Oleogels for development of health-promoting food products

Artur J.Mrtins,Antonio A.Vicente,Lorenzo M.Pstrn,Miguel A.Cerqueir

a International Iberian Nanotechnology Laboratory,Av.Mestre JoséVeiga s/n,4715-330,Braga,Portugal

b Centre of Biological Engineering,University of Minho,Campus de Gualtar,4710-057,Braga,Portugal

Keywords:

ABSTRACT

1.Introduction

Human dietary habits have changed dramatically in the past few years and as a consequence of that,the landscape of food-related disorders has changed likewise.Changes on food consumption panorama led to a considerable increase in the ingestion of cereals and later the same happened with the amount of consumed meat-based products.These dietary changes led to a decrease of the consumption of plant-based products,thus diminishing the intake levels of essential fatty acids and plant-derived antioxidants,while recording on the opposing side prohibitive levels the ingestion of saturated fats[1].One of the consequences of these changes,that are associated to the widespread consumption of trans and saturated fats,are the considerable number of adverse consequences on human health,namely detrimental effects on lipoprotein profile.Coronary heart disease,inflammation,oxidative stress,endothelial dysfunction,increased body weight and metabolic syndrome are among the health disorders that are resultant of a high-fat food consumption[2-4].Fats are one of the food ingredients with major flavour,mouth-feel and overall eating-pleasure capabilities,which make it quite difficult to remove them from consumers’diet.Therefore,manipulating food formulations without compromising the mentioned characteristics remains an extremely difficult task.Interesterified fats,which are able to structure organic solvents through chemical modifications at the molecular level,are less appreciated by consumers.Also,the introduction of a healthier alternative based on fat-rich ingredients must perform highly in regard to nutritional value,while presenting good sensorial and textural attributes at the same time.Initially,dietary lipids were said to affect the brain through an indirect pathway,namely by means of the cardiovascular physiology.Nowadays,these lipids are also being acknowledged for their direct actions on the brain,as omega-3 polyunsaturated fatty acids that regularly constitute the membranes of cells and exert essential tasks for normal brain function.Some of the features that are attributed to these fatty acids comprise:plasma membrane fluidity at the synaptic region(which is responsible for maintaining membrane integrity and,consequently,the neuronal excitability and synaptic function);the maintenance of membrane ionic permeability and the function of transmembrane receptors(supporting synaptic transmission and cognitive capacities);activation of energy-generating metabolic pathways that will produce effect on insulin-like growth factor at the liver and brain levels[5].

The human gut can be considered as our second brain and the bridge that is established between the epithelial surface and the brain is currently one of the most interesting scientific endeavours towards a better understanding of how human microbiota influences the human hormonal and nervous systems[6].Food preventive and protective aptitudes are now generally acknowledged,namely the influence of omega-3 fatty acid rich diets in regard to human cognitive process and the already reported genetic upregulation in rodents that is responsible for synaptic function and plasticity maintenance[7,8].Lipids consumption throughout time impacts modulation of mental health in different sectors of the society[5].Interconnected evidences like the decline in dietary intake of omega-3 polyunsaturated fatty acids(n-3 PUFA),in the past 30 years(subsequently increasing the values of omega-6 PUFA)and the registered intensification of the incidence of depression in the Western world in the same period,strongly points towards the involvement of the ingested fatty acid composition on health issues[9].The all or nothing dietary approaches,produced important consequences in today’s society dietary habits.Popular nutritional advices are committed to be impactful and are commonly supported by limited evidence and more often than not,the outcomes are undesired.Supplemental nutrition is often advertised after health-promoting strategies,where consumers can get their hands in the so called over-the-counter weight loss foodstuffs that are focused in the premise of a one-size-fits all solution.This seems appealing but ignores the complexities of each individual and of a proper diet[10].

