Food Engineering and Physical Properties

Microencapsulation by Spray Drying:Influence of Emulsion Size on theRetention of Volatile CompoundsA. SOOTTITANTAWAT, H. YOSHII, T. FURUTA, M. OHKAWARA, AND P. LINKO

ABSTRACT: The influence of the mean emulsion droplet size on flavor retention during spray drying was studied. d-Limonene, ethyl butyrate, and ethyl propionate were used as the model flavors. Gum arabic, soybean water-solublepolysaccharides, or modified starch blended with maltodextrin, were used as the wall materials. The increasingemulsion droplet decreased the retention of flavors. The distribution curve containing larger emulsion dropletsshifted toward a smaller size after atomization, indicating that the larger emulsion droplets would be changed insize during atomization and result in decreasing flavor retention. For ethyl butyrate and ethyl propionate, theretention had a maximum at the mean emulsion diameter of 1.5 to 2 and 2.5 to 3.5 �m, respectively.

Keywords: microencapsulation, spray drying, flavor retention, emulsions, volatiles

Introduction

The microencapsulation of flavoris the technology of converting liquid

flavor materials into easy-to-handle solids.It also provides protection against degra-dative reactions and prevents the loss of fla-vor. In addition, it can be used to controlthe release of flavors during food processingand storage. Various encapsulation meth-ods have been previously proposed (Dziezak1988; Shahidi and Han 1993; King 1995;Gibbs and others 1999). Among them, spraydrying is the most popular method of pro-ducing flavor powders, which presents thechallenge of removing water by vaporiza-tion while retaining the flavors that aremuch more volatile than water. According tothe selective diffusion concept (Thijssenand Rulkens 1968), the diffusion coefficientof flavor decreases at a higher rate than thediffusion coefficient of water during drying.

Numerous studies have been conductedto evaluate the retention of flavor duringspray drying. The properties of the volatilecompounds (Rulkens and Thijssen 1972;Reineccius 1988), the capsule wall material(Inglett and others 1988; Sheu and Rosen-berg 1995; McNamee and others 1998, 2001;Buffo and Reineccius 2000), and the emul-

sion (Reineccius 1988; Risch and Reineccius1988; Mongenot and others 2000; Liu andothers 2000, 2001), along with the dryingprocess conditions (Rulkens and Thijssen1972; Zakarian and King 1982; Anker andReineccius 1988; Chang and others 1988;Rosenberg and others 1990; Bhandari andothers 1992; Finney and others 2002), andthe powder morphology during and afterdrying (El-Sayed and others 1990; Moreauand Rosenberg 1993; Hecht and King 2000)have been already reported. Recently, themain emphasis of the microencapsulationof flavors has concentrated on preventingflavor losses during spray drying and ex-tending the shelf life of the products. Thisis intended to produce high quality flavorpowders. Buffo and Reineccius (2000) re-ported the optimization of the gum acacia/modified starch/maltodextrin blending. Theattempt to replace commonly used gum ar-abic with other carbohydrates has been in-vestigated by McNamee and others (2001).Furthermore, work has also been done onimproving the properties of flavor (that is,combining flavor with additive materials).For example, Liu and others (2001) im-proved flavor retention by adding a weight-ing agent to the flavor. Finally, because pro-cess development should be carried out tocontrol the loss of flavor, Finny and others(2002) studied the effects of the type of at-omization and processing temperature onthe properties of the encapsulated flavor.

The emulsion size in the feed liquid alsoinfluences flavor retention and the stabili-ty of the encapsulated flavors. Using gumArabic or Amiogum 23 as the carrier, Risch

and Reineccius (1988) studied the effect ofemulsion sizes of orange peel oil (0.9 mm ~4.0 mm) on flavor retention and shelf life.Their results suggested that a smaller emul-sion size yields a larger percentage retentionof the orange oil with a smaller amount ofsurface oil, but did not produce a longershelf life. Sheu and Rosenberg (1995) sug-gested that the retention of volatiles duringmicroencapsulation by spray drying couldbe enhanced by reducing the mean emul-sion size of the dispersed core material dur-ing emulsification. Ethyl caprylate retentionwas improved by reducing the mean emul-sion size when a combination of whey pro-tein and maltodextrin was used as the carri-ers. Furthermore, Ré and Liu (1996)correlated an improved eugenol retentionby measuring the difference between themean emulsion size and the mean particlesize of the dry powder.

