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Keywords:

  • bioactive lipids;
  • delivery systems;
  • emulsions;
  • functional foods;
  • nutraceuticals;
  • structural design

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Bioactive Lipids
  5. Desirable Characteristics of Delivery Systems
  6. Emulsion Science and Technology
  7. Delivery Systems Based on Emulsion Technology
  8. Conclusions
  9. Acknowledgments
  10. References

ABSTRACT:  There is a pressing need for edible delivery systems to encapsulate, protect, and release bioactive lipids within the food, medical, and pharmaceutical industries. The fact that these delivery systems must be edible puts constraints on the type of ingredients and processing operations that can be used to create them. Emulsion technology is particularly suited for the design and fabrication of delivery systems for encapsulating bioactive lipids. This review provides a brief overview of the major bioactive lipids that need to be delivered within the food industry (for example, ω-3 fatty acids, carotenoids, and phytosterols), highlighting the main challenges to their current incorporation into foods. We then provide an overview of a number of emulsion-based technologies that could be used as edible delivery systems by the food and other industries, including conventional emulsions, multiple emulsions, multilayer emulsions, solid lipid particles, and filled hydrogel particles. Each of these delivery systems could be produced from food-grade (GRAS) ingredients (for example, lipids, proteins, polysaccharides, surfactants, and minerals) using simple processing operations (for example, mixing, homogenizing, and thermal processing). For each type of delivery system, we describe its structure, preparation, advantages, limitations, and potential applications. This knowledge can be used to facilitate the selection of the most appropriate emulsion-based delivery system for specific applications.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Bioactive Lipids
  5. Desirable Characteristics of Delivery Systems
  6. Emulsion Science and Technology
  7. Delivery Systems Based on Emulsion Technology
  8. Conclusions
  9. Acknowledgments
  10. References

A number of industries need to deliver lipophilic functional components in an edible form, including the food, pharmaceutical, and medical industries. These lipophilic components may be a diverse range of substances, including bioactive lipids, flavors, antimicrobials, antioxidants, and drugs (Ubbink 2002; Shefer and Shefer 2003; Chen and others 2006; Ubbink and Kruger 2006). In this article, we will primarily focus on the delivery of bioactive lipids that provide specific health benefits to humans, such as ω-3 fatty acids, carotenoids, and phytosterols. Nevertheless, the same types of delivery system can also be used to encapsulate other types of lipophilic functional components. In many cases, it is advantageous to deliver bioactive lipids in an aqueous medium because this increases their palatability, desirability, and bioactivity. For example, a bioactive lipid might be incorporated into a beverage or food that could be consumed by drinking or eating. Nevertheless, there are often technical challenges that need to be overcome before a lipophilic component can be successfully incorporated within an aqueous-based delivery system.

Bioactive lipids differ widely in their molecular properties (for example, molecular weights, structures, functional groups, polarities, and charge), which leads to differences in their physicochemical and physiological properties (for example, solubility, physical state, rheology, optical properties, chemical stability, surface activity, and bioactivity). Consequently, delivery systems often have to be designed to address the specific molecular, physicochemical, and physiological concerns that are unique to each bioactive lipid. An edible delivery system for bioactive lipids must perform a number of different roles. First, it should serve as a means of encapsulating the bioactive lipid into a physical form that can conveniently be incorporated into foods or beverages. Second, the delivery system should be compatible with the particular food or beverage that it will be incorporated into; that is, it should not adversely affect the appearance, flavor, texture, or shelf life of the product. Third, it may have to protect the bioactive lipid from chemical degradation during preparation, storage, transport, and utilization so that it remains in its active state. Fourth, it may have to be capable of controlling the release of the functional agent at a certain rate, at a particular site, and/or in response to a specific environmental stimulus (for example, pH, ionic strength, or temperature). Finally, it should be prepared from food-grade ingredients using simple, cost-effective processing operations if it is going to be practically utilized by the food industry. A wide variety of different types of delivery system have been developed to encapsulate lipophilic functional agents, including simple solutions, association colloids, emulsions, biopolymer matrices, powders, and so on. Each type of delivery system has its own specific advantages and disadvantages for encapsulation, protection, and delivery of functional agents, as well as in its cost, regulatory status, ease of use, biodegradability, biocompatibility, and so on. In this review article, we focus on the various types of emulsion-based delivery systems that are available for delivering bioactive lipids due to the large interest within the food industry in the development of nutraceutical or functional foods.

Bioactive Lipids

  1. Top of page
  2. Abstract
  3. Introduction
  4. Bioactive Lipids
  5. Desirable Characteristics of Delivery Systems
  6. Emulsion Science and Technology
  7. Delivery Systems Based on Emulsion Technology
  8. Conclusions
  9. Acknowledgments
  10. References

The most important types of bioactive lipids that need to be delivered are briefly discussed below, along with a discussion of their physicochemical characteristics, and the current challenges to their application within functional foods (Table 1).

Table 1—.  Summary of major lipophilic nutraceutical components that need to be delivered into foods.
NameTypesPotential nutritional benefits
Fatty acidsω-3 Fatty acids, conjugated linoleic acid, butyric acidCoronary heart disease, bone health, immune response disorders, weight gain, stroke prevention, mental health, cancer, and visual acuity
Carotenoidsβ-Carotene, lycopene, lutein, and zeaxanthinCancer, coronary heart disease, macular degeneration, and cataracts
AntioxidantsTocopherols, flavonoids, polyphenolsCoronary heart disease, cancer, and urinary tract disease
PhytosterolsStigmasterol, β-sitosterol, and campesterolCoronary heart disease

Carotenoids

Carotenoids are a diverse group of lipophilic compounds that contribute to the yellow to red colors of many foods. They are polyenes consisting of 3 to 13 conjugated double bonds and in some cases 6 carbon ring structures at one or both ends of the molecule. Carotenoids containing oxygen are known as xanthophylls (for example, lutein and zeaxanthin) while those without oxygen are known as carotenes (for example, lycopene and β-carotene). The carotenoids have been proposed to exhibit several potential health benefits: lutein and zeaxanthin are thought to decrease age-related macular degeneration and cataracts (Stringham and Hammond 2005); lycopene is thought to decrease the risk of prostate cancer (Basu and Imrhan 2007). Endogenous carotenoids in foods are generally stable. However, as food additives, carotenoids are relatively unstable in food systems because they are susceptible to light, oxygen, and autooxidation (Xianquan and others 2005). Consequently, dispersion of carotenoids into ingredient systems can result in their rapid degradation (Heinonen and others 1997; Ribeiro and others 2003). Carotenoids can be degraded by reactions that cause the loss of double bonds or scission of the molecule. In addition, the double bonds in carotenoids can undergo isomerization to the cis configuration (Xianquan and others 2005). Isomerization reactions might actually be beneficial since cis isomers of carotenoids such as lycopene are thought to be more bioavailable and bioactive (Schieber and Carle 2005). An additional challenge to using carotenoids as ingredients in functional foods is their high melting point, making them crystalline at food storage and body temperatures.

Omega-3 fatty acids

Omega-3 (ω-3) fatty acids are unsaturated fatty acids that have a double bond that is 3 carbon atoms from the methyl end of the molecule. The most common ω-3 fatty acids are α-linolenic acid (ALA, 18:3), eicosapentaenoic acid (EPA, 20:5), and docosahexaenoic acid (DHA, 22:6). Of these three, the long chain ω-3 fatty acids EPA and DHA are the most bioactive. Since humans are not efficient at converting ALA to the long chain ω-3 fatty acids (Francois and others 2003; Ismail 2005), health benefits have been mainly attributed to dietary EPA and DHA (Hibbeln and others 2006). Omega-3 fatty acids can have a major impact on health because they have numerous physiological roles such as impacting cell membrane fluidity, cellular signaling, gene expression, and eicosanoid metabolism. The important physiological role of ω-3 fatty acids has been attributed to their ability to decrease the risks of cardiovascular disease, diseases affected by immune response disorders (for example, type 2 diabetes, inflammatory bowel diseases, and rheumatoid arthritis), and mental disorders, as well as benefit infant development (Cleland and others 1988; Belluzzi and others 1996; Fidler and others 2000; Makrides and Gibson 2000; Marchioli and others 2002; Giannuzzi and others 2006; Hibbeln and others 2006; Jensen 2006).

The growing list of disorders positively affected by dietary ω-3 fatty acids strongly suggests that large portions of the population would benefit from increased consumption of ω-3 fatty acids, making them an excellent candidate for incorporation into functional foods. However, numerous challenges exist in the production, transportation, and storage of ω-3 fatty acid fortified functional foods, since these lipids are extremely susceptible to oxidative deterioration. For example, DHA has been estimated to be over 50 times more susceptible to oxidation than oleic acid (Frankel 2005). In addition, the breakdown products of ω-3 fatty acid oxidation have very low sensory threshold values, meaning that they can be detected at very early stages of oxidation. Oxidation of ω-3 fatty acids is a complex chemical reaction that often requires multiple antioxidant hurdle technologies for adequate stabilization (McClements and Decker 2000). Encapsulation of omega-3 fatty acids has been found to be an excellent method for stabilization (Garg and others 2006).

