Preparation, characterization, physicochemical property and potential application of porous starch: A review

Journal Pre-proof Preparation, characterization, physicochemical property and potential application of porous starch: A review

Jiahui Chen, Yuexi Wang, Jun Liu, Xinglian Xu PII:

S0141-8130(19)39775-2

DOI:

https://doi.org/10.1016/j.ijbiomac.2020.02.055

Reference:

BIOMAC 14672

To appear in:

International Journal of Biological Macromolecules

Received date:

28 November 2019

Revised date:

10 January 2020

Accepted date:

6 February 2020

Please cite this article as: J. Chen, Y. Wang, J. Liu, et al., Preparation, characterization, physicochemical property and potential application of porous starch: A review, International Journal of Biological Macromolecules(2020), https://doi.org/10.1016/ j.ijbiomac.2020.02.055

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© 2020 Published by Elsevier.

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Preparation, characterization, physicochemical property and potential application of porous starch: a review Jiahui Chen a, b, c, d, e, Yuexi Wang b, c, d, e, Jun Liu a, *, Xinglian Xu b, c, d, e, * a

College of Food Science and Engineering, Yangzhou University, Yangzhou 225127,

Jiangsu, China b

Key Laboratory of Animal Products Processing, Ministry of Agriculture, Nanjing

210095, China Key Lab of Meat Processing and Quality Control, Ministry of Education, Nanjing

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c

d

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210095, China

Jiangsu Collaborative Innovation Center of Meat Production and Processing, Nanjing

College of Food Science and Technology, Nanjing Agricultural University, Nanjing

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e

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210095, China

*

Corresponding

authors.

E-mail

addresses:

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[email protected] (X. Xu).

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210095, China

1

[email protected]

(J.

Liu),

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Abstract In recent years, porous starch has attracted increasing attention due to its valuable functions and potential applications. Here, we present a comprehensive review of the recent advances of porous starch in different aspects, including the preparation and structural

characterization

methods,

physicochemical

properties,

and

potential

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applications. In general, different techniques including physical, chemical and enzymatic as well as their synergic methods have been extensively used to prepare porous starch.

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The prepared porous starch can be characterized by several instrumental methods

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including pore structure analyzer, scanning electron microscopy, X-ray diffractometer,

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Fourier-transform infrared spectroscopy, and nuclear magnetic resonance. As compared to

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native starch, porous starch shows enhanced adsorption efficiency, solubility and swelling

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power. Moreover, porous starch can be used to adsorb, encapsulate and release different substances. This review will provide a guideline for the rational preparation and

Keywords:

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applications of porous starch in the future. Porous

starch;

Preparation

Physicochemical property; Applications.

2

method;

Structural

characterization;

Journal Pre-proof Contents 1. Introduction 2. Preparation methods of porous starch 2.1. Physical methods 2.2. Chemical methods 2.3. Enzymatic methods 2.4. Synergic methods 3. Structural characteristics of porous starch

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3.1. Pore structure analyzer (PSA)

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3.2. Scanning electron microscopy (SEM)

3.3. Transmission electron microscopy (TEM) and atomic force microscope (AFM)

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3.4. X-ray diffractometry (XRD)

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3.5. Fourier-transform infrared (FT-IR) spectroscopy

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3.6. Nuclear magnetic resonance (NMR)

4. Physicochemical properties of porous starch

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4.1. Adsorption property

4.2. Solubility and swelling power

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4.3. Rheological property 4.4. Pasting property

4.5. Thermal property

4.6. Gelatinization property

5. Potential applications of porous starch 5.1. Food industry 5.2. Pharmaceutical industry 5.3. Environmental industry 6. Conclusions and future perspectives Acknowledgements References

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1. Introduction Starch is a renewable carbohydrate in nature [1-2]. Native starch naturally occurs in the form of semi-crystalline granules consisting of amylose and amylopectin. Amylose is a linear polymer composed of glucopyranose units linked by α-(1,4)-glycosidic linkages, while amylopectin is a highly branched polymer with short α-(1,4)-glycosidic chains

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linked by α-(1,6)-glucosidic branching points occurring every 25–30 glucose units [1]. Generally, the alternating amorphous and crystalline layers in the inner of starch granules

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result in different types of crystallization (e.g. A-, B- and C-types). Because of its low cost

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and unique physicochemical properties, native starch has been widely used in food,

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chemical, pharmaceutical and environment industries [3]. In addition, native starch

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usually exists in different forms of granules with their sizes at the micron (μm) level. The

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morphology of native starch granules is polygonal, round, spherical, elongated and kidney shapes, depending on the botanical source of starch [4]. Notably, some small holes only

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exist on the surface of cereal starch granules, providing spaces for the adsorption and encapsulation of external substances to some extent. However, practical applications of cereal starch granules are restricted by their small porous volumes and specific surface areas. In addition, no holes exist on the surface of starch from other sources, which is not conducive to the practical application [2, 5-6]. Therefore, it is necessary to prepare porous starch to overcome the disadvantage of native starch. Porous starch is a non-toxic and economical adsorbent that has been extensively used in food, pharmaceutical and environment industries [7]. In recent years, porous starch has received increasing attention due to its high adsorption and slow release properties [8-9].

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Till now, several approaches including physical, chemical, enzymatic, and synergic methods have been used to prepare porous starch (Fig. 1). Among these methods, the enzymatic method has been the most commonly used due to its gentle reaction conditions, high catalytic efficiency and substrate specificity [10]. Moreover, the combination of enzymatic methods with other methods (e.g. ultrasound, cross-linking, extrusion,

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freeze-thawing and heat moisture) further promotes the formation of pores [11-14]. Therefore, the selection of proper preparation method of porous starch is very important.

