The composition and structural organization ofemulsions ultimatelydeterminetheirdesirablesensoryattributes.Inthis section,wethereforefocusonhowthephysicochemicaland sensorypropertiesofemulsionsareinfluencedbydispersed phase,continuousphase,andinterfacialregioncharacteristics(Fig.
2) (Akhtar, Stenzel, Murray, & Dickinson, 2005; Benjamins etal.,2009;Chojnicka-Paszunetal.,2012;Chung,Olson,etal., 2013;vanAkenetal.,2011;vanVlietetal.,2009).Theinitial sensory perceptionofa foodproduct, suchas appearance, texture,andaroma,dependontheinitialpropertiesofafood emulsion.However,thebehaviorofafoodemulsionwithin themouthduringmasticationalsoplaysamajorroleinits sensory perception dueto changes in its composition and structurebroughtaboutbychewing,dilutionwithsaliva,and surfacecoating(Cakiretal.,2012;Rancetal.,2006;Togashi, Morita,&Nakazawa,2000).
Ingeneral,theterm‘‘flavor’’hasbeenusedtorefertothe combinedperceptionofaroma,taste,mouthfeel,andtexture (British Standards Institute, 1975). However, consumers mainly associate this term with aroma and taste. In this review,weusetheterm‘‘flavorprofile’’torefertoaromaand taste attributes,and ‘‘texture’’and ‘‘mouthfeel’’ toreferto those sensations that arise from the tactile stimuli in the tongueandpalateduringmastication.
4.1. Opticalpropertiesandappearance
Themainopticalpropertiesofemulsionsaretheiropacityand color.Theoverallappearanceofemulsionsdependsontheir compositionandmicrostructure(Chungetal.,2012b;Chung, Degner,&McClements,2013c;McClements,2002,2005).The opacity and color of emulsions are mainly determined by particleconcentration,size,andrefractiveindexcontrast,as wellasthepresenceofanychromophoresthatabsorblight.
4.1.1. Dispersedphasecharacteristics
The perceived lightness of an emulsion increases with increasing oil content, especially from 0 to5% fat, dueto increasedlightscatteringbythe fatdroplets(Fig.4) (Chan-trapornchai, Clydesdale,& McClements,1999;Chung et al., 2012b; McClements, 2002). This phenomenon may have important implicationsforthe creationofreduced-fatfood products,since decreasedlightnessmaybeassociatedwith undesirable loss of ‘‘creaminess’’ (McClements, 2002). The decreaseinlightnessofreducedfatproductsduetoreduction offatdropletconcentrationcanoftenbecompensatedforby optimizing formulation and/or processing conditions. For example,theparticlesizedistribution(PSD)canbecontrolled Fig.3–Confocalmicrograph(60T)ofmixedsystem
containing8%oil-in-wateremulsionand3.5%swollen starchgranules(38mm).Thedarkspotsarestarch granules(unstained)whiletheredspotsarestainedoil droplets.(Forinterpretationofthereferencestocolorin thisfigurelegend,thereaderisreferredtothewebversion ofthearticle.)
so as to increase the light scattering efficiency of the oil droplets by fabricating droplets with sizes similar to the wavelength of light (500nm). Dropletsizes below and/or above this optimal size scatterlesser light and hence give lowerlightness(McClements,2002).
4.1.2. Continuousphasecharacteristics
Variouscomponentsinthecontinuousphasecanalsoalter theopticalpropertiesofemulsionsduetotheirabilitytoscatter orabsorblightwaves.Forparticulatematter(suchasstarch granules,proteinparticles,orairbubbles)themagnitudeofthis effectdependsontheirrelativerefractiveindices, concentra-tions,andsizes(McClements,2002,2005).Incorporationof non-fatparticlesthatscatterlightinasimilarmannertooildroplets increases the opacity of emulsions, and hence may be a potential alternative for reduced-fat products, e.g., titanium dioxide (Chantrapornchai, Clydesdale, & McClements, 2000) and protein microparticles (Ako, Nicolai, & Durand, 2010;
Dissanayake, Kelly, & Vasiljevic, 2010; Gulzar, Lechevalier, Bouhallab,&Croguennec,2012;Kennedy,Mounsey,Murphy,
Duggan,&Kelly,2006).Thepresenceofwater-solublecolorants inthecontinuousphasewillalsoinfluencetheappearanceofan emulsionduetotheirabilitytoselectivelyabsorblightwaves.
