1. Introduction
Recently, food systems have undergone significant changes due to advances in food processing, and the current fast-paced way of living has increased the demand for more available and affordable food products. Traditional diets, featuring whole or minimally processed foods, were gradually replaced in modern society by industrialized and pre-prepared food products [
1]. For technological reasons, saturated fats are common in many processed foods. The addition of fats to processed food products allows for the exhibit of interesting organoleptic properties, such as texture, mouth-feel, and flavor. This creates a barrier to its substitution in foods since it can significantly overcome the mentioned characteristics and, consequently, hinder the overall eating pleasure. The intake of essential fatty acids and antioxidants naturally occurring in traditional diets has consequently given place to the consumption of unhealthy fats, which are deeply related to health disorders, such as coronary heart disease, inflammation, oxidative stress, and metabolic syndrome [
2]. Currently, food trends have been shifting towards healthy eating and plant-based diets. Consumers are increasingly aware of the negative environmental impacts caused by animal source food production and of the dangers that are associated with the consumption of overly processed animal products, therefore generating new challenges and roadblocks in the food sector in order to meet consumers’ demands [
3].
The pursuit of healthy substitutes for fats is not recent; the hydrogenation process has been developed as early as the beginning of the 20th century, as a strategy for replacing the consumption of animal saturated fats and cholesterol intake by vegetable and marine oils, rich in healthy polyunsaturated fatty acids (PUFAs). This process changes the degree of saturation and confers to these oils the firmness and plasticity desired by food manufacturers and consumers [
4]. During the second part of the century, it took over the everyday diet in the United States and several other western countries. At first, this seemed to be a positive alternative to saturated fats, thus being promoted by health advocates. However, a side effect of the incomplete hydrogenation of fats is the isomerization of the remaining double bonds, converting them to the trans-configuration [
5]. In the 1990s, the first studies on the dietary impact of trans fats appeared, associating them with unfavorable effects on the serum lipoprotein profile. Trans fats were proven to not only raise low-density lipoprotein (LDL) cholesterol levels but also to lower high-density lipoprotein (HDL) cholesterol levels [
5]. Since then, the World Health Organization (WHO) has strived to eliminate artificial trans fats from food supply chains, with some countries being pioneers in entirely eradicating the manufacture of food products using trans fats [
6]. In the United States, trans fats are no longer considered Generally Recognized as Safe (GRAS), and the Food and Drug Administration (FDA) and the European Union (EU) have established the deadline of 1 January 2021 for food producers to adopt the new guidelines regarding trans fats [
7,
8].
The liquid nature of vegetable oils limits their utility in some applications, since they do not share the same properties as solid fats [
9]. Considering this, and within governments’ frameworks for removing unhealthy fats from the market, new developments in oil structuring systems have appeared in the food industry, with oleogels being at the forefront of the scientific quest. Oil structuring is based on the formation of a gelator network that allows for the formation of a self-standing thermo-reversible viscoelastic structure without affecting the chemical structure of the oil. Unlike the hydrogenation process, oleogels are a viable way of structuring oils that are rich in PUFAs without undermining their health potential [
2]. The acceptability of oleogels by consumers depends on their capability of mimicking the characteristics of solid fats. Likewise, oleogels can be tailored to match a specific purpose, with structural and textural characteristics being an essential factor. Other than the replacement of non-healthy fats with healthy oils, oleogels can also have added nutritional value through the addition of bioactive compounds to the formulation. This functionality can be an advantage in nutritional terms and in making the product more attractive in terms of stability and shelf-life.
The main hindrance to using oleogels in food matrices is their low compatibility with water-based food products. As a way of circumventing this problem, oleogels can be used in emulsion templates by applying conventional emulsification techniques, such as high-shear homogenization [
10]. The interplay between oleogelation and emulsification expands the possible applications of oleogels in the food industry, making them a suitable tool for many new products. These biphasic systems are capable of constituting a structured system with the benefit of decreasing the fat content of a product and might be more suitable for certain applications as opposed to pure oleogels [
11]. Furthermore, this approach may broaden their potential to introduce hydrophilic bioactive compounds, rather than lipophilic compounds, or even allowing for the co-encapsulation of bioactive compounds in both phases of an oleogel-based emulsified system.
