Driven both by real industrial needs and curiosity for fundamental research, edible oil structuring has emerged as a subject of growing interest with applications in real food systems. With contributions from leading research groups around the world, this book provides a comprehensive and concise overview of the field with special emphasis on the updates from the last 5 years. New insights into the mechanism of gelation in mono- and multicomponent gels are discussed for several categories of previously known structuring agents along with the potential food applications of some of these systems. In addition, use of alternative methods to explore structuring properties of hydrophilic biopolymers are presented with illustrative examples. Some new concepts such as bio-based synthesis of supergelators, foamed oleogels and use of innovative dispersion techniques give a broader picture of the current research in edible oil structuring.
This book will be of interest to students, academics and scientists involved in the research of edible oil structuring. It will be an important reference as it provides current information on the state-of-the-art of the field.
About the Author
Prof. Ashok Patel is an Associate Professor in Biotechnology and Food Engineering at Guangdong Technion Israel Institute of Technology in Shantou, China where he is currently setting-up a state-of-the-art Food Innovation Lab. He prides himself in being an internationally mobile researcher who has been active in the field of food colloids within different sectors including industry (Unilever R and D Vlaardingen, Netherlands), University (Ghent University, Belgium) and research organization (International Iberian Nanotechnology Laboratory, Portugal). His past and current research is focused on using food-grade ingredients to create novel structured systems including oleogels, foams, colloidal particles and complex emulsions to solve formulation issues in food product development. He has published more than 50 ‘first-authored’ publications in the area of applied colloid science including original research papers, reviews, book chapters and patents. For his research in applied colloid science, he has received prestigious and highly competitive individual funding from the European Commission and other Young Scientist Awards and nominations. In 2015, he was selected for a once-in-a-lifetime opportunity to participate at the Lindau Nobel Laureate meeting as a visiting scientist.
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Oil Structuring: Concepts, Overview and Future Perspectives
ASHOK R. PATEL
Sci Five Consulting Services, Coupure 164F, 9000 Gent, Belgium
Emerging evidence related to the negative cardiovascular effects of increased fat consumption has resulted in increased regulation of trans fats in food products by regulatory authorities the world over. Starting from the mandatory labelling of the amount of trans fats in food products in the mid-2000s, to concrete steps taken towards complete removal in recent times, there has been a consistent decline in the use of trans fats in food products over the last few years.' Accordingly, the food manufacturing industry has been under pressure to innovate and find alternative solutions to formulate products with the complete absence of trans fats. The solution currently used by the food industry is replacing trans fats with saturated fats from natural sources, such as palm oil. Although quite effective in terms of replicating the functionality of trans fats, the use of saturated fats is criticized for two main reasons: (a) negative image of palm oil owing to the ecological damage that is often linked to palm plantations, and (b) possible negative cardiovascular effects of the long-term consumption of saturated fats. The latter point has seen its share of debate in recent times where some authors have argued the scientific validity of such claims based on empirical evidence. However, it is widely accepted that consumption of polyunsaturated fats over saturated fats is beneficial for cardiovascular health. The current dietary guideline with respect to saturated fat consumption is to restrict the daily consumption to less than 10% of total calories and this is unlikely to change in the near future. As the food industry is currently phasing out trans fats in formulations after the ban imposed by the US FDA in 2015, the demand for palm oil has increased. In order to shed the negative image linked to palm oil, many food manufacturers have publicly shared information about their engagement with sustainable palm oil product suppliers (Roundtable on Sustainable Palm Oil or RSPO) to minimize environmental damage done by palm plantations. However, recent scientific opinion from the EFSA panel on Contaminants in the Food Chain (CONTAM) regarding the negative health effects of 3- and 2-monochloropropanediol, as well as their fatty acid esters and glycidyl fatty acid esters, has somewhat tarnished the image of palm oil in the eyes of consumers. Hence, there is a great deal of interest in finding ways to formulate products with a better nutritional profile (i.e. trans fat-free and high in unsaturated fats), and preferably without the use of palm oil.
