Introduction
Food gels are viscoelastic compounds and several gelled items are produced across the world having several applications in the food, drug, cosmetic and bio-medical industry (Kaplan, 1998). Polysaccharides and proteins are the most common gelling agents in food. These gelling agents are held together with weak inter-molecular forces such as hydrogen bonds, electrostatic forces, Van der Waals forces and hydrophobic interactions. However, some specific protein gels being the exception produce highly ordered gel structure with disulphide bonds. Polysaccharides, especially hydrocolloids are strongly hydrated in water, although their structures are less ordered (Banerjee & Bhattacharya, 2012). The mechanism of gelation is determined by the type of the gelling agent(s) and the conditions of gel formation. Major factors affecting gel formation are temperature, ion presence, pH and gelling agent concentration. (Saha & Bhattacharya, 2010)
Gels can be classified in a variety of ways, the most common of which being rheological measurements. A multi-component or mixed gel system is an important field of research in which two or more gelling components are used simultaneously to obtain certain structural and functional properties (Buerkle & Rowan, 2012). Likewise, the gels can further be classified based on chemical nature of the hydrocolloid used and the process used for the activation of structure formation.
The most important classification for the food gels is based on the technique used for the modification of the food gels (Gulrez et al., 2011). The modifications are usually done in order to modify the physical properties of these gels, which lead to the formation of various types that mainly include hydrogel, cryogel, xerogel and aerogel (Ajdary et al., 2021). This article is mainly focused on the formation of these gels and their applications mainly into the food system.
Hydrogel
Polymer networks with hydrophilic characteristics are called hydrogels. These hydrogels are a type of soft, wet material that have three-dimensional network of crosslinked polymers beholding water in intermolecular spaces (Dumitriu & Dumitriu, 1994). Because of their diverse functional properties, these hydrogels find their utility in a wide range of applications, such as soft robots and actuators, stretched electronic devices, tissue engineering materials, controlled-release drug delivery vehicles, biomedicine materials, food science and bio-sensors (Li et al., 2021). The specific applicability of a hydrogel can be achieved by a rational structural designing, as the function and properties of hydrogel are highly dependent on its structure (Li et al., 2020). The hydrogel can be obtained by several mechanisms that are explained henceforth and which contribute towards their applicability in food and agricultural systems. (Chouhan & Mandal, 2021)
Hydrogels are made from a variety of materials. Monomers that are hydrophilic or hydrophobic are occasionally utilized in the manufacturing of hydrogels to control the characteristics and are tailored to specific applications that increase the versatility of hydrogel application (Jose et al., 2020). The application of the polymer is also extremely common for the designing and manufacturing of hydrogels. The materials which are used for the development of hydrogel are roughly classified into two types based on their origin: natural and synthetic polymers (Vedadghavami et al., 2017). Natural polymers used in the manufacture of hydrogels include hyaluronic acid, chitosan, heparin, alginate and fibrin. Several hydrogels are produced by the combination of natural and synthetic polymers to attain desirable properties (Y. Li et al., 2012). The processing of such modified hydrogel can be done by solubilizing both natural and synthetic linear polymers. The cross-linking energy is provided with the application of several techniques, such as ionizing radiation (Chmielewski et al., 2005). The free-radical formation enables the chain combination between and within natural and synthetic biopolymer and causes the cross linking of the same (Muir & Burdick, 2020). Furthermore, the physical interactions such as entanglement, electrostatics and crystallite formation are observed. This cross linking recombines the molecules and forms hydrogels (Lu et al., 2018). Depending on the hydrophilicity conferred on the hydrogel by some hydrophilic residues of the polymer(s), as well as the nature and density of the network joints, such network structures can absorb up to several thousand times their dry weight in the swollen state to form chemically stable or biodegradable gels (Fernandes et al., 2013). Further section is dedicated towards elaboration of different techniques that are used for the development of hydrogel.