Scanning Electron Microscopy (SEM)techniques have been developed for thestudy of the outer and inner structures offood microencapsulation, including proce-dures for embedding and sectioning or pol-ishing the embedded specimen by Rosen-berg and others (1985). SEM techniqueshave since been further developed and be-come an important technique in studyingmicroencapsulation as a tool for the selec-tion of wall materials, for the study of corematerial distribution in microcapsules, andfor elucidating the mechanisms of capsuleformation.

The objective of this study was to find thereasons for decreasing emulsion size yield-ing a higher retention of orange oil. The en-

MS 20030050 Submitted 01/28/03 Revised 05/28/03 Accepted 05/31/03. Authors Soottitantawat,Yoshii, and Furuta are with Dept. of Biotechnol-ogy, Tottori Univ., Tottori 680-8552, Japan. AuthorOhkawara is with Ohkawara Kakouki Co. Ltd.,Yokohama 224-0053 Japan, and author Linko iswith Dept. of Chemical Technology, Helsinki Univ.of Technology, Espoo, Finland. Direct inquiries toauthor Furuta (E-mail: takeshi@bio.tottori-u.ac.jp).

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capsulation of model flavors, d-limonene(insoluble flavor) in the emulsion state wasinvestigated by spray drying, concentratingon emulsion droplet size in both the feed-liquid emulsion and in the spray-dried pow-der. The effect of emulsion droplet size onthe flavor retention was also studied for themoderately soluble flavors, ethyl butyrateand ethyl propionate. The SEM techniquewas also used to investigate the inner struc-ture of the encapsulated powder and thearrangement of flavor droplets in the micro-capsules.

Materials and Methods

Materialsd-Limonene, ethyl butyrate, ethyl propi-

onate, and gum arabic (GA) were purchasedfrom Nacalai Tesque, Inc. (Kyoto, Japan).Maltodextrin with 15-20 DE (MD, AmycolNo. 1) and modified starch (HI-CAP 100)were gifts from Nippon Starch ChemicalsCo., Ltd. (Osaka, Japan), and Nippon NSCCo., Ltd. (Tokyo, Japan), respectively. Thebasic properties and characteristics of soy-bean-soluble polysaccharide (SSPS), a newtype of emulsifier which was extracted andrefined from soybeans, were reported byMaeda (2000). In this study, SSPS(SOYAFIBE-S EN100) was also a gift fromFuji Oil Chemical Co., Ltd. (Osaka, Japan),which has been marketing SSPS under thebrand name ‘SOYAFIBE-S,’ and sucrose ace-tate isobutyrate (SAIB) was a gift from TaiyoKagaku Co., Ltd. (Mie, Japan). The organicchemicals used in the analysis were of ana-lytical grade.

Preparation of a liquid flavoremulsion

d-Limonene, ethyl butyrate, and ethylpropionate were used as the model flavors.Based on a previous study (Liu and others2000, 2001), the emulsion of ethyl butyrateand ethyl propionate with the GA emulsifierwas unstable. Therefore, the weightingagent, SAIB (Chanamai and McClements2000; Liu and others 2001), was blendedwith ethyl butyrate and ethyl propionate.The mixing mass ratio of the esters flavor toSAIB was 0.55 to 0.45 to regulate the densi-ty. The resulting densities were 0.98 ± 0.02g/cm3 for both the ethyl butyrate and ethylpropionate. The carrier solution was pre-pared by blending and rehydrating the sol-id powders in warm distilled water, followedby cooling to room temperature. The totalconcentration of dissolved solid was 40% w/w on a wet basis, which consisted of 10%emulsifier (GA, SSPS, or HI-CAP 100) and30% additive wall materials, MD, as shownin Table 1. d-Limonene, the density-adjust-

