Structural Design Principles for Improved Food Performance_Nanolaminated Biopolymer Structures in Foods

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Chapter 1Structural DesignPrinciplesfor ImprovedFood

Performance: NanolaminatedBiopolymer StructuresinFoods

David Julian McClementsBiopolymersand ColloidsResearch Laboratory, Departmentof Food

Science,Universityof Massachusetts,Amherst, MA 01003

The bulk physicochemical, sensory and physiologicalattributes of most foods are determined by the characteristics,interactions and structural organization of the variousingredients they contain.Biopolymers are important functionalingredients in many foods, contributing to theiroverall texture,stability, appearance, flavor and nutritional quality. Animproved understanding of the molecular andphysicochemicalbasis of biopolymer functionality in foods can lead to thedesign of improved or novel functional attributes into foods.This chapter describes how nanolaminated layers can beformed from food biopolymers, and highlights their potentialapplications within the food industry. Electrostatic layer-by--layer ( LbL ) deposition of charged biopolymers can be used toformnano-structured interfacial layerswith specific properties,e.g., charge, thickness, porosity, permeability, responsiveness.These layers may be formed around macroscopic, microscopicor nanoscopic materials, and are therefore applicable to a widerange of food categories. Systematic manipulation ofinterfacial properties can be used tocreatematerials withnovelfunctional attributes, e.g., improved stability to environmentalstresses or controlled release characteristics. The potential ofthis technique is highlighted using recent studies on theformation of nanolaminated coatings on microscopic lipiddroplets and macroscopic hydrogel surfaces.

2009American Chemical Society 3

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4Introduction

There are a wide number of different lipophilic components within the foodindustry that need to be delivered in an edible form, including bioactive lipids,vitamins, flavors, antimicrobials and antioxidants e.g., -3 fatty acids,phytosterols, lycopene, lutein,-carotene, coenzyme A , vitamins A and D, citrusoils, essential oils [1-4]. In many cases, it is advantageous to deliver theselipophilic components in an aqueous medium because this increases theirstability, palatability, desirability and bioactivity. For example, an activelipophilic component might be incorporated into a beverage or food that couldeasily be consumed by drinking or eating. Nevertheless, thereare often a varietyof technical challenges that need to be overcome before an active lipophiliccomponent can be successfully incorporated into an aqueous-based deliverysystem. L ipophili c active components come in a wide variety of differentmolecular forms, which lead to differences in their physicochemical andphysiological properties, such as chemical stability, physical state, solubility,rheology, optical properties, and bioactivity. Consequently, different deliverysystems are usually needed to address specific molecular, physicochemical andphysiological concerns associated with each active component. In general, anedible delivery system must have a number of attributes:

It must be capable of efficiently encapsulating an appreciable amountof functional agentand keeping it entrapped. It may have to protect the functional agent from chemical degradation

sothat it remains in its activestate. It may have to control the release of the functional agent, e.g., the

release rateor the specific environmental stimuli that triggers release. It may have to be compatible with the surrounding food or beverage

matrix, without causing any adverse affects on product appearance,rheology, mouth feel, flavor or shelf life.

It may have to resist the environmental stresses foods or beveragesexperience during their production, storage, transport and utilizatione.g., heating, chilling, freezing, dehydration, or shearing.

It should be prepared completely from generally recognized as safe(GRA S ) ingredients using simple cost-effective processing operations. It should not adversely impact the bioavailability of the encapsulatedmaterial.

A wide variety of different types of delivery system have been developed toencapsulate lipophilic functional agents, including simple solutions, associationcolloids, emulsions, biopolymer matrices, powders, etc. Each type of deliverysystem has its own advantages and disadvantages for encapsulation, protection

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5and delivery of functional agents, aswell as in its cost, regulatory status, easeofuse, biodegradability, biocompatibility etc.

This chapter wil l begin by introducing the basic principles of structuraldesign for creating delivery systems with improved stability and novelfunctional performance. We wil l then focus on a particular structural designprinciplebased on layer-by-layer(LbL) electrostatic depositionthatcan be usedto form nanolaminated coatings around microscopic and macroscopic objects.The potential of this method for creating emulsion-based delivery systems withimproved stability to environmental stresses wil l then be demonstrated. Finally,the potential of using the L bL technique to form laminated functional coatingson macroscopic food surfaces (such as fruits, vegetables, fish and meats) wil l behighlighted.

Structural DesignPrinciplesIn this section, a brief outline of the major building blocks available to

create food grade delivery systems, as well as the major molecular interactionsand structural design principles that can be used to assemble them intofunctional systems is given.

BuildingBlocksThe major building blocksthatcan be used to assemble food-grade delivery

systems are outlined below: Lipids. Lipids are predominantly non-polar substances that are highly

hydrophobic. In the food industry, the main sources of lipids aretriacylglycerols, which may come fromanimal, fish, or plantorigins. Lipidscan be used to solubilize non-polar lipophilic components in foods, and arecommonly used in delivery systems based on emulsions or microemulsions.

