Oleogels—Innovative Technological Solution for the Nutritional Improvement of Meat Products

Food products contain important quantities of fats, which include saturated and/or unsaturated fatty acids. Because of a proven relationship between saturated fat consumption and the appearance of several diseases, an actual trend is to eliminate them from foodstuffs by finding solutions for integrating other healthier fats with high stability and solid-like structure. Polyunsaturated vegetable oils are healthier for the human diet, but their liquid consistency can lead to a weak texture or oil drain if directly introduced into foods during technological processes. Lately, the use of oleogels that are obtained through the solidification of liquid oils by using edible oleogelators, showed encouraging results as fat replacers in several types of foods. In particular, for meat products, studies regarding successful oleogel integration in burgers, meat batters, pâtés, frankfurters, fermented and bologna sausages have been noted, in order to improve their nutritional profile and make them healthier by substituting for animal fats. The present review aims to summarize the newest trends regarding the use of oleogels in meat products. However, further research on the compatibility between different oil-oleogelator formulations and meat product components is needed, as it is extremely important to obtain appropriate compositions with adequate behavior under the processing conditions.


Introduction
The consumption of saturated and trans-fats has been shown to have negative effects on human health, which have been highlighted in many studies and articles published in the popular press and scientific journals over the last decades [1][2][3][4].
Due to this unfavorable publicity, food producers are under pressure to seek other techniques for structuring lipid-based food items without considerable quantities of saturated and trans-fats. As the consumption of trans-fatty acids and saturated fatty acids have been linked to health issues, the Food and Agriculture Organization (FAO) recommends reducing the consumption of saturated fatty acids [5].
Siri-Tarino et al. concluded in 2010 that there is no sufficient evidence for linking saturated fat intake with cardiovascular disease [11]. Later, the Schwab research group showed that the replacement of saturated fats with polyunsaturated fatty acids (PUFA) has a beneficial role in the reduction of cardiovascular disease, especially in men [10], while Chowdary et al. [6] concluded that there is not sufficient evidence to sustain the high consumption of PUFA to the detriment of total saturated fats.
Another interesting study shows that the mortality causes associated with cardiovascular disease, coronary heart disease, ischemic stroke and diabetes are not directly linked to saturated fat intake but rather to industrial trans fats intake [7].
In a paper published in 2015, Hooper et al. showed that it might be possible to reduce cardiovascular events if the total intake of saturated fat is reduced [8], and later, in 2020, The present review focuses on the latest research on replacing animal fats in meat products with healthy, fat-rich, vegetable oil-based, nutritionally balanced ingredients with good sensorial and textural characteristics so that consumer acceptability is not significantly influenced. Therefore, the use of oleogels for this purpose is seen as an innovative solution for the formulation of nutritionally improved meat products.

