Open access peer-reviewed chapter
Abstract
A new issue for the scientific community is to find efficient replacements for unhealthy fat without damaging the organoleptic qualities of the food product in light of growing concerns about the consumption of harmful trans fats in the diet. Bigel is supposedly a novel structured fat system utilised for industrial purposes due to their nutritional advantages, one of numerous solutions intended to replace trans fats in food. These have a lot of potential in the food industry, and are composed of an aqueous phase known as a hydrogel and an organic phase known as an organogel or oleogel. A gel known as an oleogel has oil as its liquid component. Oleogelators, which aid in the development of gels, frequently have low molecular weight, whereas typical hydrogelators have large molecular weight since they are polymeric. A hydrogel is a gel in which water serves as the immobilised phase. Therefore, a bigel is a biphasic system made up of an oleogel and a hydrogel. This chapter will concentrate on the various bigel formulation techniques and chemistry, as well as their latest food uses, and other industries that fit their requirements.
Keywords
- bigel
- organogel
- hydrogel
- food applications
- trans-fat replacement
1. Introduction
For several food products, fats and oils are important raw materials with some beneficial minerals and vitamins. Saturated fatty acids (SFAs) and trans fatty acids (TFAs), which are present in the form of triacylglycerols, cause the assembly of a colloidal or supracolloidal particle network, which is responsible for structuring the fat into a solid or solid-like substance. This network is largely responsible for the functionality and properties of solid fat [1]. However, solid fats, specifically SFAs and TFAs, raise certain health issues [2]. An increase in low-density lipoprotein cholesterol levels is associated with the consumption of SFAs and TFAs rather than polyunsaturated fatty acids (PUFAs). There is still dispute over the harmful effects of SFAs on health, but if we assume that customers want food free of SFAs, then alternative oil structurants are urgently needed [3]. Since consuming TFAs, a subgroup of unsaturated fatty acids, is associated with increased levels of low-density lipoprotein cholesterol and decreased levels of high-density lipoprotein, they are particularly detrimental [4]. There are few solutions now for removing TFAs while preserving the desired physical characteristics of foods. In this context, the bigel (also known as hybrid gel) is a unique formulation that resembles a solid and is created by combining an oleogel and a hydrogel at a high shear rate [5].
Gels are semisolid mixtures that typically contain two ingredients: liquid and solid, where the solid component is referred to as a gelling agent or gelator and the liquid component is referred to as a solvent. The gelator raises surface tension and is frequently employed at concentrations below 15% w/v to stop solvent flow. Gels are divided into two categories based on the polarity of the solvent: hydrogel and organogel [6, 7]. In contrast to organogels (also known as oleogels), which have apolar liquids as their continuous phase such organic solvents or mineral or vegetable oils, hydrogels are gels whose continuous phase is often a polar solvent, such as water. Cross-linked hydrogels may absorb large amounts of water without dissolving themselves in it. Hydrogels are 3D hydrophilic networks of homopolymeric or heteropolymeric chains. An organic liquid is confined inside a thermoreversible 3D network to create an organogel, a system that resembles a solid. Organogels are extremely simple to make and naturally greasy [8, 9]. Numerous organogelators have been studied, including fatty acids, fatty alcohols, lecithin, physterol and oryzanol mixtures, waxes, steroids, 12-hydroxystearic acid (HSA), L-lysine-based gelators, cyclodextrins, and others. For this type of system, a number of solvents including benzene, hexane, and food oils like sunflower oil, corn oil, sweet almond oil, cod liver oil, and olive oil have also been investigated as liquid phases [10, 11, 12].
