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pullulan blend films
BIOMAC-13185; No of Pages 8 International Journal of Biological Macromolecules 140 (2019) xxx
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Preparation, properties, and structural characterization of β-glucan/pullulan blend films Jinyu Chang a, Wanrong Li a, Qin Liu a, You Zhou a, Xuan Chen a,b, Qingyun Lyu a,b,⁎, Gang Liu a,b,⁎ a b
School of Food Science and Engineering, Wuhan Polytechnic University, Wuhan 430023, China Key Laboratory for Deep Processing of Major Grain and Oil, Wuhan Polytechnic University, Ministry of Education, Wuhan 430023, China
a r t i c l e
i n f o
Article history: Received 13 July 2019 Received in revised form 18 August 2019 Accepted 23 August 2019 Available online 27 August 2019 Keywords: Pullulan β-Glucan Structural properties Edible film
a b s t r a c t This study investigates the physico-mechanical and structural properties of β-glucan (BG)/pullulan (PUL) composite edible films successfully prepared with 0–0.3 g of BG. Results demonstrated that BG addition significantly increases the elongation at break (p b 0.05), tensile strength, and water dissolution time of the resulting films. The transparency of the 0.2PUL:0.1BG film and the oxygen barrier property of the 0.15PUL:0.15BG film decreased remarkably compared with those of the plain films (0.3PUL:0BG and 0PUL:0.3BG) and other composite films (p b 0.05). FTIR indicated hydrogen bonding interactions between PUL and BG molecules, and microstructural observations showed that aggregated BG is homogeneously dispersed in the PUL continuous matrix. Among the films tested, the thermal stability of the 0.15PUL:0.15BG film was the best. A PUL:BG mixing ratio of 0.15:0.15 is thus suggested to provide the best film properties. This research offers an alternative method to improve PUL-based edible films. © 2019 Published by Elsevier B.V.
1. Introduction The hidden dangers of plastic packaging, such as their nondegradability, non-renewability, and risk to food safety [1], have been widely reported in the food packaging industry. Plastic packaging not only brings about white pollution to the environment but also harms human health. Thus, many consumers have opted to use environment-friendly, safe, green, and organic food packaging [2,3]. Non-toxic, harmless, and degradable edible films as packaging materials are a popular research topic [4]. In practical applications, different freshkeeping items require different edible film performances. Edible films are mainly used to preserve fruits and vegetables, meat products, convenience food, confectionery, and baked goods. Pullulan (PUL), a linear extracellular polysaccharide secreted by Aureobasidium pullulans, is composed of α-1,6 glycoside linkages based on maltotriose [5]. It is widely used to prepare edible films and even used as a coating for tissue scaffolds [6] because of its high water solubility, plasticity, adhesiveness, non-toxicity, edible properties, and film forming properties. PUL film is rapidly soluble in water, colorless, odorless, transparent, oil repellent, oxygen blocking, heat sealable, and can prevent the oxidation of oils and vitamins in food [7]; unfortunately, its flexibility is relatively poor. Glycerol (Gly) presents strong
⁎ Corresponding authors at: School of Food Science and Engineering, Wuhan Polytechnic University, Wuhan 430023, China. E-mail addresses: [email protected] (Q. Lyu), [email protected] (G. Liu).
hygroscopicity and can be used as a plasticizer to improve the ductility and flexibility of films [8]. Films composed of a single polysaccharide are limited in application by a number of weak characteristics, such as frangibility, easy breakage, and high cost. Composite films prepared by mixing two or more polysaccharides or different high-molecularweight substances are believed to be able to address these issues [9]. β-Glucan (BG), a component of microbial and plant cell walls [10], is a linear non-starch polysaccharide consisting of continuous β-(1 → 4)D-glycoside bonds and non-continuous β-(1 → 3) bonds derived from D-glycoside linkages to glucose [11]. BG is a white powder that is soluble in water and insoluble in organic solvents, such as ethanol and acetone. It has certain swelling characteristics, water holding capacity, and gel properties [12,13]. In addition, grain BG plays a very important role in improving body health and preventing diseases. It remarkably reduces low-density lipoprotein cholesterol, improves high-density lipoprotein cholesterol, balances blood pressure, improves blood lipid levels, and balances the postprandial blood glucose and insulin response. Despite its many benefits, however, the application of BG to edible packaging films has not been reported. Therefore, if BG is introduced to PUL films, its characteristics may improve the flexibility of the latter and provide novel functional properties to the final edible film. BG and PUL are water-soluble polysaccharides that are compatible with glycerin. In the present experiment, BG and PUL were used to prepare BG-PUL (BP) composite membranes with Gly as a plasticizer, and the properties and formation mechanism of the resultant films were explored. This study aims to develop BG-PUL composite films.
https://doi.org/10.1016/j.ijbiomac.2019.08.208 0141-8130/© 2019 Published by Elsevier B.V.
Please cite this article as: J. Chang, W. Li, Q. Liu, et al., Preparation, properties, and structural characterization of β-glucan/pullulan blend films, https://doi.org/10.1016/j.ijbiomac.2019.08.208
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2. Materials and methods
2.6. Color of the films
2.1. Materials
Film color was tested by a portable CR-10 colorimeter (Shengguang Company, Suzhou, China) in accordance with the method of Zavareze [16]. Lab color parameters were used to identify color via the following three attributes: L* (white: 100; black: 0), a* (positive: red; negative: green), and b* (positive: yellow; negative: blue). The color difference (E*) was determined by comparison with a white standard tile and calculated using Eq. (3).