Bulk Polymerization
Most hydrogel formulations contain a small amount of cross-linking agent. Chemical cross linkage method uses covalent bonding to produce permanent hydrogel between polymer chains (Maitra & Shukla, 2014). The polymerization reaction is initiated by radiation, UV light or chemical catalyst. Polymerized hydrogels are made up into a variety of forms such as rods, particles, films and membranes and emulsions (Ahmed, 2015). Also, bulk hydrogels can be formed from one or more kinds of monomers, like vinyl monomers.
Free Radical Polymerization
The primary monomers required to produce hydrogels are acrylates, vinyl lactams and amides. The polymers include polymerization friendly functional groups or are functionalized with radically polymerizable groups (Kaczmarek et al., 2020). It also employs the chemistry of conventional free radical polymerizations, including propagation, chain transfer, initiation and termination phases (Moad et al., 2000). Thermal, visible and redox initiators can be employed to generate radicals. Subsequently, the radicals react with monomers, ending up transforming them into active forms. (Truong et al., 2021)
Solution Polymerization/Cross-Linking
The primary advantage of solution polymerization over bulk polymerization is the use of solvent as a heat sink (Kiatkamjornwong, 2007). Solvents like water-ethanol mixtures, ethanol and benzyl alcohol are used. Hydrogels are rinsed with distilled water to remove initiator, soluble monomers, oligomers, cross linking agent, extractable polymer and other impurities (Chmielewski et al., 2005). They can be used in agriculture for controlled fertilizer distribution to enhance soil efficiency and decrease soil water loss which increases the availability for plant feeding even in dry soils and also water efficiency which ultimately improves crop attributes while posing no environmental concern (Kirchmann et al., 2017). Highly absorbent hydrogels can help with agricultural issues, including irrigation, erosion and water run-off. Their high absorbency can also help with soil aeration and microbial activity. (Neethu et al., 2018)
Cryogels
Cryogels are referred to as supermacroporous gel that are developed by the cryogelation of opposite monomers at subzero temperature (Nayak & Das, 2018). The cryogel has a high porosity, sufficient osmotic stability and superior mechanical strength. These cryogels are heterophase, non-transparent porous materials. Moreover, they can hold a large amount of liquid. Cryogels are also known to have higher elasticity giving it a sponge like morphology that can withstand large deformation (Dainiak et al., 2007). The response time of cryogels is faster than that of ordinary gels of the same chemical composition, owing to fast mass and heat transfer through short distances in the thin walls of the cryogel macro-porous structure in comparison with long distances in the ordinary gels. These properties enable its application in various biomedical and food applications. The simplest types of cryogels are made from natural and synthetic polymers. Natural polymers mainly include the polymers that are derived from plants or animals, namely polysaccharides, proteins and polyesters. To name a few, Polyethylene glycol 15 (PEG), Poly-(l-lactic acid) (PLLA) and Poly (vinyl alcohol) (PVA) are synthetic polymers that are widely used for the fabrication of cryogels. These synthetic polymers are mainly used, owing to their ability to degrade in biological environments.
Like composite hydrogels, composite cryogels are made up of a mixture of one or several types of polymers, which are either of natural or synthetic origin. The chemical cross-linking of monomer precursors is one of the most extensively-used pathways for cryogel fabrication. Among the different methods available for chemical cross-linking, free radical polymerization is a widely used methodology. Similar to the common irradiation process, free radical polymerization is carried out by way of a three-step process: initiation, propagation and termination. The initiation step includes formation of free radicals, due to the presence of an initiator. Different types of natural and synthetic polymers, such as hyaluronic acid, gelatin, alginate and PEG — can be functionalized with vinylic groups, in order to cross-link through a free radical polymerization pathway. Ammonium persulfate (APS) is the most common initiator system used for free radical polymerization of cryogels. Photopolymerization is another type of free radical polymerization, in which a photo-initiator decomposes when subjected to UV light in order to produce free radicals for cross-linking. This is the most common cross-linking method for generating nanoporous hydrogels. Cryogelation is the only fabrication method through which a highly interconnected 3D macroporous matrix can be obtained by cryoactivation of the gel. The process of crosslinking is commonly followed by removal of extra solvent through the application of cryogenic drying, most commonly freeze drying. In addition, cryogelation is the easiest and least time consuming technique (Carvalho et al., 2014). Cryogels have recently been used as adsorbents for the efficient separation and purification of biomacromolecules, such as proteins, DNA, cell organelles, viruses and enzymes that are present in solutions. Cryogels are also used as chromatographic supports.