ed ethyl butyrate, or the density-adjustedethyl propionate was added to the carriersolution to produce a flavor mass ratio to thetotal solid of 0.25 to 1. An emulsion was pre-pared by homogenizing the flavor with thecarrier solution using a Polytron homogeniz-er (PT-10, Kinematica GA, Littau, Switzer-land) and a Microfluidizer (Model 110T,Microfluidics Corp., Newton, Mass., U.S.A.)at 12000 psig (82.8 MPa). The rotationalspeed and time of the homogenization werecontrolled in order to vary the emulsiondroplet size distribution. In general, a larg-er emulsion size was prepared at a low rota-tional speed (at dial 5 to 8 of the Polytronhomogenizer) for 1 to 3 min. For preparing afine emulsion, the emulsion solution madeby Polytron homogenizer (at dial 8 for 3min) was passed through the Microfluidizer.The size distribution of the liquid emulsiondroplets was measured immediately afterthe homogenization.

Preparation of encapsulated flavorpowders

The powders were obtained by spray dry-ing the flavor emulsion in an Ohkawara-L8spray dryer (Ohkawara Kakouki Co., Ltd.,Yokohama, Japan) equipped with a centrif-ugal atomizer. The spray dryer was com-posed of a cylindrical chamber with an 800mm I.D. and 560 mm height, followed by aconical chamber of 650 mm high with a 60 °angle. The operational conditions of thespray drying were: air inlet temperature of200 °C, air outlet temperature of 110 ± 10 °C,rotational speed of the atomizer of 30000rev/min, feed rate of 45 mL/min, and air flowrate of 110 kg/h. The finished powder wasstored in a hermetically sealed bottle at–30 °C until analysis.

Emulsion droplet size analysisThe droplet size distribution of the

emulsions was measured by using a laserdiffraction particle size analyzer (SALD-3000A, Shimadzu Corporation, Kyoto, Ja-pan) connected to a flow sample unit cell.Distilled water was used as the dispers-ant. The refractive index was set to thestandard value at 1.70 to 0.2i. Measure-ments were reported either as the full par-

ticle size distribution or as the mean diam-eter. In this study, the volume average lip-id globule size, D43, was used as the meandiameter based on the volume frequency.Each sample was analyzed in duplicate,and the data presented as an average. Inorder to determine the flavor droplet sizedistribution in the powders, the spray-dried powders were redispersed in dis-tilled water at a concentration of 10% (w/w) by mixing with a magnetic stirrer for 30min. Subsequently, the flavor droplet size(reconstituted emulsion size) was also de-termined by SALD-3000A.

Determination of flavor dropletsize in emulsion after atomization

In order to investigate the change in theemulsion droplets during atomization, theemulsion size distribution in the atomizeddroplet was compared with the feed emul-sion before the atomization. Plastic bagswere used to cover the rotating atomizer inthe drying chamber without air flow andheating in order to collect the atomizedemulsion. The atomized emulsions werethen determined for the emulsion dropletsize by the laser scattering method as de-scribed above.

Powder particle size analysisThe size distribution of the spray-dried

powders was determined by dispersingthem in 2-methyl-1-propanol and analyz-ing by the laser light scattering method. Alaser diffraction particle size analyzer with abatch cell unit was used. D43 was used as amean diameter. Each sample was analyzedin duplicate and the data were reported asan average.

Rheological measurementsThe viscosities of the emulsion and carri-

er solution were measured by a dynamicshear rheometer, Brookfield viscometer(Model DV-II, Brookfield Engineering Labo-ratories, Inc., Middleboro, Mass., U.S.A.),with a small sample adapter, SC4-18, and 8mL sample volume. The samples wereplaced in the temperature-controlled mea-surement vessel and allowed to equilibrateto the required temperature (25 °C).

