InfluenceofOxidantsontheStabilityofTocopherolinModel ...downloads.hindawi.com/journals/jchem/2018/6438787.pdfsize distribution of the emulsions was then measured by usingacommercialstaticlightscatteringinstrument(BT-9300ST;

Research ArticleInfluence of Oxidants on the Stability of Tocopherol in ModelNanoemulsions Role of Interfacial Membrane Organized byNonionic Emulsifiers

Jinhyuk Kim1 Ha Youn Song2 and Seung Jun Choi 13

1Department of Food Science and Technology Seoul National University of Science and Technology Seoul 01811Republic of Korea2Department of Agricultural Biotechnology Seoul National University Seoul 08826 Republic of Korea3Department of Interdisciplinary Bio IT Materials Seoul National University of Science and Technology Seoul 01811Republic of Korea

Correspondence should be addressed to Seung Jun Choi choisjseoultechackr

Received 17 August 2018 Accepted 16 October 2018 Published 12 November 2018

Guest Editor Hiroshi Umakoshi

Copyright copy 2018 Jinhyuk Kim et al is is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Nanoemulsions were prepared by using emulsifiers with various sizes of hydrophilic and hydrophobic groups to determine theimpact of interfacial characteristics on the stability of α-tocopherol incorporated into the nanoemulsions e α-tocopherolconcentration remaining after 3 weeks of storage at 25degC depended greatly on the type of oxidative stress which indicated that theenvironment surrounding the oil droplets could determine the stability of α-tocopherol in nanoemulsions α-Tocopherol wasgradually degraded by radical-mediated oxidation over storage and approximately 60 of its initial concentration remained after3 weeks of storage However under acid- and iron-mediated oxidation α-tocopherol concentration steeply decreases for theinitial 3-day storage but the degradation rate of α-tocopherol decreased after 3 days of storage and over 90 of the initialα-tocopherol remained after 3 weeks of storage Interestingly and contrary to our expectations the thickness andor density of thedroplet interfacial membrane rarely affected the stability of α-tocopherol incorporated into nanoemulsions Although it is difficultto generalize beyond α-tocopherol we conclude that the properties of oil droplet surfaces had no influence on the storage stabilityof α-tocopherol encapsulated in the droplets

1 Introduction

Lipid oxidation is one of the greatest concerns for oil-containing food products because of its negative influenceon nutritional quality and consumer health [1] In additionto lipid oxidation lipophilic functional compounds in-corporated into emulsion-based delivery systems can bedecomposed by various oxidative stresses [2] To inhibit lipidoxidation and to prevent lipophilic compound de-composition several synthetic and natural antioxidants aregenerally incorporated into foods containing considerableamounts of lipids [3] However as a result of consumersrsquodemands for clean food (ie food products that do notcontain synthetic additives) food manufacturers have beenmaking various attempts to replace synthetic antioxidants

with natural ones Among the natural antioxidants per-mitted for food use tocopherols are important because theyexist naturally in many vegetable oils [4] and because theyhave the ability to retard lipid oxidation by reacting withseveral radicals generated from lipid molecules therebyprotecting functional compounds from oxidative degrada-tion α-Tocopherol radicals can form nonradical products ifthey are reduced by other coexisting antioxidants withregeneration of α-tocopherol

In food systems oil-in-water emulsions generally consistof water and oil with the oil being dispersed as smalldroplets in the water [5] Emulsifiers have surface activity sothey can create kinetically stable emulsions by absorbing atthe surfaces of droplets newly formed during homogeni-zation [6] Emulsifiers absorbed at the oil droplet surfaces

HindawiJournal of ChemistryVolume 2018 Article ID 6438787 8 pageshttpsdoiorg10115520186438787

create an interfacial membrane comprised of a layer formedof their hydrophobic tails and a layer formed of their hy-drophilic heads Physical destabilization processes such ascoalescence flocculation and Ostwald ripening are greatlyaffected by the characteristics of interfacial membranesformed with emulsifiers [7] Interfacial membranes also alterthe rates of chemical reactions such as lipid oxidation[8ndash10] between oil- and aqueous-phase compounds Ad-ditionally when a functional lipophilic compound is in-corporated into emulsion droplets the interfacial propertiesof the oil droplet surface are the main factors that control thestability of the functional compound incorporated therein[3 11] Because the interfacial membrane is formed withemulsifiers the structural and physicochemical properties ofemulsifiers play important roles in the emulsion stability andin the storage stability of functional compounds in-corporated into the oil droplets [12]

Oil-in-water (OW) emulsions are widely used as de-livery systems in a variety of industries because of theirabilities to encapsulate functional lipophilic compoundsGenerally OW emulsions are classified into conventionalemulsions (usually called ldquoemulsionsrdquo) and nanoemulsionsaccording to the size of the emulsion droplets [13] Becausenanoemulsions have much larger specific surface areas thanconventional emulsions chemical degradation reactions atthe oil-water interface can occur more quickly in nano-emulsions than in conventional emulsions [14] ereforewhen OW nanoemulsions are used as nanocarriers indelivery systems the interfacial membrane formed by theemulsifier is an important factor in controlling the ability ofthe emulsion to protect the encapsulated functional com-pounds and to inhibit their diffusion from the oil dropletsinto the aqueous phase