Definition and Description
Oleogels or structured oils can be defined as micro-structured three-dimensional systems obtained by the solidification of edible liquid oils, in the presence of so-called oleogelator(s) [26]. They are considered to be semi-solid materials in which the liquid oil fraction is immobilized in a network of structuring molecules [27].
Oleogels are a healthier alternative to saturated fats and trans-fats, and they are finding significant uses in many different sectors of the food industry [26]. As shown in Figure 1, these areas include the baking industry, meat industry, confectionery industry, and dairy industry, amongst others. The baking industry has benefited the most from the formulation of oleogels since these can be used as replacers for shortenings and spreads which then contain no trans-fats and a decreased quantity of saturated fats. Therefore, it is extremely important to investigate different methods of changing liquid oil into solid fats that include no trans-fatty acids and just a small amount of saturated acid. The innovative and potentially fruitful alternative technique, known as oleogelation, has lately attracted significant interest [28].
Oleogel is a thermo-reversible lipid combination with high viscoelastic properties that may exist in a semi-solid state. The oleogelators are combined with a lipophilic liquid (often vegetable oil) to form the emulsion. Liquid oil may be captured by oleogelators, which then crystallize or self-assemble into different shapes. This results in a three-dimensional network structure and gelatinization of the entire system, which stops lipophilic liquid from flowing through the system [29].
Oleogel has the ability not only to preserve the solid features of foods, but also to provide the health benefits associated with low levels of saturated fatty acids. A suitable Therefore, it is extremely important to investigate different methods of changing liquid oil into solid fats that include no trans-fatty acids and just a small amount of saturated acid. The innovative and potentially fruitful alternative technique, known as oleogelation, has lately attracted significant interest [28].
Oleogel is a thermo-reversible lipid combination with high viscoelastic properties that may exist in a semi-solid state. The oleogelators are combined with a lipophilic liquid (often vegetable oil) to form the emulsion. Liquid oil may be captured by oleogelators, which then crystallize or self-assemble into different shapes. This results in a three-dimensional network structure and gelatinization of the entire system, which stops lipophilic liquid from flowing through the system [29].
Oleogel has the ability not only to preserve the solid features of foods, but also to provide the health benefits associated with low levels of saturated fatty acids. A suitable oleogel for edible purposes should have changeable physical and chemical characteristics to fulfil other intended functions, such as controlled release and excellent bioavailability [30].
Researchers consider that oleogels have a wide range of applications and a significant unexploited potential. Their use allows the obtaining of products with acceptable technological properties and may combat oil leaks in a wide range of different foods. Other important applications for oleogels include their use as carriers for lipophilic bioactive substances [31,32].
Several studies show that oleogels boost lipid-soluble molecule bioavailability. For example, the incorporation of β-carotene in oleogels benefits from increased bioavailability and retardation of lipid oxidation. Moreover, it is possible to control the release of this nutraceutical in the bloodstream [33,34].
Another example is the release of curcumin, a water-insoluble nutraceutical molecule with several health benefits, which is included in oleogels. For the oleogel preparation, medium-chain triacylglycerols (canola, coconut, or maize oils) were used, in which curcuminoid was dissolved under heating. Thereafter, mono-stearin, as an artificial trans fat oleogelator, was added. It was shown that the bioavailability was greater for the curcuminoid-oleogel than for the curcuminoid powder distributed in the water [35].
Besides improving nutritional value, the incorporation of oleogels into food products can be attractive in terms of stability and shelf life. Due to their three-dimensional structures, oleogels constitute a good matrix for the delivery of bioactive molecules [36], such as antioxidants [37], probiotics [38] or antimicrobial compounds [39][40][41]. However, this last area is not yet widely studied and existing reports generally focus on the use of fat-soluble compounds, due to the lipophilic nature of oleogels [42].
Currently, there are very few studies on the microbiological aspects of oleogels. Due to their composition, high oil content and low water activity, oleogels are not a favorable environment for microorganisms. Furthermore, several studies have shown that beeswax, a commonly used oleogelator, has inhibitory effects on a variety of microbes such as Salmonella enterica, Staphylococcus aureus, Candida albicans or Aspergillus niger [43][44][45].
According to Pintado et al. (2020) [46], the strategy of reducing and replacing animal fat in dry fermented sausages with an oleogel obtained by structuring a mixture of olive and chia oils with 10% beeswax resulted in microbiologically safe products. Another study showed that it is possible to produce microbiologically safe fermented sausages by partially substituting the pork backfat with olive oil/monoglycerides oleogel [47]. Bei et al. (2015) [39] developed a stable stearic acid and peanut oil oleogel-based emulsion supplemented with D-limonene and nisin, two natural compounds with antimicrobial activity. The results indicate that the addition of the two antimicrobial agents showed remarkable inhibitory effects against all target microorganisms (Escherichia coli, Bacillus subtilis and S. aureus). Moreover, the addition of the organogel-nano-emulsion with D-limonene and nisin as food preservatives in milk samples also demonstrated notable antimicrobial activity.
In another study, modified whey protein isolate was used to prepare emulsion-based oleogels with thyme essential oil and coconut oil. The resulting product exhibited a regular, ordered, porous network structure with good antimicrobial activities against Escherichia coli [40]. Yadav et al. (2017) developed an oleogel from rice bran oil and candelilla wax supplemented with ciprofloxacin hydrochloride, a well-known antibiotic [41]. Antimicrobial studies showed that this compound maintained its antimicrobial properties and the oleogel was able to resist the growth of Escherichia coli.
In recent years, the possibility of using oleogels as an alternative method of structuring oils has been recognized and this possibility has been deeply examined by using a variety of gelator-edible oil systems. These unique oil-based oleogels contain a lipidic continuous phase and display the physical features typical of hydrogels, which have a continuous liquid water phase. To differentiate these edible oil gels from conventional organo-gels, which are typically organic solvent gels used in various industrial applications in the chemical sector, they are now referred to as oleogels [31].

