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Trends in Food Science & Technology
journal homepage: www.elsevier.com/locate/tifs
Fabrication and application of starch-based aerogel: Technical strategies
Qinyue Zhenga, Yuan Tiana, Fayin Yea, Yun Zhoua,∗∗, Guohua Zhaoa,b,∗
a College of Food Science, Southwest University, Chongqing, 400715, People's Republic of Chinab Chongqing Engineering Research Centre for Sweet Potato, Chongqing, 400715, People's Republic of China
A R T I C L E I N F O
Keywords:Starch-based aerogelTechnical strategiesPropertiesSupercritical dryingPorosity
A B S T R A C T
Background: Studies of starch-based aerogels have attracted widespread attention over the last decade, moti-vated by their environmental friendliness, biodegradability and unique properties. The diversity sources andconcentrations of natural starch, the difference in regulation of processing parameters, the employment ofgelling 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 in-ternal and external factors modulating the structure and properties, and also described the application progressof 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 theirshapes: 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 con-centrations 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-basedaerogels have been extensively explored in food ingredients delivery, food packaging, and thermal isolation. Theinvestigation to date on starch-based aerogel is driven by laboratory-scale fundamental researches, requiring afirmer theoretical foundation and pilot research to narrow the gap between basic research and realistic appli-cations.
1. Introduction
Aerogels, a kind of highly coherent porous solid materials with lowdensities and high specific surface areas (Zanini et al., 2016; Zhang,Zhai, & Turng, 2017; Zhang, Feng, Feng, & Jiang, 2017), were firstlyprepared by replacing the liquid of the jellies with gas via increasing thetemperature and applying pressure beyond its critical point by StevenKistler in 1931 (Kistler, 1931). In this way, direct evaporation of theliquid was avoided, and thus the connected structure was preservedfrom shrinkage. Since Kistler synthesized a series of aerogels such assilica, stannic oxide and cellulose (Kistler, 1931), the fabrication andcharacterization of aerogel have aroused great interest among scholars.Nevertheless, the definition of aerogel is still equivocal because of itsdiverse physiochemical properties, which are highly dependent on thefabrication routes, especially the drying procedure (Ganesan et al.,2018). According to a latest review of aerogel, any sol-gel derivedmaterial of low-density and predominantly mesopores (pore diameterbetween 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 materialswith low density, large inner surface area and high porosity, preparedby 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, AlBustami, & Alnaief, 2015) named cryogels can also be included in thescope of aerogels. At present, aerogels are widely used as thermal in-sulation 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 (Chenet 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., 2017;Meng et al., 2017; Xu, Zhou, Jiang, Li, & Huang, 2017), air filtrationmaterials (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 anddrug (Bhandari et al., 2017; De Oliveira et al., 2019; Franco,
https://doi.org/10.1016/j.tifs.2020.03.038Received 6 July 2019; Received in revised form 24 January 2020; Accepted 24 March 2020
∗ Corresponding author.College of Food Science, Southwest University, Chongqing, 400715, People's Republic of China.∗∗ Corresponding author.E-mail addresses: [email protected] (Y. Zhou), [email protected] (G. Zhao).
Trends in Food Science & Technology 99 (2020) 608–620
Available online 30 March 20200924-2244/ © 2020 Elsevier Ltd. All rights reserved.
T
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 organicaerogels according to the source of gel precursors. Among inorganicmaterials 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 inorganicaerogel 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), iswidely studied and applied. Inorganic aerogels, silicon aerogels forexample, are chemically inert and harmless to humans for its bio-compatibility (Smirnova, Mamic, & Arlt, 2003). However, such mate-rials are not biodegradable (De Marco & Reverchon, 2017). Due to theirintrinsic fragility, inorganic aerogels are probably cracked into fragilemonolith or powder during drying, thus being limited in applicationsrequiring high toughness and strength (Tkalec et al., 2015b). In thesearch of green materials for aerogel fabrication, biopolymers havebeen thought to be the next promising precursors (Antonyuk, Heinrich,Gurikov, Raman, & Smirnova, 2015; Wang, Sánchez-Soto, Abt,Maspoch, & Santana, 2016). The mechanical toughness and biode-gradability of bio-aerogels are of the most remarkable advantages(Goimil et al., 2017; Zhang, Feng, et al., 2017). At present, bio-aerogelsresearches 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; Kleemannet al., 2018; Selmer, Kleemann, Kulozik, Heinrich, & Smirnova, 2015).The polysaccharide aerogels based on agar, nitrocellulose or cellulosewere prepared previously (Kistler, 1932), followed by many otherpolysaccharides being successfully fabricated into aerogels with su-perior 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; Tkalecet al., 2015b; Veronovski, Tkalec, Knez, & Novak, 2014), alginate (Chenet 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) andκ-carrageenan (Obaidat, Alnaief, & Mashaqbeh, 2018; Quignard et al.,2008). The unique biodegradability, biocompatibility, sustainabilityand renewability of polysaccharide aerogels at comparatively low costmake 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, couldform 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 bedivided into two types: amylose and amylopectin. Amylose is essen-tially a linear starch molecule consisting of (1 → 4)-linked α-D-gluco-pyranosyl units. Amylopectin is a highly-branched starch molecule of α-D-glucopyranosyl units primarily linked by (1→ 4) bonds with branchesresulting from (1 → 6) linkages. Amylose tends to provide amorphouslamellae for starch granules, while amylopectin builds ordered crys-talline lamellae (Jenkins & Donald, 1995). Starch-based aerogel, pre-viously named as starch-based microcellular foam, had displayed lowdensity (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 ofstarch raw materials was then expanded to corn, potato, tapioca, peaand 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 to95%.
Recently, the progress of starch-based aerogel researches has beenreviewed, focusing on their production properties, applications andcomparison with other similar porous materials (Zhu, 2019). In thisreview, the fabrication of starch-based aerogel, the factors affecting itsfinal performance and the application status are discussed in detail. Thetechnical strategies are delicately provided for directing further ex-ploration and application of starch-based aerogel.