In recent years, oleogelation has gained popularity as an approach for formulating food products where the functionality provided by saturated fats (i.e. texturing, oil binding, rheological characteristics, organoleptics and stabilizing properties) can be replicated by non-fat components used as structuring agents. The possibility of gelling >90 wt% of liquid oil at a relatively lower mass fraction of gelator molecules makes oleogelation a very efficient approach for oil structuring. This efficient structuring in oleogels is usually achieved by supramolecular assemblies (building blocks) of gelator molecules that organize into a three-dimensional network that can physically imprison a large volume of mobile liquid oil into an 'arrested' gel-like structure. In the following sections, a general concept and an overview of the field of oleogelation is presented with special emphasis on the research done in the recent years.
1.2 Oleogelation: Concepts
By definition, oleogels are a sub-class of a broader class of colloidal structures called organogels and are defined as a gel system where an oil continuous liquid phase is immobilized in a network of self-assembled molecules of an oleogelator or a combination of gelators. In the last few years, oleogelation has changed from a relatively unknown term into a heavily investigated research domain. Academic research in this area has progressed rapidly as researchers from a wide range of backgrounds (such as applied chemistry, colloid science, material science and process engineering) have taken up the challenge of identifying novel ingredients and innovative processing methods to create oleogels. At the same time, industrial scientists have shown increased interest in investigating edible applications of these oleogels in a range of food products, including chocolates, baking fats and meat fat replacements. Fundamentally, it is fascinating to fabricate and characterize this new class of edible soft matter systems as there is still a lot to be learnt about their structuring mechanisms and the 'tunability' of their bulk properties through microstructural alterations. From an application point-of-view, the oleogelation approach has the potential to cater to a number of generalized and 'niche' applications. Such applications include: (a) reduction of saturated fat levels in food products, (b) stabilization of surfactant-free emulsions, (c) decreasing oil mobility and migration in chocolate products (controlling fat bloom in filled chocolates), (d) improving temperature stability in certain food products (heat-resistant chocolates), (e) developing controlled-release systems for delivery of low molecular weight bioactives, and (f) transforming liquid oils into a new range of edible materials, such as foams, films and soft pliable solids.
1.2.1 Oleogelation from a Colloidal Gel Perspective
188.8.131.52 Oleogelators and Monocomponent Gels
One of the main bottlenecks in the field of oleogelation is finding structuring agents (oleogelators) that are effective at low concentrations, cheap, readily available and, most importantly, have the required regulatory approval for use in edible products. Mechanistically, the most important requirement for a material to act as an oleogelator is to display a suitable balance between its affinity for edible oils (i.e. weak interactions with unsaturated triacylglycerols) and sufficient insolubility in these solvents in order to first trigger a 'bottom-up' phenomenon leading to the formation of primary particles as a function of super saturation and external factors, such as temperature, that can alter the solute-solvent interactions. These primary particles then need to undergo molecular self-assembly (a process by which individual molecules form defined aggregates) and subsequent self-organization (a process by which the aggregates create higher-ordered structures) to create supramolecular structures depending on the delicate balance between solute-solute and solute-solvent interactions. These supramolecular structures include: crystal lattice, liquid crystals, micelles, bilayers, fibrils, and agglomerates, which may form a three-dimensional network to physically trap the liquid oil in a gel-like system. These supramolecular structures or molecular assemblies or building blocks are usually stabilized by non-covalent interactions, such as H-bonding, van der Waals attractive interactions (London dispersion forces) and π-π stacking. The growth of molecular clusters or assemblies into a continuous network is further governed by the anisotropy of the specific surface free energy (at active sites) as well as the anisotropy of the mobility of the diffusing molecules (or the kinetic coefficient).