Table 1—Composition of the carrier solutions and their viscosities

Malto- Gum HI-CAP Viscosity ofdextrin Arabic SSPS 100 solution (mPa·s)(% w/w) (% w/w) (% w/w) (% w/w) at 25 °Ca

Blend GA-MD 30 10 96Blend SSPS-MD 30 10 1500Blend HI-CAP 100-MD 30 10 45ameasured for 2 h at the proper rotational speed; the uniform values were used.

Food Engineering and Physical Properties

Effect of emulsion size on retention . . .

Total flavor content determinationOne 10th of a gram of the spray-dried pow-

der was dispersed in 4 mL of water in a glassbottle, and then 2 mL of hexane was added,followed by violent mixing with a vortex mixerfor 1 min. To extract the flavor into the organ-ic solvent, the mixture was heated in a heat-ing block with intermittent shaking. The ex-traction time and temperature were 30 minand 90 °C, respectively. The extracted mixturewas then centrifuged at 3000 rev/min for 10min to separate the organic phase from thewater. Two microliters of the organic phasewere injected twice for each sample into a gaschromatograph (GC-14B, Shimadzu Corp.,Kyoto, Japan) equipped with a PEG-20Mpacked column (2m × 3.2 mm). The chro-matographic conditions were as follows:flame ionization detector (FID) at 230 °C withN2 as the carrier gas. The column tempera-ture was controlled at constant values of 120°C, 100 °C, and 80 °C for d-limonene, ethylbutyrate, and ethyl propionate, respective-ly. The external standard method was usedto calculate the flavor quantity. Two bottles ofthe standard 1 ml/mL of flavors in hexanewere prepared, and 2 microliters of this stan-dard were injected into the gas chromato-graph twice for each bottle of standard. Theaverage peak area was used to calculate theamount of flavor. To determine the solid con-tent in the powder, 1 gram of spray-driedpowder was dried in the vacuum dryer (AD-VANTEC VR-320, Toyo Seisakusho Co., Ltd.,Chiba, Japan) at 90 °C for 20 h and the fla-vors quantified in the same manner as men-tioned above. The solid content in the spray-dried powder was calculated based on theassumption of a totally dried powder aftervacuum drying. Flavor retention was definedas the ratio of the flavor in the powder on adry basis to the original flavors in the feedemulsion. All of the samples were analyzed induplicate and the data were presented as anaverage.

Surface oil determinationOne 10th of a gram of powder was washed

in 2 mL of hexane containing the internalstandard cyclohexanone, 1 �l/mL, in a glassbottle. The mixtures were slowly mixed on arotary shaker for the optimum time of 30 sat ambient temperature. The solvent wasthen filtered. The flavor content in the or-ganic phase was measured by gas chroma-tography as described above. The resultswere the average of the duplicates in eachsample.

Scanning electron microscopy(SEM)

A JSM 5800 model (JEOL Co., Ltd., Tokyo,Japan) scanning electron microscope was

used to investigate the microstructuralproperties of the spray-dried microencap-sulated products. The powders were placedon the SEM stubs using a 2-sided adhesivetape (Nisshin EM Co. Ltd., Tokyo, Japan). Inorder to examine the inner structure, thepowders (attached to the stub) were frac-tured by attaching a 2nd piece of adhesivetape on top of the microcapsules and thenquickly ripping it off (Moreau and Rosen-berg 1993). Because of the weakness of thiscutting method (only the weak particles,especially the hollow particles, might becut), 5 specimen stubs were prepared foreach sample using strong adhesive tape. Inall cases, the specimens were subsequent-ly coated with Pt-Pd using a Model MSP-1Smagnetron sputter coater (Vacuum DeviceInc., Tokyo, Japan). The coated sampleswere then analyzed using the SEM operat-ing at 15 kV. Twenty pictures of each samplewere taken to represent the structure of thepowders.