erefore in nanoemulsion-based oral delivery systemsfor functional lipophilic compounds it is important to knowhow the interfacial membrane affects the stability of α-to-copherol incorporated into the emulsion for the preventionof lipid oxidation and to understand the effectiveness ofα-tocopherol in inhibiting the degradation of the functionallipophilic compounds by oxidative stress from the aqueousphase Over the past decades scientists have investigatedhow the stability and effectiveness of emulsion systems arealtered during the incorporation of α-tocopherol [15 16]e charge of the emulsion droplet surfaces is one of themajor factors influencing the oxidative stability of theemulsified oil and also the stability of the α-tocopherol echemical stability of α-tocopherol is effectively improved inemulsions droplets with positively charged surfaces Whenα-tocopherol is incorporated into an emulsion with nega-tively charged interfaces the α-tocopherol stability can beincreased by adding a biopolymer layer with a positivecharge to the negatively charged droplet surfaces Howeverdespite this most studies on the influence of interfacialmembrane properties have shown that the charge of theemulsion droplet surfaces is a key factor influencing thestability of α-tocopherol in emulsions and in lipid oxidationerefore the objective of this work was to determinewhether the structural properties of the interfacial mem-brane could be involved in α-tocopherol decomposition in

emulsions particularly emulsion-based delivery systemsis evaluation was accomplished by using model emulsion-based delivery systems stabilized by emulsifiers with dif-ferent hydrophilic head sizes which led to various dropletinterfacial thicknesses

2 Materials and Methods

21 Materials Polyoxyethylene alkyl ether-type emulsifiers(polyoxyethylene 10 lauryl ether (P10L) polyoxyethylene 10stearyl ether (P10S) polyoxyethylene 20 stearyl ether (P20S)and polyoxyethylene 23 lauryl ether (P23L) and polyoxy-ethylene 100 stearyl ether (P100S)) were purchased fromSigma-Aldrich (St Louis MO USA) e molecular struc-tures of the polyoxyethylene alkyl ether-type emulsifiers usedin this study are presented in Figure 1 α-Tocopherol 22prime-azobis(2-methylpropionamidine)dihydrochloride (AAPH)ferrous sulfate heptahydrate (FeSO4middot7H2O) and ferric chlo-ride hexahydrate (FeCl3middot6H2O) were also purchased fromSigma-Aldrich Medium-chain triglyceride (Delios S) com-prised caprylic (70) and 30 capric (30) acids and wereobtained from BASF (Ludwigshafen Germany) All otherchemicals used were of analytical grade

22 Emulsion Preparation e aqueous phase was preparedby dissolving the emulsifiers in the phosphate buffer (10mMand pH 7) to a predetermined concentration and the oilphase was prepared by dissolving α-tocopherol in medium-chain triglycerides at a final concentration of 5mmolkgCoarse emulsions were prepared by homogenizing the oil(5 ww) and aqueous (95 ww) phases (025mmolα-tocopherolkg emulsion) in a high-speed blender (T18Basic Ultra-Turrax Ika Staufen Germany) for 2min atroom temperature e oil droplet sizes in the coarseemulsions were then reduced with 5 passes througha microfluidizer (MN400BF Micronox Seongnam Korea)at 100MPa After adjustment of the pH level of the emul-sions to a predetermined value the emulsions were purgedwith nitrogen with gentle stirring for 30min before thesubsequent step To determine the effect of transition metalson α-tocopherol stability in the emulsions ferrous sulfate orferric chloride solution was added to the emulsions at a finalconcentration of 1mmolkg emulsion Furthermore toevaluate the effect of free radicals on α-tocopherol stability inthe emulsions AAPH solution was added to the emulsionsto a final concentration of 1mmolkg emulsionen 10 g ofthe emulsion sample were transferred into 12mL of a glassvial closed airtight and were stored in the dark at 25degC

23 Droplet Size Measurement e mean emulsion dropletdiameters were measured by using static light scattering(laser diffraction) To avoid multiple scattering effects allemulsion samples were diluted to a droplet concentration ofapproximately 0005 (ww) with a buffer solution of thesame pH value as the sample and samples were stirredcontinuously throughout the measurements to ensure ho-mogeneity e refractive index values for MCT and buffersolution were set at 147 and 133 respectively e particle

2 Journal of Chemistry

size distribution of the emulsions was then measured byusing a commercial static light scattering instrument (BT-9300ST Bettersize Instruments Dandong China) eparticle size data are reported as the volume-weighted meandiameter d43 1113936 ni middot d4

i 1113936 ni middot d3i with ni representing the

number of particles with diameter di

24 α-Tocopherol Concentration Measurement α-Tocopherolconcentration was measured according to the methoddescribed by Yang et al [17] with slight modificationα-Tocopherol concentrations in emulsions were determinedby first vigorously vortexing 2 g of emulsion with 4 g ofdichloromethane for 2min e mixture was then centri-fuged at 1842 times g for 10min at 25degC and the solvent layerwas collected α-Tocopherol concentrations were deter-mined with HPLC by using an Agilent 1100 instrument(Palo Alto CA USA) A Triart C18 column (250mm times

46mm times 5 μm YMC Tokyo Japan) was used with amethanol mobile phase at a rate of 1mLmin e wave-length for detection was 295 nm Concentrations ofα-tocopherol were calculated on the basis of a calibrationcurve generated by using authentic α-tocopherol

erefore the decomposition rate (k) of α-tocopherolwas calculated by assuming a first-order reaction