2. Hydrogels, Oleogels, Bigels, and Emulgels
Gels represent a type of colloid that consists of a solid-like three-dimensional network, in which a liquid phase is entrapped. A gel can be defined as a coherent system of at least two components, which exhibits mechanical properties of a solid, where both the dispersed component and the dispersion medium extend themselves continuously throughout the whole system [
12]. Hermans proposed this definition as the author of the first recorded effort to connect the macroscopic and microscopic properties of a gel, which helped to define its hybrid characteristics between liquid and solid materials [
12]. Gel formulations can be divided into two major classes according to the solvent used for their production; hydrogels refer to the case where the liquid phase is water, and organogels (or oleogels) when the dispersed liquid is an organic solvent and is structured by an organogelator [
13]. Hydrogels consist mainly of a hydrophilic polymeric network, which can absorb high quantities of liquid (
Figure 1). The capability of hydrogels of absorbing fluids arises from the hydrophilic functional groups on the backbone of the crosslinked polymer chains, therefore facilitating the diffusion of liquid and important solute molecules [
14]. Hydrogels have certain characteristics, i.e., their hydrophilicity, flexibility, elasticity, softness, and high swelling capability, which allow them to be applied in a plethora of different applications [
15]. Usually, hydrogels are commonly associated with pharmaceutical applications since their features make them highly efficient for transdermal drug delivery [
13]. As such, many novel hydrogel-based delivery matrices have been designed for pharmaceutical and medical fields, playing a vital role in diagnosis and treatment [
16,
17,
18]. Recently, hydrogels have been explored in other areas, such as tissue engineering, cosmetics, and food technology, with an increasing number of publications on the subject [
19,
20,
21,
22,
23,
24].
Organogels are semi-rigid formulations considered bicontinuous systems, comprising two phases: the gelator and the organic solvent (
Figure 1). The gelator, when used in the formulation of organogels in concentrations of <15%, may experience physical and chemical transformations that create self-assembled structures; these structures entangle with each other, forming a three-dimensional network. The organic solvent is retained and immobilized within the spaces of the gelator network. If the used solvent is a liquid oil, then the term oleogel is also appropriate for these formulations. Therefore, oleogels allow properties to be explored that hydrogels are not compatible with, such as hydrophobicity of compounds [
13]. One of the main advantages of oleogels is the possibility of carrying lipophilic bioactive compounds, which is of great utility in both pharmaceutical and food applications [
11]. The combined action between structure and health benefits supports the important role that oleogels can have in novel food products, as they can be tailored to meet the ideal properties for a food product, acting as a healthy substitute for solid fats [
25,
26]. Great attention from the scientific and industrial communities towards oleogels has risen since they were first suggested as a possible substitute for fats.
Certain types of gels are developed in a way that combines certain characteristics of both hydrogels and oleogels. These hybrid gels, or bigels, are systems that, in general, contain two immiscible liquid phases that are individually stabilized by independent gelators (
Figure 1) [
13]. Bigels display the merits of both the aqueous and oil phase, including the ability to deliver simultaneously hydrophilic and lipophilic active agents, and improved viscoelasticity [
26]. Bigels have been mainly applied in cosmetic/pharmaceutical formulations, since their hybrid characteristics are proven to maximize moisturizing benefits for the skin and simplify the penetration of active agents to deeper layers of the skin tissue [
27,
28,
29,
30]. These benefits may also be very valuable for food applications in the delivery of bioactive compounds [
31]. Despite their recognized potential, bigels are still underexplored in terms of their microstructure, which has been scrutinized only in recent years [
32].
On the other hand, emulgels may be considered a type of hybrid between emulsions and gels (
Figure 1). Emulgels result from an initial emulsification process followed by a gelation process, through gelation and/or crosslinking of the compounds that are present in the mixture. The amphiphilic behavior of the emulgels, which is potentiated by the hydrophilic and lipophilic affinities of its constituents, makes them a good option for the delivery of active agents, in a similar way to bigels [
26,
33,
34,
35]. In this way, emulgels have been at the forefront of the topical and transdermal delivery of drugs, due to characteristics such as their easy removability, emollient action, ease of extrusion, and spreadability. However, these are the same characteristics that hinder emulgels from being as appealing for food applications, in addition to their stickiness and proneness to phase separation [
13,
14].
