Shortening substitutes and novel strategies for nanostructuring liquid oils into functional fats

Alejandro G. Marangoni
Department of Food Science, University of Guelph, Guelph, ON N1G2W1, Canada

Increasing public concerns over excessive saturated and trans fat intake from manufactured food products has lead to the search for alternative strategies to structure liquid oils into semisolid fats without addition of large amounts of unhealthy trans and saturated fats. Surfactant-like small molecules have been shown to self-assemble into long fibrils, effectively causing oil gelation at concentrations as low as 0.5%. Phytosterols, ceramides, and 12-hydroxystearic acid have been shown to be effective organogelators. Liquid oils can also be structured by microencapsulation within multilamellar vesicles, with walls composed of monoglyceride hydrates in the alpha-gel state. The surface potential of these monoglyceride vesicles is then adjusted so as to maximize inter-vesicle interactions and the formation of a cellular solid with oil-filled cells. These monoglyceride gels have recently been proven to have excellent functional characteristics in baking applications as well as for omega-3 oil stabilization. High-molecular weight polymers such as ethylcellulose have also been successfully used by our group to gel oil in the absence of water. This development of a polymer-stabilized organogel is very promising since these polymers are widely available and are food-grade. The development of a new way to make fat exploiting the self-assembly properties of food-grade molecules is at hand.

Keywords: Organogels, polymers, cellular solid, alpha gel, monoglycerides,shortening substitute.

INTRODUCTION

Diets high in trans and saturated fats have numerous deleterious health effects ranging from adverse effects on lipoprotein (cholesterol) profiles, increased incidence of heart disease and metabolic syndrome [1]. By 2010, it is projected that in excess of 230 million people will be affected with metabolic syndrome, a diet related disease, associated with an elevated risk of developing type II diabetes mellitus, cardiovascular disease and premature death [2].

Negative health implication associated with consuming trans and saturated fats may be reversed by altering the intake of these heart unhealthy fats and replacing them with polyunsaturated fats. It is estimated that replacing 5% of our daily energy intake from saturated fats with either equivalent energy from carbohydrates, mono- or polyunsaturated fats would be associated with a decreased risk of CVD in the range of 22 to 37% [3].

In an attempt to curb these diet related epidemics, governments across the globe are passing aggressive legislation to limit and in some cases ban the use of trans fats. Denmark was the first country which restricted the amount of trans fatty acids in industrially produced fats and oils to 2% (w/w) or less (Danish order no. 160 of March 2003). The United States’ Food and Drug Administration and the Canadian Food Inspection Agency enacted mandatory labeling of trans fatty acid content in food products in January of 2006, for any products containing more than 0.5g of trans fatty acids. As well, California and Connecticut recently passed state legislation banning all artificial trans fats from food products (CA AB 97, 2008, enacted, Chapter No. 207). Florida has proposed restrictions on trans fats in schools via the “Florida Healthier Child Care and School Nutrition Act”; and state after state are considering enacting “hard-line” legislation restricting or banning trans fat [4].

The American Heart Institute believes even greater restrictions are necessary advocating that no more than 10% of daily energy should be consumed in the form of trans AND saturated fats combined [5].

With this in mind, it becomes more evident than ever that novel technologies need to be implemented in an attempt to curb the associated epidemics and to address the gigantic food manufacturing problem this will represent.

In order to fulfill these new legislative requirements, the food industry must vigorously investigate alternatives to traditional triacylglyceride (TAG) structuring. TAGs provide structure in numerous food products including ice cream, cheese, butter, lard, etc.

Unfortunately, it is the trans and saturated lipids which provide the structure and solid-like properties of these foods. Although the structure they confer on products is desirable, and indeed required, in many products, both types of fatty acids have been shown to deleteriously influence human health.

Since it is the hardstock TAGs that responsible for network structure, it is often difficult or impossible to eliminate these ingredients in order to improve the health characteristics of a food product without sacrificing some of the quality characteristic associated with that particular food product.