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The structure and physicochemical properties of natural starch can be greatly altered

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after pore-formation. Numerous holes extend from the surface to the interior of starch

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granules, which greatly increase the specific surface area and total pore volume of starch

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granules [15]. Notably, the formation of pores can remarkably widen the application of

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starch in different fields. Up to now, porous starch has been used as the carrier for drugs, the adsorbent for pollutants, and the encapsulating agent of dietary supplements [16]. In

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addition, other polysaccharides (e.g. pectin and chitosan) and inorganic materials (e.g. graphene oxide and CoFe2O4) have been selected to functionalize porous starch [17-18]. For the first time, this review summarizes the recent advances of porous starch in different aspects,

including

the

preparation

and

structural

characterization

methods,

physicochemical properties, and potential applications in food, pharmaceutical and environmental industries. Fig. 1

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2. Preparation methods of porous starch 2.1. Physical methods Porous starch can be easily prepared through conventional physical hole-forming approaches (e.g. mechanical extrusion, microwave and ultrasonic methods). Mechanical extrusion is performed through different physical unit operations, including mixing,

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stirring, heating, spraying, and puffing [19]. This method utilizes a strong pressure difference in the spraying process of starch paste, causing the rapid vaporization of water

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and the formation of loose porous structure in starch. As summarized in Table 1, porous

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starch with better porosity can be obtained at 18% water content, 160 ºC and 200 rpm

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screw speeds [20]. The water content is negatively correlated with the porosity of

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extruded starch, whereas temperature is positively correlated with the porosity of extruded

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starch [21]. Notably, porous starch prepared by mechanical extrusion method usually shows uneven pore size. The yield of porous starch prepared by this method is very low.

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In addition, the high extrusion temperature (> 95 ºC) can cause the gelatinization and structural damage of starch granules. Therefore, mechanical extrusion method is frequently used to manufacture precooked cereal products. Recently, microwave method has been used to prepare porous starch because this method is highly efficient and environmentally friendly. This method is based on the microwave-generated thermal effect that can penetrate starch granules by "molecular friction" in an alternating electromagnetic field [22]. As shown in Fig. 2A, swelled starch granules are arranged orderly under electric field. Microwave can cause starch granules to vibrate at high frequency by constantly changing the positive and negative polarities of

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the alternating electromagnetic field. As a result, microwave energy is converted into thermal energy in starch granules. Because of the heat loss effect on the surface of starch, thermal energy can be accumulated in the inner of starch granules. Meanwhile, the moisture in starch granules can be evaporated rapidly, generating a high pressure inside starch granules. When the pressure is high enough, starch granules will swell and break to

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create pores [23]. In general, the structure of porous starch is influenced by many factors, such as microwave power, treatment time and starch dosage. Nawaz et al. [24] found

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starch granules could still maintain their integrity when the microwave power was in the

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range of 400−1600 W, the treatment time was less than 5 min, and the amount of starch

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exceeded 10 g. Kraus et al. [25] further suggested that simultaneously increasing

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microwave power and reducing system pressure could accelerate the formation of

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microporous starch. Although microwave method is simple and economical, porous starch prepared by this method has limited holes. Thus, the microwave method is not suitable to

Fig. 2

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prepare fine porous starch.

The application of ultrasonic irradiation in the preparation of porous starch is widely accepted because this method has low energy consumption and simple operation. As displayed in Fig. 2B, small cavitation bubbles are created in the liquid after ultrasonic wave treatment. Within an acoustic cycle, cavitation bubbles grow to their maximum sizes and collapse violently. The instantaneous collapse of cavitation bubbles around starch granules produces a strong shock wave in the solution, causing the appearance of pores on the surface of starch granules [26]. The ultrasonic operation conditions including the

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power and frequency of ultrasonic wave and treatment time the main factors affecting the pore size of porous starch. As summarized in Table 1, porous starch can be prepared under the following conditions: ultrasonic frequency > 20 KHz, ultrasonic power > 100 W, and treatment time > 4 min. As compared with single frequency ultrasound (20 kHz or 25 kHz) treatment, dual frequency ultrasound (20 kHz + 25 kHz) treatment is more efficient

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to prepare porous starch [27]. Carmona-García et al. [28] found that starch granules with large sizes were more sensitive to ultrasound. However, the size distribution of starch was

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little influenced by ultrasound treatment. When the ultrasonic wave propagates to the

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starch solution, it will reflect back at the gas-liquid interface to form a standing wave.

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This will result in uneven holes for porous starch, which is not beneficial to the industrial

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Table 1

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application of porous starch [29].

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Journal Pre-proof 2.2. Chemical methods Porous starch can also be prepared through different chemical methods (e.g. solvent exchange, acid hydrolysis and molecular insertion). Among them, the solvent exchange method has been the most widely used [22]. Solvent exchange method is based on the exchange of water in starch hydrogel network with solvents (e.g. ethanol and acetone), which can avoid the collapse and contraction of hydrogel structure under the direct air

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drying. As displayed in Fig. 2C, water-soluble amylose forms network structure after

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gelation, and a large number of water molecules are absorbed between the gel network of

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starch. Subsequently, ethanol is used to displace water. Finally, ethanol is removed by

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vacuum drying to form porous starch. In general, the structure of porous starch prepared

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by the solvent exchange is influenced by the solvent concentration and drying conditions. Oliyaei et al. [37] found a high ethanol concentration (100 %) and freeze-drying result in

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the ideal porous starch. Notably, the surface tension of liquid or gas during drying is a

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considerable factor affecting the formation of continuous and uniform holes in porous starch. A process with the least surface tension is beneficial to produce porous starch with high porosity. Supercritical drying is a process that can extract liquid from hydrogel rather than evaporation [38]. In addition, researches further suggested that the porosity of porous starch depended on the pressure, temperature and flow rates of supercritical fluid [39-40]. Ubeyitogullari et al. [41] suggested the optimal supercritical drying conditions for the preparation of porous starch by using supercritical carbon dioxide were at 40 °C, 10 MPa and flow rate of 0.5 L/min. Notably, the solvent exchange method usually produces different shapes of holes. Besides, the porous starch prepared by this method has limited

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adsorption capacity. Thus, the solvent exchange method is not a perfect approach for the preparation of porous starch, especially when porous starch is applied in food industries. Hydrochloric acid has been sometimes utilized in the preparation of porous starch. Hydrochloric acid allows the partial hydrolysis of native starch based on maintaining the integrity of starch granules under proper conditions. However, the indirect use of

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hydrochloric acid, instead of direct hydrolysis, is employed more frequently in the preparation of porous starch. Pourjavadi et al. [42] chose calcium carbonate (CaCO3)

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particles as the porogens to prepare porous starch followed by removing CaCO3 using

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hydrochloric acid. Mercaptosuccinic acid, different from hydrochloric acid, can form

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intermolecular hydrogen bonds with starch, leading to the formation of porous structure

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[43]. The number of holes increases with the increase of mercaptosuccinic acid content.