Bigels’ main advantages include the capacity to distribute both hydrophilic and hydrophobic agents, spread easily, be prepared easily, be more stable at room temperature, and be used to control the system’s properties by varying the proportion and structural distribution of each phase. Bigels are an appropriate and intriguing formulation for a variety of applications, including medicinal, cosmetic, and food systems [13, 14, 15]. These systems can be divided into three categories based on how both phases (organogel and hydrogel) are distributed within bigels: 1. Organogel-in-hydrogel type 2. Hydrogel-in-organogel type 3. Bi-continuous/matrix-in-matrix type. The system that contains organogel as a dispersed phase and hydrogel as a continuous phase is known as an organogel-in-hydrogel system, and it may be the most studied type in the literature. Various researches extensively discussed this kind of bigel. In this bigel type, the oleogel is present as dispersed phase and the hydrogel is present in the continuous phase (Figure 1).
Figure 1.
a. Oleogel dispersed in hydrogel. b. Hydrogel dispersed in oleogel.
This type of bigel has been reported by various researchers by taking into account various hydrogel systems with various hydrogelators/gelling agents, such as gelatin-agar mixture, guar gum, xanthan gum, and acacia gum, gelatin, whey protein, pectin, starch, sodium alginate, sodium carboxy methyl cellulose, hydroxyl propyl-methylcellulose, polyvinyl alcohol, and polyvinyl pyrrolidone. Different organogel systems were also taken into consideration to construct bigels in addition to these hydrogels [16, 17, 18, 19, 20].
In the last 10 years, a number of researchers have conducted substantial study on bigels, which are developed by combining organogel (oil phase) and hydrogel (aqueous phase) [21, 22]. As a result, this study will focus mostly on this bigel system type. This book chapter’s objective is to give readers a solid understanding of various bigel systems. Types and properties of bigels, various bigel synthesis procedures, and various characterisation techniques that can be used to evaluate these systems are the key subjects covered. Additionally, the characterisation techniques of bigel are also discussed using specific instances from the literature as a special system for food applications.
2. Preparation of Bigel
Figure 2 illustrates the methods for making bigels by combining hydrogel and organogel. The three most crucial variables for synthesising bigels by combining hydrogel and organogel are the bigels’ storage conditions, mixing speed, and temperature. The temperature needs to be held steady during mixing for making bigel. In order to prevent degradation of the hydrogelators during the mixing process, the hydrogelators must be very thermostable. While other study found at mixing the two phases at room temperature while stirring continuously, others reported inclusion of the aqueous phase/hydrogel into the organic phase/organogel at a significantly higher temperature (like 50°C) [23].
Figure 2.
Schematics of preparation of bigels by mixing hydrogel and organogel.
The way gels are stored before or after mixing can also have an impact on the final bigels’ characteristics. Some bigel systems, as documented in the literature, have been manufactured by storing the final bigel system for a specific amount of time and at a specific temperature after storing the component gels separately for a specific amount of time (e.g., 24 hours). The second strategy, which involves storing the final system after mixing, may result in a more stable system for characterisation. Storing individual gels before combining may assure complete gelling of the individual system [24, 25].
In order to prepare bigels, Satapathy et al. [26] described mixing the individual systems at a somewhat high temperature (50°C), as well as mixing the two phases at room temperature while stirring continuously. Rehman et al. [27] looked at the characteristics of bigels made by combining the two separate systems (hydrogel and organogel) after storing them separately at a specific temperature and time interval. On the other hand, bigels systems can also be made by mixing the various gels before storing the finished product [28]. Figure 3 depicts the experimental block design for the synthesis of bigels utilising two various approaches. Recent research by Fasolin and Vicente [29] describes how the rheological and microstructural characteristics of bigels are affected by the speed of mixing. To create the bigel system, emulgel/emulsion hydrogel was also combined in various proportions with the organogel phase at room temperature in place of hydrogel. Additionally, a number of models have been mentioned that can be used to relate the various characteristics of bigels, in particular the rheological models that have recently been proposed in the literature to relate the rheological characteristics of bigels with the dispersed phase fraction as well as with the characteristics of individual phases (organogel and hydrogel) [30, 31].
Figure 3.
Flow diagram for synthesis of bigels (a) by storing individual gels before bigel preparation and characterisation (b) by mixing individual phases and then storing bigel formulations before characterisation.