Oleogelators: Properties and Classification
The conversion of the liquid oil into a solid structure is ensured by the oleogelator, which is responsible for forming the three-dimensional network in which the oil is trapped. The oleogelation process and the properties of oleogels depend mainly on the oleogelator, but these cannot be discussed without also considering the oil characteristics and the processing conditions [27,48]. Regardless of the oleogelator's type, when used in food processing, it must possess certain properties including food grade, GRAS status (Gen- erally Recognized as Safe), no negative effects on sensory attributes of the food, similar characteristics to solid fats, stability and, last but not least, economic feasibility [29,49].
In recent years, a large number of oleogelators have been investigated with promising results. These can be classified according to two major criteria (i) gelation strategy on the one hand [50] and (ii) their molecular weight on the other [51].
The gelation strategy depends on the solubility of the oleogelator. Thus, lipophilic gelators (e.g., fatty acids, monoglycerides, waxes, phytosterols, ethyl-cellulose) and hydrophilic gelators (generally polymers, including carbohydrates and proteins) can be distinguished [52]. Solubility is an important characteristic of gelators. According to Co and Marangoni [53], there must be a balance between the solubility and insolubility of the gelator. If the solubility is too high, there is a risk of forming a solution instead of a gel, while if the gelator is too insoluble, it will form a precipitate.
Considering their molecular weight, oleogelators can be grouped into low molecular weight molecules and high molecular weight molecules, as shown in Figure 2.

Oleogelators: Properties and Classification
The conversion of the liquid oil into a solid structure is ensured by the oleogelator, which is responsible for forming the three-dimensional network in which the oil is trapped. The oleogelation process and the properties of oleogels depend mainly on the oleogelator, but these cannot be discussed without also considering the oil characteristics and the processing conditions [27,48]. Regardless of the oleogelator's type, when used in food processing, it must possess certain properties including food grade, GRAS status (Generally Recognized as Safe), no negative effects on sensory attributes of the food, similar characteristics to solid fats, stability and, last but not least, economic feasibility [29,49].
In recent years, a large number of oleogelators have been investigated with promising results. These can be classified according to two major criteria (i) gelation strategy on the one hand [50] and (ii) their molecular weight on the other [51].
The gelation strategy depends on the solubility of the oleogelator. Thus, lipophilic gelators (e.g., fatty acids, monoglycerides, waxes, phytosterols, ethyl-cellulose) and hydrophilic gelators (generally polymers, including carbohydrates and proteins) can be distinguished [52]. Solubility is an important characteristic of gelators. According to Co and Marangoni [53], there must be a balance between the solubility and insolubility of the gelator. If the solubility is too high, there is a risk of forming a solution instead of a gel, while if the gelator is too insoluble, it will form a precipitate.
Considering their molecular weight, oleogelators can be grouped into low molecular weight molecules and high molecular weight molecules, as shown in Figure 2. Low molecular weight gelators are small particles with a molecular weight of less than 3000 [54]. According to their gelling behavior, low molecular weight oleogelators can also be subdivided into crystalline particles and self-assembled structures [55]. Low molecular weight gelators are small particles with a molecular weight of less than 3000 [54]. According to their gelling behavior, low molecular weight oleogelators can also be subdivided into crystalline particles and self-assembled structures [55].

Crystalline Particles
The most important crystalline particle gelators are mono-and diglycerides, fatty acids and fatty alcohols [56], plant or animal waxes (e.g., rice bran wax, soy wax, sunflower wax, carnauba wax, candelilla wax, beeswax) [30,57], shellac, wax esters, oligopeptides, sorbitan esters (monostearate and tri-stearate), lecithin [58] and ceramides [55]. The gelation mechanism consists of the formation of a crystalline network, by completing three specific stages: nucleation, self-assembly and self-organization. First, the nucleation centers are formed as a result of the precipitation or/and crystallization gelator's molecules, due to the oversaturation of the solvent [55]. Secondly, through the self-assembly process, the primary particles can grow and form different shapes of crystals (fibers, tubules, rods or ribbons). Finally, the gel is formed as a result of crystalline particles' self-organization into a continuous network, where the liquid oil is immobilized [29]. The mechanical and rheological properties of the network depend on the crystals' configuration, which can be controlled through the cooling and shearing processes [59].
Proteins and polysaccharides are the two major classes of high molecular weight oil gelators that have attracted attention in the field because they can be used as food ingredients, are widely available, are inexpensive, and some have potential nutritional value [50]. Excepting ethyl-cellulose and chitin [60], the high molecular weight oleogelators are difficult to use for structuring hydrophobic oils due to their hydrophilic nature. Thus, their usage requires a different technology, as described in the following paragraph. Polysaccharides such as alginate, chitosan, Arabic gum, galactomannans [59], k-carrageenan [61], xanthan and Guam gum [62], pectin [63] and proteins such as zein [64], caseinate, lactoglobulin, soy protein and gelatin [13] are the most commonly used high molecular weight compounds used in oleogelation.

Methods of Obtaining Oleogels
Depending on the solubility of the oleogelator, oleogels can be obtained by two distinct methods: direct or indirect dispersion. An important aspect in this regard is that neither of these methods produces changes in the chemical or structural properties of the oils so that their nutritional value is not affected [59].