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Although chemical methods are more efficient than physical methods in the preparation of porous starch, these methods require different chemical reagents that are not

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environmentally friendly. Thus, it is still necessary to explore safer and more environmentally friendly methods. 2.3. Enzymatic methods

Recently, enzymatic catalysis has been widely used to prepare porous starch due to its gentle reaction conditions, high catalytic efficiency and substrate specificity [44]. Among different enzymes, α-amylase (EC 3.2.1.1), glycogen branching enzyme (EC 2.4.1.18), amyloglucosidase (EC 3.2.1.3) and cyclodextrin-glycosyltransferase (EC 2.4.1.19) are often selected [45]. The mechanism for the preparation of porous starch through enzymatic hydrolysis is shown in Fig. 2D. Firstly, the saccharification enzyme 52

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hydrolyzes the irregular portion and amorphous region of starch granules, which is more susceptible to hydrolysis. Then enzyme hydrolyzes along the non-reducing end of the starch molecule. In addition, α-amylase is more accessible to the interior of starch granules due to the water swelling of starch. The random endo-cleavage of α-amylase provides new non-reducing ends for the saccharification enzyme, allowing hydrolysis to

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progress continuously into the interior of starch molecules. Interestingly, the alternating structure of amorphous and crystalline layers of starch guarantees the continuity of this

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hydrolysis behavior. Finally, starch is in the porous state and forms a hollow structure

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inside. As compared with physical and chemical methods, enzyme catalysis method is

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more efficient and environmentally friendly. Therefore, preparation of porous starch by

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the enzymatic hydrolysis has received increasing attention in the past decade [46-47].

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However, this method is mainly based on starch-matched amylases. Notably, the amylases with high catalytic efficiency are expensive, which limits the industrial application of

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enzymatic methods in the preparation of porous starch. The yield of porous starch mainly depends on the type of enzyme and starch as well as reaction conditions. Benavent-Gil et al. [10] compared porous starches prepared by different enzymes and suggested amyloglucosidase (EC 3.2.1.3) catalysis produced porous starch with the largest holes whereas cyclodextrin-glycosyltransferase (EC 2.4.1.19) catalysis resulted in porous starch with the smallest holes. Moreover, multi-enzyme treatment (e.g. the combination of α-amylase and amyloglucosidase) has been used more frequently than single enzyme treatment. Benavent-Gil et al. [48] demonstrated cereal starch was more susceptible to enzymatic hydrolysis than tuber starch. 52

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Dura et al. [49] investigated the reaction conditions of cyclodextrin-glycosyltransferase (EC 2.4.1.19) on porous corn starch, and found pH 6.0 was suitable for the formation of porous starch was effected by the pHs, being higher at pH 6.0. Table 2 summarizes different reaction conditions for the enzymatic preparation of porous starch. Regardless of the type of enzyme used, the reactions are achieved in the temperature range of 30−50 ºC

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and less than 2 h.

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Table 2 2.4. Synergic methods

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Undoubtedly, there are some certain limitations when a single method was used for

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the preparation of porous starch. Therefore, more and more studies have been carried out

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on the combination of different methods to prepare porous starch with better

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performances. The synergic methods for the preparation of porous starch are summarized in Table 3. Obviously, enzymatic catalysis has been the most widely used in the

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synergistic synthesis of porous starch, which is due to the high efficiency of enzymatic catalysis. The combination of enzyme and ultrasound has been successfully applied in the preparation of porous starch in recent years [53-55]. Barton et al. [56] found ultrasound could enhance the catalytic efficiency of α-amylase and amyloglucosidase. In addition, the enzymatic method has been combined with other methods including cross-linking, extrusion, freeze-thawing and heat-moisture treatments [13]. To obtain porous starch with good quality, the optimal preparation conditions are pH 4.6−6.0, reaction temperature 40−55 °C and reaction time 2−24 h. Notably, extrusion is usually conducted at a temperature higher than 88 ºC, and thus starch should be pre-treated with enzymes before 52

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extrusion to avoid the deactivation of enzymes [57]. Apart from enzymatic hydrolysis, some researchers have combined other methods to prepare porous starch. Porous starch microspheres were prepared through alkaline-alcohol treatment and inverse cross-linking emulsion [58]. This method mainly consists of four steps: gelatinization, emulsification, pore-forming and cross-linking. Epichlorohydrin is used as the crosslinking agent to

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establish covalent linkages between starch chains, which enhance the structural stability of porous starch. Nasri-Nasrabadi et al. [59] prepared a new type of porous starch

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composite (size of 40−90 nm) by combining film casting, salt leaching and freeze-drying.

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Table 3

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3.1. Pore structure analyzer (PSA)

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3. Structural characterization methods of porous starch

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An important feature of porous starch is that its adsorption/desorption isotherm is type IV according to the classification of the International Union of Pure and Applied

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Chemistry (IUPAC) [62]. In addition, porous starch has a typical hysteresis loop in N2 adsorption/desorption isotherms, suggesting the pores are slit plate-like without any adsorption restrictions in the high-pressure regions. Notably, the hysteresis loop is the result of capillary condensation of N2 in the starch, and the large hysteresis loop corresponds to the large pore size. The pore character of porous starch can be intuitively judged through the size of the hysteresis loop as analyzed by pore structure analyzer. The specific surface area (SBET, m2/g) of porous starch can be measured indirectly by Brunauer–Emmett–Teller (BET) adsorption isotherm equation as follows [12, 63]: P 1 C 1 P    V ( P0  P ) CVm CVm P0 52

(1)

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where P0 is the saturated vapor pressure of N2 at the adsorption temperature, P is the partial pressure of N2, V is the actual adsorption amount of porous starch, Vm corresponds to single layer saturated adsorption capacity, and C is the constant related to the adsorption capacity of porous starch. Typically, the BET curve can be plotted with P/P0 as the x-axis and P/(VP0-VP) as

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the y-axis based on BET adsorption isotherm equation. As a result, it is easy to calculate

(SBET) can be determined by the following equation:

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Vm  N A A MW

M

(2)

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S BET 

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the adsorption amount Vm of N2 single layer saturation. Finally, the specific surface area

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where NA is the avogadro constant (6.02×1023), AM means the surface area taken by one molecule of N2 , and MW is the molecular weight of N2.