Barley BG (medium viscosity, 90% purity) was purchased from Megazym Co., Ltd. (Ireland). PUL (99% purity) was acquired from Yuanye Biological Co., Ltd. (Shanghai, China). CaCl2 and glycerin were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Soybean oil was purchased from Wushang Merchants Co., Ltd. (Wuhan, China).
E ¼
2.2. Preparation of the BP blend films Blend film-forming solutions were first prepared by dissolving BG, PLU, and Gly in 20 ml of distilled water at different ratios (Table 1) by using the solvent casting method with continuous stirring in a thermostat water bath at 80 °C for 2 h. The blend films were refrigerated at 4 °C overnight to eliminate air bubbles and then poured into a polystyrene plate with a diameter of 9 cm. Subsequently, the films were dried at 35 °C for 10 h by using a DHG-9075A electric thermostatic drying oven (Yiheng Scientific Instrument Co., Ltd.; Shanghai, China). The total content of BP in all films was maintained at 0.40 g. All blend films were conditioned at 53% relative humidity (RH) for 2 days before further testing. 2.3. Thickness measurement Following GB/T 6672-2001 [14], we measured the thickness of the films by using a CH-10-AT thickness gauge (Liuling Instrument Corporation; Shanghai, China) to the nearest 0.001 mm at 13 positions, one of which was the center of the film. The average value was calculated. 2.4. Mechanical property measurement We used a TA.XT2i instrument (Testometric, S.M.S. Corporation, USA) to determine the tensile strength (TS, MPa) and percentage elongation at break (E, %) of the films according to ASTM D882-1991 [15]. Three tensile specimens (width 10 mm, length 20 mm) were cut from each film for tensile testing. During testing, the measuring speed was 1 mm/min, the measured speed was 10 mm/min, the initial grip separation was 26 mm, and the force was 5 × g. TS and E were calculated on the basis of Eqs. (1) and (2):
TS ¼
E¼
F S
ð1Þ
L 100 L0
ð2Þ
where F is the maximum force (N), S is the cross-sectional area of the film (width × thickness, mm2), L is the length of the film at failure (mm), and L0 is the initial grip separation (mm).
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 L2 þ a2 þ b2 ¼ ðL1 −L0 Þ2 þ ða1 −a0 Þ2 þ ðb1 −b0 Þ
ð3Þ
where L1, a1, and b1 are the color attributes of the samples and L0, a0, and b0 are the color attributes of the white standard tile. 2.7. Barrier characteristics of the films 2.7.1. Water vapor permeability (WVP) measurement The WVP (g·mm/m2·day·kPa) of the composite membranes were determined on the basis of Eq. (4) in accordance with GB 1037-1988 [17]. WVP ¼
m0 d S ΔP
ð4Þ
where m0 (g) is the weight of moisture absorbed by the desiccant after 1 day, S (m2) is the area of the moisture permeable cup diameter, d is the thickness of the film (mm) and ΔP is the water vapor pressure difference across the membrane (kPa). 2.7.2. Oxygen barrier property (OP) measurement The OP of the films was determined by sodium thiosulfate titration [18]. Approximately 3 g of vegetable oil was weighed into an Erlenmeyer flask, which was then sealed with a film and placed in an incubator at 50 °C for 5 days. OP was determined on the basis of the peroxide value (PV) of the oil in accordance with the standard GB 5009.227 [19]. The lower the peroxide value, the better the oxygen barrier of the membrane A set of blanks without any film served as the control. PV (g/ 100 g) was calculated as follows (Eq. (5)): PV ¼
ðV−V0 Þ C 0:1269 100 M
ð5Þ
where V and V0 are the volumes of sodium thiosulfate consumed by the oil of the film sample and control sample (ml), respectively, C is concentration of sodium thiosulfate standard solution (mol/l), 0.1269 represents mass of iodine corresponding to 1 ml of sodium thiosulfate, and M is the quality of the oil to be titrated (g). 2.7.3. Grease permeability measurement A test tube was added with 5 ml of soybean oil and sealed with a film. Filter paper was then weighed and folded onto the tube. After 5 days, we sought to discover whether oozing had occurred and weighed the filter paper.
2.5. Water solubility of the films 2.8. Formation mechanism of the composite film Film samples (1.5 cm × 1.5 cm) were placed in boiling water to observe and record the time required for complete dissolution. Table 1 Films formulas were prepared by BG, PLU, and Gly at different ratios. Groups BG/g PUL/g Gly/g
PUL1
BP2
BP3
BP4
BP5
BP6
BG7
0 0.3 0.1
0.05 0.25 0.1
0.1 0.2 0.1
0.15 0.15 0.1
0.2 0.1 0.1
0.25 0.05 0.1
0.3 0 0.1
2.8.1. Rheological measurement Frequency–modulus scanning of all film-forming solutions was conducted by using a rheometer (Kinexus Pro+; Malvern, USA) with a cone-and-plate geometry (40 mm diameter; 150 mm gap; cone angle 40°) in accordance with the method of Xiao [20]. The frequency sweep interval ranged from 0.1 Hz to 100 Hz at 25 °C with 1% strain. The strain–viscosity relationship of all film-forming solutions was studied by using a parallel plate clamp (40 mm diameter; 1 mm gap) at 25 °C. The films were sheared at rates ranging from 0.1 s−1 to 100 s−1.
