Aerogels are defined as a special type of nanostructured material endowed with special physical features and obtained from a gel by removing the pore fluid (García-González et al., 2019). Solid, low bulk density and open porosity (mainly in the mesoporous range) stand out as specific physical properties that the material should fulfill to fit in the consensual definition of aerogel. Aerogel networks are formed by bonded particles or nanometric fibers that are loosely packed leading to high porosities (typically in the 95–99.99 % range) and very high specific surface areas (150 m²/g and above) (Fig. 1). 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 (www.cost-aerogels.eu), including functional food and packaging (Lehtonen et al., 2020; Plazzotta, Calligaris, & Manzocco, 2018; Plazzotta, Calligaris, & Manzocco, 2018; Plazzotta, Calligaris, & Manzocco, 2020; Selmer et al., 2019).

 

 

Fig. 1. SEM images of the microstructures of (a) alginate and (b) pectin aerogels obtained by supercritical drying, (c) cellulose aerogels by freeze-drying, and (d) silica-cellulose composite aerogels by ambient drying.

 

Adapted from https://doi.org/10.1016/j.cej.2018.09.159, https://doi.org/10.1016/j.carbpol.2018.05.026, https://doi.org/10.1021/acsami.5b05841, https://doi.org/10.1007/s10853-016-0514-3 with permissions.

 

 

Aerogels are obtained by the extraction of the solvent of wet gels (hydrogels or organogels) using a method that allows the preservation of the solid network structure, typically supercritical carbon dioxide (scCO₂)-assisted drying being the gold-standard (Fig. 1a,b). Fig. 2 shows a schematization of the possible strategies to turn hydrogels into aerogels. Prior to supercritical drying, a solvent exchange may be needed depending on the gel solvent, as the solubility of water in supercritical CO₂ is low but the solubility of ethanol or acetone is high (Şahin, Özbakır, İnönü, Ulker, & Erkey, 2017). Other drying techniques, such as ambient/oven drying or freeze drying, can also lead to aerogel structures in very specific cases (Fig. 1c,d). For ambient/oven drying, surface functionalization (mainly with silanes) or flexible gels followed by the sequential use of solvents with low surface tension (hexane, pentane, ethanol, acetone) may be needed to avoid the shrinkage or cracking of the dry gel due to capillary forces (Budtova, 2019). For freeze drying of hydrogels, the higher volume of water in the solid than in the liquid form results in solvent expansion upon solidification with severe porous damages usually leading to the formation of foams with large macropores, microchannels, cracks and loss of mesoporosity after sublimation (Fig. 3b) (Baudron, Gurikov, Smirnova, & Whitehouse, 2019; Rodríguez-Dorado et al., 2019). Despite these limitations, certain biopolymer-based aerogels, typically nanofibrous cellulose aerogels but also chitin aerogels, may be also obtained by atmospheric drying or freeze-drying (Budtova, 2019; Gao, Lu, Xiao, & Li, 2017; Jiménez-Saelices, Seantier, Cathala, & Grohens, 2017; Li et al., 2019; Nemoto, Saito, & Isogai, 2015).

 

 

Fig. 2. Flow sheet (continuous line) and addition of materials (dashed line) to obtain aerogels and functionalized aerogels. Steps at which target molecule is impregnated or oil is absorbed are also shown.

 

 

Fig. 3. Aerogel engineering strategies: (a) Shape preservation; visual appearance of starch, and pectin hydrogels and their corresponding aerogels. (b) Effect of drying technique; optical appearance of a whey protein hydrogel prepared at pH 7 and the corresponding alcogels, aerogels and cryogels (above). The microstructure of the whey protein aerogels (bottom left) and cryogels (bottom right) obtained by supercritical drying and freeze-drying differed significantly in morphology and textural properties (S_a=specific surface area determined from N₂ adsorption-desorption analysis). (c) Customized morphology; aerogels monoliths with personalized shape, aerogel particles from the millimetric to the micrometric sizes and coated aerogels.

Adapted from https://doi.org/10.1016/j.supflu.2013.03.001, https://doi.org/10.1016/j.carbpol.2011.06.066, https://doi.org/10.1016/j.supflu.2012.08.019, https://doi.org/10.1038/s41586-020-2594-0, https://doi.org/10.1016/j.cej.2018.09.159, https://doi.org/10.1016/j.powtec.2015.