Surfactants. Surfactants are surface-active molecules that consist of ahydrophilic head group and a lipophilic tail group. The functionalperformance of a specific surfactant depends on the molecularcharacteristics of its head and tail groups. Food-grade surfactants come in avariety of different molecular structures. Their head groups may vary inphysical dimensions and electrical charge (positive, negative, zwitterionicor non-ionic),while their tail groups may vary in number (typically one ortwo), length (typically 10 to 20 carbons per chain) and degree of saturation(saturated or unsaturated). Surfactants are typically used to formemulsions,

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6association colloids or biopolymer complexes that are suitable for use asdelivery systems.

Biopolymers. The two most common classes of biopolymer used asstructure forming materials in the food industry are proteins andpolysaccharides. Ultimately, the functional performance of foodbiopolymers (e.g., solubility, self-association, binding and surface activity)is determined by their unique molecular characteristics (e.g.,molecularweight, conformation, flexibility, polarity, hydrophobicity and interactions).These molecular characteristics are determined by the type, number andsequence of the monomers that make up the polymer chain. The monomersvary according to their polarity, charge, physical dimensions, molecularinteractions and chemical reactivity. Biopolymers may adopt a variety ofconformations in food systems, which can be conveniently divided intothree broad categories: globular, rod-like or random coil. Globularbiopolymers have fairly rigid compact structures, rod-like biopolymers havefairly rigid extended structures (usually helical), and random-coilbiopolymers have highly dynamic and flexible structures. In practice, manybiopolymers do not have exclusively one type of conformation, but havesome regions that are randomcoi l , some that are rod-like and some that areglobular. Biopolymers can also be classified according to the degree ofbranching of the chain. Most proteins have linear chains, whereaspolysaccharides can have either linear or branched chains. In solution,biopolymers may be present as individual molecules or they may be presentas supra-molecular structures where they are associated with one or moremoleculesof the same or different kind. Finally, it should be mentionedthatbiopolymers may undergo transitions fromone conformation to another, orfrom one aggregation state to another, if their environment is altered, e.g.,pH , ionic strength, solvent composition or temperature. The conformationand interactions of biopolymers play a major role in determining theirability to formstructured delivery systems.Some of the most important food-grade components that are available asbuilding blocks to form structured delivery systems are listed in Table 1. The

choice of a particular food-grade component depends on the type of structurethat needs to be formed, as well as its legal status, cost, usage levels, ingredientcompatibility, stability andeaseof utilization.

Molecular Interactions

Knowledgeof theoriginandnatureof the various molecular forces that actbetween food components is also important for understanding how to assembledelivery systems with specific structures from foodgrade ingredients:

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7

Table1. Major food-gradestructural components that can beusedto construct deliverysystemsfor nutraceuticals.

Name ImportantCharacteristics Examples

Lipids Chemical stabilityM elting profilePolarity

Animal fats:beef, pork, chickenFish oils: cod liver, menhedan,salmon, tunaPlant oils: palm, coconut, sunflower,safflower, corn, flax seed, soybeanFlavor oils: lemon, orange

Surfactants Solubility ( H L B )ChargeMolecular geometrySurface load

Non-ionic: Tween, SpanAnionic: S L S , D A T E M , C I T R E MCationic: Laurie ArginateZwitterionic: lecithin

Biopolymers Molar MassConformationChargeHydrophobicityFlexibility

Globular Proteins: whey, soy, eggFlexible Proteins: casein, gelatinNon-ionic Polysaccharides: Starch,Dextran, Agar, Galactomannans,CelluloseAnionic Polysaccharides: Alginate,Pectin,Xanthan, Carrageenan,Gellan, Gum ArabicCationic Polysaccharides: Chitosan

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8 Electrostatic interactions. Electrostatic interactions are important for food

components that have anelectrical charge under theutilizationconditions,e.g., proteins, ionic polysaccharides, ionic surfactants, phospholipids,mineral ions, acids and bases. Electrostatic interactions may be eitherattractive or repulsive depending on whether the charge groups involvedhave opposite or similar signs. Thesign andmagnitude of thechargeonfood components usually depends on solution pH, since they have weakacid or base groups. Thestrength and range of electrostatic interactionsdecreases with increasing ionic strength due to electrostatic screeningeffects. The most common means of manipulating the electrostaticinteractions between food components aretherefore to alter the pH and/orionic strength of the aqueous solution.Alternatively, electrostatic bridginginteractions maybeusedtoassemble food components.

Hydrophobic interactions. Hydrophobic interactionsare important for foodcomponents that have appreciable amounts of non-polar groups, andtheymanifest themselves as a tendency for the non-polar groups to associatewith each other in water. Hydrophobic interactions may bemanipulatedsomewhat by altering the temperature or changing the polarity of anaqueous solution(e.g., by addingalcohol).

Hydrogen bonding. Hydrogen bonding is important for food componentsthat have polar groups that are capable of forming relatively stronghydrogen bonds with other polar groups on the same or on differentmolecules. Hydrogen bonds tendtodecrease instrength as the temperatureis increased, and they often form between helical orsheet-like structuresonthe sameordifferent biopolymers.