The color of emulsions will also be influenced by any chromophoresdissolvedwithintheoilphase.
Additionofcomponentsthatalterthestructural organi-zation,andthereforelightscatteringpattern,offatdroplets may also influence the appearance of emulsion-based products. For example,addition oflocust beangum (LBG) toemulsionspromoteddropletaggregation,whichreduced theirlightness(Fig.5)(Chung,Degner,&McClements,2013a).
Alternatively,novelstructuraldesignprinciplescanbeused tocreatereducedfatproductswithhighopacity,e.g.,hydrogel particles(Chung,Degner,&McClements,2013b;Zhangetal., 2013).Moredetailsonthesestructuraldesignapproachesand theirpotentialuseinfoodproductdevelopmentaregivenin Section6.
4.1.3. Interfacialregion
Theinterfacialregionwouldnotbeexpectedtohavealarge directeffectontheopticalpropertiesofanemulsionbecause itsthickness(d)ismuchsmallerthanthewavelengthoflight (dl). In addition, in most conventional emulsions the volumeoccupiedbytheinterfacialregion onlymakes upa small contribution to the overall particle volume (McCle-ments, 2011), and hence to the overall optical properties.
Nevertheless,theopticalpropertiesoftheinterfacialregion maybeimportantinnanoemulsions,sincethenitmakesup anappreciablecontributiontotheoverallparticlevolume.In thiscase,informationaboutthethicknessandcompositionof theinterfacialregionareneeded,aswellasknowledgeofits refractiveindexandabsorptionproperties.Thepropertiesof theinterfacialregionmayalsohaveanindirecteffectonthe opticalpropertiesofemulsions-basedproductsbyalteringthe aggregation state ofthe droplets, and therefore their light scatteringefficiency.
4.2. Flavorprofile
Theoverallflavorprofileofafoodemulsiondependsonthe distributionofflavormoleculesamongstthevariousphases present(e.g.,oil,water,interface,headspace),andtheirrelease
Fig.5–Opticalmicrographs(60T)of(a)oil-in-wateremulsion(5%oil)and(b)mixedsystemsoflocustbeangum(0.4%)and oil-in-wateremulsion(5%oil).
Fig.4–Dependenceofthelightnessofoil-in-water emulsionsondropletconcentration.
profileduringconsumption.Thereleaseofflavormolecules from food emulsions is determined by their equilibrium partitioncoefficientsandtheirmasstransportkinetics(Frank, Appelqvist,Piyasiri,Wooster,&Delahunty,2011;Karaishou, Blekas, & Paraskevopoulou, 2008; Leksrisompong, Barbano, Foegeding,Gerard,&Drake,2010;McClements,2005).Flavor release is usually characterized by an increase in the concentrationofflavormoleculesintheaqueousphase(taste) orheadspace(aroma)asafunctionoftime(flavorintensity–
time relationship) (McClements, 2005; Seuvre, Philippe, Rochard,&Voilley,2007).
4.2.1. Dispersedphasecharacteristics
Theflavorprofileofemulsion-basedfoodsdependsonvarious oildropletproperties,e.g.,oiltype,concentration,size,and physicalstate.Fatsandoilsfromdifferentsources(e.g.,corn oil,oliveoil,vegetableoil,andanimalfats)differinthetype andconcentrationofvolatilecompoundstheycontain,and hence contribute differently to the flavor profile of foods (Cunha,Grimaldi,Alcantara,&Viotto,2013;Vingerhoeds,de Wijk,Zoet,Nixdorf,& vanAken,2008;Vitova, Loupancova, Sklenarova,Divisova,&Bunka,2012).