Please cite this article as: J. Chang, W. Li, Q. Liu, et al., Preparation, properties, and structural characterization of β-glucan/pullulan blend films, https://doi.org/10.1016/j.ijbiomac.2019.08.208
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2.8.2. Differential scanning calorimetry (DSC) The thermal stability of the film was analyzed by DSC in accordance with the method of Ye [21]. Each film sample was analyzed by using a Q2000 DSC analyzer (TA Corporation, USA) under the conditions of 25 ml/min, 30–280 °C, and 15 °C/min. 2.8.3. Scanning electron microscopy (SEM) The surface and cross-sectional morphologies of the films were observed by a SEM S-3000N instrument (Hitachi, Japan) at 1000× and 600×, respectively. 2.8.4. Fourier transform infrared spectroscopy (FT-IR) analysis The infrared absorption spectra of the samples were measured at 650–4000 cm−1 by an FT-IR NEXUS 670 instrument (Nichols Instruments, USA) in accordance with the method of Xiao [5]. The resolution was 4 cm−1, the beam precision was 0.01 cm, the number of scans was 32, and the ambient temperature was 25 °C. 2.8.5. X-ray diffraction Samples were scanned with an Empyrean X-ray diffractometer (Panaly Corporation, Netherlands) at 25 °C, 40 kV voltage, 40 mA current, and 5°–50° scan angle in accordance with the method of Xiao [5].
Fig. 1. Water dissolution time of PUL film, BG film and blend films.
weakened. In general, the chemical and aggregate structures of polysaccharides could affect their absorption of water [27]. These results suggest changes in the chemical structure of the BP composite films.
3. Results and discussion 3.3. Film color 3.1. Mechanical property measurement Mechanical properties are important characteristics of materials, especially membrane materials. TS and E influence the end-use handling properties and mechanical performance of films [22]. Addition of increasing amounts of BG to the films increased their TS and E (Table 2). Thus, BG addition could increase the mechanical properties of the films, likely due to the formation of hydrogen bonds between PUL and BG. However, the TS and E of BP6 (PUL:BG = 0.05:0.25) was low, probably because of its low PUL content and weak phase separation. Among the films tested, the pure BG film revealed the highest TS and E. The difference in thickness among the composite films was not significant (p N 0.05) (Table 2). As BG content increased, the thickness of the films gradually increased, which reveals that the film thickness is linked to the composition of the film. These results are in agreement with those reported by Abugoch [23], Sebti [24], and Di Pierro [25], who respectively studied buckwheat protein–chitosan, Hypromellose–chitosan, and chitosan–whey protein blend films. The thickness of the films was similar to that obtained by Pan [26].
Film color is another important indicator of the overall appearance and consumer acceptance of an edible film [26]. The pure PUL film was the most transparent among the films studied (Fig. 2). Significant differences (p b 0.05) in L* and E* were observed among the films. As BG addition increased to 0.1 g (BP3), the color of the composite films gradually changed to black. The transparencies of the blend films increased with further addition of BG, which means the higher the amount of BG added, the more transparent the blend film becomes. Zhu [9] revealed similar result that the pure PUL film is more transparent than PUL/carboxymethyl–gellan composite films. 3.4. Barrier characteristics The barrier properties of edible films play an important role in their use as packaging materials and help ensure food quality during food storage and transport. The barrier properties of edible films include moisture resistance, oxygen resistance, and oil resistance. 3.4.1. WVP measurement The results of the WVP testing are shown in Fig. 3. Data analysis indicated no significant difference (p N 0.05) in WVP among all composite
3.2. Water solubility of the films Water solubility is an indicator of the water resistance of edible films [8]. The fields of application of films with different water solubilities vary. Among the BP composite films, BP2 (PUL:BG = 0.25:0.05) and BP3 (PUL:BG = 0.2:0.1) revealed the best solubility (Fig. 1). As the content of BG increased, the water solubility of the composite films
Table 2 Mechanical properties of BP blend films. Films
TS (MPa)
E (%)
Thickness (mm)
PUL1 BP2 BP3 BP4 BP5 BP6 BG7
7.58 ± 1.04e 17.00 ± 1.69d 25.17 ± 2.93c 33.81 ± 0.67b 34.62 ± 0.76b 30.77 ± 0.68b 48.44 ± 0.92a
23.47 ± 2.64d 38.73 ± 1.65c 57.27 ± 1.21b 64.61 ± 7.06b 75.86 ± 8.30a 54.72 ± 5.04b 79.74 ± 10.67a
0.042 ± 0.003b 0.044 ± 0.001b 0.049 ± 0.001a 0.051 ± 0.002a 0.051 ± 0.002a 0.052 ± 0.003a 0.052 ± 0.002a
Values are means ± standard deviation. The different letters in a column indicate significant differences at p b 0.05.
Fig. 2. The color of PUL film, BG film and blend films.
