a stable oil-in-water emulsion to be prepared, with very low oil droplets diameters (below 1 Am) and to protect the active component during the spray drying operation. Other compounds used as fillers must form a dry matrix around the oil droplets. This function requires materials capable of forming low-viscosity concentrates, which dry readily and yield powders with specific properties depending on the further uses (non-hygroscopic, nonporous, soluble, stable, etc.). Acacia gum, a hydrocolloid produced by natural exudation of acacia trees, is ideally suited to the encapsulation of lipids. It is composed of a highly branched arrangement of the simple sugars (galactose, arabinose, rhamnose) and glucuronic acids and contains a protein component (~2% w/w) covalently bound within its molecular arrangement. This protein fraction plays a significant role in determining the functional properties of acacia gum: high water solubility, low viscosity of concentrated solutions compared to other hydrocolloid gums, bland flavour, very good oil-in-water emulsifier. To produce stable acacia gum emulsions, oil droplets must have diameters below 1 Am (Thevenet, 1988). In the case of soy oil in 10% w/w acacia gum solutions, such oil droplet diameters are obtained for gum/oil weight ratios higher than 1.0 (oil contents inferior to 50% w/w of dry matter in the emulsion). Below this value, the quantity of acacia gum available to act as an emulsifier becomes limiting, resulting in the production of larger oil droplets and the decrease of the encapsulation efficiency in spraydried powders (McNamee, O’Riordan, & O’Sullivan, 1998). However, acacia gum is an expensive ingredient, its production and costs are susceptible of climatic and political turbulence and the viscosity of concentrated solutions is sometimes troublesome for spraying; hence, there is a need to find total or partial substitutes. In most of the oil encapsulation applications, oil contents are greatly below 50% w/w of the total dry matter corresponding to amounts of wall material far in excess of what is required for the emulsification role of acacia gum (gum/ oilz1). Consequently, the large proportion of the gum fulfilling the matrix forming function is thought to be effectively replaceable by a less expensive and less viscous ingredient. Hydrolysed and chemically modified starches have been proposed to replace acacia gum. Hydrolysed starches (called maltodextrin when the dextrose equivalent DE is less than 20) are often used as wall material due to their low cost, bland flavour, high water solubility (up to 75%) and the low viscosity of solutions. But their major shortcoming is a virtual lack of emulsifying capacity. Chemically modified starches have been partially hydrolysed and derivatized to impart lipophilic properties and
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therefore improved emulsification properties. But their cost is high and there is potential off-flavour. The use of mixtures of acacia gum and maltodextrin appears therefore to offer a good compromise between cost and effectiveness. It has been shown that the replacement of 50% of acacia gum with maltodextrins of various DE values (between 5.5 and 38) in emulsions with wall material/oil weight ratios of 2.0 (oil content 33% w/w of total dry matter) had no significant effect on the diameter of soy oil droplets and very little influence on the spray drying encapsulation efficiency (McNamee, O’Riordan, & O’Sullivan, 2001). However, higher DE maltodextrins with lower molecular weight are known to form a denser more oxygen impermeable matrix providing longer shelf life for orange oil and h-carotene for example. But, at the same time, because of the lowering of the glass transition temperature as the molecular weight decreases, the material has higher hygroscopicity and can cake destabilizing the product (Desobry et al., 1997). In the case of flavour encapsulation by spray drying, it has been shown that flavour retention depends on the ratio between maltodextrin and acacia gum with an optimal retention for weight ratios between 2/3 and 3/2 (Bhandari, Dumoulin, Richard, Noleau, & Lebert, 1992). The objective of this work was first to investigate the feasibility of encapsulation in powder of a vegetable oil used as a model for oil-based compounds. Acacia gum was used as emulsifier and protective support in association with maltodextrin. Solid oil dosed particles were produced by spray drying oil-in-water/gum/maltodextrin emulsions. The spray-dried powders produced had to be stable, dosed in the active component (5% w/w of dry matter as the initial emulsion) and exhibit good handling properties. Spray-dried particles therefore had to be as large as possible with a regular shape and with a low water content and a low water activity. The first part of the work dealt with the evaluation of the influence of the operating conditions during spray drying on these properties. In a second part, the quality of encapsulation and the impact of further fluid bed agglomeration were estimated from the oil content and oil protection (surface oil, lipid oxidation) and the handling properties of the powders.