Lupi et al. [32] discovered olive oil as a solvent and mixture of glyceryl stearate and policosanol as an organogelator, sorbitan monopalmitate-sunflower oil based organogels were investigated by Behera et al. [33], sorbitan monostearate and sesame oil were utilised to prepare organogels as studied by Singh et al. [34], span 60, cetyl alcohol and lecithin-pluronic as organogelators and soya-bean oil as a solvent were found by Ibrahim et al. [7], soya-bean oil-stearic acid based organogels were investigated by Sagiri et al. [35] and Wakhet et al. [36], sorbitan monostearate/medium chain triglyceride based organogels were discovered by Rodrigues et al. [37], sorbitan monostearate as an organogelator and almond oil as a solvent was reported by Andonova et al. [38].
The hydrogel-in-organogel system is a second type of bigel in which the hydrogel phase is dispersed throughout the continuous matrix of the organogel. According to Patel et al. [39], the examination of bigels made by combining organogels made of silica and sunflower oil-fumed organogels at various ratios was done. The results of confocal microscopy supported the morphology of bigels that is of the hydrogel-in-organogel type. The third form of bigel can be thought of as a system with a complex structure where it is challenging to distinguish between the continuous and dispersed phase. A cosmetic system (O/W) was combined with organogels that contained monoglycerides of fatty acids as the organogelator and olive oil as the solvent to create bigels, according to Lupi et al. [30]. The results demonstrated the presence of a complex matrix-in-matrix structure at the highest fraction of organogel.
3. Different formulation methods of edible bigel with potential to replace trans-fat
A novel edible biphasic gel system known as bigel was developed and its mechanical, microstructural, and thermal characteristics was studied by Acevedo & Saffold [40]. The bigel was created using a gelatin hydrogel, a rice bran wax (RBW)-based oleogel, and soybean oil. Bigels were created using three different concentrations of gelatin (5, 7, and 10% (w/w)) and four different oleogel-to-hydrogel (OG:HG) ratios (50,50, 40,60, 30:70, and 20:80). The 10% (w/w) concentration of RBW remained steady. Bigels were examined using confocal laser scanning microscopy (CLSM), small deformation rheology, diferential scanning calorimeter (DSC), and Fourier transform infrared spectroscopy (FTIR). For all bigel formulations, CSLM pictures confirmed an oleogelin-hydrogel system, with an increase in oleogel proportion resulting in larger oleogel droplets and improved stability. All bigel formulations, regardless of gelatin concentration and the ratio of oleogel to hydrogel, exhibited more solid than liquid (G′ > G′′) behaviour and frequency independence at 20°C, according to rheological analysis of the systems. Greater elastic modulus (G′) values were consistently seen in bigels with higher OG:HG ratios, such as 50:50 and 40:60, than in those with lower OG:HG ratios, demonstrating that increased oleogel droplet contact results in improved mechanical properties. The Boltzmann Sigmoidal model successfully described the rheological behaviour of all bigels. In all bigel samples, FTIR and DSC analyses revealed discrete peaks for the oleogel and hydrogel phases without any additional thermal events, demonstrating a lack of interactions between the parts of both phases. Overall, the system benefits from having two separate phases and is kinetically stable, making it a “true” bigel.
