http://informahealthcare.com/mncISSN: 0265-2048 (print), 1464-5246 (electronic)

J Microencapsul, Early Online: 1–11! 2013 Informa UK Ltd. DOI: 10.3109/02652048.2013.778909

Matrix structure selection in the microparticles of essential oil oreganoproduced by spray dryer

Joyce Maria Gomes da Costa1, Soraia Vilela Borges1, Ariel Antonio Campos Toledo Hijo1, Eric Keven Silva1,Gerson Reginaldo Marques1, Marcelo Angelo Cirillo2, and Viviane Machado de Azevedo1

1Department of Food Sciences, Federal University of Lavras (UFLA), Lavras, MG, Brazil and 2Department of Exact Sciences, Federal University of

Lavras (UFLA), Lavras, MG, Brazil

Abstract

The goal of this work was to select the best combination of encapsulants for themicroencapsulation of oregano essential oil by spray dryer with the addition of Arabic gum(AG), modified starch (MS) and maltodextrin (MA). The simplex-centroid method was used toobtain an optimal objective function with three variables. Analytical methods for carvacrolquantification, water activity, moisture content, wettability, solubility, encapsulation efficiency(ME) and oil retention (RT) were used to evaluate the best combination of encapsulants. Theuse of AG as a single wall material increased ME up to 93%. Carvacrol is the major phenoliccompound existent in the oregano essential oil. Carvacrol exhibits a maximum concentration of57.8% in the microparticle with the use of 62.5% AG and 37.5% MA. A greater RT (77.39%) wasobtained when 74.5% AG; MS 12.7% and 12.7% MA were applied, and ME (93%) was improvedwith 100% of gum.

Keywords

Additives, physical properties, simplexcentroid, volatile compounds

History

Received 8 September 2012Revised 7 February 2013Accepted 18 February 2013Published online 25 March 2013

Introduction

Essential oils are extracted from herbs or spices and are sources ofbiological active compounds, such as terpenoids and phenolicacids (Bakkali et al., 2008). Many records report that suchcompounds exhibit good antimicrobial activity (Burt, 2004; Burtet al., 2005; Lopez et al., 2007). The oregano essential oil containscarvacrol and thymol as its major compounds (Al-Bandak andOreopoulou, 2007). Carvacrol is a phenolic monoterpene thatshows significant antibacterial activity in vitro (Didry et al.,1994), as well as antifungal (Burt and Reinders, 2003; Giordaniet al., 2004; Burt et al., 2005), antitoxigenic, insecticide andantiparasitic activities (Veldhuizen et al., 2006). Thus, puttingoregano essential oil in foods as a natural alternative ensures thepreservation and safety of foods (Souza et al., 2005).

However, the use of essential oils in their conventional formmay have limited applications because of the oils’ high volatility.The microencapsulation process provides several benefits toessential oils, such as the protection and stability of releasedvolatiles and storage that can be applied in textile products,pesticides, pharmaceuticals, cosmetics and food (Leimann et al.,2009; Murua-Pagola et al., 2009). For example, for microencap-sulation of compounds found in foods, spray drying is one of themost commonly used technologies in the food industry (Borgeset al., 2002; Shefer and Shefer, 2003; Fuchs et al., 2006;Reineccius, 2006; Murua-Pagola et al., 2009; Souza et al., 2009,2011; Ahmed et al., 2010) and has been widely used for essentialoils (Reineccius, 1988; Beristain et al., 2001; Bylaite et al., 2001;Baranauskiene_ et al., 2006, 2007; Yang et al., 2009), and in recent

years, ultrasound applications have been investigated as deviceson uses of atomisation stage in encapsulation processes in order toovercome the limitations typical of common equipments such asthe lack of versatility and high consumption of resources(Dalmoro et al., 2012a).

The main advantage of microencapsulation is the formation ofa barrier between the compound and the environment. This barriercan protect against oxygen, water and light and can preventcontact with other ingredients in a prepared meal or, for example,in a controlled diffusion of the encapsulated compound. Theefficiency of controlled release or protection depends mainly onthe composition and structure of the established wall and on theprocess conditions (temperature, pH, pressure and moisture)during the production and use of such particles. The barrier isgenerally formed by components that create a network through thehydrophilic or hydrophobic properties (Fuchs et al., 2006).

Despite the ease of manufacturing on an industrial scale, themicrospheres may have several disadvantages. The disadvantagesinclude the low capacity of encapsulation and removal of the corematerial during storage that can occur because of the crystallinestructure and polymorphic arrangements characteristic of manylipid materials during solidification and crystallisation withreduction in the amorphous regions of the polymer matrix (Satoand Ueno, 2005; Chambi et al., 2008).

