Artificial pores can be created using many approaches. For example, a portion of the macromolecules within a granule can be hydrolyzed by enzymes or acids to low molecular fragments, and pores are formed when these hydrolyzed products dissolve. Another approach involves starch chains in a microemulsion which are cross-linked to form microspheres, and pores are present in some of the resulting products. In addition, starch gels can exhibit a porous structure when treated with freeze-thaw processes. In recent years, a solvent exchange method and other methods have been used to produce Porous Starch (PS) or to enhance the pore-forming efficiency.
- Introduction to Porous Starch (PS)
- Porous Starch Structure and Properties
- Mechanism of the Formation of Pores in Starch
- Application of Porous Starch
- Porous Starch Wastewater Treatment
Hydrolysis by Enzyme or Acid
Enzymatic hydrolysis is one of the most commonly used methods for preparing Porous Starch (PS). When hydrolysis is conducted below the gelatinization temperature of starch, enzymes can act on the granules without damaging the granule integrity.
Uthumporn U Compared the granular change of different starches during enzymolysis at sub-gelatinization temperature. The research group dispersed starch slurry in sodium acetate buffer and hydrolyzed the starch using a mixture of the enzymes alpha-amylase (from Aspergillus kawachi) and glucoamylase (from Aspergillus niger). The temperature of hydrolysis was set at 35 °C, and pH was controlled at 1.5–1.6. After 24 h reaction, the pH of the starch dispersion was adjusted to 5–6 to stop further enzymolysis. Porous Starch (PS) was collected by filtering the hydrolyzed starch with distilled water and then dried at 40 °C for 2 days. When compared with hydrolyzed mung bean, cassava, and sago starches, more porous granules were observed in hydrolyzed corn starch. After hydrolysis, corn and mung bean starch had deeper holes and a more porous structure compared with hydrolyzed cassava and sago starch. In addition, hydrolyzed cassava and sago starch had a rough surface, and some of the granules remained intact.
Chen XY prepared PS granules with a pore diameter ranging from 620 to 1150 nm. For this reaction, the starch was dispersed in sodium acetate buffer (pH 4.6), and glucoamylase was added to hydrolyze the starch at 50 °C. Most starch granules were first hydrolyzed superficially, and then the pores on the surface enlarged and subsequently enzymes penetrated the granules. The distribution of pores on the surface and pore size was controlled by the enzyme/granule ratio and hydrolysis time.
Yussof NS compared the granular structure of PS prepared from native and cross-linked starches. Corn, tapioca, and sweet potato starches were cross-linked with epichlorohydrin, and STARGENTM 001 (a blend of α-amylase and glucoamylase) was used to hydrolyze these starches at 35 °C for 24 h. Compared with native starches, all cross-linked starches had fewer porous granules after hydrolyzation. Enzymatic erosion occurred mainly on the surface of starch granules, with deep pores in the interior part of the granules.
Lecorre D compared the hydrolysis process of different enzymes in preparing porous waxy maize starch. They found that α-amylase was highly effective in hydrolyzing waxy maize starch, but produced fewer porous granules than β- and γ-amylases. β-Amylase and glucoamylase seemed to produce equally porous granules.
Błaszczak W studied the different effects of fungal α-amylase and amyloglucosidase on corn starch at sub-gelatinization temperature to obtain PS. Corn starch was suspended in NaH2PO4 buffer (pH 6.0) or sodium acetate buffer (pH 4.0), and the two enzymes were added to each suspension, respectively. The reaction mixtures were placed in a shaking water bath at 50 °C for 24 h. The results showed that at sub-gelatinization temperatures, hydrolysis led to the porous structure of starch, but the microstructure surface and internal morphology differed in the α-amylase and amyloglucosidase suspensions. Enzymatic modification of the starch resulted in porous structures with more agglomerates in the amyloglucosidase suspension.
