Emulsions are formed when one immiscible phase is dispersed in another through mechanical action such as homogenization. Oil-in-water (O/W) emulsions are formed when oil is dispersed in an aqueous phase (e.g., mayonnaise) while water becomes the dispersed phase in water-in-oil (W/O) emulsions (e.g., margarine). Surfactants help keep the immiscible phases in an emulsion together because they are amphiphilic and thus have properties that are compatible with both the hydrophilic and lipophilic portion of the mixture. Non-polar ends of an emulsifier align themselves within the lipid phase, while the polar ends align in the water phase. As a result, an electrically charged surface forms between the two immiscible phases, or at the interface, causing particles to repel one another rather than coalesce.
The reduction of interfacial surface tension is just one possible function of an emulsifier. Other functions associated with this class of ingredients are varied and application-specific. A product developer's decision on which emulsifier or combination of emulsifiers to use often is determined by the complexity of the food matrix and other processing-related issues.
Emulsion Basics
Mixtures of immiscible fluids are not only stabilized by amphiphilic molecules composed of lipids that have been chemically modified through esterification with alcohols and acids, but also by proteins. Proteins, which are large, complex molecules containing various amino acids with different degrees of hydrophobicity, exhibit their surface active tendencies by adsorbing at the interface that lies between the two immiscible phases. It is during the adsorption process that the protein begins to unfold, thereby exposing its more hydrophobic groups to the hydrophobic phase of the emulsion.The stabilization mechanism for lipid-based emulsifiers differs somewhat from proteins in that proteins form a viscoelastic gel at the interface as they interact with one another. This viscoelastic gel stretches and deforms as it accommodates deformations in the interface and, as such, prevents coalescence. On the other hand, lipid-based emulsifiers provide stability by causing the rapid diffusion of liquid from areas of high surface tension to regions of low tension, which occurs when there is a gradient in the surface tension at the interface. This phenomenon is known as the Gibbs-Marangoni effect. (See sidebar “Protein and Surfactants 101.”)
Destabilization of an emulsion can occur in various ways. Creaming is a gravitational separation of phases (i.e., oil phase floats to the surface). During flocculation, clumping occurs without a disruption to the interfacial film, whereas with coalescence, the interfacial film is disrupted as droplets collide and form a separate phase. Flocculation is reversible, while coalescence is not.
The complexity of emulsion stability is compounded by the many factors that influence it. Aside from interfacial tension, emulsion stability is affected by the viscosity of the continuous phase; density differences between phases (e.g., ester gum added to flavor oil in beverages to “weight it” and prevent ringing); droplet size in the internal phase (i.e., the smaller the size, the more stable the emulsion); temperature extremes and the presence of solids (i.e., finely divided solids “wetted” equally well by both phases at the interface will stabilize the emulsion, whereas a higher percentage of solids dispersed in the internal phase will destabilize the emulsion).
Multi-tasking
Stabilization of emulsions is just one function associated with emulsifiers. Whey ingredients, for instance, aid in the dispersion of fat in sauces and soups, which enhances the perception of creaminess, reports Dairy Management Inc. Thus, efficient dispersion of oil can help reduce the fat level in certain formulations of sauces, soups and salad dressings. Confections such as mousse, meringue and nougat also benefit from whey proteins' ability to stabilize whipping and foaming action where the protein aligns itself at the interface between air and aqueous phase.Most hydrocolloid gums stabilize emulsions by increasing the viscosity of the continuous aqueous phase (i.e., water phase in an O/W emulsion such as salad dressing), notes Mar Nieto, PhD, director of technical services for an ingredients company specializing in gum technology. “In a high-oil product such as mayonnaise, the emulsification of the oil with an emulsifier such as egg yolk and/or lecithin results in significant viscosity buildup; hence, a 'viscofier' is not needed,” adds Nieto.
Nieto also points out that certain gums, such as propylene glycol alginate, gum Arabic and tragacanth gum are amphoteric in nature and thus function as true emulsifiers.
The hydrophilic/lipophilic balance (HLB) is one of the means used to select the appropriate emulsifier for a particular type of food or beverage application. HLB values range from 0 to 20 and dictate the relative amphiphilicity of a surfactant. Low HLB ingredients are more oil-soluble, while those with higher values are more water-soluble. “An oil/fat continuous product would need a lower HLB emulsifier, and an aqueous system would require a higher HLB,” explains Bruce R. Sebree, PhD, manager of emulsifiers and texturizers for an ingredients company specializing in a range of products including emulsifiers, baking enhancers and acidulants. “Many times, a median HLB emulsifier may work well in both systems. The developer needs to keep in mind that several types of emulsifiers have modified versions (physical blends, chemically modified, enzymatically modified, etc.) available, which widens the HLB offerings and, in turn, broadens the range of possibilities for that emulsifier type. Lecithin, monoglyceride, polyglycerol esters and sucrose esters are among the more common emulsifiers that have multiple HLB versions available. At times, high- HLB/low-HLB combinations are required, as are mixtures of charged/uncharged or large/small molecular weight emulsifiers. Coverage of the oil/water interface is essential in emulsion systems, and a combination of unlike emulsifiers can improve packing [i.e., molecule arrangement at the surface] by sitting differently at the interface.”
Emulsifiers also perform a variety of other essential functions in food and beverage applications. These ingredients act as aerating and foaming agents, defoaming agents, crystallization promoters or inhibiters, viscofiers, dispersants, dough strengtheners and even as flavor moderators. For instance, while lecithin's main use is to improve hydration and dispersion of a cocoa powder in an aqueous system, it also reduces bitterness [of the cocoa-flavored product], notes Sebree.
