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Fabrication and characterization of starch nanohydrogels via reverse emulsification and internal gelation Na Ji, Yang Qin, Man Li, Liu Xiong, Lizhong Qiu, Xiliang Bian, and Qingjie Sun J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.8b02601 • Publication Date (Web): 15 Aug 2018 Downloaded from http://pubs.acs.org on August 16, 2018
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Journal of Agricultural and Food Chemistry
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Fabrication and characterization of starch nanohydrogels via reverse emulsification
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and internal gelation
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Na Ji† Yang Qin† Man Li†
Liu Xiong†
Lizhong Qiu‡ Xiliang Bian‡ Qingjie Sun†*
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† College of Food Science and Engineering, Qingdao Agricultural University, Qingdao,
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Shandong 266109, China
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‡ Zhucheng Xingmao Corn Developing Co., Ltd (Weifang, Shandong Province, 262200, China)
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*Correspondence
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[email protected]), College of Food Science and Engineering, Qingdao Agricultural University,
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266109, 700 Changcheng Road, Chengyang District, Qingdao, China.
author
(Tel:
86-532-88030448,
Fax:
86-532-88030449,
e-mail:
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ABSTRACT: Biopolymer-based nanohydrogels have great potential for various applications,
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including in the food, nutraceutical, and pharmaceutical industries. Herein, starch nanohydrogels
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were prepared for the first time via reverse emulsification coupled with internal gelation. The
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effects of starch type (normal corn, potato, and pea starches), amylose content, and gelation time
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on the structural, morphological, and physicochemical properties of starch nanohydrogels were
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investigated. The diameter of starch nanohydrogel particles was around 100 nm after 12 h of
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retrogradation time. The relative crystallinity and thermal properties of starch nanohydrogels
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increased gradually with increasing amylose content and with increasing gelation time. The
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swelling behavior of starch nanohydrogels was dependent on the amylose content, and the
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swelling ratios were between 2.0 and 14.0, with the pea starch nanogels exhibiting the lowest
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values and the potato starch nanogels the highest values.
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KEYWORDS: Hydrogels; Preparation; Retrogradation; Swelling, Amylose
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INTRODUCTION
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Nanohydrogels, as one of the most attractive soft materials used in the food, nutraceutical,
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and pharmaceutical industries, possess a unique three-dimensional network structure and
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nanoscale size.1 The network structure of nanohydrogels can hold a large amount of water within
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an inter-connected porous structure, which has swelling and de-swelling properties.2–4 They have
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drawn increasing international research attention because of their widespread applications in
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modern high-tech fields, such as in the food and biotechnological industries,5–6 as drug delivery
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carriers,7 and in tissue engineering.8
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Generally, hydrogels are formed by chemically or physically cross-linking polymers. The
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utilization of chemical hydrogels may have limited applications because of the use of toxic
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chemicals or cross-linking agents during hydrogel processing.9–10 To address this issue, several
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research groups have developed attractive strategies to introduce reversible and non-covalent
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interactions—such as hydrogen bonding,3 ionic bonding,9 and hydrophobic11 and electrostatic
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interactions12—to replace sacrificial covalent bonds.4 Moreover, hydrogels based on natural
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biopolymer materials, especially polysaccharides, have attracted increasing attention due to their
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biocompatibility, innate biodegradability, and critical biological functions.13–14 For instance,
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hyaluronic acid,15 chitosan,13 pullulan,16 cellulose,17 and gelatin,18–19 are widely used for
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fabricating hydrogels. Moreover, alginate,20 pullulan,21 chitosan,22 and dextran23 are used to
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prepare nanogels.
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Among the various types of natural biopolymers, starch is the most commonly consumed
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polysaccharide. It is obtained from renewable sources and offers important advantages, such as
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cost-effectiveness, biocompatibility, biodegradability, and nontoxicity.24–25 During retrogradation,
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gelatinized starch can readily form a three-dimensional network via physical entanglement of
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amylose and/or amylopectin, which is particularly suitable for fabricating hydrogels.26 Recently,
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physical cross-linking, starch-based zwitterionic hydrogels are being developed as a promising
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material for biomedical applications.25 Moreover, starch has been used to prepare nanoparticles,27–
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micelles and vesicles,30 and nanocapsules31–32 for delivery of active substances.
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In this study, we developed physically cross-linked nanohydrogels using starch prepared by
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reverse emulsification coupled with an internal gelation process. There is no detailed information
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about the starch types on the formation and characterization of starch nanohydrogels. Therefore,
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the objective of this investigation was to undertake a comprehensive study of characterization of
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starch nanohydrogels fabricated by normal corn (A-type), potato (B-type) and pea (C-type)
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starches. The microstructures and physicochemical properties of starch nanohydrogels were
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investigated via transmission electron microscopy (TEM), Fourier transform infrared (FTIR)
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spectroscopy, X-ray diffraction (XRD), and differential scanning calorimeter (DSC). This study
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outlines a facile assembly approach to the preparation of physically cross-linked starch
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nanohydrogels, which could have wide applications in the food, medical, and cosmetic industries.