Cross-linked Porous Starch (PS) samples were produced by partially hydrolyzing cross-linked corn starch with a mixture of α-amylase and glucoamylase. The cross-linking agent sodium trimetaphosphate was added to corn starch slurry containing Na2CO3 and NaCl. The mixture was stirred at 50 °C for the cross-linking reaction. After this reaction, the pH of the slurry was adjusted to 6.5, and the cross-linked starch product was washed with deionized water. The cross-linked starch was then dispersed in sodium acetate buffer (pH 4.6), and the slurry was preheated at 40 °C for 20 min. A mixture of α-amylase and glucoamylase at a ratio of 1:4 was added for hydrolysis. The starch product in the slurry was then separated by centrifugation, washed, and lyophilized. The hydrolysis caused an obvious increase in pore area, pore diameter, and porosity. The magnitude of the total pore area of cross-linked PS was increased 10–20 times compared with that of the native starch.
Jiao A investigated the impact of combined α-amylase and hydrochloric acid hydrolysis on the structure of starch granules. The α-amylase was dissolved in sodium phosphate buffer (pH 6.9) containing CaCl2. Waxy rice starch was dispersed in the enzyme solution and incubated at 37 °C. After hydrolysis, absolute ethanol was added to stop the reaction, and the solution was then centrifuged. The precipitates were dried overnight at 35 °C. The hydrolyzed starch was treated with 2.2 mol/L HCl at 35 °C. After this reaction, the resulting mixture was neutralized, and ethanol was added to precipitate starch. The final solution was centrifuged and the residues were dried at 35 °C. Numerous pinholes appeared on the enzyme-treated waxy rice starch granules, which were openings to channels that provided access to the granule interior. As the hydrolysis time increased, the holes became larger. The surface pores showed no visible change with increased hydrolysis time. Furthermore, the granular size sharply decreased, and many fragments were formed when the enzyme-pretreated starch sample was exposed to the mild acid for 24 h during combined hydrolysis.
Shariffa YN used mild heat to treat starch before enzymolysis to prepare Porous Starch (PS). The pretreated starch was obtained by incubating the starch slurry (sodium acetate buffer) in a water bath at 60 °C for 30 min. The temperature of the slurry was then decreased to 35 °C for hydrolysis. The mild heat treatment caused the starch to swell, resulting in the enlargement of the pinholes on the surface of the starch granules. This permitted the enzyme to penetrate the granules more extensively and form pores and channels in the starch granules during hydrolysis.
Microporous wheat starch was prepared using different levels of α-amylase, different sonication treatments, and a combination of both methods. A starch suspension was left at ambient temperature overnight to fully hydrate before ultrasonic treatment. The suspension was then poured into an ultrasound bath and sonicated at a frequency of 35 kHz. During sonication, the temperature was maintained at 40 °C. α-Amylase was added to the starch suspension, and the hydrolysis temperature was set at 45 °C. The resulting product was centrifuged, and the pellet was recovered and washed three times with distilled water and then dried at 80 °C. The PS product was ground and sieved to obtain an average particle size of 100 μm. The combination of α-amylase and ultrasound treatment increased the quantity and size of the micropores. However, sonication of the enzyme-treated samples destroyed some of the starch granules. This effect was more pronounced when samples were sonicated for 40 and 60 min.
Porous Starch (PS) granules were formed by partial hydrolysis of starch using amylase. An ultrasonic technique to assist enzymatic hydrolysis was used to pretreat raw starch. The starch was suspended in pH 4.6 buffer solution and then treated with ultrasonic waves at temperatures below the gelatinization point. Following pretreatment, fungal amylase (or beta-amylase) was added to treat the starch at 55 °C. The solution was then centrifuged, washed with distilled water, dried in a vacuum dryer at 60 °C, and then comminuted. PS prepared by fungal amylase had better pores than that prepared with beta-amylase. Compared with raw starch, ultrasonic pretreatment facilitated the attack by fungal amylase on starch and increased the size and depth of pores. The optimal ultrasonic power and time of ultrasonic action were 400 W and 15 min, respectively.