Ct C0 middot eminuskmiddott

(1)

where C0 is the initial α-tocopherol concentration (mmolkgemulsion) and Ct is the α-tocopherol concentration remainingat time t (day) e k value was calculated by performinga linear regression on the plot ln(CtC0) versus t e equalityof coefficients of different linear regressions was analyzed bythe Chow test [18] If time-dependent changes in the degra-dation of α-tocopherol in emulsions were observed (the fastdegradation of α-tocopherol in the early stage of storage andthe slow its degradation in the late storage period in thisstudy) the initial α-tocopherol decomposition rate (mmolkgemulsionday) was determined e initial α-tocopherol de-composition rate could be determined by calculating the

tangential slope at t 0 because the instantaneous rate at time tis determined by calculating the tangential slope at t onα-tocopherol concentration versus time curve

25 Statistical Analysis All the experiments were performedin triplicate and the data are expressed as mean plusmn standarddeviation Analysis of variance (ANOVA) was performedand the mean separations were performed using Duncanrsquosmultiple range test (plt 005) e statistical analyses de-scribed above were all conducted using SAS (version 94SAS Institute Inc Cary NC USA)

3 Results and Discussion

To minimize the negative effect of micelles on the stability ofthe emulsions and α-tocopherol therein [19] the minimumemulsifier concentrations (MECs) required to preparehighly stable emulsions with mostly small droplets weredetermined in our previous study [7] e MECs for P10LP10S P20S P23L and P100S were 2903 3165 2926 1784and 0994mM respectively As all emulsions prepared at theMECs had similar initial droplet diameters (d43 029 028029 032 and 028 μm for P10L- P10S- P20S- P23L- andP100S-stabilized emulsions respectively) and the dropletsizes rarely changed after 21 days of storage and any sig-nificant difference in the α-tocopherol decomposition ratebetween emulsions could not stem from an effect of the oildroplet interfacial area Although the emulsifier concen-tration in emulsions were different from each emulsion thefacts that all emulsions had a similar oil droplet size in-dicated that emulsions had the different emulsifier loading(emulsifier concentration per unit droplet surface area)values which could be attributed to droplet interfacialdensity e emulsifier loading values for P10L- P10S-P20S- P23L- and P100S-stabilized emulsions were calcu-lated as 191 290 187 124 and 062 μmolm2 respectivelyindicating that the interfacial density of emulsions differedWhen oils are stabilized by emulsifiers to spherical dropletsthe interfacial membrane of oil droplet surfaces are

OOH

10

(a)

OH10

O

(b)

OH20

O

(c)

OH23

O

(d)

OH100

O

(e)

Figure 1 Molecular structures of polyoxyethylene alkyl ether-type emulsifiers used in this work (a) polyoxyethylene 10 lauryl ether (P10L)(b) polyoxyethylene 10 stearyl ether (P10S) (c) polyoxyethylene 20 stearyl ether (P20S) (d) polyoxyethylene 23 lauryl ether (P23L) (e)polyoxyethylene 100 stearyl ether (P100S)

Journal of Chemistry 3

comprised of the inner layer formed with the hydrophobictails of emulsifiers and the outer layer formed with theirhydrophilic heads Considering the molecular structures ofemulsifiers used in this work it means that the thickness ofthe outer layer of the interfacial membrane could be mainlyattributed by a number of oxyethylene groups of the hy-drophilic groups of emulsifiers and that the length of alkylchains of the hydrophobic tails of emulsifiers could de-termine the thickness of its inner layer

Since the partition coefficient of α-tocopherol is ap-proximately 12 [20] it is likely that the concentration ofα-tocopherol in the aqueous phase was negligible ereforeif α-tocopherol degradation is observed after a certain periodof storage and prooxidants are present in the aqueous phasemost of α-tocopherol must have decomposed at the emul-sion droplet surface rather than in the aqueous phase

31 Influence of the pH Level on α-Tocopherol DegradationVitamin E compounds including α-tocopherol exhibitfairly good stability in the absence of oxygen and lipidperoxides [21] However with the consideration thatcommercially available emulsion-based foods are generallyacidic [10] and molecular oxygen is never completely re-moved from them it is important to understand the in-fluence of the pH level on the chemical stability ofα-tocopherol in emulsions erefore to examine how thecharacteristics of emulsion droplet surfaces affect thechemical decomposition of α-tocopherol in acidic envi-ronments the pH level of the emulsions was adjusted to 7 or3 and the emulsions were then stored Because medium-chain triglycerides consist of only saturated fatty acids theyare exceptionally stable to oxidation [22] In addition mostof the oxygen molecules were removed by nitrogen purgingerefore if α-tocopherol degradation is observed toa considerable level it could be the result of factors otherthan lipid peroxides derived from the oxidation of themedium-chain triglyceride carrier oil One possible reasonfor the reduction of α-tocopherol during storage is thepresence of a trace amount of oxygen molecules in theemulsions In this study to minimize the effect of oxygenmolecules on α-tocopherol decomposition nitrogen purgingwas carried out to remove oxygen molecules However itseems that the oxygen molecules in the aqueous phase werenot completely removede effects of the emulsifier and pHlevel on α-tocopherol stability in emulsions are shown inFigure 2 As indicated in Table 1 regardless of the pH levelP100S-stabilized emulsions showed the highest initial de-composition rate of α-tocopherol among the emulsionsP100S has the largest hydrophilic head size among theemulsifiers used and the P100S-stabilized emulsion con-tained the smallest amount of the emulsifier among theemulsions prepared in this work so it appears that the thickandor loosely-packed interface is disadvantageous for thestability of α-tocopherol encapsulated in the emulsionsBecause the P10S- and P20S-stabilized emulsions havedroplet surfaces of similar density it was expected thatα-tocopherol in the emulsion stabilized with P20S whichhas a hydrophilic head size that is twice as large as that of

