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EffectofWaterActivityontheStabilitytoOxidationofSpray‐DriedEncapsulatedOrangePeelOilUsingMesquiteGum(ProsopisJuliflora)asWallMaterial

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206 JOURNAL OF FOOD SCIENCE—Vol. 67, Nr. 1, 2002 © 2002 Institute of Food Technologists

Food Engineering and Physical Properties

JFS: Food Engineering and Physical Properties

Effect of Water Activity on the Stability toOxidation of Spray-Dried EncapsulatedOrange Peel Oil Using Mesquite Gum(Prosopis Juliflora) as Wall MaterialC.I. BERISTAIN, E. AZUARA, AND E.J. VERNON-CARTER

ABSTRACT: Mesquite gum solutions (30% w/v) were used to emulsify orange peel oil in an oil-gum solids ratio of0.25. Emulsions were spray dried in laboratory scale equipment. The powders were stored in aw (s) from 0.108 to0.743 at 35 88888C. Quantitative analysis of limonene oxide indicated that the sample at 0.628 showed a very goodstability against oxidation after thirty d, without caking and stickiness occurring. At this water activity the systemwas within rubbery state, and the moisture content corresponded to that of the minimum integral entropy.

Keywords: mesquite gum, microencapsulation, minimum entropy, water activity, and glass transition

Introduction

NUMEROUS MATERIALS ARE COMMERCIALLY AVAILABLE TO EN-capsulate flavor using the spray drying technique. The

most widely used encapsulating agents are gum Arabic andmodified or hydrolyzed starches. Mesquite gum (Prosopisjuliflora) has been reported as having the ability to encapsu-late orange peel oil (80.5% of the starting oil) but to a lesserextent than gum Arabic (93.5% of the starting oil) (Beristainand Vernon-Carter 1994). Goycoolea and others (1998) foundthat the orange peel oil encapsulating capacity of gum Arabicwas slightly higher than that of a native and low-tannin mes-quite gum (Prosopis spp.), when used in combination withmodified cornstarch carrier matrices. A blend of 60:40% wtof gum Arabic to mesquite gum was able to encapsulate thesame amount of orange peel oil than pure gum Arabic(Beristain and Vernon-Carter 1995), whereas a 3:2 ratio ofmaltodextrin 10DE to mesquite gum retained 84.6% of thestarting orange peel oil, providing a better encapsulating ca-pacity than either polysaccharide on its own (Beristain andothers 1999). However, mesquite gum is not yet commercial-ly available. In most studies, temperature has been consid-ered as the main parameter affecting the stability of encap-sulated orange peel oil, whereas aw effects have beenneglected (Risch and Reineccius 1988; Chang and others1988). One exception is the report by Anker and Reineccius(1988), who monitored the formation of limonene oxides inencapsulated spray dried orange peel oil stabilized in the awrange of 0.001 to 0.536, finding that the slowest rates of for-mation of limonene oxide occurred at water activities of0.536.

The glass transition has been proposed to accelerate dif-fusion-controlled reactions, such as oxidation, enzymatic re-actions, and nonenzymatic browning (Slade and Levine1991). These reactions in food systems are related to changesin water content, water activity, temperature, pH, reactantconcentration, and structure (Bell 1996).

Diffusion in glassy systems is claimed to be virtually non-existent. It is believed that low mobility in the glassy state dueto high viscosity makes chemical reactions improbable (Belland Hageman 1994). However, oxidation is dependent on

proper matrix formation as well as exclusion of oxygen fromthe matrix (Bell and Hageman 1995). The physical state ofthe encapsulation matrix and its moisture absorption is criti-cal in stopping oxidation development during storage (Shi-mada and others 1991). An amorphous glass entraps flavorcompounds and protects them from oxidation due to slowdiffusion (Onwulata and Holsinger 1995). Ross (1995)claimed that flavor compounds that are susceptible to oxida-tion become protected from oxygen due to decreased diffu-sion of gases through the glassy structure below Tg. On theother hand, Ma and others (1992) found that oxidation of oilwithin a maltodextrin M 100 encapsulating matrix occurredwithin the wall material’s glassy state. Roozen and others(1991) showed that the rotational mobility of tempol in-creased within the glass transition temperature of the sys-tem. Glassy state was not rate limiting. Bell and Hageman(1994) found that the rate of aspartame degradation depend-ed upon water activity rather than upon the state of the sys-tem, and that mobility, as dictated by the glass transitiontemperature of the system, was not rate limiting. Andersenand others (2000) followed the permeation of oxygenthrough a glassy matrix encapsulating oil, finding that oxygenpermeation increased with temperature and was the rate-de-termining step.