Food manufacturers are very reluctant to change the characteristics of successful industrially produced food products to any extent!

Thus, the task of replacing a major ingredient responsible for many of the quality attributes of a food product is a very difficult task, and an even harder sale.

Pernetti so eloquently states that finding alternatives to TAGs with healthy properties, versatility and performance is a tremendous challenge [6].  Without being overly critical of the food manufacturing industry, the “solution” to replacing trans fats adopted by industry has been the replacement of partially hydrogenated fats with saturated fats from palm “oil”.

Palm oil is a misnomer since it should, by definition, be referred to a palm “fat”, since it is solid at room temperature (20-25oC).

This is obviously true for palm kernel “fat” and coconut “fat” as well. Calling these materials “oils” is not correct and should not be encouraged. For the record, the fatty acid composition of palm fat is included in Table 1 below.

Table 1: Fatty acid composition (% w/w) of palm fat

Source: Firestone, Physical and Chemical Characteristics of Oils, Fats, and Waxes, AOCS Press, 1999

Of particular concern is the high content of palmitic acid (16:0), which has been shown to be the most atherogenic saturated fatty acid [7,8].

The aim of this review is to examine strategies for replacing colloidal fat crystal networks comprised of trans and saturated fats including small-molecule organogels of unsaturated oils, structured oil-in-water emulsions and polymer gels. This short paper is not meant to be an exhaustive review, but an overview of the work carried out in our laboratory.

STRUCTURED EMULSIONS

At the core of this technology is the creation of an oil-in-water emulsion stabilized by hydrated saturated monoglyceride multilayers. These multilayers are composed of monoglyceride bilayers with large amounts of water “sandwiched” in between.

The monoglyceride bilayers are formed at temperatures above their Krafft temperatures, and are in the L-alpha state. Upon cooling, the fatty acid chains of the hydrated monoglycerides crystallize into a hexagonal conformation ( alpha subcell polymorph) to an L-beta phase. A co-surfactant is added along with the saturated monoglyceride to enhance the formation and stabilization of this, so called, alpha-gel state.

At the same time as these 9hydrated monoglyceride multilayers are formed, oil is added to the system and exposed to an external shear field. A monoglyceride monolayer then surrounds the oil droplets upon which the multilayers can deposit. Thus, in the end, the oil becomes encapsulated within hydrated monoglyceride multilayers, forming and oil-in-water emulsion.

Upon crystallization of these multilayers, the oil becomes effectively microencapsulated within hydrated crystalline spheres. By adjusting the surface charge of the vesicles by judicious addition of a charged co-surfactant and/or medium pH adjustments, it is possible to fine-tune vesicle-vesicle interactions to create a range of rheological behaviors, from liquid to solid and also prevent the Lβ to Lβ’ transformation [9-12]. Figure 1 summarizes this structured emulsion technology.

Figure 1: Principle behind the technology involved in the creation of a structured oil-in-water emulsion that can be effectively used as an all-purpose shortening.

Figure 2: Polarized light micrograph of a cellular solid composed of hydrated crystalline monoglyceride walls where liquid oil is encapsulated within the cellular lumen.

This structured emulsion, now a commercial product marketed under the brand CoaSunTM can be very effectively used as an aloo-purpose shortening substitute to manufacture a variety of bakery products, particularly cookies, cakes, scones, muffins, waffles, among many others.

The great advantage of these materials is its very low saturated fatty acid content, which makes it possible to both manufacture healthier food products and also make claims on food labels.

If prepared with rapeseed oil, CoaSunTM will contain between 7 and 8% total saturates and can usually replace traditional shortenings on a 1:1 basis [13].

Moreover, it is possible to encapsulate and partially protect omega-3 fatty acids effectively using this shortening alternative technology.

Limitations of these oil-in-water structured emulsions are the fact that water is present (30-40%) in the material and baking parameters have to be adjusted accordingly.

On the other hand, this represents a net decrease in the total amount of fat used and thus consumed. This is another advantage from an economic and nutritional point of view.