Among food ingredients,triacylglycerols present a very important structural role as a food formulator.Fats commonly present in food products in the crystallized(solid)form,serving as structure modifiers,affecting the texture and therefore affecting consumers’perception.Food stability during storage is certainly a characteristic that would be affected by such formulation conditions.Oil structuring leads to the formation of a gelator network that is able to produce a self-standing thermo-reversible viscoelastic structure[11].The structuring of selected oil media and the development of oleogels could be an interesting approach towards the development of nutritional balanced foods,with capability to undertake important features in human nutrition and even serve as brainfoods.

Industries such as pharmaceutical,food,and cosmetics have established a remarkable use for oleogels in the last years[12].Critical developments for oleogel structures have been demonstrating their aptitude to function as drug delivery matrices and as trans and saturated fat replacers in processed foods.The research and the subsequent introduction of oleogels as a food ingredient is foreseen as a complementary route for different needs.They can be both used for edible oil structuring and for saturated fat substitution purposes,and at the same time as a way of tailoring the fatty acid profile of food products[13,14].As mentioned before,the polyunsaturated fatty acids have been considered of great interest for human health,due to their anti-inflammatory action potential,that can act in a protective way in a number of chronic degenerative diseases with an inflammatory pathogenesis[15].Structuring oils that are rich in PUFA is a technological breakthrough that has been under the scientific scope,as this approach seems to be an advantageous way for their incorporation in foods.Another important aspect is that not only the oil medium could function as a source of functionality and wellbeing,but also the molecules that exert the oil gelling functionalities play an equal important role due to their possible intrinsic bioactivity[16].

Considering the efforts that scientists and industries have been demonstrating in the reduction and elimination of saturated and trans fats from our diets,this paper will explore some of the strategies that are in place to develop and produce oleogels that can be introduced in foods.Edible oils conformation changes(impersonating texture and rheology of frequently used fats)and their use as nutrition providers will be likewise explored.It also reviewed in what way oleogels can act on the human overall metabolic health.Fig.1 portrays a summary of the event chain towards oleogel incorporation in foods,pinpointing some of the challenges and outputs of this process.

2.Materials and methodologies for oleogels production

Different characteristics like the hydrophobic nature,crystallinity or glass transition temperature can make a certain molecule,or combination of molecules,efficient oleogelators[13].The nature of such gelling molecule will be responsible for the final characteristics of the resulting oleogels(e.g.opacity,texture,melting temperature,oil binding capacity)[17].The oil phase used is also extremely relevant,since the fatty acid profile can also influence the oleogels’final properties.The selection of oils rich in PUFA or the use of organic solvents with shorter carbon-chain backbones will determine how the absorption of lipid droplets or bioactive molecules occurs at the intestinal level.Based on what was described above,distinct approaches can be implemented in order to successfully impart a gel-like structure to edible oils.Also,some derivations of such strategies can be employed,namely when using the versatile emulsion-templated approaches.Fig.2 presents some examples of oleogelators which can be applied to perform direct and indirect oil structuring.