 

 

A limitless customization of the format, shape, size and texture of the aerogels is possible and is mainly carried out during the gelation step using different molds and gelation conditions (gel source concentration, cross-linker concentration, etc.) (Fig. 1, Fig. 3) (García-Gonzalez, Alnaief, & Smirnova, 2011). For the specific case of aerogel particles, the use of molds is usually replaced by technological combinations of the sol-gel processing with powder technologies (e.g., emulsion-gelation, spraying, prilling, inkjet printing, jet-cutting) or by postprocessing (grinding) (Auriemma et al., 2020; Ganesan et al., 2018). The size and texture of the aerogels can be also partially modulated during the solvent exchange step (solvent choice, direct or sequential procedure) or by post-processing (aerogel compression) (García-Gonzalez et al., 2011; Plappert, Nedelec, Rennhofer, Lichtenegger, & Liebner, 2017). Finally, aerogels with dual formats like core-shell aerogel particles and coated aerogel particles are also possible by technological combinations (Antonyuk, Heinrich, Gurikov, Subrahmanyam, & Smirnova, 2015; Auriemma et al., 2020; Bugnone, Ronchetti, Manna, & Banchero., 2018; Veronovski, Knez, & Novak, 2013).

 

Most aerogels are inorganic or synthetic polymer-based, being often made of silica, metal oxides or polystyrenes (Du, Zhou, Zhang, & Shen, 2013; Gesser & Goswami, 1989). However, according to different authors (Kistler, 1931; Pierre & Pajonk, 2002, 1932; Zhao, Malfait, Guerrero‐Alburquerque, Koebel, & Nyström, 2018), not only inorganic polymerizing agents but all biopolymers are potential candidates to form aerogels. Namely, the second generation of aerogels eases the penetration of these materials in the food market since it comprises biopolymer-based aerogels, including food-grade polysaccharides and proteins (El-Naggar, Othman, Allam, & Morsy, 2020; García-Gonzalez et al., 2011; Nita, Ghilan, Rusu, Neamtu, & Chiriac, 2020). These new aerogel sources provided new opportunities for food applications due to their compatibility with human diet, absence of adverse health effects and peculiar physical properties. Aerogels can be used as edible delivery systems for nutraceuticals, nutritional supplements, flavors and other additives or as intelligent components for food packaging. Aerogels can be designed to serve as hosts or carriers of food ingredients and can increase the stability of the loaded ingredient, mask its odor and allow for a controlled or pH-triggered release after intake (Betz, García-Gonzalez, Subrahmanyam, Smirnova, & Koluzik, 2012; Del Gaudio et al., 2013; García-González, Jin, Gerth, Alvarez-Lorenzo, & Smirnova, 2015; Tkalec, Knez, & Novak, 2016). Food-grade aerogels can also contribute to an increased shelf life of the product by the encapsulation of labile or sensitive components (De Oliveira et al., 2020; García-González et al., 2021; Miranda-Tavares, Croguennec, Carvalho, & Bouhallab, 2014). The use of natural polymers can be also considered as an economical and environmentally-friendly approach to penetrate the food packaging sector.

 

Finally, aerogels as nanostructured materials should be assessed regarding food safety, mainly with respect to the overproduction of reactive oxygen species inducing cell oxidative stresses (Eleftheriadou, Pyrgiotakis, & Demokritou, 2017; Fu, Xia, Hwang, Ray, & Yu, 2014; Pathakoti et al., 2017). Accordingly, the changes in physicochemical and biological properties of aerogels with respect to the bulk unprocessed counterpart should be critically studied in food systems.

 

In this review article, the current state-of-the-art of aerogel processing for food applications is compiled for the first time. This work will firstly focus on different biopolymer-based aerogel sources obtained from hydrogels, organogels and tissues as the main sources of food-grade aerogels. Then, the evaluation of these aerogels for their potential direct uses as functional ingredients by themselves, as delivery systems and as fat replacers in food products, as well as their indirect food application in packaging are discussed. Finally, current gaps and challenges in aerogel research for food are identified, and future niche markets for food-grade aerogels are prospected.

 

 

 

2. Bio-based materials for aerogel preparation

The possibility of using aerogels as novel carriers has shown great promise and is certainly the most studied application in the food sector. Basically, the loading of the target compound in the aerogel can be performed at any step of its preparation (Fig. 2). Depending on this choice, two main strategies of aerogel functionalisation can be identified: wet and post drying impregnation. Table 2 compares the efficacy of these techniques when applied to impregnate differently prepared aerogels.