P10S would be more stable than that in the P10S-stabilizedemulsion however there was no significant difference in theinitial decomposition rate of α-tocopherol in these twoemulsions (pgt 005) P20S- and P23L-stabilized emulsionshave interfacial membranes of similar thickness because thedifference in the oxyethylene group number of the hydro-philic heads of P20S and P23L is only three while the P20S-stabilized emulsion has a denser interfacial membrane thanthe P23L-stabilized emulsion as described above Howeverboth of these emulsions showed very similar initial de-composition rates of α-tocopherol independent of the pHlevel It was apparent that the thickness andor density of thedroplet surfaces did not affect the initial α-tocopherol de-composition rate In addition considering the content(gt90) of α-tocopherol remaining after 21 days of storagethe variation in initial α-tocopherol decomposition ratesamong the emulsions did not have much effect

32 Influence of Transition Metals on α-TocopherolDegradation e previous findings indicate that iron ionscould be the direct or indirect reasons for the degradation ofthe several food components including lipids [23] and theprecursors of vitamins such as carotenoids [24] because oftheir electron transfer reaction e cation radicals could beformed by the interaction of iron ions with those foodcomponents It means transitionmetals like iron could act asoxidizing agents erefore in this study iron ions werechosen as oxidants for studying the degradation of α-to-copherol e initial decomposition rates of α-tocopherol inemulsions with iron were different from those of α-to-copherol in iron-free emulsions As shown in Table 1 theemulsions stabilized with different emulsifiers in the absenceof iron had different initial α-tocopherol decompositionrates whereas little difference was observed in the initialα-tocopherol decomposition rates for emulsions stabilizedwith different emulsifiers in the presence of iron Howeversimilar to the observation mentioned above there was nosignificant difference in the α-tocopherol content remainingin the emulsions after 21 days of storage (plt 005) whichsuggests that iron did not have an influence on the stabilityof α-tocopherol in the emulsions (Figures 3 and 4)

Irrespective of the oxidative state of the iron the initialdecomposition rates of α-tocopherol in emulsions in thepresence of ferrous iron were not significantly different fromthose in emulsions stored with ferric iron (plt 005) In ad-dition when emulsions contained iron with the same oxi-dative state they showed very similar initial decompositionrates of α-tocopherol regardless of the pH level Although allof the emulsifiers used in this work were nonionic the dropletsurface charges of the emulsions were slightly negative andtheir values changed depending on the pH level (minus73 minus68minus55 minus94 and minus17mV for P10L- P10S- P20S- P23L- andP100S-stabilized emulsions at pH 7 respectively and aroundminus15mV at pH 3) Because the droplet surfaces were morenegatively charged at pH 7 except those in the P100S-stabilized emulsion and could attract iron molecules to thesurface of the emulsion droplets it was expected that α-to-copherol would be rapidly decomposed at pH 7 because the


ironmolecules would accumulate around the more negativelycharged droplet surfaces at this pH levelis suggests that theiron did not decompose α-tocopherol by direct interaction atthe interfacial membrane erefore the stability of α-to-copherol in emulsions was not influenced by the thicknessandor density of the droplet interfaces

33 Influence of Radicals on α-Tocopherol DegradationAAPH may be a suitable material for studying the influenceof radicals on the stability of α-tocopherol encapsulated inemulsions because it is a self-generator of free radicalsthrough spontaneous decomposition at room temperatureQuite different from the previous findings in this case the

Storage time (days)0 3 6 9 12 15 18 21

Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

80

85

90

95

100

P10L-stabilized emulsionP10S-stabilized emulsionP20S-stabilized emulsion

P23L-stabilized emulsionP100S-stabilized emulsion

(a)




Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

80

85

90

95

100

(b)

Figure 2 Change in concentration of α-tocopherol in emulsions at pH 7 (a) and 3 (b) stored at 25degC P10L polyoxyethylene 10 lauryl etherP10S polyoxyethylene 10 stearyl ether P20S polyoxyethylene 20 stearyl ether P23L polyoxyethylene 23 lauryl ether P100S polyoxy-ethylene 100 stearyl ether




Storage time (days)

Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

80

85

90

95

100

0 3 6 9 12 15 18 21

(a)


Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

80

85

90

95

100



(b)

Figure 3 Change in concentration of α-tocopherol in emulsions in the presence of ferrous iron at pH 7 (a) and 3 (b) stored at 25degC P10Lpolyoxyethylene 10 lauryl ether P10S polyoxyethylene 10 stearyl ether P20S polyoxyethylene 20 stearyl ether P23L polyoxyethylene 23lauryl ether P100S polyoxyethylene 100 stearyl ether

Table 1 e initial decomposition rate (mmolkg emulsionday) of α-tocopherol in emulsions stabilized with emulsifiers having varioussizes of hydrophilic and hydrophobic groups