By combining gelatin hydrogel and stearic acid-based organogel using a heated emulsification technique, Pal et al. [35] created bigel. Sesame oil and soy bean oil based stearate organogels were used to create two distinct types of bigels. Sesame oil and soy bean oil-based gelatin-based emulgels served as the controls. Microscopic examinations showed that the emulgels displayed distributed droplets inside the continuum phase, in contrast to the bigels, which retained droplet clumps. While the interior phase of the bigels barely leached at all, the emulgels revealed a higher level of oil leaching. By using XRD, FTIR, and DSC techniques, the presence of an organogel matrix within the bigels was verified. In comparison to emulgels, bigels demonstrated higher mucoadhesive and mechanical qualities. Studies on cyclic creep-recovery and stress relaxation supported the formulations’ viscoelastic properties. The cyclic creep-recovery data were analysed using the four-element Burger’s model. Studies on cyclic creep-recovery revealed that the presence of organogels within the structure of bigels may have reduced the amount of deformation. The formulations’ nearly complete recovery following the creep stage can be attributed to their increased elastic nature. According to a stress-relaxation study, emulgels had a longer period of relaxation than bigels. Additionally, emulgels had a higher relaxation percentage, indicating a fluid dominance. Human epidermal keratinocyte cell line was used to test the bigels’ in vitro biocompatibility (HaCaT). Bigels and emulgels were both biocompatible substances. Based on the findings, it was determined that the created bigels may have a very high potential for usage as emulgel substitutes.
By combining a glycerol monostearate (GMS) oleogel with a gelatin hydrogel with the addition of lecithin and glycerol as surfactant and co-surfactant, respectively, Pinhas et al. [41] developed a unique in-situ bigel system. A bi-continuous system made up of an oleogel crystalline network and a hydrogel polymeric network was created as a result of this combination. On bigel structure, hardness, and stability, the effects of bigel composition, water:oil ratio, and homogenisation time were investigated. Gel hardness and stability were shown to be positively correlated with structuring agent concentration, with the maximum enhancement occurring at 2% weight gelatin and 25% weight GMS. At 5%wt. of the oil phase, lecithin showed maximal strength increase and stabilisation; above that point, a considerable drop was seen. Hardness increased with an increase in oil concentration up to 50%wt, but producing stable bigels was impossible with higher oil contents. When the samples were homogenised for 3 min, better mechanical and stability qualities were seen. With a slight decline in OBC, optimised bigel systems kept their mechanical integrity after 90 days. In addition, compared to the oil sample, low PV, < 10 meqkg-1, was sustained for 30 days at 4°C, 25°C, and 40°C. However, the TBA readings showed a distinct pattern, pointing to a variable transition kinetics between the main and secondary oxidation products. These findings offer a thorough picture of the composition-structure-function relationship of the bigel system and should be taken into account when creating textured food products in the future.
Bigel was created by homogenising at high shear an oleogel emulsion made of soy lecithin, stearic acid, soybean oil, and water, as well as a hydrogel made of whey protein concentrate and water, according to Acevedo et al. [42]. Small angle x-ray scattering, rheology, and fluorescence microscopy were used for characterisation. The oleogel emulsion kept its fundamental structural properties with the addition of the hydrogel component but lost higher order structuring. It was discovered that the bigels’ G’ values were temperature-dependent. Despite their susceptibility to temperature, the bigels displayed G > G at all ranges from 8 to 98°C. At equal ratios of oleogel emulsion and hydrogel, fluorescence microscopy showed that a bi-continuous bigel was created; however, as either of those phases increased, one of them became the dominant continuous phase. The oleogel emulsion and hydrogel may have interacted to some extent at 10 weight percent water and 15 weight percent protein usage, respectively. This synergy enhanced the mechanical properties of the bigel. On the other hand, the connection between phases changed to be hostile to the mechanical properties of the bigel at protein and water contents outside of those mentioned above.
The objective of the study conducted by Pal et al. [43] was to create and characterise novel bigels for controlled drug delivery applications by combining guar gum hydrogel and sorbitan monostearate-based organogel. The confocal microscopy revealed the presence of a bigel composed of the aqueous and oil phases. Shear-thinning and viscoelastic properties of the bigels were indicated by micro-scale deformation (viscometric) analyses in combination with macro-scale deformation research. Thermal analysis revealed that as the quantity of organogel in the bigels increased, so did their thermal stability.
The bigels that were created had a biocompatible composition. According to an in vitro drug release investigation, the amount of ciprofloxacin (a lipophilic medication) released rose as the amount of organogel content decreased. Further investigation revealed that all of the bigels’ drug releases adhered to zero order diffusion kinetics, which is ideal for a controlled release system. The drug-loaded gels effectively combatted Bacillus subtilis’ microbes. Finally, the created bigels might be used as matrices for topical medication administration.