Direct Dispersion
Direct dispersion, due to its simplicity, is one of the most widely used methods of producing oleogels. The method is specific for hydrophobic agents and consists of direct dispersion of the oleogelator into the oil, heated to a temperature above its melting point. Then, to achieve the gelation, the mixture is cooled, at which point the nucleation and crystal growth leads to the formation of the gelator network and the entrapment of the oil in this solid structure [65].
In terms of texture, stability, rheological or thermal properties, oleogels' characteristics might be improved by using mixtures of two or even more oleogelators with synergistic interaction. Some of the most representative oleogelators' systems include stearyl alcohol and stearic acid, stearyl alcohol, stearic acid and ethyl-cellulose [66], monoglycerides and phytosterols [67], adipic acid and ethyl-cellulose [68], candelilla wax and glyceryl monostearate [69], tripalmitin with candelilla wax, lecithin and fruit wax [70], γ-oryzanol and β-sitosterol [71], lecithin and stearic acid [72]. Table 1 summarizes some notable examples found in the scientific literature regarding the procedure for the direct dispersion method for obtaining oleogels. Table 1. Examples of oleogels obtained by direct dispersion method.

Oleagelator Oil Gelation Conditions Application Reference
Candelilla wax (CW) 3 and 6% Canola oil (CO) Heating to 150 • C, under gentle agitation for 15 min and cooling at room temperature.
Improving physical properties and oxidation stability of Camellia oil (CO) by oleogelation. Carnauba wax/adipic acid oleogel characterization for fat replacement in cake and beef burger. [81] Ethyl-cellulose (EC) 5 wt%; 10 wt% Corn oil (CO) EC powder was mixed with heated CO (150 • C), stirred (15 min) and cooled at room temperature.

Indirect Dispersion
Indirect methods of obtaining structured oils have attracted considerable attention in recent years because they can diversify the range of oleogelators, to include also hydrophilic compounds, such as proteins and polysaccharides, which cannot be directly dispersed in oil. Indirect dispersion involves the formation of a structural network or building blocks in a water-continuous system, followed by the removal of the aqueous phase, as shown in Table 2. This can be achieved by various techniques, such as the biphasic emulsion method, solvent exchange, or foaming method [29,30].
(i) Biphasic emulsion method As described by Alvarez et al. [88] and Li et al. [26], this method first involves the preparation of a concentrated oil in water emulsion, by using a protein as an emulsifier. Then, the emulsion is dried and sheared, resulting in stable oleogels with a hydrophilic gelator and a high concentration of edible oil (above 97%).
(ii) Solvent exchange The solvent exchange method consists primarily of forming a hydrocolloid by dispersing a polymeric material in water, followed by heating to expose its hydrophobic groups. This results in a strong network due to the physical cohesive forces, ionic or covalent bonds between the polymeric chains of proteins or polysaccharides. Next, water is removed and replaced by an organic solvent with medium polarity (acetone, tetrahydrofuran) [89] so as to prevent any disruption of the network by the solvent exchange procedure. The solvent is then substituted with oil, leading to the oleogel formation [60,90].
The foam-templated method is based on the preparation of aqueous protein foams, followed by the removal of water through drying, which leads to an aerogel structure. The dried foam is immersed in oil until saturation and sheared to break the polymeric structure and to obtain oleogels [13]. Sometimes, for obtaining stable foams, polysaccharides such as pectin, alginate, carrageenan [50], xanthan gum and Tara gum [91] can be added. Study about the ability of G and XG to produce oleogel through foamtemplated method. [93]