3. Materials and methods 3.1. Materials Acacia gum (IRX 40693) was provided by CNI (France). Maltodextrin Glucidex with a dextrose equivalent of 12 was purchased from Roquette (France). The vegetable oil (ISIO 4R, Lesieur, France) was a market oil commonly used in cooking. It is composed of 35% sunflower oil, 25% sunflower oil enriched in oleic acid, 7% rapeseed oil and 3% grapes pip oil and contains 45 to 60 mg/100 g atocopherol. Some trials were also performed with pure a-
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tocopherol (Fluka, Switzerland) instead of vegetable oil to check that for the same operating conditions, powders obtained with both oils had similar properties confirming that vegetable oil can be used as a model for other lipophilic compounds encapsulation. 3.2. Emulsification Acacia gum (AG) was first dissolved in demineralised water at room temperature under slow mixing then maltodextrin (MD) was slowly added. When total dissolution was obtained oil was added to the solution and the mixture was homogenized for 10 min (Polytron Pt 3100, Kinematica, Switzerland). Emulsions were prepared with different dry matter concentrations (DM) comprised between 30% and 50% w/w and two different weight ratios MD/AG (3/2 and 2/3) (Bhandari et al., 1992). In all the emulsions, the oil represented 5% w/w of the dry matter (DM) corresponding to gum/oil weight ratio of 7.6 and 11.4 for MD/AG ratios of 3/2 and 2/3 respectively. Before use, emulsions were stored at 4 8C. 3.3. Spray drying Emulsions were spray-dried in a Niro Minor dryer (A/S Niro Atomizer) equipped with a rotating disc for the atomization of the emulsion into small droplets at the top of the chamber. The dryer was operated at air temperatures of 200 and 220 8C (F2 8C) for inlet and 100–130 8C for outlet. Emulsions were fed by means of a peristaltic pump with flow rates between 22 and 68 ml min1. 3.4. Fluid bed agglomeration Agglomeration of the spray-dried powders was performed in a pilot scale batch fluid bed Uni Glatt (Glatt, Germany). 300 g of powder was agglomerated by top spraying water at room temperature by means of a two fluid nozzle fed by a peristaltic pump. The sprayed water flow rate was adapted to keep constant the air temperature in the fluid bed of particles (45F2 8C). The fluidising air flow rate was regularly increased to ensure a good fluidisation of the particles. In order to assess powder properties along the trial, samples (i1g) were regularly picked up in the fluid bed of particles. Agglomeration trials were stopped when no more size increase was observed. Three trials were performed to test the repeatability. 3.5. Characterization methods 3.5.1. Emulsions The size of the oil droplets in the initial emulsions and in the reconstituted emulsions obtained by dissolving the final powder in water was measured under a light microscope (Olympus BX60) equipped with a CCD camera (Sony, XC-711P).
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3.5.2. Powders The particle size distribution and median diameter were measured by laser diffraction (Mastersizer, Malvern, France) for the spray-dried powders and by sieving for the larger agglomerated particles. Internal porosity (e int) of particles was calculated from the apparent (q app) and true (q true) densities of dried powder samples, measured with an air pycnometer (ACCUPYC 1330, Micrometric, France) using a pressure of 1.34 bar, respectively, before and after grinding in a mortar: eint ¼ 1
qapp qtrue
The wettability was estimated by the time required for 1 g of powder to disappear from the surface of water (100 ml, 20 8C). The flowability was estimated by the time required for 100 g of powder to flow out of a calibrated glass funnel (AFNOR NF B 35032). The Carr index (Table 1) was calculated from the bulk and tapped densities of the powder estimated from the volume occupied by a given amount of powder in a 25 cm3 test tube, respectively, before and after ~1000 knocks (AFNOR NF T 51042). The water content was measured by drying a powder sample (1 g) in an oven at 102 8C during 24 h to 48 h. The water activity at 20 8C was measured using a Novasina Thermoconstanter (RTD-32/TH2, Defensor, Switzerland). 3.5.3. Oil Powder samples for oil characterization were stored at 20 8C in sealed plastic bags under vacuum (850 mbar). The total oil content of the powder (2.5 g) was measured after dissolution in water and oil extraction with a methanol/chloroform mixture (30/60 v/v) (adapted from Imagi et al., 1992). The amount of non-encapsulated oil was estimated as the surface oil content. It was measured, after washing the surface of the solids particles (20 g) with petroleum ether (Bolton reflux extraction). The solvent was evaporated to dryness and the extracted oil was determined gravimetrically. The amount of oxidized oil was estimated from the quantity of conjugated dienes contained in oil (AFNOR NF T 60-223). In order to check the efficiency of protection, an accelerated oxidation test was performed storing powder samples (25 g) in air at 60 8C for 8 weeks. For conjugated Table 1 Estimation of powder flowability from the Carr index value (Carr index=(q tappedq bulk)/q tapped) Carr index
Flowability
0.05 to 0.15 to 0.18 to 0.22 to 0.35 to N 0.40
Very good Good Poor Bad Very bad Awful
0.15 0.18 0.22 0.35 0.40
dienes dosage, oil in powders had to be separated by dissolving the powder in water and extracting oil as described before.