Steric exclusion. Stericexclusioneffects areimportant forfood componentsthat occupy relatively large volumeswithin asystem, because they excludeother components from occupyingthe same volume, thereby alteringtheconfigurational and/or conformational entropy of the system.Therelative importance of these interactions inaparticular system depends

on the types of food components involved (e.g., molecular weight, chargedensity vs. pH profile, flexibility, hydrophobicity), the solution composition(e.g., pH, ionic strength and dielectric constant) and the environmentalconditions, (e.g., temperature, shearing). Bymodulatingthese parameters it ispossible to control the interactions between the food components and thereforeassemblenovel structures thatcanbeusedasdelivery systems.

Structural DesignPrinciplesIn this section, some of themajor structural design principlesthat can be

usedtoassemblenovel structures from foodcomponentsarehighlighted.

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9 Phase separation. When twodifferent materials are mixed together they

may becompletely miscibleandform asingle phase, or they may separateinto anumber ofdifferent phases, depending on therelative strength of theinteractions between thedifferent types ofmolecules present (comparedtothe thermal energy). There are anumber ofexamples ofphase separationinvolving food components that can beused tocreatenovel structures.Themost common example is thephaseseparation of oi l andwater due to thefact that oil-water interactions are strongly unfavorable (compared to theaverage of water-water andoi l -oi l interactions), which is thebasis for theformation of emulsion systems. Another example is thephaseseparationofmixed biopolymer solutions as a result of relatively strong thermo-dynamically unfavorable interactions between the different types ofbiopolymers (e.g., electrostatic repulsion and/or steric exclusion). As aresult of this type of phase separation the mixed biopolymer systemseparates into twodifferent phases: onephase isenrichedwith onetypeofbiopolymer anddepleted with theother type, while theopposite situationoccurs intheother phase.

Spontaneous Self-assembly. Under appropriate environmental conditions,certain types offood components spontaneously assemble into well-definedstructures since this minimizesthe free energy of the system, e.g.,micelles,vesicles, fibers, tubes, liquid crystals. Thedriving force forself-assembly issystem dependent, butoften involves hydrophobic attraction, electrostaticinteractions and/or hydrogen bond formation.Association colloids, suchasmicelles, vesiclesand microemulsions,aresome of the most common typesof self-assembled structures infood materials. The primarydriving forceforthe spontaneous formation of these structures is the hydrophobic effect,which causes thesystem to adopt amolecular organizationthatminimizesthe unfavorable contact area between thenon-polar tails of the surfactantmoleculesand water.

Directed self-assembly. Directed self-assembled systems do not formspontaneously i fall thecomponents aresimply mixed together. Instead, thepreparation conditions (e.g., order of mixing, temperature-, pH-or ionicstrength-time profiles) must be carefully controlled to direct the differentcomponents sothat they areassembled intoaparticular metastable structure.The driving force for directed-assembly of food structures isalso system-dependent, but again hydrophobic, electrostatic and hydrogen bondinginteractions arecommon. A widely used directed self-assembly method islayer-by-layer (LbL) electrostatic deposition of polyelectrolytes and othercharged substances onto oppositely charged surfaces due to electrostaticattraction (whichwil l becovered inthis chapter). Another example ofthismethod is theformation of hydrogels frombiopolymers. For example, whena solutionof gelatin iscooled below acertain temperature a coil-to-helixtransition occurs, which isfollowed byextensive hydrogen bond formation

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10between helices on different gelatin molecules. These hydrogen bondedregions act as physical cross-links between the gelatin molecules that mayeventually lead to gelation. The gelatin molecules therefore self-assembleunder the prevailing environmental conditions, but the precise details of thestructures formed (i.e., the number, position and length of the cross-links)dependson the specific preparation conditions used (e.g., time-temperatureprofile, shearing).

Directed assembly. In principle, it is also possible to form structureddelivery systems by physically bringing molecules together in well-definedways, e.g., by micro-manipulation methods. Nevertheless, most of thesetechnologies are unlikely to find widespread use in the food industry, atleast in the foreseeable future, due to a variety of economic and practicalconstraints, such as the fact that expensive equipment is needed to fabricateand characterize the structures formed, and the throughout of fabricatedstructures is likely to be extremely low.

Structured DeliverySystemsStructural design principles can be used to create a variety of different

delivery systems that can be utilized to encapsulate l ipophilic components.Some structured delivery systems that can be created using food-gradeingredients and common unit operations that are based on emulsion technologyare highlighted in Figure 1.

Conventional Emulsions. Conventional oil-in-water (O/W) emulsionsconsist of emulsifier-coated lipid droplets dispersed in an aqueouscontinuous phase. They are formed by homogenizing an oil and water phasetogether in the presence of ahydrophilic emulsifier.

Multiple Emulsions. Multiple water-in-oil-in-water (W/O/W) emulsionsconsist of small water droplets contained within larger oil droplets that aredispersed in an aqueous continuous phase. They are normally producedusing a two-step procedure. First, a W/O emulsion is produced byhomogenizing water, oil and an oil-soluble emulsifier. Second, a W/O/Wemulsion is then produced by homogenizing the W/O emulsion with anaqueoussolution containing a water-soluble emulsifier.