Advantages and Benefits of the Use of Oleogels in Foods
The major health benefit that may be achieved by using oleogels is the substitution of saturated and trans fats with unsaturated fats. Through the process of oleogelation, it is possible to produce a managed or controlled distribution of functional ingredients and medicines that have been added to meals in the appropriate quantities [90].
The human body absorbs, in very small proportions, several fat-soluble compounds, including lycopene, coenzyme Q10, β-carotene, conjugated linoleic acid, eicosapentaenoic acid, plant sterols, isoflavone, docosahexaenoic acid and tannins [98]. Their ingestion results in medical or health advantages, including the prevention and treatment of certain illnesses, e.g., reduction in platelet aggregation, blood viscosity and fibrinogen, as well as antioxidant qualities and a reduced prevalence of chronic diseases such as cardiovascular diseases and different types of cancer. For this reason, their encapsulation or delivery via oleogels is needed, by including the latter in functional foods [99].
Numerous experimental results obtained by scientists who formulated meat products containing oleogels may be a starting point for the meat industry to implement this alternative formulation and for nutritional and technological considerations. It is generally accepted that hard=stock is a natural source of trans-fats, and it has been claimed that processed meat products contain an average of 35% saturated fats [99,100]. This is one of the primary factors that contribute to the prevalence of cardiovascular diseases among consumers [100].
Fats are responsible for both the structure and flavor of meat products. The composition of fats has binding capabilities, which further contribute to the consistency and durability of meat products. As customers' tastes are also influenced by sensorial characteristics, innovative formulations need to preserve these qualities which have traditionally been associated with meat products [90,101].
In addition to being used in the production of foods that can withstand high temperatures, oleogels can also be used to solve the problem of oil leaks in a broad variety of different products. Oleogels also have significant secondary utility as transporters for lipid-soluble bioactive chemicals, which is another important application. Recent uses of oleogels have reduced the possibility of trans-fatty acids being created during the frying of noodles, although the frying of foods in oil at a high temperature is a substantial contributor to the synthesis of trans-fatty acids [102].
In the chocolate industry, oleogels are mainly used to solve one of the most pressing problems of replacing oil binders in chocolate paste, to prevent fat bloom during storage and to make chocolate more heat-resistant. To combat this issue in hot climates, chocolates can be formulated with added high-melting oleogels, making them thermally resistant. Saturated fatty acids were reduced by 30% due to the 27% rapeseed oil instead of palm oil. During storage, no oil separation occurred [103].
Ethyl-cellulose was the oleogelator of corn-oil based oleogel shown to increase the hardness of dark chocolate when used to replace 50% of the cocoa butter, while glycerol monostearate was the only oleogelator of the named oleogel that formed solid like chocolate by replacing 100% of the cocoa butter. Based on these results, oleogels rich in unsaturated fatty acids may be useful agents for reducing cacao butter in dark chocolate [104].
Few experiments have attempted to replace milk fat with oleogel in dairy products. Oleogel, a milk fat alternative, has been extensively studied as a possible addition to cream cheese products. Different types of cream cheese, including those produced with rice bran wax and ethyl=cellulose oleogels, have been developed and compared to both regular and reduced-fat commercial cream cheese as standards. The use of oleogel improved the fatty acid profile of the cream cheese, as total fat levels were reduced by 25% in all samples compared to the full-fat commercial control [105].
In a separate study, researchers analyzed the use of oleogels instead of milk cream in artisanal ice cream. The oleogel-enriched samples showed a more optimal ratio of healthy to less healthy fatty acids. For instance, sunflower oil-based oleogel containing 12 g/100 g gelators produced ice creams with the same or even better-quality characteristics than versions produced with milk cream. Therefore, using oleogels in ice creams as milk cream substitutes have proven to be a practical way to achieve healthier products [106].
For baked products, such as cookies and bread, saturated fat is a necessary ingredient to maintain their specific taste, elasticity and texture. By replacing regular margarine with oleogels produced from edible oils and natural waxes, it is possible to produce cookies with less saturated fat. Oleogels are a lower calorie and more environmentally friendly substitute for saturated and hydrogenated fats in bakery products, without lowering the overall standard of the end product. Researchers working in the field of healthy food design are paying close attention to the many applications that structured oils can have in the baking industry [107,108].
Saturated and trans fats are often ingested through margarine consumption. As solid substitutes for margarine, it is possible to use oleogels, which offer superior texture, color and oxidation stability during storage. Several variants of oleogels have been studied, but those based on vegetable oils rich in polyunsaturated fatty acids using natural waxes as oleogelators have shown good results in obtaining healthy margarine and spreads. However, consumers prefer to consume commercial margarine instead of that prepared with wax-based oleogel because of its less waxy aftertaste. It is possible to obtain spreadable commercial margarine with a candelilla wax-beeswax content of less than 3%. In addition, combining two different waxes will make the oleogel-based margarine harder, while the melting point can be adjusted by changing the ratio of the different waxes used [109].