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On the basis of the criterion of IUPAC, macropores refer to pores with the hole

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diameter larger than 50 nm, mesopores represent pores with the hole diameter between 2 nm and 50 nm, while micropores possess the hole diameter smaller than 2 nm. Therefore, it is necessary to determine the hole diameter (D = 2r) of porous starch. Notably, the accurate hole radius (r) of porous starch can be easily measured by water vapor desorption method using the following equation [30]: r

2Vw w c os p RT ln 1 p2

(3)

where VW is the molar volume of adsorbed water vapor, γw means the surface tension of water, α (deg) corresponds to the contact angle of water solid phase, T represents temperature, R is the water vapor constant, and P2 is the vapor pressure of the equilibrium 52

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surface. The total pore volume (vt) can be calculated from the mass (Mt) and density (ρ) of the adsorbent as follows: vt 

Mt

(4)

t

Existing studies have demonstrated that total hole volume and hole diameter of

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porous starch are significantly affected by the preparation method of porous starch and the

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type of starch. Xie et al. [13] found the total hole volume of B-type wheat porous starch

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was significantly larger than those of A-type wheat porous starch. As summarized in

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Table 3, porous starch prepared by the combination of enzymes hydrolysis and repeated

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heat-moisture treatment shows the largest total hole volume. Zhao et al. [14] suggested that freeze-thawing could enhance the hole size of porous starch, while enzymatic

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hydrolysis and the combined treatment decreased the hole size.

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3.2. Scanning electron microscopy (SEM) SEM is usually used to observe the morphology of porous starch. Significant differences can be observed between native and porous starches. For native starch, its surface is generally smooth. However, the surface of porous starch is rough with visible holes extending from the surface to the interior. Benavent-Gil et al. [48] concluded that the morphological change degree of starch highly depended on the source of starch, which was due to the difference in the susceptibility of starch. The morphology of porous starch is also affected by the preparation method used [7, 64]. Porous starch prepared by physical methods has relatively larger hole diameter [65].

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3.3. Transmission electron microscopy (TEM) and atomic force microscope (AFM) Transmission electron microscopy (TEM) and atomic force microscope (AFM) have been seldom used to observe the morphology of porous starch. TEM images can supply more information on the internal structure, hole size and distribution of porous starch [37]. The molecular weight, pore depth and some surface details of porous starch can be

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evaluated by AFM [66-67]. However, the use of these technologies in porous starch seems to be disjointed in recent years. This may be related to the allocation of experimental

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equipment in different laboratories. In contrast, the configuration and application of SEM

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is more common.

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3.4. X-ray diffractometry (XRD)

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Porous starch can be characterized by XRD. Generally, native starch has the

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semi-crystalline character with several narrow and sharp diffraction peaks on the XRD pattern. Notably, native and porous starches showed the same type of crystalline pattern

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with strong diffraction peaks at the same locations. However, changes of relative crystallinity after enzymatic preparation are controversial. A little increase in the relative crystallinity of porous starch was observed by Zhang et al. [68]. However, other researches found that there was no significant difference in the relative crystallinity between native starch and porous starch obtained by enzymatic hydrolysis [46, 51]. In fact, enzymatic hydrolysis is performed simultaneously in amorphous and/or crystalline regions, depending on the type, activity and dosage of enzymes [10, 45]. As reported, the crystalline degree of starch decreases when other methods (e.g. ultrasound and alcohol-alkaline treatments) are used [58]. When enzymatic hydrolysis is combined with 52

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other methods, it is necessary to determine the crystallinity of porous starch by XRD. In the future, more XRD experiments are required to reveal the principle of crystallinity change for porous starch. 3.5. Fourier-transform infrared (FT-IR) spectroscopy FT-IR spectroscopy is sensitive to the changes in the structure of starch granules [43,

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69]. The location of the characteristic bands of porous starch is not significantly different from that of native starch. However, the intensity of the characteristic bands of porous

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starch decreased obviously after hole-formation, which is due to the decrease in the

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density of starch granules [68, 70]. FT-IR spectroscopy has been also used to reveal the

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intermolecular interactions between porous starch and other molecules [46]. Jiang et al.

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[46] found all the characteristic bands of porous starch are consistent with those of native

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starch. When chitosan was blended with porous starch, the O–H stretching vibration at 3410 cm−1 shifted significantly to 3387 cm−1 and the N–H stretching of chitosan

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decreased from 1598 cm−1 to 1565 cm−1, suggesting that the intermolecular hydrogen bonds (O–H–N and O–H–O) formed between starch and chitosan. 3.6. Nuclear magnetic resonance (NMR) NMR technology has been widely used to explore the pore connectivity and pore size distribution of porous starch. This technology relies on the fact that the surface of porous starch serves as a powerful relaxation sink for horizontal magnetization of solvent proton. Brownstein-Tarr theory can be employed to discuss the pore size distribution of porous starch by analyzing the relaxation time [71]. Generally, relaxation time is divided into longitudinal relaxation time T1 and 52

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transverse relaxation time T2. However, the measurement speed and repeatability of T1 is poor [72]. Therefore, only T2 is measured in actual experiments [73]. The transverse relaxation time T2 is further divided into volume relaxation time T2bulk, surface relaxation volume T2s and diffusion relaxation time T2D as presented in the following equation: 1 1 1 1    T2 T2b u l k T2 s T2

(5) D

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where the volume relaxation time T2bulk is an inherent property of the fluid, and the

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diffusion relaxation T2D can be ignored by using a smaller echo interval [74]. Thence, the

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equation can be simplified and used to calculate the hole radius of porous starch as

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follows:

(6)

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1 1 C   s T2 T2bulk r

and C is a constant.

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where ρs is the surface relaxation coefficient, r is to the hole radius of porous starch,

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Obviously, the pore size of porous starch can be easily evaluated by the transverse relaxation time T2 of solvent. Therefore, this method has been applied to study the structure of porous starch [67, 75]. Solvents including water and acetone are commonly selected for the analysis of porous starch by NMR [76]. In addition, NMR results show the relaxation time and proton diffusion coefficient of polar solvents decrease after the addition of porous starch, indicating strong intermolecular interactions form between porous starch and polar solvents [77-78]. 4. Physicochemical properties of porous starch 4.1. Adsorption property 52

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Adsorption property is a representative ability of porous starch. Waste (e.g. sewage, oil and gas) often requires certain adsorption treatments, while conventionally used adsorbents (such as carbon, silica gel and alumina) are expensive and non-biodegradable [12]. Meanwhile, the adsorption efficiency of some biodegradable adsorbents (e.g. sodium alginate and chitosan) is very limited. In recent years, porous starch has been considered

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as a new type of adsorbent. The production process of porous starch is simple, adaptable and adjustable.