Several auxiliary dryers (carriers or adjuvants), includingstarches (corn, cassava and rice), modified starches (MS),maltodextrins (MAs), gum Arabic (AG), cyclodextrins and cornsyrups, are often added to foods to minimise the loss of bioactivecompounds and act as the encapsulation agents to improve ormodify the physical and chemical composition of a product(De Souza et al., 2007). The encapsulating agents can be usedalone or in combination, and the ideal composition is set for eachspecific situation (Fernandes et al., 2012). The ideal selection ofencapsulants associated with microencapsulation techniques is

Address for correspondence: Joyce Maria Gomes da Costa, Department ofFood Sciences, Federal University of Lavras (UFLA), cx 3037, Lavras37200-000, MG, Brazil. Tel: +55 35 2142 2016. E-mail: joycecosta@dca.ufla.br

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valuable in minimising the loss of volatile essential oils, such ascarvacrol, that are extracted from herbs and is notably importantin the efficiency of microencapsulation.

Carbohydrates, such as starches and MAs, have beendemonstrated to be good encapsulating agents, and MA is amaterial of major application in the spray-drying process becauseof its physical characteristics, such as high solubility and lowviscosity in high solid concentrations (Reineccius, 1991; Goubetet al., 1998; Cano-Chauca et al., 2005). However, most of theseencapsulants alone have no interfacial properties necessary forgood efficiency of microencapsulation and therefore are oftenassociated with other encapsulating materials, such as gums andproteins (Yoshii et al., 2001).

AG is a notably effective encapsulating agent because of itsstabilising-colloid property. AG produces stable emulsions withmost of the oils in a wide pH range, constitutes a visible film onthe oil interface and has good retention of volatile, low viscosityand high solubility and is compatible with most of the gums,starches, carbohydrates and proteins (Yang et al., 2009). However,the cost and limited amount has restricted the use of AG forencapsulation. An increasingly interesting research field is of thedevelopment of an inexpensive alternative polymer, or combin-ations of polymers, that is capable of encapsulating flavours withan efficiency greater than or equal to that of AG.

Hydrophilic polymer blends are widely used in many pharma-ceutical and food for production of the microparticle (Dalmoroet al., 2012b). It is well described that a mixture of AG, starch andMA is suggested as a good wall material alternative formicroencapsulation of essential oil, which could result in powderswith good quality, low water activity (Aw), easier handling andstorage and also protects the active material against undesirablereactions (Anandaraman and Reineccius, 1987; Reineccius, 1988;McNamee et al., 2001; You-Jin et al., 2003; Kanakdande et al.,2007; Carneiro et al., 2013). MAs have been investigated as asubstitute for the AG in emulsions of spray dryer (Anandaramanand Reineccius, 1986), and a mixture of AG and MA proved tobe effective in the spray-dryer-mediated microencapsulation ofcardamom oil (Sankarikutty et al., 1988). The main deficiencyof MA is the lack of emulsifying capacity and low retentionof volatile compounds (Reineccius, 1988; Buffo andReineccius, 2000). The retention of volatile compounds increaseswith increasing dextrose equivalent (DE) of MAs (Anandaramanand Reineccius, 1986), suggesting the importance of DE infunctionality of the wall material. The chemically modifiedstarches have reproduced the functional properties of AG(Krishnan et al., 2005).

The aim of this study was to prepare microparticle of oreganoessential oil by spray drying using different materials and to selectthe best combination of matrix structure to be used as anencapsulating agent through analyses of the physical and chemicalproperties of the produced microparticle.

Experiment

Materials

MA GLOBE� 1905 (DE 20) and MS (Capsul Snow Flake�

E6131), both of which were obtained from Corn Products (MogiGuacu, SP, Brazil), and AG (Colloides Natureis SP, Brazil) wereused in order to form matrix structures. The material used was theessential oil of oregano (Laszlo Aromatherapy Ltd, BeloHorizonte, MG, Brazil).

Preparation of emulsion

Initially, MA and AG were hydrated in distilled water for �12 h at10–12 �C. Next, these ingredients were dissolved in distilled

water at 60 �C using the Ultra Turrax homogeniser at a speed of20 000 rpm for 30 min. Next, the Capsul was added at 82 �C,maintaining homogeneity until complete dissolution of the wallmaterials. Below 10 �C, 10% of oregano essential oil (proportionalto the presented encapsulants in the experimental design) wasadded, rotating at 20 000 rpm for 5 min, until a completelyhomogeneous emulsion was obtained.