Trends in Improving Nutritional Profile of Meat Products by Using Oleogels
Meat and meat products are an important part of people's diets everywhere, as they contain significant amounts of proteins and have a high nutritional value. However, due to their high content of saturated and trans fatty acids, consumption of these foods can be a health hazard. A high intake of saturated fats correlates with an increased risk of several diseases, such as diabetes, obesity, inflammation, high blood lipid levels, oxidative stress, metabolic syndrome and, in particular, cardiovascular diseases. The meat industry is therefore obliged to develop healthier meat product alternatives with an improved fatty acid profile to protect the health of customers. Technological improvement of the fatty acid profile can be achieved by replacing animal fat, which meat products naturally contain, with fats that have a more favorable profile [110].
Today's consumers are more health-conscious than ever and are therefore willing to pay extra for healthier meat products. In this sense, the first step could be the elimination of fat from processed meat products, but fat plays a crucial role in the ability of a meat product to retain its sensory, technical, textural, oxidative stabilization and flavor compoundproducing capabilities. Changes in fat content and composition can have adverse effects on the fundamental qualities of products, reducing their attractiveness to consumers and reducing their marketability. Therefore, fat replacers should be nutritionally superior to animal fat, but retain the same structure and other quality attributes as real fat [90].
In terms of their composition, fats and oils are made up of triacyl-glycerides, molecules formed by binding three fatty acids to glycerol. For this reason, the quality of the fat is determined by its types of included fatty acids, namely saturated fatty acids, -cis and -trans monounsaturated fatty acids and -cis polyunsaturated fatty acids. When the saturation level is high, more solids are produced and these solids can self-assemble into crystalline structures formed from triacylglycerols. The temperature may also be responsible for inducing this final form, which results in a more extensive crystal network. Solid fats contribute to the texture and stability, as well as the function of the product. Therefore, possible fat replacers should offer results not only in terms of nutritional content but also in terms of solid structure and proper flavor [2].
Designing alternatives to animal fat requires a deep analysis of the qualities of the lipid material, which in turn dictate the characteristics of the product into which it will be incorporated. Meat fat has a wide variety of quality characteristics such as appearance, texture, resistance to lipid oxidation, flavor and many others. The use of a lower-quality animal fat substitute in meat products can cause many technological problems, such as improper drying, oily appearance, rancidity and others. It is therefore very important to consider the quality requirements for new fat materials, such as their fatty acid content and physical qualities [2].
A possible alternative solution for replacing animal fats in meat products, intensively researched in recent years as a response to consumers' health needs, is presented in Figure 3. improper drying, oily appearance, rancidity and others. It is therefore very important to consider the quality requirements for new fat materials, such as their fatty acid content and physical qualities [2].
A possible alternative solution for replacing animal fats in meat products, intensively researched in recent years as a response to consumers' health needs, is presented in Figure 3. Efforts have been made to replace meat fat with liquid oils. The first mention of oleogels in meat products explains that the creation of extremely small fat globules is one of the key problems of direct oil-enrichment, along with high vulnerability to oxidative breakdown and low solubility [2].
Oleogelation, which allows the addition of vegetable oil, is one of the most promising methods for structuring by using oleogelators that include crystalline particles, fatty acids, monoacylglycerols and waxes, supramolecular structures and the polymeric molecule ethyl cellulose. The selection of the vegetable oil to be used in the development of oleogels is important not only in terms of its nutritional properties but also in terms of its fatty acid composition, which influences several characteristics of oleogels, such as crystallization and melting behavior, rheological properties and oxidative stability [101].
Oleogelation is a method of stabilizing and adjusting the consistency of food suspensions and edible oils. The use of this technology has shown encouraging results in addressing the physical and sensory difficulties in meat products, which are related to the direct replacement of saturated and trans fats with vegetable oils. The most common types Efforts have been made to replace meat fat with liquid oils. The first mention of oleogels in meat products explains that the creation of extremely small fat globules is one of the key problems of direct oil-enrichment, along with high vulnerability to oxidative breakdown and low solubility [2].
Oleogelation, which allows the addition of vegetable oil, is one of the most promising methods for structuring by using oleogelators that include crystalline particles, fatty acids, monoacylglycerols and waxes, supramolecular structures and the polymeric molecule ethyl cellulose. The selection of the vegetable oil to be used in the development of oleogels is important not only in terms of its nutritional properties but also in terms of its fatty acid composition, which influences several characteristics of oleogels, such as crystallization and melting behavior, rheological properties and oxidative stability [101].
Oleogelation is a method of stabilizing and adjusting the consistency of food suspensions and edible oils. The use of this technology has shown encouraging results in addressing the physical and sensory difficulties in meat products, which are related to the direct replacement of saturated and trans fats with vegetable oils. The most common types of oil used in processing meat products include sunflower, sesame, soybean, olive, fish and even linseed oil [29].
In terms of oleogelators used to structure edible oils, it is a common practice to use monoglycerides to generate economically viable oleogels. In the process of making healthier frankfurters, half of the pig backfat was replaced by sunflower oil-based oleogel, gelled with a combination of 1:1 monoglycerides and 3:1 phytosterols. The resulting frankfurter acquired higher hardness values compared to the control. In addition, oxidation degree and sensory parameters were not altered due to this replacement [67].
There is a wide variety of wax oleogelators available, some of them from plant provenance, e.g., candelilla, carnauba, rice bran and sunflower, and others from animals, such as beeswax. Relative to commercial meat products, it has been shown that the aroma and taste of waxes produce the least possible sensory changes in meat products formulated with wax-based oleogels [111].
Due to their ability to give meat products a crispy and elastic appearance and to serve as a stabilizer and glaze component, it is feasible to use waxes as oleogelators. Beeswax, at a concentration of 11%, added to a mixture of linseed, fish and olive oils, produced a high oleic acid oleogel, further used to create low-fat pork burgers enriched in polyunsaturated fatty acids. To reduce lipid oxidation during refrigeration and cooking procedures, curcumin was added at a rate of 0.2% [2,111,112]. In another study, linseed oil was gelled with 8% beeswax and the obtained oleogel was used to formulate frankfurters, as a substitute for 25% and 50% of pork fat. The low n-6/n-3 resulting ratio was a strong indicator of the increased frankfurters' nutritional value. In addition, fat replacement showed minor effects on textural qualities, although improvements in sensory characteristics are still needed for consumer acceptance [90]. Beeswax has also been used in high-fat meat products (liver pâté), with positive results in the case of a mixture of vegetable oils (olive, linseed and fish) [111].
Phytosterols are an interesting group of oleogelators for the production of oleogels due to their structure, which is very similar to that of cholesterol in human cells. It has also been shown that phytosterols can reduce the amount of cholesterol in human blood by up to 10%. Even though a combination of and even linseed oil [29].
In terms of oleogelators used to structure edible oils, it is a common practice to use monoglycerides to generate economically viable oleogels. In the process of making healthier frankfurters, half of the pig backfat was replaced by sunflower oil-based oleogel, gelled with a combination of 1:1 monoglycerides and 3:1 phytosterols. The resulting frankfurter acquired higher hardness values compared to the control. In addition, oxidation degree and sensory parameters were not altered due to this replacement [67].
There is a wide variety of wax oleogelators available, some of them from plant provenance, e.g., candelilla, carnauba, rice bran and sunflower, and others from animals, such as beeswax. Relative to commercial meat products, it has been shown that the aroma and taste of waxes produce the least possible sensory changes in meat products formulated with wax-based oleogels. [111].
Due to their ability to give meat products a crispy and elastic appearance and to serve as a stabilizer and glaze component, it is feasible to use waxes as oleogelators. Beeswax, at a concentration of 11%, added to a mixture of linseed, fish and olive oils, produced a high oleic acid oleogel, further used to create low-fat pork burgers enriched in polyunsaturated fatty acids. To reduce lipid oxidation during refrigeration and cooking procedures, curcumin was added at a rate of 0.2% [2,111,112]. In another study, linseed oil was gelled with 8% beeswax and the obtained oleogel was used to formulate frankfurters, as a substitute for 25% and 50% of pork fat. The low n-6/n-3 resulting ratio was a strong indicator of the increased frankfurters' nutritional value. In addition, fat replacement showed minor effects on textural qualities, although improvements in sensory characteristics are still needed for consumer acceptance [90]. Beeswax has also been used in highfat meat products (liver pâté), with positive results in the case of a mixture of vegetable oils (olive, linseed and fish) [111] Phytosterols are an interesting group of oleogelators for the production of oleogels due to their structure, which is very similar to that of cholesterol in human cells. It has also been shown that phytosterols can reduce the amount of cholesterol in human blood by up to 10%. Even though a combination of ɣ-oryzanol and β-sitosterol in linseed oil is a nutritionally favorable choice, there are additional technical considerations that need to be considered. Linseed oil gelled with this sterol combination, at a concentration of 8%, has been shown to be particularly effective in replacing 25-75% of the fatty acids in pork meatballs. For the same reason, pork meatballs have been formulated with the addition of Fucus vesiculosus extract (250-1000 ppm) to prevent oxidation of the linseed oil during storage [67,113].
By plasticizing the ethyl cellulose structure, ethyl cellulose oleogels can take on the melting and flow characteristics of various lipids. The addition of adipic acid to ethyl cellulose improved the flexibility of oleogels when used in meat products. By formulating beef burgers with 2% ethyl cellulose and 4% adipic acid oleogel, a pleasing textural profile, color and organoleptic qualities were noticed [68].