Please cite this article as: J. Chang, W. Li, Q. Liu, et al., Preparation, properties, and structural characterization of β-glucan/pullulan blend films, https://doi.org/10.1016/j.ijbiomac.2019.08.208
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oxygen barrier of the PUL film may be stronger than that of the BG film because of differences in their molecular structures. 3.4.3. Grease permeability measurement All films had oil-impermeable characteristics, which means all of their membrane structures are dense. 3.5. Formation mechanism of the composite film
Fig. 3. The water vapor permeability of PUL film, BG film and blend films.
film groups. In general, among the films tested, the PUL and BG films demonstrated the best moisture resistance; moreover, the moisture resistance of the BP2 and BP6 compound films were better than that of the BP3, BP4, and BP5 films. This result may be due to the aggregation and self-condensation of BG particles at higher concentrations, which produces voids in the composite matrix [28]. Gly also affects the water absorption of the films. The moisture barrier property of the pure PUL film was better than that of the BP compound film, similar to the result of the PUL–gelatin/carboxymethyl gelatin film studied by Zhu [9,29].
3.5.1. Solution rheological measurement The presence or absence of defects in thin liquid films after coating relies partly on their rheological properties [30]. Bubbles are difficult to eliminate when the edible film liquid is in gel form or has high viscosity, which prohibits the formation of a flat film. Liquids with low viscosity are prone to corrugation under the action of hot air during edible film formation, and the thickness of the formed film may be uneven [20]. The rheological properties of the membrane fluid depend on its internal structure [31]. In the present study, among the films tested, the viscosity of the pure PUL film was the largest (Fig. 5a). As BG content increased, the viscosity of the composite film liquid gradually decreased, and the viscosities of the BP2 and BP3 composite films were highly similar. The membrane fluid exhibited shear thinning behavior under static shear, so all film fluids could be considered to be non-Newtonian fluids. This result indicates that intermolecular junctions are disrupted at a rate faster than their reformation rate as the shear rate increases, resulting in a decrease in junction density and, hence, a drop in viscosity. Such findings reveal that the CMC FFS behaves as an entangled network system
3.4.2. OP measurement The results of OP testing of the composite membranes are shown in Fig. 4. The membrane-free group had a PV of 1.335 g/100, which is remarkably higher than the PV of the membrane-covered groups. This result indicates that the PUL, BG, and BP blend films could block oxygen transmission. Data analysis showed a significant difference (p b 0.05) in OP among the films. The PV of the PUL film was 1.193 g/100 g. Addition of BG gradually enhanced the OP of the composite films, and the highest OP was observed when the ratio of PUL and BG was 0.15:0.15 (BP4) and PV was 1.124 g/100 g. It indicates that addition of a certain amount of BG to the PUL film could improve its OP. This finding also confirms that BG undergoes intermolecular interactions with PUL. The
Fig. 4. The oxygen permeability of PUL film, BG film and blend films.
Fig. 5. (a) The viscosity characteristics of PUL film, BG film and blend film liquids. (b) The tan δ (G″/G′) of PUL film, BG film and blend films (G″ represents viscous modulus and G′ represents elastic modulus).
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J. Chang et al. / International Journal of Biological Macromolecules 140 (2019) xxx
(or a concentrated solution) [32]. The properties of the pure PUL membrane fluid were identical to those of Liu [31] but differed from those of Xiao [20]; this difference may be due to variations in the concentrations of the PUL film. Changes in the viscosity and fluid properties of the BG membrane fluid were consistent with those of the high-concentration BG solution studied by Kinnari J and Wang [33,34]. The modulus (G″) and elastic modulus (G′) characterize the viscoelasticity of a film fluid. Viscoelasticity is a state exhibited by a solution in response to some applied stress. The tangent (tan) δ (G″/G′) of the film liquids are shown in Fig. 5b. The PUL1–BP5 film liquids exhibited G′ N G″ at low vibration frequencies, which means the values are intersected when a high-frequency vibration stage is reached. This result reveals that the membrane fluid system is solid under low vibrational frequencies and may be considered elastic. The tan δ of BP6 film liquid closed to 1 in the whole scanning frequency, which expressed as viscoelasticity. For BG7 film liquid, the G″ was remarkably larger than the G′, indicating that the majority of the energy was dissipated by viscous flow, similar to that reported by de Souza on native BG [35]. The overall rheological properties of the film fluids indicate that the properties of the composite film fluid systems gradually transform from those of a solid to those of a liquid with increasing BG. Elastic behavior has been reported to be a function of the number of effective chains participating in the formation of a network structure. Increases in G′ and G″ with increasing frequency in the presence of PUL suggest that the latter is well dispersed within the PUL/BG blend solution [32]. 3.5.2. SEM The microstructure of an edible film reflects its homogeneity and compactness; indeed, an edible film with high uniformity and compactness is also believed to have relatively high barrier properties [36,37]. Fig. 6a shows that the surface of PUL1 has wrinkles, which may be caused by the movement of fluid across the membrane during drying. A crack on the flat cross-section (Fig. 6b), which may be caused by the electron beam hitting the sample, was also observed; this crack is evidence that PUL1 breaks easily. The surface and cross-section of BP2 are relatively uniform and smooth, thereby indicating that BG and PUL have good compatibility at this ratio. BP3 showed a smooth and uniform cross section; its surface was continuous but not smooth because its color was not uniform. This finding may be attributed to the decreased compatibility between BG and PUL at this ratio. The surface of BP4 was similar to that of BP3 but was not as smooth. The cross-section of BP4 was continuous and smooth. Addition of BG in the range added to BP5–BG7 caused the surface of the resulting films to become flatter and smoother, thus showing that the compatibility of BG and PUL increases gradually as the ratio of BG and PUL increases. The toothed structures observed in the cross-section of BP5, BP6, BG7 were caused by cutting. The surface of BP6 was flat but cracked, likely due to the electron beam hitting it. Overall, the BP2, BP5, and BG7 films were smooth and flat, consistent with the moisture resistance results. BP3, BP4, and BP5 showed good OP, which prevents gas permeation and reveals the presence of few microporous structures on the surface of these films. 