So far, the mechanisms responsible for the formation of the structuring units in edible oleogels have not been studied in detail. Most of the knowledge related to self-assembly and self-organization of edible oleogelators has been obtained mainly from in-depth studies done using 12-hydroxystearic acid and its derivatives as gelators in a range of organic solvents.' In rare cases, the correlation of self-assembled structure formation with solvent properties (Hansen solubility parameters) has also been explored to better understand the mechanism involved in the formation of supramolecular gels. As reported in the literature, a weak solvent-gelator interaction results in dominant gelator-gelator interactions, which may lead to the formation of a continuous network. However, a much stronger gelator-gelator interaction will eventually lead to the precipitation of crystalline or amorphous molecule clusters resulting in phase separation. Therefore, a suitable balance between solvent-gelator and gelator-gelator interactions ultimately governs the formation of a continuous (percolated) network of self-assembled gelator molecules that provides the structural framework for gelation of the solvent. As seen with the colloidal gels, arrested phase separation may be considered as one of the mechanisms that drives 'out-of-equilibrium' gelation in oleogels. Basically, the micro- to macro-phase separation is interrupted by a dynamical arrest, which leads to the immobilization of the solvent (gelation). In the majority of cases, equilibrium gelation may follow the conventional route, which involves primary particles forming transient clusters, which in turn form a transient network that further transform into a percolation of long-lived clusters that cause gelation of the solvent. Hence, oleogelation can be considered to be a rather complex process as it involves supramolecular interactions at the primary, secondary and even tertiary levels.
Based on the above discussed mechanisms of gelation, some of the important conditions that need to be fulfilled for gelation of organic solvents include: (i) directional intermolecular interactions of gelator molecules that promote unidirectional growth of supramolecular aggregates; (ii) formation of internet-work secondary interactions leading to intertwined aggregates, and (iii) prevention of neat crystallization of gelator molecules. Accordingly, based on this understanding, molecular and crystal engineering concepts can be utilized to design and identify new gelator molecules. For instance, molecular features such as having moieties with hydrogen bonding functionalities and long alkyl chains that are capable of self-assembling (via London dispersion forces) could provide an early indication of gelation properties of components based on their molecular structures. On the other hand, molecules displaying crystallization properties such as unidirectional crystal growth (and suppression of lateral growth) and/or minimal post-crystallization events (such as crystal aggregation) may result in a more 'spread-out' crystalline mass in the continuous phase, forming a space-spanning network of crystals at a very low volume fraction. Additionally, information about thermodynamic dissolution parameters (enthalpies and entropies) of structuring agents in the chosen solvents could offer new insights into the gelation behaviour of gelators as well as the critical role played by the solvent in gelation. This information could also pave the way to creating novel 'solvent-mediated' gels where the poor solubilizing capacity of solvents and/or the difference in the crystallizing/aggregating/self-assembling properties of the solute in different solvents could be exploited to generate new types of gels. One of the most common examples in this category is lecithin-based oleogels, which are typically formed at certain critical ratios of vegetable oil and water as solvents. A similar concept could even be exploited to create water-free oleogels by investigating the crystallization/self-assembly behaviour of gelators in mixed solvent systems consisting of different types of oils (long chain triglycerides, medium chain triglycerides, oils rich in diglycerides and essential oils). For instance, natural waxes have been known to display different gelation behaviours (critical gelling concentration, sol-gel transformation temperature etc.) in vegetable oils with differing fatty acid profiles, and this is attributed to the difference in the gelator-solvent interactions that affects their crystallization properties (kinetics, molecular packing and crystal aggregation). A thorough investigation focusing on different gelator-solvent combinations may provide information on creating multi-solvent gels where the formation of molecular assemblies and their subsequent organization into a continuous network could be controlled.
Another molecular feature that may play a vital role in the molecular ordering of gelators is the chirality of the molecules. As demonstrated by Kim and co-workers, the presence of a stereogenic centre in the molecular structure strongly influences the molecular ordering, resulting in preferential one-directional growth of crystalline units, which in turn causes a decrease in the minimum gelation concentration of the structuring agent. Close to the edible field, the concept of chirality can be explored to probe the gelation properties of monoacylglycerols (MAGs). MAGs are esters of glycerol in which only one of the hydroxyl groups is esterified with a fatty acid, and they can exist in three different structural forms: sn-1, sn-2 and sn-3 isomers, depending on the location of the fatty acids on the glycerol backbone. Of these, sn-1 and sn-3 are not distinguished from each other and are termed 'α-MAGs', while the sn-2 isomers are called 'β-MAGs'. The second carbon atom on the glycerol backbone is a stereogenic centre, while P-MAGs are devoid of such a stereogenic centre. Commercial MAGs used in the food industry have more than 90% α-MAGs with β-MAGs at less than 5%. Owing to their broad functionalities, commercial MAGs are among the most common food emulsifiers used in a range of edible products. They have also been explored for their oil structuring properties both in monocomponent and mixed component gels. However, one of the drawbacks associated with MAGs includes polymorphic transition to gritty β-crystals on aging. This drawback could be solved by using β-MAGs instead of α-MAGs as the non-chiral nature of the former may help with a better crystallization profile and reduce the unwanted polymorphic transitions that cause stability issues in gels. Moreover, β-MAGs are considered to have higher surface activity compared to α-MAGs (which is reflected in their anti-bacterial effects), which may further enhance their self-assembly in oil medium. It is important to note, however, that the acyl migration phenomenon may pose an issue in obtaining a high yield of β-MAGs synthesized through conventional processes.
