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j.foodhyd.2017.10.028

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Accepted Manuscript
Preparation and characterization of a novel edible film based on Artemisia
sphaerocephala Krasch. gum: Effects of type and concentration of plasticizers
TieqiangLiang, LijuanWang
PII:
S0268-005X(17)31434-0
DOI:
10.1016/j.foodhyd.2017.10.028
Reference:
FOOHYD 4115
To appear in:
Food Hydrocolloids
Received Date:
20 August 2017
Revised Date:
22 October 2017
Accepted Date:
22 October 2017
Please cite this article as: TieqiangLiang, LijuanWang, Preparation and characterization of a novel
edible film based on Artemisia sphaerocephala Krasch. gum: Effects of type and concentration of
plasticizers, Food Hydrocolloids (2017), doi: 10.1016/j.foodhyd.2017.10.028
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
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ACCEPTED MANUSCRIPT
Highlights
(1) Edible film was prepared from Artemisia sphaerocephala Krasch. gum (ASKG).
(2) Sorbitol, glycerol and triethyl citrate were added as plasticizers.
(3) The effects of the type and concentration of plasticizers were investigated.
(4) Physical properties of ASKG film were significantly affected by plasticizers.
(5) Sorbitol is a suitable plasticizer for ASKG film.
ACCEPTED MANUSCRIPT
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Preparation and characterization of a novel edible film based on Artemisia
2
sphaerocephala Krasch. gum: Effects of type and concentration of
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plasticizers
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5
TieqiangLianga, b, LijuanWanga, b, *
6
aCollege
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b
of Material Science and Engineering, Northeast Forestry University, Harbin, PR China
Research Center of Wood Bionic Intelligent Science, Northeast Forestry University, Harbin, PRChina
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9
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*Corresponding
author: Lijuan Wang, E-mail: [email protected], Tel.: +86-451-82191693, Fax: +86-
451-82191693
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Abstract:
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The aim of this work was to evaluate the influence of type (glycerol, sorbitol and triethyl
14
citrate (TEC)) and concentration of plasticizers, on the physical properties of a novel film
15
from Artemisia sphaerocephala Krasch. gum (ASKG). The results showed that the tensile
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strength and the light transmission of the film decreased, whereas the elongation at break, the
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oxygen permeability, and the water vapor permeability of the film increased with increased
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plasticizer content. It was found that ASKG film containing TEC as plasticizer was brittle and
19
less flexible. Fourier-transform infrared analysis and rheological measurement revealed that
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plasticizer molecules may disrupt the intermolecular interaction and the entanglement network
21
between ASKG molecules, and form hydrogen bonds with ASKG chains. Thermogravimetric
22
analysis showed that the addition of plasticizer decreased the stability of the film. Scanning
23
electron microscopy showed a homogeneous cross-section of ASKG film with glycerol or
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sorbitol, however this became porous with the addition of TEC. Among the three plasticizers,
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sorbitol was found to be the most suitable for the film, making it more flexible and
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hydrophilic, and the tensile strength of the film reached 29.92–54.04 MPa. It is suggested that
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ASKG is a promising packaging material, and that ASKG-based film containing sorbitol as a
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plasticizer has potential as a packaging film.
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Key words: Artemisia sphaerocephala Krasch. gum; Plasticizers; Rheology; Physical
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properties; Film
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1. Introduction
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Petroleum-derived plastics have been used extensively in food packaging due to their
35
excellent properties for food protection, their ease of manufacture and their low weight.
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However, they are not biodegradable and remain in the environment for long periods because
37
of their stable bonding and very large polymer molecules (Muscat, Adhikari, Adhikari, &
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Chaudhary, 2012). The non-biodegradability and non-renewability of these plastics waste
39
from food packaging has resulted in the so-called “white pollution” phenomenon. Recently,
40
extensive efforts have been made to investigate the use of biodegradable and eco-friendly
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packaging materials made from renewable natural resources. The development of
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biodegradable and edible packaging film from biopolymers has the potential to increase
43
sustainability and decrease the adverse effects of plastic packaging on the environment
44
(Yakimets, et al., 2007). Biopolymers are natural materials with significant advantages over
45
synthetic materials in food, pharmaceuticals, biomedical and packaging applications due to
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their range of sources and safety. However, existing materials have some limitations such as
47
their cost, availability and functional properties (Vieira, da Silva, dos Santos, & Beppu, 2011).
