Recently, there has been great interest in extending the functional performance of food emulsions using various
structuraldesignapproaches.Inthissection,abrief introduc-tiontosomeofthesestructuraldesignapproachesisprovided, and potential applications within the food industry are highlighted.
6.1. Multipleemulsions
Amixtureofwater-in-oilandoil-in-waterelementscontained within the same system isknown as a multiple emulsion.
Structurally,multipleemulsionsconsistofsmalldropletsofone phase embeddedwithin larger droplets of a secondphase, which itselfisdispersedwithinacontinuous phase (Figs. 1 and10).Thetwomajortypesofmultipleemulsionsare water-in-oil-in-water (W1/O/W2) and oil-in-water-in-oil (O1/W/O2) emulsions (Dickinson, 2011; Leal-Calderon et al., 2007;
McClements,2012a;Muschiolik,2007).Thesubscripts1and 2refer tothe innerandouterregionsofoneofthe liquid phases (either oil or water). For example, for W1/O/W2
emulsions the W1 indicates the inner aqueousphase and the W2 indicates to the outer aqueous phase. These two aqueous phases may have different compositions and properties (McClements, 2012a). W1/O/W2 emulsions are suitableforvariousapplicationswithinaqueous-basedfoods sincetheycanbeusedtoproducereduced-fatproductsand forencapsulationandcontrolledreleaseofbioactive mole-cules(Dickinson,2011;Leal-Calderonetal.,2007).
Multiple emulsions can beprepared using manyof the same toolsusedforpreparationofconventionalemulsions, e.g.,highshearmixers,highpressurehomogenizers, sonica-tors, and membrane homogenizers (McClements, 2012a;
Muschiolik,2007).W1/O/W2emulsionsarenormallyproduced using a two-stage process.In brief, thefirst stageinvolves formationofaW1/Oemulsionbyhomogenizingwaterandoil together in the presence of an oil-soluble surfactant. The secondstageinvolveshomogenizationoftheW1/Oemulsion withwaterinthepresence ofawater-solublesurfactantto Fig.9–Schematicrepresentationsandphotographicimagesoftheinstrumentalmasticationdevicedesignedtosimulate thetongueandpalateinthemouth.
form a W1/O/W2 emulsion (Dickinson, 2011; McClements, 2012a).Usually,thesecondhomogenizationstagehastobe less intense than the first otherwise the system may breakdown(McClements,2012a;Pal,2011).Multipleemulsions arethermodynamically unstable systems that break down overtimeandtherefore theymust becarefully designedto givethemsufficientkineticstabilityforpracticalapplications.
W1/O/W2emulsionsbreakdownduetothesamemechanisms asconventionalemulsions(suchasgravitationalseparation, droplet aggregation, and Ostwald ripening), plus some additionalmechanism(suchaswaterdiffusionanddroplet expulsionfromtheoildroplets)(Dickinson,2011;McClements, 2012a). Further details of the formation and instability of multipleemulsionsareavailablefromthecitedreferences.
A number of studies have investigated the potential applicationofmultipleemulsions(W1/O/W2)infoodproducts (Giroux et al., 2013; Lobato-Calleros et al., 2008; O’Dwyer, O’Beirne, Ni Eidhin, Hennessy, & O’Kennedy, 2013). For example, it was shown that reduced fat cheeses could be prepared that had properties similar to full fat cheeses (Lobato-Calleros,Rodriquez,Sandoval-Castilla, Vernon-Cart-er,&Alvarez-Ramirez,2006;Lobato-Callerosetal.,2008).
6.2. Filledhydrogelparticles
Filledhydrogelparticlesaresystemsinwhichtheoildroplets areembeddedwithinahydrogelmatrixthatisitselfdispersed within an aqueous continuous phase (Figs. 7 and 10). The hydrogel particlescan bemadefromproteinsand/or poly-saccharidesthatarecapableofformingagelmatrix under
controlledconditions(e.g.,heating,cooling,pHadjustment,or enzyme addition). Filled hydrogel particles can be formed using a variety of methods, including injection methods, moldingmethods,geldisruption,andcontrolledbiopolymer phase separation (Burey, Bhandari, Howes, & Gidley, 2008;
Matalanis,Jones,& McClements,2011;McClements, 2012a).
MostofthesemethodsinvolvemixinganO/Wemulsionwitha biopolymer solution and then adjusting the preparation conditionstopromotehydrogelparticleformationandmatrix gelation(McClements,2012a).