Multilayer Emulsions. Multilayer oil-in-water (M-O/W) emulsions consistof small oil droplets dispersed in an aqueous medium, with each oil dropletbeing surrounded by a nano-laminated interfacial layer, which usuallyconsists of emulsifier and biopolymer molecules. They are normally formedusing a multiple-step procedure. First, an oil-in-water emulsion is preparedby homogenizing an oil and aqueous phase together in the presence of anionized water-soluble emulsifier. Second, an oppositely chargedpolyelectrolyte is added to the system so that it adsorbs to the droplet

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12surfaces and forms a two-layer coating around the droplets. This procedurecan be repeated to form oil droplets coated by nano-laminated interfacescontaining three or more layers by successively adding polyelectrolyteswith opposite charges.

Solid Lipid Nanoparticles. Solid lipid nanoparticles (SLN) are similar toconventional emulsions consisting of emulsifier-coated l ipid dropletsdispersed in an aqueous continuous phase. However, the lipid phase iseither fully or partially solidified, and the morphology and packing of thecrystals within the lipid phase may be controlled. SL N are formed byhomogenizing an oil and water phase together in the presence of ahydrophilic emulsifier at atemperatureabove the melting point of the l ipidphase. The emulsion is then cooled (usually in a controlled manner) sothatsome or all of the lipids within the droplets crystallize.

Filled Hydrogel Particles. Filled hydrogel particle emulsions consist of oi ldroplets contained within hydrogel particles that are dispersed within anaqueouscontinuous phase. They can therefore be considered to be a type ofoil-in-water-in-water (0/W1/W2) emulsion. There are a number of differentways to form this kind of system based on aggregative or segregative phaseseparation of biopolymers in solution.The functional performance of a particular delivery system can be

controlled by varying the properties of the structured particles (Figure 1):composition (e.g., ratio of oi l , water and biopolymer, oil type, biopolymer type);dimensions (e.g.,particle radii, film thicknesses), physical state (e.g.,solid orliquid); permeability; polarity etc.

Multilayer Emulsion-based DeliverySystems: LipidDropletsCoatedbyNanolaminated Coatings

In this section, the focus wil l be on the development of multilayeremulsions that could be used as delivery systems for lipophilic functionalcomponents. Conventionally, oil-in-water (O/W) emulsions are created byhomogenizing oil and aqueous phases together in the presence of an emulsifier[5, 6]. The emulsifier adsorbs to the surfaces of the droplets formed duringhomogenization, where it reduces the interfacial tension and facilitates furtherdroplet disruption. In addition, the adsorbed emulsifier forms a protectivecoating around the droplets thatprevents them from aggregating. Many differentkinds of emulsifiers are available for utilization in food products, with the mostimportant being small molecule surfactants, phospholipids, proteins, andpolysaccharides. Each type of emulsifier varies in its effectiveness at producingsmall droplets during homogenization, and its ability to prevent dropletaggregation under different environmental stresses, such as pH, ionic strength,

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13heating, freezing and drying. Food emulsifiers also differ in their cost, reliability,ease of utilization, ingredient compatibility, label friendliness and legal status.For thesereasons, there is no single emulsifier that is ideal for use in every typeof food product. Instead, the selection of a particular emulsifier (or combinationof emulsifiers) for a specific food product dependson the type and concentrationof other ingredients that it contains, the homogenization conditions used toproduce it, and the environmental stresses that it experiences during itsmanufacture, storageand utilization.

Using conventional food emulsifiers and homogenization techniques thereare only a limited range of functional attributes that can be achieved inemulsion-based delivery systems. This has motivated a number of researchers toexamine alternative means of improving emulsion stability and performance.One strategy has been to createoil-in-water emulsions containing lipid dropletssurrounded by multi-component nano-laminated interfacial coatings consistingof emulsifiers and/or biopolymers [7-23]. In this "layer-by-layer" (LbL)electrostatic deposition approach, an ionic emulsifier that rapidlyadsorbs to thesurface of lipid droplets during homogenization is used to produce a primaryemulsion containing small droplets, then an oppositely charged biopolymer isadded to the systemthat adsorbs to the droplet surfaces and produces secondaryemulsions containing droplets coated with an emulsifier-biopolymer interfaciallayer (Figure 2). This latter procedure can be repeated to form lipid dropletscovered by coatings consisting of three or more layers, e.g., emulsifier -biopolymer 1 - biopolymer 2. Emulsions containing lipid droplets surroundedby multi-layered interfacial coatings have been found to have better stability toenvironmental stresses than conventional oil-in-water emulsions under certaincircumstances (18-22). They can also be used to protect lipophilic functionalcomponents within lipid droplets from chemical degradation [16, 17], or todevelop controlled or triggered releasesystems [8,21].

The , ,-electrostatic deposition method therefore offers a promising wayto improve the stability and performance of emulsion-based delivery systems.Nevertheless, the choice of an appropriate combination of emulsifier andbiopolymers is essential to the success of this approach, aswell as determinationof the optimum preparation conditions (e.g.,droplet concentration, biopolymerconcentration, pH, ionic strength, order of addition, stirring speed, washing, floedisruption, and temperature) [18, 20, 22]. The purpose of this section is toprovide an overview of recent research that has been carried out in ourlaboratory on the development, characterization and application of O/Wemulsions containing lipid droplets surrounded by nanolamiated coatings ofemulsifier and biopolymer. In particular, we will focus on the use of the LbLelectrostatic deposition technique to createemulsions with improved resistanceto environmental stresses, such as pH, ionic strength, thermal processing,freezing, dehydration, and lipid oxidation. These multilayer emulsions may beuseful for the encapsulation and delivery of lipophilic functional ingredients.