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Therefore, new biological sources are being continuously explored (Haq, Hasnain, Jafri,
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Akbar, & Khan, 2016). Usually, biopolymers, such as carbohydrates and protein derivatives,
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are sourced from plants, animal tissues and various microorganisms. Among them,
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polysaccharides are the preferred choice due to their higher stability, compatibility, and the
52
ease with which their physicochemical properties can be altered by mixing with other
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biopolymers (Pan, Jiang, Chen, & Jin, 2014). Many kinds of polysaccharides have been used
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to develop packaging film, such as tara gum (Antoniou, Liu, Majeed, Qazi, & Zhong, 2014),
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ghatti gum (Zhang, Zhao, & Shi, 2016), arabic gum (Dickinson, Elverson, & Murray, 1989),
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xanthan gum (Veiga-Santos, Oliveira, Cereda, Alves, & Scamparini, 2005), decolorized
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hsian-tsao leaf gum (Chen, Kuo, & Lai, 2009), gellan gum (Lee, Chen, & Tsao, 2010),
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cashew tree gum (Azeredo, et al., 2012) and guar gum (Saurabh, Gupta, Variyar, & Sharma,
59
2016). However, most polysaccharide films are brittle, and crack during use and storage. To
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increase the flexibility of polysaccharide films, non-volatile substances with low molecular
61
weight have been added as plasticizers (Antoniou, et al., 2014). The efficiency of these
62
plasticizers is related to their molecular size, shape, number of oxygen atoms, spacing of
63
oxygen atoms and water binding ability (Haq, Hasnain, & Azam, 2014). To date, polyols
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(glycerol, sorbitol, xylitol, polyethylene glycol and propylene glycol) (Antoniou, et al., 2014;
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Li & Huneault, 2011; Muscat, et al., 2012), esters (triethyl citrate, poly(3-hydroxybutyrate)
66
and diethyl phthalate) (Jae Shin Choi & Park, 2004; Li, et al., 2016) or vegetable oil (corn oil,
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castor oil and epoxidised soybean oil) (Cerqueira, Souza, Teixeira, & Vicente, 2012; Choi &
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Park, 2003; Mekewi, et al., 2017) have been applied to packaging films and significant effects
69
on their mechanical properties found.
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Artemisia sphaerocephala Krasch. (ASK) is a perennial psammophyte subshrub specie
71
found in desert and semi-desert regions of China. Its seeds have been used in Chinese
72
medicine to alleviate hyperglycemia, as an anticancer agent, as an antioxidant (Bai, Yong, &
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Yun, 2000; Guo, et al., 2011; Hu, et al., 2011), and as a food additive to increase viscosity and
74
improve mouth-feel (Liu, Li, Gu, & Tan, 2006; Huang, Liu, & Gu, 2007). Artemisia
75
sphaerocephala Krasch.gum (ASKG) is obtained from the outer layer of ASK seeds and is
76
widely used as a thickener, stabilizer, water retention agent or film forming agent (Bai, et al.,
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2000). ASKG is a heteropolysaccharide, composed of L-arabinose, D-xylose, D-lyxose, D-
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mannose, D-glucose and D-galactose in a ratio of 1.74: 1: 4.64: 27.6: 3.90: 14.4, with a
79
molecular weight of approximately 1.42×105 g/mol (Zhang, et al., 2007). The structure of
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ASKG is a linear backbone with branches which adopts a network structure through
81
entanglement of the branches (Zhang, et al., 2007). To date, reports on ASKG have focused
82
on its composition (Guo, et al., 2012), physicochemical properties (Liu, et al., 2006; Zhang, et
83
al., 2007), extraction methods (Guo, et al., 2011; Chen, et al., 2014), super-absorbent
84
polymers (Zhang, Zhang, Yuan, & Wang, 2007) and pharmacy applications (Xing, Zhang,
85
Hu, Wu, & Xu, 2009; Wang, Zhao, Wang, Yao, & Zhang, 2012; Wang, et al., 2016). There
86
has been no report about films formed from ASKG.