Environmental stressEmulsifier used for emulsion preparation

P10L P10S P20S P23L P100S

pH 7No AB00185 plusmn 00108ab B00034 plusmn 00007b B00083 plusmn 00094b A00098 plusmn 00109ab A00266 plusmn 00095a

Ferrous iron ABC00142 plusmn 00003c AB00210 plusmn 00000b A00250 plusmn 00009a A00242 plusmn 00028a A00211 plusmn 00002bFerric iron A00210 plusmn 00093a AB00231 plusmn 00028a A00215 plusmn 00035a A00258 plusmn 00323a A00230 plusmn 00061a

pH 3No ABC00173 plusmn 00004b B00030 plusmn 00000c B00034 plusmn 00005c A00030 plusmn 00005c A00188 plusmn 00011a

Ferrous iron A00226 plusmn 00017a AB00225 plusmn 00021a A00250 plusmn 00009a A00217 plusmn 00042a A00107 plusmn 00011bFerric iron A00231 plusmn 00060a A00330 plusmn 00407a A00264 plusmn 00101a A00364 plusmn 00467a A00198 plusmn 00236a

P10L polyoxyethylene 10 lauryl ether P10S polyoxyethylene 10 stearyl ether P20S polyoxyethylene 20 stearyl ether P23L polyoxyethylene 23 lauryl etherP100S polyoxyethylene 100 stearyl ether e values with different small letter superscripts in the same row are significantly different (plt 005) by Duncanrsquosmultiple range test e values with different capital letter superscripts in the same column are significantly different (plt 005) by Duncanrsquos multiple range test


Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

80

85

90

95

100



(a)


Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

80

85

90

95

100



(b)

Figure 4 Change in concentration of α-tocopherol in emulsions in the presence of ferric iron at pH 7 (a) and 3 (b) stored at 25degC P10Lpolyoxyethylene 10 lauryl ether P10S polyoxyethylene 10 stearyl ether P20S polyoxyethylene 20 stearyl ether P23L polyoxyethylene 23lauryl ether P100S polyoxyethylene 100 stearyl ether


α-tocopherol concentration gradually decreased during the21-day storage period (Figure 5) e values of k for α-to-copherol in emulsions stored at pH 3 ranged from 00293 to00344 dayminus1 and the values of k for α-tocopherol inemulsions stored at pH 7 ranged from 00218 to 00252 dayminus1(Table 2) Although α-tocopherol decomposed more quicklyin acidic conditions than neutral conditions the lack ofcorrelation between the k value and the properties (thicknessandor density) of the interfacial membranes suggests thatthe interfacial characteristics played little or no role inimproving the stability of emulsified α-tocopherol againstradical-mediated oxidation

During the design of this experiment we expected thatthe properties of the interfacial membranes of oil dropletswould affect the storage stability of α-tocopherol in-corporated in emulsions Although there is a lack of in-formation about the influence of the density of interfacialmembranes on the oxidative stability of emulsified oils andthe storage stability of encapsulated functional lipophiliccompounds according to previous studies the emulsioninterfacial thickness could be one of the important

determinants of the oxidative stability of food emulsions[16] Song et al [11] reported that the storage stability ofβ-carotene in emulsions varied depending on the dropletinterfacial thickness and they also revealed that the dropletinterfacial density may be a factor to consider for im-proving β-carotene stability As described above all of theemulsions analyzed in this study had different densities andthicknesses for their interfacial membranes For examplethe P100S-stabilized emulsion had the thickest interfacialmembrane but its density was the lowest among theemulsions whereas the P10L- and P10S-stabilized emul-sions had the opposite properties e stability of α-to-copherol in the emulsions greatly depended on theenvironmental conditions surrounding the emulsiondroplets and the denseness andor thickness of the in-terfacial membrane of the oil droplets did not play a crucialrole in improving the stability of the encapsulated α-to-copherol In conclusion our findings together with thoseof previous studies suggested that the data are still in-sufficient to generalize the influence of droplet interfacecharacteristics on the oxidative stability of emulsified oils


Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

20

40

60

80

100



(a)

Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()


20

40

60

80

100



(b)

Figure 5 Change in concentration of α-tocopherol in emulsions in the presence of radicals at pH 7 (a) and 3 (b) stored at 25degC P10Lpolyoxyethylene 10 lauryl ether P10S polyoxyethylene 10 stearyl ether P20S polyoxyethylene 20 stearyl ether P23L polyoxyethylene 23lauryl ether P100S polyoxyethylene 100 stearyl ether

Table 2 e decomposition rate (k) of α-tocopherol in emulsions stabilized with emulsifiers having various sizes of hydrophilic andhydrophobic groups in presence of radicals

Emulsifier used for emulsion preparationP10L P10S P20S P23L P100S

k (dayminus1) r2 k (dayminus1) r2 k (dayminus1) r2 k (dayminus1) r2 k (dayminus1) r2

pH 7 B00218b 0989 B00252a 0984 B00235ab 0967 B00226ab 0985 B00230ab 0951pH 3 A00312bc 0996 A00319b 0994 A00310bc 0988 A00344a 0999 A00293c 0995P10L polyoxyethylene 10 lauryl ether P10S polyoxyethylene 10 stearyl ether P20S polyoxyethylene 20 stearyl ether P23L polyoxyethylene 23 lauryl etherP100S polyoxyethylene 100 stearyl ethere α-tocopherol decomposition rate values with different small letter superscripts in the same row are significantlydifferent (plt 005) by the Chow teste α-tocopherol decomposition rate values with different capital letter superscripts in the same column are significantlydifferent (plt 005) by the Chow test


and the chemical stability of encapsulated oil-solublecomponents

Data Availability

e data used to support the findings of this study areavailable from the corresponding author upon request