Stability is greatly influenced by the moisture sorptioncharacteristics of the product. The thermodynamics of watersorption in dried foodstuffs has drawn interest because itprovides a more thorough interpretation of the sorption iso-therm phenomenon and helps to understand better thesorption mechanism. There exists a controversial issue ofwhether moisture equilibration represents a true state ofthermodynamic equilibrium, as certainly are all the spray-dried materials. Although true equilibrium in thermodynam-ic terms is not achieved, the pseudo equilibrium states at-tained may be considered stable for practical time frames.Thermodynamic parameters have been used to study thestability of foods. Le Maguer (1985) presented the basicframework for the use of solution thermodynamics and dis-cussed its application to starch water systems. Rizvi (1986)discussed general principles associated with hystereses and

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how these affect thermodynamic calculations. Braibanti(1990) calculated the chemical potential of binding watermolecules to reveal the specific features of adsorbing surfac-es for cod, bee, pectin, and microcrystalline cellulose.Beristain and others (1994, 1996) used Othmer’s equation tocalculate both differential and integral thermodynamic pa-rameters to study stability of coffee beans, and applied theenthalpy-entropy compensation for water adsorption ofstarchy foods and dried fruits, finding that adsorption pro-cesses in microstructures with low moisture content wereentropy controlled and modified by temperature.

The purpose of this study was to investigate the relativedegree of protection against oxidation provided by mesquitegum to orange peel oil encapsulated via spray drying storedat several water activity conditions.

Materials and Methods

Raw materialsMesquite gum, collected in the form of tears in the Mexi-

can state of San Luis Potosi, was pulverized in a hammer mill(Glen Mills Inc., Clifton, N.J., U.S.A.). In order to stop enzy-matic activity, the powder was dissolved in water at 77 8C for1 h in a stainless jacketed vessel (Polinox, Mexico, D.F., Mexi-co) with a propeller-type agitator, filtered with a high-flowsupercel in a Shriver filter press (EIMCO Process Equipment,Houston, Texas, U.S.A.), and dried in a Bowen BLSA pilotplant spray-drier (Somerville, N.J., U.S.A.) with an inlet airtemperature of 175 ± 5 8C and an outlet air temperature of87 ± 5 8C.

Single strength orange peel oil was obtained from Deriva-dos Veracruzanos, S.A. de C.V. (Xalapa, Ver., Mexico). All waterused in the experiments was double distilled and deionized.

Emulsion formation and spray dryingAn aqueous mesquite gum solution was prepared from

spray-dried gum with a concentration of 30% (w/v). The solu-tion was heated and held at 80 8C with constant stirring for 30min, then allowed to cool, covered, and left to stand overnightat room temperature. Emulsions were prepared by addingdrop wise and dispersing the requisite amount of orange peeloil into the aqueous mesquite gum solution in order to obtaina ratio of 25 g of oil/100 g gum solids, using a Silverson L4RHeavy Duty Laboratory Mixer Emulsifier (Silverson Machines,Ltd., Chesam., Bucks., U.K.) at a speed setting of 5. The result-ing crude emulsions were then homogenized by increasingthe speed setting in the Silverson to 10 until a mean volumetricglobule size smaller than 2.5 mm was obtained, as measuredby a Malvern Droplet and Particle Size Analyzer series 2600(Malvern Instruments, Malvern, Worcs., U.K.). Emulsion sam-ples (about 400 g) were spray dried in a Büchi Mini Spray Dry-er model 190 (Büchi Laboratoriums Technik AG, Flawil, Swit-zerland), using an inlet temperature of 200 ± 5 8C and anoutlet air temperature of 110 ± 5 8C.