87
In this study, ASKG was selected as a novel biopolymer to prepare films. The effects of
88
plasticizer type (glycerol, sorbitol and triethyl citrate (TEC)) and plasticizer concentration, on
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film properties were investigated. Fourier-transform infrared (FTIR) spectroscopy,
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thermogravimetric analysis, oxygen permeability (OP), water vapor permeability (WVP),
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scanning electron microscopy (SEM), mechanical tests and opacity were used to characterize
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the ASKG film. The rheological properties of film-forming solutions were also measured to
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analyze the detailed effects of the plasticizers.
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2. Materials and methods
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2.1 Materials
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ASKG (food grade) was purchased from Anhui Zhongnan Biotechnology Co., Ltd. (Anhui,
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China). Glycerol and sorbitol (analytical reagent) were purchased from Yongda Chemical
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Regent Co. Ltd. (Tianjin, China). TEC (analytical reagent) was purchased from Macklin
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Biochemical Technology Co., Ltd. (Shanghai, China).
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102
2.2 Preparation of films
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Preliminary experiments revealed that a 1.0% (w/v) solution of ASKG was suitable for
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preparing a film. However, film prepared without plasticizer was brittle and hard to peel off as
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an intact film. Therefore, plasticizers were added to improve the flexibility of the films. It was
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found that films with more than 50% glycerol or sorbitol were flexible but sticky, and TEC
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separated out on the films at concentrations over 50%. So, plasticizer concentrations of 20%,
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30%, 40% and 50% (w/w of ASKG) were selected.
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Films were prepared using the solvent-cast method. Briefly, 6 g of ASKG powder was
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mixed with 600 mL of distilled water. The mixture was heated to 60 oC and maintained at that
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temperature for 30 min under constant stirring. After that, plasticizer was added and the
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mixture was stirred for 15 min. The resulting solution was cast onto a plexiglass plate (26 cm
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× 26 cm × 4 cm) after removing bubbles, and dried for 48 h at 60 oC in a vacuum oven.
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2.3 Mechanical properties
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The thicknesses of the films were measured with an ID-C112XBS micrometer (Mitutoyo
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Corp., Tokyo, Japan), and the values were the average of five random points. Tensile tests
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were performed using an auto tensile tester (XLW-PC, PARAM, Jinan, China) equipped with
119
a 500 N load cell under a strain rate of 300 mm/min at 25°C.
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2.4 Oxygen permeability
122
The oxygen permeability (OP) of the films (50 cm2) was measured by using Ox-Tran
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equipment (PERME OX2/230, Labthink, Jinan, China) according to the Chinese National
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Standard GB/T 19789-2005 in continuous mode at 25 °C and 0% relative humidity. All tests
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were performed in triplicates.
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2.5 Water vapor permeability
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The water vapor permeability (WVP) was measured according to our previous study (Ma,
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Hu, Wang, & Wang, 2016). The cup containing anhydrous calcium chloride desiccant was
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covered with the developed film. The mouths of the cup was sealed and the test assembly was
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incubated at 25 oC and 75% relative humidity (saturated NaCl solution). The driving force
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was 1753.55 Pa (ΔP), which was expressed as water vapor partial pressure. The cups were
133
weighed periodically to determine the amount of moisture transferred through the sample into
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the desiccant. WVP was realized in the steady state of the weight versus time result and
135
calculated by dividing the slope of the line by the exposed film area. Each film was tested in
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triplicate.
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2.6 Light transparency measurements
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Light transmittance was measured by using a UV–vis spectrophotometer (UV-2600,
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Shimadzu, Kyoto, Japan). The transmittance spectra of film was recorded from 200 to 800
141
nm.