2. Fabrication of starch-based aerogel
2.1. Fabrication process
Starch-based aerogels can be prepared into various shapes and sizes,such as monolith, film and microsphere, ranging from nanoscale tomicron-sized. To obtain monolith (Fig. 1a), the gelatinized starch sus-pension is poured into molds, such as multiwell plates (Antonyuk et al.,2015) or cylindrical polypropylene molds and allow to gel and dry(Ubeyitogullari & Ciftci, 2016a). The solvent casting method is oftenused for producing starch-based aerogel films (De Souza, Dias, Sousa, &Tadini, 2014). In this regime, the pre-homogenized film-forming solu-tion is poured onto the plates (usually glass plates) and then subjectedto gelling and drying. After that, the membrane was harvested bypeeling from the plates. It is also feasible to prepare microsphereaerogel by emulsification-based method (Zhu et al., 2018), which isconsidered as a promising carrier for oral administration (Goimil et al.,2017; Lovskaya, Lebedev, & Menshutina, 2015). A water-in-oil emul-sion (w/o) is prepared by using surfactants-containing mixture ofaqueous starch suspension phase and oil phase (Fig. 1b), where starch-based hydrogel microspheres covered with surfactants are formed in
Fig. 1. Fabrication routes of starch-based aerogel: (a) monolith (García-González & Smirnova, 2013); (b) microsphere (Goimil et al., 2017).
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the emulsion system upon heating. Surfactants can be selectively re-moved prior to drying. The size of microspheres can be controlled byregulating the stirring rate. In addition, the starch melt can be pumpedinto coagulation bath by atomizing nozzle to prepare microspheres(Glenn et al., 2010).
In general, the starch-based aerogel can be fabricated by two steps,the starch-based hydrogel formation and drying. Gelation of starch ischaracterized by the extraction of amylose components, irreversiblephysical changes and the destruction of the granular structure (Ismail,Irani, & Ahmad, 2013). Gel structure forms in the process of cooling andaging (usually placed at 4 °C), along with the structural recombinationand partial recrystallization, named retrogradation (Mehling,Smirnova, Guenther, & Neubert, 2009). During the retrogradationprocess, starch could not be completely recovered to its originalstructure, and instead, the molecules chains form gel networks bywinding into double helices (Chang, Chen, & Jiao, 2010). The retro-graded amylose is prerequisite for the crystal nuclei formation, whichcan promote the nuclei crystal growth, so the higher the amylosecontent, the faster the starch retrogrades (Yu, Ma, & Sun, 2009). Afternucleation, propagation and maturation, starch-based hydrogel isformed. Actually, the fabrication of aerogel-oriented starch-based hy-drogel complies with the processing methods of starch-based hydrogelfor other use.
One major challenge for the fabrication of aerogel is to maintain theexisting porous structure of the hydrogel while eliminating the sub-sequent shrinkage during the drying process (García-González et al.,2012a; Husing & Schubert, 1998). Among the drying methods mostwidely adopted in aerogel production are air-drying, freeze-drying andsupercritical drying, of which the processing procedures are indicatedin Fig. 2.
Air-drying is performed at ambient pressure at room temperature orin a constant oven until a constant weight is achieved (Ubeyitogullari,Brahma, Rose, & Ciftci, 2018). Capillary tension generated form directevaporation of solvent during gel drying is expected to be 100 MPa–200MPa (Scherer & Smith, 1995). In this process, a liquid-vapor meniscusformed in the pores would recede when the solvent is emptied withpores contract. Equipment for air-drying could be easily achieved, but ittakes more than 48 h to remove the solvent of the gel (Bakierska et al.,2017; De Souza et al., 2014).
Freeze-drying is another commonly used drying technique, bywhich wet material or solutions should be pre-frozen into a solid stateat −45 °C to −15 °C, before sublimating solvent into a gaseous stateunder vacuum to dehydrate the gel. During pre-freezing process, solutemolecules are unavoidably pushed into the interstices of the ice crys-tals, possibly triggering intermolecular self-assembly of solutes (Wang,Chen et al., 2018). At the sublimation step, these ice crystals are
removed in vacuum, resulting in the formation of aerogel pores (Niet al., 2016; Wang, Chen et al., 2018). There is no liquid-vapor me-niscus existing in this process. Prior to this step, the growth of icecrystals causes a varying degree of structural damage (Quignard et al.,2008). Volume expansion of the solvent upon crystallization inducesstresses in the gel directed from the crust toward inside, resulting inshrinkages and breakage of the crust layers (García-González et al.,2012a; Quignard et al., 2008). Besides, the nucleation and growth rateof ice crystals are not the same as a function of depth, resulting in thenonuniformity of pore structure in the aerogel (García-González, Uy,et al., 2012). Freeze-drying is a time-consuming and energy-intensivemethod for the complete removal of solvent, which takes over 10 h.
Supercritical drying is one of the most widely chosen dryingmethods in aerogels preparation and relies on the application of su-percritical fluids. Fluids with temperature and pressure above the cri-tical point are supercritical fluids. The physical properties of super-critical fluids are like gases and liquids, which are of low viscosity, highdiffusion coefficient like gases and close to the density and solvationcapacity of liquid. The density and solvent strength of supercriticalfluids are adjustable with no gas-liquid interface and surface tension(García-González et al., 2012a; Miao et al., 2008). Supercritical CO2
(scCO2) is the most used supercritical drying fluid for its characteristicssuch as non-toxicity, non-combustibility, low-cost, chemical inertnessand the mild operating conditions required to reach its critical point(7.4 MPa, 31.1 °C) (Santos, Gaudio, Landí, & García-González, 2015;Ubeyitogullari & Ciftci, 2017).
Solvent exchange is necessary prior to drying in this way. Water ishigh in critical point (22.1 MPa, 374.1 °C) and low in solubility inscCO2, which prevents aerogels from being obtained by supercriticaldrying of hydrogels directly (García-González et al., 2012b). More im-portant is that starch will degrade at the drying conditions needed.Therefore, ethanol, a poor solvent for starch, is often chosen in solventexchange, which has a high solubility in scCO2 and miscibility withwater (García-González et al., 2012b). The gel obtained by ethanolexchange is called alcogel. Solvent exchange can be completed by one-step. Starch-based hydrogel is directly transferred to absolute ethanolfor solvent exchange, which is then replaced by fresh ethanol untilremoval of water being completed to obtain the alcogel (García-González et al., 2012b; Goimil et al., 2017). Multi-step water-to-ethanolexchange is performed by immersing starch-based hydrogel in severalsubsequent baths of elevated concentration of ethanol (De Marco &Reverchon, 2017; Ubeyitogullari et al., 2018).