Vapor sorption isothermsSamples of the spray-dried encapsulated orange peel oil

were placed in desiccators containing P2O5 for 3 wk at roomtemperature. The moisture sorption data were obtained us-ing the gravimetric method described by Lang and others(1981). Two to 3 grams of samples were weighed in triplicateinto standard weighing dishes with a circular section on thebottom, where a quantitative filter paper Whatman No. 1 wasused to support the sample and at the same time allow trans-mission of moisture. In order to obtain a true moisture gain

by the sample alone, the filter paper was allowed to equili-brate over the salts solution. Samples were placed in sepa-rate desiccators containing saturated salt slurries in therange of water activity from 0.11 to 0.85 using the aw report-ed by Labuza (1985). The samples, were held at 25, 35, and45 8C until equilibrium was reached. Values of water activitywere generated using equations reported in the same paper.Equilibrium was assumed when the difference between 2consecutive weightings was less than 1 mg/g of solids. Thetime to reach the equilibrium varied from 8 to 14 d. Moisturecontent of the humidified systems was determined by differ-ence in weight after drying in a vacuum oven at 60 8C in thepresence of magnesium perchlorate desiccant.

The Guggenheim-Anderson-De Boer (GAB) equation wasused in modeling water sorption (Weisser 1985):

(1)

where aw is water activity; M is water content of the sampleon dry basis; Mo is the monolayer water content; C is theGuggenheim constant = C’exp (hm-hn)/RT; hm is the heat ofsorption of the first layer, hn is the heat of sorption of themultilayer, K is the constant correcting properties of multi-layer molecules with respect to bulk liquid = k’ exp (h1-hn)/RT; h1 is the pure water heat condensation; T is the absolutetemperature; and R is the gas constant. The parameter valuesof GAB equation (Mo,C,and K) were obtained by nonlinearregression analysis.

Goodness of fit was evaluated using the average of the rel-ative percentage difference between the experimental andpredicted values of the moisture content or mean relativedeviation modulus (P) defined by the following equation (Lo-mauro and others 1985):

(2)

where Mi is the moisture content at observation i; MPi is thepredicted moisture content at that observations; and n is thenumber of observations.

Thermodynamic propertiesThe isosteric heat of sorption is a differential molar quan-

tity derived from the temperature dependence of the iso-therm, and it represents the energies for water moleculesbinding at a particular hydration level, in contrast to the inte-gral heat, which is the average energy of all molecules al-ready bound at that level (Schneider 1981). The respectivedifferential and integral entropies are obtained from theirdifferential and integral heats, respectively. The usual entro-py discussed qualitatively or quantitatively (statistical me-chanics) in terms of order-disorder of the adsorbed mole-cules is the integral entropy and not the differential entropy(Hill and others 1951, Rizvi and Benado 1984).

Differential properties. Changes in differential enthalpy atthe water-solid interface at different stages of the adsorptionprocess were determined using Othmer’s equation (1940):

(3)

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Stability to oxidation of encapsulated orange peel oil . . .

where Pv is vapor pressure of water in the food; Pv0 is vapor

pressure of pure water at the same temperature; Hv(T) isisosteric heat for water adsorption; Hv

0(T) is heat of conden-sation of pure water; M is moisture; and C is adsorption con-stant.

A plot of lnPv against lnPv0 gives a straight line if the ratio

Hv(T)/Hv0(T) is maintained constant in the range of tempera-

tures studied.The net isosteric heat of adsorption or differential en-

thalpy is defined by Eq. 4.

(4)

Calculating Hv(T)/Hv0(T) with Eq. 3 and substituting into

Eq. 4, it is possible to estimate the net isosteric heat of ad-sorption at different temperatures using steam tables.

With values obtained for enthalpy changes, the variation inthe molar differential entropy (DSdif)T may be estimated usingEq. 5:

(5)

where S1 = (­S/­N1)TiP is molar differential entropy of wateradsorbed in the food; SL is molar entropy of pure water inequilibrium with the vapor; S is total entropy of water ad-sorbed in the food; N1 is number of moles of water adsorbedin the food; R is universal gas constant; aw is water activity;and T is temperature (K).