As the previous example illustrates, it is not uncommon for an emulsifier to perform multiple functions. The current trend of removing trans-fatty acids from existing fats and oils presents another example of how an emulsifier must multi-task within its intended system. “Removal of trans fatty-acids changes the crystallinity of fat in a system, as it mimics saturated fat, but not quite,” explains Sebree. “The emulsifier system needed for low-trans products may need to promote crystallization of the fat in the product (e.g., distilled monoglyceride).”
Rising to the Challenge
It would be simple to merely say, “If you use this particular emulsifier in this application, all of your problems will be solved,” but the reality is often far more complex for product developers who must choose the right ingredient for their formulation. The product developer must consider the oil and water content of the food to determine whether the emulsion is an O/W or W/O type, as the usage level of the emulsifier depends on the oil-to-water ratio, notes Nieto. Other considerations include the viscosity of the finished emulsion, the temperature (i.e., emulsifiers are usually unstable at high temperature and will tend to separate if the wrong emulsifier is used), the flavor profile of the application to ensure that the emulsifier does not impart a taste to it and whether or not an “all natural” label is required. The selection of an emulsifier needs to be compatible not only with the other ingredients used in the product, but with the equipment available in manufacturing, adds Sebree.Development of line extension products may require more than a simple and straightforward emulsifier replacement across the board, but instead require a different system of emulsifiers for each product variation. “A good example would be a solid margarine system (80% fat) to that of a spread (60% fat or lower),” explains Sebree. “The full-fat margarine would use either a standard lecithin or monoglyceride, or a combination of the two. When you go to a spread, you may need to look at using a modified lecithin (i.e., lysolecthin, acetylated lecithin) or something with an even higher HLB, such as polyglycerol ester (PGE), to form a stable emulsion.”
It is important to consider regulatory issues early in the development process, notes Sebree. “The E.U., Japan, Australia/New Zealand and, many times, even Canada have different regulations [from the U.S.] for types of emulsifiers that can be used in food products. Sometimes, the regulations prohibit certain types while, other times, there are restrictions on use for certain food products. Regulatory issues concerning genetically modified organisms (GMO) and/or allergenicity need to be addressed as well. Some emulsifiers have non-GMO versions; others do not. Some are considered as possible allergens in some countries, and others not.”
“Making a clear food emulsion and developing a super-emulsifier that is capable of making micro-emulsions where oil droplets are much smaller are current challenges being faced in the area of food emulsions,” observes Nieto. “Micro-emulsions, in addition to being clear, also are thermodynamically stable and will not separate. Currently, emulsifiers that are used to make micro-emulsions are either non-food grade or impart an off-flavor to foods.” These issues and many more surely will be addressed as the complex world of emulsions and those ingredients that stabilize them continues to evolve.
A “thank you” to Tyre Lanier, PhD, professor of Food Science, North Carolina State University, tyre@ncsu.edu, for help with this article.
Website Resources:
www.PreparedFoods.com -- Type in “emulsifiers”www.ncbi.nlm.nih.gov/entrez -- Type “proteins
and emulsifiers at liquid interfaces” into the PubMed search field
www.mpikg.mpg.de/kc/scripts/Micelles_and _Emulsions_Tauer_WS_2005_06/Emulsions_-2.pdf -- Theory of Gibbs-Marangoni Effect
www.iseo.org -- Institute of Shortening and Edible Oils
www.ticgums.com -- A site that matches properties of branded ingredients with the applications for which they are of use
www.admworld.com -- Keyword search fields can be used to find information on ingredient use for specific products
www.innovatewithdairy.com -- Dairy Management Inc. site providing application information
Sidebar: Protein and Surfactants 101
Emulsions can be stabilized by both proteins and low-molecular weight surfactants (LWS) such as monoglycerides; however, they function differently. When both are used, an emulsion can be destabilized.LWS stabilize by congregating at the surface interface between an aqueous and air or oil phase. When oil droplets (e.g., in a salad dressing) or air cells (e.g., in a whipped cream) are made smaller and more numerous, the film (e.g., aqueous interlamellar phase) thins, and the interface surface area increases. When this happens, LWS molecules are thinly spread at the interface surface, and surface tension increases. Surface tension equilibrium is restored as more LWS are pulled from the interior of the aqueous phase to the interface surface. This is called the “Gibbs effect.” Surface tension equilibrium also is restored when LWS move sideways or diffuse along the interface surface to help fill in the thinning ranks of LWS and, as they do, drag along the aqueous liquid with which each LWS molecule is associated. This is called the “Marangoni effect.”
In contrast, proteins stabilize emulsions through protein-protein interactions that form a stiff, gel-like film (sometimes multi- layered), which has viscoelastic properties. When films are thinned, protein molecules stabilize emulsions by their cohesive nature which dissipates changes across the film (and possibly also due to protein molecule deformation) rather than by diffusion of the protein molecules themselves.
The “trouble” starts when LWS are mixed with proteins, since the two stabilize in incompatible ways. LWS molecules rely on their lateral diffusion (mobilization) and protein molecules on their immobilization to provide stability. It is thought that, in mixed systems, proteins compete with LWS for adsorption onto the phase interfaces. Additionally, proteins may complex with LWS. These actions may destabilize and break the emulsion.
—Claudia D. O'Donnell, Chief Editor