Moisture content determinationThe moisture content of the spray-dried

powder was determined using a MKS-510NKarl Fischer Titrator (Kyoto Electronics Man-ufacturing Co., Ltd., Kyoto, Japan). The pow-der sample, 0.03 to 0.08 g, was weighed andmixed with the Hayashi solvent FM contain-ing 26% methanol and < 0.2 mg H2O/mL(Hayashi Pure Chemical Ind., Co., Ltd., Os-aka, Japan) and titrated against a 5 mg/mLHydranal composite solvent (Riedel-de-Haën, Sigma-Aldrich, Seelze, Germany). Thewater content was reported in % w/w of sam-ple. The samples were measured 3 timesand the averages reported.

Statistical analysisThe effects of the emulsion droplet size

were studied in 3 types of wall materials and

3 types of model flavors. The quantificationof the flavor by gas chromatography, emul-sion droplet, and powder size measure-ments, and the rheological measurementswere done in duplicate for each treatment,except for determining the water contentwhich was analyzed in triplicate. Averagevalues were used for calculations and fig-ures. The calculations and graphs weremade using the computer program MicrocalOrigin 6.1 (Microcal Software Inc.,Northampton, Mass., U.S.A.).

Results and Discussion

Effect of the feed emulsion size onthe properties of encapsulatedspray dried powder of insolubleflavor

Table 2 shows the properties of the en-capsulated insoluble flavor (d-limonene)powder, including the water content andparticle droplet size as a function of the ini-tial mean emulsion size. The viscosity of theblend of SSPS-MD was much higher thanthe other 2 systems, but the fine emulsionand good properties of powder were alsoobtained. The water content of the powderwas similar for each system. The resultsshow no effect of the emulsion droplet sizeand type of wall materials on the water con-tent properties of the powder that shoulddirectly relate to the drying conditions. Fur-thermore, the results also show that thepowder size was similar in each system asrelated to the properties of the feed liquidemulsion, viscosity, density, and surface ten-sion (Marshall 1954). These properties ofthe feed liquid will influence the size of theliquid droplet formation during atomiza-tion.

Figure 1 shows the influence of the feedemulsion droplet size on the retention of the

Table 2—Properties of feed liquid emulsion and the spray dried powder forencapsulating d-limonene

Viscosity D43 ofD43 of feed of emulsiona flavor in Water Powder

Wall material emulsion size (mPa.s) powder content sizesystem (µm) at 25 °C (µm) % (w/w) (µm)

Blend GA - MD 1.05 180 1.55 0.88 42.681.26 120 1.65 2.53 44.812.97 180 2.62 2.51 47.194.09 180 2.94 1.27 44.83

Blend SSPS - MD 0.88 3200 1.14 1.88 52.590.99 2600 1.29 1.74 53.931.55 2600 1.60 2.65 55.192.95 2600 2.55 2.09 49.04

Blend HI-CAP 100 - MD 0.65 90 0.90 2.48 53.420.90 65 0.91 1.35 42.992.05 61 2.10 2.26 46.474.07 60 2.21 1.79 44.88

ameasured for 2 h at the proper rotational speed; the uniform values were used.

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insoluble flavor, d-limonene, during spraydrying. For all combinations of the carriersolid, the increasing emulsion diameter re-sulted in a decreasing retention of d-li-monene. Flavor retention markedly de-creased, especially in the range of the 0.5 to2.0 �m mean emulsion size. This impliesthat a fine emulsion is stable during boththe atomization and spray drying, and alsoindicates that, for the proper wall materials,the emulsion droplet size is a significant fac-tor for the retention of flavor. Risch and Rei-neccius (1988) have also shown in similarresearch that a smaller emulsion size yieldsa higher retention of orange oil.