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15Preparationof multilayeredemulsions

Oil-in-water emulsions containing oil droplets surrounded by multi-layeredinterfacial coatings can be prepared using a multiple-step process [12, 18]. Forexample, the following procedure could be used to create emulsion dropletscoated by three layers, e.g., emulsifier-biopolymer 1-biopolymer 2 (Figure 2).First, aprimary emulsion containing electrically charged droplets surrounded bya layer of emulsifier is prepared by homogenizing oil, aqueous phase and awater-soluble ionic emulsifier together. Second, a secondary emulsioncontaining charged droplets stabilized by emulsifier-biopolymer 1 layers isformed by incorporating biopolymer 1 into the primary emulsion. Biopolymer 1normally has to have an opposite electrical charge than the droplets in theprimary emulsion (although this is not always necessary i f thereare significantlybig patches of opposite charge on the droplet surface). If necessary mechanicalagitation is applied to the secondary emulsion to disrupt any floes formedbecause of bridging of droplets by biopolymer molecules. In addition, thesecondary emulsion may be washed (e.g., by filtration or centrifugation) toremove any free biopolymer remaining in the continuous phase. Third, tertiaryemulsions containing droplets stabilized by emulsifier-biopolymer 1-biopolymer2 interfacial layers are formed by incorporating biopolymer 2 into the secondaryemulsion. Biopolymer 2 normally has to have an opposite electrical charge thanthe droplets in the secondary emulsion (but see above). If necessary mechanicalagitation is applied to the tertiary emulsion to disrupt any floes formed, and theemulsion may be washed to remove any non-adsorbed biopolymer. Thisprocedure can be continued to add more layers to the interfacial coating. Theadsorption of the biopolymers to the droplet surfaces can be convenientlymonitored using -potential measurements (Figures 3 and 4), whereas thestability of the emulsions to flocculation can be monitored by light scattering,microscopy or creaming stability measurements (Figures 3 and 4) [12-14, 20].Since the major driving force for adsorption of biopolymers to the dropletsurfaces is electrostatic in origin, it is important to control the pH and ionicstrength of the mixingsolution.Unless stated otherwise, the results reported below are for oil-in-wateremulsions containing lipid droplets coated by interfacial layers comprising of -lactoglobulin (primary) and-lactoglobulin-pectin (secondary). These emulsionswere formed by mixing a -lactoglobulin-stabilized emulsion with a pectinsolution at pH 7 where the protein and polysaccharide were both negativelycharged so that no adsorption occurred. Then, the pH of the solution wasadjusted so that the protein-coated droplets became positively charged (or hadpositive patches), which promoted pectin adsorption [11, 19, 24]. The pectinconcentration was controlled to avoid both bridging and depletion flocculation[11,18,19].

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17Improved Stability to Environmental Stresses

Food emulsions experience a variety of different environmental stressesduring their manufacture, storage, transport and utilization, including pHextremes, high ionic strengths, thermal processing, freeze-thaw cycling,dehydration, and mechanical agitation [5]. Many of the emulsifiers currentlyavailable for utilization within the food industry provide limited stability totheseenvironmental stresses. In this section, some of the recent work carried outin our laboratories on utilizing the ,-electrostatic deposition technique toimproveemulsion stability to various environmental stresses is reviewed.

pHThe influence of pH on the -potential, mean particle diameter and

creaming stability of primary (-lactoglobulin) and secondary (-lactoglobulin-pectin) emulsions was measured (Figure 4). The-potential of the protein-coateddroplets in the primary emulsions changed from negative to positive as the pHdecreased from 7 to 3, with the point of zero charge being around pH 5 (Figure4). This can be attributed to the fact that the isoelectric point of the adsorbedproteins is around pH 5. The -potential of the -lactoglobulin-pectin coateddroplets in the secondary emulsions had a similar negative charge as the -lactoglobulin-coated droplets in the primary emulsions at pH values >6, whichindicates that the anionic pectin molecules did not adsorb to the anonic droplets.When the pH was decreased below 6 the charge on the secondary emulsiondroplets was more negative than that on the primary emulsion droplets, whichindicatedpectin adsorption. Indeed, the primary emulsion droplets were cationicat low pH, whereas the secondary emulsion droplets were anionic. This may beimportant for designing delivery systems that have tunable charge characteristicsso that they can adsorb to specific charged sites, or to alter the mouthfeel ofdelivery systems.