2.1.Direct dispersion

Excluding the use of non-polar lecithin,which forms reversed tubular micelles in the presence of water with capability to structure up to 90%(m/m)of oil medium[18],direct dispersion methodologies represent the least problematic and highly upscalable strategies for oleogel production.Such methods consist in a direct dispersion of the oleogelator into the liquid oil,at temperatures above the melting point(glass transition in the case of some polymers such as ethylcellulose).After that,a cooling period is promoted in which the gelator network is formed,entrapping the oil in a solid structure and forming the oleogel[19].Depending on the type of oleogelator used,crystallization kinetics can be different.Rheological and textural characteristics,in line with increased bioactive protection,seem to be a positive and decisive factor for their implementation when incorporated in certain food matrices[20,21].However,these bulk systems can also present some limitations,such as the use of only lipophilic compounds that could be a problem not only for incorporation in food products,due to compatibility and binding properties,but also regarding the digestibility response,which could lead to lower values of bioacessibility and bioavailability during digestion.Crystal platelets,formed after wax material crystallization,in the presence of oil medium at higher temperatures,comprise one of the most interesting and adaptable structural conformations that are able to induce edible oil structuring.The existence of high-and low-melting(relating to temperature)waxes make it possible to produce oleogels with distinct transition temperatures,different viscoelastic,visual and textural properties.Moreover,the lower critical concentration needed to induce oleogelation(＞3%)is a positive characteristic,when considering waxes as oleogelators,once this can represent a significant reduction of production cost and an increase of the bioactive solubilization within the oil medium[20,22].In another perspective,the networks formed by the combination(in certain ratios)of phytosterols,such as β-sitosterol,with?-oryzanol(composed by esters and phytosterols)self-assemble into tubular conformations,that are able to form oleogels with elevated mechanical properties still exhibiting high levels of transparency[23].Such fibrilar structures establish an helical architecture due to the presence of the ferulic acid moiety within the?-oryzanol molecule[24].Recent reports suggest that the increase of the concentration of gelator(?-oryzanol+β-sitosterol)develops highly branched structures,altering the microstructure formed by the fibrils[25,26].An inherent functionality can be associated to these oleogels due to the phytosterol amount that is provided by the gelators and represents an important feature due to its bioactivity after digestion[27].Ethylcellulose is a widely studied gelator and,due to its capacity to form oleogels with interesting bioacessibility performance(for lipophilic compounds),makes this oleogelator a source of functionality through direct oil structuring methods[28].This polymeric oleogel presents mechanical properties directly associated to the polymer molecular weight as well as to the cooling rate and the gelling setting time,where a faster cooling yield a less stable oleogel,with a lower elastic modulus[29].

Fig.1.Event chain diagram for oleogel development and introduction in the food industry.

Fig.2.Direct and indirect strategies for oil gelation and materials used in each methodology.

2.2.Indirect dispersion

Indirect methods aiming at oleogelation are getting considerable attention in recent years[30].The possibility of using amphiphilic molecules to serve as building blocks for oil structuring purposes considerably widens the field of application for such structures.Carbohydrates,for instance,are usually biocompatible,present low cost and their chiral properties makes them suitable to develop indirect dispersion strategies for oil gelation while in the presence of water[17].Some examples of emulsion complexes,that are able to serve as templates for oleogel formation can be:the complexation of regenerated cellulose with carboxyl methyl cellulose[31];combination of proteins and polysaccharides[32,33];use of chitin complexes with surfactant or chitin nanoparticles[34];and the gelation of high internal phase emulsions(HIPEs)done with a combination of different polysaccharides that develop synergistic interactions with other food-grade hydrocolloids like carrageenan and xanthan gum[35].One of the shortcomings that are associated to the mentioned approaches is the drying step,which is necessary to remove the aqueous phase in order to expose the formed gelling network.With the purpose of diminishing the impact of such procedure,Wijaya et al.[36]used HIPEs,stabilized by hydrocolloids(whey protein isolate combined with low methoxy pectin),as template.When using HIPEs,the accumulation of material that occurs in the bulk phase is responsible for a decrease of the aqueous content,accelerating the drying process of the gels and resulting in the production of high volume oil-in polymer gels.Oleogels formed with emulsions stabilized by the incorporation of polyphenols are attracting growing attention in the field.These natural antioxidant molecules have been widely used as interfacial reinforcement agents,due to their covalent or noncovalent interaction with carbohydrates or proteins[33].Their ability to form crystals that can act as stabilizers for W/O(water in oil)or O/W(oil in water)emulsions(producing Pickering emulsions)makes them appropriate for oil structuring purposes in addition to their intrinsic bioactivity[37,38].Some studies comprise the association between tannic acid(polyphenol)and gelatine.This colloidal complexes(assembled via hydrogen bonding)showed capability to inhibit lipid oxidation after emulsification[39]and the same was observed for fish oil stabilization with the same complex[40].In fact,the later reported a slower fatty acid hydrolysis when digested in vitro;despite of that,the simulation under intestinal conditions allowed to conclude that the emulsion was considerably hydrolysed by the lipase enzymatic action.HIPE gels development,through pH-driven complexed particles of zein and tannic acid,was studied by Zou et al.[41],who reported that the interfacial network induced by particle concentration has influenced the total oil content of the gel,therefore the study suggested a great tunability of these structures.This is still an underexplored ternary complex for the development of oleogels but promising features are recognized.Qiu et al.[33]characterized oleogels that were formed under emulsion stabilization performed by gelatine-polyphenolpolysaccharide ternary complex.Fig.3 shows the soft solids that were developed by emulsification using high-energy homogenization and drying step by freeze-drying to remove the aqueous phase.Oven dried samples were likewise produced and in contrast to the high hardness evidenced by the freeze-dried oleogels,these showed easier spreadability,demonstrating the importance of the polysaccharide network in the texture properties of these oleogels[33].