Thefatdropletconcentrationinfluencestheperceptionof volatileandnon-volatileflavorcompoundsbyalteringtheir partitioning between the oil,water, and headspace phases (Fig.6a).Withincreasingfatcontent,theconcentrationof non-polarflavorsintheheadspaceaboveanemulsionisreduced, and therefore their perceived flavor intensity decreases (Bayarri,Smith,Hollowood,& Hort,2007;Bayarri,Taylor,&
Joanne,2006;Christiansenetal.,2011;Leksrisompongetal., 2010;Mao,Roos,&Miao,2013;Weeletal.,2004),whereasthe oppositeistrueforpolarflavors(Fig.6a).Thisphenomenonis particularlyimportanttotakeintoaccountwhendesigning reduced-fatproducts.
Theflavorintensityoffullfatproductsmaybe character-ized asbalancedand sustainablethroughout the courseof consumption(‘‘sustained release’’), whereasin reduced fat productstheflavorintensitymaybeunbalanced duetoan initial spike (‘‘burst release’’) of flavor immediately after consumption,followedbylowflavorintensityatlatertimes (Fig.6b)(Frank,Eyres,Piyasiri,&Delahunty,2012;Ma&Boye, 2013;Malone&Appelqvist,2003).Theinitialflavorburstin reducedfatproductsisduetoanappreciablefractionofthe non-polarflavormoleculesbeingpresentinthewaterphase ratherthanintheoilphase(Malone&Appelqvist,2003).The composition and structure of reduced-fat products may thereforehavetoberedesignedsothattheirreleaseprofiles more closely match those of high fat products. Sustained release may be achievedby encapsulating non-polar flavor moleculesindeliverysystems,suchasfilledhydrogelparticles or microencapsulated particles. As shown in Fig. 7, the individualoildroplets(1%,w/w)inconventionaloil-in-water emulsionaredistributedrandomlywhilethoseinthehydrogel particlesystemsareembeddedinthehydrogelparticlematrix.
Withtheentrapmentoftheoildropletsthereleaseofflavor compoundstotheheadspacecanbedelayedandsustaineddue tothelongerdiffusionpath-lengthofthehydrogelparticles.
Fatdroplet sizemay also influencethekinetics offlavor releasefornon-polarflavorcompoundssincetheymustdiffuse outoftheoildropletsbeforebeingreleasedintotheaqueous
phaseandheadspaceabovetheemulsion(Charles,Rosselin, Beck, Sauvageot, & Guichard, 2000; Karaishou et al., 2008;
Miettinen,Tuorila,Piironen,Vehkalahti,&Hyvonen,2002).In systemscontaininglargerdroplets(afewmicrometers)there may beadelayedreleaseofthenon-polar flavormolecules withinthemouthduringmastication,asthediffusion path-lengthincreaseswithdropletsize(Charlesetal.,2000).Droplet sizemayalsoindirectlyinfluencetheflavorprofileofsomefood systems through its influence on other physicochemical properties.Forexample,aninvestigationofthebreakdownof emulsion-filledgelsduringmasticationfoundthatdropletsize influence perceived sweetness because it influenced the fracturepropertiesofthegels(Sala&Stieger,2013).