3.5.3. FTIR The FTIR data were analyzed to identify the main functional groups of the films [38]. The infrared spectra of the composite films are shown in Fig. 7. The absorption peak in the 1800–1200 cm−1 region is characteristic of the polysaccharide. FTIR spectra showed the characteristic absorption bands of the α-anomeric and β-anomeric configurations at 850 and 890 cm−1, respectively [39]. The absorption peak at 850 cm−1 in the BG7 film is due to the purity of the BG sample, which is approximately 95%. The absorption band at 3354 cm−1 is attributed to the \\OH stretching vibrations of the polysaccharide [37]. The absorption peak of BG at 1073 cm−1 is attributed to the \\OH stretching vibrations of C6. The absorption of PUL at 1028 cm−1 is due to the stretching vibrations of C\\O\\C [38]. The absorption peaks of PUL at 1610 cm−1 and the absorption peaks of BG at 1688 cm−1 reflect the
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stretching vibrations of C\\O linked\\OH. The absorption band around 1070 cm−1 broadened and shifted to a lower wave number with the increase of PUL, indicating the gradual increase of intermolecular hydrogen indicating the gradual increase in interactions between PUL and BG. The peak at 1688 cm−1 shifted toward a lower wavenumber with increasing BG, thereby indicating a gradual increase in intermolecular hydrogen bonds between PUL and BG. These results are in agreement with the mechanical properties of the edible film.
3.5.4. DSC The endothermic peaks (Tg) of a polymer blend can reveal useful information related to the miscibility of its constituent components [40]. In the current study, the DSC curves of all films were comparable, which indicates that no chemical reaction occurs between PUL and BG, as shown in Fig. 8. The Tg values of BP3 and BP4, at 134.95 and 135.03 °C, respectively, were the highest among the composite films. This result indicates that the two films have good thermal stability and compatibility and that hydrogen bonds are formed between the molecules, consistent with their OP results. Among the films, the Tg of BP6 was the lowest, indicating unsatisfactory compatibility between PUL and BG in this film. This result may be due to stronger hydrogen bonding interactions between BG molecules than between BG and PUL molecules when the PUL:BG ratio is 0.05:0.25, which causes microscopic phase separation, consistent with the mechanical properties of the composite membrane. The Tg of the PUL film obtained in this experiment was higher than that obtained by Sakata [41] and Xiao [40], likely due to differences in the sample amounts and heating rates employed.
3.5.5. X-ray diffraction (XRD) XRD is used to assess the apparent crystallite sizes of polymers, determine crystallization processes and kinetics, characterize the fine structure of materials, and explore interactions between molecules. A completely incompatible polymer blend system will show separate crystalline regions owing to the absence of or weak interactions, and the resulting XRD patterns could be expressed as a proportional diffraction spectrum of each component. The XRD results are shown in Fig. 9. The crystal peaks of all edible films are observed at approximately 20° and demonstrate an amorphous structure, thus revealing the formation of a dense network between molecules [28]. This finding is consistent with the SEM results and similar to those reported by Xiao, Zhang, and Trovatti [5,42,43].
4. Conclusion BP films were successfully prepared with different ratios of PUL and BG by using the solvent casting method. The optimum PUL:BG mixing ratio of 0.15:0.15 yielded good optical transparency, reduced the WVP, and enhanced tensile properties, all of which are essential properties required for packaging applications. The experiments showed that the thickness of the BP composite films increases with increasing BG content. The overall rheological properties of the film fluids indicate that the properties of the composite film fluid systems gradually transform from those of a solid to those of a liquid with increasing BG. The microstructure of the final edible film indicated that BG and PUL have good compatibility at some ratio. Addition of increasing amounts of BG to the films increased the mechanical properties of the films, likely due to the formation of hydrogen bonds between PUL and BG, confirmed by the FTIR data. All of the films could block the permeation of grease. The BP3, BP4, and BP5 films revealed the best barrier properties and could be used as cling film. However, the sealing strength of these films requires further improvement if they are to be used as packaging materials.
Please cite this article as: J. Chang, W. Li, Q. Liu, et al., Preparation, properties, and structural characterization of β-glucan/pullulan blend films, https://doi.org/10.1016/j.ijbiomac.2019.08.208
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Fig. 6. (a) SEM micrographs of the surface of PUL film, BG film and blend films. (b) SEM micrographs of the cross-section of PUL film, BG film and blend films.
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Acknowledgment This study was supported by the Chinese National Science Foundation (Grant No. 31771925) and China Central Government Guide the Development of Local Science and Technology Special Funds (2017 No. 88). References
Fig. 7. FTIR spectra of PUL film, BG film and blend films at wavenumbers between 400 and 4000 cm−1.
Fig. 8. DSC curves of PUL film, BG film and blend films.
Fig. 9. X-ray diffraction patterns of PUL film, BG film and blend films.