Fig.3.Visual appearance of freeze-dried products stabilized by GLT(A),GLT-0.075(m/m)TA(B),GLT-0.15(m/m)TA(C),GLT-0.3(m/m)TA(D),GLT-FG(E),GLT-0.075(m/m)TA-FG(F),GLT-0.15(m/m)TA-FG(G),GLT-0.3(m/m)TA-FG(H).(GLT-gelatine;TA-tannic acid;FG-flaxseed gum).Reprinted with permission from literature[33].

Solvent exchange is an additional approach that can be considered for oleogel development using protein aggregates and polysaccharides as building blocks.As introduced by de Vries et al.[42],the first degree of gelation consists in the development of an hydrogel,that is achieved through a protein(polymeric)network which formed in aqueous medium.The solvent exchange is then sustained in the presence of organic solvents that will remove the water content,later substituted by the oil phase through a sequence of dipping or immersion steps,which will be responsible for oil incorporation within the previously formed polymeric network.Later was reported that the final oleogel properties,obtained using such technique,could be affected by parameters,such as used oil type,temperature,water content and also the type of the organic solvent used in the intermediate step[43].This same solvent-exchange route was manipulated in order to create oleogels(without protein)using κ-carrageenan hydrogels.These were later transformed,with the addition of alcohol,and a supercritical CO2process that served as drying material forming a high-porosity surface gel(aerogel)with capability to absorb up to 80%of oil[44].

3.Gut-brain interaction and possibilities for delivery of functionality through ingestion of oleogels or oleogel-based systems

Preclinical and clinical studies published in the last decade have disclosed highly convincing indications of the existence of bidirectional gut-brain interactions.This communication path is established through the central nervous system with channels involving nervous,endocrine,and immune signalling mechanisms.Consequently,the brain is capable for affecting the microbiota structural and functional role through modulation of gut motility,intestinal transit and secretion,gut permeability and possibly through the luminal secretion of hormones which are responsible for microbial gene expression.Preclinical observations link alterations in the brain-gut-microbiome communication to pathogenesis and pathophysiologies of irritable bowel syndrome,obesity,and also psychiatric and neurologic disorders[45].

Dietary omega-3 fatty acids are said to interfere in the synaptic and cognitive functions through their function in plasma membrane fluidity at the synaptic level[5].Fig.4 displays the role of fatty acids in brain energy metabolism.