Production of the microparticle

The formed emulsion was subjected to drying using a LABMAQBrazil 1.0 MSD spray dryer (Ribeirao Preto, Sao Paulo, Brazil),equipped with a 1.2� 10�3 m diameter nozzle. The pressure ofcompressed air for the spray flow was adjusted to 5 bars. The inletand outlet air temperatures were maintained at 180� 2 �C and105� 2 �C, respectively, and the feed rate was adjusted to2.97� 10�7 m3 s�1. The rate of inlet air was maintained at5.8� 10�4 s�1 m3.

The powders obtained for each treatment were stored underrefrigeration (4–7 �C) in glass vials protected from light and gaspermeation to minimise possible changes in the material, such asagglomeration, caused by water absorption and oxidation.

Experimental design

The interactions among the encapsulants of the variable part werestudied using a simplex centroid experimental design for amixture following methods described by Perez-Alonso et al.(2003) and Abdullah and Chin (2010). The mixture of theencapsulants was composed of AG (X1), MS (X2) and MA 20 DE(X3 or MA). The proportions of ingredients were expressed asfractions of the mixture with the sum of X1, X2 and X3 equal toone. The three encapsulants and their levels, respective ofthe experimental design in terms of pseudo-components with10 combinations, are shown in Table 1.

In the representation of the adjustment of the response values(water activity (Aw), moisture (X0), wettability (Wett), solubility(Sol), encapsulation efficiency (ME) and oil retention (RT)) of themicroparticle, a linear equation, which did not fit the responsevariables, was used initially; next, the quadratic equation(Equation (1)) was used in terms of pseudo-components. Thestatistical significance of the equations was determined byvariance analysis at a 10% confidence.

Y ¼ �1X1 þ �2X2 þ �3X3 þ �12X1X2 þ �13X1X3

þ �23X2X3,ð1Þ

where Y is the response-variable for each treatment (Aw, X0, Wett,Sol, ME and RT), bn are the regression coefficients determined byCornell (2002) and Garcia et al. (2010), and Xn are the

Table 1. Composition of the mixtures in the formulation ofmicroparticles of oregano essential oil with gum arabic, modifiedstarch and maltodextrin in a simplex centroid.

Proportions of wall materials

Tests X1 (AG) X2 (MS) X3 (MA)

1 1.00 0.00 0.002 0.00 1.00 0.003 0.00 0.00 1.004 0.50 0.50 0.005 0.50 0.00 0.506 0.00 0.50 0.507 0.33 0.33 0.338 0.66 0.16 0.169 0.16 0.66 0.16

10 0.16 0.16 0.66

Note: AG, gum Arabic; MS, modified starch; MA, maltodextrin.

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proportions of pseudo-components in which X1 is the proportionof AG, X2 is the proportion of MS and X3 is the proportion of MA.

The response variables of each experiment were analysedusing Statistica (StatSoft, Tulsa, OK, 2007) to analyse the mixtureusing a simplex centroid design. The variance analysis was usedto test the fit of the models. The validation and analysis ofregression models were performed by such observations as thelack of fit of the model, the estimation of the regression modelvariance and the degree of fit and significance of the model. Todetermine the effect of independent variables on the responsesevaluated, plots with a contour line in the experimental area wereconstructed.

The optimisation of the selection of the encapsulants in themicroencapsulation process was aimed at maximising the effi-ciency of encapsulation and oil retention using ResponseDesirability Profiling of the software Statistica (StatSoft, Tulsa,OK, 2007) according to the methodology described by Derringerand Suich (1980).

Analysis of the microparticle

Identification and quantification of the major constituent of themicroencapsulated oregano essential oil

To determine the best composition of wall materials used in thepreparation of the microparticle, the identification and quantifi-cation of the major constituent of the oregano essential oil and ofthe obtained microparticle was performed according to theexperimental design. This determination was performed by gaschromatography, and the quantification was obtained by stand-ardisation of the areas (%). The analyses were performed using agas chromatograph Varian CP 3380 equipped with a flameionisation detector (FID). The identification and quantification ofthe major constituent of the oregano essential oil was performedwith the standard injection (C10–C17) comparing the retentiontime of the standard with the compounds of the samples. Theoperating conditions were the following: a high-performancecapillary column (hp), split 1:100, injector temperature of 200 �C,temperature of FID detector of 200 �C, programming of thecolumn with initial temperature of 50 �C (3 min), 3 �C min�1 until145 �C, carrier gas hydrogen (2.0 mL min�1) and injection volumeof l mL (1% solution in dichloromethane).

Water activity

The water activity (Aw) was measured by direct lecture in a digitaldevice AQUALAB, CX-2 model (Decagon Devices Inc., Pullman,WA) with controlled temperature of 25� 0.5 �C.

Moisture

The moisture determination (wet basis) of the microparticle wasperformed by the gravimetric method at a temperature of 105 �Cuntil a constant weight was reached (AOAC, 2000).