4. Results and discussion 4.1. Emulsification Whatever the dry matter content in the emulsion (30% to 50%) and the weight ratio between maltodextrin and acacia gum (3/2 or 2/3), oil droplets in the emulsions had diameters inferior to 1 Am ensuring emulsion stability and a good dispersion of the active component in the wall material. This result indicates that conditions chosen for the emulsification (homogenizer, rotation speed, duration) were satisfying and it is in agreement with the high gum/oil weight ratio in the emulsions (7.6 and 11.4), greatly above 1.0, corresponding to an excess of acacia gum for emulsification. 4.2. Powders production 4.2.1. Spray drying Different spray drying trials were performed with different inlet air temperatures, emulsion flow rates, dry matter contents and two weight ratios between maltodextrin and acacia gum (Table 2). The objectives were to get a stable free flowing powder with low water content, low water activity and particles as large as possible with a regular shape. In most of the trials the vegetable oil ISIO 4R was used as the active component. For some of them (quoted (1) in Table 2) it was replaced by a-tocopherol in the same proportion (5% w/w DM). Two trials (quoted (2) in Table 2) were performed with both vegetable oil ISIO 4R and atocopherol and it was checked that in both cases the powders obtained were similar (size, shape) confirming that the vegetable oil can be used as a model for such lipid-based products. For all the trials, the temperature of the powder collected at the bottom of the spray dryer chamber was lower than 70 8C indicating that the very short residence time of particles in the chamber allowed to keep their temperature below 70 8C during spray drying. Whatever the conditions chosen, the water activity of the spray-dried powder obtained was close to 0.2 and the water content lower than 5 g/100 g DM. As a result, particles were not sticky and a good bacteriological stability during storage could be ensured. The powder water content was found to increase with the emulsion flow rate. And, whatever the emulsion flow rate and dry matter content, it was linearly correlated to the temperature of outlet air (Fig. 1) known to be a good indicator of the final powder water content. The emulsion dry matter content (DM) was found to influence the size and shape of the spray-dried particles obtained. When it was increased from 30% to 50%, the
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Table 2 Operating conditions for the different spray drying tests Parameter studied
Inlet air temperature (8C)
Emulsion flow rate (ml min1)
Dry matter (%)
Weight ratio MD/AG
Oil
Inlet air temperature (8C) Emulsion flow rate (ml min1) Dry matter (%) Weight ratio MD/AG Oil
200–220 220 220 220 220
24 24–64–68 24 24 24
40 40 30(1)–35(2)–40(2)–50 40 35(2)–40(2)
3/2 3/2 3/2 3/2(2)–2/3(1) 3/2
IS IS TC TC TC–IS
Each line corresponds to one parameter studied. Values for this parameter are underlined (TC: a-tocopherol, IS: ISIO4 ; MD: maltodextrin, AG: acacia gum). Trials were performed with IS except those with (1) performed with TC only and (2)performed with TC and IS.