Integral properties. Molar integral enthalpy is calculatedusing an expression similar to that for differential enthalpy,maintaining diffusion pressure constant:

(6)

where Hvi(T) is integral molar heat of water adsorbed infood and F can be found by (Nunes and Rotstein 1991):

(7)

(8)

where F is diffusion pressure or surface potential of thefood; ma is chemical potential of the adsorbent in the con-densed phase; map is chemical potential of the pure adsor-bent; Wap is molecular weight of the adsorbent; Wv = molec-ular weight of water; F/a1 constant is similar to a process atF constant.

When values for (DHint) are obtained, changes in molarintegral entropy can be calculated using Eq. 7:

(9)

where SS = S/N1 is integral entropy of water adsorbed in thefood.

Storage stabilityThirty-gram samples were placed in desiccators contain-

ing saturated solutions of LiCl, KC2H3O2, MgCl2, K2CO3,Mg(NO3)2, NaNO2, NaCl for 30 d. At 35 8C the water activitiesof the desiccants were 0.108, 0.215, 0.318, 0.436, 0.515, 0.628,and 0.743 respectively. Portions of the samples were with-drawn every 5 d for gas chromatographic analysis. The sam-ples were put into the desiccants immediately after theywere spray-dried, and this was taken as the zero point time.

Gas chromatographic analysisSamples were prepared for gas chromatography as de-

scribed by Anker and Reineccius (1988). In short, 0.15 g pow-der were dispersed in 0.85 g distilled water in a screw capvial, then mixed slowly with a vortex into 4 ml of acetonecontaining 0.25 mg/ml of 2-octanone as internal standard.The resultant supernatant was injected without further prep-aration into a Hewlett-Packard model 6890 gas chromato-graph (GC) (Hewlett Packard Co., Wilmington, Del., U.S.A.).The GC was equipped with flame ionization detector andHP-5 column. The carrier gas was helium at a flow velocityof 1 ml/min. The temperature of the column was pro-grammed initially at 50 8C for 1 min and then increased atrate of 8 8C/min to a final temperature of 200 8C. Theamount of limonene oxide was determined by the internalstandard method (Anandaraman 1984).

Glass transitionThe glass transition temperature was determined by a dif-

ferential calorimeter (DSC 2910, TA Instruments, New Castle,Del., U.S.A.), equipped with a refrigerated cooling accessory.The Thermal Solutions Instrument Control and UniversalAnalysis Software were used (TA Instruments, New Castle,Del., U.S.A.). The samples (8 to 10 mg) were transferred toaluminum pans, sealed hermetically, and weighed. The calo-rimeter was calibrated with indium (melting point156.598 8C) and mercury (melting point -38.834 8C). Threereplicates were carried out for each analysis. Samples wereheating at 20 8C/min to a temperature that was 30 8C abovethe estimated Tg from preliminary runs. Then it was cooledat 10 8C/min and reheated at 5 8C/min. Glass transition tem-perature was determined as the onset point of the stepchange on the heat flow curve.

Results and Discussion

THE GAB MODEL WAS FOUND TO AGREE VERY WELL WITH THEexperimental data. The parameters from fitting the GAB

model were determined (Table 1). The mean relative devia-tion modulus value was less than 5% at 25, 35, and 45 8C. Wa-ter activity of foods can be related to stability and the ratesof deteriorative reactions. It has also been assumed that themonolayer value is the critical water content where dehy-

Table 1—Estimated parameters of the GAB equation fororange peel oil microcapsules

Temperature 88888C

Parameter 25 35 45

C 12.04 15.33 18.9K 0.918 0.933 1.02Mo (% dry basis) 5.74 5.34 4.45P% 4.96 3.41 4.75

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drated foods are most stable. Encapsulated orange peel oilprepared with mesquite gum at 35 8C had a monolayer valueof 5.34 g water/100 g solids calculated by the GAB equation.