In order to explain the decreasing d-li-monene retention of the larger dropletemulsion, the emulsion droplet size distri-butions before and after atomization wereinvestigated The flavor droplet distribu-tions in the powder (emulsion droplet afterdrying) were also measured. Figure 2ashows the distribution curve of the initialcoarse emulsion droplet size before and af-ter atomization when a blend of GA and MDwas used as the wall material. The distribu-tion of the emulsion in the spray-dried pow-der is also shown in the figure. The distribu-tion curve after atomization shifted towarda smaller size, while the distribution of theemulsion droplets after drying appearedunchanged compared with the droplets dis-tribution after atomization. This implies thatthe larger emulsion droplets would besheared into smaller droplets because ofthe large velocity gradient and the turbu-lence in the thin liquid film on the surfaceof the rotating atomizer. Some of thesheared droplets were then broken downand evaporated during atomization. Theseresults explain the greater loss of flavorfrom the larger emulsion droplets during

spray drying, as shown in Figure 1. On theother hand, the distribution curve of thefine emulsion before and after atomizationand after drying are shown in Figure 2b. Thedistribution curve of fine emulsion dropletsize seemed unchanged during the atomi-zation, implying that the fine emulsiondroplets were redistributed into a lower ra-tio than the coarse emulsion droplet sizeafter atomization, thus resulting in higherretention.

In Figure 2, the emulsion droplet distri-butions after atomization seemed un-changed after spray drying. Therefore, therelationship between the mean flavor sizein the feed emulsion and in the spray-driedpowder for all combinations of wall materi-als was investigated as shown in Figure 3.The larger droplets became smaller afterdrying, especially the mean emulsion sizewhich was greater than 2 mm. On the other

hand, for the fine droplet size, the flavordroplet size remained unchanged after dry-ing. These results also supported the reasonfor the lower retention of the coarse emul-sion droplet as previously explained.

Effect of the feed emulsion dropletsize on the surface oil content

The surface oil can be easily oxidized toform off-flavor compounds. The amount ofsurface oil in the spray-dried powder isquite important for stable storage. Figure 4shows the amount of surface oil for differ-ent carrier solutions as a function of the fla-vor emulsion droplet size for d-limonene.The ordinate is the percentage of surface oil(flavor) to the total amount of flavor in thepowder. The amount of surface oil increasedwith the increasing mean emulsion dropletsize for most of the wall materials; that is,surface oil values were in the range of 0 to

Figure 1—Effect of feed emulsion drop-let size on the retention of d-limonene,an insoluble flavor, during spray dry-ing

Figure 2—Change in distribution of emulsion droplet size: Wall materials: BlendGA-MD, Flavor: d-limonene (a) Coarse droplet size (b) Fine droplet size: � Feedemulsion, � Emulsion after atomization, � Redispersed encapsulated powder

Figure 3—The relation between d-li-monene flavor droplet size in the feedemulsion and after spray drying for thevarious types of wall materials

Figure 4—Effect of feed d-limoneneemulsion droplet size on the amountof the surface oil after spray drying

Food Engineering and Physical Properties

Effect of emulsion size on retention . . .

6% of the total oil content with an emulsionsize range of 0 to 5 mm. Risch and Reineccius(1988) have also shown similar results—thata larger emulsion size yielded a higher sur-face oil content. Higher remaining flavor onthe surface of the atomized droplets mightbe explained by the breakdown of the largeemulsion particles during atomization, asshown in Figure 2a.

Effect of the feed emulsion dropletsize and the type of flavors onflavor retention during spraydrying