The main source of free fatty acids(FAs)crossing the bloodbrain-barrier(BBB)likely derives from non-esterified long chain FA/albumin complexes,after their dissociation from albumin and,to a lesser extent,from circulating lipoproteins.Once entered into cells,the conversion to acyl-CoA by acyl-CoA synthetases allows their intracellular entrapment.Once FAs enter astrocytes,they are translocated into mitochondrial matrix for β-oxidation and ketone body(KB)production.Branched-and very long chain FA can be also metabolized by peroxisomal α-and β-oxidation.KBs(acetoacetate,beta-hydroxybutyrate,and their spontaneous break down metabolite,acetone)are produced(mainly by the liver)from two acetyl-CoA units when glucose is less available.These KBs are readily used by extra-hepatic tissues that convert the mintoacetyl-CoA,which then enters the citric acid cycle to produce energy.Glucose is any way necessary to provide Krebs cycle substrates(i.e.succinyl-CoA)for the complete oxidation of KBs.Medium chain FAs(MCFAs),such as octanoic acid(C8:0)and decanoic acid(C10:0)can modulate astrocyte metabolism,by promoting ketogenesis and glycolysis,respectively.Reprinted from[46]with permission of Elsevier.

The maintenance of neuronal excitability and synaptic function is mediated by docosahexaenoic acid(DHA),which within the total phospholipid composition of plasma membranes can reach values higher than 40%[47].The brain capacity to locally synthetize DHA is extremely low,hence the necessity to increase the uptake levels of DHA from circulating lipids in order to maintain the homeostatic levels[47].Monounsaturated fatty acids(MUFA),namely cis-MUFA oleic acid 18:1,which high consumption is normally associated to the Mediterranean diet,are associated also to beneficial effects on the cardiovascular system.When associated to olive oil anti-oxidants(i.e.tyrosol,hydroxytyrosol,and oleuropein)a decrease in the chronic inflammation of human glioblastoma was observed[46].Positive effects on diseases such as inflammatory bowel disease are being attributed to the production of anti-inflammatory compounds,as a result of microbiota composition reversion promoted by n-3 PUFA ingestion[48].Investigation on microbiota-depleted animals,which received microbiota from patients exhibiting depression symptoms,showed intensification of depressive behavioural and physiological signs[49,50].Also,the increasing production of acetate in mice and rats were exposed to an altered gut microbiota,showed signs of parasympathetic nervous system activity with consequently increase of glucose-stimulated insulin and ghrelin secretion levels,extended hyperphagia and increased obesity[51].Opposite from the initial though,the microbial imbalance or dysbiosis is actually not only connected to the intestinal pathologies(e.g.colorectal cancer,inflammatory bowel disease or celiac disease)but is likewise associated to liver and brain disorders.Chronic liver diseases,such as chronic hepatitis B,chronic hepatitis C,alcoholic liver disease,non-alcoholic fatty liver disease,non-alcoholic steatohepatitis,development of liver cirrhosis,and hepatocellular carcinoma have shown direct relationship with gut microbiota,indicating that new gut-associated therapeutics could potentiate the treatment of such diseases in the near future[52].Brain disorders like Parkinson’s disease,Alzheimer’s disease,autism,depression and even schizophrenia are also associated to gut microbiota[53].La Rosa et al.[54],lately reviewed the need for longer term trials with increasing omega-3 fatty acids supplementation should be tested in early stages of Alzheimer’s disease.The prenatal and early postnatal periods are crucial stages in infants,both for brain and for gut microbiota development.A report on the impact of maternal and early life n-3 PUFA intake is suggests a critical stage,where nutrition may be crucial in regard to brain and brain-related behavioural outcomes[55].This in vivo study demonstrated the high dependency of cognitive-related neurobehavioural development,anxiety and social behaviours upon in utero and lifelong n-3 PUFA availability.

Fig.4.Scheme on how dietary fatty acids can affect synaptic plasticity and cognition.