Hydrogel particles are often formed using controlled biopolymer phase separation, which can be divided into segregation-basedandaggregation-basedmethods(Schmitt&
Turgeon, 2011). Segregation-basedseparation occursdueto thermodynamic incompatibility of the two biopolymers (Norton&Frith,2001).Thermodynamicincompatibilityarises whenthereisarepulsiveinteractionbetweentwodifferent kinds of biopolymers. Above a critical concentration, the mixed systems separate into two biopolymer phases with differentcompositions,onephasethatisrichinonetypeof biopolymeranddepletedintheother,andviceversa(Norton&
Frith,2001;Norton&Norton,2010).Hydrogelparticlesarethen formedbyblendingthesystemorusinginjectionmethods.
Thehydrogelparticlesarethencross-linkedusingamethod that depends on biopolymer type, such as adjustment of temperature, pH, salt,or enzyme activity. Thepreparation methodisdesignedsothattheoildropletsarelocatedwithin thebiopolymerphasethatformsthehydrogelparticles.The viscosity of filled hydrogel particles is higher than that of conventionalemulsionswiththesameoilconcentrationdue Fig.10–Examplesofstructuredemulsionscreatedbystructuraldesignprinciplesusingemulsiondropletsasabuilding block.
to the increased effective volume fraction of the hydrogel matrices(Matalanisetal.,2011).Filledhydrogelparticlesare thussuitableforapplicationinreducedfatproducts,which needtoattainhigh viscositywhenfatsareremoved.Filled hydrogelparticlesmayalsobeusedtomanipulatetherelease offlavorcompoundsfromtheoilphase(Malone&Appelqvist, 2003;Raschip,Hitruc,Oprea,Popescu,&Vasile,2011).Withthe oiltrappedinthe hydrogel matrices, thereis anincreased path-length through which the flavor compounds need to diffusebeforetheycanbereleasedfromthesystemsduring mastication.Thus,theflavorreleaseprofileofareduced-fat productmaybematchedtothatofthefull-fatcounterpart(Fig.
6b).
6.3. Multilayeremulsions
Theproperties of O/W emulsions can alsobe modified by buildinglaminatedcoatingsaroundtheoildropletsusingthe layer-by-layer(LbL)depositionmethod(McClements,2010a).
Theoildropletscanbesurroundedbyanano-laminatedshell consistedoftheemulsifierinterfaciallayerandoneormore polyelectrolytecoatingsaroundtheadsorbedemulsifier(Fig.
10).Thecharacteristicsofthelayers(e.g.,charge,thickness, integrity, and permeability) can be tailored for specific functionalperformances(e.g.,improvedstabilityorcontrolled release)bychoosingappropriateemulsifiers,polyelectrolytes, and preparation conditions (Guzey & McClements, 2006;
McClements,2012a;Shchukina&Shchukin,2012).
Inbrief,thepreparationofmultilayeremulsionsinvolves mixinganO/Wemulsionstabilized byachargedemulsifier withanaqueoussolutioncontaininganoppositelycharged polyelectrolyte under conditions where the polyelectrolyte formsacoatingaroundthedropletsurfaces.Additionallayers can be added by mixing the coated droplets with more oppositelychargedpolyelectrolytesolutions(Guzey& McCle-ments, 2006; McClements, 2012a; Shchukina & Shchukin, 2012).Moredetailedinformationonfabricationofmultilayer emulsionscanbefoundinthecitedreferences.Theformation ofmultilayeremulsionsmustbecarefullycontrolledtoavoid bridgingflocculation,e.g.,bycontrollingoilcontent, polyelec-trolyte concentration, pH, and ionic strength (Guzey &
McClements,2006;Shchukina&Shchukin,2012).
6.4. Microclusters
Theviscosityofemulsionscanbeincreasedconsiderablyby inducing dropletflocculation, whichincreases the effective volume ofthe particles dueto theaqueous phasetrapped withintheflocs(Fig.10)(Dickinson,2009a;McClements,2005).
Atsufficiently highdropletconcentrations, flocculationcan formathree-dimensionalnetworkthatresultsingel-likeor paste-like characteristicsina productand inhibits gravita-tionalseparation(McClements,2005).Controlledflocculation isusefulinreducedfatfoodswhereahighviscosityneedsto beattainedafterfatdropletsareremoved(Mao&McClements, 2011, 2012). The formation of microclusters with specific propertiescan be achieved byhomoaggregation or hetero-aggregation.Homoaggregationinvolvesaggregationofsimilar particles,whereasheteroaggregationinvolvesaggregationof dissimilarparticles.