Differential and integral entropies changed with respectto moisture content at 35 8C for the encapsulated orangepeel oil prepared with mesquite gum (Figure 1). The inter-section of the curves is found at the minimum integral entro-py, and it can be observed that, as the microcapsules adsorbmoisture, their integral entropy falls to a minimum. It is atthis point that maximum stability can be assumed, since wa-ter molecules are more ordered within the microencapsulat-ing material (Nunes and Rotstein 1991). In our experiments,the point of maximum stability against oil oxidation wasfound at 12.8 g water/100 g soluble solids for microcapsulesprepared with mesquite gum, corresponding to a water ac-tivity close to 0.628. The point of minimum integral entropycan be interpreted as the moisture content that correspondsto the monolayer (Hill and others 1951; Nunes and Rotstein1991). Minimum entropy values may be expected wherestrong bonds between adsorbate and adsorbent occur(Nunes and Rotstein 1991), and therefore, water is less avail-able to participate in spoilage reactions.

The point of minimum integral entropy, and theoreticallyof maximum stability, is different than the calculated valuesfor the GAB monolayer for the mesquite gum microcapsuleswith 5.34 g water/100 g soluble solids (Figure 1).

Differential entropy had a minimum at 5.7 g water/100 gdry solids that was similar to the GAB monolayer value. Nev-ertheless, this parameter does not mean order or disorder ofthe total system. The differential entropy represents the alge-braic sum of the integral entropy at a particular hydration lev-el, plus the change of order or disorder after new water mole-cules were adsorbed by the system at the same hydrationlevel. If the values of moisture content corresponding to mini-mum integral entropy and minimum differential entropy are

different, this particular hydration level at the minimum dif-ferential entropy cannot be considered as the maximum sta-bility point, because not all available active sites have been oc-cupied at that particular water content, and therefore it ispossible to obtain after this point lower differential changesthat provide a better ordering of the water molecules ad-sorbed on food. The water content at the minimum differen-tial entropy in this case was similar to the GAB monolayer val-ue because the maximum entropy change was provoked bythe water sorption on the most active sites. Monolayer value isthe saturation of polar groups corresponding to water ad-sorbed at the most active sites.

The temperature (Tg) at which microcapsules have theirglass transition depends strongly on the water content. TheTg temperatures decreased with increasing water content ascan be observed in Table 2. Glass transition was difficult todetect in mesquite gum at low moisture content < 4.91 g wa-ter/100 g of dry solids. The inability to find the glass transi-tion at low moisture contents was consistent with previousfindings for complex food systems (Bell and Touma 1996).

Table 2—Glass transition temperature (Tg) of encapsulatedorange peel oil with mesquite gum storage at 35 88888C as afunction of water activity (aw) and moisture content (M)

aw Tg 88888(C)a M (g water/100 g s.s.)

0.215 86.41 ± 0.28 4.910 ± 0.150.318 71.35 ± 0.32 6.850 ± 0.170.436 52.07 ± 0.26 8.900 ± 0.160.515 30.65 ± 0.20 10.06 ± 0.220.628 20.35 ± 0.21 12.23 ± 0.230.743 - 4.10 ± 0.20 15.83 ± 0.290.821 - 10.51 ± 0.21 22.90 ± 0.25a Mean of 3 replicates 6 standard deviation

Figure 1—Differential and integral entropy changes as afunction of moisture content for encapsulated orange oilstored at 35 88888C

Figure 2—Glass transition temperature of orange peel oilmicroencapsulated in mesquite gum stored at 35 88888C as afunction of water activity

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Stability to oxidation of encapsulated orange peel oil . . .

The Tg values were also plotted against water activity. Thisplot showed a linear relationship between water activity andglass transition temperature (Figure 2). Prediction of the Tgwith the linear relationship between Tg and aw allows a rapidand fairly reliable method for locating the Tg of materialsstored at various conditions before experimental verificationof the Tg is obtained (Roos 1995).

This GAB monolayer value corresponds to a water activityof 0.26 and a Tg = 79 8C, and this Tg is above the storage tem-perature 35 8C. Based on the properties associated with theglassy state, where diffusion is claimed to be virtually nonex-istent (Levine and Slade 1989), it would be expected that nooxidation of the orange oil should occur at all. Nevertheless,the rate of oxidation of the orange oil, contained within themesquite gum-encapsulating matrix, occurred rapidly withinthe glassy state. Oxidation rates increased as the water activi-ty of the microcapsules decreased from 0.436 to 0.108, butwere still within the glassy state (Figure 3). In contrast, themicrocapsules in the aw range of 0.515 and 0.628, within therubbery state, showed the greatest stability, when an acceler-ated rate of oxidation was expected based on the resultsfound for the microcapsules in the aw range from 0.436 to0.108. The mesquite gum microcapsules exposed to aw of0.743 showed the best stability against oxidation, but the datafrom this water activity were not included in Figure 3 be-cause microcapsules showed visual structural changes andwater gain by the powders led to a change in their flow prop-erties as a result of caking and agglomeration.