Such a complex network of communication between the gut microbiota and the central nervous system is mediated by the autonomic nervous system,the enteric nervous system,the immune system,and the bacterial metabolites[56].Focusing on the bidirectional gut-brain axis,specific bioactives(e.g.polyphenols)can affect the microbiota through inhibition or the increase of specific bacteria proliferation,rising the probability of altering brain functions that are related to neurodegenerative diseases[53,57].This principle is now widespread for prebiotics and probiotics.The non-covalent dietary fiber-polyphenol interactions in the digestive tract,can positively affect the polyphenols bioaccessibility and as consequence their bioavailability.In this way,dietary fibers(e.g.cellulose derivatives,xyloglucan,cell wall material,dextrin,among others)are susceptible to play the role of“controllers”for the amount of bioaccessible polyphenols in the upper or lower parts of the digestive tract[58].Studies(generally in vivo)established the association between changes in the bacterial gut population and consequential neurological disorders.Bercik et al.[59]concluded that brain biochemistry modifications were able to modulate behaviour in adult mice,in an independent way from the autonomic nervous system,pointing towards the direct involvement of substances produced by gut bacteria.Recently,Sampson et al.[60]reported on the movement disorders which are associated to Parkinson’s disease,suggested that the human microbiome is influential on it,as modifications in mice gut bacteria showed to regulate movement disorders.Microbiota analysis in Parkinson’s disease patients revealed a decrease of Prevotella strains and increase of Enterobacteria supporting the possibility that the disease itself begins to develop in the gut and spreads to the brain[61].As addressed earlier,polyphenols can be used during the production of oleogel systems(by the emulsion template method).Apart from that structural function,important bioactivity is associated to polyphenols as these plant-based molecules are capable of acting as natural antioxidants due to metal-chelating and free-radical scavenger capabilities.Their incorporation in the formulation of oleogel systems could be interesting in a way that such environmental conditions could favour polyphenols absorption in the gut,since their interaction with proteins,fibres and polysaccharides will determine their easy absorption[62].Gut bacteria metabolism produce neurotransmitters and bioactive metabolites from polyphenols,that can reach the brain after crossing the intestinal barrier[62].Fig.5 shows a scheme of the gut-brain interactive bidirectional pathway.

The food in our diet is broken down into carbohydrates,protein and lipids,which can be further metabolized by the gut microbiota.The by-products from carbohydrate fermentation can result in the synthesis of SCFA,which have the possibility to induce epigenetic modulation of the intestinal epithelial cell in addition to direct effects on GPCRs(GPR43/41)on EECs.Bile acids derived from fatty acid metabolism can also have multiple effects including interacting with GPCR TGR5(also known as G protein-coupled bile acid receptor 1[GPBAR1])and the nuclear receptor farnesoid X receptor(FXR)on the(EECs).Both SCFA and bile acids can thus stimulate the modulation of gut hormones secretion,including PYY,GLP-1 and CCK as well as having immunomodulatory responses.The satiety hormones can modulate CNS function and regulate appetite and food intake.Finally,a myriad of neurotransmitters and neuroactive substances produced by the gut microbiota can regulate a host of peripheral and central functions via indirect and direct mechanisms.In addition,some metabolites can pass into the blood and through the circulatory system,indirectly via receptors on cells or directly through the blood brain barrier,modulate brain function.CCK,Cholecystokinin;EECs,Enteroendocrine cells;FXR,Farnesoid X receptor;GABA,Gamma-aminobutyric acid;GLR-1,Glycogen like protein;GPCR,G protein-coupled receptor;HAT,Histone acetyltransferase;HDAC,Histone deacetylases;PYY,Peptide YY;SCFA,Short chain fatty acid.Reprinted from[56]with permission from Elsevier.