Microporous starch was prepared from corn starch by glucoamylase catalysis combined with ultrasonic treatment. Three different ultrasonic treatments were performed; sequential sonication was performed before, simultaneously, or after hydrolysis. Corn starch was dispersed in Na2HPO4–citric acid buffer (pH 4.0), and glucoamylase was added for the reaction at 40 °C. After the reaction, the dispersion was centrifuged to isolate hydrolyzed starch, which was then washed with deionized water and dried at 55 °C. A combination of glucoamylase and ultrasound treatment accelerated the formation of holes and increased the specific surface area in microporous starch granules. Of the three different ultrasound treatments, simultaneous ultrasound treatment during glucoamylase digestion efficiently facilitated the micropore formation process, followed by ultrasound treatment before digestion, while the efficacy of ultrasound treatment after digestion was unremarkable.
The susceptibility of cereal starches (normal maize, waxy maize, amylomaize V and VII, rice, and oat) to hydrolysis with high concentrations of acid was examined. Starches were hydrolyzed with 2.2 mol/L HCl at 35 °C. The granular residue was separated by centrifugation and washed three times with deionized water. The products were resuspended in a small amount of water and lyophilized. In hydrolyzed oat starch, many of the granules were deformed, and their surfaces were completely covered with small pores. In contrast, hydrolyzed waxy maize starch exhibited a total loss of granular shape, and the whole mass was covered with pores and cracks. Granule shape was clearly discernible in both amylomaize V and amylomaize VII starches, but their external appearances were different. The surface of some amylomaize V granules was extensively corroded with numerous pores and cracks. In contrast, the surface of amylomaize VII granules was wrinkled and devoid of cracks. Very few pores were present on the granule surface of amylomaize VII starch.
After freeze-drying, Porous Starch (PS) xerogels can be formed by inserting mercaptosuccinic acid molecules into the starch chains. Mercaptosuccinic acid was dissolved in water, and potato starch was added to the solution. The mixtures were heated to 90 °C to form gelatinized composites of starch and mercaptosuccinic acid. The composites were first frozen at −4 °C and then freeze-dried at −52 °C under pressure of 20 Pa. The porous structure was observed, and the number of pores (6–7 μm in diameter) increased with increasing mercaptosuccinic acid content. The researchers indicated that the introduction of mercaptosuccinic acid broke the intrinsic hydrogen bonds within starch chains and intermolecular hydrogen bonds were formed between mercaptosuccinic acid and starch, which resulted in the formation of the pore structure.
Magnetic Porous Starch (PS) was prepared using a freeze-thaw process. An aqueous suspension of native corn starch was heated with a magnetic iron oxide suspension or magnetic fluid. The resulting magnetic gel was frozen at −18 °C and thawed at room temperature. After three freeze-thaw cycles, the insoluble magnetic starch gel formed was disintegrated and sieved to obtain particles with a diameter of less than 0.5 mm. The size of porous magnetic starch particles was 300–500 μm.
Calcein-containing starch acetate microparticles were prepared by a modified water-in-oil-in-water (W/O/W) double-emulsion technique. Potato starch acetate was dissolved in chloroform, and the solution was cooled in an ice bath. The aqueous calcein solution was emulsified in the organic polymer solution using an Ultra Turrax Homogenizer. The primary W/O emulsion was poured into an aqueous PVA solution as homogenization was carried out using Ultra Turrax. The W/O/W emulsion was stirred to allow the evaporation of chloroform. After this, the microparticles were sieved (Ø 15 μm). The microparticles were isolated by centrifugation, washed, and dried under vacuum at room temperature. The mean diameter of starch acetate microparticles was 11 μm, with small pores distributed in the microparticles.