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2.7 Fourier-transform infrared spectroscopy
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Fourier-transform infrared (FTIR) spectroscopy was conducted by using a Nicolet 6700
145
spectrometer (Thermo Fisher Scientific, MA, USA), with an attenuated reflectance (ATR)
146
Fourier transform mode. The FTIR spectra of the films were measured in the range from 4000
147
to 600 cm−1 and recorded at a resolution of 4 cm−1.
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2.8 Thermogravimetric analysis
150
Thermogravimetric analysis (TGA) was carried out by using a TA Instruments TGA Q500
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(TA Instruments, USA). The heating rate was 20 oC/min in the temperature range from room
152
temperature to 600 oC under nitrogen protection. The curves of derivative thermogravimetric
153
analysis (DTG) were obtained from the first-order derivative of curves.
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2.9 Scanning electron microscopy
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Morphology of the film was observed by using a Quanta 200 scanning electron microscope
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(Philips-FEI Co., The Netherlands) under an accelerating voltage of 5 kV. Films were
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fractured in liquid nitrogen, subsequently, fixed on the stage by using a double-sided tape.
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Finally, a thin gold layer was sprayed onto the film prior to observation.
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2.10 Rheological measurement
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Rheological evaluations of the film-forming solution was performed with a rheometer
163
(AR2000ex, TA Instrument, Newcastle, DE, USA) equipped with a cone and plate geometry
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(40 mm diameter, cone angle 2 o) at a shear rate from 0.1 to 100 s−1 at 25 oC.
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The flow behavior was analyzed by using the Cross model (Eq. [1]):
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 = ∞ + (0 ‒ ∞) [1 + ()]
(1)
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Where η is the viscosity (Pa·s),  is the shear rate (s−1), η0 is the zero shear rate viscosity
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(Pa·s), η∞ is the infinite shear rate viscosity (Pa·s), K is a time constant and p is a
169
dimensionless exponent.
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The dynamic rheological properties were measured in the linear viscoelasticity range. Then,
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the storage modulus (G′, Pa), loss modulus (G″, Pa) and complex viscosity (η*, Pa·s) were
172
measured at frequency sweeps from 0.1 to 100 rad/s under a strain of 0.1%.
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2.11 Statistical analysis
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The data obtained were analyzed by analysis of variance using SPSS (version 17.0, SPSS
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Inc., Chicago, IL, USA). Duncan’s multiple-range test (p< 0.05) was used to compare the
177
differences between the films’ properties.
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3. Results and discussion
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3.1 Mechanical properties
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The tensile strength (TS) and elongation at break (EB) of ASKG films containing
182
plasticizers are shown in Fig. 1. The effects of glycerol on the film are shown in Fig. 1a; with
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an increase in glycerol content from 20% to 50%, the TS decreased from 47.53 to 29.86 MPa
184
but the EB increased from 19.27 to 66.87%, possibly due to the presence of glycerol
185
increasing the spatial distance between ASKG chains. Furthermore, the hydrogen bonds also
186
increased because of the increase of hydroxyl groups. In this way, glycerol would decrease the
187
intermolecular forces, soften the rigidity of the film’s structure and increase the mobility of
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ASKG (Cerqueira, et al., 2012). Furthermore, the presence of glycerol also increased the
189
water content of the films, which affected TS and EB (Ziani, Oses, Coma, & Maté, 2008). The
190
effects of sorbitol on the film are shown in Fig. 1b; with an increase in sorbitol content from
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20% to 50%, the TS decreased from 54.04 to 29.92 MPa and the EB increased from 4.20% to
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44.13%. The effects were similar to those of glycerol, but the flexibility was lower than that
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of the glycerol plasticized films due to the different molecular weight of glycerol and sorbitol,
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and because glycerol has a higher water-holding ability (Ghasemlou, Khodaiyan, &
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Oromiehie, 2011). With addition of TEC from 20% to 50% to the films, the TS decreased
196
from 72.56 to 40.73 MPa. The EB was very low, and little variation in this property was
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observed for TEC plasticized films (around 2.5%). This may be explained by the chemical
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structure and the immiscibility of the plasticizer, so that it could not form hydrogen bonds
199
with ASKG in the film (Choi, et al., 2004; Jost & Langowski, 2015). It was notable that the
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TS of TEC plasticized film was higher than with the other plasticizers, possibly because TEC
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had little interaction with ASKG. Thus, the TS of TEC plasticized film was closer to that of
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the pure ASKG film. Because of the smaller molecular size, more hydroxyl groups and
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hydrophilic nature, glycerol and sorbitol could easily penetrate the network of ASKG
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polymers and weaken the intermolecular interactions between the polymer chains, which
205
altered the mechanical properties of ASKG film (Seyedi, Koocheki, Mohebbi, & Zahedi,
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2014; Ebrahimi, Koocheki, Milani, & Mohebbi, 2016).