Several strategies,using oil structuring systems,can be applied to target the delivery of functionality to food consumers and therefore exert an active role on gut-brain interaction path.Oleogels rich in omega-3 or other bioactive compounds constitute a real possibility for the delivery of bioactivity,while at the same time can act as fat substitutes.Fish oil is certainly one of the richest sources for omega-3 PUFA,however,it sustains an extremely high risk of oxidation that results in a severe loss of functionality[63].The production of fish oil-based oleogels can be an alternative,however despite the protection that is provided by the gelator network,additional strategies can be used to ensure proper protection for such sensible compound.Lee et al.[64],engineered a scheme that apart from the already studied micro-and nano-emulsions,created an oleogel-based structure exhibiting internal oil structuring(with beeswax)and external protein coating(with whey protein isolate).This method produced high efficiency loading of the particles,with sizes ranging from 173 nm to 200 nm,revealing a decrease in the rate of oil oxidation.Oleogels produced with blends of rice bran and flaxseed oil,gelled with combinations of palm stearin and cetyl laurate or cetyl caprylate were subjected to an in vivo study and showed efficacy towards bad cholesterol reduction.Oleogels formed with palm stearin and cetyl caprylate showed more pronounced positive changes[65].The consumption of coconut oil in oleogel form(structured by ethylcellulose)or in liquid form,affected differently blood triglycerides,glucose,insulin,and appetite when co-ingested with a carbohydrate-rich meal.Still another structure-dependent metabolic response was verified after the consumption of monoglycerides MAG gels,after incorporation in whole wheat toasts,resulting in a tempered postprandial metabolic responses(disparities in serum triglyceride and free fatty acid levels,and insulin response as well)when compared to the ones from consumption of an equivalent composition using an oil suspension(liquid form)[66].Tp-palmitate particles in conjunction with citrus pectin showed feasibility to stabilize of O/W emulsions and the corresponding camellia oil-based oleogels,by means of the interfacial accumulation of Tp-palmitate[67].Previous research with oleogels using increasing beeswax concentration,demonstrated a follow-up improvement in regard to the oxidative stability of these oleogels when loaded with β-carotene[21].Oleogel strength as an effect of beeswax concentration and different crystal arrangements(demonstrated by small angle X-ray spectroscopy)resultant from the usage of different fatty acid chain lengths(medium and long chain triglycerides oils)are features that can be relevant for bioactive delivery in the human gut[68].Regarding the oleogelation of protein-stabilized O/W emulsions using rice bran wax,Guo et al.[69]reported on a highly stable conformation and as a consequence of that kinetic stability,the release profile of free fatty acids revealed a delayed intestinal lipid digestion due to the intra-droplet rice bran crystallization.Tan et al.[70]performed studies where the subjects were exposed to dietary fats in different physical forms and evaluated how this impacted on postprandial energy expenditure,oxidation and glycemic responses and found that oleogels prevented the glycemic-lowering and fat-oxidation effects,which were induced by simple liquid oil presence.Still in this regard,Tan et al.[66]suggested that the incorporation in solid forms of oils foods,like coconut oil oleogel or sunflower oil oleogel,are associated to the increase of postprandial triglyceride,could act in a reversed way.This effect was firstly demonstrated by this study,where a significant reduction in postprandial triglyceride excursion level was verified after the consumption of a coconut oil-based oleogel.Coenzyme Q10 is a mitochondrial respiratory cofactor,and also is an endogenous antioxidant that can suppress the progression of renal diseases and neurological ailments.This lipophilic molecule was incorporated in medium chain triglyceride-based oleogels by Masotta et al.[71],and apart from a high loading capability,the ease demonstrated by dysphagia CoQ10-deficient patients towards swallowing the oleogels proved to be beneficial.The residence time that bioactive compounds show along the different stages of the gastro-intestinal apparatus is now accepted to be dependant of the food matrix characteristics and consequently this will affect the outcome in terms of nutrients physiological conditions[72].With that in mind,materials can be applied in different food systems in order to retard in vivo lipid digestion and deliver in situ bioactivity at a slow rate.Recent reports on oleogel digestibility demonstrated how lipolysis and consequentially the bioactivity of lipophilic compounds can be affected by the oleogelator nature as well as its concentration.The recorded behaviour between low and high molecular weight oleogelators(i.e.ethyl cellulose,mono-and di-glycerides(E471),and a mixture of β-sitosterol+γ-oryzanol),under simulated intestinal conditions,showed dissimilar levels of break-down and free fatty acids release as the increase of the molecular weight of the gelators led to a lower extent of lipid hydrolysis[73].The ability to control or delay the release of carotenoids is an important scientific undertaking,since the micellization rate of β-carotene,at the intestinal level seems to be related to the lipidic structure that carries such molecules.Hughes et al.[74]used an in vitro model of the human digestive system to show that the formation of canola oil oleogels(gelled with 12-hydroxystearic acid)affected the micellization rate of the bioactive,when compared with liquid canola oil.Since it was registered that only a negligible amount of β-carotene was transferred during the gastric phase(evidencing structural maintenance of the gelator network and consequent bioactive protection)the major changes(bioactive transfer)occurred during the intestinal stage consequently evidencing the delivery of highly aqueous-insoluble molecules through digestion of oleogels,representing a significant achievement.