ORGANOGELS

An organogel can be defined as an organic liquid entrapped within a thermo-reversible, three-dimensional gel network. This gel network is formed by the self assembly of a relatively low concentration of organogelator molecules into long crystalline fibers (Figure 3), and is thus called Self-assembled fibrillar networks or SAFINs.

Figure 3: Polarized light micrograph of a 12-hydroxystearic organogel at 2% (w/w) concentrations in rapeseed oil. Notice the alternating dark and bright areas indicative of a helical twist in the fibers.

 Organic solvents can actually be gelled at organogelator concentrations as low as 0.5%. Depending upon the chemical properties of the organogelator, gels can be formed from organic solvents (benzene, hexane) or liquid oils.

The wide variety of both current and future applications of organogels has been outlined in two comprehensive reviews [14,15]. While some industries are already making use of organogel technology, many of its potential applications are still in the research and development phase.

The potential applications of organogels in food system are numerous and offer a revolutionary option for nano and microstructuring edible oils into functional fats. A number of organogelator systems have the ability to gel edible oils at very low concentrations (0.5-2.0% wt). General categories of network-forming edible oil structurants include: Triacylglycerols (TAGs), Diacylglycerols (DAGs), Monoacylglycerols (MAGs), fatty acids, fatty alcohols, waxes, wax esters, sorbitan mono-stearate; as well as the following mixtures: fatty acids and fatty alcohols, lecithin and sorbitan tri-stearate, and phytosterols and oryzanol [6]. More specifically, the following organogelators or mixtures thereof are well-known for their ability to structure in edible oils: 12-hydroxystearic acid [14,16-20], ricinelaidic acid [21,22], candelilla wax [23], mixtures of β-sitosterol and γ-oryzanol [24], mixtures of stearic acid and stearyl alcohol [25,26], and mixtures of lecithin and sorbitan tri-stearate [27], and more recently mixed ceramides [28]. Regardless of the network composition, organogels have numerous potential different functionalities in food products including (but not limited to): the restriction of oil mobility and migration, the replacement of saturated and trans- fats, the stabilization of emulsions, and the ability to control the rate of nutraceutical release. Figure 4 shows one of the most promising applications of these organogel systems, the purely physical stabilization of water-in-oil emulsions. This emulsion gel was stable for over a month at room temperature conditions regardless of the large size of the water droplets. The birefringent network of 12-HSA fibers can be appreciated throughout the continuous oil gel.

The greatest challenge in this field is to find an organogelator that is food grade and that offers a functional advantage. The phytosterol-oryzanol system developed in Bot’s group is the most promising system in this respect. On one hand, the material forms optically transparent gels structured by crystalline nanotubes of 7nm diameter [29], while also having the added value of the proven cholesterol-reducing properties of phytosterols. Even though the work is still at a preliminary stage, the ceramide system developed in our group follows in the footsteps of the Bot’s phytosterol-oryzanol work – self-assembled fibrillar network structuring and a powerful pharmacological effect.

Figure 4: Polarized light micrograph of a 12-hydroxystearic organogel at 2% (w/w) concentrations stabilizing large water droplets. Notice the birefringent fibers in the continuous oil gel phase.