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3.2 Barrier properties
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The barrier properties of film are important factors for estimating product shelf-life, and
210
good barrier properties may increase the quality of products to some extent. OP and WVP are
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two key parameters in film applications, because oxygen and water may transfer from the
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internal or external environment through the film, resulting in deterioration of product quality
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and shelf-life (Kanatt, Rao, Chawla, & Sharma, 2012). The OP and WVP values are shown in
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Fig. 2. The WVP values of films containing sorbitol are the lowest because the films
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plasticized by sorbitol were more compact than those plasticized by the other two plasticizers.
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Similar results were reported in cordia gum film (Haq, et al., 2014).
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As shown in Fig. 2a. The OP values of films increased with increasing content of
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plasticizer. The OP values of film containing glycerol, sorbitol and TEC were from 0.0137 to
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0.0239 cm3 mm m−2 atm−1 day−1, 0.0108 to 0.0236 cm3 mm m−2 atm−1 day−1 and 0.0116 to
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0.0243 cm3 mm m−2 atm−1 day−1 respectively, as the plasticizer concentration was increased
221
from 20% to 50%. As expected, the OP of films was good, due to their tightly packed and
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ordered hydrogen-bonded network structures (Kanatt, et al., 2012). The WVP values are
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shown in Fig. 2b. The WVP values of films containing glycerol, sorbitol and TEC ranged
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from 10.9518 to 27.7636 × 10−11 g s−1 m−1 Pa−1, 3.2849 to 7.5027 × 10−11 g s−1 m−1 Pa−1 and
225
6.0900 to 8.4129 × 10−11 g s−1 m−1 Pa−1 respectively, as the plasticizer concentration was
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increased from 20% to 50%. The WVP of films containing glycerol was consistently higher
227
than that of films containing sorbitol or TEC, and the WVP values of films containing sorbitol
228
were the lowest. The increase in WVP values of films with increasing plasticizer content is
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likely due to the plasticizer reducing the intermolecular forces between ASKG polymers
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chains, and thus increasing free volume and segmental motions, allowing water molecules to
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diffuse more easily (Sothornvit & Krochta, 2001).
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3.3 Light transmission properties
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The light transmission properties of the ASKG films are important for their application in
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food packaging. Fig. 3 shows the light transmission spectra for film containing glycerol,
236
sorbitol and TEC. The light transmission decreased with increasing plasticizer content,
237
presumably because the light-barrier properties of the plasticizers governed the transparency
238
of the films (Zhang & Han, 2006). Films containing glycerol and sorbitol were more
239
transparent (the light transmission values decreased from 57.30% to 51.40% and from 61.34%
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to 47.60% at 600 nm with an increase in glycerol and sorbitol content, respectively). The light
241
transmission of the films containing TEC was the lowest, the light transmission rate value
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decreased from 24.40% to 2.33% at 600 nm with increasing TEC content. This is likely due to
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the large number of pores, as shown in SEM images of the films.