4.2.2. Continuousphasecharacteristics
Non-fatparticlesandothercomponentswithintheaqueous phasemayalsoinfluencetheperceivedflavorcharacteristics Fig.6–(a)Schematicrepresentationofflavordependence inoil-in-wateremulsionsondropletconcentration.(b) Schematicdiagramofdifferencesinflavorreleaseprofiles oflowfatandhighfatemulsion-basedfoods.Anon-polar flavormoleculewillgivea‘‘burstrelease’’inalow-fat product,butamore‘‘sustainedrelease’’inahighfat product.
ofemulsion-basedproductsduetotheirabilitytoalterthe partitioningandmasstransportofvolatileand non-volatile molecules.Thickeningorgellingagentscandelaydiffusionof flavormoleculestothetastereceptoronthetongueandhence affectthe overallflavorprofile(Arancibia,Jublot, Costell,&
Bayarri,2011;Cook,Hollowood,Linforth,&Taylor,2002;Frank et al., 2012; Gonzalez-Tomas, Carbonell, & Costell, 2004;
Hollowoodetal.,2008).Somebiopolymersaltertheperceived flavorofafoodbybindingflavormolecules(Cook,Linforth,&
Taylor,2003;Rosett,Kendregan,Gao,Schmidt,&Klein,1996).
4.2.3. Interfacialregion
Thenatureofthe interfacialregionmayalsoinfluencethe kineticsof flavorrelease (Charles et al., 2000;Christiansen etal.,2011;Karaishouetal.,2008;Miettinenetal.,2002).Some emulsifierscanbindflavorcompoundsanddelaytheirrelease intothemouthandheadspace.Thenatureoftheinterfacial regionalsodetermines howfatdropletsbehave withinthe mouth(e.g.,theirabilitytosticktothetongueortoaggregate), whichwillalterthereleaseofflavormolecules.
4.3. Textureandmouthfeel
The ‘‘texture’’ and ‘‘mouthfeel’’ of emulsion-based food products plays an important role in determining their perceived flavor. Texture is ‘‘the sensory and functional manifestation of the structural, mechanical and surface propertiesoffoodsdetectedthroughsensesofvision,hearing, touchandkinesthetic’’(Szczesniak,2002),while‘‘mouthfeel’’
is generally defined as the sensations arising from the interactionsofaningestedfood(mixedwithsaliva)withthe receptorsinthemouththatrespondtotactilestimuliduring mastication(Smith&Margolskee,2001).Arangeofdescriptors is commonly used to describe the texture and mouthfeel characteristics of food emulsions, such as ‘‘creaminess’’,
‘‘richness’’, ‘‘smoothness’’, ‘‘sliminess’’, ‘‘thickness’’, ‘‘thin-ness’’,‘‘watery’’,‘‘firmness’’,‘‘hardness’’,and‘‘astringency’’
(Chen & Eaton, 2012; Guinard & Mazzucchelli, 1996). The influenceofthevariousphasesinanemulsiononthetexture andmouthfeelofemulsion-basedfoodproductsisdiscussed inthefollowingsections.
4.3.1. Dispersedphasecharacteristics
Thetypeand concentrationoffatdropletsinan emulsion-based food greatly influences its perceived texture and mouthfeel. Increasing the fat droplet concentration in a productincreasestheperceptionof‘‘richness’’,‘‘creaminess’’,
‘‘smoothness’’,‘‘thickness’’,and‘‘fattiness’’ (Chojnicka-Pas-zunetal.,2012;Chung,Olson,etal.,2013;deWijk &Prinz, 2005;deWijk&Prinz,2007;Kilcast&Clegg,2002;vanAken etal.,2011).Thedifferencebetweenreduced-fatandfull-fat versions of food products can beused todemonstrate the influence offatcontent ontheir textureand mouthfeel.In manyproducts,thereducedfatversionsareperceivedasnot beingas‘‘rich’’,‘‘creamy’’or‘‘thick’’astheirfull-fat counter-parts, which has been partly attributed to their lower viscosities (Chung et al., 2012b;Chung,Olson, et al., 2013).