Xerogel
A Xerogel is obtained when the liquid phase of a gel is removed by evaporation, which is usually understood as gel without water. It is a solid formed from a gel that is obtained by drying with no control or hinderance in shrinkage. Xerogels usually retain high porosity (15–50%) and enormous surface area (150–900 m2/g), along with very little pore size (1-10 nm) (Nayak & Das, 2018). This makes it suitable for its application as an absorbent. Considering the materials of construction, silica is the most commonly used natural material that is used for the development of xerogel, followed by several other biomolecules namely pectin, cellulose, chitosan, etc. Examples of xerogels include silica gel and dried out, compact macromolecular structures such as gelatin or rubber. Silicon dioxide is synthetic amorphous silica which may be additionally referenced as silica, silica gel, silica xerogel or silica aerogel. Silica is the inorganic material considered to be Generally Recognized as Safe (GRAS) under FDA regulations and meets their requirements for direct and indirect contact with food and drugs (Quintanar-Guerrero et al., 2009). Amorphous silica particles (in contrast to crystalline silica) are not toxic and are regularly used as food additives and components of vitamin supplements as colloidal suspensions.
The initial process usually involves mixing of the gelling agent with solvent. Further, the mixture is jellified by different methods such as acid- and/or base- catalyzed hydrolysis and by condensation of alkoxysilanes, such as tetramethyl orthosilicate and tetraethyl orthosilicate. The manufacturing process usually involves application of conventional drying to the hydrogel like compound. Due to application of heat, change in vapour pressure with surrounding occurs, leading to the removal of moisture or solvent from hydrogel like structure and converting to a xerogel. Heat treatment of a xerogel at elevated temperature produces viscous sintering (shrinkage of the xerogel due to a small amount of viscous flow) and effectively transforms the porous gel into a dense glass like structure. Xerogel may retain its original shape, but often cracks due to the extreme shrinkage. The applications of the xerogel are mainly dependent on the type of material used for development along with its physico-chemical properties. Pectin-based xerogel beads possess good stability and can be utilized for the biosorption and removal of heavy metals such as copper, lead and cadmium from aqueous solutions. The pectin xerogels have a compact structure and higher stability than the pectin hydrogels (Ishwarya S & Nisha, 2022). Silica xerogel finds its application in drug delivery systems. This activity involves the application of carbon xerogels in the removal of two emerging contaminants, being caffeine and diclofenac from aqueous solutions.