In their study, Quilaqueo et al. [44] assessed how the type of hydrogel affected the creation of bigels that would be utilised to substitute fat in cookies. Beeswax/canola oil oleogel, sodium alginate, and carboxymethylcellulose hydrogels were used to make bigels. The outcomes demonstrated that the type of hydrogel utilised had an impact on the peroxide value and binding ability of bigels. They did not change in terms of fatty acid composition, p-anisidine value, oxidative stability, or texture, though. Cookies made with bigels as fat substitutes had a hardness equivalent to that of cookies made with shortening, demonstrating the potential of bigels for usage in food.
4. Characterisation technique
4.1 Organoleptic evaluation
Bigels is left undisturbed once the formulation phase is complete until it reaches room temperature. They are then assessed for a number of factors, including uniformity, colour, pH, viscosity, and phase segregation [30]. Bigel has a high spreading and white colour intensity when the oleogel concentration is high.
4.2 Determination of pH
Digital pH meter are used to determine the pH of the formulation [45].
4.3 Spreadability
The spreadability of the formulation is assessed by layering 0.1 g of gel between two glass slides with the same dimensions (e.g., 25 mm, 1 mm, or 75 mm). Following that, specific weights of 10 g, 20 g, 50 g, or 100 g are loaded for 60 seconds on the upper glass slide. The initial and final spreading diameters are measured before and after each weight is placed [46].
4.4 Extrudability
A particular quantity of gel is placed into an ointment tube. The length of the gel ribbon that emerged from the ointment tube after uniform pressure was applied is used to calculate the extrudability of the gel [47]. In (cm/s), extrudability is measured.
Extrudability = Distance travelled by gel in cm/10 s
4.5 Thermal analysis
To ascertain the gel-sol transition temperature of organogels, the falling ball method is applied (Tgs). The surface of the organogel is securely supported by a metal ball that weighed around 250 mg. The gel is then heated at a specified rate while a thermometer is inserted, causing the temperature to rise by 1°C per minute-by-minute until it reached 70°C. The temperature of the gel-sol transition is determined as the point at which the ball started to travel through the gel (Tgs). Bigel situations prevent the use of this approach over 50°C because phase separation develops and they become unstable [48].
4.6 Drug content determination
To completely allow for drug leaching, the drug-incorporated bigel is dissolved in phosphate buffer and left undisturbed for at least 48 hours. The drug-containing dispersion is then filtered using Whatmann filter paper. The resulting solution is suitably diluted, and the absorbance is evaluated using a UV spectrophotometer set to the drug’s maximum wavelength [49].
4.7 In vitro drug release
A modified Franz diffusion apparatus is used to assess the in vitro release of the medication from the gels using a dialysis membrane (HIMEDIA® LA 330-5MT). A specified quantity of the drug-loaded sample is deposited in the donor compartment, and the phosphate-containing chamber (also known as the receptor chamber) is kept at 32 0.5°C. For 7 hours, a sample of 1 ml is taken every hour, replaced with a new buffer system, and the process repeated. The acquired data is further examined using the Higuchi equation, zero-order, first-order, and Korsmeyer-Peppas models [50].
4.8 Accelerated stability studies
The accelerated stability data are gelled using freeze-thaw (F-T) thermocycles (20 minutes of freezing at 20°C and 20 minutes of thawing at 70°C). The bigels is examined for colour change, viscosity, phase segregation, and homogeneity after each of the five cycles of this procedure. These investigations provide forecasts for long-term stability [51].
4.9 Droplet size distribution
The distribution of droplet size is one of the most important factors in determining how well a medicine is absorbed from gels. Smaller droplet sizes provide us more interfacial surface area, which increases medication absorption and offers great stability. To analyse the droplet size distribution, utilise ImageJ programme [52].