Phytosterols

Phytosterols are a group of phytochemicals that include compounds such as stigmasterol, β-sitosterol, and campesterol. Plant stanols, which are found naturally at lower concentrations than sterols, can be produced by the hydrogenation of phytosterols. Phytosterols concentrations in vegetable oils range from 0.1% to 1.0% (Chaiyasit and others 2007) and typical phytosterol consumption is in the range of 200 to 400 mg/day. The production of phytosterol fortified foods has become popular due to the ability of phytosterols to decrease total and low-density lipoprotein cholesterol in humans by inhibiting the absorption of dietary cholesterol (Wong 2001; Ostlund 2004). Intake of 1.6 g phytosterols/day results in an approximately 10% reduction in LDL cholesterol (Hallikainen and others 2000). The intestinal absorption of phytosterols is very low so dietary phytosterols do not have adverse effects on health.

Incorporation of phytosterols into foods is difficult due to their high melting point and tendency to form insoluble crystals. This has been overcome by esterification of phytosterols to polyunsaturated fatty acids, which increase sterol solubility. Upon ingestion of phytosterols esters, lipases hydrolyze the fatty acid to produce free phytosterols. Phytosterols were originally added to high-fat foods (for example, margarine) where solublization and dispersion are relatively simple. For phytosterols to be introduced into aqueous-based foods, they need to be either suspended or emulsified. Phytosterols oxidation products have been observed in model systems, oils, and food products (Dutta 1997; Bortolomeazzi and others 2003; Lambelet and others 2003; Soupas and others 2004; Cercaci and others 2007). It is not clear whether oxidized phytosterols lose their bioactivity or are toxic in a manner similar to oxidized cholesterol. As with other bioactive lipids that are susceptible to oxidation, encapsulation of phytosterols could increase their oxidative stability.

Desirable Characteristics of Delivery Systems

  1. Top of page
  2. Abstract
  3. Introduction
  4. Bioactive Lipids
  5. Desirable Characteristics of Delivery Systems
  6. Emulsion Science and Technology
  7. Delivery Systems Based on Emulsion Technology
  8. Conclusions
  9. Acknowledgments
  10. References

There are a number of characteristics that an edible delivery system must have if it is going to be suitable for utilization by the food and other industries. Some of the most important attributes are listed below:

  • • 
    Food grade: The delivery system must be fabricated entirely from permitted food ingredients using processing operations that have regulatory approval. Some of the most common food-grade ingredients that can be used to assemble emulsion-based delivery systems are listed in Table 2.
  • • 
    Economic production: The delivery system should be capable of being economically manufactured from inexpensive ingredients. The benefits gained from encapsulating the bioactive lipid within a delivery system (for example, improved shelf life, enhanced marketability, novel functionality, better bioavailability) should outweigh the additional costs associated with encapsulation.
  • • 
    Protection against chemical degradation: The delivery system may have to protect an encapsulated bioactive lipid against some form of chemical degradation, for example, oxidation and hydrolysis. Knowledge of the mechanism of the chemical degradation reaction and the factors that impact it (for example, oxygen, pH, and heat) may facilitate the design of a more protective delivery system.
  • • 
    Loading capacity and retention: Ideally, a delivery system should be capable of encapsulating a relatively large amount of bioactive lipid per unit mass of carrier material, and should efficiently retain the encapsulated component until it needs to be delivered.
  • • 
    Delivery mechanism: The delivery system may have to be designed so that it releases the bioactive lipid at a particular site-of-action, at a controlled rate or in response to a specific environmental stimulus (for example, pH, ionic strength, enzyme activity, or temperature).
  • • 
    Food matrix compatibility: The delivery system should be compatible with the surrounding food matrix, that is, it should not adversely affect the appearance, texture, flavor, or stability of the final product.
  • • 
    Bioavailability/bioactivity: A delivery system should enhance (or at least not adversely affect) the bioavailability/bioactivity of the encapsulated component.
Table 2—.  Major food-grade structural components that can be used to construct delivery systems for bioactive components.
NameImportant characteristicsExamples
Lipids ChemicalNonpolarity stabilityAnimal fats: beef, pork, chicken Fish oils: cod liver, menhedan, salmon, tuna
Plant oils: palm, coconut, sunflower, safflower, corn, flax seed, soybean
Flavor oils: lemon, orange
SurfactantsSolubility (HLB)Non-ionic: Tween, Span
Head group chargeAnionic: SLS, DATEM, CITREM
Molecular geometryCationic: lauric arginate
Surface load at saturationZwitterionic: lecithin
BiopolymersMolar massGlobular proteins: whey, soy, egg
ConformationFlexible proteins: casein, gelatin
ChargeNonionic polysaccharides: starch, dextran, agar, galactomannans, cellulose
HydrophobicityAnionic polysaccharides: alginate, pectin, xanthan, carrageenan, gellan, gum arabic
FlexibilityCationic polysaccharides: chitosan

Emulsion Science and Technology

  1. Top of page
  2. Abstract
  3. Introduction
  4. Bioactive Lipids
  5. Desirable Characteristics of Delivery Systems
  6. Emulsion Science and Technology
  7. Delivery Systems Based on Emulsion Technology
  8. Conclusions
  9. Acknowledgments
  10. References

Generally, an emulsion consists of at least 2 immiscible liquids (usually oil and water, but not always), with one of the liquids being dispersed as small spherical droplets in the other (Dickinson and Stainsby 1982; Dickinson 1992; Friberg and others 2004; McClements 2005a). Typically, the mean diameter of the droplets within food systems is somewhere between 100 nm and 100 μm, but in some systems it may be either smaller or larger than these values. Emulsions can conveniently be classified according to the spatial organization of the oil and water phases. A system that consists of oil droplets dispersed in an aqueous phase is called an oil-in-water (O/W) emulsion, whereas a system that consists of water droplets dispersed in an oil phase is called a water-in-oil (W/O) emulsion. The substance that makes up the droplets in an emulsion is referred to as the dispersed phase, whereas the substance that makes up the surrounding liquid is called the continuous phase. In addition to the simple O/W or W/O emulsions described previously, it is also possible to prepare various types of multiple emulsions, for example, oil-in-water-in-oil (O/W/O) or water-in-oil-in-water (W/O/W) emulsions (Garti and Bisperink 1998; Benichou and others 2004; van der Graaf and others 2005). Moreover, it is possible to form “emulsions” based on the thermodynamic incompatibility of mixed biopolymer solutions, for example, water-in-water (W/W) or oil-in-water-in-water (O/W/W) emulsions (Norton and Frith 2001; Kim and others 2006; Norton and others 2006). Emulsions are thermodynamically unfavorable systems that tend to break down over time due to a variety of physicochemical mechanisms, including gravitational separation, flocculation, coalescence, and Ostwald ripening (Dickinson 1992; Friberg and others 2004; McClements 2005a). Consequently, they are different from microemulsions, which are thermodynamically stable systems. It is possible to form emulsions that are kinetically stable for a reasonable period of time by including substances known as stabilizers, for example, emulsifiers or texture modifiers. Emulsifiers are surface-active molecules that adsorb to the surface of freshly formed droplets during homogenization, forming a protective layer that prevents the droplets from aggregating. Texture modifiers thicken or gel the continuous phase, which improves emulsion stability by retarding or preventing droplet movement. Selection of the most appropriate stabilizer(s) is one of the most important factors determining the shelf-life and physicochemical properties of emulsion-based delivery systems. In this article, we will focus on the properties of emulsions whose continuous phase is aqueous (that is, O/W, W/O/W, W/W, or O/W/W), since these are commonly used as delivery systems for bioactive lipids in the food and other industries. In addition, we will focus primarily on liquid systems, rather than dried systems, and so will not consider microencapsulation by spray drying, which has been reviewed recently elsewhere (Desai and Park 2005; Madene and others 2006; Vega and Roos 2006).

Droplet characteristics

The bulk physicochemical properties of emulsions (such as rheology, optical properties, stability, molecular partitioning, and release characteristics) are highly dependent on the properties of the droplets that they contain (McClements 2005a). Some of the most important droplet characteristics that can be controlled in emulsions are listed below:

Droplet concentration The droplet concentration is usually expressed as the number, mass, or volume of droplets per unit volume or mass of emulsion (McClements 2005a). For example, the disperse phase volume fraction (ø) is the volume of droplets per unit volume of emulsion. The droplet concentration of an emulsion can usually be controlled by varying the proportions of the 2 immiscible liquids used to prepare it. Alternatively, an emulsion may be prepared with a particular droplet concentration and then be either diluted (for example, by adding continuous phase) or concentrated (for example, by gravitational separation, filtration, or centrifugation). Consequently, a manufacturer normally has good control over the droplet concentration of an emulsion-based delivery system.

Particle size distribution The particle size distribution (PSD) of an emulsion represents the fraction of particles in different size classes (McClements 2005a). It is typically represented as either a table or plot of particle concentration (for example, volume or number percent) compared with droplet size (for example, diameter or radius). It is often convenient to represent a PSD of an emulsion by a measure of the central tendency (for example, mean, median, or modal) and a measure of the width of the distribution (for example, standard deviation). The PSD of an emulsion can usually be controlled by varying homogenization conditions (for example, intensity or duration of energy input) or system composition (for example, the type and concentration of emulsifier used). Smaller droplets can usually be produced by increasing the intensity or duration of homogenization, or by increasing the concentration of emulsifier used (Walstra 1993; Schubert and Engel 2004).