Figure 5 shows the flavor retention duringspray drying as a function of the emulsionsize of d-limonene, ethyl butyrate, and eth-yl propionate when a blend of GA and MDwas used as the wall material. The physico-chemical properties of each flavor are shownin Table 3. The results show that the flavorretention of d-limonene (80 to 90% flavorretention) was higher than the esters, ethylbutyrate (40 to 60% flavor retention), andethyl propionate (40 to 50% flavor reten-tion). The same results were also reported inour previous study (Liu and others 2000,2001) with the lower retention of ester fla-vors. The lower stability of the ester emul-sion was mentioned as the reason for thelower flavor retention during spray drying.They also showed the improvement of theflavor emulsion stability and flavor reten-tion by adjusting the density of the esterflavor with a weighting agent, sucrose ace-tate isobutyrate (SAIB). However, althoughthe stable emulsion with the adjusted den-sity esters were used in this study, the reten-tion of the ester flavor during spray dryingwas still lower than the retention of the sta-ble emulsion of d-limonene. This indicatesthat there might be other factors which in-

fluence the loss of the ester flavor duringdrying. The molecular dimensions seem toplay a significant role in the loss of flavorbecause they are directly related to the dif-fusion of molecules. Furthermore, the highvolatile and high solubility of flavors mightencourage a higher loss of flavor duringspray drying. As Rosenberg and others(1990) commented in their study, the dis-solved flavor can diffuse with water throughthe wall material matrix during the earlystages of the drying process. They also saidthat, besides the loss from the moleculardiffusion that controls retention, the loss ofthe water-soluble flavor should be a result ofdroplet stripping during the early stages ofdrying. Therefore, the dissolved flavors inthe feed emulsion were easier to lose thanthe flavor droplets in the emulsion duringthe drying, especially during the semiper-meable membrane formation period.

When the effect of the emulsion size isconsidered, the increasing emulsion diam-eter resulted in a decreasing retention of fla-vor during spray drying of the insoluble d-limonene, as already explained. Also, in thecase of the moderately soluble ethyl bu-tyrate and ethyl propionate, larger emulsiondroplets decreased the retention. The reten-tion had a maximum at the optimal value ofthe mean emulsion diameter, and a fineemulsion caused a decrease in the retentionof both the ethyl butyrate and ethyl propi-onate. Ethyl butyrate showed a maximum of60% retention at the mean emulsion size inthe range 1.5 to 2.0 �m. In Figure 5, it wasdifficult to reach a conclusion about ethylbutyrate since there are few data points inthe range of the fine small droplet size, 0.5to 1.0 �m. However, the retention seemedto decrease in the range of the small drop-lets, although the small emulsion in therange of 0.5 to 1.0 �m could not be pre-pared. The emulsion droplet size was alsolimited by the emulsified properties of thecarrier solution (blend of GA and MD at 40%w/w solid content). Moreover, ethyl propi-onate showed a maximum of about 55% re-tention for the mean emulsion size around2.5 to 3.5 �m. This would be due to an in-crease in the surface area of the flavor drop-lets in the emulsion, resulting in an acceler-ated dissolution of the soluble flavor into thecarrier solution and diffusion through the

liquid to evaporate on the surface of thesprayed droplets during the early stage ofthe drying period as already explained. Thedifference in the optimal emulsion dropletsize of the ethyl propionate and ethyl bu-tyrate, which showed the highest retention,might be explained by the difference in theirsolubility as shown in Table 3. The solubilityof ethyl propionate is 3 times greater thanthe solubility of ethyl butyrate.

Internal microstructure ofencapsulated powder for differenttypes of model flavor

The morphology of encapsulated flavorpowders during spray drying affects someof their properties, such as the flavor releaserate and the flow properties of the spray-dried powder. The flavor release character-istics depend upon the porosity, surfaceintegrity, and the distribution of flavordroplets in the capsule which is directly re-lated to the morphology of the atomizeddroplet during drying. The flow propertiesof spray-dried powders are also closelylinked to the outer topography of the parti-cles. Therefore, it is very important to char-acterize the outer and inner structures ofthe encapsulated flavor powders.