The influence of pH on the mean particle diameter and creaming stability ofthe droplets in the primary and secondary emulsions is also shown in Figure 4.The primary emulsion was stable to flocculation and creaming at low and highpH due to the strong electrostatic repulsion between the droplets, but wasunstable to flocculation and creaming around the isoelectric point of theadsorbed proteinbecauseof the relatively low net charge on the droplets. On theother hand, the secondary emulsions were stable across the whole pH range,which can be attributed to the increased electrostatic and steric repulsionbetween the droplets, and the decreased van der Waals attraction [11, 18, 19].These results clearly show that the multilayer technique can be used to improvethe pH stability of protein-coated droplets, which may be useful in developingdelivery systems for lipophilic functional components thatcan be used in a widerrangeof products than is currently possible usingonly protein-coated droplets.

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19NaCl

Many food systems contain significant amounts of mineral ions in them,which can negatively impact the physical stability of emulsion-based deliverysystems. The influence of NaCl concentration (0 to 300 mM) on the meanparticle diameter and creaming stability of diluted primary and secondaryemulsions at pH 3.5 has been measured [24]. The primary emulsions wereunstable to droplet aggregation and creaming when the salt concentration was >50 mM (Figure 5), which can be attributed to screening of the electrostaticrepulsion between the droplets. On the other hand, the secondary emulsionswere relatively stable to droplet aggregation up to 200 mM NaCI (Figure 5),which can be attributed to the increased electrostatic and steric repulsion, anddecreased van der Waals attraction [11, 18, 19, 24]. These results show that themultilayer technique can be used to improve the salt stability of protein-coatedlipid droplets, thus extending the range of food matrices that this kind ofdelivery systemcould be used in.

Thermal ProcessingThe influence of thermal processing on the stability of primary (-lacto

globulin) and secondary (-lactoglobulin-carrageenan) emulsions has beenstudied at pH 6 [8, 10]. These emulsions were held isothermally at temperaturesranging form 30 to 90 C, cooled to room temperature, and then stored for 24hours. The -potential, mean particle diameter and creaming stability of theemulsions were then measured (data not shown). In the absence of added salt,therewas no significant change in the -potential or mean particle diameter ofthe secondary emulsions upon heating, and there was no evidence of creaming,which indicated that they were stable to thermal processing in the temperaturerange used. Nevertheless, at 150 mM NaCl , therewas evidence of desorption ofcarrageenan from the droplet surfaces at temperatures exceeding the thermaldenaturation of the adsorbed protein molecules [8, 10]. This suggested that theconformational change of the adsorbed globular protein caused by heatingweakened the attraction between the carrageenan and -lactoglobulinmoleculesleading to polysaccharide desorption. The emulsions where the carrageenanmolecules became detatched from the droplet surfaces were more unstable toflocculation after heating.

Freeze-Thaw CyclingPrimary (-lactoglobulin) and secondary (-lactoglobulin-pectin) emulsion

samples (2 wt% oil, pH 3.5) were transferred into cryogenic test tubes andincubated in a -20 C freezer for 22 hours. After incubation the emulsionsamples were thawed by incubating them in a water bath at 30 C for 2 hours.

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Figure 5. The salt-dependence of themeanparticle diameterand creamngstability of primary (-lactoglobulin) and secondary (-lactoglobulin-pectin)emulsions. Theprimary emulsion flocculates at lower saltconcentrationsthan

the secondary emulsion.

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Maltodextrin (wt%)Figure6. Impactof addedmaltodextrin on the creamng stability of primary(-lactoglobulin) and secondary (-lactoglobulin-pectin) emulsionsafterfreezing and thawing (3 cycles). Lessmaltodextrin is required to stabilize

the secondary emulsionthantheprimary emulsion.

This freeze-thaw cycle was repeated two times and its influence on emulsionstability (creaming index) was measured (Figure 6). The emulsions were allunstable to droplet aggregation in the absence of maltodextrin. We thereforecarried out experiments to determine the minimum amount of maltodextrinrequired to stabilize the primary and secondary emulsions against dropletaggregegation during freezing and thawing. We found thatsecondary emulsionscontaining pectin required only 1 wt% maltodextrin to stabilize them againstaggregation, whereas the primary emulsions required 4 wt% maltodextrin. Wealso found that there were differences between polysaccharides, with pectinbeingmore effective than carrageenan. Hence, the multilayer technique may beuseful for reducing the amount of sugars that are needed in frozen products toprotect lipid droplets against aggregation.

Freeze DryingWe have recently carried out preliminary experiments comparing the

stability of primary (-lactoglobulin) and secondary (-lactoglobulin-pectin)emulsions to freeze-drying at pH 3.5 [25]. Emulsion samples (30 mL) weretransferred into Petri dishes and frozen by placing them overnight in a -40 Cfreezer. A laboratory scale freeze-drying device (Virtis, the Virtis Company,Gardiner, N Y ) was used to dry the frozen emulsions. After finishing the dryingprocess the dried products were ground using a mechanical device (HandyChopper, Black & Decker Inc., Shelton, CT). The secondary emulsions had amuch better stability to droplet aggregation than the primary emulsions after

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22freeze-drying and reconstitution in buffer solution, especially when maltodextrinwas incorporated into the emulsions. For example, we found that secondaryemulsions containing pectin were stable to aggregation at maltodextrinconcentrations of 2 wt% and higher, whereasmore than 8 wt% maltodextrin wasrequired to stabilize primary emulsions. The multilayer technique may thereforeprove useful for increasing the lipid load of dehydrated emulsions that could beused as powdered delivery systems.