Homoaggregationcanbeinducedinfoodemulsionsusing various approaches: (i) reduction of electrostatic repulsion betweenfatdropletsbyalteringthepHtoreducethecharge on the droplets or by adding salts to screen the charges (Dickinson, 2010; McClements, 2012a); (ii) increase depletion attraction – by adding non-adsorbing polymers or colloidal particles to increase the osmotic pressure and generate a depletion attraction between thedroplets (Dickinson,2010;
McClements, 2012a; Udomrati,Ikeda, & Gohtani, 2011);(iii) increase hydrophobic attraction in globular-protein-stabilized droplets by heating the emulsions above the thermal denaturation temperature of the adsorbed proteins, which increases the surfacehydrophobicity(Dickinson, 2010;Kim et al., 2002); (iv) bridging flocculation– by adding oppositely chargedpolyelectrolytesorbiopolymerstoformionicbridges between differentdroplets(Cho& McClements,2009; Dick-inson,2010;Guzey&McClements,2007).
Heteroaggregationcanbeachievedbymixingtwo emul-sionstogethercontainingoppositelychargedfatdroplets(Mao
& McClements, 2011). This has been achieved by mixing positive droplets (protein-coated) with negative droplets (modifiedstarch-coated)underacidicconditions,orbymixing twoprotein-coateddropletswithdifferentisoelectricpointsat an intermediate pH where they have opposite charges (b-lactoglobulinandlactoferrin)(Fig.11).Themixedemulsions produced havemuch higherviscosities thanthe individual non-flocculated emulsions they were prepared from. The functionalpropertiesoftheflocculatedemulsionsdependon thenatureoftheflocsformed,suchastheirconcentration, size,shape,internalstructure,andbondstrength. Neverthe-less,controlledflocculationshouldbecarriedoutwithcareso astonotcauseadverseeffects,suchasdecreasedcreaming stability.
6.5. Air-filledemulsions
Air-filledO/Wemulsionshaverecentlybeeninvestigatedasa novel approach for producing food products with novel properties or reduced calorie contents (Dickinson, 2013; Le Reverendetal.,2010;Tchuenbou-Magaia,Al-Rifai, Mohamed-Ishak,Norton,&Cox,2011;Tchuenbou-Magaia&Cox,2011;
Tchuenbou-Magaia, Norton, & Cox, 2009, 2010). In this approach,micron-sizedairbubblesareincorporatedintoO/
Wemulsionstoformtriphasicair-in-oil-in-wateremulsions, whereasignificantamountofthefatphaseisreplacedbyair bubbles (Fig. 10). Studies with model salad dressings have shownthattriphasicemulsionscangiverheological (Tchuen-bou-Magaiaetal.,2009)andtribological(Tchuenbou-Magaia&
Cox, 2011)propertiessimilar tothose ofconventional O/W emulsions.
Initially,micronsizedairbubbleswerestabilizedusinga novelgroupofproteins(hydrophobins)thatassembleatthe air/water interface usinga sonicationprocess (Tchuenbou-Magaiaetal.,2009,2010,2011).Theinterfacialcoatingformed around the airbubblesby theseproteins was showntobe relativelyrobustandcapableofsurvivingvariousprocessing steps duringfood production.Theair-filledemulsions also showedlittlechangeinairbubblesizewhenstoredforupto4 monthsatroomtemperature(Tchuenbou-Magaiaetal.,2009).
However,hydrophobinsarerelativelyexpensiveingredients
that are not currently suitable for widespread application within the food industry, and so alternative sources of proteins are being examined (Le Reverend et al., 2010).
Recently,ithasbeenshownthatcysteine-richproteins(such asbovineserumalbuminandeggwhiteproteins)canalsobe usedtoformstableairbubblesusingasonochemicalmethod (Tchuenbou-Magaia & Cox, 2011). The majority of the air bubblesformedbytheseproteinshaddiametersbetween0.5 and10mm.Thisstudyalsofoundthattheairbubblesizeand stabilityishighlydependentontheprocessingconditionsand systemcomposition.Thesepreliminarystudiessuggestthat air-filledemulsionshavepotentialapplicationin manufactur-ing reduced- or low-fat products withcomparable sensory propertiesasfull-fatversions.