A aw of 0.821 resulted in the complete destruction of themicrocapsules. At this humidity level, sufficient water wasdrawn up by the microcapsules to form a paste-like mass.

These results seem to indicate that storing microcapsulesat aw(s) corresponding to the zone of minimum integral en-tropy provides them with the best stability against oxidation.

Levine and Slade (1989) claimed that low mobility in glassystate makes chemical reactions improbable. However, the nu-merous mechanisms of chemical reactions make such a gen-eralization unfounded (Bell and Hageman 1995). Oxidation isdependent upon proper matrix formation as well as exclusion

of oxygen from the matrix. Glassy characteristics may delayoxidation only if the oxygen is suitable to be entrapped by themolecular structure of the glassy matrix. Andersen and others(2000) showed that in oil encapsulating glassy food matrix,some oil particles might oxidize rapidly and some more slow-ly, due to heterogeneity in the degree of encapsulation. Therate of oxidation should be regarded as an average of all indi-vidual oxidation processes taking place in each of the oil parti-cles. In the present work, probably the major determinant ofthe microcapsules shelf life is the porosity of the dried matrixto oxygen diffusion, independently of the supercooled liquidstate of the matrix. With values of aw (s) > 0.5, that were defi-nitely above the calculated values for the GAB monolayer re-gion, an increase in the rate of oxidation is expected. Likewise,the protection and retention of volatiles is expected to occuras long as the microcapsules wall structure remains intact.The gel-like structure arising from the water, adsorbed by themesquite gum, acts as a shell opposed to oxygen diffusion intothe microcapsule core, inasmuch as water is adsorbed with-out initiating the wall dissolution process. However once thecapsule structure is damaged by water uptake, volatiles are in-creasingly exposed. Once the structure is destroyed, as in thecase of water activity at aw of 0.821, the capsules lose their vol-atiles content.

Conclusion

MESQUITE GUM PROVIDED A VERY GOOD PROTECTIONagainst oxidation to orange peel oil as long as micro-

capsules remained stored at the proper water activity. Oxida-tion processes occurred with different intensities at the vari-ous water activities. Microcapsules in the aw range of 0.515and 0.628 showed the greatest stability. At water activity be-low 0.628, the structure of the capsules was not damaged; in-creasing water activity to between 0.743 and 0.821 led togradual dissolution of the walls. Results suggest that oxida-tion of orange peel oil occurred when the samples were inglassy state, and oxidation was slower when they were rub-bery. Based on the properties of the glassy state of the mi-crocapsules, glassy systems were stable to physical changes(agglomeration, caking), whereas rubbery systems were sta-ble to oxidation and physical changes as long as the capsulestructure was intact.

The minimum integral entropy can be interpreted as thewater activity (0.628) at which the microcapsules have thebest stability to oxidation. The system at this water activitywas within the rubbery state, showing the best stability.

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Figure 3—Effect of water activity on limonene oxidation ofencapsulated orange peel oil stored at 35 88888C

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MS 20001536, Submitted 10/9/00, Revised 4/6/01, Accepted 4/8/01, Received 5/01/01

The authors gratefully acknowledge financial aid by the Consejo Nacional de Ciencia y Tecnologiade Mexico, who supported this study through the projects 25924-B and G33565-B.

Authors Beristain and Azuara are with the Instituto de Ciencias Básicas,Universidad Veracruzana, Apdo. Postal 575, Xalapa, Ver., México. AuthorVernon-Carter is with the Universidad Autónoma Metropolitana-Iztapalapa, San Rafael Atlixco #186, 09340 México, D.F. Direct correspon-dence to author Beristain (E-mail: cberista@bugs.invest.uv.mx).

Stability to oxidation of encapsulated orange peel oil . . .