Droplet charge The electrical properties of a droplet are usually characterized in terms of its surface electrical potential (Ψ0), surface charge density (σ), and/or ζ-potential (ζ) (Hunter 1986). The surface charge density is the number of unit electrical charges per unit surface area. The surface electrical potential is the free energy required to increase the surface charge density from zero to σ by bringing charges from an infinite distance to the surface through the surrounding medium. The surface electrical potential depends on the ionic composition of the surrounding medium due to electrostatic screening effects and usually decreases as the ionic strength of the aqueous phase increases. The zeta-potential (ζ) is the electrical potential at the “shear plane,” which is defined as the distance away from the droplet surface below which the counter-ions remain strongly attached to the droplet when it moves in an electrical field. Practically, the ζ-potential is often a better representation of the electrical characteristics of an emulsion droplet because it inherently accounts for the adsorption of any charged counter ions. In addition, the ζ-potential is much easier to measure than the electrical potential or the surface charge density, and therefore droplet charges are usually characterized in terms of ζ-potential (Hunter 1986). The electrical characteristics of emulsion droplets can be controlled by careful selection of particular emulsifier types. Droplets stabilized by non-ionic surfactants tend to only have a small droplet charge (for example, Tweens and Spans), those stabilized by anionic surfactants have a negative charge (for example, lecithin, DATEM, CITREM, fatty acids), those stabilized by polysaccharide emulsifiers tend to have a negative charge (for example, gum arabic, modified starch, and beet pectin), and those stabilized by proteins have a positive charge below the isoelectric point and negative charge above it (for example, whey protein, casein, soy proteins).

Interfacial characteristics The droplets in most emulsions are coated by a layer of adsorbed species in order to protect them from aggregation (for example, emulsifiers or biopolymers). The properties of the interfacial region are determined by the type, concentration, and interactions of any surface-active species present during homogenization, as well as by the events that occur before, during and after emulsion formation, for example, complexation, competitive adsorption, and layer-by-layer formation (Dickinson 2003). It is sometimes possible to control the characteristics of the interfacial layer, such as tension, charge, thickness, permeability, rheology, and environmental responsiveness, by altering system composition or processing conditions. Controlling the interfacial characteristics is one of the most powerful methods of designing delivery systems with specific functional performances. The interfacial characteristics of emulsion droplets can be controlled by selection of specific emulsifier types. For example, caseinates tend to form thick fluid-like interfacial layers, whereas whey proteins form thin elastic-like layers (Dickinson 1992; Dickinson 2003).

Physical state By definition, the droplets that make up the dispersed phase of an emulsion are liquid, but in some emulsion-like systems they are either partially or fully solidified, for example, solid lipid particles (Walstra 2003; Muller and Keck 2004; Wissing and others 2004; McClements 2005a). For example, the oil droplets in an oil-in-water emulsion can be made to crystallize by reducing the temperature sufficiently below the melting point of the oil phase. It should be noted that the crystallization temperature may be appreciably less in an emulsified fat than a bulk fat because of supercooling effects. In addition, the nature of the crystals formed by an emulsified fat may be different from those formed by a bulk fat because of curvature effects and the limited volume present in an individual emulsion droplet, for example, crystal structure, dimensions, and melting behavior (Muller and Keck 2004; Wissing and others 2004). The concentration, nature, and location of the fat crystals within the lipid droplets in an O/W emulsion can be controlled by careful selection of oil type (for example, solid fat content compared with temperature profile), thermal history (for example, temperature compared with time), emulsifier type, and droplet size (Muller and others 2000; Walstra 2003; Muller and Keck 2004).

Controlling droplet characteristics for improved performance Manipulation of the droplet concentration, particle size distribution, interfacial properties, and physical state can be carried out to create delivery systems with specific functional performances suitable for different types of lipophilic components and food matrices. For example, some bioactive lipids are susceptible to chemical degradation through oxidative reactions, for example, ω-3 fatty acids, CLA, and carotenoids. In these systems, it is important to protect the bioactive lipids from coming into contact with pro-oxidants such as transition metals. Previous studies have shown that this can be achieved by engineering the properties of the interfacial layer surrounding the lipid droplets, for example, by making it cationic so that it repels cationic transition metal ions or by increasing its thickness to form a steric boundary (McClements and Decker 2000). Another problem that has to be overcome for certain bioactive lipids is that they are crystalline at ambient temperatures, for example, phytosterols and carotenoids. Crystalline lipids often cause instability problems in traditional emulsions because they promote partial coalescence (Walstra 2003). It may be possible to restrict the degree of partial coalescence in an emulsion containing a crystalline bioactive lipid by increasing the strength of the repulsive colloidal interactions operating between the droplets, by increasing the thickness of the adsorbed layer to inhibit the penetration of crystals, or by crystallizing the carrier oil phase (Coupland 2002; Vanapalli and others 2002; Muller and Keck 2004; Thanasukarn and others 2004).

Physicochemical Properties of Emulsions

An emulsion-based delivery system should be compatible with the food matrix that it is going to be incorporated into; that is, it should not adversely affect the appearance, texture, stability, or taste. Some of the most important bulk physicochemical characteristics of emulsions are briefly outlined below:

Rheology Emulsions exhibit a wide variety of different rheological behaviors depending on their composition, structure, and droplet interactions: viscous liquids, viscoelastic liquids, viscoelastic solids, plastics, or elastic solids (Walstra 2003; McClements 2005a; Genovese and others 2007). For relatively dilute emulsions the rheology is normally characterized in terms of their apparent shear viscosity. The shear viscosity of an emulsion is largely determined by the continuous phase viscosity (ηC), the droplet concentration (φ), and the nature of the droplet–droplet interactions (w): η=ηC×f(φ,w) (McClements 2005a; Genovese and others 2007). Normally, the viscosity of an emulsion increases with increasing droplet concentration, gradually at first and then steeply as the droplets become more closely packed (Figure 1). Around and above the droplet concentration where close packing occurs (typically around 50% to 60% for a nonflocculated O/W emulsion), the emulsion exhibits solid-like characteristics such as visco-elasticity and plasticity (McClements 2005a). The droplet concentration where the steep increase in emulsion viscosity is observed depends on the nature of the droplet interactions in the system, decreasing for either strong attractive or strong repulsive interactions (McClements 2005a). The viscosity of an emulsion tends to increase when the droplets are flocculated because the effective particle concentration is increased due to the continuous phase trapped within the floc structure. In addition, shear thinning behavior is observed in flocculated emulsions because of deformation and breakdown of the floc structure as shear stresses increase. The impact of the droplet characteristics on the overall rheology of an emulsion may be an important consideration when designing a delivery system for a particular food application. Some food systems have a relatively low viscosity (such as beverages) and therefore the delivery system itself should not significantly increase the viscosity. Other food systems are highly viscous or gel-like (for example, dressings, dips, desserts) and in these cases the delivery system should not decrease the viscosity or disrupt the gel network.

image

Figure 1—. The optical properties, rheology, and creaming stability of emulsions are determined by their droplet concentration.

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Optical properties The most important optical properties of emulsions are their opacity and color, which can be quantitatively described using tristimulus color coordinates, such as the L*a*b* system (McClements 2005a). In this color system, L* represents the lightness, and a* and b* are color coordinates, where +a* is the red direction, –a* is the green direction; +b* is the yellow direction, –b* is the blue direction, low L* is dark, and high L* is light. The opacity of an emulsion can therefore by characterized by the lightness (L*), while the color intensity can be characterized by the chroma: C= (a*2+b*2)1/2. The color intensity is usually inversely related to the lightness; that is, when the lightness increases, the chroma decreases. The optical properties of emulsions are mainly determined by the relative refractive index, the droplet concentration, and the droplet size (McClements 2002a, 2002b, 2005a). The lightness of an emulsion tends to increase with increasing refractive index contrast and increasing droplet concentration, and has a maximum value at a particular droplet size. For O/W emulsions, the lightness increases steeply as the oil droplet concentration is increased from 0 to 5 wt%, but then increases more gradually at higher droplet concentrations (Figure 1). The impact of the droplet characteristics on the overall appearance of an emulsion may be an important consideration when designing a delivery system for a specific food application. Some food products are transparent or only slightly turbid (for example, soft drinks and fruit beverages) and so the delivery system should not cause a large increase in opacity. Other food products are optically opaque (for example, dressings, sauces, mayonnaise) and therefore the opacity of the delivery system may be less important.

Stability Emulsions are thermodynamically unfavorable systems that tend to break down over time due to a variety of physicochemical mechanisms, including gravitational separation, flocculation, coalescence, and Ostwald ripening (Dickinson 1992; Friberg and others 2004; McClements 2005a). Gravitational separation is one of the most common forms of instability in food emulsions, and may take the form of either creaming or sedimentation depending on the relative densities of the dispersed and continuous phases. Creaming is the upward movement of droplets due to the fact that they have a lower density than the surrounding liquid, whereas sedimentation is the downward movement of droplets due to the fact that they have a higher density than the surrounding liquid. Liquid edible oils normally have lower densities than liquid water and so creaming is more prevalent in oil-in-water emulsions, whereas sedimentation is more prevalent in water-in-oil emulsions. Nevertheless, this may not be the case in emulsions that contain fully or partially crystalline lipids because the density of oils usually increases when crystallization occurs.