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The adsorption efficiency (Qt) of porous starch is often calculated by the following

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w  w0 w0

(7)

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Qt 

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equation [79]:

where w0 is weight of dried porous starch sample, and w is to the weight of porous

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starch under adsorption equilibrium.

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The adsorption isotherm is usually determined to analyze the relationship between adsorbate and porous starch. Monolayer and multilayer adsorption are two possible conditions during the absorption, which can be analyzed through Langmuir and Freundlich isotherm, respectively [80-81]. Langmuir isotherm model (Equation 8) is generally used in the monolayer adsorption. This model assumes a homogeneous surface with regard to the energy of adsorption, in which there is no interaction exists between adsorbate and all the adsorption sites are equally available to the adsorbate [82]. 1 1 1 1    Qe K l Qm Ce Qm

52

(8)

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where Qe is the total adsorption amount, Qm is the saturated adsorption amount of the single layer molecule, Ce is the equilibrium concentration of the adsorbate, and Kl is the Langmuir adsorption coefficient. Unlike Langmuir isotherm model, the Freundlich isotherm model (Equation 9) is generally used in the multilayer adsorption. This model assumes the non-ideal adsorption

1 l nQe  l nK f   l C ne n

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on heterogeneous surfaces. (9)

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where Qe is the total adsorption amount, Kf is the Freundlich adsorption coefficient,

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Ce is the equilibrium concentration of adsorbate, and n is the constant.

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Generally, the adsorption isotherm is preliminarily studied by calculating adsorption

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efficiency (Qt) as a function of the equilibrium concentration (Ce). Then, the obtained data

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are fitted with Langmuir and Freundlich isotherm models. The correlation coefficients of the two isotherms are used to judge which model the isotherm belongs to. Xiang et al.

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[82] found the adsorption of Cu2+ by porous starch fitted the Langmuir model. In general, the adsorption process of porous starch includes three main steps. Firstly, the adsorbate transfers from the solution to the adsorbent surface. Subsequently, the adsorbate is adsorbed on the surface and further movements of the adsorbate occur. Finally, the adsorption of porous starch reaches dynamic equilibrium. However, the adsorption kinetics of porous starch at different stages is different. Therefore, it is essential to study the adsorption kinetics of porous starch through pseudo first-order and pseudo second-order dynamic models [83-84]. The equation of pseudo first-order model is as follows: 52

Journal Pre-proof ln  Qe  Qt   ln Qe  k1t

(10)

where Qe is the total adsorption amount, t is to the time, Qt is the amount of adsorption at time t, and K1 is the constant of pseudo first-order model. The equation of pseudo second-order model is as follows: t 1 1   t 2 Qt K 2  Qe Qe

(11)

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where Qe is the total adsorption amount, t is to the time, Qt is the amount of

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adsorption at time t, and k2 is the constant of pseudo second-order model.

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Table 4 summarizes the adsorption conditions and efficiency of porous starch for

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different substances. Notably, the adsorption is generally achieved at suitable conditions

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(25−30 °C, 2−12 h, 100−300 rpm, pH 5.0). The adsorption kinetics of porous starch is more relevant to the pseudo second-order model when it is used to adsorb heavy metal

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ions, gardenia yellow and methylene blue dye [46, 85]. By contrast, the adsorption

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kinetics of porous starch is relevant to the pseudo first-order model when it is applied to adsorb functional substances, such as procyanidins [51]. Qian et al. [86] compared the adsorption ability of porous starch prepared by single freezing process with that prepared by dual freezing process, and found porous starch from the dual freezing process showed better adsorption capacity for methylene blue adsorption, but lower adsorption capacity for moisture and oil. Therefore, it can be concluded that the adsorption property of porous starch is affected by the adsorption conditions and the preparation method of porous starch as well as the nature of adsorbate. Table 4 4.2. Solubility and swelling power 52

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One of the main purposes to prepare porous starch is to increase the solubility of native starch. The complete dissolution of starch is a relatively slow process, which can be divided into two stages (i.e. swelling and dissolution). Swelling refers to the process by which solvent molecules diffuse into the interior of starch granules, increasing starch volume. If the solvent molecules continue to infiltrate, the macromolecular segment

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movement of starch increases gradually. The macromolecular segment slowly enters the solution through the coordinated motion to form a stable dissolution phase [87-88].

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Many studies have proved that solubility and swelling power of native starch are

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enhanced after hole-formation [89-90]. The amorphous regions of native starch are

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disrupted during the hole-forming process, allowing starch granules to expand freely.

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Singh et al. [91] reported that acid treatment significantly increased the swelling power of

4.3. Rheological property

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starch due to the structural weakening and de-polymerization of starch.

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Rheological property of starch reflects the flow behavior of starch as a function of stress [83]. The native starch solution shows a poor rheological property at room temperature even at a high sample concentration [92-93]. Therefore, it is necessary to heat starch suspension to the gelatinization temperature, where starch granules begin to lose crystalline structure, absorb water and showed improved rheological properties [94]. So far, only very few studies have focused on the rheological property of porous starch [95-96]. Benavent-Gil et al. [95] revealed that storage modulus (G'), loss modulus (G") and loss tangent (tan δ = G"/G') were related to the porosity of porous starch. In general, G' and G" of starch significantly increase with the increase of temperature. The initial 52

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increase of G' can be attributed to the expansion of starch granules. As the temperature further increasing, a decrease in G' happens because of the melting of remaining crystallites. Obviously, the loss of structural integrity in porous starch leads to lower G' and G'' values. Kochkina et al. [96] found the tan δ value increased with the increase in the porosity of porous starch, revealing porous starch exhibited solid-like behavior. In the

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future, the effects of temperature, pH and the source of starch on the rheological property

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of porous starch can be studied. 4.4. Pasting property

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Pasting property of porous starch is usually studied following the heating-cooling

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cycle by a rapid visco analyzer [97]. Several parameters can be observed through the

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pasting curve, reflecting the degree of starch changes and retrogradation after porous

na

modification [98]. In general, the integrity of starch granules is disrupted after hole-formation, making them more susceptible to breakage and less resistance to shear