Fig.5.Schematic representation of the gut microbiota-brain interaction.

4.Food product applicability now and in the future

Replacing saturated triacylglycerols by healthier alternatives,configures one of the major opportunities towards the use of oleogels in food products[75].Also,ailments like dysphagia and malnutrition,which are easily identified in some sectors of society(e.g.elderly and children)can be addressed with such technological implementation[76].Oleogels that show compatibility within food matrices by providing structure,bioactive protection and bioactive delivery will raise interest from the academic and industrial sectors.All of this combined with the versatility to implement oleogels in already established regular industrial processes will increase the interest from the industry in the following years.Table 1 presents some of the applications of oleogels in food products that were explored during the last decade and recent proposals for the incorporation of oleogels in food products.mented and such trend may function as a catalyst for a makeover of the nutritionally enhanced food market.

Table 1 Food products where oleogels were tested and the corresponding oleogelator and oil type.

5.Conclusions and future considerations

The design of oleogels using specific gelation mechanisms can be exploited so that lipid digestibility can be tailored.This can be performed by promoting a controlled release of nutraceuticals through oleogel structure breakdown in association with lipolysis rate control.Because the personalized nutrition concept is one of the main future trends,it is expected an increasing number of in vivo studies focused on functionality delivery,through food ingestion aiming at individual microbiota composition and genetic(DNA)nutritional need.Targeting the improvement of therapeutic efficacy of oleogels or oleogel-based systems in the future,the nano-structures in oleogels production seem to be a possibility to achieve the introduction of novel prospects for oil structuring.In situ biodegradable oleogels were evaluated for their applicability as vehicles for controlled drug delivery.Such thermo-reversible oleogels formed in situ,after injection,represent an innovative approach that targets the solubility and burst limitations of hydrogels that would result in faster drug release.In vivo results reported by Wang et al.[90]showed the feasibility to gel soybean oil applying relatively low fatty acids concentrations,while sustaining a long term(one week)rate release of an injectable drug.Structures like solid lipid nanoparticles(SLNs)or nanostructured lipid carriers(NLCs)[91]can be re-designed using oil structuring approaches for bioavailability improvement directing and/or tailoring the delivery rate of selected molecules.Apart from the fatty acids that constitute the edible oil backbone of oleogels,other molecules can be incorporated in these systems(e.g.polyphenols),possibly aiming at delivering them through intestinal absorption.Another possibility is targeting the reduction of the hazardous effects of saturated fats by diminishing their absorption after digestion.Mucoadhesive biopolymers have the capability to play a relevant role with their introduction in oleogel stabilization.These biopolymers can benefit(if that is the main objective)from the fact that their surface properties can be used as a means for controlled release and increased residence time at the intestinal level[92].

It is clear that the structuring of lipids has still not reached its full potential.Changes within oil structuring strategies can be imple-

Declaration of Competing Interest

The authors declare no conflict of interest.

Acknowledgements

We would like to acknowledge the H2020-MSCA-RISE project FODIAC—Food for Diabetes and Cognition(reference number 778388).