In this study, an SEM technique wasused to investigate the inner microstructureof the encapsulated flavor powders. Figures6, 7, and 8 show SEM photographs of theencapsulated powders of d-limonene, ethylbutyrate, and ethyl propionate in a GA andMD blended capsule, respectively. A centralvoid was observed. Formation of the centralvoid is related to the expansion of the cap-sule. This has been commonly observed inspray-dried powders and has been previ-ously studied (Verhey 1972; Greenwald andKing 1982). The mechanisms associatedwith the formation of these central voids arerelated to the expansion of the particlesduring the latter stages of the drying pro-cess.

Furthermore, the inner structure of thecapsules indicated that the flavors were lo-cated in the form of small droplets embed-ded in the wall matrix. The inner structure ofthe capsules was similar to that reported ingeneral for spray-dried flavor microcapsules(Chang and others 1988; Rosenberg andYoung 1993; Sheu and Rosenberg 1995; Rei-

Figure 5—Effect of feed emulsion drop-let size and properties of model flavorson the retention flavor during spraydrying. Wall materials: Blend GA-MD

Table 3—Physicochemical properties of flavors

Molecular weight Solubility (v/v) Boiling temperature (°C)

d-limonene 136 insoluble 136Ethyl butyrate 116 6.7x10-3 116Ethyl propionate 102 1.7x10-2 102

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Figure 7—Internal microstructure of encapsulated adjusted density ethyl butyrate in dried powder. Wall materials:Blend GA-MD, D43 of flavor in feed emulsion: 1.31 �m, D43 of flavor in powder 1.35 �m

Figure 8—Internal microstructure of encapsulated adjusted density ethyl propionate in dried powder. Wall materials:Blend GA-MD, D43 of flavor in feed emulsion: 1.32 �m, D43 of flavor in powder 1.46 �m

Figure 6—Internal microstructure of encapsulated d-limonene in dried powder. Wall materials: Blend GA-MD, D43 offlavor in feed emulsion: 1.71 �m, D43 of flavor in powder 1.88 �m

Food Engineering and Physical Properties

Effect of emulsion size on retention . . .

neccius and others 1995; Kim and Morr 1996;Finney and others 2002). Figure 6 showsmany d-limonene droplets embedded in thewall matrix and the high concentration of fla-vor droplets. On the other hand, for the mod-erately soluble flavors of ethyl butyrate andethyl propionate, Figures 7 and 8 show a fewof the encapsulated flavor droplets and a lowconcentration of the droplets, especially inthe area near the outer surface of the powder.These observations support the assumptionthat the losses in soluble flavor might be en-hanced by internal mixing that may bring theester to the surface of the capsule and thuscause its partial loss to the drying air, espe-cially during the early stage of the dryingperiod resulting in the low retention of flavor.Rosenberg and others (1990) also mentionedthe same assumption.

Conclusions

The emulsion droplet size influencedthe retention of volatiles after encapsu-

lation by the spray-drying method. The re-sults indicate that there are advantages tocreating a smaller emulsion for the insolubleflavor when preparing a solution for spraydrying, even though larger emulsions aremuch easier to prepare and are less expen-sive. The smaller emulsion of the insolubleflavor shows the higher retention and lowersurface oil content after the encapsulation.The change in the size of the larger emul-sion droplet during atomization shows themajor loss of flavors. However, for the mod-erately soluble flavor, the preparation of asmaller emulsion is not required in the pro-cess. The highest retention of an encapsu-lated moderately soluble flavor can be ob-tained at the optimal value of the meanemulsion droplet size.

AcknowledgmentsThe authors thank Fuji Oil Chemical Co.,

Ltd. (Osaka, Japan), Nippon NSC Co., Ltd.(Tokyo, Japan), Nippon Starch ChemicalsCo., Ltd. (Osaka, Japan), and Taiyo KagakuCo., Ltd. (Yokkaichi, Mie, Japan) for their

kind gifts of SSPS, HI-CAP 100, MD, andSAIB, respectively.

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Bhandari BR, Dumoulin ED, Richard HMJ, Noleau I,Lebert AM. 1992. Flavor encapsulation by spraydrying: Application to citral and linalyl acetate. JFood Sci. 57(1):217-21.

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