Lipid OxidationWe have recently compared the oxidative stability of primary (-lacto

globulin) and secondary (-lactoglobulin-pectin) emulsions at pH 3.5. Theprimary emulsion droplets were cationic, whereas the secondary emulsiondroplets were anionic, so we would have expected the primary emulsions tohave been more oxidatively stable due to repulsion of the positively charged ironions from the positively charged protein-coated droplets [26]. Nevertheless, wefound that secondary emulsions containing lipid drolets coated by citrus pectinactually had slightly better oxidative stability (data not shown). This suggeststhat the relatively thick polysaccharide layer may have been able to prevent theiron ions from reaching the lipid surfaces.

Other SystemsIn addition to the systems described above we have examined the suitability

of other types of emulsifier and biopolymer combinations for preparing stableoil-in-water emulsions containing droplets surrounded by multi-layeredinterfacial layers. We have shown that laminated lipid droplets can be formedusinga variety of different emulsifiers (lecithin, SDS, -lactoglobulin, caseinate)and biopolymers (chitosan, gelatin, pectin, carrageenan, alginate, gum arabic) [8,9, 11, 15-20, 27-31]. Recently we have used a similar technique to prepare"colloidosomes", which consist of large oil droplets surrounded by a layer ofsmall oil droplets [32].

Nano-laminated Coatingson MacroscopicObjectsApplicationsof Laminated EdibleCoatings

Potentially, the LbL deposition technology can also be used to formmultifunctional laminated coatings on macroscopic objects, such as fruit,vegetables, meat and fish. For example, there is currently a need for high-performance edible coatings for application on fresh-cut fruits & vegetables that

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23are capable of exhibiting a variety of different functions, e.g., control ofmoisture or gas migration; anti-microbial, anti-oxidant and anti-browningactivity; prevention of textural degradation; encapsulation of nutraceuticals,colors or flavors; controlled or triggered release of active components [33-36].Conventional technologies used by the food industry have limted scope forprecise engineering of novel functionalities into edible coatings. The LbLdeposition technique could be used to design and fabricate lamnated ediblecoatings with greatly improved functional properties. These coatings could becreated from food-grade ingredients (e.g.,proteins, polysaccharides and lipids)using simple and inexpensive processing operations (e.g., dipping, spraying andwashing). In addition, they could be designed to have a range of functionalattributes that are difficult to achieve using conventional coating methods, suchas selective permeability to water and other volatile components; encapsulationof active components (such as antioxidants, antimicrobials, anti-browningagents,colors, flavors or nutraceuticals); andtexturestabilization.

Formationof LaminatedEdibleCoatingsThe principle of using the LbL technique to coat macroscopic objects is

highlighted in Figure 7. The object to be coated is dipped sequentially into a seriesof solutions containing substances that adsorb to its surface. (Alternatively, thesolutions containing the adsorbing substancescould be sprayed onto the surface ofthe object). Between each dipping step it may be necessary to have a washingand/or dryingstep to remove the excess solution attached to the surface prior tointroduction of the object into the next dipping solution. The composition,thickness, structure and properties of the lamnated coating formed around theobject could be controlled in a number of ways, including: (i) changing the type ofadsorbing substances in the dipping solutions; (ii) changing the total number ofdipping steps used; (iii) changing the order that the object is introduced into thevarious dipping solutions; (iv) changing the solution and environmental conditionsused, such as pH, ionic strength, solvent, temperature, dipping time, stirring speedetc. The driving force for adsorption of asubstanceto a surface would depend onthe natureof the surface and the natureof the adsorbing substance, and could beelectrostatic, hydrogen bonding, hydrophobic interactions, etc. Nevertheless, themajor driving force utilized by the L bL deposition method is electrostaticattractionbetween electrically chargedsubstances.

In general, a variety of different adsorbing substances could be used tocreatethe different layers (Figure 8), including: Natural Polyelectrolytes. Any food-grade polyelectrolyte that is capable ofadsorbing to the exposed surface of the object could be used, such as

proteins (e.g., whey, casein, soy) or polysaccharides (e.g., pectin, alginate,xanthan, carrageenan, chitosan).

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Figure 8.Possible components thatcould beusedtoassemblemultilayered ediblefilmsor coatings

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26 Surface-Active Lipids. Any food-grade lipid that iscapable ofadsorbingto

the exposed surface of the object, e.g. phospholipidsandsmall moleculesurfactants. These surface active lipids could form single layers, bi-layers,multiple layers, micelles, vesiclesorother associationcolloidsat thesurface.