INTERESTERIFIED HIGH-STEARIC ACID FATS

Interesterification, either chemical or enzymatic, can improve the functionality and physical properties of fats and oils [30]. This method can replace hydrogenation as a major source of trans fatty acids (TFA) and has been a primary tool in the creation of plastic fats for baked goods [31]. Reducing the dietary intake of trans fatty acid has been recommended by various scientific studies, as well as public and regulatory policy [32,33]. For example, the American Heart Association recommends that the intake of TFA be less than 1% of total daily energy [34]. A recent study showed that a 2% increase in energy intake in trans fatty acids forms was associated with a 23% increase in the risk of incidence of coronary heart disease [33].
In order to achieve a similar functionality in fat product as hydrogenated fat, a saturated hard stock such as fully hydrogenated rapeseed oil or fully hydrogenated soybean oil rich in stearic acid can be used. Early studies of Hegsted et al. [35] and Keys et al. [36] noted that stearic acid (18:0) has a neutral effect on serum cholesterol levels. Another study examined the effect of cocoa butter and structured fat with high stearic acid content and showed that stearic acid elicits a hypocholesterolemic effect compared with other saturated fatty acid [37]. More recently, the work of Mensink et al. [7] strongly suggests the stearic acid does not affect lipoprotein metabolism to a significant extent. Furthermore, consumption of oleic acid has been shown to have beneficial effects on human health, such as decreased vulnerability of plasma low density lipoprotein (LDL) to oxidation [38]. Moreover, oleic acid is stable towards oxidation relative to linoleic and linolenic acids [39,40].

Thus, a possible strategy for structuring food products less harmful fats is to use mixtures of fully hydrogenated vegetable stocks with high-oleic acid oils. However, these fats tend to have a very poor functionality and thus interesterification using sodium methoxide or lipases is necessary.

Figure 5 shows the solid fat content vs. temperature profiles for non-interesterified (NI), chemically interesterified (CI) and enzymatically interesterified (EI) mixtures of 30% fully hydrogenated canola oil in high oleic sunflower oil. The enzymatic interesterification was carried out using a random lipase from Candida antartica.

Figure 5: Solid fat content vs. temperature profiles for 30% mixtures of fully hydrogenated canola oil in high oleic sunflower oil (CI: chemically interesterified, EI:enzymatically interesterified; NI:non-interesterified).

The SFC-temperature profile of the interesterified samples suggests a much more functional fat. Moreover, the NI crystals were in the β polymorphic form, while the EI and CI samples were in the β’ form [41,42]. It is evident, thus, that these mixtures require a chemical randomization of the fatty acids on the TAG backbone in order to be functional in food applications.

Figure 6 shows the changes in the storage modulus G’ (A) and Yield Force (B) upon chemical and enzymatic randomization of some of the mixtures. The increase in G’ and yield force of the chemically interesterified samples was due to a decrease in crystal size [43].

Figure 6: Changes in the storage modulus (G’) and yield force due to chemical (CI) and enzymatic (EI) interesterification of mixtures of fully hydrogenated canola oil (FHCO) and high-oleic sunflower oil (NI).

POLYMER ORGANOGELS

In this author’s opinion, this is the next frontier of liquid oil structuring. There is a wide availability of food-grade polymers (polysaccharides, proteins) that could fulfill the promise of gelling oil. Being a frugal industry by nature, this “cheap” structuring of oils should help the implementation of these new technologies in the marketplace.

Currently, many biopolymers are being assessed in our laboratory for their potential as organogelators at concentrations below 10%.

Figure 7: Polymer organogel of ethylcellulose in vegetable oil. The fat of the future!

 Very little published work is available on this topic. We could only identify one paper on the gelation of ethylcellulose-oil mixtures into a plastic solid by addition of olive oil-derived surfactants [45]. The motivation for this work was the creation of a cream for the topical delivery of pharmaceuticals.

CONCLUSIONS
In this work we have presented some of the most promising strategies for structuring liquid oils. The motivation for this work is to create a semisolid structure with the functionality of fats, but with the nutritional profile of liquid oil – low in saturates, devoid of trans fatty acids and possibly containing some nutritionally beneficial fatty acids. Structured emulsions, SAFINs, polymer organogels and high-stearic acid fats will all be part of the solution to the problem of improving on the nutritional quality of our manufactured food products, in consideration of sensory quality and price.

ACKNOWLEDGMENTS

The author acknowledges the financial assistance of the Natural Sciences and Engineering Research Council of Canada, the Ontario Ministry of Agriculture and Food, the Advanced Food Materials Network, the Canadian Foundation for Innovation and the Canada Research Chairs Program. The author would like to thank Dr. Michael Rogers and Ms. Naomi Hughes for help with the manuscript.

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