particles median diameter increased from 18 to 85 Am. A regular spherical shape can only be achieved for dry matter contents below 40%. Above this value some threads appeared as well as when the inlet air temperature was lowered to 200 8C or when the weight ratio between maltodextrin and acacia gum was set to 2/3 instead of 3/2. This is probably due to the fact that, high dry matter contents (N40%) as well as the increase of the amount of acacia gum cause the increase of the viscosity of the emulsion leading to inhomogeneities in the sprayed droplets size and shape. The operating conditions finally chosen for the spray drying of the emulsion were: 40% DM in the emulsion with a weight ratio MD/AG of 3/2, an emulsion flow rate of 68 ml min1, and inlet and outlet air temperatures of 220 and 100 8C, respectively. Three production trials were performed with these conditions with a mean powder flow rate of 1150 g h1. The whole product yield still had to be improved since only 65F4% of the dry matter sprayed was recovered as a powder. The other part was stuck on the dryer walls probably due to a too large angle of the spray that
could be changed by modifying the spraying conditions (rotation speed or type of the atomizer). 4.2.2. Fluid bed agglomeration For the fluid bed agglomeration of the fine spray-dried powders (b100 Am), the difficulty was to find operating conditions allowing agglomerates to grow without uncontrolled agglomeration. Actually, if the spraying flow rate is too high, or if the sprayed liquid drops are too large compared to the solids particles and drying is not sufficient, particles stick together or to the agglomerator walls and the fluid bed collapses. On the other hand, if the spraying flow rate is too low and drying is too important, liquid droplets and wet particles dry before colliding with another particle. No liquid bridge between particles can be built and no size increase is possible. It was found here that the best way to control the process was to keep a constant air temperature of 45F2 8C in the fluid bed by varying the flow rate of sprayed water. Three agglomeration trials were performed with each 300 g of the spray-dried powder. Trials were stopped when no more size increase was observed (samples taken all along the trial). At the end of the three trials, agglomerates had reached a diameter of about 200 Am. Due to the way of operating chosen, the duration and the amount of water sprayed were not exactly the same for each trial. For the three trials performed, duration was comprised between 65 and 115 min and the amount of sprayed water between 500 and 780 g. In the three cases, more than 87% of the powder was recovered at the end of the agglomeration process. The losses corresponded to the fine particles (from the initial powder or formed by attrition during the trial) dragged to the filters. 4.3. Powders properties
Fig. 1. Relation between outlet air temperature and spray dried powder water content for emulsion flow rates between 22.0 and 68.2 ml min1 and dry matter contents in emulsions between 30% and 50% (weight ratio maltodextrin/acacia gum=3/2, oil 5% w/w of dry matter, inlet air temperature 220 8C, mean drying air flow rate=105 kg h1).
Spray-dried powders were made of spherical particles (Fig. 2a) with a median diameter of 32 Am (Fig. 3). After fluid bed agglomeration, larger irregular agglomerates were obtained (Fig. 2b) with a median diameter of about 200 Am. As can be seen in Fig. 3, the particle size distributions of the
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Fig. 2. Photos of spray dried particles (a) before and (b) after fluid bed agglomeration.
three agglomerated powders were very close despite the difficulty to reproduce exactly the operating conditions from one trial to another. The properties of the spray-dried powders, before and after fluid bed agglomeration are summarised in Table 3. 4.3.1. Oil protection The total oil content was 4.7 g/100 g DM in the initial spray-dried powder and reduced to 4.4 g/100 g DM after agglomeration to compare with 5 g/100 g DM in the initial emulsion. Oil losses during spray drying were therefore low (i6%) and increased during further agglomeration (i13%). After reconstitution of the emulsion by dissolving the powder in water, oil drops with diameters comparable to those in the initial emulsion (b1 Am) were obtained confirming a good dispersion in the solids matrix and that no coalescence of oil drops had occurred either during spray drying or agglomeration. Oil dispersion within the solid matrix is very important to avoid any propagation of oxidation within the assembly. The amount of non-encapsulated oil at the surface of the solids particles was low (less than 2% of the total oil) and was not changed after agglomeration. The internal porosity of spray-dried particles was inferior to 3% and slightly increased due to agglomeration (5%). In both powders, it corresponds to very low amounts of air entrapped and
Fig. 3. Particle size distribution of spray dried powders (- - - -) before and (—) after fluid bed agglomeration.
subjected to cause oil oxidation. Results of conjugated dienes dosage after powder production by spray drying or direct agglomeration (Initial), and during the accelerated oxidation test are given in Fig. 4. Conjugated dienes concentration in encapsulated oil just after powders production is slightly higher than in initial oil (unprotected) used to produce the emulsion. Oil is therefore slightly oxidized during processing. After 8 weeks, the conjugated dienes concentration in unprotected oil has considerably increased compared to that in oil encapsulated in powder by spray drying with and without agglomeration.