Wettability

This property was determined by the method proposed by Fuchset al. (2006). One gram of powder was spread over 100 mL ofdistilled water at 25 �C without stirring. The time required for thepowder particles to sediment, or sink and disappear from thesurface of the water, was measured and used for a relativecomparison between the samples.

Solubility

Solubility was determined by the method described by Cano-Chauca et al. (2005), where 25 mL of distilled water wastransferred to a beaker and submitted to agitation in a

homogeniser at 2500 rpm. One gram of powder (dry basis) wasgently added to the water, and the agitation was maintained for5 min. The solution was transferred to a tube and centrifuged for5 min at 2600 rpm. One aliquot (20 mL) of the supernatant wastransferred to a petri dish and dried for 5 h at 105 �C. The percentsolubility (mass of powder/volume of solution) was calculated bythe weight difference.

Encapsulation efficiency

Encapsulation efficiency was determined by the fraction ofencapsulated oil over the total amount of oil (Equation (2)).

ME ¼ Oiltotal�Oilsurfaceð ÞOiltotal

� 100 ð2Þ

where ME is the encapsulation efficiency, Oiltotal is the totalamount of oil and Oilsurface is the amount of non-encapsulated oilexistent on the surface of the microparticle. The non-encapsulatedoil existent on the particle surface was determined according to themethod described by Bae and Lee (2008). Fifteen millilitres ofhexane was added to 2 g of powder in a sealed glass bottle, whichwas shaken for 2 min at room temperature to extract the free oil.Next, the solvent mixture was passed through a number 1 Whatmanpaper filter. The powder collected in the filter was washed threetimes with 20 mL of hexane. The solvent was evaporated at 60 �Cuntil constant, and the weight of the non-encapsulated oil wascalculated based on the difference in weight between the initialclean flask and the flask containing the extracted oil residue (Jafariet al., 2007). Total oil content of the spray-dried encapsulatedproducts was determined by distilling 10 g of encapsulated powderfor 3 h in a Clevenger-type apparatus (Jafari et al., 2007).

Oil retention

RT was determined by dividing the total oil quantified in theparticles after spray drying by the total oil initially added to theemulsion preparation (Equation (3)).

RT ¼ Oiltotal

Oilinitial

ð3Þ

where Oilinitial is the concentration of oil before spray drying andOiltotal is the concentration of oil after spray drying.

Particle morphology

Particle morphology was evaluated by scanning electron micros-copy (SEM). Powders were attached to a double-sided adhesivetape mounted on SEM stubs with 1 cm diameter and 1 cm height,coated with gold under vacuum and examined with a MEV 1430VP – LEO scanning electron microscope (Electron MicroscopyLtd., Cambridge, UK). SEM was operated at 20 kV withmagnifications of 900–1200.

Results and discussion

Identification and quantification of the major constituentof the microencapsulated oregano essential oil

The results showed that carvacrol is the major phenolic compoundexistent in the oregano essential oil, and Table 2 shows theconcentration of carvacrol on the microparticle produced in thedifferent proportions of wall material and the concentrationcarvacrol in the oregano essential oil before the microencapsula-tion process.

Silva et al. (2010) and Dardioti et al. (2012) obtained levels ofcarvacrol in oregano essential oil ranging from 61.66% to 93.42%and 55.2% to 62.3%, respectively. From these results, it can be

DOI: 10.3109/02652048.2013.778909 Matrix structure selection in essential oil oregano 3

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inferred that the microencapsulation process was more efficientwhen using combinations of wall materials, such as AG and MA.

Table 3 shows the correlation coefficients of the quadraticmodel (Q), R2 and the best fit obtained for the responses(carvacrol quantification, water activity, moisture, wettability,solubility, encapsulation efficiency and oil retention) of theexperimental results, as analysed using the simplex centroiddesign.

Through the data presented in Table 3, contour curves for thequantification of carvacrol in the oregano essential oil micro-particle were constructed as shown in Figure 1, and theinterpretation of the contour lines showed that AG at

concentrations of �60% (as one moves from the AG vertextowards the peak of the triangle) and MA at a concentration of43.5% (as one moves from the MA vertex towards the base of thetriangle) caused an increase in the carvacrol concentration.

The results obtained in this work corroborate the reportsindicating that AG and MA can be used as encapsulants forimproving the ME (Anandaraman and Reineccius, 1987;Reineccius, 1988; McNamee et al., 2001; You-Jin et al., 2003;Kanakdande et al., 2007).

The concentration of carvacrol near 45% can be explained bythe loss of non-encapsulated volatiles and thus not protected fromevaporation during the process, but these values are still greaterthan those obtained in other studies. Baranauskiene et al. (2006)obtained the levels of carvacrol equal to 15.4% and 27.4% fororegano essential oil microencapsulated with milk powder andwhey protein, respectively; these results are lower than the levelsfound in this work.