The rate at which gravitational separation occurs decreases with increasing droplet concentration because the movement of a droplet is hindered by the presence of the surrounding droplets. At sufficiently high droplet concentrations gravitational separation may be completely retarded (Figure 1), as it is in mayonnaise.

Molecular distribution and release characteristics When a delivery system is incorporated into a food matrix there may be a redistribution of the various types of molecule present among the different phases (for example, oil, water, and interfacial phases), which is governed by their equilibrium partition coefficients and the kinetics of molecular motion (McClements 2005a). For example, if an emulsion-based delivery system is incorporated into a fatty food, then some of the bioactive lipid may move from the oil droplets in the delivery system to the fat phase in the food. This molecular transfer process may have an adverse, neutral, or beneficial impact on the functionality of the bioactive lipid within the system, and should be considered when designing delivery systems for specific food matrices. Another important physicochemical property of emulsion-based delivery systems is their ability to release encapsulated materials. In particular, it is important to establish any potential trigger mechanisms for release (for example, pH, ionic strength, temperature, and enzymes), as well as the rate and extent of release. In an emulsion, release is usually characterized in terms of the increase in concentration of the bioactive compound in the continuous phase (or in some target material such as the mouth, stomach, or gastrointestinal tract) as a function of time. A number of parameters can be derived from such curves, such as the area under the curve (AUC), the maximal concentration released (Cmax), and the time to reach the maximum concentration (tmax) (Aguilera 2006). The release rate of encapsulated components from within emulsions depends on many factors, including their equilibrium partition coefficients, their original location, the mass transfer coefficients of the components in the different phases, mechanical agitation, and the microstructure of the system, for example, droplet size, layer thickness (McClements 2005a). Consequently, it is possible to structurally design emulsion-based systems that are capable of controlling the release of encapsulated components by selecting appropriate ingredients and microstructures (Lian and others 2004).

Implications for design of delivery systems The colloidal properties of emulsions (droplet size, concentration, charge, interactions, and physical state) determine their physicochemical properties (optical properties, rheology, molecular partitioning, and stability), which in turn determine the overall quality attributes of food products (appearance, texture, mouthfeel, flavor, and shelf-life) (McClements 2005a). Consequently, it is important when designing an emulsion-based delivery system to ensure that it does not adversely impact the quality attributes of the food product that it is going to be utilized in. For example, some food products are required to have a low overall viscosity yet contain particulate matter that is stable to gravitational separation (for example, beverage emulsions), so that an emulsion-based delivery system used in these products should contain small droplets that do not cream or sediment during the shelf-life of the product and that do not appreciably increase product viscosity. On the other hand, other food products are highly viscous or gelled (for example, dressings, dips, sauces, and desserts) so that the size of the droplets in the delivery system may be less crucial since gravitational separation is not a major problem. An important factor to be aware of when designing a delivery system is that changing the droplet characteristics to achieve one goal (for example, reducing gravitational separation), may have an impact on all the other quality attributes of the final product (for example, appearance, rheology, and flavor) (McClements and Demetriades 1998). Consequently, one should take an integrated approach where the emulsion-based delivery system is carefully designed to meet all the required quality specifications.

Delivery Systems Based on Emulsion Technology

  1. Top of page
  2. Abstract
  3. Introduction
  4. Bioactive Lipids
  5. Desirable Characteristics of Delivery Systems
  6. Emulsion Science and Technology
  7. Delivery Systems Based on Emulsion Technology
  8. Conclusions
  9. Acknowledgments
  10. References

In this section, we provide a brief overview of the major kinds of delivery systems that can be assembled from food-grade components based on the principles of emulsion technology. We will only focus on emulsion-based delivery systems that are dispersible in aqueous solutions, that is, those in which water is the continuous phase. In addition, we will only consider emulsion systems (thermodynamically unstable), and not microemulsion systems (thermodynamically stable).

Conventional emulsions

Structure Conventional oil-in-water (O/W) emulsions consist of oil droplets dispersed in an aqueous continuous phase, with the oil droplets being surrounded by a thin interfacial layer consisting of emulsifier molecules (Figure 2) (Dickinson 1992; Friberg and others 2004; McClements 2005a). The concentration and particle size distribution of the oil droplets in O/W emulsions can be controlled, as can the nature of the emulsifier used to stabilize the system. The oil droplets in most food emulsions typically have diameters somewhere between 0.1 and 100 μm, although larger and smaller droplets are possible in certain applications. The interfacial layer is typically between about 1 and 10 nm thick for food-grade emulsifiers (for example, surfactants, phospholipids, proteins, or polysaccharides), but it may be appreciably thicker if biopolymer multilayers are formed around the droplets (Guzey and McClements 2007). The electrical charge on the droplets can be controlled by selecting an appropriately charged emulsifier, which may be positive, noncharged, or negative.

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Figure 2—. Different types of delivery systems that can be created based on emulsion technology.

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Preparation Conventionally, oil-in-water (O/W) emulsions are prepared by homogenizing an oil phase and an aqueous phase together in the presence of a water-soluble emulsifier (Figure 3). A variety of different homogenizers are available, including high shear mixers, high-pressure homogenizers, colloid mills, ultrasonic homogenizers, and membrane homogenizers (Walstra 1993, 2003; McClements 2005a). The choice of a particular kind of homogenizer and of the operating conditions used depends on the characteristics of the materials being homogenized (for example, viscosity, interfacial tension, shear sensitivity) and of the required final properties of the emulsion (for example, droplet concentration, droplet size, and viscosity). For example, the size of the droplets in an O/W emulsion produced by a high-pressure homogenizer can usually be decreased by increasing the homogenizer pressure or number of passes. A bioactive lipid would normally be dispersed in the oil phase prior to homogenization with the water phase. If the bioactive lipid was crystalline (for example, phytosterols or carotenoids), then it may be necessary to ensure that it is used at a level below its saturation concentration in the carrier oil, or to warm the lipid phase prior to homogenization to melt any crystals present (since fat crystals can cause fouling of homogenizers). If the bioactive lipid was susceptible to chemical degradation (for example, ω-3 fatty acids, CLA, and carotenoids), then it may be necessary to carefully control homogenization conditions to avoid exposure to factors that increase the degradation rate, for example, high temperatures, oxygen, light, or transition metals.

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Figure 3—. Schematic representation of the formation of conventional and multilayer emulsions. Conventional O/W emulsions are formed by homogenizing oil and aqueous phases together in the presence of a water-soluble emulsifier. Multilayer emulsions are formed by adding polyelectrolytes to an emulsion containing oppositely charged droplets so that they adsorb and form a nano-laminated coating.

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Advantages and limitations Oil-in-water emulsions have a number of potential advantages as delivery systems for bioactive components. First, they contain a nonpolar region (the oil phase), a polar region (the aqueous phase), and an amphiphilic region (the interfacial layer). It is therefore possible to incorporate bioactive agents that are polar, nonpolar, and amphiphilic within the same delivery system. For example, a lipophilic bioactive component could be dispersed in the oil phase prior to homogenization, while a hydrophilic functional component could be incorporated in the aqueous phase either before or after homogenization (McClements 2005a). Second, the heterogeneous structure of emulsions means that it is possible to develop novel strategies for controlling the chemical stability of encapsulated components, for example, by engineering the interface or controlling the physical location of different reactants (Coupland and McClements 1996; McClements and Decker 2000). Third, by controlling composition and microstructure it is possible to create emulsions with different rheological properties (ranging from viscous liquids, to plastic pastes, to elastic solids), so that they can be prepared in a form that is most convenient for each specific application (Genovese and others 2007). Fourth, emulsions can either be used directly in their “wet” state or they can be dried to form powders, which may facilitate their transport and utilization in some applications (Soottitantawat and others 2003; Desai and Park 2005; Klinkesorn and others 2005c; Vega and Roos 2006). Finally, emulsions can be created entirely from food-grade ingredients (such as water, oil, surfactants, phospholipids, proteins, and polysaccharides) using fairly simple processing operations (mixing and homogenization).

Conventional emulsions should usually be the 1st system considered when one is thinking of using an emulsion-based system for delivery of bioactive lipids because of their relative ease of preparation and low cost, compared to the more sophisticated systems discussed below. Nevertheless, there are also certain limitations of conventional emulsions that may mean that more sophisticated systems are needed for particular applications. Conventional emulsions are often prone to physical instability when exposed to environmental stresses such as heating, chilling, freezing, drying, pH extremes, and high mineral concentrations. In addition, one often has limited control over the ability to protect and control the release of functional components because the small size of the droplets (approximately μm) and the interfacial layers (approximately nm) means that the time scales for molecular diffusion of substances are extremely short. Finally, there are a limited number of emulsifiers that can be used to form the interfacial layers that surround the oil droplets, which limits one's ability to create delivery systems exhibiting a wide range of protection and release characteristics. Consequently, there are continuing attempts to identify novel emulsifiers or for developing new approaches to utilizing existing emulsifiers more effectively to facilitate emulsion formation and improve emulsion stability (Benichou and others 2002; Dickinson 2003; Drusch and Schwarz 2006; Drusch 2007).