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force during the heating process. In addition, porous starch displays a higher setback than native starch, which may be related to the recrystallization of amylose chains. This phenomenon suggests that fractional amylose chains are leached from starch granules, and they are able to form helical structure [99]. Compared with native starch, porous starch obtained from α-amylase (EC 3.2.1.1) catalysis exhibits a similar swelling and hydration, but a lower maximum viscosity after the heating-cooling cycle. However, starch treated by amyloglucosidase (EC 3.2.1.3) shows a higher maximum of viscosity after cooling [45]. Therefore, the pasting property of porous starch is greatly influenced by the type of enzyme. Fortuna et al. [100] proposed the pasting temperature of porous 52

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starch was closely related to the pore volume and the specific surface area of porous starch. However, no relationship was found between the pasting property and the average pore diameter of porous starch. Benavent-Gil et al. [10] further proved that peak viscosity is negatively correlated with the pore area and the size of porous starch. Moreover, porous starch prepared by the combination of enzymatic hydrolysis and freeze-thaw showed a

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lower peak viscosity as compared to that prepared by enzymatic hydrolysis. This further

ro

suggests the pasting property of porous starch is related to its preparation method. 4.5. Thermal property

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The thermal property of starch, usually determined by thermogravimetric analysis

re

(TGA), reveals the changes in the internal chemical bonds (e.g. hydrogen bonding) and

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the crystalline structure of starch. As reported, a high crystalline degree makes starch

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granules more resistant towards the temperature change [93, 101]. TGA provides information on the thermodynamic property of porous starch. The initial weight loss of

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porous starch is caused by the loss of water, and the following weight loss stages are owing to the decomposition of porous starch [102]. The peak maxima of differential thermogravimetric (DTG) analysis reveal the maximum loss rate of starch. Lastly, the residual char yield in TGA curves is generally used to analyze the thermal stability of starch. Lei et al. [103] found the peak maximum of starch moved to lower temperatures after hole-formation, indicating porous starch had a lower thermal stability than native starch. The similar phenomenon was also observed by Wu et al. [104]. When porous starch is blended with gelatin, the obtained composite materials show enhanced thermal stability [105]. 52

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4.6. Gelatinization property The gelatinization property, usually evaluated by differential scanning calorimetry (DSC), provides information for the industrial applications of starch. Notably, the crystalline double helix structure of starch is unwrapped during the gelatinization process. Then, starch granules begin to lose the maltese cross and finally become amorphous. Till

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now, several studies have demonstrated the gelatinization enthalpy of porous starch is higher than that of native starch [14, 70, 106]. This is because the hydrolysis of starch will

ro

lead to the decrease in the proportion of amorphous region and the increase in the

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proportion of crystalline region. In addition, the hydrogen bonds and double helix

re

structure of starch are broken with the increase of temperature. As a result, more energy is

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required to destroy the crystalline region in starch granules.

5.1. Food industry

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5. Potential applications of porous starch

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Due to its biodegradable, non-toxic and biocompatible properties, porous starch has been used to encapsulate and slowly release food ingredients. The encapsulation can greatly improve the stability of different substances, such as oils, flavoring agents and probiotics. Almost all the oils are easily oxidized when exposed to high temperature, light, oxygen and moisture during storage, which eventually leads to unpleasant taste and odor. The encapsulation oil in porous starch prolongs the shelf-life and the stability of oil [81]. In most cases, porous starch is mechanically agitated with oil to promote the osmosis and diffusion of oil under suitable conditions (oil/starch = 4/1, 40 °C, 70−80 min) [107]. The encapsulation can greatly improve the stability of flavor [108]. In order to guarantee a 52

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sufficient number of viable bacteria, porous starch was used to encapsulate Lactobacillus plantarum. Briefly, bacterial culture and porous starch were stirred for 3 h. The sample was precipitated for 2 h, followed by lyophilized overnight after cooled in a refrigerator for 4 h. Native maize starch is gelatinized and coated as the coating material on the surface of porous starch. The encapsulated polymer was recrystallized overnight, and then

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freeze-dried. Results showed that L. plantarum encapsulated in porous starch could resist against different stress conditions without affecting the survival ratio of the bacteria [52].

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starch (10%, w/v) showed enhanced stability.

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Xing et al. [64] also demonstrated that Lactobacillus acidophilus encapsulated in porous

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Common additives in foods (such as flavoring agents and nitrogen-containing

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volatiles) require slow release to achieve optimal performances. The liquid tomato flavor

na

was plated on porous starch and then spray-dried to achieve the desired performance. Results revealed the importance of solvents for porous starch in carrying liquid flavors

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[108]. Qiao et al. [109] suggested that potato porous starch-based polymer showed the best slow-release property for nitrogen, which contributed to the production of various food products in the future. Undoubtedly, all applications of porous starch in food industry is to improve the stability of food materials. Notably, the structure of porous starch is prone to change when the environmental conditions change (e.g. temperature, and humidity). However, there have been no studies focus on improving the stability of porous starch, which may be an interesting topic in the future. 5.2. Pharmaceutical industry Porous starch-based hydrogels can be used as the carrier of controlled drug delivery 52

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and the substitute of bone/cartilage tissue in pharmaceutical industry since they are non-toxic, plastic and biocompatible. Controlled drug release technology is developed to improve the efficacy and convenience of drug. The preparation process of melatonin-loaded porous starch is shown in Fig. 3 [11]. Melatonin molecules are absorbed into the pores of porous starch in acetone solution. At the molecular levels, melatonin and

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porous starch can interact with each other through hydrogen bonds. In addition, amorphous melatonin forms during the drying process of acetone. Since the free energy of

ro

amorphous melatonin is higher than that of crystalline melatonin, the solubility of

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amorphous melatonin is remarkably increased. Therefore, the introduction of porous

re

starch significantly improves the bioavailability, solubility and bioactivity of melatonin.

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This technology has been also applied in the delivery of poorly water-soluble and

Fig. 3

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solid-state lipid-based drugs, such as lovastatin, probucol and carbamazepine [110-111].