Water activity and moisture

The water activity and moisture content of the microparticleare important variables for the shelf life of the dust becausethey ensure microbiological stability during storage in theapplication of food products. In this work, the microparticle oforegano essential oil showed average values of water activityand moisture of 0.13–0.17% and 0.92–3.27%, respectively.These two variables were significantly influenced by theindependent variables and showed similar responses under theconditions evaluated. It was observed that higher concentrations

Figure 1. Contour curves of carvacrol quan-tification of microcapsules of oreganoessential oil.

Table 3. Results of the adjusted models for the chemical and physical properties of encapsulated oregano oil.

Carvacrol (%) Aw (–) X0 (%) Wett (s) Sol (%) ME (%) RT (%)

R2 (%) 87.54 86.45 90.97 76.80 97.61 86.80 84.11p Value 0.6027 ns 0.0463* 0.1048 ns 0.2305 ns 0.0058* 0.0724* 0.5650nsb1 46.98� 2.20* 0.16� 0.02* 2.95� 0.35* 21.55� 1.96* 73.71245� 0.23* 92.28� 1.35* 35.72� 0.67*b2 46.57� 2.20* 0.10� 0.02* 1.13� 0.35* 13.67� 1.96* 76.15432� 0.23* 85.65� 1.35* 61.12� 0.67*b3 45.51� 2.20* 0.18� 0.02* 3.29� 0.35* 14.08� 1.96* 77.12011� 0.23* 87.10� 1.35* 70.65� 0.67*b12 28.22� 10.27* �0.14� 0.08 ns �2.94� 1.64 ns �9.40� 9.14 ns �2.86646� 1.07* 11.83� 6.28 ns 98.86� 2.70*b13 41.64� 10.25* �0.12� 0.08 ns �5.44� 1.64* �20.30� 9.12* �0.52589� 1.07ns �15.96� 6.27* 95.78� 2.60*b23 0.61� 10.25 ns �0.14� 0.08 ns �3.77� 1.64* 10.41� 9.12 ns �5.60494� 1.07* 2.52� 6.27 ns �56.08� 2.60 ns

Notes: *Significant (p50.1); not significant (p40.1).Aw, water activity; X0, moisture; Wett, wettability; Sol, solubility; ME, encapsulation efficiency; RT, oil retention.

Table 2. Levels of carvacrol microencapsulated with differentproportions of wall materials.

Tests AG (%) MS (%) MA (%) Carvacrol content (%) w/v

1 100 0.0 0.0 46.02 25.0 75.0 0.0 46.53 25.0 0.0 75.0 45.64 62.5 37.5 0.0 54.75 62.5 0.0 37.5 57.86 25.0 37.5 37.5 48.27 49.7 25.1 25.1 51.28 74.5 12.7 12.7 56.29 36.2 51.0 12.7 50.1

10 36.2 12.7 51.0 49.9Oregano essential oil 74.5

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of MS had a positive impact on the dependent variables. Figures 2and 3 show that proportions of the regions near to 25% of AG,25% of MS and 75% of MA had values of and moisture less than0.1 and less than 1.2, respectively, which is useful to ensure thestability of the microparticle. The water removal is important toensure the formation of microparticle, but the quantity and therate of removal must be controlled. The main constituent in thespray of emulsion is water, which evaporates during the dryingprocess (490%); however, the volatile constituents may bemaintained when optimum drying conditions are used(Reineccius, 1998, 2004). Therefore, it was found that themicroparticle produced with higher concentrations of MS hadlower water adsorption together with the MA 20DE; in contrast,the samples produced with high concentrations of AG were themost hygroscopic. These differences in adsorption of water can beexplained by the chemical structure of each agent. AG and MA20DE have a large number of branches with hydrophilic groups,

and thus can easily adsorb moisture from the ambient air. MS isless hydrolysed and has fewer hydrophilic groups and thus is lessadsorbent of the water (Tonon et al., 2009). Frascareli et al. (2012)observed that increasing the content of AG caused an increase inhygroscopicity of the microparticle of coffee oil, and this increasewas attributed to the hygroscopic nature of AG. The resultsobtained in this work confirm that the Aw and humidity of themicroparticle were increased when higher concentrations of AGwere used due to the susceptibility of AG to adsorbing water.Adhikari et al. (2004) evaluated the effect of the addition of MAon the drying kinetics and stickiness properties of the sugar usingsolutions with different combinations of fructose, glucose,sucrose, citric acid, MA and water. The authors observed thatthe addition of MA significantly reduced the tackiness on thesurface of the sugar solution of low molecular weight, showing itseffective application in the drying process of fruit juices.Baranauskiene et al. (2007) evaluated various MS in the retention

Figure 2. Contour curves of water activity ofmicrocapsules of oregano essential oil.