Figure 1 presents a schematic representation of oleogels, hydrogels, bigels, and emulgels structures.
5. Oleogel-Based Systems as a Vehicle for the Delivery of Bioactive Compounds
As aforementioned, the structuring of liquid oils rich in PUFAs can bring significant benefits for human health, since they act as fat substitutes and have a much richer constitution than typical solid fats. Furthermore, the structure of oleogels constitutes a good matrix for the delivery of bioactive molecules, with it being both a way of protecting the integrity of bioactive compounds against oxidation or loss of functionality and a way of controlling their release. However, this area is still scarcely explored, and most of the studies rely on the integration of liposoluble compounds in the oleogel structure rather than hydrosoluble compounds. This seems to be the most straightforward approach due to the lipophilic nature of oleogels [
57]. Oleogels also often serve as a starting point for the development of more complex structures, such as oleogel-based emulsions or oleogel-based Pickering emulsions, either to obtain additional benefits or to increase the formulation’s versatility.
Table 1 presents some of the food-grade oleogel systems with proven effectiveness in the delivery of bioactive compounds.
Curcuminoids are a class of compounds that have been successfully studied in oleogels. Their water insolubility and rapid metabolism greatly affect their bioaccessibility and bioavailability, which hinders the rea** of its health-promoting benefits. Despite previous efforts to encapsulate curcuminoids, such as regular O/W emulsions, microemulsions, and solid lipid particles, the problem of its bioavailability was not explored thoroughly. The first oleogel system established for the delivery of curcuminoids dates from 2012 [
84]. This study by Yu et al. encompassed a comparison between different oils and additives; in the end, the oleogel was prepared with medium-chain TAGs with added Span 20 and monostearin. This selection was based on the metastable solubility and bioaccessibility of the curcuminoids after lipolysis. Although the metastable solubility of the curcuminoids was not the highest on the medium-chain TAGs, after in vitro lipolysis, these were observed to generate higher bioaccessibility. The gelation process was proven not to affect the bioaccessibility, and oleogels were formed with a successful loading of 2.6% (
w/w) of curcuminoids with a bioaccessibility of 80% in a fasted state. These oleogels were used for the fabrication of rapid-digestion emulsions, further proving that the delivery of poorly water-soluble nutraceuticals can be achieved through oleogel-based systems [
85]. Since then, other types of formulations have been developed for the delivery of curcuminoids, with authors making use of the diversity of edible gelators suitable for oil structuring. Li et al. [
86] developed a novel curcumin-loaded oleogel formulation, making use of the capability of β-sitosterol and lecithin to form self-assembled fibers, and studied its oxidative stability and release behavior. The developed structure protected the curcumin from being oxidized; on the other hand, a reciprocal effect was observed, where curcumin-loaded oleogels featured higher shelf-life stability when compared to non-loaded β-sitosterol + lecithin oleogels. The curcuminoids did not interfere with the gel network assembly, resulting in oleogels very similar in structure to the non-loaded oleogels.
Figure 5 shows the oleogels developed with β-phytosterol and lecithin unloaded and loaded with curcumin.