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Since porous starch is almost completely hydrolyzed when it reaches the gastrointestinal tract, some drugs that work in the colon are difficult to have an effect. By contrast, some biopolymers (e.g. chitosan and pectin) are able to pass the gastrointestinal tract and then decomposed by secreted enzymes in the colon. Zhu et al. [18] designed a colon targeted drug (doxorubicin) delivery system based on porous starch and pectin-chitosan complex. As shown in Fig. 4, porous starch was first mixed with doxorubicin at 40 ºC for 2 h. Then, pectin and chitosan were added into the mixture, mixed with high-speed shearing, and then degassed under vacuum. Finally, the composite material is injected into the calcium chloride (CaCl2) solution by the needle. The pectin on 52

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the surface of composite material could immediately form gels through the action between Ca2+ and the carboxylic groups of pectin. The obtained composite material showed controlled release ability of doxorubicin in simulated stomach and small intestine conditions. Fig. 4

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Porous starch is also an important biomaterial as bone/cartilage tissue substitute. Wu et al. [67] provided a new idea for the application of porous starch in bone/cartilage tissue

ro

substitute. Firstly, nano-sized graphene oxide is covalently attached to tapioca starch

-p

through an esterification reaction. The nano-sized graphene oxide can act as an effective

re

anchoring site to induce the recrystallization of calcium phosphide (CaP) in the simulated

lP

body fluid. As a result, the functionalized porous starch scaffold can induce the

na

production of hydroxyapatite, which is close to human bone. Although this bone/cartilage tissue substitute is similar to human bones in terms of the mechanical strength, this study

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did not consider whether it would cause rejection of the human body in practical applications. Unfortunately, most researches of porous starch in the pharmaceutical industry only reveal its excellent properties, ignoring the problems it may face in practical applications (e.g. metabolism, emergency, etc.). Therefore, more in-depth animal and cellular experiments are needed to verify the feasibility of porous starch applications. 5.3. Environmental industry Numerous studies indicate the adsorption of porous starch is an alternative method among various environmental techniques since porous starch-based adsorption has high efficiency, low operating cost and ease of handling [15, 112]. Till now, porous starch has 52

Journal Pre-proof been used for the treatment of different kinds of heavy metals (such as Zr4+, Al3+, Fe3+, La3+, Pb2+ and Cu2+) and methylene blue in the wastewater [46, 51] (Table 4). In general, the surface of porous starch is homogeneous with regard to the energy of adsorption. Therefore, single molecule adsorption occurs easily in the surface of porous starch. In addition, there is an adsorption field in a certain space near the adsorbent, and the adsorbed molecules are prone to re-adsorption, namely, multilayer adsorption. Notably,

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heavy metal ions with high valences (e.g. Zr4+, Fe3+ and La3+) tend to multi-layer

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adsorption, while those with low valences (e.g. Pb2+ and Cu2+) are prone to single-layer

-p

adsorption.

re

In order to enhance the adsorption efficiency of porous starch towards metal ions, a

lP

novel porous composite material was developed based on porous starch and three

na

dimensions graphene oxide (3D GO) through a facile hydrothermal method [17]. 3D GO was sonicated with porous starch and the obtained mixture was reacted in a special

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stainless steel autoclave for 10 h to afford the composite material. Oxygen-containing groups (e.g. hydroxyl and carboxyl groups) in the surface and sheet of 3D GO could react with the hydroxyl groups of starch molecules (Fig. 5). The 3D GO/porous starch composite material showed good adsorption capacity towards different heavy metals, such as Pb2+, Cu2+, Cd2+, Yb3+ and Nd3+. Although porous starch is a kind of safe, non-toxic and highly efficient adsorbent, it also has some certain problems. For example, recent researches only focus on its adsorption properties, while the thermal and storage stability of porous starch have been ignored for a long time. In addition, the adsorbent requires a certain degree of mechanical 52

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strength and wear resistance, and it is obvious that porous starch does not have these characteristics. However, recent researches only focus on new preparation methods and simple applications, and there has been no study reveals the possibility of improving the mechanical strength and wear resistance of porous starch. Therefore, this may also be an interesting point in the research of porous starch.

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Fig. 5 6. Conclusions and future perspectives

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Porous starch can be obtained through physical, chemical, enzymatic and synergic

-p

methods. Among these methods, extrusion and freeze-thawing methods usually produce

re

porous starch with large pore sizes. Notably, the combination of enzymatic catalysis and

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other methods is the most effective approach to prepare porous starch. In the future, it is

na

necessary to optimize the preparation conditions of porous starch to obtain a product with an ideal pore structure. The structure of porous starch is generally revealed by PSA, SEM,

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XRD, FT-IR and NMR. The determination of the pore size, specific surface area and total pore volume of porous starch by PSA are very important since these characters are closely related to the adsorption property of porous starch. The other physicochemical properties including solubility, swelling power, rheological, pasting and gelatinization properties are associated with the application of porous starch in the food industry. Meanwhile, the preparation and properties of porous starch might be closely related to the source and particle size of native starch granules, which should be deeply investigated in the future. Till now, porous starch has been used to encapsulate instable food ingredients and drugs and adsorb heavy metals. Therefore, porous starch has potential applications in food, 52

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pharmaceutical and environmental industries. In the future, porous starch can be functionalized with other biopolymers or inorganic materials to enhance the thermal and storage stability of porous starch. Moreover, more in-depth animal and cellular experiments are needed to verify the feasibility of porous starch in the applications of the pharmaceutical industry. Notably, researches on mechanical strength and wear resistance

of

of porous starch are disjointed and limited to some extent, which may be an interesting

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topic in the study of porous starch in the future. Acknowledgments

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This work was supported by Grants-in-Aid for scientific research from National

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Natural Science Foundation of China (No. 31571788), Qing Lan Project of Jiangsu

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Province, China Agriculture Research System (CARS-41) and the Priority Academic

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Program Development of Jiangsu Higher Education Institution.

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starch

crosslinked

with

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Figure captions Fig. 1. A summary of different preparation methods of porous starch. Fig. 2. Mechanism diagrams for the preparation of porous starch by microwave method (A), ultrasonic method (B), solvent exchange (C) and enzymatic hydrolysis (D). The a and b represent high and low pressure areas in starch granules, respectively.

of

Fig. 3. The preparation process and internal structure of melatonin-loaded porous starch (adapted from Li et al. [11]).

ro

Fig. 4. The preparation scheme of pectin-chitosan/porous starch complex for the

-p

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Fig. 5. The structure of graphene oxide derived porous starch scaffold.

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Journal Pre-proof Table 1 The preparation conditions and structural features of porous starch obtained by different physical methods. Methods Extrusion method

Starch source Lentil flour Rice starch

Microwave method Ultrasonic method

Lotus starch Corn starch Waxy maize starch Rice, corn, wheat and potato starches Corn starch Plantain and taro starches Waxy corn starch Potato starch Jicama starch Corn starch

ND, not determined.