Designing of Innovative Oleogel-Based Meat Products
The stability of a meat emulsion depends on several factors, such as the size of the fat globules (smaller globules are more stable), the degree of breakdown of the cell structure to release the fat and the amount of dissolved protein covering the contact area of the fat droplets. The fat content of processed meat products varies between about 20% and 30% and has significant interactions with the other elements of the emulsion. Fat plays a key role in the quality of meat products, as it gives the proper tenderness, moisture, taste and appearance. It is a challenge for meat processors to create low-fat meat products, both healthier and with satisfying sensorial properties. When solid fats are replaced by liquid oils, the fatty acid profile of meat products improves, as this increases the amount of mono and polyunsaturated fatty acids and improves the ratio of n-6 to n-3 acids [114].
-oryzanol and β-sitosterol in linseed oil is a nutritionally favorable choice, there are additional technical considerations that need to be considered. Linseed oil gelled with this sterol combination, at a concentration of 8%, has been shown to be particularly effective in replacing 25-75% of the fatty acids in pork meatballs. For the same reason, pork meatballs have been formulated with the addition of Fucus vesiculosus extract (250-1000 ppm) to prevent oxidation of the linseed oil during storage [67,113].
By plasticizing the ethyl cellulose structure, ethyl cellulose oleogels can take on the melting and flow characteristics of various lipids. The addition of adipic acid to ethyl cellulose improved the flexibility of oleogels when used in meat products. By formulating beef burgers with 2% ethyl cellulose and 4% adipic acid oleogel, a pleasing textural profile, color and organoleptic qualities were noticed [68].