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3.4 FTIR analysis
246
The FTIR spectrum of ASKG powder (0% plasticizer) showed typical bands at 3700–2800
247
cm−1 and 1800–650 cm−1 associated with carbohydrate and protein (Fig. 4). The band at
248
around 3307 cm−1 is attributed to the stretch of hydrogen-bonded hydroxyl groups associated
249
with free, inter, and intra-molecular bound hydroxyl groups. The band at 2922 cm−1 is
250
characteristic of C-H stretching vibration. The band at 1737 cm−1 has been reported to be of
251
ester carbonyl (C=O) groups and carboxyl ion stretching vibration (COO−) (Kamnev, Colina,
252
Rodriguez, Ptitchkina, & Ignatov, 1998). The bands at 1635 and 1544 cm−1 are characteristic
253
of amide I (C=O) and II (N–H and C–N) in protein, respectively (Haq, et al., 2014). The
254
bands at 1058, 1015 and 867 cm−1 are attributed to C–O stretching vibration. In particular, the
255
bands at 1058 and 1015 cm−1 are characteristic of the anhydroglucose ring O–C stretching
256
vibration (Zhang, et al., 2006).
257
The FTIR spectra of glycerol and films containing glycerol are shown in Fig. 4a. The bands
258
at 3307, 2922 and 2883 cm−1 were found to be increased with the increase of glycerol
259
addition, indicating a change in the nature of the hydrogen bonding in the film. Three types of
260
hydrogen bonds are likely in the films between the polymer chains, between the polymer and
261
glycerol, and within glycerol itself. At low glycerol concentration, all the hydroxyl groups in
262
the polymer would interact with hydroxyl groups in the glycerol molecules to replace the
263
interaction between polymer molecules, owing to the solvation effect of the plasticizer. With
264
increased glycerol concentration, the free hydroxyl groups in glycerol dominated (Haq, et al.,
265
2014) and would decrease the intermolecular forces between the ASKG chains. These results
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indicate that a good compatibility existed between ASKG and glycerol.
267
Fig. 4b shows the FTIR spectra of sorbitol and films containing sorbitol. There is no
268
significant difference between the spectra of ASKG powder and those of the plasticized films.
269
These results are similar to those reported previously, and indicate a good compatibility
270
between ASKG and sorbitol (Haq, et al., 2014; Zhang, et al., 2006). The reasons are similar to
271
the addition of glycerol.
272
Fig. 4c shows the FTIR spectra of TEC and films containing TEC. The bands at 3497,
273
2982, 1731, 1500 to 1250 and 1250 to 950 cm−1 are attributed to O-H stretching vibration, C–
274
H stretching vibration, C=O stretching vibration, C–H bending vibration and C–O stretching
275
vibration, respectively (Hou, Feng, Wu, & Gao, 2014). After adding TEC, the band at 3307
276
cm−1 shifted to 3497 cm−1, suggesting that the hydrogen bonds between the polymer chains
277
were broken by TEC because there is only one hydroxyl group in TEC molecule. The new
278
bands at 2983, 1736, 1370 and 1184 cm−1 occurred with the increase of TEC, indicating the
279
presence of TEC in the film.
280
281
3.5 Thermogravimetric analysis (TGA)
282
The TG and DTG curves of ASKG powder and films containing plasticizers are shown in
283
Fig. 5. As shown in the ASKG curve (Control), the first weight loss peak at 73.99 oC
284
represents the loss of adsorbed water, and the second weight loss peak at 310.49 oC is that for
285
the decomposition of ASKG. Figs. 5(a) and 5(a′) show the TG and DTG curves of films
286
containing glycerol. There are three peaks in the DTG curve; the first peak at 76.35 oC being
287
that for loss of adsorbed water. The second peak at 198.02 oC indicates the decomposition of
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glycerol, and this peak increased with the increasing content of glycerol. The third peak from
289
304.16 oC is for the decomposition of ASKG, and this peak shifted to lower temperature
290
compared with that of ASKG powder, indicating the effect of glycerol in destabilizing the
291
interaction between the ASKG chains.
292
The TG and DTG curves of films containing sorbitol are shown in Figs. 5(b) and 5(b′).
293
There are two peaks of decomposition, the first peak at 98.57 oC is for the loss of adsorbed
294
water, this peak is shifted to a higher temperature compared to that of ASKG powder,
295
possibly due to the hydrophilicity of sorbitol. The second peak at 310.15 oC is the
296
decomposition of ASKG, it is similar to that of ASKG powder, indicating that sorbitol has
297
little effect on the structure of ASKG.