Fig. 3 demonstrates a typical equipment flow chart of dynamicscCO2 drying. The alcogels are placed in the high-pressure vessel.Continuous CO2 outlet flow from the tank is cooled in a cryostat toavoid cavitation before being pumped (De Marco, Iannone, et al., 2017;
Fig. 2. Different drying methods for starch-based aerogels: (a) air-drying; (b) freeze-drying (c) supercritical drying (Abhari, Madadlou, & Dini, 2017; Ubeyitogullari &Ciftci, 2016a).
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De Marco et al., 2019). When the required pressure and temperature arereached, the drying process begins (Franco et al., 2018). After a fewhours (usually less than 2 h), the system is slowly decompressed back toatmospheric pressure (Obaidat et al., 2015). Due to the lower solvationcapacity of gaseous CO2, two-phase splitting occurs in the separator.The gaseous CO2-rich stream is vented to atmosphere, while the sol-vent-rich stream is collected (García-González & Smirnova, 2013).Following the releasing of pressure at constant temperature, scCO2 ingel is converted to gaseous form and subsequently replaced by air whenopening the vessel at ambient conditions (Zhang, Zhai, et al., 2017).The equipment of dynamic drying can also be used for static super-critical drying. Discontinuous scCO2 is in full contact with the starch-based alcogel to saturate the scCO2 and ethanol. The scCO2 that hasalready been enriched with ethanol is replaced with a fresh batch(Milovanovic, Jankovic-Castvan, Ivanovic, & Zizovic, 2015). The dryingprocess may run over several cycles selectively. At the end of the dryingprocess, the vessel is decompressed slowly. The supercritical dryingprocess of aerogel microspheres is faster than the case of cylindricalaerogel because the minimum diffusion path length of the microspheresis much lower than that of the monoliths (García-González & Smirnova,2013).
Although solvent exchange inevitably introduces organic solvent tothe system and forms possible chemical residues in the gel pores(García-González et al., 2012a; García-González & Smirnova, 2013;Ivanovic, Milovanovic, & Zizovic, 2016; Ni et al., 2016), supercriticaldrying is still considered to be optimal for aerogel drying at present. Itsadvantages include but not limited to: (1) extremely low surface tensionand capillary pressure gradient existing during drying to preserve gelstructure as much as possible (García-González et al., 2012a); (2)moderate temperature employed in drying, whereby conformationalchanges and intermolecular interaction are expected to be minimized(García-González et al., 2011; Obaidat et al., 2015).
Apart from the above aerogel drying methods discussed, some otherdrying methods have also been studied recently, such as infrared dryingtechnology (Peng et al., 2018). After gelatinization, the cooled foammaterials were dried in an infrared drying oven and then in a vacuumdry oven, both at 80 °C for 8 h. Eventually, the water content was lessthan 0.5 wt% (Peng et al., 2018). It is worth exploring more other
drying methods in the future for the drying of aerogels.Carbon aerogel with a continuous solid framework and an open pore
structure is a novel class of nanostructured carbon material (Bakierska,Molenda, Majda, & Dziembaj, 2014). Carbon aerogels exhibits extra-ordinary properties, including well-defined and controllable porosity,large surface area, high chemical stability, and low electrical resistance,which make them desirable materials for a wide range of applications atextreme conditions (Bakierska et al., 2017). The initial synthetic routeof carbon aerogel is as the same as that of other starch-based aerogel,but the pyrolysis of aerogel at elevated temperature (from 600 °C to1000 °C) under an inert atmosphere such as argon or nitrogen flow isnecessary to obtain carbon aerogel (Bakierska et al., 2014; Chang et al.,2010). The pyrolysis temperature is critical to control the surface areaof carbon aerogel. It was reported that the surface area of the starch-based carbon aerogel increased from 400 m2/g to about 500 m2/g asthe pyrolysis temperature was increased from 700 °C to 800 °C(Bakierska et al., 2017).
2.2. Key points
Several factors, such as the ratio of amylose/amylopectin, the con-centration of starch suspension, gel forming conditions and solventremoval methods have a significant influence on physical properties ofstarch-based aerogel, shown in Table 1 and Table 2.
2.2.1. Selection of raw materialsThe physicochemical characteristics of starch granules, such as
amylose/amylopectin ratio, granule size and distribution, molecularorder, mineral content and crystallinity, are related with the plantorigin of starch, which determines the hydration, swelling and gelationbehaviors of starch in water (De Marco, Baldino, Cardea, & Reverchon,2015). Amylose fraction of the starch is responsible for the meso-porosity of the aerogels to a large extent (Ubeyitogullari, Moreau, Rose,Zhang, & Ciftci, 2019), based on the fact that higher specific surfacearea is accordingly obtained with a higher amylose content starch(Druel, Bardl, Vorwerg, & Budtova, 2017; García-González, Jin, Gerth,Alvarez-Lorenzo, & Smirnova, 2015; Ubeyitogullari et al., 2019). Lessshrinkage for the amylose-rich hydrogel is evident during retro-gradation process and solvent exchange (Druel et al., 2017; Mehlinget al., 2009). It could be explained as the structural characteristics ofamylose are only slightly changed, whereas the stretched amylopectinis partially degenerated during the retrogradation process (Mehlinget al., 2009). Besides, higher amylose content leads to more developedsupramolecular structure and stronger network. However, monolithicaerogels could not be achieved by pure amylose at the tested condition(Druel et al., 2017). Retrogradation process is highly intensive withpure amylose sample, leading to cracks of networks which do not sus-tain pressure during drying. It is observed that aerogels of extremelyhigh amylose content present inferior thermal insulation properties,and meanwhile aerogels of very large amount of amylopectin is lessevaluated in morphology (Druel et al., 2017).
The concentration of starch suspension is another factor whichshould be taken into account in the fabrication of starch-based aerogel.Starch-based aerogels could not be formed at very low concentration(＜5 wt%). Appropriate increasing the starch concentration contributesto aerogels of superior mechanical strength and less shrinkage (García-González & Smirnova, 2013; Ubeyitogullari & Ciftci, 2016a;Ubeyitogullari et al., 2019). However, increasing starch concentrationto 15 wt% or even higher would increase the density of the aerogel andrestrict the thermal insulation. If the pore wall is thickened with in-creasing starch concentration, the pore size could also be enlarged andthe specific surface area decreased as a result (Druel et al., 2017;Jiamjariyatam, Kongpensook, & Pradipasena, 2015; Miao et al., 2008).