Applications Conventional oil-in-water emulsions have been used to encapsulate and deliver a variety of different bioactive lipids. O/W emulsions have been used as a delivery system for encapsulating ω-3 fatty acids and incorporating them into food products such as milk, yogurts, ice cream, and meat patties (McClements and Decker 2000; Chee and others 2005, 2007;Sharma 2005; Lee and others 2005, 2006a, 2006b). These emulsion-based delivery systems were specifically designed to prevent oxidation of the polyunsaturated fats within the lipid droplets (McClements and Decker 2000). The emulsions were prepared at acidic pH using a dairy protein as emulsifier so that the lipid droplets were cationic and therefore repelled cationic transition metal ions (Fe2+) that normally catalyze lipid oxidation. In addition, a chelating agent (EDTA) was added to the aqueous phase to sequester the transition metal ions, thereby preventing them from coming into contact with the lipid phase and promoting oxidation. Finally, antioxidants that partition into the interior or surface of lipid droplets were selected so that they were located at the site where the lipid oxidation reactions occurred, thereby increasing their effectiveness. These studies showed that emulsion-based delivery systems are particularly useful vehicles for encapsulating and protecting polyunsaturated lipids during storage, as well as offering a convenient means of directly incorporating these bioactive lipids into aqueous based foods. Conventional O/W emulsions have also been used to incorporate various other types of bioactive lipids, such as lycopene (Tyssandier and others 2001; Ribeiro and others 2006), astaxanthin (Ribeiro and others 2005, 2006), lutein (Losso and others 2005; Santipanichwong and Suphantharika 2007), β-carotene (Santipanichwong and Suphantharika 2007), plant sterols (Sharma 2005), and conjugated linoleic acids (CLA) (Jimenez and others 2004). Many of these bioactive lipids are crystalline in their pure form at ambient temperatures, which potentially leads to problems with emulsion formation and/or stability. For this reason, these bioactive lipids are either used at levels below their critical saturation concentration in the carrier oil, or the lipid phase is melted prior to homogenization. For example, emulsion-based delivery systems have been prepared containing lycopene, which is crystalline at ambient temperatures (melting point = 173 °C) (Ribeiro and others 2003). To prepare stable emulsions it was necessary to disperse the lycopene crystals into carrier oil (medium chain fatty acid triacylglycerols) and then heat it to a temperature where the crystals melted (approximately 140 to 210 °C). The hot oil phase was then homogenized with a hot aqueous phase containing a water-soluble emulsifier to form an oil-in-water emulsion. Lycopene is highly susceptible to chemical degradation and therefore it was necessary to minimize the time that the oil phase spent at the elevated temperatures and to reduce the oxygen content in the system during homogenization. The chemical stability of the emulsified lycopene was studied after it had been incorporated into different food matrices: milk, orange juice, and water (Ribeiro and others 2003). It was found that the lycopene was more stable in orange juice than in milk or water, but that its stability could be improved considerably when α-tocopherol was added as an antioxidant. One of the advantages of using emulsions as delivery systems for bioactive lipids is that they can usually be converted into a powdered form by spray drying, which may increase their long-term stability, facilitate their transport, and improve their ease of utilization (Beristain and others 2001; Soottitantawat and others 2003, 2005).

Multiple emulsions

Structure Water-in-oil-in-water (W/O/W) emulsions consist of small water droplets contained within larger oil droplets that are dispersed in an aqueous continuous phase (Garti 1997a, 1997b; Garti and Bisperink 1998; Garti and Benichou 2004) (Figure 2). Water-in-oil-in-water emulsions are more accurately designated as W1/O/W2 emulsions, where W1 is the inner water phase and W2 is the outer water phase (which usually have different compositions). There are 2 different interfacial layers in this type of emulsion: the W1-O layer surrounding the inner water droplets, and the O-W2 layer surrounding the oil droplets. Consequently, 2 different types of emulsifier are usually needed to stabilize W/O/W emulsions: an oil-soluble emulsifier for the inner water droplets and a water-soluble emulsifier for the oil droplets. In this system, it is possible to control the particle size distribution and concentration of both the inner water droplets and the oil droplets, as well as the interfacial properties of the W1-O and O-W2 layers surrounding the droplets (for example, thickness, charge, permeability, and environmental responsiveness).

Preparation Multiple emulsions of this kind are normally produced using a 2-step procedure (Figure 4): (1) a W1/O emulsion is produced by homogenizing water, oil, and an oil-soluble emulsifier; (2) a W1/O/W2 emulsion is then produced by homogenizing the W1/O emulsion with an aqueous solution containing a water-soluble emulsifier. Similar kinds of homogenizers can be used to produce W1/O/W2 emulsions as to produce O/W emulsions, for example, high shear mixers, high-pressure homogenizers, colloid mills, ultrasonic homogenizers, and membrane homogenizers (McClements 2005a). Nevertheless, the homogenization conditions used in the 2nd stage are often less intense than those used in the 1st stage, so as to avoid disruption or expulsion of the W1 droplets within the oil phase. The size of the water droplets in the W1/O emulsion can be controlled by varying emulsifier type, emulsifier concentration, and homogenization conditions (for example, energy intensity and duration) in the 1st stage. Similarly, the size of the oil droplets in the final W1/O/W2 emulsion can be controlled by varying emulsifier type, emulsifier concentration, and homogenization conditions in the 2nd stage. The concentration of water droplets in the W1/O emulsion can be controlled by using a different ratio of W1 to oil phase in the 1st homogenization step, whereas the concentration of oil droplets in the final W1/O/W2 emulsion can be controlled by using a different ratio of W1/O emulsion to W2 phase in the 2nd homogenization step. Consequently, one has great scope for creating structured delivery systems with different compositions and microstructures based on multiple emulsions.

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Figure 4—. Schematic representation of the formation of W/O/W emulsions. First, a W/O emulsion is formed by homogenizing an oil and aqueous phase together in the presence of an oil-soluble emulsifier. Second, a W/O/W emulsion is formed by homogenizing the W/O emulsion with an aqueous phase containing a water-soluble emulsifier.

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Bioactive components can potentially be located in a number of different molecular and physical environments within a W1/O/W2 emulsion. Hydrophilic components can be incorporated into the inner water phase by dispersing them in the W1 phase prior to the 1st homogenization step, or in the outer water phase by dispersing them in the W2 phase either before or after the 2nd homogenization step. Lipophilic components can be incorporated into the oil droplets by dispersing them in the oil phase either before or after the 1st homogenization step. Surface-active components could be located at either the W1/O or O/W2 interfaces depending on when they were incorporated during the emulsion preparation procedure.

Advantages and limitations Potentially, W/O/W emulsions have some advantages over conventional O/W emulsions as delivery systems for bioactive lipids, although they are normally more suitable for encapsulation, protection, and release of hydrophilic components. Hydrophilic bioactive components (for example, minerals, vitamins, enzymes, proteins, bioactive peptides, fibers) could be trapped within the internal water phase, which may have benefits in a number of applications. For example, (1) functional components could be trapped inside the inner water droplets and released at a controlled rate or in response to specific environmental stimuli, for example, in the mouth, stomach, or small intestine; (2) functional components could be protected from chemical degradation by isolating them from other water-soluble ingredients that they might normally react with. Another potential advantage of W/O/W systems is that the overall fat content of food products that normally exist as oil-in-water (O/W) emulsions (for example, dressings, mayonnaise, dips, sauces, desserts) could be reduced by loading the oil phase with water droplets. A W/O/W emulsion could be produced that had the same overall dispersed phase volume fraction and droplet size distribution as a conventional O/W emulsion, but with a reduced fat content. Consequently, it should be possible to produce reduced-fat products with similar physicochemical and sensory properties as full-fat products, for example, appearance, texture, mouthfeel, and flavor. Foods based on W/O/W emulsions may therefore be particularly useful for developing delivery systems for health-promoting bioactive lipids.

There have been numerous articles highlighting the great potential of multiple emulsions as edible delivery systems (Garti 1997b; Garti and Bisperink 1998; Benichou and others 2004). Despite this potential there are few examples of multiple emulsions actually being used in food products at present. The main reason for this is that multiple emulsions are highly susceptible to breakdown during storage or when exposed to environmental stresses commonly used in the food industry, such as mechanical forces, thermal processing, chilling, freezing, and dehydration. A variety of instability mechanisms are responsible for W/O/W emulsion breakdown, with some of these being similar to those in conventional O/W emulsions and some being unique to multiple emulsions. The oil droplets in W/O/W emulsions are susceptible to creaming, flocculation, coalescence, and Ostwald ripening just as they are in O/W emulsions. The inner water droplets in W/O/W emulsions are also susceptible to conventional flocculation, coalescence, and Ostwald ripening processes; however, they may also become unstable due to diffusion of water molecules between the inner and outer aqueous phases or due to the expulsion of whole water droplets from the oil droplets. A variety of different strategies have been developed in an attempt to overcome these problems (Garti 1997b; Garti and Bisperink 1998; Benichou and others 2004; Garti and Benichou 2004), including identification of appropriate combinations of oil and water soluble emulsifiers, incorporation of biopolymers into the W1 phase, solidification of the oil phase, and osmotic balancing of the inner and outer water phases to prevent water diffusion (Garti 1997b; Garti and Bisperink 1998; Benichou and others 2004; Garti and Benichou 2004).