 

 

Table 2. Main functionalisation strategies and amount of loaded compound in aerogels of different nature and shape, and prepared according to different drying techniques. Literature references are also reported.

 

n.r. Not reported. M: Monoliths; P: Particles; S: Spheres; FD: freeze-drying; SCD: supercritical drying.

 

 

 

3.2.1. Wet impregnation

 

If target molecule incorporation is performed before drying, the process is generally referred to as wet impregnation (Fig. 2). In this case, the target compound is dissolved in one of the solvents that come into contact with the biopolymer during aerogel preparation (water or ethanol). Alternatively, the target molecule can be vehiculated by the SC-CO₂ flow during supercritical drying of the alcogel following a drying impregnation approach (Comin et al., 2012). For instance, the target molecule could be simply inserted in the aqueous or ethanol solution used for hydrogel and alcogel preparation, respectively. This approach requires the molecule to exert a certain affinity to the selected solvent as well as to be resistant to environmental conditions of polymer gelation (e.g., high temperature, extreme pH and ionic force) and/or subsequent steps (solvent exchange and drying). Wet impregnation can be also performed into the alcogel. In this case, the alcogel is soaked in an ethanol solution containing the target molecule for a specific time. Following, drying is carried out using supercritical CO₂, which extracts ethanol and causes precipitation of the target molecule within the aerogel pores by an antisolvent mechanism (Miguel, Martín, Gamse, & Cocero, 2006). The efficacy of aerogel wet impregnation is strongly dependent on the target molecule-solvent affinity (Table 2). In fact, aerogel impregnation with phenol compounds, which are characterized by a high solubility in ethanol, seems quite effective. By contrast, oil impregnation by mixing with the aqueous phase of the hydrogel is efficacious only if water removal is performed by freeze drying, and thus avoiding possible oil transfer to supercritical CO₂ (Comin et al., 2012; Lehtonen et al., 2020). A higher oil loading can be achieved by performing alcogel drying with a mixture of supercritical CO₂ and oil (Comin et al., 2012). It has been postulated that the presence of oil in the supercritical CO₂ may also assist in the removal of ethanol from the gel pores, although the mechanisms is not clear (Comin et al., 2012).

 

 

3.2.2. Post-drying impregnation

 

In the post-drying impregnation, the active substance is loaded in the dried aerogel. This can be performed by simply immersing the aerogel in the liquid target molecule (Table 2). Such procedure was used for loading aerogels with an anti-fungal volatile (trans-2-hexanal) and can only be applied when the target molecule is a liquid that does not solubilize the aerogel polymer. In most cases, the target molecule to be loaded in the aerogel is generally solubilised in an assisting solvent, which is then allowed to diffuse into the aerogel pores. The subsequent removal of the solvent from the aerogel causes solute precipitation/absorption into the matrix pores. Depending on the characteristic of the target molecule, adequate solvents are selected. For instance, Ahmadi, Madadlou, and Saboury (2016) performed post drying oil impregnation by soaking a starch aerogel in a solution of hexane-oil. Following, hexane was evaporated under hood. Nevertheless, the most efficient and common methodology for post-drying impregnation is currently based on the use of supercritical CO₂ as assisting solvent (Table 2). In this case, a supercritical CO₂ solution saturated with the target molecule is allowed to diffuse into the aerogel pores. Molecule impregnation would result from chemical adsorption onto the aerogel pores as well as by capillary condensation and local precipitation upon depressurization (Gurikov & Smirnova, 2018). The depressurization of supercritical CO₂ is a critical step for impregnation: although fast depressurization is generally associated to higher loadings, slow depressurization allows avoiding the precipitation of the delivered compound onto the surface of the material (Selmer et al., 2019). The latter is certainly undesired when particle agglomeration should be avoided to maintain the typical free-flowing property of dried materials.

 

Post drying loading assisted by supercritical CO₂ has been applied with reference to pharmaceutical compounds (Betz et al., 2012; García-Gonzalez & Smirnova, 2013) as well as food ingredients (Table 2). The solubility of the target compounds in the solvent (i.e. supercritical CO₂) is a critical factor controlling the impregnation efficacy (Table 2) (Viganó et al., 2020).