Vellido-Pérez et al. [
87] designed a formulation comprising a fish oil concentrate as the lipid phase, aiming to stabilize and transport curcumin as a bioactive and the protection of the lipid phase. A powdered form of fully hydrogenated rapeseed oil with crystallization properties was used as a gelator in a concentration of 12% (
w/w); both the gel structure and curcumin content in a minimum concentration helped to retard the oil oxidation. The oleogel structure and the manufacturing conditions were further optimized by a statistical experimental design and multi-response surface methodology. The curcumin content, the amount of gelator, and the manufacturing temperature were studied as independent variables, whereas the oxidation degree of the lipid matrix and the amount of loaded curcumin were selected using response surface methodology. The objective of this study was the simultaneous minimization of the lipid oxidation and maximization of loaded curcumin, with the optimum parameters for the studied variables being 0.150% (
w/w) curcumin and 4.461% (
w/w) oleogelator. Calligaris et al. [
88] recently studied the effect of the oleogelator type and its resultant structure on oil lipolysis and the bioaccessibility of the curcuminoids during in vitro digestion. The liquid oil consisting of high oleic sunflower oil was enriched with turmeric extract, which is very dense in several curcuminoid types with well-known health benefits. Oleogels were prepared by mixing the enriched oil with 5% (
w/w) of saturated MAGs, RBX, or a mixture of β-sitosterol and γ-oryzanol (2:3
w/w). Considering the rheology of the produced oleogels, all of them exhibited gel behavior, with the storage modulus being higher than the loss modulus. For a fixed frequency of 1 Hz, the RBX sample was shown to be the firmest, with the MAGs sample exhibiting the lowest storage modulus value. On the other hand, the β-sitosterol and γ-oryzanol samples demonstrated the highest yield stress value, which represents the value at which there is a structural breakdown of the gel. Regarding the bioaccessibility of the curcuminoids, the nature of the oleogelator significantly affected the outcome. The bioaccessibility of the curcuminoids included in the β-sitosterol and γ-oryzanol oleogel was comparable to the values referring to non-structured oil, suggesting that the gelator structure does not hinder the release of the bioactive molecules, and therefore does not compromise their bioaccessibility. On the other hand, the presence of crystalline networks (MAGs and RBX) presented lower bioaccessibility of the loaded curcuminoids, possibly due to interference of the crystalline particles with the bioactive compounds. The extent of lipolysis was also affected by the oleogelator options, and the lipid digestion was evaluated by measuring the free fatty acid release during the intestinal phase of the in vitro digestion, to which β-sitosterol and γ-oryzanol demonstrated the lowest extent. On the other hand, MAGs and RBX oleogels demonstrated lipolysis values closer to those reported for non-structured oil. In fact, different structural networks affect lipid digestion in different ways, possibly due to difficulties in lipase accessing the TAG digestion sites. The most up-to-date application of curcumin in an oleogel-based system was developed by Liu et al. [
89]; the aim was to study the influence of a surface-active agent in the gelation process of ethylcellulose, in order to increase the loading of curcumin by reducing lipid oxidation and simultaneously improving curcumin solubility and chemical stability. At a morphological level, the samples prepared without the surface-active agent (sorbitan monopalmitate) had a wide distribution of pore size, and the gel network was very heterogeneous, with some areas being close to collapsing due to the connection of bigger pores. The addition of a small content of sorbitan monopalmitate reduced the number of large pores, an observation that was gradually noticeable with the increase in sorbitan monopalmitate content. This is evident in
Figure 6, where the microstructures of the developed oleogels are shown. Evidently, with larger pores, there are bigger oil droplets entrapped within the network, which affects the macroscopic properties. The rheological data showed improved viscoelastic properties of the oleogels prepared with the surface-active agent, which might be associated with the interaction between the surface-active agent and the polar entities of the EC backbone. The results also showed that the creation of a more compact network had a direct influence on the inhibition of the formation of curcumin crystals, the slowing down of lipid oxidation, and resistance to UV radiation exposure.
Curcumin was also used by Ojeda-Serna et al. [
90], in addition to two other poorly water-soluble compounds, betulin and quercetin, for a water-in-oleogel emulsion formulation, with the aim of increasing their bioaccessibility and cell permeability. The bioactive compounds were incorporated in the oleogel preparation, which was done using a MAGs blend as the gelator in a 10% (
w/w) concentration. It was shown that not only the gelator can induce variations at the lipolysis and bioaccessibility level, but also the type of bioactive molecule influences these parameters, with distinct observations for each compound. Results showed that the use of emulsified oleogels enhanced the apparent permeability of betulin, which was not observed when curcumin and quercetin were used. On the other hand, regarding the bioaccessibility of each compound, the formulation was shown to enhance the bioaccessibility of quercetin but not of the other bioactive compounds. This was not the first oleogel formulation aiming at the delivery of quercetin, with Rocha-Amador et al. [
91] focusing on the preparation of quercetin-loaded oleogels with canola oil, corn oil, and soybean oil. Quercetin is a flavonoid that can be widely found in most edible plants, typically in the glycoside-bound form. The authors aimed at assessing the effect of quercetin’s degree of glycosylation on its in vitro bioaccessibility. The results showed that, independently of the type of oil, lower bioaccessibility was observed for the higher degree of glycosylation. This might be associated with the impact of the glycoside group on the oleogel structure, which creates a more elastic and resistant network, hindering the release of the bioactive compounds. The results indicated an improved bioaccessibility of the quercetin when compared to bulk oil for both glycosylation states.