It is clear that multiple emulsions do have potential as delivery systems for bioactive lipids, but that there are still a number of important challenges that need to be addressed before this technology can be successfully employed within the food industry. In particular, it is important that they can be produced from all food-grade ingredients using simple and economic processing operations, and that they have the physical stability to resist the stresses typically encountered in foods, for example, mechanical stresses, heating, chilling, freezing, and drying.

Applications As mentioned previously, there are few examples of multiple emulsions actually being utilized in the food industry to encapsulate lipophilic functional compounds. Some of the few examples that the authors were able to find where W/O/W emulsions have been used to encapsulate lipophilic bioactive components are β-carotene (Rodriguez-Huezo and others 2004) and ω-3 fatty acids (Onuki and others 2003; Cournarie and others 2004). These emulsions were prepared using the 2-step method discussed previously; that is, initially a W/O emulsion was formed by homogenizing a water phase with an oil phase containing the bioactive lipid and an oil-soluble emulsifier, and then a W/O/W emulsion was formed by homogenizing the W/O emulsion with a water phase containing a water-soluble emulsifier. There are many more examples of W/O/W emulsions being used to encapsulate hydrophilic bioactive components such as vitamin B (Owusu and others 1992; Fechner and others 2007; Kukizaki and Goto 2007), immunoglobulins (Chen and others 1999; Lee and others 2004), insulin (Cournarie and others 2004), proteins (Su and others 2006), and amino acids (Owusu and others 1992; Weiss and others 2005). The main challenges with the commercial utilization of multiple emulsions have been to make products that have a sufficiently long-shelf life for utilization within the food industry, and which are capable of withstanding the fairly harsh processing operations mentioned previously. In conclusion, W/O/W emulsions are most suitable for encapsulation of hydrophilic bioactive components, but they may be useful if one wants to prepare a delivery system that contains both lipophilic and hydrophilic bioactive components in the same system (Cournarie and others 2004).

Multilayer emulsions

Structure Multilayer oil-in-water (M-O/W) emulsions consist of small oil droplets dispersed in an aqueous medium, with each oil droplet being surrounded by a nano-laminated interfacial layer, which usually consists of emulsifier and biopolymer molecules (Figure 2 and 3). The particle size distribution and concentration of the oil droplets in M-O/W can be controlled, as can the characteristics of the interfacial layer surrounding the droplets, for example, composition, thickness, charge, permeability, and environmental responsiveness.

Preparation The principle behind the formation of multilayer emulsions is shown schematically in Figure 3 (McClements 2005b). Initially, an oil-in-water emulsion is prepared by homogenizing an oil and aqueous phase together in the presence of an ionized water-soluble emulsifier. The resulting “primary” emulsion consists of small charged oil droplets dispersed in an aqueous continuous phase. An oppositely charged polyelectrolyte is then added to the system so that it adsorbs to the droplet surfaces and produces a “secondary” emulsion consisting of oil droplets coated by a 2-layer emulsifier-polyelectrolyte interface. This procedure can be repeated to form oil droplets coated by nano-laminated interfaces containing 3 or more layers. It should be noted that each polyelectrolyte layer can be deposited onto the droplet surfaces using either a 1- or 2-step mixing procedure:

1. One-step mixing. An oil-in-water emulsion containing electrically charged droplets is prepared, and it is then mixed with an oppositely charged polyelectrolyte that directly adsorbs to the droplet surfaces through electrostatic attraction.

2. Two-step mixing. An oil-in-water emulsion is prepared containing a polyelectrolyte at a pH where there is not a strong electrostatic attraction between the droplets and the polyelectrolyte. The pH of the solution is then varied to change the electrical charge on the droplets and/or polyelectrolyte so that the polyelectrolyte adsorbs to the droplet surfaces through electrostatic attraction.

Studies have shown that 2-step mixing often produces more stable multilayer emulsions than 1-step mixing (Guzey and others 2004). It should also be noted that a washing step may be required between each electrostatic deposition step in order to remove any excess non-adsorbed polyelectrolyte remaining in the continuous phase. This washing step can be conveniently achieved by centrifugation or filtration. Alternatively, the solution conditions can be optimized so that there is little or no free polyelectrolyte remaining in the aqueous phase after electrostatic deposition. In some cases, it is possible to breakdown any flocs formed during the preparation of multilayer emulsions by applying mechanical agitation to the system, such as sonication, blending, or homogenization.

One of the major advantages of using multilayer emulsions as delivery systems is that the properties of the interfacial layer surrounding the oil droplets can be carefully controlled, for example, composition, structure, charge, thickness, permeability, rheology, and environmental responsiveness (Decher and Schlenoff 2003). This can be achieved by careful control of system composition and preparation conditions during the production of the multilayer emulsions, for example, emulsifier type and concentration, polyelectrolyte type and concentration, pH, ionic strength, order of ingredient addition, and mixing conditions (Guzey and McClements 2006). The ability to systematically control interfacial properties in a rational manner enables one to design droplets with improved stability or novel functional performance (see below).

In multilayer emulsions, bioactive components could be trapped within the oil droplets or within the nano-laminated layer surrounding the droplets. For example, a bioactive lipid could be incorporated into the oil phase prior to homogenization, whereas a charged hydrophilic component could be incorporated into one or more of the polyelectrolyte layers surrounding the oil droplets. The functional components could then be retained within the delivery system until they were released at the site of action in response to a specific environmental stimulus such as pH, ionic strength, or temperature. This can be achieved by designing the interfacial layer so that its permeability or integrity change in a well-defined way in response to the environmental trigger (Ogawa and others 2003a; Gu and others 2006; Guzey and McClements 2007).

Advantages and limitations Some of the potential advantages of multilayer emulsions over conventional emulsions for application as delivery systems are listed below:

It should also be noted that the interfacial engineering technology used to produce multilayer emulsions utilizes food-grade ingredients (such as surfactants, proteins, polysaccharides, and phospholipids) and processing operations that are already widely used in the manufacture of food emulsions (such as homogenization and mixing). Nevertheless, it should be stressed that the formation of stable multilayer emulsions requires careful control over the system composition and preparation procedures in order to avoid droplet aggregation through bridging, depletion, and other effects (Ogawa and others 2003b; Guzey and others 2004; Aoki and others 2005; McClements 2005b). The main limitations of this method are that additional ingredients and processing steps are required over conventional emulsion formation, and that at present it is only possible to directly prepare relatively dilute emulsions using this method (< 5 wt%) because of the tendency for flocculation to occur.

More recently, it has been shown that a similar approach can be used to form “colloidosomes,” which consist of large lipid droplets surrounded by a layer of smaller lipid droplets (Gu and others 2007). These systems may also be useful for the encapsulation and release of bioactive lipids, for example, one or more component could be dispersed within either the inner or outer droplets.

Applications So far, the potential of multilayer emulsions as delivery systems for bioactive lipids has only been demonstrated in the laboratory for ω-3 fatty acids (Klinkesorn and others 2005a, 2005b, 2005c). In these studies, primary emulsions were prepared by homogenizing fish oil with an aqueous phase containing anionic lecithin (pH 3). Secondary emulsions were then prepared by mixing the primary emulsion (which contained anionic droplets) with a chitosan solution (which contained cationic polymers). The preparation conditions had to be carefully controlled to avoid bridging and depletion flocculation in the emulsions, for example, droplet concentration and size, chitosan concentration, and stirring. The primary emulsions contained lipid droplets coated by an anionic lecithin layer, whereas the secondary emulsions contained lipid droplets coated by a cationic lecithin–chitosan layer. The addition of chitosan to the emulsions therefore changed the droplet charge from negative to positive, and increased the thickness of the interfacial coating surrounding the droplets. Lipid droplets coated by the lecithin–chitosan coating had better oxidative stability than those coated by the lecithin only coating, which was attributed to the thick cationic lecithin–chitosan coatings being able to prevent transition metals ions from coming into close contact with the emulsified ω-3 fatty acids (Klinkesorn and others 2005b). It was also shown that these systems could be converted into a powdered form by spray drying, which would facilitate their utilization in some food products (Klinkesorn and others 2005c, 2006). An additional advantage of the multilayer emulsions is that the amount of wall material (carbohydrates) required to form stable powders is considerably less when the lipid droplets have a polysaccharide coating around them (unpublished work). Multilayer emulsions may also be suitable for encapsulating and protecting other kinds of bioactive lipids in food systems. For example, the tendency for partially crystalline lipid droplets to aggregate due to partial coalescence could be prevented by building a thick biopolymer layer around them (Walstra 2003). Consequently, the electrostatic deposition technology may prove particularly useful for improving the stability of emulsions in which the bioactive lipids have a tendency to crystallize, for example, carotenoids or phytosterols. The major advantage of the multilayer technology is that interfacial layers with specific physicochemical properties (for example, thickness, charge, permeability, composition) can be rationally designed to achieve particular functional performances (for example, antioxidant properties, stability to crystallization).

