Fabrication and application of starch-based aerogel: Technical strategies

Highlights

• The type and concentration of starch are critical to the performance of starch-based aerogels.

• Gelling conditions of hydrogels are important to control the pore structure of aerogels.

• Supercritical drying is widely used for preserving the internal structure of starch-based aerogels.

• Starch-based aerogels have been extensively explored as food ingredients.

Abstract

Background

Studies of starch-based aerogels have attracted widespread attention over the last decade, motivated by their environmental friendliness, biodegradability and unique properties. The diversity sources and concentrations of natural starch, the difference in regulation of processing parameters, the employment of gelling improvers or not makes it kind of confusion in fabricating starch-based aerogels for specific uses.

Scope and approach

This review summarized the fabrication routes of starch-based aerogel, evaluated the internal and external factors modulating the structure and properties, and also described the application progress of starch-based aerogel, mainly focused on food industry. Technical strategies are given for above topics.

Key findings and conclusions

There are two main fabrication routes of starch-based aerogels based on their shapes: one is for monolith aerogel, another is for microsphere. The parameters including specific surface area, density, pore size, total pore volume, and porosity should be optimized by changing the sources and concentrations of natural starch, the conditions of starch-based hydrogel formation, the methods of solvent removal, and the employment of gelling improvers for different performance requirements. Applications of starch-based aerogels have been extensively explored in food ingredients delivery, food packaging, and thermal isolation. The investigation to date on starch-based aerogel is driven by laboratory-scale fundamental researches, requiring a firmer theoretical foundation and pilot research to narrow the gap between basic research and realistic applications.

Introduction

Aerogels, a kind of highly coherent porous solid materials with low densities and high specific surface areas (Zanini et al., 2016; Zhang, Zhai, & Turng, 2017; Zhang, Feng, Feng, & Jiang, 2017), were firstly prepared by replacing the liquid of the jellies with gas via increasing the temperature and applying pressure beyond its critical point by Steven Kistler in 1931 (Kistler, 1931). In this way, direct evaporation of the liquid was avoided, and thus the connected structure was preserved from shrinkage. Since Kistler synthesized a series of aerogels such as silica, stannic oxide and cellulose (Kistler, 1931), the fabrication and characterization of aerogel have aroused great interest among scholars. Nevertheless, the definition of aerogel is still equivocal because of its diverse physiochemical properties, which are highly dependent on the fabrication routes, especially the drying procedure (Ganesan et al., 2018). According to a latest review of aerogel, any sol-gel derived material of low-density and predominantly mesopores (pore diameter between 2 nm and 50 nm) could be considered as an aerogel (Zhao, Malfait, Alburquerque, Koebel, & Nystrom, 2018). To our knowledge, aerogels could be extended to any kind of highly porous solid materials with low density, large inner surface area and high porosity, prepared by substituting the liquid in the three-dimensional networks with gas. Porous materials produced by air-drying commonly called xerogels (Quignard, Valentin, & Di Renzo, 2008; Ubeyitogullari & Ciftci, 2016a), or prepared through freeze-drying (Obaidat, Tashtoush, Bayan, Al Bustami, & Alnaief, 2015) named cryogels can also be included in the scope of aerogels. At present, aerogels are widely used as thermal insulation materials (Chen, Shen, Chen, Zhao, & Schiraldi, 2016; Chen, Wang, Sánchez Soto, & Schiraldi, 2012; Shang et al., 2017; Shang, Lyu, & Han, 2019; Yang et al., 2017), CO₂ or organic dye adsorbents (Chen et al., 2017; Kutty et al., 2018; Li, Wan, Lu, & Sun, 2014; Maatar & Boufi, 2016; Wang, Motuzas, et al., 2018; Wang, Li, Li, Zheng, & Du, 2019), oil-water separation materials (Cao et al., 2017; Li et al., 2017a, Li et al., 2017b; Meng et al., 2017; Xu, Zhou, Jiang, Li, & Huang, 2017), air filtration materials (Wang, Chen, Kuang, Xiao, Su, & Jiang, 2018; Zeng et al., 2019), catalysis (Keshipour & Khezerloo, 2017; Su et al., 2016; Wang, Wang, Chen, Cai, & Zhang, 2017), and delivery system for food and drug (Bhandari et al., 2017; De Oliveira et al., 2019; Franco, Aliakbarian, Perego, Reverchon, & De Marco, 2018; García-González, Alnaief, & Smirnova, 2011; Kleemann, Selmer, Smirnova, & Kulozik, 2018; Ulker & Erkey, 2014).