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Contents lists available at ScienceDirect
International Journal of Biological Macromolecules journal homepage: http://www.elsevier.com/locate/ijbiomac
Preparation, properties, and structural characterization of β-glucan/pullulan blend films Jinyu Chang a, Wanrong Li a, Qin Liu a, You Zhou a, Xuan Chen a,b, Qingyun Lyu a,b,⁎, Gang Liu a,b,⁎ a b
School of Food Science and Engineering, Wuhan Polytechnic University, Wuhan 430023, China Key Laboratory for Deep Processing of Major Grain and Oil, Wuhan Polytechnic University, Ministry of Education, Wuhan 430023, China
a r t i c l e
i n f o
Article history: Received 13 July 2019 Received in revised form 18 August 2019 Accepted 23 August 2019 Available online 27 August 2019 Keywords: Pullulan β-Glucan Structural properties Edible film
a b s t r a c t This study investigates the physico-mechanical and structural properties of β-glucan (BG)/pullulan (PUL) composite edible films successfully prepared with 0–0.3 g of BG. Results demonstrated that BG addition significantly increases the elongation at break (p b 0.05), tensile strength, and water dissolution time of the resulting films. The transparency of the 0.2PUL:0.1BG film and the oxygen barrier property of the 0.15PUL:0.15BG film decreased remarkably compared with those of the plain films (0.3PUL:0BG and 0PUL:0.3BG) and other composite films (p b 0.05). FTIR indicated hydrogen bonding interactions between PUL and BG molecules, and microstructural observations showed that aggregated BG is homogeneously dispersed in the PUL continuous matrix. Among the films tested, the thermal stability of the 0.15PUL:0.15BG film was the best. A PUL:BG mixing ratio of 0.15:0.15 is thus suggested to provide the best film properties. This research offers an alternative method to improve PUL-based edible films. © 2019 Published by Elsevier B.V.
1. Introduction The hidden dangers of plastic packaging, such as their nondegradability, non-renewability, and risk to food safety [1], have been widely reported in the food packaging industry. Plastic packaging not only brings about white pollution to the environment but also harms human health. Thus, many consumers have opted to use environment-friendly, safe, green, and organic food packaging [2,3]. Non-toxic, harmless, and degradable edible films as packaging materials are a popular research topic [4]. In practical applications, different freshkeeping items require different edible film performances. Edible films are mainly used to preserve fruits and vegetables, meat products, convenience food, confectionery, and baked goods. Pullulan (PUL), a linear extracellular polysaccharide secreted by Aureobasidium pullulans, is composed of α-1,6 glycoside linkages based on maltotriose [5]. It is widely used to prepare edible films and even used as a coating for tissue scaffolds [6] because of its high water solubility, plasticity, adhesiveness, non-toxicity, edible properties, and film forming properties. PUL film is rapidly soluble in water, colorless, odorless, transparent, oil repellent, oxygen blocking, heat sealable, and can prevent the oxidation of oils and vitamins in food [7]; unfortunately, its flexibility is relatively poor. Glycerol (Gly) presents strong
⁎ Corresponding authors at: School of Food Science and Engineering, Wuhan Polytechnic University, Wuhan 430023, China. E-mail addresses: [email protected] (Q. Lyu), [email protected] (G. Liu).
hygroscopicity and can be used as a plasticizer to improve the ductility and flexibility of films [8]. Films composed of a single polysaccharide are limited in application by a number of weak characteristics, such as frangibility, easy breakage, and high cost. Composite films prepared by mixing two or more polysaccharides or different high-molecularweight substances are believed to be able to address these issues [9]. β-Glucan (BG), a component of microbial and plant cell walls [10], is a linear non-starch polysaccharide consisting of continuous β-(1 → 4)D-glycoside bonds and non-continuous β-(1 → 3) bonds derived from D-glycoside linkages to glucose [11]. BG is a white powder that is soluble in water and insoluble in organic solvents, such as ethanol and acetone. It has certain swelling characteristics, water holding capacity, and gel properties [12,13]. In addition, grain BG plays a very important role in improving body health and preventing diseases. It remarkably reduces low-density lipoprotein cholesterol, improves high-density lipoprotein cholesterol, balances blood pressure, improves blood lipid levels, and balances the postprandial blood glucose and insulin response. Despite its many benefits, however, the application of BG to edible packaging films has not been reported. Therefore, if BG is introduced to PUL films, its characteristics may improve the flexibility of the latter and provide novel functional properties to the final edible film. BG and PUL are water-soluble polysaccharides that are compatible with glycerin. In the present experiment, BG and PUL were used to prepare BG-PUL (BP) composite membranes with Gly as a plasticizer, and the properties and formation mechanism of the resultant films were explored. This study aims to develop BG-PUL composite films.
https://doi.org/10.1016/j.ijbiomac.2019.08.208 0141-8130/© 2019 Published by Elsevier B.V.
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2. Materials and methods
2.6. Color of the films
2.1. Materials
Film color was tested by a portable CR-10 colorimeter (Shengguang Company, Suzhou, China) in accordance with the method of Zavareze [16]. Lab color parameters were used to identify color via the following three attributes: L* (white: 100; black: 0), a* (positive: red; negative: green), and b* (positive: yellow; negative: blue). The color difference (E*) was determined by comparison with a white standard tile and calculated using Eq. (3).