Solid lipid particles

Structure In some aspects, solid lipid particle (SLP) emulsions are similar to conventional emulsions consisting of emulsifier-coated lipid droplets dispersed in an aqueous continuous phase (Figure 2). However, the lipid phase is either fully or partially solidified, and the morphology and packing of the crystals within the lipid phase are usually controlled (Figure 5) to obtain particular functional attributes (Wissing and Muller 2002; Souto and others 2004; Uner and others 2004; Wissing and others 2004; Saupe and others 2005). As with conventional emulsions, the size and concentration of the droplets can be controlled, as well as the nature of the interfacial coating surrounding the lipid phase.

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Figure 5—. Different types of solid lipid particles that can be formed by controlling the crystallization of the lipids within O/W emulsions.

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Preparation Emulsions containing SLP are usually formed by homogenizing an oil and water phase together in the presence of a hydrophilic emulsifier at a temperature above the melting point of the lipid phase (Wissing and Muller 2002; Souto and others 2004; Uner and others 2004; Wissing and others 2004; Saupe and others 2005; Schubert and Muller-Goymann 2005). The emulsion is then cooled so that some or all of the lipids within the droplets crystallize. It is of key importance that the temperature of the emulsion remains substantially above the crystallization temperature of the highest melting lipid to prevent any fat solidification during homogenization. The stability of the droplets produced and the spatial organization of the lipid crystals within the droplets are usually controlled by careful selection of the number and type of lipids present, the nature of the surfactant(s) used to stabilize the droplets, the initial droplet size and concentration, and the cooling conditions (Muller and others 2000; Muller and Keck 2004). In principle, it is possible to create a variety of different internal structures within solid lipid particles, for example, homogeneous, core-shell, or crystal dispersions (Figure 5). In addition, it is possible to control the relative location of the different phases within the droplets; for example, the core could be solid and the shell liquid, or vice versa. The structure and spatial organization of the fat crystals within the lipid droplets has important consequences for designing SLP systems for encapsulating, protecting, and delivering lipophilic components. For example, in some applications it may be beneficial to locate the lipophilic component within the core (away from any hydrophilic reactive species), whereas in other situations it may be more important to locate it close to the droplet surface (to enhance its release). Typically, 2 or more lipids with different melting profiles are used to create specific microstructures within SLP, for example, mixtures of purified triglycerides, complex triglyceride mixtures, or waxes (Wissing and Muller 2002; Dubes and others 2003; Souto and others 2004; Uner and others 2004; Wissing and others 2004; Saupe and others 2005). Utilization of a number of lipids, rather than an individual lipid, usually increases the loading capacity and retention of encapsulated lipophilic components because they can fit better into a more imperfect crystalline structure. The nature of the emulsifier used to stabilize the lipid droplets may also be important in creating specific internal structures. For example, the tail groups of certain surfactants act as templates that promote nucleation within lipids located at the oil–water interface, that is, heterogeneous surface nucleation (Sonoda and others 2006). This principle can be used to form core-shell particles with a solid shell and a liquid core. A lipophilic bioactive component is usually dissolved or dispersed in the lipid phase at approximately 10 °C above the melting temperature of the highest melting lipid. The hot lipid phase is then homogenized with hot aqueous phase in the presence of a hydrophilic emulsifier to produce an oil-in-water emulsion, which is then cooled in a controlled manner to promote lipid crystallization.

Advantages and limitations Some potential advantages of solid lipid particle emulsions over conventional emulsions for application as delivery systems are listed below (Muller and others 2000; Muller and Keck 2004; Wissing and others 2004):

  • • 
    The ability to improve the stability of chemically labile lipophilic components by trapping them within a structured solid matrix. The molecular mobility of bioactive lipids and/or reactive chemical species (such as oxygen) can be altered by controlling the physical state and structure of the lipid matrix. For example, a bioactive lipid that normally reacts with hydrophilic components in the aqueous phase could be encapsulated within the lipid core and surrounded by a protective shell of inert lipid.
  • • 
    The ability to control the delivery of lipophilic functional components. For example, a solid lipid phase could be designed to melt at a particular temperature thereby releasing an encapsulated bioactive lipid.
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    The ability to create stable emulsion-based systems containing crystalline lipophilic components. In conventional O/W emulsions, a crystalline lipophilic component may promote physical instability due to partial coalescence (McClements 2005a), but this may be prevented by trapping a solid bioactive lipid within a solid lipid matrix.

The major limitation of SLP emulsions is that they must be prepared at elevated temperatures to avoid crystallization of the lipid phase during the homogenization process. If crystallization does occur, then the homogenizer will be blocked and potentially damaged. In addition, the use of high temperatures may cause chemical degradation of certain types of heat-sensitive lipophilic components, and should therefore be limited as much as possible (Ribeiro and others 2003). Finally, the lipid phase usually has to be highly saturated so that it has a sufficiently high melting point to form SLP emulsions, which may have an adverse impact on health.

Applications Research in the pharmaceutical industry has shown that delivery systems based on SLP emulsions can be used to increase the bioavailability and stability of a variety of highly lipophilic drugs (Muller and others 2000). The potential of using SLP emulsions as delivery systems in foods has only just begun to be explored. Nevertheless, research work carried out in other areas has already shown that SLP emulsions can be used to encapsulate, protect, and deliver fat soluble vitamins such as vitamin A, D, and K (Iscan and others 2005; Souto and Muller 2005; Uner and others 2005; Jee and others 2006; Pople and Singh 2006). These SLP emulsions were formed using the hot homogenization method described previously. The authors stress the importance of selecting an appropriate lipid carrier and processing conditions to avoid expulsion of the encapsulated lipophilic component during storage. Potential future applications of SLP emulsions in the food industry include delivery of chemically sensitive lipophilic components, such as ω-3 fatty acids or lycopene. One potential drawback of using solid lipid particles as delivery systems is that the in vivo digestibility of the lipid phase may be reduced when it is crystalline, thereby reducing the bioavailability of any encapsulated lipophilic component. Nevertheless, studies have shown that solid lipid particles are degraded by lipase using in vitro models, albeit at a slower rate than liquid lipid droplets (Olbrich and others 2002a, 2002b).

Filled hydrogel particles

Structure Filled hydrogel particle emulsions consist of oil droplets contained within hydrogel particles that are dispersed within an aqueous continuous phase (Figure 2). They can therefore be thought of as a type of oil-in-water-in-water (O/W1/W2) emulsion. The particle size distribution, concentration, and spatial location of the oil droplets within the hydrogel particles can be varied, as well as the size and properties of the hydrogel particles themselves (for example, charge, stability, compatibility, permeability, environmental responsiveness).

Preparation There are a number of methods that can be used to form filled hydrogel particles (Norton and Frith 2001; Chen and others 2006; Pich and Adler 2007; Zhang and others 2007), with the major ones applicable to the food industry being summarized subsequently. As a 1st step, most of these methods involve preparing an oil-in-water emulsion by homogenizing an oil phase together with an aqueous phase containing a water-soluble emulsifier. The size, concentration, and charge of the droplets in these emulsions can be controlled by selecting an appropriate emulsifier (type and concentration) and appropriate homogenization procedure (homogenizer type and operating conditions). A filled hydrogel particle can then be created by combining this O/W emulsion with an appropriate biopolymer solution and then adjusting the solution or environmental conditions to promote hydrogel particle formation. A number of methods that can be used to form filled hydrogel particles are outlined below:

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    Phase separation and gelation of single biopolymer solutions. Under certain conditions, biopolymer molecules dispersed within an aqueous solution can be made to self-associate and form a separate phase by adjusting the “quality” of the solvent, for example, by changing dielectric constant, temperature, pH, or ionic strength (Chen and others 2006). This process is usually referred to as simple coacervation and can be used to form filled hydrogel particles. For example, the lipid droplets could be mixed with the biopolymer solution prior to adjusting the solvent quality. The solvent quality could then be adjusted to promote the formation of small hydrogel particles that trapped the lipid droplets inside them (which would require careful selection of lipid droplet surface characteristics and biopolymer type). If necessary, the solution conditions could then be further adjusted to promote gelation of the biopolymer molecules within the hydrogel droplets. In this case, it is important to ensure that the biopolymer concentration is not too high, or else gelation of the whole system may occur.
  • • 
    Injection and gelation of single biopolymer solutions. Many biopolymer solutions are capable of forming gels when the solution or environmental conditions are altered in a specific manner, for example, heating, cooling, mineral addition, pH adjustment, or enzyme treatment. Small hydrogel particles can therefore be formed by starting off with a solution of biopolymers that is liquid, and then injecting it (for example, injecting, spraying, atomizing, or extruding) into another phase that promotes biopolymer gelation (Senuma and others 2000; Zhang and others 2007). The size of the particles produced can be controlled by varying the injection conditions, for example, injection flow rates, pore sizes, or stirring conditions (Zhang and others 2007). This process can be adapted to form filled hydrogel particles (Zhang and others 2006). The lipid droplets are mixed with the biopolymer solution prior to gelation, and the resulting mixture is then injected into another liquid that promotes rapid biopolymer gelation.
  • • 
    Gelation of single biopolymer solutions within double emulsions. In this approach, an oil-in-water-in-oil (O/W/O) emulsion is formed by homogenizing an O/W emulsion with an (outer) oil phase containing a lipophilic emulsifier (Ribeiro and others 1999; Cho and others 2003). The (inner) oil phase of the O/W emulsion contains the lipophilic component that is to be encapsulated, while the water phase contains a biopolymer that is capable of gelling. After the O/W/O emulsion has been produced the gelation of the biopolymer is induced, for example, enzymatically, chemically, or thermally (Ribeiro and others 1999; Cho and others 2003; Hwang and others 2005). The oil phase is then removed from the O/W/O emulsion by centrifuging or filtering to collect the W/O droplets, followed by washing with an organic solvent and drying to remove any excess external oil. Alternatively, the O/W/O emulsion can be formed by homogenizing a O/W emulsion with an organic solvent that acts as the outer oil phase, which can then be removed by evaporation (Freitas and others 2005). Finally, the filled hydrogel particles are collected and can be used directly or dispersed in water. The size and properties of the internal oil droplets can be controlled by varying the composition (for example, emulsifier type and concentration) and homogenization conditions (for example, duration and intensity) used to produce the initial O/W emulsion, whereas the size and properties of the hydrogel particles can be controlled by varying the composition and homogenization conditions used to produce the O/W/O emulsion.
  • • 
    Aggregative separation and gelation of mixed biopolymer solution. If an aqueous solution contains 2 biopolymers that have a sufficiently strong attractive force between them (Figure 6), then it will separate into 2 phases: a phase that is rich in both biopolymers and a phase that is depleted in both biopolymers (Burgess 1990; Renard and others 2002; Weinbreck and others 2003; de Kruif and others 2004; Cooper and others 2005). The major driving force for this type of phase separation is usually an electrostatic attraction between oppositely charged biopolymers, for example, an anionic polysaccharide and a cationic protein. The biopolymer-rich phase may either be a coacervate or a precipitate depending on the strength of the electrostatic attraction and the charge densities of the 2 biopolymers (Cooper and others 2005). Nevertheless, coacervates are usually more suitable for use as delivery systems because they tend to form particles with more well-defined properties (size and charge) and they have better stability to aggregation and sedimentation. Small coacervate droplets tend to be formed when a mixed biopolymer solution is adjusted to conditions where complex coacervation is favored (Cooper and others 2005). These coacervate droplets are usually susceptible to coalescence, and may dissociate when either the pH or ionic strength of the solution is adjusted. Consequently, it may be necessary to stabilize them so that one or both of the biopolymers forms a gel, for example, by heating, cooling, mineral addition, or enzyme treatment. In this case, filled hydrogel particles could be created by mixing an O/W emulsion with a mixed biopolymer solution before inducing complex coacervation. After coacervation is induced, the coacervate phase forms around the oil droplets and traps them within the coacervate particles. Alternatively, the O/W emulsion could be mixed with a preformed coacervate phase and then the filled coacervate phase could be injected into or blended with an aqueous solution (Figure 7).
  • • 
    Segregative separation and gelation of mixed biopolymer solution. If an aqueous solution contains 2 biopolymers (A and B) that have a sufficiently strong repulsive force between them (Figure 6), then it may separate into 2 phases (Schmitt and others 1998; Norton and Frith 2001; Benichou and others 2002; Tolstoguzov 2002, 2003). One of the phases is rich in Biopolymer A and depleted in Biopolymer B, whereas the other phase is rich in Biopolymer B and depleted in Biopolymer A. Typically, the driving force for this kind of phase separation is steric exclusion or electrostatic repulsion. When the solution conditions are adjusted so that segregative separation is promoted in a mixed biopolymer solution, the system often initially forms droplets of one phase dispersed in a continuous medium of the other phase. This kind of system is often referred to as a water-in-water (W/W) emulsion. As with coacervates, the water droplets formed due to segregative separation are often unstable to coalescence and gravitational separation and so it may be necessary to stabilize them by adjusting the solution or environmental conditions so that one or both of the water phases gels, for example, by heating, cooling, mineral addition, or enzyme treatment. In this case, filled hydrogel particles could be formed by mixing the O/W emulsion with the mixed biopolymer solution prior to inducing segregative phase separation, or by mixing the O/W emulsion with the biopolymer phase that will become the dispersed phase of the W/W emulsion and then incorporating this into the biopolymer phase that will become the continuous phase (for example, by mixing, injecting, or spraying) (Figure 7).
image

Figure 6—. Proteins and polysaccharides may form a variety of different phases in aqueous solutions due to segregative or associative separation.

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image

Figure 7—. Schematic representation of production of an oil-in-water-in-water (O/W/W) emulsion from a 2-phase system consisting of 2 aqueous phases and an O/W emulsion.

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Filled hydrogel particles with different microstructures and properties can be created by varying the nature of the biopolymers involved (for example, charge, molecular weight, flexibility, hydrophobicity), the solution composition (for example, pH, ionic strength), the mixing conditions (for example, order of addition, stirring speed, homogenization pressure), and environmental conditions (for example, temperature). Thus, it is possible to prepare hydrogel particles that have different sizes, electrical charges, porosity, oil droplet loading capacity, release properties, and environmental responsiveness.

Bioactive lipids can be encapsulated within the oil phase prior to formation of the O/W emulsion. In addition, hydrophilic functional components can be encapsulated within the hydrogel particles or the surrounding aqueous phase, for example, peptides, minerals, chelating agents.

Applications The ability to form filled hydrogel particles has been demonstrated in a number of studies utilizing single biopolymer gelation (Malone and others 2003; Chen and others 2006), aggregative phase separation (Renken and Hunkeler 1998; Schmitt and others 1998; Benichou and others 2002; Weinbreck and others 2004; Desai and Park 2005; Madene and others 2006), and segregative phase separation (Norton and Frith 2001; Malone and Appelqvist 2003; Lian and others 2004; Kim and others 2006). Filled hydrogel particles based on aggregative phase separation have been used to encapsulate and protect ω-3 fatty acids (Lamprecht and others 2001; Wu and others 2005) and flavor oils (Weinbreck and others 2004), while those based on segregative phase separation have also been used to encapsulate fish oils (Kim and others 2006). As a specific example of this approach, we will briefly review the formation of filled hydrogel particles based on complex coacervation to stabilize ω-3 fatty acids (Lamprecht and others 2001). In this study, an oil-in-water emulsion stabilized by gelatin was initially formed, and then it was mixed with a solution containing gum acacia. The pH was then adjusted to promote complex coacervation of the gelatin and gum acacia, which leads to the formation of a thick biopolymer shell around the lipid droplets. Finally, different methods were used to cross-link or harden the biopolymer shells in order to increase the long-term stability of the filled hydrogel particles, for example, ethanol, dehydroascorbic acid, glutardialdehyde, or spray drying (Lamprecht and others 2001). The authors of this study found that the filled hydrogel particles hardened by ethanol were the most stable to lipid oxidation. The encapsulation of lipids within hydrogel particles may be beneficial for the protection and delivery of a variety of lipophilic bioactive components in foods. The biopolymer shell could be constructed so that it provides chemical or physical protection of the encapsulated lipids. For example, the composition, thickness, permeability, and environmental responsiveness of the biopolymer shell could be designed to prevent lipid oxidation or to prevent instability due to partial coalescence (which may occur in conventional emulsions). The shell could also be designed so that it released the bioactive components at the appropriate site within the human digestive system, for example, the mouth, stomach, or small intestine. This kind of system is widely used in the pharmaceutical industry for the delivery of drugs, and is likely to gain increasing utilization within the food industry once suitable formulations and preparation conditions have been identified.

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Bioactive Lipids
  5. Desirable Characteristics of Delivery Systems
  6. Emulsion Science and Technology
  7. Delivery Systems Based on Emulsion Technology
  8. Conclusions
  9. Acknowledgments
  10. References

A wide variety of emulsion-based delivery systems are currently available for the encapsulation of bioactive lipids, each with its own advantages and disadvantages. This review article has provided an overview of the major types of emulsion-based delivery system that are available, and has highlighted some of the advantages and disadvantages of each one. Conventional oil-in-water emulsions are currently the most widely used method of encapsulating bioactive lipids, but they are often susceptible to breakdown over time or when they are exposed to certain environmental stresses during production, transport, storage, or utilization. In addition, they have limited ability to encapsulate, protect, and deliver certain types of bioactive lipids. Consequently, there is a need for certain applications to have alternative types of delivery systems. Other types of lipid-based delivery systems, such as multiple emulsions, multilayer emulsions, filled hydrogel particles, and solid lipid particle emulsions, have certain advantages over conventional emulsions, but they are often more complicated to prepare and are sometimes more unstable.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Introduction
  4. Bioactive Lipids
  5. Desirable Characteristics of Delivery Systems
  6. Emulsion Science and Technology
  7. Delivery Systems Based on Emulsion Technology
  8. Conclusions
  9. Acknowledgments
  10. References

This article is based upon work supported by the Cooperative State Research, Extension, Education Service, U.S. Dept. of Agriculture, Massachusetts Agricultural Experiment Station (Project nr 831), and a U.S. Dept. of Agriculture, CREES, NRI Grant (Award nr 2005-01357).

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  6. Emulsion Science and Technology
  7. Delivery Systems Based on Emulsion Technology
  8. Conclusions
  9. Acknowledgments
  10. References
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