Technical strategies for raw material selection:
i. It is necessary to coordinate the ratio of amylose/amylopectin in
Fig. 3. Process flow diagram of the equipment used for the supercritical dryingof starch-based aerogels (V1–V4 = valves; CS = cryostat; P = pump;HPA = high-level pressure alarm; PG1-PG2 = pressure gauges;TC = thermocouple; HPV = high pressure vessel; S = separator; FM = flowmeter).
Q. Zheng, et al. Trends in Food Science & Technology 99 (2020) 608–620
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Table1
Fabricationan
dch
aracterization
ofpu
restarch
-based
aeroge
l.
Starch
source
Amylose
conten
t(%
)Con
.(wt
%)
Form
ationof
aeroge
l-orien
tedge
lDryingof
starch
-based
gel
Cha
racterizationan
alysis
Referen
ces
Gelatinization
tempe
rature
(°C)
Retrograd
ationtempe
rature
(°C)an
dtime(h)
Drying
metho
dDryingco
ndition
S BET
(m2/g
)Den
sity
(g/
cm3)
Pore
size
(nm)
Pore
volume
(cm
3/g
)Po
rosity
(%)
corn
808
130
4,48
scCO2
drying
37°C,8
MPa
254
0.14
––
–Druel
etal.(20
17)
7010
120
40°C,1
0MPa
220.5
0.16
7.4
0.36
89.4
Ube
yitogu
llari
etal.(20
19)
537
45°C,1
1MPa
233
––
–89
.5García-Gon
zálezan
dSm
irno
va(201
3)10
221
––
–87
.015
234
––
–85
.152
.67.5
87.8
–freeze-
drying
−45
°C,1
0h
–0.09
8–
–93
.46
Yild
irim
etal.(20
14)
1012
14,
48scCO2
drying
40°C,1
2MPa
235
0.18
217
.51.12
87.0
Zamora-Se
queira,Ardao
,Starbird,
andGarcia-Gon
zalez(201
8)15
9540
°C,1
1MPa
66–
–0.37
–García-Gon
zálezet
al.,20
12b
120
112
––
0.32
–12
7<
0.25
––
>85
García-Gon
zálezet
al.(20
15)
45°C,1
1MPa
274
–9.4
0.78
–(G
arcía-Gon
zálezet
al.,20
12a)
121
40°C,1
2MPa
205
–17
0.98
–Sa
ntos
etal.(20
15)
188
–24
1.25
87.7
Goimilet
al.(20
17)
140
40°C,1
1MPa
40–
–0.18
–García-Gon
zálezet
al.,20
12b
–40
°C,1
1MPa
217
–9.4
––
Lovska
yaet
al.(20
15)
–11
04,
7245
°C,2
0MPa
90–
––
–DeMarco
andRev
erch
on(201
7)80
––
––
Fran
coet
al.(20
18)
995
4,48
40°C,9
–12MPa
90.3
0.34
1.9
0.37
78Meh
linget
al.(20
09)
2810
100
–air-drying
ambien
tco
ndition,
10d
5.52
–13
.30.02
38–
Milo
vano
vicet
al.(20
15)
pea
408
130
4,48
scCO2
drying
37°C,8
MPa
221
0.14
––
–Druel
etal.(20
17)
355
120
45°C,1
1MPa
230
––
–92
.2García-Gon
zálezan
dSm
irno
va(201
3)7
225
––
–90
.410
222
––
–88
.215
204
––
–84
.7whe
at25
512
04,
48scCO2
drying
40°C,1
0MPa
53.5
0.05
19.2
0.27
–(U
beyitogu
llari
&Ciftci,20
16a)
1059
.70.11
200.20
–59
.70.12
19.6
0.27
91.9
Ube
yitogu
llari
andCiftci(201
6b)
61.6
0.11
17.8
0.27
92.78
Ube
yitogu
llari
andCiftci(201
7)61
.50.11
19.0
0.27
92.5
Ube
yitogu
llari
etal.(20
19)
60.9
0.11
17.6
0.27
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Q. Zheng, et al. Trends in Food Science & Technology 99 (2020) 608–620
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order to tune the properties and improve the performance of starch-based aerogel. To obtain starch-based aerogels of higher specificsurface area, starch with comparatively high amylose is required.
ii. The concentration of starch suspension or proportion of amylosecould be appropriately increased for starch-based aerogels in re-quirement for good compression resistance.
2.2.2. Regulation of the formation of starch-based hydrogelThe gelatinization temperature exhibits a significant influence on
the surface area and density of starch-based aerogel. Corn starchgranules could not completely burst even being heated for 20 min at95 °C (García-González et al., 2012a). The highest surface area(58.3 m2/g) was obtained at 120 °C since the releasing of amylose waspromoted, but increasing temperature to 130 °C and even higher led toa decrease in the surface area of the starch-based aerogels, which couldbe explained by the swollen starch granules bursting and structurecollapsing (Mehling et al., 2009; Ubeyitogullari & Ciftci, 2016a). Be-sides, gelatinization at higher temperature results in more rigid gelstructures and a denser network (Mehling et al., 2009). The density ofaerogel fell from 0.4 g/cm3 to 0.2 g/cm3 with the gelatinization tem-perature increasing from 100 °C to 140 °C, which could be ascribed tohigher gelatinization degree and the formation of more fibrous starchnetwork (Ubeyitogullari & Ciftci, 2016a).
Physical promoters like pH or temperature could assist in inducinggel formation of starch suspension (García-González et al., 2011). A5.5% dispersion of the high amylose corn starch-sodium palmitate in-clusion complex was loaded into a 50 mL gas tight syringe and deliv-ered dropwise to an unstirred solution of HCl to form hydrogel beadsinstantly (Kenar, Eller, Felker, Jackson, & Fanta, 2014). Moreover, themixture of alginate-starch was diluted with water and treated withpressurized gaseous CO2 at 5 MPa at room temperature in a high-pressure autoclave to prepare hydrogels (Antonyuk et al., 2015; Martinset al., 2015). Besides, scCO2 could be considered as a promisingblowing agent to promote gelation (Peng et al., 2018). By increasing thedepressurization rate from 0.01 MPa/min to 3 MPa/min, the macro-porosity of aerogel was increased significantly from 2% to 25%(Martins et al., 2015).