Figure 3. Contour curves of moisture contentof the microcapsules of oregano essential oil.

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of aroma peppermint essential oil and observed that the wateractivity in the microparticle without removal of the oil surfacereached an average of 0.54 and 0.76.

Wettability

It was observed that the microparticle of oregano essential oilshowed better instantisation in water when 36.2% of AG, 12.7% ofMS and 51% of MA (DE 20) were applied than with thecombination of 25% of AG and 75% of MA (Figure 4). Thewettability was decreased due to increased concentration of AG,and the time taken for the particles of all wet tests ranged from 11to 22 min. Fuchs et al. (2006) reported 28 min in determining thewettability of microparticle of vegetable oil mixed with sunfloweroil, rapeseed and grape seed encapsulated by AG and MA (DE12). In this work, the combination of 25% of AG, 37.5% of MSand 37.5% of MA resulted in lower capacity of instantisation.Arbuto et al. (1998) verified that the MS (capsul) showed slowdissolution in water at 25 �C and was optimal when thetemperature was 82 �C. A combination that favours capillaryeffects and morphological characteristics could explain the lowvalues found for the time of wetness of all the encapsulantmaterials. According to Schubert (1993), among the materialsused in this work, the MAs have larger particle diameters and caninfluence the dispersibility, promoting wetting. According toShittu and Lawal (2007), the relation between the instantaneousproperties and physicochemical characteristics is not linear andinvolves the interaction of both. The solubility of cocoa bever-ages, for example, was primarily dependent on the chemicalconstituents (content of sugar and lipids), while the wetting timewas more affected by the physical properties. Using MAs withdifferent DEs, Fazaeli et al. (2012) found that the solubilityincreased by increasing the DE because of the large number oframifications with hydrophilic groups. In spite of the goodwettability of the encapsulants used in this study, the reconsti-tution of such materials in water can occur distinctly because ofthe chemical and structural properties of the wall materials used.Research by Quek et al. (2007), Pitalua et al. (2010) andFrascareli et al. (2012) suggest the use of AG as an encapsulantand additive in the spray-drying process mostly because of AG’semulsifying properties and high water solubility. However, MA isan encapsulant with the best solubilisation properties, and becausethis material is more accessible than AG. McNamee et al. (2001)

suggest the partial replacement of AG by the MA. However, theauthors found that MA (DE 18.5) was considered the mostappropriate suitable replacement for AG because it showed rapidreconstitution of the emulsion in water (Gharsallaoui et al., 2007).

Solubility

High solubility is a desirable property of the powder particles andis a result of a good encapsulating agent used in the spray drying.The produced microparticle reached a solubility of 74.2–77.2%(powder weight/solution volume), which is considered high,because the encapsulants that were used (amorphous solid) havehigh solubility in relation to the crystalline state (Yu, 2001;Gombas et al., 2003; Cano-Chauca et al., 2005; Lu, 2012). It wasobserved that increasing the concentration of MA resulted in anincreased solubility of the microparticle in water, and similarresults were obtained by Moreira et al. (2009) and Fazaeli et al.(2012), who obtained the solubility of mulberry juice powder anddry extract of acerola bagasse, respectively, with results near 87%soluble. These authors also observed that increasing the replace-ment of AG by MA caused the increased solubility of the powderin water. This finding can be explained by the high degree ofsolubility (490%) of MAs and varies depending on its molecularweight; the solubility of MAs increases with decreasing degree oframification of the a (1–6) (Cano-Chauca et al., 2005). Thus, theMAs DE 20 show high solubility (Gidley and Bulpin, 1987).However, it was found that the combination of MA with AG andMS influences the functional property of microparticle solubil-isation because of a possible change in the microstructure of thesystem. It was observed that the combination of MA with AG andMS in the evaluated proportions resulted in a decrease insolubility of microparticle, especially when AG was used inhigher concentrations (Figure 5). It was also observed that thesolubility was lower (73.6%) when high concentrations of thecombination of AG and MS were applied. This result may beobserved because of the low solubility of starch in cold water(�35–40%), which is attributed to the higher organisation of theparticles (crystalline state).

Oil retention and encapsulation efficiency

It was observed that the application of the mixture of theencapsulants (Figure 6) in the emulsion resulted in a positive

Figure 4. Contour curves of wettability of themicrocapsules of oregano essential oil.