Aerogel
Hydrogel is a system with at least two components (solid and water) that no longer flows and can support its own weight exhibiting solid-like behaviour. Aerogels are highly porous, nanostructured sol-gel derived materials which can find applications as low-calorie ingredients, able to tune nutrient release and modulate satiety (Stergar & Maver, 2016). Their porosity extends from about 1 to 100 nm, which is the reason why properly made aerogels are highly transparent. They have a sponge like, open-pore structure with a large inner surface (Fricke & Tillotson, 1997). Aerogels combine the properties of being highly divided solids with their metastable character (Mavelil-Sam et al., 2018). One of the most interesting properties of them includes their extraordinary flexibility of sol-gel processing, coupled with original drying techniques (Pierre & Pajonk, 2002). These structures result in unique thermal and sound insulation properties and high loading capacities that are being exploited in many industrial fields (aerospace, buildings, petrochemical) and are under research in the recent years for environmental and biomedical applications. Aerogels can accommodate several components begetting a full range of functionalized derivatives. Graphene-based aerogels, silica-based aerogels and zeolite-based aerogels are the most common examples of aerogels. Aerogels may be produced through liquid (water) extraction, keeping original solid structure using super critical drying conditions. (Dhua et al., 2022)
The critical step in aerogel production is drying of wet gels (hydrogels), which is done by various methods such as supercritical drying, ambient drying and freeze drying. A solvent exchange may be needed depending on the solvent used for gel preparation and supercritical drying, as the solubility of water in supercritical CO2 is low, but the solubility of ethanol or acetone is more. Ambient dried aerogels can be dried at room temperature and pressure which allows many materials, such as plastics and certain metal alloys that cannot survive supercritical conditions, to be directly immersed in liquid aerogel precursor and then encapsulated in the final, dried aerogel. Freeze drying of hydrogels gives superior gels as compared to ambient drying. However, drying of gels with the higher volume of solvent than that of the solid form results in solvent expansion upon freezing causing severe porous damages that lead to the formation of foams with large macropores, microchannels, cracks and loss of mesoporosity after sublimation. Despite these limitations, certain biopolymer-based aerogels, typically nanofibrous cellulose aerogels, chitin aerogels, may also be obtained by atmospheric drying or freeze-drying. (Takeshita et al., 2019)
It has applications in low-calorie ingredients which are able to tune nutrient delivery and changes the satiety. In addition, because of the large surface area and open pore structure, aerogels can accommodate several components, begetting a full range of functionalized derivatives. Aerogels can be used to protect and deliver target molecules, potentially triggered by adverse environmental conditions or undesired tastes and odours. Based on their capacity to entrap large amount of unsaturated lipids, aerogels can also be regarded as promising sources for the preparation of fat substitutes with health protecting capacity. They are also considered for use in laser experiments, sensors, thermal insulation, waste management, apart from being used for molten metals, optics and light guides, electronic devices, capacitors, imaging devices, catalysts, pesticides and cosmic dust collection. (Manzocco et al., 2021)
Though, numerous applications are being studied, aerogels as nanostructured materials should be assessed regarding food safety, mainly with respect to the overproduction of reactive oxygen species, inducing cell oxidative stresses. (Pathakoti et al., 2017)
Conclusion
Gel formation is basically a transformation of sol to the state of gel during which the viscoelasticity changes abruptly with simultaneous development of solid characteristics. The gelation process depends on the interaction of two or more polymer molecules forming gel (Banerjee & Bhattacharya, 2012). They are viscoelastic materials and their stability is of utmost importance for commercial use. A large number of factors such as pH, time, temperature, etc. can affect the process of gel formation and its texture being the most critical factor for consumer acceptance. Gels are used to thicken and stabilize various foods like jellies, desserts and candies. Various types of gels hold different applications, such as hydrogels possessing remarkable characteristics and biocompatibility that makes them a powerful candidate to use in biological and environmental applications as implants or materials for removal of toxic pollutants (Fitrawan et al., 2021). Cryogels are produced at the subzero temperature and have demonstrated an efficiency for processing cell and virus suspensions, cell separation and cell culture applications. They have unique properties such as microporosity, tissue like elasticity and biocompatibility, physical and chemical stability. The extraordinary flexibility of the sol-gel processing is combined with drying techniques for the production of aerogels and xerogels (Boersma et al., 2002). Xerogels and aerogels are described as dried gels which retain their porous texture after drying (Pierre & Pajonk, 2002). New types of gelled products like multi-component or mixed gels, aerated gels and emulsion gels are the new directions that are expected to have a good future soon. The application of these findings lies in developing restructured foods and new types of foods having adequate mechanical integrity, long shelf-life, nutritional status and desirable consumer acceptability. (Banerjee & Bhattacharya, 2012)
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