Temperature and duration time for retrogradation process are an-other important factors in regulating the performance of starch-basedaerogel. Retrogradation at 4 °C favors crystal nucleation rather thancrystal growth (Ubeyitogullari et al., 2019). A large amount of smallcrystals could be achieved at lower temperature, which is favorable toreach a larger surface area of the aerogel (Mehling et al., 2009). Foraerogels with the same density, longer retrogradation time could de-crease the specific surface area and increase the thermal conductivity,most probably due to the increase of crystallinity and thickened porewalls with chain aggregation (Druel et al., 2017). However, it was re-ported previously that an extension of retrogradation time from 2 daysto 6 days caused an increase of the specific surface area from 52 m2/g to83 m2/g under the test gelation conditions (Mehling et al., 2009). It isworth noting that the efficacy of controlling retrogradation time is re-lated to the intrinsic starch parameters such as amylose/amylopectinratio. Probably unlike to amylose molecules exhibiting a pronouncedtendency to retrograde during the cooling process, amylopectin mole-cules are known to recrystallize over an extended period (Mehlinget al., 2009; Milovanovic et al., 2015).
The emulsification-based method is regarded as a practical tech-nology for the preparation of microspheres. The morphology of thestarch-based microspheres can be tuned by controlling the processingconditions and modifying chemical formulation by introducing surfac-tants. Common oil phases such as paraffin oil or vegetable oil could bemixed with starch suspension to form a water-in-oil emulsion (García-González et al., 2015; Goimil et al., 2017). The increase of oil-to-starchaqueous solution ratio lead to a subsequent reduction of the volume ofthe dispersed phase and a decrease in the frequency of collisions andparticle coalescence upon stirring, resulting in the formation of lessTa
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Q. Zheng, et al. Trends in Food Science & Technology 99 (2020) 608–620
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aggregated microspheres (García-González et al., 2012a). Moreover,the viscosity of the emulsion is decreased in this case, favoring thehomogenization of the emulsion. However, no significant effect of theoil-to-starch aqueous solution ratio on the mean particle size is ob-served (García-González et al., 2012a). Stirring rate plays an importantrole in the particle size of starch-based aerogel microspheres ((García-González et al., 2012b); Zhu et al., 2018). The diameter was reducedfrom 821 μm to 519 μm and 399 μm, as the stirring rate of 600 rpmincreased to 1400 rpm and 2000 rpm, respectively (García-González &Smirnova, 2013). The introduction of the surfactants also reduces thesurface tension between the aqueous phase and the oil phase by ad-sorption at the liquid-liquid interface, thereby reducing the droplet sizeof the dispersed phase (García-González et al., 2012a). Surfactant re-moval treatment made the specific surface area (188 m2/g) and BJHpore volume (1.25 cm3/g) of the aerogel doubled (Goimil et al., 2017).
Technical strategies for starch-based hydrogel formation:
i. The gelatinization temperature is critical to density and specificsurface of the starch-based aerogel. Aerogels of low density could beachieved at higher gelatinization temperature above 100 °C. Andaerogel of higher specific surface is suggested to be prepared at120 °C.
ii. The temperature of retrogradation can be controlled at 4 °C forproducing starch-based aerogels with small pores.
iii. Prolonging the retrogradation time could change the morphology ofstarch-based aerogel with pore walls to be thicker and stronger.
iv. To obtain the starch-based aerogel with low thermal conductivity,the retrogradation process should be limited within 48 h.
v. Increasing the stirring rate of the emulsion could reduce the particlesize of aerogel microspheres.
vi. Introducing surfactants increases the stability of the emulsion foraerogel microspheres fabrication. It should be aware that thesesurfactants should be removed prior to drying to increase the spe-cific surface area and minimize the safety risk.
2.2.3. Selection of solvent removal methodsThe air-dried aerogel is revealed to be less porous or even non-
porous (Fig. 2), of which the density is quite close to that of the driedpolysaccharide and the specific surface area is lower than 0.05 m2/g(Quignard et al., 2008; Ubeyitogullari & Ciftci, 2016a). The tempera-ture and duration of air-drying processing could be modulated to alle-viate a certain extent the negative effect of capillary force caused bydirect evaporation of the solvent (García-González et al., 2012a;Quignard et al., 2008; Ubeyitogullari & Ciftci, 2016a). The humidity ofdrying medium could be crucial in determining the performance ofdried starch-based gels (Xiang, Ye, Zhou, Wang, & Zhao, 2018). How-ever, the relationship between the humidity of the drying medium andthe performance of the starch-based aerogel has not been exploitedcomprehensively. Direct evaporation of solvent leads to obvious porecollapse, gel structure shrinkage and crack formation. Macropores ap-pearance and even non-porous materials are expected for the air-driedaerogel, since the structures of the hydrogels are so compliant thatcould be drawn together by capillary tension (Quignard et al., 2008).Therefore, air-drying is often used to produce starch-based aerogel oflow porosity, such as thin packing films (De Souza et al., 2014).
The pre-freezing temperature is critical to the size of ice crystals ofthe frozen gel, which plays a maple role in determining the pore size ofthe freeze-dried aerogel. As pre-freezing temperature is reduced from−15 °C to −40 °C, the size of the ice crystals decreases, and transformsinto uniformly spherical from polygons, and eventually grows irregu-larly and layered along with the rapid heat transfer (Ni et al., 2016).The nucleation rate is enhanced and, in contrast, the ice crystal growthrate is lower than the nucleation rate at lower pre-freezing temperature.This led to the water molecules rapidly form numerous small ice crys-tals (Ni et al., 2016). Temperature and vacuum of drying process are theparameters that can be controlled during the sublimation of ice crystals.
The temperature and vacuum were usually controlled to be −55 °C to−40 °C and 0.1 Pa–1 Pa, respectively. To our knowledge, there arecurrently few studies on the effects of these two parameters on starch-based aerogels.
Another method to remove the solvent is supercritical drying.Solvent exchange is necessary before this drying process. The water inthe gel system is replaced by ethanol, which is more easily soluble inscCO2, with a lower critical point than that of water. Multi-step and lowfrequency water-to-solvent exchange makes a smaller extent ofshrinkage of aerogels possible (Martins et al., 2015; Raman, Gurikov, &Smirnova, 2015). The nanostructured aerogel cannot be obtained whenthe solvent exchange time is not enough for the formation of the alcogel(De Marco et al., 2015). Actually, the solvent exchange is a time-con-suming but fairly vital procedure.