184.108.40.206 Oleogelators and Multi-component Gels
Multi-component supramolecular gels have not yet been explored to their full potential. As the name indicates, these gels are composed of multiple solute components that directly or indirectly assist in gelling the solvents. The simplest of these gels are two-component gels where synergistic interactions of two solutes are exploited to alter the formation of microstructure as well as the gel-supporting structural framework. What makes this category of gels particularly interesting is the possibility of tuning their properties by simple alteration of the proportions of the components. The two-component gels can be categorized into three general classes (Figure 1.1) including: (a) two component gel-phase materials where both components are required for gelation as the individual components either cannot form structured materials (higher ordered self-organized structures) or gel the solvents on their own; (b) two gelator component gels where both components are themselves gelators (having properties of forming structured materials) and when used in combination they are capable of organizing into assembled structures either together (co-assembly) or independently of each other (self-sorting), and (c) gelator plus additive component gels, which are formed by a combination of a gelator and a non-gelling additive. The additive is required to either impact the self-assembling properties of the gelator or to promote an effective spatial distribution of building blocks formed by the gelator or to strengthen the network linkages among formed building blocks.
Lecithin–sorbitan tristearate oleogels are good examples of two-component gel phase systems as both lecithin and sorbitan tristearate (STS) are incapable of forming gels on their own but, when mixed at certain ratios, undergo synergistic association to form gels. The crystalline units formed in these systems are based on STS, while the surfactant (lecithin) plays an important role in influencing the morphology of the crystalline units and strengthening the network junctions among the formed units. Similarly, this kind of synergistic association is also observed when lecithin is combined with sucrose oligoesters (Figure 1.2). As seen from the photographs in the figure, both components are incapable of gelling oil on their own but they form a gel when mixed together at a specific ratio. Microstructure studies suggest that the crystalline units in the gels are based on sucrose oligoesters and the inclusion of lecithin influences the crystallization and aggregation behaviour of the units, leading to a finer network that is capable of physically immobilizing the liquid oil. Some other examples explored for creating this class of gels are oleic acid + sodium oleate and lecithin + α-tocopherols In most cases, the main component is responsible for providing structured materials (crystalline particles, lamellar phases) in the bulk oil phase, while the presence of the other component is necessary for modifying these basic structured materials into units that can form a space-filling network.
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Table of Contents
Section 1 – Introduction: Oil Structuring: Concepts, Overview and Recent Progress; Section 2 - Structuring Units: Bio-Based Molecular Structuring Agents; Biomimicry: An Approach For Oil Structuring; Section 3 - Structuring Units: Crystalline Particles and Self-Assembled Structures: New Insights into Wax Crystal Networks in Oleogels; Structuring Edible Oil Phases with Fatty Acids and Alcohols; Gelation Properties of Gelator Molecules Derived from 12-Hydroxystearic Acid; Section 4 - Structuring Units: Polymeric Strands and Network: Thermo-Gelation of Ethylcellulose Oleogels; Proteins as Building Blocks for Oil Structuring; Oleogels from Emulsion (HIPE) Templates Stabilized by Protein-Polysaccharide Complexes; Cereal Protein Based Emulsion Gels for Edible Oil Structuring; Section 5 - Edible Applications: Edible Applications of Wax-Based Oleogels; Edible Applications of Ethylcellulose Oleogels; Section 6 - Functional Colloids from Structured Oils: Non-Aqueous Foams Based on Edible Oils; Innovative Dispersion Strategies for Creating Structured Oil Systems; Subject Index