Starch/cyclodextrin bioadhesive porous microspheres were prepared as a platform for the nasal administration of drugs. Starch and cyclodextrin were dissolved in NaOH solution in the presence of NaBH4. After the complete removal of air bubbles under vacuum, the solution was added to 1,2-dichloroethane-containing cellulose acetate butyrate CAB. Epichlorohydrin (ECH) was added to the obtained W/O emulsion, and the cross-linking reaction was carried out at 50 °C. The cross-linked microspheres were recovered by filtration, which was then washed with 1,2-dichloroethane, acetone, water/acetic acid solution (30%, v/v), water, and methanol. The microspheres were then completely dried at 60 °C under a vacuum. These microspheres were characterized by a narrow size distribution with 70% of microparticles between 50 and 160 μm. The microspheres displayed a homogeneous and dense internal structure with small pores.
Soluble starch-based biodegradable and microporous microspheres were prepared by an emulsion chemical cross-linking technique using trisodium trimetaphosphate as cross-linker. Starch and trisodium trimetaphosphate were mixed in sodium hydroxide aqueous solution, which was used as the water phase. Liquid paraffin-containing emulsifier (Span 80 and Tween 80 at a volume ratio of 1:1) was used as the oil phase. The water phase was then added dropwise into the oil phase, and the cross-linking reaction took place at 45 °C. The obtained microspheres were collected and washed three times with petroleum ether, acetone ethanol, and deionized water, respectively. Finally, the products were dried in a vacuum. The surfaces and the internal structure of the microspheres had a compacted and continuous network with certain microporosities. The research group indicated that the formation of microporosities may be related to the mechanisms of air bubbles or entrapped organic solvent evaporating during the cross-linking and drying processes.
Solvent Exchange Method
The exchange solvent is an important factor in displacing water from the hydrogel to maintain the porous structure of the gel. Alcohol as the exchange solvent can avoid contraction and collapse of the aquagel due to direct air drying. A biodegradable Porous Starch (PS) foam (BPSF) was prepared by the solvent exchange method. A suspension of soluble amylum was heated to 100 °C for 0.5 h, and the melt was lowered to 85 °C and then poured into a Petri dish. The resulting slurry was chilled to 5 °C overnight to facilitate gelation. The gel was then transferred to four volumes of 40%, 60%, 90%, and 100% ethanol/water solution, respectively, and equilibrated for 24 h in order to maintain the porous structure of the gel, whereby ethanol displaced the water in the aquagel to form an alcogel. The resulting foam was obtained by rotary evaporation drying at 30 °C. The dry PS foam was milled in a mortar and passed through an 80 mesh sieve, and the BPSF particles were stored in a vacuum dryer. The pore size distribution of the PS foam was narrow (lower than 200 nm), and 60% of the pores had a diameter of 20–80 nm. The pore size distribution of PS foam was between that of mesopores and large pores.
Porous Starch (PS) microspheres were prepared by the W/O emulsion-freeze-thawing method/m. The water phase was a soluble starch aqueous suspension, which was boiled to 100 °C for 0.5 h and then lowered to 85 °C for use. The water phase was then added to the oil phase which was formulated with a methylbenzene and chloroform solution (2:1) and Span 80 as the emulsifier. The mixed liquid was homogenized to form a stable W/O emulsion. The emulsion was stored at −20 °C for the emulsion drops to gelatinize. Using the solvent exchange technique, the products were equilibrated with four concentrations of ethanol (40%, 60%, 90%, and 100%), which displaced the water in the aquagel to form an alcogel. The products were filtered and dried under a vacuum at room temperature. Particle size distribution of the PS microspheres was 20–100 μm. The PS microspheres possessed a nanometer porous-connected structure (500 nm).
Treated gel microspheres were prepared by emulsion cross-linking with the solvent exchange method to obtain a porous structure. Oil/starch emulsions were prepared by mixing corn starch aqueous dispersion with vegetable oil. The resulting emulsion was then heated and pressurized at 0.1–0.2 MPa in an autoclave. The pressure in the autoclave was then released and the temperature of the emulsion was lowered using an ice bath. After centrifugation, particles were separated from the oil phase, soaked in ethanol, and placed in the refrigerator for retrogradation. After retrogradation, starch particles were transferred to a fresh ethanol solution. The resulting starch alcogels were dried by extraction of the solvent using supercritical carbon dioxide. The starch alcogel was then loaded into the autoclave and immersed in ethanol to prevent shrinkage. The autoclave was heated to 40 °C, and pressure was set at 11.0– 12.0 MPa. After treatment, the starch microspherical alcogel was dried to remove the solvent. The spherical particles of starch in the range of 200–400 μm with a nanoporous texture characterized by a high specific surface area and total mesopore volume were obtained.
