The role of solvent composition (polar minor components, PC) on β‐sitosterol/γ‐oryzanol oleogel formation, appearance, and properties is studied. Solvent composition is altered via thermal treatment and the elimination of natural PC from untreated sunflower and canola oil. A maximum in oleogel hardness is found. DSC reveals that sol‐gel transition is increasingly suppressed with the level of PC. This is attributed to two distinct mechanisms: enlarged interactions of PC with the structuring elements and a reduction in diffusion due to a higher solvent viscosity. Gel‐sol‐transition consists of two concurrent processes: decomposition of tube bundles (peak fronting) and the dissolution of tubes (peak). The peak area decreases with increasing solvent permittivity while simultaneously the area of the fronting increases. Gel‐sol‐transition temperatures are practically invariable for all samples indicating that the maximum in gel hardness cannot be related to changes in the solubility of the sterols. AFM microscopy reveals that the arrangement of the elements of scaffolding changes considerably with the level of PC. This work demonstrates the strong impact of PC on the self‐assembly of β‐sitosterol/γ‐oryzanol and oleogel properties. A detailed characterization of the oils is thus inevitable to perform trustworthy research. Practical Applications: It is widely known that a high intake of saturated fatty acids increases the risk of suffering from cardiovascular diseases. Unfortunately, their ability to provide unique texture to food products can hardly be met. Oleogelation has the potential to deliver the solid structure necessary for various fat‐based food products by transferring an oil rich in essential fatty acids into a solid‐like structure. Moreover, the nutritional value of these oils remains nearly unchanged. It is found that oil composition, precisely the fatty acid composition of the TAGs and minor oil components have a profound impact on oleogel properties. Fundamental understanding of network properties and formation of oleogels helps to maximize their capability for industrial application. The work presented reveals that detailed documentation of the quality of the oils, in particular the fatty acid composition and presence of polar minor components, is a necessary prerequisite to conduct reliable scientific work in the field oleogel research. Hardness and spatial arrangement of network building blocks of of β‐sitosterol/γ‐oryzanol oleogels is modified in the presence/absence of polar minor components.
Oleogels offer the possibility to replace conventional saturated fatty acid (SAFA)-based lipids with a healthier alternative by immobilizing liquid edible oils in a 3D-network which is provided by an oleogelator. Numerous molecules which can structure oils rich in (poly)unsaturated fatty acids have been identified. These differ greatly in their chemical composition, network formation, and interactions and thus macroscopic properties of the respective oleogels. Oleogels have been a focal point of food research for over 20 years, yet product applications are lacking. Hence, the question arises whether the application of oleogels is unfeasible or if science lost sight of its objective. This review aims to assess different structuring systems concerning their availability, their potential for the utilization in food products and, if possible, their prices. Moreover, recent studies comprising the application of oleogels in food products are reviewed with special emphasis on the state and the function of the lipid phase during processing and in the final product. Therefore, the physical properties and preparation methods of different oleogels need to be considered in connection with the respective food application. Finally, it is discussed whether the application of oleogels is justified in these products and advantageous in comparison to liquid oil. Practical Applications: A diet rich in mono-and polyunsaturated fatty acids which make up the majority of liquid edible oils lowers the risk to suffer from cardiovascular diseases. Unfortunately, these oils cannot provide texture to food products in their native state. Oleogelation has the potential to deliver the solid structure necessary for numerous food products by transferring an oil rich in essential fatty acids into a solid-like structure. Besides, the nutritional value of these oils remains practically unchanged. Although oleogelation has been the objective of various research groups for more than 20 years, product applications are scarce. This review aims to stimulate the mindfulness of research concerning the successful application of oleogels in food products. This hopefully enables a better connection between science and industry.
Oleogels or, more precisely, non-triglyceride structured lipid phases have been researched excessively in the last decade. Yet, no comprehensive knowledge base has emerged, allowing technology elevation from the laboratory bench into the industrial food application. That is partly due to insufficient characterization of the structuring systems studied. Examining a single composition decided upon by arbitrary methods does not stimulate progress in the research and technology area. A framework that gives much better guidance to product applications can easily be derived. For example, the incremental structure contribution concept is advocated as a parameter to compare the potency of structuring systems. These can straightforwardly be determined by combining solubility data and structural measurements in the recommended manner. The current method to determine the oil-binding capacity suffers from reproducibility and relevance. A newly developed method is suggested to overcome these shortcomings. The recommended new characterization of oleogels should contribute to a more comprehensive knowledge base necessary for product innovations.
The role of the fatty acid (FA) composition of triglycerides (TAGs) on sterol/sterol ester oleogels has been studied. Minor oil components of three vegetable oils with varying degrees of unsaturation (iodine values, IV) were removed. Typical oil quality parameters were determined before and after the treatment, and oleogels were produced using all six oils. Characteristic gel properties such as transition temperatures, mechanical properties and microstructure were tested. The results were compared regarding the impact of IV and the stripping procedure. Minor components were essentially removed during stripping, resulting in significantly different oil properties such as peroxide value, free FA and viscosity. However, peroxides formed rapidly in stripped flaxseed oil. Gel–sol transition temperatures and enthalpies were higher in gels from untreated oils and decreased with IV in samples with stripped oils. In contrast, the sol–gel transition was suppressed due to minor oil components in untreated oils. The effect of IV on gel formation was much less and linked to a lower solvent viscosity in more unsaturated oils. Nevertheless, gel firmness was significantly higher in oleogels from untreated oils and decreased slightly with IV in stripped oils. That was associated with differences in the arrangement of network building blocks, which was confirmed using atomic force microscopy. This study showed that the FA composition of TAGs has a limited effect on oleogel properties compared to those of minor oil components. The next part of this study focuses on modifying oleogel properties by adding selected minor components to stripped oils at varying concentrations.
The role of selected minor oil components on sterol/sterol ester oleogels was studied. Therefore, oleic acid, tocopheryl acetate and monoglycerides were admixed with three vegetable oils, having different fatty acid compositions. Before that, minor natural components were removed from untreated oils (purification). Moreover, purified oils were subjected to a humidity treatment to increase their water content. All additives retarded the molecular self‐assembly of sitosterol with oryzanol, and the effect was dose‐dependent. Gel hardness only increased at low concentrations of tocopheryl acetate and decreased in all gels at higher concentrations. In contrast, Gmax′ was invariable in samples with oleic acid and monoglycerides and increased in gels containing tocopheryl acetate and water. Therefore, Gmax′ does not necessarily relate to the gels' compression firmness. Atomic force microscopy showed that the microstructure of oleogels was considerably modified by the additives. In general, a packed surface of twisted, thick bundles of tubules may be associated with a stiffer gel. Moreover, a composite structure in gels with monoglycerides was visible and confirmed by differential scanning calorimetry (DSC). DSC was used to determine gel–sol transition temperature and was associated with the number of tubules in the gel. The gel–sol temperature increased in samples 1.0% w/w oleic acid and tocopheryl acetate and decreased in gels with monoglycerides and water. The results show that oleogel properties can be significantly modified by minor components with functional groups. That was associated with interactions with the sterol and sterol ester in solution and with the surface of the tubules (ferulic acid moieties of oryzanol) in oleogels.
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