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effect on the retention of oil, ranging from 33.10% to 77.39%.The higher RT was obtained when the proportions of 74.5%AG, 12.7% MS and 12.7% MA were used with an emphasis inthe combination of AG and MS, which are important for astabilising effect. This property can be attributed to the immediateformation of a semi-permeable membrane that must result fromapplying a greater concentration of AG (Re, 1998; Reineccius,2004; Fernandes et al., 2008). However, it was found that the use ofa single encapsulation (100% AG) resulted in lower RT in themicroparticle. Although AG was as effective as the wall material inencapsulating five different monoterpenes (citral, linalool, b-myr-cene, limonene and b-pinene) in the study by Bertolini et al.(2001), this encapsulant has a limited barrier ability againstoxidation because it acts as a semi-permeable membrane to oxygenand is therefore a limiting factor in the shelf life of the activecompound in the core and in the retention of volatile compounds.

Thus, similar results were obtained where AG wasineffective in the microencapsulation of orange oil compared

to the whey protein and soy protein isolates (Kim andMorr, 1996).

However, AG combined with other encapsulants may have apositive effect on RT. Krishnan et al. (2005) tested combinationsof AG, MS and MA, and obtained a high stability and volatileretention in cardamom oil resin when the concentration of AGwas higher. These combined materials represent an encapsulatingmatrix with improved properties in the retention of volatilecompounds, improvement in emulsion stability and protectionagainst oxidation (Buffo et al., 2002; Krishnan et al., 2005).

The encapsulation of oregano essential oil using gelatin andsucrose was also studied by Da Costa et al. (2012), and theseauthors concluded that the spray drying method provided aneffective retention of essential oil because of the protectionprovided by the microcapsules in the spray-drying process,resulting in less degradation and loss of volatile compounds.According to the ‘‘selective diffusion’’ theory, when the waterconcentration at the droplet surface decreases to �7–23%

Figure 5. Contour curves of solubility of themicrocapsules of oregano essential oil.

Figure 6. Contour curves of retention oil ofthe microcapsules of oregano essential oil.

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(Aw50.90), this dry surface acts as a semi-permeable membranepermitting the continued loss (or diffusion) of water but efficientlyretaining flavour molecules (Reineccius, 2001, 2004). IncreasedRT during high feed rates can also be related to the rapid formationof the semi-permeable membrane because of the higher solidcontent inside the drying chamber. Further research should focus ona detailed investigation of the impact of mixture of encapsulant onRT, and the acoustic levitation technique is an option to analyse thedrying behaviour of single emulsions (Serfet et al., 2013). Theminor retention obtained in other tests was probably because ofdifferent combinations and proportions of the used encapsulant andthe shape in which the core material (volatile) is arranged in themicroparticle. As water is removed by drying the surface of thedroplets, concentration gradients of water are formed. Thus, the RTdepends on the partial pressure of water in the system because thecore of the microparticle is directly accessible to water vapour(Karel, 1990; Beristain et al., 2001).

The use of different matrix materials may have a significantinfluence on emulsion droplet size. Many studies have shown thatthe droplet size is related to the viscosity, stability and may resultin great retention of active material (Soottitantawat et al., 2005;Jafari et al., 2008). However, Carneiro et al. (2013) in the study ofthe microencapsulation of flaxseed oil by spray drying withdifferent wall materials (MA, AG, whey protein concentrate andMS) reported that the ME could not be related to the emulsiondroplet size or viscosity and the differences between them can beattributed to the differences between the polymer matrices formedby each of the materials used, which have different retentionproperties and emulsion capacity. Further studies about theinfluence of using different matrix material on the oil drop sizedistribution will be developed in future work.

ME ranged from 85.3% to 93% and was significantlyinfluenced by the concentration of AG; in fact, the maximumefficiency was obtained when 100% AG was applied (Figure 7).The combination of AG and MS also favourably influenced MEbecause of the film-forming properties of these materials (Yanget al., 2009).

The encapsulation of the curcumin with MS has been studiedby Paramera et al. (2011), and these authors obtained an ME of56.2%. Through the response surface methodology, Ahn et al.(2008) obtained 96.6% efficiency in the microencapsulation ofsunflower oil through a structural matrix composed of 19.0% ofmilk protein isolate, 2.5% of soy lecithin and 54.8% of dextrin.

Physicochemical properties of the resulting powders and themicroencapsulation performance depend on many process vari-ables, such as the type of core and wall materials, emulsionproperties, the drying air characteristics and the type of atomiser.Therefore, for each oil/encapsulant system, it is important toselect the main variables and to optimise the drying process toobtain good-quality powders. As a result of the data obtained,mixtures that optimise the response variables are described inTable 4.

According to the results showed in Table 4, the samples thatcontain AG and MA (in higher proportions) were more appro-priate in the microencapsulation of oregano essential oil, andthese data are in accordance with the discussions presented in thisarticle, which reported that AG combined with MAs and MS oralone are encapsulants good applied to improve the ME andstability of the microparticle.