Lipid Droplets. Any food-grade lipid droplet that iscapable of adsorbingtothe exposed surface on the object. Theemulsion droplet would usuallyconsistof a liquid oil droplet coated by afood grade emulsifier,but itcouldalsobe apartly or fully crystallized oil droplet, or anoil droplet containingsmall water dropletsorother material.Thechoice of the type of adsorbing substances usedtocreateeach layer, the

total number of layers incorporated intotheoverall coating, thesequence of thedifferent layers, and thepreparation conditions used to prepare each layer wil ldetermne thefunctional performance of the final coatings: permeability(e.g., togasses, organic substances, minerals or water); mechanical properties (e.g.,rigidity, flexibility, brittleness); swelling and wetting characteristics;environmental sensitivity (e.g., to pH, ionic strength and temperature). Inaddition, theabove procedure enables one to encapsulate various hydrophilic,amphiphilic or lipophilic substances within the coatings, e.g., non-polarsubstances could be incorporated in micelles or lipid droplets, while polarsubstances could be incorporated in biopolymer layers. Thus, it would bepossible to incorporate active functional agents such as antimicrobials, anti-browning agents, antioxidants, enzymes, flavors, colors andnutraceuticals intothe coatings. These functional agentscould beusedto increasetheshelf-lifeandquality of thecoated fresh-cut fruit andvegetables. An example of apossiblemulti-component, multi-layered coatingisshown in Figure9.

Preliminary StudiesRecently, wehave carriedoutstudies that have shown that theelectrostatic

layer-by-layer technique can be used to form laminated coatings on planarhydrogels (agar-pectin) that were designed to mimic cut-fruit surfaces (whichcontain asignificant amount ofpectin) [37]. However, wehave also observedsome interesting physicochemical phenomena that need to beconsidered whenforming these coatings on macroscopic objects. An anionic hydrogel wasprepared in aPetri dish andthen cationic protein-coated droplets were broughtinto contact with thehydrogel for aspecified period. The contact emulsion wasthen removed and theplate waswashed with buffer solution (Figure 10). Theturbidity of the plates was measured tomonitor theadsorption of droplets to thehydrogel surfaces (Figure 11), and thecharge andsize of the droplets in thecontact emulsion removed from thehydrogel surfaces were measured (Figure12). Theturbidity measurements indicatedthat the lipid droplets did adsorb tothe droplet surfaces fairly rapidly, but the results were not what would be

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TimeFigure IL Turbidity of emulsionsover time.

expected for a simple adsorption process (e.g., a Langmuir isotherm). Forexample, we observed increases and then decreases in plate turbidity over time,and no clear dependence of plate turbidity on the initial droplet concentration inthe deposition emulsion. In addition, the droplets in the contact emulsionremoved from the hydrogel plates became increasingly negative and aggregatedover time, with the effect being more pronounced in the more dilute emulsions(Figure 12). We postulate that anionic pectin molecules diffused out of thehydrogels and adsorbed onto the surfaces of the cationic protein-coated droplets,which made them become more negative and aggregate. Indeed,spectrophotometry measurements indicatedthat the pectin molecules did diffuseout of the hydrogels over time. Thediffusion of biopolymers out of macroscopicsurfaces may be an important consideration when forming laminated coatings onfruits and vegetables.

In other preliminary studies, we have examined the impact of biopolymertype, pH, contact time, stirring speed, salt concentration and surfactants on theformation of edible coatings on hydrogel surfaces. For example, Figure 13shows the turbidity of anionic hydrogels (carrageenan/agar) when they havebeen brought into contact with emulsions containing whey protein coateddroplets at different pH values. The droplets only stick to the hydrogel surfaceswhen the droplets are positively charged (pH < pi), i.e., they have oppositecharges to the hydrogel surfaces. Recently, we have shown that laminatedcoatings (chitosan and/or eugenol droplets) can be formed on fruit (strawberryand cantaloupe) and vegetable (sweet potatoes) surfaces, and that theseprovideprotection against microbial growth.

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S" *

ax!p -40

0

03 wt%1.25 wt%5.0 wt%

5 10 15 20 25Time

0.3 wt%; wt%

0 5 10 15 20 25Time

Figure 12. -potential andmeanparticle diameter of dropletspresent in thecontact emulsion removedfrom the surfaces of hydrogels. The dropletsbecamemorenegativeover timeandaggregated, which can be attributed to diffusion

ofpectin out of the hydrogels andonto the droplet surfaces.

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Figure 13. Turbidityofanionic hydrogels(carrageenanJagar) in contact withemulsionscontaining wheyprotein coateddropletsatdifferentpH values.

ConclusionOur work so far has shown that stableemulsions containing multi-layered

lipid droplets can be prepared using a simple cost-effective method and foodgrade ingredients. These multilayered emulsions have better stability toenvironmental stresses than conventional emulsions under certain conditions.Moreresearch is still needed to establish, at a fundamental level, the factors thatinfluence the preparation of stable multilayered emulsions with specificfunctional attributes, including emulsifier characteristics (e.g., sign andmagnitude of droplet charge), biopolymer characteristics(e.g., molecular weight,charge density and flexibility), mixing conditions (e.g., order of addition,stirringspeed)and washing solution composition (e.g., ionic strength and pH).In addition, research needs to be carried out to establish where thesemultilayered emulsions can be practically used within the food industry asdelivery systems. We have also carried out preliminary experiments showingthat the L bL technique may be useful for coating macroscopic objects, such asmeat, ish, ruitandvegetables.

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Structural Design Principles for Improved Food Performance_Nanolaminated Biopolymer Structures in Foods

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