Another important bioflavonoid that was implemented in oleogel-based formulations is hesperidin, which has very low oral bioavailability due to its poor water solubility. Wei et al. [
92] studied the integration of hesperidin in an oleogel-based Pickering emulsion stabilized by ovotransferrin fibrils, which were selected due to their high abundance and nutritional value. Hesperidin-loaded oleogels were prepared using soybean oil and monostearin, which were then used for the preparation of the Pickering emulsions. Both the lipolysis rate and bioaccessibility were proven to improve in the Pickering emulsion when compared to the oleogel. The ovotransferrin fibrils were also proven to be a very effective stabilizer at a very high internal phase volume ratio. Oleogel-based emulsions may also come up as a way of delivering nutraceuticals such as capsaicin, which is characterized by an intensely pungent flavor that results in a burning sensation; Lu et al. [
93] developed a novel formulation that effectively alleviated the irritating effects of the capsaicin.
Carotenoids are a class of organic pigments that can be found in many fruits and vegetables, but also fungi, algae, and photosynthetic bacteria. Overall, carotenoids exhibit antioxidant properties, but individual carotenoids can display other characteristics. β-carotene (BC) is responsible for conferring a strong red-orange coloration to plant tissues and is also a precursor of Vitamin A. Vitamin A deficiency is a major health problem, especially in develo** countries, and supplementation with Vitamin A is not an easy strategy to implement. As such, fortification of food with BC could be a viable alternative. Moreover, the consumption of a moderate dose of BC appears to have effects on eye health and cognitive performance, which may be associated with its antioxidant properties. Other than the dietary impact of BC itself, its ability to be converted to Vitamin A expands the ground of added health benefits, such as the improvement of immune function [
108]. BC was first tested out as proof of principle for the capability of ethylcellulose oleogels to deliver bioactive compounds effectively by O’Sullivan et al. [
94]. In this first approach, properties such as mechanical strength, in vitro digestibility, BC accessibility, and stability in the oleogel matrix were assessed, reinforcing oleogels’ value as carriers. These studies were performed using ethylcellulose’s physicochemical properties, particularly its viscosity, as a variable. The viscosities refer to the polymer molecular weight distribution, being commercially available in a range of viscosity values.
Other gelators such as MAGs have also been shown to be very effective for the delivery of BC. Fan et al. [
95] developed both an oleogel and an oleogel-based emulsion for the delivery of BC. Although oleogel-based emulsions featured some benefits when compared to oleogels concerning the loaded amount, bioavailability, and biological activity of said compounds, little information was established for the case of BC. This work encompassed the preparation of oleogels using different liquid oils and the assessment of the BC accessibility in these oleogels; corn oil oleogels showed the most interesting results, which can be related to its length and unsaturation degree, serving as a starting point for the oleogel-based emulsion. The following step involved the assessment of different emulsifiers, specifically Tween 20, 40, 60, and 80, referring to an aliphatic chain of 12, 16, 18, and 18 carbons, respectively. Among the tested emulsifiers, Tween 20 facilitated the highest BC accessibility and extent of lipolysis, being chosen as the emulsifier for the nanoemulsions. Finally, the comparison between BC-loaded oil, oleogel, and oleogel-based nanoemulsions has revealed a positive tendency for both the bioaccessibility and extent of lipolysis, with the oleogel-based nanoemulsions showing the highest values. Moreover, the cellular uptake of BC-loaded in nanoemulsions was appreciably higher than in suspension, indicating that the emulsification process can improve the absorption of encapsulated BC. The results were also positive for the in vivo studies of bioavailability, which showed an increase in bioavailability of 11.5-fold compared to BC in bulk oil. Cui et al. [
96] studied the effect of MAG content on the solubility and chemical stability of oleogels, by develo** a corn oil-based oleogel, comprising different contents in MAGs. This allowed the obtaining of oleogels with different properties, and a positive relationship was observed between the concentration of MAGs and the strength of the gel network. Additionally, the solubility of BC was higher in the oleogels than in bulk oil, which is favorable for eventual food applications.