Barley BG (medium viscosity, 90% purity) was purchased from Megazym Co., Ltd. (Ireland). PUL (99% purity) was acquired from Yuanye Biological Co., Ltd. (Shanghai, China). CaCl2 and glycerin were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Soybean oil was purchased from Wushang Merchants Co., Ltd. (Wuhan, China).
E ¼
2.2. Preparation of the BP blend films Blend film-forming solutions were first prepared by dissolving BG, PLU, and Gly in 20 ml of distilled water at different ratios (Table 1) by using the solvent casting method with continuous stirring in a thermostat water bath at 80 °C for 2 h. The blend films were refrigerated at 4 °C overnight to eliminate air bubbles and then poured into a polystyrene plate with a diameter of 9 cm. Subsequently, the films were dried at 35 °C for 10 h by using a DHG-9075A electric thermostatic drying oven (Yiheng Scientific Instrument Co., Ltd.; Shanghai, China). The total content of BP in all films was maintained at 0.40 g. All blend films were conditioned at 53% relative humidity (RH) for 2 days before further testing. 2.3. Thickness measurement Following GB/T 6672-2001 [14], we measured the thickness of the films by using a CH-10-AT thickness gauge (Liuling Instrument Corporation; Shanghai, China) to the nearest 0.001 mm at 13 positions, one of which was the center of the film. The average value was calculated. 2.4. Mechanical property measurement We used a TA.XT2i instrument (Testometric, S.M.S. Corporation, USA) to determine the tensile strength (TS, MPa) and percentage elongation at break (E, %) of the films according to ASTM D882-1991 [15]. Three tensile specimens (width 10 mm, length 20 mm) were cut from each film for tensile testing. During testing, the measuring speed was 1 mm/min, the measured speed was 10 mm/min, the initial grip separation was 26 mm, and the force was 5 × g. TS and E were calculated on the basis of Eqs. (1) and (2):
TS ¼
E¼
F S
ð1Þ
L 100 L0
ð2Þ
where F is the maximum force (N), S is the cross-sectional area of the film (width × thickness, mm2), L is the length of the film at failure (mm), and L0 is the initial grip separation (mm).
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 L2 þ a2 þ b2 ¼ ðL1 −L0 Þ2 þ ða1 −a0 Þ2 þ ðb1 −b0 Þ
ð3Þ
where L1, a1, and b1 are the color attributes of the samples and L0, a0, and b0 are the color attributes of the white standard tile. 2.7. Barrier characteristics of the films 2.7.1. Water vapor permeability (WVP) measurement The WVP (g·mm/m2·day·kPa) of the composite membranes were determined on the basis of Eq. (4) in accordance with GB 1037-1988 [17]. WVP ¼
m0 d S ΔP
ð4Þ
where m0 (g) is the weight of moisture absorbed by the desiccant after 1 day, S (m2) is the area of the moisture permeable cup diameter, d is the thickness of the film (mm) and ΔP is the water vapor pressure difference across the membrane (kPa). 2.7.2. Oxygen barrier property (OP) measurement The OP of the films was determined by sodium thiosulfate titration [18]. Approximately 3 g of vegetable oil was weighed into an Erlenmeyer flask, which was then sealed with a film and placed in an incubator at 50 °C for 5 days. OP was determined on the basis of the peroxide value (PV) of the oil in accordance with the standard GB 5009.227 [19]. The lower the peroxide value, the better the oxygen barrier of the membrane A set of blanks without any film served as the control. PV (g/ 100 g) was calculated as follows (Eq. (5)): PV ¼
ðV−V0 Þ C 0:1269 100 M
ð5Þ
where V and V0 are the volumes of sodium thiosulfate consumed by the oil of the film sample and control sample (ml), respectively, C is concentration of sodium thiosulfate standard solution (mol/l), 0.1269 represents mass of iodine corresponding to 1 ml of sodium thiosulfate, and M is the quality of the oil to be titrated (g). 2.7.3. Grease permeability measurement A test tube was added with 5 ml of soybean oil and sealed with a film. Filter paper was then weighed and folded onto the tube. After 5 days, we sought to discover whether oozing had occurred and weighed the filter paper.
2.5. Water solubility of the films 2.8. Formation mechanism of the composite film Film samples (1.5 cm × 1.5 cm) were placed in boiling water to observe and record the time required for complete dissolution. Table 1 Films formulas were prepared by BG, PLU, and Gly at different ratios. Groups BG/g PUL/g Gly/g
PUL1
BP2
BP3
BP4
BP5
BP6
BG7
0 0.3 0.1
0.05 0.25 0.1
0.1 0.2 0.1
0.15 0.15 0.1
0.2 0.1 0.1
0.25 0.05 0.1
0.3 0 0.1
2.8.1. Rheological measurement Frequency–modulus scanning of all film-forming solutions was conducted by using a rheometer (Kinexus Pro+; Malvern, USA) with a cone-and-plate geometry (40 mm diameter; 150 mm gap; cone angle 40°) in accordance with the method of Xiao [20]. The frequency sweep interval ranged from 0.1 Hz to 100 Hz at 25 °C with 1% strain. The strain–viscosity relationship of all film-forming solutions was studied by using a parallel plate clamp (40 mm diameter; 1 mm gap) at 25 °C. The films were sheared at rates ranging from 0.1 s−1 to 100 s−1.