298
The TG and DTG curves of films containing TEC are shown in Figs. 5(c) and 5(c′). There
299
are three peaks of decomposition, the first peak at 73.99 oC for adsorbed water in ASKG
300
powder is broadened and shifted to 158.93 oC with the increasing content of TEC. The second
301
peak at 276.97 oC (new peak) is the characteristic decomposition of TEC and this peak
302
increased with increasing TEC content. The third peak at 299.81 oC is for the decomposition
303
of ASKG and it is shifted to a lower temperature compared to that for ASKG powder,
304
indicating that TEC decreases the thermal stability of ASKG by breaking the stable structure
305
between ASKG chains. Comparing the three plasticizers, the order of thermal stability of
306
plasticized films is: sorbitol > glycerol > TEC. However the results above indicated that the
307
films were still stable at temperatures below 100 oC and could still be sufficiently stable for
308
many food packaging applications.
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3.6 Scanning electron microscopy (SEM)
311
The surface morphology of freeze-fractured cross-sections of the films is shown in Fig. 6.
312
The appearance of films containing glycerol (Fig. 6a) and sorbitol (Fig. 6b) were
313
homogeneous and there is no remarkable structural difference, indicating that glycerol and
314
sorbitol are compatible with ASKG. Thus glycerol and sorbitol could improve the flexibility,
315
and prevent cracks in such films. Similar results have been reported in kefiran film
316
(Ghasemlou, et al., 2011). The appearance of films incorporating TEC is shown in Fig. 6c.
317
The appearance shows roughness and many pores in the films. With increased TEC content,
318
the number of pores increased significantly, owing to the incompatibility between TEC and
319
ASKG polymer.
320
321
3.7 Rheological behavior of film-forming solutions
322
3.7.1 Steady rheological properties
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In order to analyze the effects of the plasticizers on the steady state shear properties of the
324
ASKG solutions, concentrations of 0%, 30% and 50% of the plasticizers were selected for
325
analysis. As shown in Fig. 7, the viscosity decreased with the increase of shear rate, which can
326
be characterized by non-Newtonian behavior. This behavior could be caused by the network
327
of ASKG polymers and the hydrogen bonds (between the polymers and the plasticizers) being
328
altered during shear (Lin, Tsai, & Lai, 2009). Moreover, the entangled network could not
329
recover within a short time (Chauveteau, 1982). The shear-thinning behavior of the ASKG
330
solution and mixed solutions with plasticizers may be regarded as arising from the
331
modification in macromolecular organization in the solution as the shear rate changes. The
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332
viscosity of sorbitol plasticized solutions was the lowest, indicating that the entangled
333
network was altered significantly. This may be due to sorbitol having the largest number of
334
hydroxyl groups in its molecule.
335
The Cross rheological model quantifies the relationship between the apparent viscosity and
336
shear rate from experimental data. As shown in Table 1, the Cross model described well the
337
structural behavior of all the solutions, as indicated by a high correlation coefficient (R2>
338
0.9999). The η0 and the K values of film-forming solutions after adding plasticizers were
339
lower than that of the control film-forming solution, and the K values decreased with the
340
increasing plasticizer content, indicating that the hydrodynamic size of ASKG molecules was
341
smaller and the freedom of movement was less restricted, consequently requiring less time to
342
form new entanglements and hydrogen bonds (Morris, Cutler, Ross-Murphy, Rees, & Price,
343
1981). The p values of all the solutions were similar and lower than 1, which is characteristic
344
of a pseudoplastic fluid (Haddarah, et al., 2014). These results indicate that plasticizers can
345
alter the tight network between ASKG polymers.
346
347
3.7.2 Dynamic rheological properties
348
In order to evaluate the interactions between ASKG and plasticizers in film-forming
349
solution under an optimum concentration, the plasticizer of 40% was added in this study. For
350
comparison, the plasticizer-free solution was also measured. As shown in Fig. 8, the G″ of all
351
the solutions was higher than G′ at low frequency region, whereas the G′ was dominant at
352
higher frequencies, which could be considered typical weak gel systems (Long, Zhao, Zhao,
353
Yang, & Liu, 2012). The complex viscosities (η*) decreased with the increasing frequency and
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354
exhibited a shear-thinning behavior, which is in accordance with the steady rheological
355
properties. There is a crossover point at 3.17 rad/s for the control film-forming solution,
356
which shifted to higher values with the addition of plasticizers. This is because the plasticizer
357
addition promoted the formation of hydrogen bonds and rebuilt the entanglement network
358
between the ASKG polymer chains and the plasticizers. These results suggest that ASKG
359
solutions can be classified as an entanglement network system (Wu, Ding, Jia, & He, 2015).