Conflicts of Interest

e authors declare that they have no conflicts of interest

Acknowledgments

is study was supported by the Research Program fundedby Seoul National University of Science and Technology

References

[1] E N Frankel ldquoLipid oxidation mechanisms products andbiological significancerdquo Journal of the American Oil ChemistsrsquoSociety vol 61 no 12 pp 1908ndash1917 1984

[2] D J McClements E A Decker and J Weiss ldquoEmulsion-based delivery systems for lipophilic bioactive componentsrdquoJournal of Food Science vol 72 no 8 pp R109ndashR124 2007

[3] D J McClements ldquoEmulsion design to improve the deliveryof functional lipophilic componentsrdquo Annual Review of FoodScience and Technology vol 1 no 1 pp 241ndash269 2010

[4] E N Frankel ldquoAntioxidants in lipid foods and their impact onfood qualityrdquo Food Chemistry vol 57 no 1 pp 51ndash55 1996

[5] D J McClements Food Emulsions Principles Practices andTechniques CRC Press Boca Raton FL USA 2nd edition2005

[6] H D Silva M A Cerqueira B W S Souza et al ldquoNano-emulsions of β-carotene using a high-energy emulsification-evaporation techniquerdquo Journal of Food Engineering vol 102no 2 pp 130ndash135 2011

[7] S W Han H Y Song T W Moon and S J Choi ldquoInfluenceof emulsion interfacial membrane characteristics on Ostwaldripening in a model emulsionrdquo Food Chemistry vol 242pp 91ndash97 2018

[8] B Chen D J McClements and E A Decker ldquoRole ofcontinuous phase anionic polysaccharides on the oxidativestability of Menhaden oil-in-water emulsionsrdquo Journal ofAgricultural and Food Chemistry vol 58 no 6 pp 3779ndash3784 2010

[9] H Faraji D J McClements and E A Decker ldquoRole ofcontinuous phase protein on the oxidative stability of fish oil-in-water emulsionsrdquo Journal of Agricultural and FoodChemistry vol 52 no 14 pp 4558ndash4564 2004

[10] C Qian E A Decker H Xiao and D J McClementsldquoPhysical and chemical stability of β-carotene-enrichednanoemulsions influence of pH ionic strength tempera-ture and emulsifier typerdquo Food Chemistry vol 132 no 3pp 1221ndash1229 2012

[11] H Y Song T W Moon and S J Choi ldquoStorage stability ofβ-carotene in model beverage emulsions implication of in-terfacial thicknessrdquo European Journal of Lipid Science andTechnology vol 120 no 9 article 1800127 2018

[12] D J McClements ldquoNanoemulsion-based oral delivery sys-tems for lipophilic bioactive components nutraceuticals andpharmaceuticalsrdquo =erapeutic Delivery vol 4 no 7pp 841ndash857 2013

[13] T G Mason J N Wilking K Meleson C B Chang andS M Graves ldquoNanoemulsions formation structure andphysical propertiesrdquo Journal of Physics Condensed Mattervol 18 no 41 pp R635ndashR666 2006

[14] D J McClements and J Rao ldquoFood-grade nanoemulsionsformulation fabrication properties performance biologicalfate and potential toxicityrdquo Critical Reviews in Food Scienceand Nutrition vol 51 no 4 pp 285ndash330 2011

[15] J R Mancuso D J McClements and E A Decker ldquoAbility ofiron to promote surfactant peroxide decomposition andoxidize α-tocopherolrdquo Journal of Agricultural and FoodChemistry vol 47 no 10 pp 4146ndash4149 1999

[16] M P C Silvestre W Chaiyasit R G BrannanD J McClements and E A Decker ldquoAbility of surfactantheadgroup size to alter lipid and antioxidant oxidation in oil-in-water emulsionsrdquo Journal of Agricultural and FoodChemistry vol 48 no 6 pp 2057ndash2061 2000

[17] Y Yang E A Decker H Xiao and D J McClementsldquoEnhancing vitamin E bioaccessibility factors impactingsolubilization and hydrolysis of α-tocopherol acetate encap-sulated in emulsion-based delivery systemsrdquo Food andFunction vol 6 no 1 pp 84ndash97 2015

[18] G C Chow ldquoTests of equality between sets of coefficients intwo linear regressionsrdquo Econometrica vol 28 no 3pp 591ndash605 1960

[19] D J McClements ldquoUltrasonic determination of depletionflocculation in oil-in-water emulsions containing a non-ionicsurfactantrdquo Colloids and Surfaces A-Physicochemical andEngineering Aspects vol 90 no 1 pp 24ndash35 1994

[20] USEPA Estimation Program Interface Suite (TM) Ver 411United States Environmental Protection Agency Wash-ington DC USA 2012

[21] J F I Gregory ldquoVitaminsrdquo in Fennemarsquos Food ChemistryS Damodaran K L Parkin and O R Fennema Eds CRCPress Boca Raton FL USA 4 edition 2008