 

Post drying impregnation of aerogels with non-polar compounds, such as oil, is generally reported to be quite effective, providing loading ratios in the range of 0.2−0.7 g oil/g aerogel (Kleemann et al., 2020; Selmer et al., 2019). By contrast, impregnation of molecules with lower polarity (vitamins or lactulose) seems more critical (Díez-Municio, Montilla, Herrero, Olano, & Ibáñez, 2011; García-González et al., 2015; Ubeyitogullari & Ciftci, 2019). Accordingly, when impregnation involves complex mixtures of molecules with different affinity for SC-CO₂, such as oils, the relative abundance of their components in the entrapped oil can be significantly different from that of the original oil. For instance, fish oil entrapped in protein aerogels presented much higher content in triglycerides and cholesterol, and lower content in free fatty acids than fish oil used for loading (Selmer et al., 2019). Comparing data reported in Table 2, it is interesting to note that high loading efficacy of lipids was observed for impregnation into both protein- and polysaccharide-based aerogels. This suggests that the contribution of physical entrapment into the aerogel pores is probably the most critical factor controlling lipid impregnation. In other words, oil absorption into the pores would be driven by number, dimension, interconnectivity and size distribution of pores rather than by the chemical interaction of oil components with the functional groups available on the aerogel surface.

 

 

3.2.3. Stability and functionality of aerogel delivery systems

 

Despite the abundance of papers dealing with impregnated aerogels, limited information is available about their capacity of modifying stability and functionality of the entrapped components. Available data suggest that aerogels would be able to protect sensitive compounds. For instance, entrapping plant extracts into cellulose aerogels was shown to highly maintain their antioxidant activity (De Oliveira et al., 2020). Aerogel coating seems to be critical to decrease oxygen susceptibility of loaded oil. To this regard, Ahmadi et al. (2016) showed that fish oil entrapped in whey protein aerogels coated with zein presented about 60 % lower peroxide value than oil impregnated without coating.

 

The peculiar physical properties of aerogels are also expected to modify the bioavailability of loaded molecules. To this regard, in vitro bioavailability of phytosterols loaded into starch aerogels resulted significantly higher (35 %) than that of the crude phytosterols (3 %) (Ubeyitogullari, Moreau, Rose, & Ciftci, 2019). The authors also inserted these phytosterol-loaded aerogels into “real” food products, namely granola bars and puddings (Ubeyitogullari & Ciftci, 2019). Introduction of phytosterols in food in the form of aerogel-protected particles made in vitro bioavailability three times higher than when they were added as free ingredients. This effect was attributed to the lower crystallinity level of phytosterols entrapped into the aerogels.

 

Unlike polysaccharide aerogels which easily dissolve in water, those made of proteins are generally more resistant during swelling and digestion. This is due to the fact that proteins undergo substantial denaturation during hydrogel formation. In addition, hydrogel drying further promotes contraction of the protein backbone, leading to the maximization of the interactions among proteins (Tang, Wei, & Guo, 2014). Due to this water insolubility, the release of loaded molecules is generally delayed (Betz et al., 2012). As an example, fish oil loaded in protein aerogels was mainly released during intestinal digestion, whereas only a small amount was released during oral and gastric digestion (Kleemann et al., 2020).

 

 

3.3. Fat replacers

 

According to their open pore structure and large surface area, aerogels quickly uptake large amounts of oil. This capacity is particularly interesting for the preparation of oleogels, which are mainly proposed as fat substitutes to obtain healthier foods with reduced content of saturated/trans fatty acids (Patel & Dewettinck, 2016; Stortz, Zetzl, Barbut, Cattaruzza, & Marangoni, 2012). According to this application, the oil fraction is driven into the pores of aerogel particles by capillary forces and held at the aerogel surface by surface-oil interactions. For this reason, large amounts of oil closely stick onto the aerogel surface both inside the pores and outside the aerogel particle, which loses the typical dry appearance. The presence of oil at the surface of the hydrophilic particles favors particle-particle interactions, due to intense hydrogen bonding in a nonpolar environment (De Vries, Lopez Gomez, Jansen, van der Linden, & Scholten, 2017). This mechanism allows the formation of a strong particle network, where protein particles behave like building blocks able to embed oil within pores as well as to hold it tightly in the interparticle space (Plazzotta et al., 2020). Absorbed oil is generally higher than 2 g per g of aerogel, regardless of the chemical nature of the aerogel (Table 2). In the case of oleogels obtained from aggregation of whey protein aerogels, the oil content exceeded 5 times the weight of the aerogel particles. The obtained material did not lose any oil upon centrifugation and presented the typical plastic behavior of commercial solid fats (Plazzotta et al., 2020).