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MATERIALS AND METHODS
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Materials. Normal corn starch (approximately 25.9% amylose content), potato starch
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(approximately 24.3% amylose content), and pea starch (approximately 32.2% amylose content)
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were purchased from Qingdao Haidaer Starch Co., Ltd. (Qingdao, China). Soybean oil was
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obtained from a local supermarket (Qingdao, China). Span 80 was purchased from Sigma-Aldrich
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(St. Louis, MO). All reagents used were of analytical grade.
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Starch gel preparation and characterization. Starch gel was made in an RVA test tube
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according to the procedure of Lu et al. (2012).33 Starch (3.0 g, 14% moisture) was weighed
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directly in 25 ml distilled water and underwent a controlled heating and cooling cycle under
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constant shear. After RVA testing, starch paste samples were covered in Parafilm and stored at 4°C
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for 6, 12, 24, and 36 h. The textural parameters of hardness, cohesiveness, springiness, gumminess,
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and chewiness were computed using the method of Sun et al. (2014b).34
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The cooked starch pastes were placed under oscillatory shear during retrogradation and
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investigated using a dynamic rheometer (MCR102, Anton Paar, Austria) equipped with a 50-cm
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parallel plate geometry. After the RVA determinations, the samples were promptly moved onto
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the rheometer plate. Mechanical spectra were obtained by recording complex moduli as a function
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of frequency. The complex moduli were computed using the method of Sun et al. (2018).35
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Preparation of starch nanohydrogels. A suspension of 10 g of each type of starch sample
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(normal corn, potato, and pea starches) in distilled water (10%, w/w) was fully gelatinized at
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100 °C for 30 min and then cooled to 25 °C. In order to prepare W/O nanoemulsions, 30 g of
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starch solution were slowly mixed into the soybean oil phase (70 g) containing Span 80 (1% w/w,
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based on the volume of emulsions) while stirring with a high speed homogenizer (UltraTurrex T24,
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IKA Labotecnik), then subjected to ultrasonic processing for 20 s. The emulsions were stored at
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4 °C for different time intervals (6, 12, 24, and 36 h) to form starch nanohydrogels. To separate the
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formed nanohydrogels, the emulsions were dispersed in double distilled water via centrifugation at
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14,000 rpm for 10 min, and the centrifugation cycles were repeated several times to remove
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excess oil and surfactants. The nanohydrogels were vacuum freeze-dried to acquire the dried
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starch nanohydrogels.
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Particle size distribution of nanohydrogels. The mean hydrodynamic diameter of
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nanohydrogels was determined with a Zetasizer Nano-ZS (Malvern Instruments, UK). The freshly
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prepared nanohydrogels were diluted to 0.1 mg/mL with ultrapure water before analysis.
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TEM. The morphology of the nanohydrogels was characterized by TEM (Hitachi, Tokyo,
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Japan). A drop of freshly prepared sample was spread onto a carbon-coated copper grid and then
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lyophilized.
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XRD. The X-ray diffractogram was recorded using a Bruker D8 X-ray diffractometer
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(Brucker D8, Odelzhausen, Germany). All samples were tightly packed into the sample holder,
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and X-ray diffraction patterns were recorded in an angular (2θ) range from 5°–40° in 0.04° steps,
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with a count time of 1 s.
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DSC. The thermal properties were measured using a differential scanning calorimeter (DSC1,
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Mettler Toledo, Schwerzenbach, Switzerland). Ten milligrams of starch nanohydrogels were
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added to 20 µL of distilled water and sealed in aluminum pans. The specimens were heated at a
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constant rate of 10 °C/min from 25–100 °C under a nitrogen atmosphere.
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FTIR. The chemical structures of starch nanohydrogels were investigated using an FTIR
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spectrometer (NEXUS-870; ThermoNicolet Corporation, Madison, WI, USA) at wavenumbers of
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4,000–600 cm−1. The dried samples were well blended with KBr and pressed. Spectra were
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recorded as an accumulation of 256 scans at 4 cm−1 resolution.
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Swelling power. To analyze the swelling behavior of starch nanohydrogels, the samples were
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soaked in distilled water at 25 °C for 2 h. The nanohydrogel dispersion was then centrifuged for
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20 min at 2,000 rpm. The weight of the sediment was used to calculate the swelling power.
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The swelling powers were calculated on a dry basis using equation (1):
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Swelling ratio =
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(1)
where Wh is the weight of the product after hydration and Wd is the weight of the dried product.
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Statistical analysis. All results were reported as a mean ± standard, n = 3. The data were
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evaluated using the Statistical Package for the Social Sciences (SPSS) 17.0 (SPSS Inc., Chicago,
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IL). The significance level (P) was set at 0.05.
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RESULTS
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Starch gel hardness and rheological properties. In order to analyze the gel hardness of
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starch nanogels, which was difficult to measure directly, the bulk starch hydrogels were measured
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instead. The textural properties of different starch gels are shown in Table 1. For each starch gel,
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the hardness values increased during storage, and these values increased significantly (P