4.10 Optical microscopy
The mutual arrangement of oleogel and hydrogel within the bigels is investigated using a variety of microscopic techniques, including confocal microscopy, scanning electron microscope (cryo-SEM), and transmission electron microscopy. To distinguish between oleogel and hydrogel, fluorescent dyes are added into both phases [53].
4.11 Fourier transform infrared (FTIR)
The functional groups that are present in the bigel formulation are evaluated using this spectroscopy. Almost majority of the molecules’ functional groups absorb infrared light between the wavelengths of 4000 and 1500 cm−1. The spectrum, which was captured in this precise range, is utilised to ascertain if the mixture is lipophilic and hydrophilic. The hydrogel’s intra- and intermolecular hydrogen bonding cause a noticeable hump to be present in the range of 3300 to 3200 cm−1 [54].
4.12 Mechanical properties
Different approaches are used by viscometers and rheometers to study different mechanical properties of bigels, such as viscosity. The viscosity of the bigels is measured and recorded using a cone and plate viscometer. The measurement is often carried out with a shear rate ranging from 20 to 100 s−1 at conventional room temperature (i.e., 25°C). Data are calculated and measured using the Ostwald-de-Waele power law. The viscosity profiles of non-Newtonian fluids are represented by this law.
τ = K.γn (τ-shear stress, γ-shear rate, K-flow consistency coefficient and n-rate index).
The aqueous and organic phases’ interpenetrating network has a synergistic effect on the bigels’ rheological properties. Bigel formation will not happen if the hydrogel content in the formulation is greater than 50% of the organogel [55].
4.13 Electric conductivity
The electrical characteristics of the bigels are examined in order to appreciate the conductivity profiles. The electrical profiles of the bigel can be calculated using a computer-controlled impedance analyser. A specified frequency range is used to measure the data while it is at room temperature. We are better able to comprehend their transport behaviour when we consider how the current affects the formulation’s conductivity. The biggerels with a higher fraction of hydrogel display stronger conductivity because of the ions that are present in the water phase [56].
4.14 Photostability study
To evaluate the stability profile of the bigels in the presence of light, photostability investigations are carried out. This further aids us in choosing the kind of packaging that will be applied to the finished commodity (i.e Primary or secondary packaging). The ICH Q1B guideline has been followed in the execution of these research. The product is tested under four distinct conditions: first, it is wrapped in aluminium foil; second, it is packed in cardboard; third, it is kept in a container with a label; and fourth, it is kept in a container without a label. The formulation is assessed for different characteristics, including pH, contaminants, assay, and appearance after exposure to a certain amount of light [57].
5. Conclusion and future perspective
Different bigel systems have recently been created and modified to suit the requirements of various applications. This book chapter discussed the modelling of these systems in detail and presented the literature on the significant bigel properties. Additionally, it has been considered to use these systems for food applications. Bigels are a new class of materials, hence thorough analysis of these systems is necessary for applications in industry. Storage of bigels, mixing speed, mixing temperature, the amount of hydrogelator and organogelator used, the ratio of hydrogel to organogel, the addition of emulsifiers, and the use of emulgels in place of either organogel or hydrogel are some of the variables that are crucial in the synthesis of bigels. Researchers need to focus further on how the aforementioned criteria affect the final attributes of the created formulation. While the hydrogel-in-organogel and bi-continuous forms of bigels have received less attention from researchers, the organogel-in-hydrogel type has been extensively examined in the literature. In order to better propose a model to precisely anticipate the features of such a complicated system, more research is required to comprehend the reliance of the rheological properties of bigels on the moduli of dispersed and continuous phase as well as on the particle size distribution. Additionally, such systems can be simulated to compare the outcomes with real-time responses. Furthermore, bigel formulations can be created and examined for a wide range of culinary applications in the future, in addition to medicinal and cosmetic applications.
Conflict of interest
The authors declare that there is no conflict of interests among them regarding the publication of this paper.
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