 

 

4. Applications of aerogels in food packaging

 

Packaging materials have multiple purposes, being the most important to protect the packed product against mechanical stress, gases and vapors, moisture, light, temperature, microbes, and dirt (Robertson, 2010). Packaging materials are selected based on their capacity to provide this protection, taking into account the other functions that a packaging material may perform, including containing, transporting, serving, presenting the product, and providing information to consumers. Packaging materials can be used as primary packaging, i.e. consumer packaging, or secondary packaging which contains a defined number of primary packages. Secondary packaging units can also be gathered in a tertiary packaging for better transport and storage. The concept of packaging materials also includes a variety of components that can be inserted in the primary packaging to provide further information of the product quality and shelf-life (intelligent packaging), or to extend the shelf-life by adsorbing or releasing functional components (active packaging) (Dobrucka & Przekop, 2019). Other important criteria for packaging materials are their price and environmental impacts, including origin of raw materials, sustainability of processing, and recycling routes. Importantly, consumer experience and user friendliness determine the market potential of packaging materials.

 

The most important and unique property of aerogels for food packaging is their porous structure, leading to low weight and high specific surface area. This provides interesting opportunities for mechanical protection, thermal insulation, or active packaging materials capable of adsorbing or releasing specific compounds. Aerogel structures also offer an inspiring basis for designers to develop and construct material shapes (Michaloudis & Dann, 2017), which is essential in packaging design.

 

The mechanical properties of aerogels are determined by their porous morphology (Ghafar et al., 2017). Reinforcing components, such as nanoparticles or fibers can be added to increase the aerogel strength. A practical illustration of the aerogel strength is the weight an aerogel can withstand on it. For example, a chitin-based aerogel square weighing 60 mg and having ∼5.6 cm³ apparent volume withstood an object of 100 g without any shape distortion (Yan et al., 2020). Such strong materials could provide efficient protection to packed food against mechanical stress that could occur during transport or handling. Moisture can alter material properties, especially of materials derived from bio-based polymers. Repeated (minimum five cycles) mechanical milling followed by freezing in liquid nitrogen and thawing partially separated micro- and nanosized fibrils from cellulose fibers, and enabled preparation of dimensionally stable aerogels that retained their shapes and geometric sizes in solutions (Khlebnikov, Postnova, Chen, & Shchipunov, 2020). This is a highly useful property with materials in contact with moist substances, like many foods.

 

A bio-based thermal insulator, such as aerogel, could be a sustainable replacement for expanded polystyrene used widely to pack products that need either cold storage, such as fish, or maintaining the temperature of hot contents, such as ready-made meals or hot beverages (Mikkonen et al., 2013). Pectin-TiO₂ nanocomposite aerogels were proposed for the storage of temperature-sensitive food and prepared via a sol-gel process (Nešić et al., 2018). The thermal conductivity of these aerogels was 0.022–0.025 W/m K, being lower than the thermal conductivity of air (0.024−0.032 W/m K). The thermal conductivity of pectin aerogels followed the aerogel density within a U-shaped curve, where the density depended on preparation conditions, such as cross-linking degree and pH of the solvent (Groult & Budtova, 2018). Pectin aerogels with thermal conductivity as low as 0.0147 ± 0.0002 W/m K were obtained with the optimized preparation method. The conjunction of the thermal insulation and the lightweight is especially attractive for specialized food service conditions like aircraft meal services or crewed spacecraft food, where solutions with reduced fuel consumption is a must.

 

An active aerogel component was developed to be used to extend the shelf-life of fresh fruit and vegetables (Lehtonen et al., 2020). This innovation was based on in situ production and release of a volatile compound (hexanal), which affects plant metabolism by decreasing ethylene production, and prevents growth of spoilage microbes. The concept was tested with blueberries, where less mold growth was observed, and cherry tomatoes, which maintained their firmness longer when packed with the hexanal-releasing active component in comparison with control samples.

 

 

 

Acknowledgments

 

Work carried out in the frame of the COST Action CA18125 “Advanced Engineering and Research of aeroGels for Environment and Life Sciences” (AERoGELS) and funded by the European Commission. We thank Dr. Hanna Koivula (University of Helsinki) for fruitful discussions about packaging regulation.

 

 

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