[22] J A Heydinger and D K Nakhasi ldquoMedium chain tri-acylglycerolsrdquo Journal of Food Lipids vol 3 no 4 pp 251ndash257 1996

[23] E Choe and D B Min ldquoMechanisms and factors for edible oiloxidationrdquo Comprehensive Reviews in Food Science and FoodSafety vol 5 no 4 pp 169ndash186 2006

[24] C S Boon D J McClements J Weiss and E A DeckerldquoFactors influencing the chemical stability of carotenoids infoodsrdquo Critical Reviews in Food Science and Nutrition vol 50no 6 pp 515ndash532 2010


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create an interfacial membrane comprised of a layer formedof their hydrophobic tails and a layer formed of their hy-drophilic heads Physical destabilization processes such ascoalescence flocculation and Ostwald ripening are greatlyaffected by the characteristics of interfacial membranesformed with emulsifiers [7] Interfacial membranes also alterthe rates of chemical reactions such as lipid oxidation[8ndash10] between oil- and aqueous-phase compounds Ad-ditionally when a functional lipophilic compound is in-corporated into emulsion droplets the interfacial propertiesof the oil droplet surface are the main factors that control thestability of the functional compound incorporated therein[3 11] Because the interfacial membrane is formed withemulsifiers the structural and physicochemical properties ofemulsifiers play important roles in the emulsion stability andin the storage stability of functional compounds in-corporated into the oil droplets [12]

Oil-in-water (OW) emulsions are widely used as de-livery systems in a variety of industries because of theirabilities to encapsulate functional lipophilic compoundsGenerally OW emulsions are classified into conventionalemulsions (usually called ldquoemulsionsrdquo) and nanoemulsionsaccording to the size of the emulsion droplets [13] Becausenanoemulsions have much larger specific surface areas thanconventional emulsions chemical degradation reactions atthe oil-water interface can occur more quickly in nano-emulsions than in conventional emulsions [14] ereforewhen OW nanoemulsions are used as nanocarriers indelivery systems the interfacial membrane formed by theemulsifier is an important factor in controlling the ability ofthe emulsion to protect the encapsulated functional com-pounds and to inhibit their diffusion from the oil dropletsinto the aqueous phase

erefore in nanoemulsion-based oral delivery systemsfor functional lipophilic compounds it is important to knowhow the interfacial membrane affects the stability of α-to-copherol incorporated into the emulsion for the preventionof lipid oxidation and to understand the effectiveness ofα-tocopherol in inhibiting the degradation of the functionallipophilic compounds by oxidative stress from the aqueousphase Over the past decades scientists have investigatedhow the stability and effectiveness of emulsion systems arealtered during the incorporation of α-tocopherol [15 16]e charge of the emulsion droplet surfaces is one of themajor factors influencing the oxidative stability of theemulsified oil and also the stability of the α-tocopherol echemical stability of α-tocopherol is effectively improved inemulsions droplets with positively charged surfaces Whenα-tocopherol is incorporated into an emulsion with nega-tively charged interfaces the α-tocopherol stability can beincreased by adding a biopolymer layer with a positivecharge to the negatively charged droplet surfaces Howeverdespite this most studies on the influence of interfacialmembrane properties have shown that the charge of theemulsion droplet surfaces is a key factor influencing thestability of α-tocopherol in emulsions and in lipid oxidationerefore the objective of this work was to determinewhether the structural properties of the interfacial mem-brane could be involved in α-tocopherol decomposition in

emulsions particularly emulsion-based delivery systemsis evaluation was accomplished by using model emulsion-based delivery systems stabilized by emulsifiers with dif-ferent hydrophilic head sizes which led to various dropletinterfacial thicknesses

2 Materials and Methods

21 Materials Polyoxyethylene alkyl ether-type emulsifiers(polyoxyethylene 10 lauryl ether (P10L) polyoxyethylene 10stearyl ether (P10S) polyoxyethylene 20 stearyl ether (P20S)and polyoxyethylene 23 lauryl ether (P23L) and polyoxy-ethylene 100 stearyl ether (P100S)) were purchased fromSigma-Aldrich (St Louis MO USA) e molecular struc-tures of the polyoxyethylene alkyl ether-type emulsifiers usedin this study are presented in Figure 1 α-Tocopherol 22prime-azobis(2-methylpropionamidine)dihydrochloride (AAPH)ferrous sulfate heptahydrate (FeSO4middot7H2O) and ferric chlo-ride hexahydrate (FeCl3middot6H2O) were also purchased fromSigma-Aldrich Medium-chain triglyceride (Delios S) com-prised caprylic (70) and 30 capric (30) acids and wereobtained from BASF (Ludwigshafen Germany) All otherchemicals used were of analytical grade