Both temperature and pressure in the process of supercritical dryingaffect the mass transfer related properties and surface area, since thesolubility of ethanol in scCO2 increases with temperature and pressure.However, higher pressure increases the density of scCO2 and ethanolmixture, which affects the diffusion properties negatively(Ubeyitogullari & Ciftci, 2016a). Surface area of 10% wheat starchdecreased from 59.7 m2/g to 53.1 m2/g when the pressure was elevatedform 10 MPa to 15 MPa at 40 °C (Ubeyitogullari & Ciftci, 2016a).
The internal structure of aerogel is highly affected by the flow rateand depressurization rate of CO2. The surface area of starch-basedaerogel decreased from 59.7 m2/g to 51.6 m2/g as the CO2 flow ratewas increased from 0.5 L/min to 1.5 L/min during supercritical drying(Ubeyitogullari & Ciftci, 2016a). Drying the alcogel at higher CO2 flowrate could make the convective mass transfer of ethanol more effective.A higher depressurization rate disrupts the structure of starch-basedaerogel to a larger extent. Rapid depressurization (400 MPa/min)causes the gas to expand before exiting the gel, resulting in compressionof the aerogel and enlargement of the fissures. An opaquer appearanceoccurs compared to a slower depressurization (10 MPa/min) because ofthe incident light scattering caused by the larger fissures and densermatrix (Kenar et al., 2014). The volume of aerogel is inversely pro-portional to depressurization rate, while the density is opposite (Kenaret al., 2014).
Drying time is related to the content of ethanol remnants andstructural water within the aerogel. The overestimation of drying timeis a general practice to ensure little remnant liquid remained in theaerogel (García-González et al., 2012a; García-González & Smirnova,2013). Due to the energy efficient requirements of industrial produc-tion, this practice can only be allowed on a laboratory scale (De Marco,Miranda, Riemma, & Iannone, 2016; De Marco, Riemma, & Iannone,2017; García-González & Smirnova, 2013). Drying for 30 min led to anaerogel of 102 m2/g specific surface areas, which could be increased to274 m2/g if the drying time was extended to 1 h, and 91.8% of ethanolcould be removed in this case. After drying for more than 4 h, thesolvent removal efficiency was higher (up to 99%), but the specificsurface area was significantly reduced (García-González et al., 2012a).Starch-based gel is composed of structured skeleton made of semi-crystalline amylopectin, amorphous amylose, and structural water dis-persed on the scaffold (García-González et al., 2012a). Structural wateris partially removed with the scCO2 flows where ethanol is a co-solvent,even though the solubility of water in scCO2 is comparatively low. Dueto the extraction of structural water, the aerogel skeleton might bedistorted and even collapsed, resulting in a decrease in surface area,total pore volume and pore size (García-González et al., 2012a; García-González & Smirnova, 2013). Structural shrinkage takes place in ret-rogradation, solvent exchange, and supercritical drying process, but itshould be noted that shrinkage rate of supercritical drying is negligiblecompared to other processes (Mehling et al., 2009).
Technical strategies for solvent removal method selection:
i. Air-drying is secondary to other drying methods considering thecommon requirement of highly porous structure for aerogel, but it
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can also be selected if the final product is expected to be chewy andcompression resistant in some specific cases.
ii. For freeze-dried aerogel, it is suggested to limit the pre-freezingtemperature at −30 °C to −40 °C.
iii. Supercritical drying can maintain to a larger extent the porousstructure of the starch-based aerogel compared to other dryingmethods. It is appropriate to adopt a multi-step solvent exchangemethod to replace the water in the gel, and control the temperatureand pressure in the drying process at 40 °C and 11 MPa. The flowrate and depressurization rate of CO2 are controlled at 1 L/min and10 MPa/min, respectively, and the drying duration is suggested tomaintain at about 4 h.
2.2.4. Employment of gelling improversThe hydroxyl groups on the surface of starch chain can be cross-
linked by cross-linking agents, such as glutaraldehyde (Peng et al.,2018), formaldehyde (Zhang, Zhai, et al., 2017), sodium metapho-sphate (Ubeyitogullari et al., 2018), and trisodium citrate (Abhari,Madadlou, Dini, & Hosseini Naveh, 2017). The cross-linking reactionincreases the viscosity of the starch suspension and hinders the growthof ice crystals during the pre-freezing process (Wang et al., 2016). Afterthe cross-linking reaction, the mass mobility of starch molecules islimited due to their assembly, which could form an interconnected gelnetwork for fabricating an aerogel with improved mechanical proper-ties (Abhari, Madadlou, & Dini, 2017; Wang et al., 2016). Cross-linkingtreatment increased the hardness of the aerogel by 16%, reduced theadhesiveness and fracturability by 74% and 29%, respectively, andimproved the uniformity of pore distribution of starch-based aerogels(Abhari, Madadlou, & Dini, 2017; Zhu, 2019).
Some starch-based aerogels have been attempted to be fabricatedalong with non-starch hydrocolloids, acting on the morphology andproperties of the aerogel (Table 2). Agar (the mass ratio of agar tostarch is 1:1) was used as a gelling agent in starch-based aerogelsmanufacturing for biomedical and food purpose (Shamsuri, Abdullah, &Daik, 2012), which alleviated the dependence on chemical cross-linkingagent. It was reported that wheat straw strengthened the thermal in-sulation property of konjac glucomannan/starch composite aerogelsdue to the special cavity structure of wheat straw affecting aerogel porestructure and decreasing pore size (Wang, Wu et al., 2018). Gelatin(1%–3%) assisted in the aerogel preparation to prevent wheat strawprecipitating and improve the thermal insulation performance (Wang,Chen, et al., 2018; Wang, Wu, et al., 2018). Introducing cellulose na-nofibrils (0–1.5%) contributed to higher mechanical properties andpromising thermal properties of cellulose nanofibrils/starch compositeaerogel (Yildirim, Shaler, Gardner, Rice, & Bousfield, 2014).
Technical strategies for gelling improver employment:
i. Incorporation of gelling improvers to the gelling system is promisingfor improving the mechanical properties and functionality of starch-based aerogel.