Prepared wheat starch foam using the solvent exchange method. The wheat starch gel was obtained by adding starch to water in a rapid viscoamylograph specimen vial. The instrument was programmed to ramp from 25 °C at a rate of 10 °C/ min to 95 °C and then hold at that temperature for 5 min. Following the temperature treatment, the starch melt was immediately transferred to cylindrical molds and stored at 5 °C overnight. The gelled starch was removed from the molds and equilibrated in a graded ethanol series (40%, 70%, 90%, and three changes of 100% ethanol) to displace the water. Wheat starch microcellular foam was produced by simply evaporating the ethanol from the samples under a stream of dry nitrogen gas. Starch foam panels made from wheat starch consisted of a porous matrix of starch interspersed with granule remnants which appeared relatively nonporous.
Porous Starch (PS) can be prepared by replacing ice crystals in frozen starch gel with ethanol using the solvent exchange technique. Potato starch was added to distilled water, and the mixture was heated at 90 °C for complete gelatinization. It was then cooled to 5 °C to obtain the starch gel. The gel was cut into cubes and frozen at −10 °C. The frozen cubes were immersed in ethanol three times. The cubes were dried at 50 °C to remove the ethanol, and white solid PS cubes were obtained. With increasing starch paste concentration, the pore size of PS gradually decreased.
Porous carboxymethyl sago starch-acid hydrogel was prepared by an irradiation technique. Sago starch powder was added to isopropanol and sodium hydroxide to form a slurry, and sodium monochloroacetic acid was added for the etherification process at 55 °C. After the reaction mixture was cooled to room temperature, the produced carboxymethyl sago starch was washed with methanol three times, neutralized with glacial acetic acid followed by absolute ethanol, and then dried in an oven at 60 °C. Carboxymethyl sago starch-acid hydrogel was prepared by dissolving the carboxymethyl sago starch in lactic acid. This composite mixture was then transferred into a plastic mold. Irradiation was conducted using electron beam radiation. A typical hydrogel structure was obtained, which had three-dimensional networks with empty pores.
A new simple processing route to produce starch-based porous materials was developed based on a microwave baking methodology. The bowing powder was mixed with the corn starch/ethylene vinyl alcohol blend powders. H2O2, acting as a bowing agent, was then added until a consistent slurry dispersion was obtained. The mixture was then heated in a microwave oven. The oven power and treatment time were optimized to range between 250 and 400 W and 1 and 5 min, respectively. The samples were then dried at 50 °C. The samples were prepared in a cylindrical shape, which were then cut into rectangles. The microwave blowing method leads to porous materials, and the porosity increases with the amount of bowing agent.
Glenn et al. prepared porous high-amylose corn starch microspheres by spraying dissolved starch with ethanol. The starch was heated to 140 °C using a pressure reactor. The temperature was held for 10 min before cooling the starch melt to 85 °C. The starch melt was pumped through an atomizing nozzle at a rate of 100 mL/min. The air pressure supplied to the nozzle was maintained at 0.55 MPa. The atomized starch was air classified into two fractions that were collected in two separate ethanol (95%) baths. The first fraction was collected directly from the spray stream and contained the largest particle size range. The second fraction was collected from droplets suspended by air turbulence and drawn by vacuum into a second ethanol bath. Both baths were stirred constantly to provide agitation and help minimize particle-to-particle interaction. The samples collected were stored in ethanol. The porous microspheres from the first collector consisted of spheres ranging in size from approximately 10 μm to greater than 300 μm with a mean size of more than 100 μm. The sample consisted of very fine microspheres ranging in size from approximately 2 to 15 μm with a mean size of 5 μm.