Aerogels can be classified into inorganic aerogels and organic aerogels according to the source of gel precursors. Among inorganic materials being widely used for preparing aerogels are titania, alumina, zirconia, clay, and other oxides (Abramian & El-Rassy, 2009; Bandi, Bell, & Schiraldi, 2005; Gao et al., 2018; He, Li, Su, Ji, & Wang, 2016; Long et al., 2018; Madyan, Fan, & Huang, 2017; Santanu, Trikalitis, Chupas, Armatas, & Kanatzidis, 2007; Shi et al., 2018; Wu, Shao, Shen, Cui, & Wang, 2016; Xie et al., 2017). Silica aerogel, a typical inorganic aerogel with extremely low thermal conductivity (as low as 0.012 W/m·K) (Koebel, Rigacci, & Achard, 2012), high specific surface area (600 m²/g-1500 m²/g) (Gurav, Jung, Park, Kang, & Nadargi, 2010; Xu, Ren, et al., 2017), and low density (0.003 g/cm³-0.5 g/cm³) (Husing & Schubert, 1998; Mikkonen, Parikka, Ghafar, & Tenkanen, 2013; Randall, Meador, & Jana, 2011; Tkalec, Knez, & Novak, 2015b), is widely studied and applied. Inorganic aerogels, silicon aerogels for example, are chemically inert and harmless to humans for its biocompatibility (Smirnova, Mamic, & Arlt, 2003). However, such materials are not biodegradable (De Marco & Reverchon, 2017). Due to their intrinsic fragility, inorganic aerogels are probably cracked into fragile monolith or powder during drying, thus being limited in applications requiring high toughness and strength (Tkalec et al., 2015b). In the search of green materials for aerogel fabrication, biopolymers have been thought to be the next promising precursors (Antonyuk, Heinrich, Gurikov, Raman, & Smirnova, 2015; Wang, Sánchez-Soto, Abt, Maspoch, & Santana, 2016). The mechanical toughness and biodegradability of bio-aerogels are of the most remarkable advantages (Goimil et al., 2017; Zhang, Feng, et al., 2017). At present, bio-aerogels researches are commonly focused on biopolymers like polysaccharides (Comin, Temelli, & Saldaña, 2012; García-González et al., 2011; Kargarzadeh et al., 2018; Zhao et al., 2018) and proteins (Ahmadi, Madadlou, & Saboury, 2016; Chen, Wang, & Schiraldi, 2013; Kleemann et al., 2018; Selmer, Kleemann, Kulozik, Heinrich, & Smirnova, 2015). The polysaccharide aerogels based on agar, nitrocellulose or cellulose were prepared previously (Kistler, 1932), followed by many other polysaccharides being successfully fabricated into aerogels with superior properties, including but not limited to cellulose (Liao et al., 2016; Wan, Zhang, Yu, & Zhang, 2017; Yang et al., 2017), starch (Abhari, Madadlou, & Dini, 2017; De Marco, Iannone, Miranda, & Riemma, 2017; De Marco, Riemma, & Iannone, 2019; Peng et al., 2018), chitosan (Cao et al., 2017; Dong, Liu, Ma, & Liang, 2016; Quignard et al., 2008), pectin (Tkalec, Knez, & Novak, 2015a; Tkalec et al., 2015b; Veronovski, Tkalec, Knez, & Novak, 2014), alginate (Chen et al., 2012; Quignard et al., 2008; Tkalec et al., 2015b), xanthan gum (Horvat et al., 2017; Tkalec et al., 2015b), agar (Chen et al., 2017) and κ-carrageenan (Obaidat, Alnaief, & Mashaqbeh, 2018; Quignard et al., 2008). The unique biodegradability, biocompatibility, sustainability and renewability of polysaccharide aerogels at comparatively low cost make them ideal for medical, pharmaceutical and food applications (Frindy et al., 2017; García-González et al., 2011; Goimil et al., 2017; Tkalec et al., 2015b, 2015a; Wang, Shou, Lv, Kong, Deng, & Shen, 2017; Wang, Chen, et al., 2018; Wang, Wu, et al., 2018).

Starch, a food-grade biodegradable gelling agent of low cost, could form an integrated gel network structure in the absence of cross-linkers (Mikkonen et al., 2013; Ubeyitogullari & Ciftci, 2017). Chemically, starch is a homopolysaccharide composed of glucose, which can be divided into two types: amylose and amylopectin. Amylose is essentially a linear starch molecule consisting of (1 → 4)-linked α-d-glucopyranosyl units. Amylopectin is a highly-branched starch molecule of α-d-glucopyranosyl units primarily linked by (1 → 4) bonds with branches resulting from (1 → 6) linkages. Amylose tends to provide amorphous lamellae for starch granules, while amylopectin builds ordered crystalline lamellae (Jenkins & Donald, 1995). Starch-based aerogel, previously named as starch-based microcellular foam, had displayed low density (0.10 g/cm³-0.24 g/cm³) and low thermal conductivity (0.024 W/m·K-0.043 W/m·K) (Glenn & Irving, 1995). The variety of starch raw materials was then expanded to corn, potato, tapioca, pea and wheat, and different drying methods were adopted to obtain high-performance starch-based aerogels with the density as low as 0.05 g/cm³, the specific surface area reaching 480 m²/g and the porosity up to 95%.

Recently, the progress of starch-based aerogel researches has been reviewed, focusing on their production properties, applications and comparison with other similar porous materials (Zhu, 2019). In this review, the fabrication of starch-based aerogel, the factors affecting its final performance and the application status are discussed in detail. The technical strategies are delicately provided for directing further exploration and application of starch-based aerogel.