22 Emulsion Preparation e aqueous phase was preparedby dissolving the emulsifiers in the phosphate buffer (10mMand pH 7) to a predetermined concentration and the oilphase was prepared by dissolving α-tocopherol in medium-chain triglycerides at a final concentration of 5mmolkgCoarse emulsions were prepared by homogenizing the oil(5 ww) and aqueous (95 ww) phases (025mmolα-tocopherolkg emulsion) in a high-speed blender (T18Basic Ultra-Turrax Ika Staufen Germany) for 2min atroom temperature e oil droplet sizes in the coarseemulsions were then reduced with 5 passes througha microfluidizer (MN400BF Micronox Seongnam Korea)at 100MPa After adjustment of the pH level of the emul-sions to a predetermined value the emulsions were purgedwith nitrogen with gentle stirring for 30min before thesubsequent step To determine the effect of transition metalson α-tocopherol stability in the emulsions ferrous sulfate orferric chloride solution was added to the emulsions at a finalconcentration of 1mmolkg emulsion Furthermore toevaluate the effect of free radicals on α-tocopherol stability inthe emulsions AAPH solution was added to the emulsionsto a final concentration of 1mmolkg emulsionen 10 g ofthe emulsion sample were transferred into 12mL of a glassvial closed airtight and were stored in the dark at 25degC

23 Droplet Size Measurement e mean emulsion dropletdiameters were measured by using static light scattering(laser diffraction) To avoid multiple scattering effects allemulsion samples were diluted to a droplet concentration ofapproximately 0005 (ww) with a buffer solution of thesame pH value as the sample and samples were stirredcontinuously throughout the measurements to ensure ho-mogeneity e refractive index values for MCT and buffersolution were set at 147 and 133 respectively e particle









(1)






OOH

10

(a)

OH10

O

(b)

OH20

O

(c)

OH23

O

(d)

OH100

O

(e)













Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

80

85

90

95

100



(a)




Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

80

85

90

95

100

(b)





Storage time (days)

Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

80

85

90

95

100

0 3 6 9 12 15 18 21

(a)


Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

80

85

90

95

100



(b)











Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

80

85

90

95

100



(a)


Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

80

85

90

95

100



(b)







Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

20

40

60

80

100



(a)

Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()


20

40

60

80

100



(b)








Data Availability




Acknowledgments


References































Journal of

Chemistry





Journal of

Volume 2018






Volume 2018




Journal of






Enzyme Research


Journal of



MaterialsJournal of






Na

nom

ate

ria

ls











(1)






OOH

10

(a)

OH10

O

(b)

OH20

O

(c)

OH23

O

(d)

OH100

O

(e)













Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

80

85

90

95

100



(a)




Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

80

85

90

95

100

(b)





Storage time (days)

Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

80

85

90

95

100

0 3 6 9 12 15 18 21

(a)


Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

80

85

90

95

100



(b)











Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

80

85

90

95

100



(a)


Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

80

85

90

95

100



(b)







Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

20

40

60

80

100



(a)

Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()


20

40

60

80

100



(b)








Data Availability




Acknowledgments


References































Journal of

Chemistry





Journal of

Volume 2018






Volume 2018




Journal of






Enzyme Research


Journal of



MaterialsJournal of






Na

nom

ate

ria

ls














Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

80

85

90

95

100



(a)




Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

80

85

90

95

100

(b)





Storage time (days)

Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

80

85

90

95

100

0 3 6 9 12 15 18 21

(a)


Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

80

85

90

95

100



(b)











Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

80

85

90

95

100



(a)


Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

80

85

90

95

100



(b)







Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

20

40

60

80

100



(a)

Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()


20

40

60

80

100



(b)








Data Availability




Acknowledgments


References































Journal of

Chemistry





Journal of

Volume 2018






Volume 2018




Journal of






Enzyme Research


Journal of



MaterialsJournal of






Na

nom

ate

ria

ls







Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

80

85

90

95

100



(a)




Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

80

85

90

95

100

(b)





Storage time (days)

Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

80

85

90

95

100

0 3 6 9 12 15 18 21

(a)


Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

80

85

90

95

100



(b)











Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

80

85

90

95

100



(a)


Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

80

85

90

95

100



(b)







Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

20

40

60

80

100



(a)

Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()


20

40

60

80

100



(b)








Data Availability




Acknowledgments


References































Journal of

Chemistry





Journal of

Volume 2018






Volume 2018




Journal of






Enzyme Research


Journal of



MaterialsJournal of






Na

nom

ate

ria

ls






Storage time (days)

Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

80

85

90

95

100

0 3 6 9 12 15 18 21

(a)


Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

80

85

90

95

100



(b)











Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

80

85

90

95

100



(a)


Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

80

85

90

95

100



(b)







Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

20

40

60

80

100



(a)

Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()


20

40

60

80

100



(b)








Data Availability




Acknowledgments


References































Journal of

Chemistry





Journal of

Volume 2018






Volume 2018




Journal of






Enzyme Research


Journal of



MaterialsJournal of






Na

nom

ate

ria

ls








Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()

20

40

60

80

100



(a)

Rela

tive α

-toco

pher

ol co

ncen

trat

ion

()


20

40

60

80

100



(b)








Data Availability




Acknowledgments


References































Journal of

Chemistry





Journal of

Volume 2018






Volume 2018




Journal of






Enzyme Research


Journal of



MaterialsJournal of






Na

nom

ate

ria

ls





Data Availability




Acknowledgments


References































Journal of

Chemistry





Journal of

Volume 2018






Volume 2018




Journal of






Enzyme Research


Journal of



MaterialsJournal of






Na

nom

ate

ria

ls









Journal of

Chemistry





Journal of

Volume 2018






Volume 2018




Journal of






Enzyme Research


Journal of



MaterialsJournal of






Na

nom

ate

ria

ls




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InfluenceofOxidantsontheStabilityofTocopherolinModel ...downloads.hindawi.com/journals/jchem/2018/6438787.pdfsize distribution of the emulsions was then measured by usingacommercialstaticlightscatteringinstrument(BT-9300ST;