On the other hand, Martins et al. [
97] developed a novel high oleic sunflower oil-beeswax system fortified with BC. The intent was to understand the structural implications of the addition of a compound such as BC in the formulation, encompassing polarized microscopy and rheological analyses, differential scanning calorimetry, wide-angle and small-angle X-ray analysis, oil binding capacity, oxidative stability, and color evaluation. Although the polarized microscopy observations did not exhibit relevant differences between non-loaded and loaded oleogels, the rheological analysis showed that the addition of the BC promoted changes in the crystallization process. The cooling process of the non-loaded oleogels happened more abruptly, with the cooling curve displaying a more pronounced ‘step’, possibly influenced by the heterogeneity of the beeswax composition. On the other hand, the loaded oleogels exhibited a more gradual increase of the viscoelastic properties during the cooling period and a less abrupt ‘step’, masking the effect of the heterogeneous beeswax structuring. The isothermal frequency curves showed the presence of a strengthened conformation in the oleogels prepared with BC, in comparison with that in oleogels prepared without BC. The X-ray results confirmed that the BC-loaded oleogels suffered dissimilarities at a structural level, affecting the positioning and size of the lamellar structures. The oil binding capacity is related to the capacity of beeswax crystalline structure to retain the oil, which helps to identify the relationship between oleogel strength and oil binding capacity. The observed tendency was that for low concentrations of beeswax, there is a clear improvement of the oil binding capacity with the addition of BC. While this type of relationship was not observed for higher concentrations of beeswax, the addition of BC did not hinder the oil binding capacity of the formulation. Qi et al. [
98] also selected beeswax as an oleogelator in a novel oleogel-in-water Pickering emulsion approach for BC delivery. The study aimed at comparing the role of the beeswax versus a conventional Pickering emulsion (prepared without an oleogelator) on the oxidative stability and bioavailability of the BC. The oleogel-in-water Pickering emulsion demonstrated improved stability when subjected to a range of pH and salt concentrations and freezing-thawing stability, in addition to the enhanced chemical stability and bioavailability of the BC.
Although BC is the most widely studied carotenoid for food applications, other carotenoids such as lutein ester also possess strong health benefits. Jiang et al. [
99] aimed at resolving one of the major problems with the stabilization of lutein ester in food applications, which is its poor light stability, by the oleogelation approach. By using monostearin as an oleogelator, it was concluded that the oleogel structure effectively prevented the degradation of lutein ester by UV irradiation, which was positively related to the content in monostearin.
One important property that should be modulated in the delivery of nutraceuticals via oral administration is the kinetics of the release, which is why oleogel structures are useful since they are capable of controlled release. Ferulic acid (FA) is known for its widespread use in anti-ageing creams, featuring strong antioxidant and anti-inflammatory properties. Its application in edible oleogels met its start in 2013 when an oleogel formulation was defined using olive oil as a vehicle. FA was submitted to a drastic acidic ambient to mimic stomach conditions, and the oleogel structure was capable of protecting the integrity of the FA to fulfil its nutraceutical function, with great rheological properties. In vitro release tests proved that the control samples, prepared without policosanol, were almost completely released after 2 h in stomach conditions, while by adding as little as 1% of policosanol, the delivery was controlled and delayed [
100].