Somestudieshavealsoreportedthatdropletsizeinfluences the perceived ‘‘creaminess’’ offood emulsions(Ciron, Gee, Kelly,& Auty, 2012;de Wijk& Prinz, 2005;Kilcast& Clegg, 2002).deWijkandPrinz(2005)foundthatincreasingthefat dropletsizeinmayonnaisesincreasedthemeasuredfriction, whichisaparameterassociatedwithhighroughnessandlow creaminess.However,otherstudiesreportedlittleinfluenceof fatdropletsizeoncreaminessorthickness(Akhtaretal.,2005;
Frost,Dijksterhuis,&Martens,2001).Thesedifferingfindings suggest that fats can influence the sensory properties of differentfoodproductsindistinctivemanners.
Dropletinteractions:Colloidalinteractionsbetweendroplets haveasignificantimpactontherheologyandtextureoffood emulsion products. These interactions affect the effective volumefractionofthedispersedphaseinanemulsion,and thereforeinfluencetheoverallviscosityandtexture proper-ties. Attractive interactions between droplets can lead to Fig.7–Confocalmicrographs(60T)of(a)conventionaloil-in-wateremulsion(1%,w/woil)stabilizedbysodiumcaseinate (1%,w/woil)(atpH7)and(b)oil-filledhydrogelparticles(1%,w/woil)withsodiumcaseinateasthegelmatrixandpectinas thecontinuousphaseatpH5.
droplet flocculation, which increases the effective volume fractionofthe dispersed phasedueto thepresenceofthe continuousphasetrappedwithinflocstructures(McClements, 2012a).Flocculationandtheassociatedincreaseinviscosity canbeinducedthroughseveralmethods,includingadjusting solutionpHorionicstrengthtoreduceelectrostaticrepulsion, addition of biopolymers to increase depletion or bridging attraction, and heating to increase hydrophobic attraction between globular protein-coated droplets (Aben, Holtze, Tadros, & Schurtenberger, 2012; Protonotariou et al., 2013;
Tangsuphoom&Coupland,2008).
Dropletsize:Thesizeofthedropletsinanemulsiononlyhas anappreciableinfluenceontheviscosityofnon-flocculated systems when the dispersed phase volume fraction is relatively high,f>0.45 (McClements, 2005). The degree of polydispersityoftheoildropletsalsoinfluencestheviscosity ofemulsionssinceitaffectstheeffectivepackingparameter.
Thepackingofthedropletsinpolydisperseemulsionsismore efficient than in monodisperse emulsions, since small particlescanfitinthespacesbetweenlargeparticles,thereby leading to lower viscosities at similar fat contents (Chung etal.,2012b;McClements,2005,2012a).
4.3.2. Continuousphasecharacteristics
Thepropertiesoftheaqueousphasemayalsoinfluencethe perceived texture and mouthfeel of food emulsions. The presenceofbiopolymers,particularlythosewiththickeningor gelling properties,influences thetexture and mouthfeelof emulsions(Akhtar,Murray,&Dickinson,2006;Flett,Duizer,&
Goff, 2010; Meyer, Bayarri, Tarrega, & Costell, 2011). The viscosityofemulsionsisdirectlyproportionaltotheviscosity of the continuous phase, and therefore any components within the aqueous phase that enhance its viscosity will influencetheoverallrheologyandtexturalpropertiesofthe system(Cerimedoetal.,2010;Chungetal.,2012b;Huck-Iriart etal.,2011;Perrechil&Cunha,2010;Protonotariouetal.,2013;
Rao, 2007). The degree of the increase depends on the concentration,conformation,andinteractionsofthe compo-nents present and the spatial arrangement of the overall system(Fig.3).Additionofbiopolymerscanthereforebeused asameansofreplacingsomeofthetexturalattributeslost when fat droplets are removed from reduced-fat products (Chungetal.,2013a;Dickinson,2013;Flettetal.,2010;Mun etal.,2009;Torres,Janhoj,Mikkelsen,&Ipsen,2011).
Recently, a novel approach to regulate the rheological propertiesofemulsion-based products hasbeen developed based on the incorporation of micron-sized air bubbles.