3. Applications of starch-based aerogel
3.1. Loading and controlled release of active compounds
Starch-based aerogel is capable to decrease the caloric value of foodfor its ultralow density and high porosity (Ubeyitogullari & Ciftci,2016a). Such starch-based aerogel exhibits higher resistant starchcontent than native starch in food system prior to digestion(Ubeyitogullari et al., 2018). High amylose starch in its native state isclassified as type II resistant starch and transformed into type III oncebeing gelatinized and retrograded (Ubeyitogullari et al., 2019). Thedigestibility of starch-based aerogel is critical in both food and phar-maceutical industry not only when the nutritional value of the aerogelmatrix is under consideration, but also when the delivery of loadedcompounds is addressed.
Aerogel with the open structure and the large surface area could bean outstanding matrix for loading and controlled release of activecompounds. Starch aerogel-based delivery system has been reported totransport various pharmaceutical compounds such as anti-bacterialagents and anti-inflammatory medicines, as well as bioactive in-gredients like vitamins and antioxidants in the food industry. The typesof compounds loaded on the starch-based aerogel matrix and theirloading capacities are shown in Table 3. Biodegradable starch-basedaerogel is more desired for therapeutic applications, compared to silicaaerogel which cannot be degraded by the in vivo enzymatic system. Thecompounds loading capacity is related to the internal structure of thestarch-based aerogel and the chemical interaction of the compounds-aerogel matrix. Loading capacity of aerogel increases with increasingspecific surface area. Proper reducing the pore size is also helpful be-cause smaller pore size can enhance the capillary forces, which couldcontribute to a higher loading (Mehling et al., 2009). The content of OHgroups in starch-based aerogel is about 4–10 times that of silica aerogel(García-González et al., 2012a). These OH groups could form hydrogenbonds with pharmaceutical and nutriceutical compounds rich in car-bonyl and carboxyl groups, thus increasing the loading efficiency.Moreover, amylose forms a helical structure with a hydrophobic cavitythat could host the active compounds and form an inclusion complex(García-González et al., 2015). Reducing the crystal size of loadedcompounds could increase their bioaccessibility and in turns the bioa-vailability (Ubeyitogullari & Ciftci, 2017). Porous starch-based aerogelmatrix acts as a model and physical barrier to prevent the formation oflarge compounds crystals (Ubeyitogullari et al., 2019).
The methods of incorporating the compounds into a starch-basedaerogel matrix are summarized in Fig. 4. The compounds are added tothe starch suspension, thoroughly mixed to form a starch-based gelcarrying these compounds, and then dried to form an aerogel loadedwith the compounds (Fig. 4a). This method is suitable for water-solublecompounds. It is noted that the particle size of those compounds shouldbe taken into account in this way. The drying method selected not onlyaffects the specific surface area and porosity of aerogel matrix, but alsothe suspension stability of loaded compounds in the matrix. The ad-sorption of loaded compounds could take place during the solvent ex-change step as well (Mehling et al., 2009), where water in gel is re-placed by the compounds saturated ethanol solution and then dried(Fig. 4b). Another method of loading is to immerse the porous aerogelin the loaded compounds solution. The excessive solution is drained,and the compounds loaded aerogel is obtained after drying (Fig. 4c)(Abhari, Madadlou, & Dini, 2017). Supercritical solvent impregnation(SSI), a technique favors final product free of any residual solventcontamination, is widely employed for loading compounds intopolymer carriers for the pharmaceutical, cosmetic, and food industries(Fig. 4d) (Milovanovic et al., 2015). In this process, active molecules
Table 3Types of loaded compounds in starch-based aerogels and their loading content.
Active compounds Loading capacity References.
trans-2-hexenal 1.45 ± 0.15 ml/g Abhari, Madadlou, and Dini (2017)ketoprofen 16 wt% García-González, Camino-Rey, et al.
(2012)8 wt% Santos et al. (2015)12.84 wt% García-González et al. (2015)11.53 wt% García-González and Smirnova (2013)
ibuprofen 30 wt% Lovskaya et al. (2015)10-22 wt% Mehling et al. (2009)
benzoic acid 21.54 wt% García-González et al. (2015)paracetamol 10-25 wt% Mehling et al. (2009)α-tocopherol 19.99 wt% De Marco and Reverchon (2017)
12.5 wt% De Marco et al. (2019)menadione 8.76 wt% De Marco and Reverchon (2017)phytosterol 5.5 wt% Ubeyitogullari and Ciftci (2016b)
19.5 wt% Ubeyitogullari et al. (2019)thymol 4.02 wt% Milovanovic et al. (2015)
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are dissolved in scCO2 and can contact with the aerogel substrate at amolecular level (Mehling et al., 2009). The rapid equilibration andmicropore penetration of the fluid phase in the porous matrix is im-proved by the peculiar characteristics of scCO2, such as low viscosityand high diffusivity. After removing scCO2 through a slow depressur-ization, a compound loaded aerogel free of any solvent is obtained (DeMarco & Reverchon, 2017). SSI is considered superior to other loadingmethods at different aspects: firstly, SSI is suitable for certain com-pounds, such as ketoprofen, which is more soluble in scCO2 than inwater (Santos et al., 2015); Secondly, the crystallinity of loaded com-pounds is reduced in this way, thereby increasing the water solubility ofthese compounds entrapped in aerogel due to the increase of lattice freeenergy (García-González et al., 2015; Ubeyitogullari et al., 2019).
Technical strategies as an active compounds delivery system.
i. Starch-based aerogels could be incorporated in food system as anadditive to reduce its energy density.
ii. Starch-based aerogels are desirable matrixes for loading compoundsrich in carbonyl and carboxyl groups.
iii. Aerogels of a higher specific surface area and a suitable reduction inpore size could support higher loading content.
iv. Supercritical solvent impregnation could be employed as a loadingstrategy to ensure better solubility and bioavailability of the loadedcompounds.
3.2. Thermal insulation materials
Applications of starch-based aerogel as thermal insulation materialhave been widely reported in recent years. The heat transfer mechanismof aerogel could be expressed by the combination of heat conduction inboth solid backbone and gaseous phase, as well as the thermal radiationbetween the interior surfaces (Wang, Wu, et al., 2018). Low density andporous starch-based aerogels possess superinsulation property as the airis confined in the pores when the pore size of aerogel is smaller than themean free path of air molecules (around 70 nm, at 25 °C, 1 ATM) (Druelet al., 2017). This leads to the conduction of gas phase lower than thatof ambient air in line with Knudsen diffusion.