D-limonene is one of the main constituents of all citrus-derived essential oils, being GRAS for use as a flavoring agent and food preservative. However, it is very prone to oxidative degradation, causing the loss of its lemon-like flavor. As it is a hydrophobic compound, an oleogel-based approach seems very suitable for its protection in food products. Zahi et al. [
101] developed a stable oleogel-based nanoemulsion using the method described by Yu et al. [
85] with variations and performed an iterative method of selection of the oil phase, the emulsifier, and oleogelator. The final formulation, prepared with MCT oil, stearic acid, and Tween 80 at 10% (
w/w), showed good stability in terms of Ostwald ripening and coalescence of the droplets during storage. Bei et al. [
102] developed a novel system for the co-loading of D-limonene and nisin, both having reported antimicrobial activity. The preparation of an oleogel-based emulsion allowed for the dispersion of the D-limonene into aqueous phases in the form of small droplets, with an overall improvement of the antimicrobial properties. This approach may substitute current food preservative options as a more ’natural’ alternative.
Most of the above-described alternatives encompass an enrichment of the product’s nutritional level through the use of bioactive compounds. However, some authors focused on the aromatization of oleogels and oleogel-based emulsions, mostly envisioning their eventual applications as breakfast spreads and other similar products. Yilmaz et al. [
103] focused on both the aromatization and the nutritional enrichment of the oleogels, develo** an oleogel based on hazelnut oil and waxes. None of the included compounds undermined the gelation process, and their concentrations were intact after 3 months of storage. Through the performance of sensory descriptive analysis of the oleogels, parameters such as appearance, texture, aroma, flavor, and mouth-feel were assessed; this type of data is a first in the oleogel literature and is crucial for analyzing the acceptability of novel food products by the consumers. Chen et al. [
104] developed a new strategy for the controlled release of volatiles with an oleogel-based emulsion approach, using a γ-oryzanol + β-sitosterol oleogelator system. The release behavior was registered under dynamic conditions using a self-designed model mouth cell, and the role of the microstructural characteristics on the volatiles’ release was assessed. The sitosterol adsorption to the droplet interface acted as an enhancer for the barrier properties of the droplet membrane, strengthening the barrier properties. Moreover, the co-operative self-assembly of the oryzanol–sitosterol molecules demonstrated its value in delaying the release of the volatiles to the interface. This constitutes an advantage over conventional emulsions, promising a system with an improved flavor profile in low-fat and sterol-rich food products. Yang et al. [
105] also investigated the potential of the delayed release of volatiles in an oleogel formulation comprising a mixture of phytosterols and MAGs as the oleogelator.
Tea polyphenols are known for their antioxidant properties and free-radical scavenging capability. However, due to their low solubility in oils, their use in food products with high lipid content is hindered. Shi et al. [
106] developed an approach that encompasses the inclusion of tea polyphenols in emulsion-based oleogels, to benefit from its properties for preservation of the oleogels during storage. To allow the dispersibility of the tea polyphenols in the oil matrix, initially, a stearic acid–surfactant–tea polyphenol complex was prepared through emulsification and lyophilization, which was later dissolved in liquid oil in different concentrations. The tea polyphenols’ antioxidant activity was comparable to chemically synthesized food additives, and this approach seemed to be effective in delaying the onset of oxidative rancidity, serving up as a good strategy for combining the potential of water-soluble ingredients with lipid-rich food products. This approach marked a very significant difference regarding previous works, where only hydrophobic compounds were studied for oleogel-based formulations. Andrade et al. [
107] also developed a novel mechanism for the delivery of hydrophilic vitamins, in a double-emulsion approach. This system envisioned the co-delivery of both hydrophilic and lipophilic compounds with increased stability when compared to conventional double-emulsions, and the assessment of the double emulsion behavior during in vitro digestion. A solution containing water, ions, and Vitamin B
12 was used as the internal phase; the oil phase was partially crystallized with the addition of trimyristin to soybean oil and using phytosterol and Vitamin D
3 as bioactive compounds. The external water phase was prepared with water and ions for the maintenance of the osmotic and Laplace pressures between the two water phases. Comparing the double emulsions with and without a gelled oil phase, the first samples exhibited increased lipid digestibility and an extended release of the lipophilic bioactive compounds. Additional experiments were performed with a gelled internal aqueous phase, which was proven to affect the release behavior of the compounds. This study suggested that the double-emulsion approach is not effective in protecting the hydrophilic compounds dispersed in the water phase, unless the water phase is gelled, which garnered more positive results.