Powder morphology

Figure 8 shows the SEM microstructure (internal and external) ofpowder produced with different matrix materials combination.Observing the external morphology, most of the microparticlesshowed a spherical shape and various sizes with diameter varyingbetween micrometre and millimetre. This trait depends on thematerial and method used to prepare it; it was observed to formconglomerate, and according to Bhandari et al. (1992) andChambi et al. (2008) this happens with the atomisation processalso. Similar structures were obtained by Sansone et al. (2011)that studied morphology and other proprieties of the MA/pectinmicroparticle as carrier for nutraceutical extracts produced byspray drying. Some microparticles when broken show a porouswall (Figure 8). According to Fang and Bhandari (2010) andTeixeira et al. (2004), microparticles produced by spray dryingusually have a spherical shape and in some cases, they can behollow (Yao et al., 2008; Sun et al., 2009; Gu et al., 2013). It canbe explained because of a shrinkage process after the solidifica-tion of the materials followed by the expansion of the bubblesembedded into the drop. The mechanisms associated with theformation of this empty space inside the particle are related withthe movement of the material at the last stage of the dryingprocess. Analysing the internal morphology, all microparticleswere hollow and the active material was adhered to the surfaceas small droplets embedded in the wall material matrix.

Figure 7. Contour curves of encapsulationefficiency of the microcapsules of oreganoessential oil.

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This emptiness is a result of the quick particles expansion duringthe final stages of drying (Jafari et al., 2008). According to Khoand Hadinoto (2010), to produce the large hollow spherical micro-aggregates, the spray drying condition (e.g. drying temperature,feed rate) and the formulation ingredients (i.e. encapsulants typeand its concentration) must be meticulously determined. Thephysical mechanism behind the hollow micro-aggregate formationis described as follows. Liquid evaporation from the dropletsurface exposes the microparticles at the receding liquid–vapourinterface to the vapour phase. As the surface energy of a solid–vapour interface is greater than that of a liquid–vapour interface,the microparticles migrate towards the droplet centre to minimisetheir surface energy. A fast convective drying rate in which theliquid evaporation time is shorter than the time needed by themicroparticles to diffuse back towards the droplet centreis required to produce the hollow morphology. Themicroparticles with porous and hollow structure have moreexcellent properties such as low density and large encapsulationcapacity. The results also show a preponderance of themicroparticle with rough surface.

Conclusion

In agreement with the proposed objectives, it was concluded thatmixtures model was adequate in the optimisation study andselection of wall materials used in microstabilisation of oreganoessential oil by spray drying, carvacrol was the major phenoliccompound existing in the oregano essential oil with 57.8%concentration maximum in microparticle with the use of 62.5%AG and 37.5% MA. Better RT occurred when using 74.5% AG,12.7% MS, 12.7% MA and AG as a single wall materialconcentration of 100%. These conditions increased the ME upto values of 93%.

Therefore, according to the results a mixture of AG, MS andMA could be suggested as a good alternative for stabilisation oforegano essential oil which result in good microparticle withphysical and chemical properties and better ME.

Declaration of interest

The authors report no conflicts of interest. The authors alone areresponsible for the content and writing of the article.

Figure 8. Microphotograph of particles of oregano essential oil produced by spray drying.

Table 4. Summary of results in the optimisation for each response-variable.

AG (%) MS (%) MA (%) Observed mean Predicted mean

Mixtures which provided maximal responsesCarvacrol (%) 50.0 0.0 50.0 57.801 56.663Aw (–) (%) 0.0 0.0 100.0 0.179 0.178X0 (%) 0.0 0.0 100.0 3.270 3.290Wett (s) 100.0 0.0 0.0 22.000 21.550Sol (%) 0.0 0.0 100.0 77.200 72.120RT (%) 68.0 16.0 16.0 77.380 65.110ME (%) 100.0 0.0 0.0 93.00 92.280

Mixtures which provided minimal responsesCarvacrol (%) 0.0 0.0 100.0 45.630 45.510Aw (–) (%) 16.0 68.0 16.0 0.096 0.092X0 (%) 16.0 68.0 16.0 0.733 0.852Wett (s) 16.0 16.0 66.0 11.330 13.890Sol (%) 100.0 0.0 0.0 73.600 73.710RT (%) 100.0 0.0 0.0 33.100 35.720ME (%) 0.0 100.0 0.0 85.330 85.650

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The authors thank FAPEMIG (Fundacao de Amparo a Pesquisa doEstado de Minas Gerais, Brazil), and CNPq (Centro Nacional deDesenvolvimento Cientıfico e Tecnologico) for providing funding.

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DOI: 10.3109/02652048.2013.778909 Matrix structure selection in essential oil oregano 11

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