Preparation conditions 900 rpm, 95−180 ºC, 10% and 20% of water content 80 rpm, 110−160 ºC, 10–30% of water content 500−800 W, 1−5 min, 50 g 400−800 W, 2−19 MPa, 100-300 g 1600 W, 5 min, 10 g 20 kHz, 170 W, 30 min

References [20]

8−15 nm, 1950−3850 nm

[30]

f o

15−30 nm 0.1−40 nm 100−300 nm 1.7−300 nm

[24] [25] [31] [32]

ND ND

[27] [28]

20−60 nm < 100 nm ND 20−200 nm

[33] [34] [35] [36]

o r p

e

r P

20 kHz, 25 kHz, 20 kHz + 25 kHz 25 kHz, 80 W, 20 and 50 min

l a

Pore size ND

40 kHz, 500 W, 180 min 22 kHz, 100 W, 30 min 20 kHz, 240, 320, 400, 560 W, 5 min 24 kHz, 30 W, 4, 8 and 16 min

n r u

o J

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Table 2 The preparation conditions and structural features of porous starch obtained by enzymatic methods. Enzyme type α-AM, AMG, CGTase, GBE α-AM, AMG

Starch source Corn starch

Reaction conditions pH 6.0 and pH 4.0, 50 ºC, 2 h

Pore size 0.01−1.15 μm

References [10]

Corn starch

pH 6.0 and pH 4.0, 50 ºC, 24 h

[45]

α-AM, AMG α-AM, AMG, CGTase

Rice starch Potato, rice, wheat and cassava starches Corn starch Corn and rice starches

pH 5.4, 45 ºC, 3 h pH 6.0 and pH 4.5, 50 ºC, 2 h

0.11−0.19 μm 1.54−1.90 μm 0.005−0.145 μm 0.01−1.50 μm

CGTase α-AM, AMG

e

o r p

r P

f o

pH 6.0 and pH 4.0, 50 ºC, 24 and 48 h 30 ºC, 24 h

[46] [48]

ND [49] 2 0.03−0.13 μm [50] 2 0.19−0.59 μm α-AM, AMG Corn starch pH 5.4, 50 ºC, 4 h 1.0 μm [51] α-AM Maize starch pH 4.6, 37 ºC, 2 h ND [52] ND, not determined. α-AM, α-amylase; AMG, Amyloglucosidase; CGTase, cyclodextrin-glycosyltransferase; GBE, glycogen branching enzyme.

l a

n r u

o J

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Table 3 The preparation conditions and structural features of porous starch obtained by synergic methods. Method 1 Enzyme

Reaction Method 2 condition 1 1.5 h, pH 5.6, Ultrasound 60 ºC 24 h, pH 6.0, 45 ºC 5−10 h, pH 5.0−6.0, 55 ºC 6 h, pH 5.4, 45 ºC 14 h, pH 4.6, 40 ºC 2 h, pH 5.4, 37 ºC 11 h, pH 4.6, 55 ºC 8 h, pH 5.8, 50 ºC

Inverse cross-linkin g

55 °C, 10 h

Cross-linkin g

Extrusion

Reaction condition 2 40 kHz, 420−540 W, 40−60 ºC, 20−40 min 35 kHz, 240 W, 40 ºC, 20−60 min 40 kHz, 100 W, 55 ºC, 2−10 min 50−100 rpm, 2 h, 40 ºC 200 rpm, 80 min, 50 ºC 150 rpm, 50−88 ºC, 2.5 kg/h −15 ºC,3 h

l a

n r u

o J

Freeze-thaw ing Heat-moistu re Alcohol-alk aline

45 ºC, 24 h, 25 ºC, 20 h, 100 ºC, 5 h 65 °C, 10 min 55 °C, 5 min

Starch source Corn starch

HD

THV

ND

ND

Wheat starch Cassava starch Corn starch Corn starch Corn starch Corn starch Wheat starch

ND

53

References

ND

[60]

ND

[54]

ND

ND

[11]

10−12 nm 800−15 00 nm 10−20 μm 4−30 μm 3−75 nm

9.79 ± 0.10

684.50 ± 0.12

[12]

7.05 ± 0.34

ND

[61]

ND

ND

[57]

ND

ND

[14]

30.45 ± 0.79

10.75 ± 0.14

[13]

ND

ND

ND

[58]

ND

f o

ro

p e

r P

Potato starch

SSA

ND

Journal Pre-proof Film casting

100 min

°C,

30 Salt leaching

40−90 nm

40 °C, 50% relative Modifie humidity d starch

ND

ND

ND, not determined. HD, hole diameter. THV, total hole volume (cm3/kg). SSA, specific surface area (m2/g).

f o

l a

e

o r p

r P

n r u

o J

53

[59]

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Table 4 The adsorption conditions and efficiency for different substances adsorbed by porous starch. R2 2

References

0.999

0.992

[51]

0.996

0.987

[46]

0.888 0.964 0.635 0.789 ND

1.000 0.998 0.999 0.977 0.999

[85]

ND ND

0.999 0.999

[79]

20.5

0.945

0.999

[82]

72.0

0.983

0.999

[43]

Synthesis method

Starch source

Adsorption conditions

Adsorbents

Enzymatic hydrolysis

Corn starch Rice starch Corn starch

25 °C, 6 h, 120 rpm 25 °C, 2 h, 120 rpm 25 ºC, 2 h, 300 rpm, pH 5.0

Grape seed Proanthocyanidins Procyanidins 96.0

Potato starch Potato starch

30 °C, 2 h, 100 rpm 30 °C, 2 h, 100 rpm, pH 5.0 25 °C, 12 h, pH 5.0 25 °C, 2−11 h, 100 rpm

Solvent exchange

Insertion of mercaptosuccinic acid

Corn starch Potato starch

Zr4+ Al3+ Fe3+ La3+ Pb2+

l a

rn

J

u o

Pb2+ Methylene dye Cu2+

Adsorption efficiency (mg/g) 18.9

r P

277.0 blue 180.8

Gardenia yellow

f o

o r p

e 173.8 18.9 109.5 22.6 109.1

R12

[8]

R12, correlation coefficient of pseudo first-order model; R22, correlation coefficient of pseudo second-order model; ND, not determined.

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Highlights: Recent advances of porous starch are summarized. Porous starch can be produced by physical, chemical, enzymatic and synergic methods. Porous starch shows enhanced adsorption efficiency, solubility and swelling power.

Jo ur

na

lP

re

-p

ro

of

Porous starch can be used to adsorb, encapsulate and release different substances.

53

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5