Starch-based aerogel is acceptable being employed as thermal in-sulation material for comparatively low temperature. Silica aerogel isoften treated as a reference in the research of thermal insulation aerogelmaterials (Druel et al., 2017; Li, Cheng, et al., 2017; Raman et al.,2015), of which the thermal conductivity can be as low as 0.012 W/m·K-0.015 W/m·K, much lower than that of air (0.025 W/m·K) (Druelet al., 2017). It must be admitted that the thermal conductivity ofstarch-based aerogel is higher than that of silica aerogel. The thermalconductivity of pea-based starch aerogel containing 40% amylose was0.021 W/m·K (Druel et al., 2017). Thermal insulation properties couldbe promoted with a higher mesoporosity, a uniform pore-size, and alower density of starch-based aerogel. Another way to regulate thethermal conductivity of starch-based aerogel is to fabricate mesoporouscomposite thermal insulated aerogels by incorporating silica, clay, andcellulose nanofibrils (Wang et al., 2016; Yildirim et al., 2014). How-ever, the conduction of the solid phase would increase as the densityincreases. A delicate balance is required to achieve the desired thermalconductivity.
3.3. Food packaging
Starch-based aerogel can be used for food packaging. Lignocellulosenanofibrils reinforced the waxy corn starch-based aerogel and reducedits water absorption from 15 g/g to 12 g/g (Ago, Ferrer, & Rojas, 2016).The mechanical properties of composite aerogel are comparative topolystyrene foam. In this regard, the composite aerogel is potential tobe a sustainable and green alternative material for packaging.
Aerogels loaded with antibacterial agents and antioxidants are re-ferred as active packaging, which process multiple functions in foodpreservation other than merely providing an inert barrier to externalconditions (De Souza et al., 2014). Regarding the inhibition of lipidoxidation, active packaging with controlled release behaviors is moreefficient than simply adding antioxidants directly into the food for-mulation (Franco et al., 2018). The pore size of starch-based aerogel isadjustable to control the release rate of antibacterial agents and anti-oxidants. That is, the larger the pore size, the faster the release of thefunctional substance. Bio-composite films that were made of cassava
Fig. 4. Four approaches of loading compounds into a starch-based aerogel matrix.
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starch incorporated with cinnamaldehyde via SSI presented goodproperties for the development of food active packaging (De Souzaet al., 2014). Quercetin, a flavonoid with outstanding antioxidant andantibacterial efficacy, was adsorbed onto maize starch-based aerogel,which could be applied in active packaging for food preservation(Franco et al., 2018). However, it is essential to know the oxygen pe-netration rate of aerogels as food packaging materials, which has notbeen studied before.
Technical strategies as food active packaging:
i. The mechanical strength of aerogel food packaging could be en-hanced by adding material improvers such as lignocellulose nano-fibrils.
ii. The pore size of the starch-based aerogel could be adjusted to con-trol the release rate of antibacterial agents and antioxidants forversatile food packaging applications.
3.4. Other applications
In addition to these applications, starch-based aerogels are potentialin biomedical research, air pollution control, sewage treatment andnovel materials development. In an in vitro bone tissue engineeringexperiment, macroporous alginate/starch based composite aerogel wasprepared to be a bioactive, non-cytotoxic scaffold, on which cells areeasy to be adhered and freely proliferate (Martins et al., 2015). Mac-ropores provided the space necessary for cell growth in tissue en-gineering and acted as transport pores in catalytic applications (Ramanet al., 2015). Starch-based aerogel with wheat straw incorporatedshowed a dramatic reduction of filtration resistance and air intake load,displaying eminent filtration performance due to its unique porestructure (Wang, Chen, et al., 2018). This result demonstrated thepossibility of applying starch-based composite aerogel enhanced withwheat straw in air filtering. The adsorption of water (Zhu et al., 2018),CO2 (Anas, Gönel, Bozbag, & Erkey, 2017), organic dye and oil (Changet al., 2010; Qian, Chang, & Ma, 2011) was efficient by applying starch-based aerogel of high specific surface area, low density and high por-osity. Moreover, the starch-based aerogel network was employed as adirecting template to prepare the hierarchically ordered TiO2 network,which provided opportunities for developing multi-functional novelmaterials (Miao et al., 2008). Starch-based aerogel is not electricallyconductive by itself, but the carbon aerogel derived from pyrolysis turnsto be conductive, which broadens its energy application range(Bakierska et al., 2017; Bakierska et al., 2014).
4. Summary and prospective
As a kind of novel bio-aerogel, starch-based aerogel presents ex-cellent mechanical toughness and biodegradability compared with in-organic aerogel. It is still in progress and dominated by fundamentalresearches. These studies are primarily advanced in the fabrication andproperties, including but not limited to how they can be constructedbased on a given starch precursor or how the gelation or drying pro-cedure affects the final properties. The sources and concentrations ofnatural starch, the conditions of starch-based hydrogel formation, theemployment of gelling improvers, and the methods of solvent removalshould be carefully considered when optimizing the properties ofstarch-based aerogel. Controlling these factors has the potential to ob-tain low-cost, energy-efficient and high-performance starch-basedaerogels.
For starch-based aerogel used in food and drug industry, its com-patibility and stability should be characterized within a food or phar-maceutical matrix subjected to processing and storage under differentexternal conditions in thermal treatment, lighting, mechanicalshearing, shelf life etc. There are not enough studies about the releaseand absorption of loaded compounds in vivo and in vitro yet, nor itssafety risk of intaking starch-based aerogel or aerogel packaged foods.
Besides, modified starch-based aerogel with special mechanical re-sponsiveness and compounds affinity can be promising as versatiledelivery system and food packaging, which has not been extensivelyexplored yet. It could not be denied that there is still a gap betweenbasic research and realistic application in these realms.
The flexibility of parameters already enables the preparation ofstarch-based aerogels with unique properties that target a wide varietyof applications. However, the production of starch-based aerogels hasnot yet expanded from laboratory scale to industrial scale. Given therapid acceleration in the field of aerogels mass production, it is likelythat starch-based aerogel will be successfully devoted to advancing foodindustry, especially as food packaging materials and food deliverysystem.
Declaration of competing interest
The authors declare no conflict of interest and no competing fi-nancial interest.
Acknowledgements
This work was supported by National Key Research andDevelopment Plan (2016YFD0400204-2), Fundamental Research Fundsfor the Central Universities (XDJK2020C051), the Venture & InnovationSupport Program for Chongqing Overseas Returnees (cx2019119) andDevelopment and Research Center of Sichuan Cuision (CC18Z13).
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