360
361
4. Conclusions
362
We prepared ASKG films containing glycerol, sorbitol and TEC as plasticizer, and their
363
mechanical, barrier, thermal and optical properties were measured. The TS of the film
364
decreased and the EB increased after the addition of plasticizer. The order of TS of the films
365
containing the plasticizers was: TEC > sorbitol > glycerol; and the order of flexibility of the
366
films containing plasticizer was: glycerol > sorbitol > TEC. The OP and WVP increased, but
367
the light transmission decreased with increasing plasticizer content. FTIR results showed that
368
plasticizer molecules decreased the intermolecular forces between the ASKG chains and
369
formed hydrogen bonds with the ASKG molecules. Plasticizers reduced the thermal stability
370
of the film and the maximum decomposition temperature of the film decreased with increased
371
glycerol or sorbitol content, but increased with the increasing TEC content. Rheological
372
results suggested that hydrogen bonds between plasticizer molecules and ASKG molecules
373
disrupted the entanglement network of ASKG chains. SEM observation showed that glycerol
374
and sorbitol are more miscible with ASKG than TEC. Sorbitol appears to be the most suitable
375
plasticizer for ASKG. ASKG film containing sorbitol has higher elongation at break, light
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376
transmission, flexibility and hydrophilicity. ASKG has potential as an alternative film-
377
forming material for food packaging.
378
379
380
381
Acknowledgments
This work was supported by the National Natural Science Foundation of China (31770618)
and the Fundamental Research Funds for Central Universities (2572017AB11).
382
383
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Figure captions
527
Fig. 1 TS and EB curves of ASKG film containing glycerol (a), sorbitol (b) and TEC (c).
528
Fig. 2 OP (a) and WVP (b) data of ASKG film containing different type and concentration of
529
plasticizer.
530
Fig. 3 Light transmission rate of ASKG film containing glycerol (a), sorbitol (b) and TEC (c).
531
Fig. 4 FTIR spectra of plasticizers and ASKG film containing glycerol (a), sorbitol (b) and
24
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532
TEC (c).
533
Fig. 5 TG and DTG curves of ASKG powder and ASKG film containing glycerol (a and a′),
534
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535
Fig. 6 Morphology of the cross-section of ASKG film containing glycerol (a), sorbitol (b) and
536
TEC (c). All the magnifications are 500×.
537
Fig. 7 Steady rheological spectra of film-forming solutions.
538
Fig. 8 Dynamic rheological spectra of film-forming solution containing 0% plasticizer (a),
539
40% glycerol (b), 40% sorbitol (c) and 40% TEC (d).
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Table 1 Cross model parameters of film-forming solutions.
Sample
η0 (Pa·s)
K (s)
p
R2
Control
30% glycerol
50% glycerol
30% sorbitol
50% sorbitol
30% TEC
50% TEC
1.3560 ± 0.0592
1.3480 ± 0.0670
1.3349 ± 0.0595
1.2267 ± 0.0646
1.1459 ± 0.0343
1.3760 ± 0.0703
1.2644 ± 0.0822
1.3962 ± 0.0101
1.3248 ± 0.0101
1.2606 ± 0.0096
1.0251 ± 0.0107
0.9322 ± 0.0063
1.3329 ± 0.0109
1.2715 ± 0.0131
0.7444 ± 0.0030
0.7445 ± 0.0035
0.7475 ± 0.0033
0.7581 ± 0.0052
0.7409 ± 0.0034
0.7478 ± 0.0035
0.7420 ± 0.0045
0.99999
0.99999
0.99999
0.99998
0.99999
0.99999
0.99999
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