Table 3 Properties of the spray dried powders before and after fluid bed agglomeration Properties Particles Median diameter [Am] Shape Density Apparent [g cm3] True [g cm3] Internal porosity [%] Oil Total oil content [% w/w DM] Surface oil content [% w/w total oil] Oil drop size Initial emulsion Reconstituted emulsion (in water) Powder Water content [g/100 g DM] Water activity Flowability [s] Carr index [–] Wettability [min] Friability [%] Density Bulk [g cm3] Tapped [g cm3]
Spray dried powder
Agglomerated spray dried powder
32F8 Spherical
190F40 Irregular
1.35F0.02 1.39F0.01 2.6F1.2
1.35F0.02 1.42F0.01 5F1.8
4.7F0.45
4.4F0.7
1.2F0.5
1.7F0.5
b1 Am b1 Am+some larger drops (b5 Am)
b1 Am b1 Am+some larger drops (b3 Am)
3.8F0.9
6.0F2
0.1F0.03 l 0.44F0.01 28F9 n.d.
0.16F0.06 l 0.27F0.01 3F1 89F17
0.33F0.01 0.59F0.03
0.27F0.08 0.38F0.12
n.d.: not determined; l: infinite time (no powder flow).
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Fig. 4. Oil oxidation measured by conjugated dienes dosage during accelerated oxidation test (air, 60 8C).
4.3.2. Handling properties Spray-dried powders had poor handling properties. Their wettability (28 min) was poor, probably due to the small particle size (32 Am). And the Carr index of 0.44 confirmed a bad flowability. Fluid bed agglomeration was expected to improve these properties by increasing the particle size and modifying the structure of particles. The wettability was actually improved and decreased to a satisfying mean value lower than 3 min. But, despite a small decrease of the Carr index (0.27), no real improvement of the powder flowability was obtained after agglomeration. This may be due to the irregular shape of the agglomerates with many asperities (Fig. 2b). The friability test showed that the mechanical resistance of these agglomerates was low (about 90% were destroyed). This may be linked to their very open structure. It has to be improved to keep agglomerates properties during powder handling. This may be done modifying agglomeration conditions. For example, instead of spraying pure water, a solution of maltodextrin and/or acacia gum or other binder could be used to create stronger solid bridges between particles.
5. Conclusion Operating conditions were found for the spray drying of a formulated emulsion allowing encapsulation of liquid oil in a powder (5% w/w of dry matter). Oil drops were well dispersed within the solid matrix and only 1.2% of the total oil content was found unprotected at the surface of the solid particles. During accelerated oxidation test, oil oxidation was reduced when the oil was protected, compared to unprotected one. However, handling properties of the spraydried powder (e.g. flowability and wettability) were very poor probably due to their small size (32 Am).
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In a next step, fluid bed agglomeration of these particles with help of spraying of water was performed. Optimal operating conditions to agglomerate fine particles were found to consist in controlling the air temperature within the fluid bed of particles by varying the flow rate of sprayed water. Whilst the powder wettability was actually increased, its flowability was not sufficiently improved. It has to be further improved as well as the mechanical resistance of agglomerates that were found to be very fragile. In this study, a vegetable oil was used as a model for other lipid-based ingredients. The behaviour of vegetable oil during spray drying was similar to that of a-tocopherol. This proposed encapsulation method could therefore be used for other oils in which it is also possible to incorporate oilsoluble substances such as aromas, antioxidants, etc. Instead of pure acacia gum or maltodextrin, a 3/2 mixture of maltodextrin DE12 and acacia gum was used as the solid wall material. Further trials could be performed with higher oil contents and other wall materials (maltodextrins with different DE, modified starches, proteins, etc.).
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