Preliminarystudiesontheseair-filledemulsions demonstrat-edthattheirrheologicalpropertieswerecomparabletothose offull-fatemulsions(Tchuenbou-Magaia&Cox,2011).Amore detaileddiscussionoftheformationandpropertiesofair-filled emulsionsingiveninSection6.
4.3.3. Interfacialregion
Theperceived texture and mouthfeel ofemulsions can be alteredbymodulatingthewaythatthefatdropletsbehave within the oral cavity, e.g., whether they coalesce and/or spreadonthetongue.Studieshavereportedthatfatdroplets thatarepronetocoalescencewithinthemouthhavecreamier mouthfeel and fatty sensations than those that do not
(Benjaminsetal.,2009;Dresselhuis,deHoog,CohenStuart, Vingerhoeds, & van Aken, 2008a). The attachment of fat dropletstotheoralcavityandtheirtendencytocoalesceand spread within the mouth canbe controlled byaltering the nature ofthe emulsifierused tostabilize them(Benjamins etal.,2009;Dresselhuis,deHoog,CohenStuart,Vingerhoeds,
&vanAken,2008a;vanAken,2007).
4.4. Foodemulsionstability
Emulsionsarethermodynamicallyunstablesystemsthatare inclined to break down over time through a variety of physicochemical mechanisms, includinggravitational sepa-ration, flocculation, coalescenceand Ostwald ripening (Fig.
8a). Practically, an emulsion-based food product can be considered to be physically unstable when there is visual separationorinhomogeneityinitsoverallappearance,oran undesirablechangeinitstexturalattributes.
Control of emulsion stability is important in the food industryinthecreationofcommercialproductsthatmaintain their desirable sensory and physicochemical properties throughouttheproductsshelflife.Therearevariouspossible strategiesavailabletoretardemulsioninstabilityby modulat-ingthepropertiesofthedispersed,continuousandinterfacial phases.
4.4.1. Dispersephasecharacteristics
The structural and physicochemical characteristics of the disperse phase affect a number of different emulsion instabilitymechanismse.g.,gravitationalseparation, floccu-lation,coalescenceandOstwaldripening(Fig.8a).Thedensity ofthedispersed phaseinfluencesthe tendencyfor gravita-tionalseparationtooccur.Theconcentrationofdropletsinan emulsion also influences their stability to gravitational separation–thehigherthedropletconcentration,theslower thecreamingrate(Fig.8b).Increasingthedroplet concentra-tionmaynotbeafeasiblesolutiontoinhibitinggravitational separationinmostfoodproducts,butitmaybepossibleto introduce other non-fat particles to inhibit creaming by a similarmechanism(e.g.,hydrogelparticles). Thesizeofthe dropletsalsoplaysanimportantroleingravitational separa-tion,withtherateofcreamingincreasingwiththesquareof the particle radius. Thus, gravitational separation can be retarded bydecreasingthe dropletsizee.g., byaltering the homogenizationconditions(e.g., increasingthe intensityor duration ofhomogenization) and productformulation (e.g., emulsifier type and concentration) (Cerimedo et al., 2010;
Huck-Iriartetal.,2011;Perrechil&Cunha,2010).
4.4.2. Continuousphasecharacteristics
Thepropertiesofthecontinuousphasemayalsoinfluencea variety of different instability mechanisms in emulsions.
Addition of substances that increase the viscosity or gel strengthofthecontinuous phaseretardsdropletmovement e.g.,starches,gums,sugars,and/orproteins(Cerimedoetal., 2010;Chungetal.,2012b;Huck-Iriartetal.,2011;Perrechil&
Cunha, 2010;Protonotariou etal., 2013).Consequently,they mayreducetheratesofbothdropletgravitationalseparation and aggregation. Changes inthe density ofthe continuous phaseinfluencetherateofgravitationalseparation.Alterations
inthepHorionicstrengthoftheaqueousphasesurroundingfat
inthepHorionicstrengthoftheaqueousphasesurroundingfat