Designing of Innovative Oleogel-Based Meat Products
The stability of a meat emulsion depends on several factors, such as the size of the fat globules (smaller globules are more stable), the degree of breakdown of the cell structure to release the fat and the amount of dissolved protein covering the contact area of the fat droplets. The fat content of processed meat products varies between about 20% and 30% and has significant interactions with the other elements of the emulsion. Fat plays a key role in the quality of meat products, as it gives the proper tenderness, moisture, taste and appearance. It is a challenge for meat processors to create low-fat meat products, both healthier and with satisfying sensorial properties. When solid fats are replaced by liquid oils, the fatty acid profile of meat products improves, as this increases the amount of mono and polyunsaturated fatty acids and improves the ratio of n-6 to n-3 acids [114].
For this purpose, it is essential to discover economically feasible procedures that can lower the amount of fat considered to be included in meat products. In this sense, several recent studies regarding oleogels' use in various meat products as animal fat replacers are briefly presented in Table 3.  Higher level of unsaturated fatty acids and a more compact structure that affected the sliceability. Reducing the pork fat by 50% proved to be the best option, which not affected the hardness of the sausages.  Lower sensory hardness when adding 1.5% or 3.0% SMS than the sample with 0.0% SMS, at the low EC levels. Similarity between the oleogel sample with 8% EC and the control and significantly increased hardness at higher EC concentrations. The acceptance of the product unaltered by the replacements.

Breakfast sausages
Canola oil (CO) Ethyl cellulose (EC) Substitution of pork fat so as to obtain 20.8% fat provided by oleogel in sausages. 8% rusk was added Similar objective hardness of the most SMS oleogels compared with the pork fat control sample. No rancidity and chemical taste for the final product, prevented by the addition of BHT and rosemary oleoresin to CO. No changes in the water and fat contents during the heating of sausages, due to the presence of the rusk that contributed to their binding.
The sensory characteristics not significantly affected by using BW oleogel, but a negative effect on sensorial properties when using EC oleogel, in direct relation with the substitution level. The information presented in Table 3 proves that the substitution of the animal fat with differently designed oleogels and their incorporation in meat products resulted in better fatty acid profiles and more acceptable sensory and technical properties.

Conclusions and Future Perspectives
In concordance with the recent trends in attention to human health, many researchers have shown the possibility of totally or partially replacing animal fat in meat products. Without minimizing the solution of incorporated cereal flours, fibers, natural bio-polymers, or plant-based emulsions, notable results were obtained by using oleogels.
Oleogel production and integration into foods presents a growing interest among researchers, constituting an important advance in food technology. This is due to their ease of obtaining, affordability and versatility. The understanding of oleogels' characteristics and their capacity of controlling phase separation and of decreasing the mobility of the oil phase is extremely important for developing novel applications. There is still a real need to investigate different formulations of oleogels from the point of view of component type and ratio, as well as properties that make them suitable for meat products. The influence of different processing parameters on the oleogels' properties also needs to be examined further. Despite the promising results of obtaining healthier meat products with a higher fatty acid profile, some negative aspects, in terms of textural parameters, flavor or oxidative stability, have also been reported, thus further studies are needed to eliminate these drawbacks. However, promoting the benefits of meat products reformulation with oleogels to producers is highly recommended and can be considered a future direction in obtaining healthier foods.