A view of a standard emulsion by confocal scanning laser microscopy.

In moderation, fats and oils are a necessary part of the diet. It has been found these materials can deliver fat-soluble vitamins, unsaturated fatty acids, like omega-3 and omega-6, and a host of other beneficial nutrients. Of course, nutrition is not their only role; fats and oils also provide functional roles in food, such as lubricity, structure, flavor, mouthfeel and much more. Fats and oils are often delivered in the form of emulsions that make up many traditional foodstuffs. Mayonnaise, milk and cream represent conventional oil-in-water emulsions, while butter and margarine represent the opposing water-in-oil type. More recently, there is another category of emulsions called double phase or multiple emulsions, which are somewhat of a combination of those mentioned above. Water-in-oil-in-water (WOW) emulsions are a specific type of multiple emulsion.

What is a WOW Emulsion?
WOW emulsions have received a notable share of press in the past few years, touting their potential opportunities for the food and beverage industry, especially in the area of fat reduction, but the potential extends far beyond fat reduction.

WOW emulsions are a kind of double or multiple emulsion, with three distinct phases. There are two types of double emulsions; water-in-oil-in-water (WOW) and its opposite, oil-in-water-in-oil (OWO). Of the two, WOW emulsions are more relevant to the food and supplement industry, since most food systems are composed of a continuous aqueous phase (as opposed to a continuous oil phase). With respect to the WOW type, the three distinct phases consist of (1) internal water droplets that are dispersed in (2) an oil phase, which is then enclosed in a (3) continuous water phase.

Confocal images that accompany this article (see page 83 of the print or digital version of Prepared Foods) show the structure of these novel emulsions. The first photo shows a standard emulsion, while the second photo is a double phase WOW emulsion. In the images, fat appears white and water appears black. The black internal water droplets of the WOW emulsion dot the larger oil droplets, creating a speckled appearance.

Often, literature refers to WOW emulsions as microemulsions. Although some components of WOW emulsions may be in the micro and nano range, they do not meet the definition for microemulsions. WOW emulsions do not spontaneously form and, in fact, are difficult to make.  Most WOW emulsions are processed through a high-pressure homogenizer, to create a stable emulsion. The internal water droplets are often in the nano range.

Making WOW Emulsions
Forming WOW emulsions is a tricky task. Typical methods involve two steps: first, making a traditional water-in-oil emulsion and then dispersing and emulsifying this entire mixture in water, using a combination of surfactants and shear to create a stable, aqueous emulsion.

Two types of surfactants are needed: 1) a low hydrophilic-lipophilic balance (HLB) for the interface between the interior water droplet and the encompassing oil droplet; and 2) a high HLB for the interface between the oil droplet and continuous water phase.

Recent research has resulted in new and improved methods for creating these emulsions. One example is using single-component, synthetic amphiphilic diblock copoly-peptide surfactants. According to Nature's September 4, 2008, issue, Hanson et al. reported: "These surfactants even stabilize droplets subjected to extreme flow, leading to direct, mass production of robust double nanoemulsions that are amenable to nanostructured encapsulation applications in foods, cosmetics and drug delivery." The resulting emulsions were stable for nine months, which also addresses the inherent stability issue of WOW emulsions.

WOW's Role in Fat Reduction
The somber news that obesity is on the rise and is often out of control has been very noticeable recently. Statistics abound that inform people the population is not merely overweight, but actually obese. Figures from the National Center for Health Statistics showed 34% of Americans age 20 and older were classified as obese (body mass index of 30 or higher) in 2007-2008. Thus, food formulators are constantly searching for ways to create great-tasting foods, without all the fat.

Leatherhead Food International (LFI), based near London, is looking at the ability of WOW emulsions to create fat-reduced products that look and taste like their conventional counterparts. LFI has been one of the forerunners in WOW applications research, working with the technique for almost four years. 

Kathy Groves, project leader for LFI, explains that WOW is where formulators make the water-in-oil emulsion first and then place that into a water phase. This end-product contains nano-sized internal water droplets, surrounded by an oil layer, which is then surrounded by a continuous water phase. This structural arrangement allows consumers to still perceive a creamy, full-fat mouthfeel, without the full-fat content. The hidden inner water droplets allow for higher water content, thus reducing the total fat content. 

LFI worked with mayonnaise as a model to demonstrate WOW's fat-reduction principles. They were able to achieve a 40% reduction in fat from that of the standard product, using this unique emulsion technology. Trained panelists were not able to detect a difference between reduced-fat and the full-fat control. Dr. Stuart Clegg, a project manager in LFI's Food Innovation Group, further explains that, in principle, WOW will work in any food application which is an oil-in-water emulsion, such as creams, salad dressings, sauces and dips.

Other institutions, including Wageningen University and Research Center (Netherlands) and the UK Institute of Food Research (Norwich), are also researching WOW emulsion applications in foods. Wageningen University was involved with the launch of a commercial reduced-fat mayonnaise marketed as Nanonaise, which utilized WOW technology.

Technical Hurdles
One of WOW's drawbacks is often instability. These emulsions can be difficult to prepare and tend to be thermo-dynamically unstable, requiring complex processing techniques and/or surfactant mixtures. The mixtures tend to collapse under osmotic pressure, since there is a tendency for the internal water droplet to migrate to the continuous aqueous phase. Some of these issues are being addressed by protein hydrocolloid research to form strong adsorbent layers at the interface that prevents droplet coalescence and improves emulsion stability. For example, some researchers have been experimenting with sodium caseinate/maltodextrin and whey protein isolate/pectin combinations, to combat unwanted creaming and water transport between the emulsion's "compartments."

Another challenge associated with WOW emulsions is scale-up. WOW emulsions can be sensitive to shear stress commonly encountered in commercial large-scale food processing. Although these double phase emulsions have extraordinary performance in the lab, the trick is to create the same creamy texture in a full-scale food plant. Commercial food processing, with unit operations, such as homogenization and inline mixing, can take a toll on these emulsions. Creaming of the oil globules (separation of the oil-and-water phase) is often the result of such stress.

Other Potential WOW Applications
WOW emulsions have been identified to provide other exciting functions, such as delivery vehicles for flavors and active ingredients. As the demand for functional foods and beverages rises, formulators are hard at work to include promising, health-promoting ingredients in mainstream foods. These functional foods offer specific health benefits beyond the realm of normal foodstuffs. Due to their compartmentalized structure, WOW emulsions have the ability to carry both polar and non-polar bioactives. This translates into an innovative delivery vehicle for both water- and fat-based ingredients. Several studies have looked at delivering water-soluble vitamins, botanicals, polyunsaturated fatty acids and other fashionable active ingredients through WOW technology.

Salt reduction is another application where WOW double emulsions may have potential. By having two water phases in an emulsion, it is possible to preferentially add salt to either phase, thereby creating the possibility for modifying the distribution of salt within the emulsion. For example, if some of the water content is trapped in the internal water droplet, and all of the salt is concentrated in the continuous water phase, then consumers will experience more saltiness--though the total salt content has been reduced.

Other interesting applications exist in the natural color industry. Blueberry anthocyanins are blue, until the pH of the product is dropped, and then they turn red. If blueberry pigments are incorporated into neutral-pH internal water droplets that are protected within a WOW emulsion, then it may remain blue, even if incorporated into a low-pH beverage.

Outside the food industry, extensive research has enabled the incorporation of WOW technology into pharmaceutical, medical, cosmetic and ink products, to name just a few. pf

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