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2.8.2. Differential scanning calorimetry (DSC) The thermal stability of the film was analyzed by DSC in accordance with the method of Ye [21]. Each film sample was analyzed by using a Q2000 DSC analyzer (TA Corporation, USA) under the conditions of 25 ml/min, 30–280 °C, and 15 °C/min. 2.8.3. Scanning electron microscopy (SEM) The surface and cross-sectional morphologies of the films were observed by a SEM S-3000N instrument (Hitachi, Japan) at 1000× and 600×, respectively. 2.8.4. Fourier transform infrared spectroscopy (FT-IR) analysis The infrared absorption spectra of the samples were measured at 650–4000 cm−1 by an FT-IR NEXUS 670 instrument (Nichols Instruments, USA) in accordance with the method of Xiao [5]. The resolution was 4 cm−1, the beam precision was 0.01 cm, the number of scans was 32, and the ambient temperature was 25 °C. 2.8.5. X-ray diffraction Samples were scanned with an Empyrean X-ray diffractometer (Panaly Corporation, Netherlands) at 25 °C, 40 kV voltage, 40 mA current, and 5°–50° scan angle in accordance with the method of Xiao [5].
Fig. 1. Water dissolution time of PUL film, BG film and blend films.
weakened. In general, the chemical and aggregate structures of polysaccharides could affect their absorption of water [27]. These results suggest changes in the chemical structure of the BP composite films.
3. Results and discussion 3.3. Film color 3.1. Mechanical property measurement Mechanical properties are important characteristics of materials, especially membrane materials. TS and E influence the end-use handling properties and mechanical performance of films [22]. Addition of increasing amounts of BG to the films increased their TS and E (Table 2). Thus, BG addition could increase the mechanical properties of the films, likely due to the formation of hydrogen bonds between PUL and BG. However, the TS and E of BP6 (PUL:BG = 0.05:0.25) was low, probably because of its low PUL content and weak phase separation. Among the films tested, the pure BG film revealed the highest TS and E. The difference in thickness among the composite films was not significant (p N 0.05) (Table 2). As BG content increased, the thickness of the films gradually increased, which reveals that the film thickness is linked to the composition of the film. These results are in agreement with those reported by Abugoch [23], Sebti [24], and Di Pierro [25], who respectively studied buckwheat protein–chitosan, Hypromellose–chitosan, and chitosan–whey protein blend films. The thickness of the films was similar to that obtained by Pan [26].
Film color is another important indicator of the overall appearance and consumer acceptance of an edible film [26]. The pure PUL film was the most transparent among the films studied (Fig. 2). Significant differences (p b 0.05) in L* and E* were observed among the films. As BG addition increased to 0.1 g (BP3), the color of the composite films gradually changed to black. The transparencies of the blend films increased with further addition of BG, which means the higher the amount of BG added, the more transparent the blend film becomes. Zhu [9] revealed similar result that the pure PUL film is more transparent than PUL/carboxymethyl–gellan composite films. 3.4. Barrier characteristics The barrier properties of edible films play an important role in their use as packaging materials and help ensure food quality during food storage and transport. The barrier properties of edible films include moisture resistance, oxygen resistance, and oil resistance. 3.4.1. WVP measurement The results of the WVP testing are shown in Fig. 3. Data analysis indicated no significant difference (p N 0.05) in WVP among all composite
3.2. Water solubility of the films Water solubility is an indicator of the water resistance of edible films [8]. The fields of application of films with different water solubilities vary. Among the BP composite films, BP2 (PUL:BG = 0.25:0.05) and BP3 (PUL:BG = 0.2:0.1) revealed the best solubility (Fig. 1). As the content of BG increased, the water solubility of the composite films
Table 2 Mechanical properties of BP blend films. Films
TS (MPa)
E (%)
Thickness (mm)
PUL1 BP2 BP3 BP4 BP5 BP6 BG7
7.58 ± 1.04e 17.00 ± 1.69d 25.17 ± 2.93c 33.81 ± 0.67b 34.62 ± 0.76b 30.77 ± 0.68b 48.44 ± 0.92a
23.47 ± 2.64d 38.73 ± 1.65c 57.27 ± 1.21b 64.61 ± 7.06b 75.86 ± 8.30a 54.72 ± 5.04b 79.74 ± 10.67a
0.042 ± 0.003b 0.044 ± 0.001b 0.049 ± 0.001a 0.051 ± 0.002a 0.051 ± 0.002a 0.052 ± 0.003a 0.052 ± 0.002a
Values are means ± standard deviation. The different letters in a column indicate significant differences at p b 0.05.
Fig. 2. The color of PUL film, BG film and blend films.
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oxygen barrier of the PUL film may be stronger than that of the BG film because of differences in their molecular structures. 3.4.3. Grease permeability measurement All films had oil-impermeable characteristics, which means all of their membrane structures are dense. 3.5. Formation mechanism of the composite film
Fig. 3. The water vapor permeability of PUL film, BG film and blend films.
film groups. In general, among the films tested, the PUL and BG films demonstrated the best moisture resistance; moreover, the moisture resistance of the BP2 and BP6 compound films were better than that of the BP3, BP4, and BP5 films. This result may be due to the aggregation and self-condensation of BG particles at higher concentrations, which produces voids in the composite matrix [28]. Gly also affects the water absorption of the films. The moisture barrier property of the pure PUL film was better than that of the BP compound film, similar to the result of the PUL–gelatin/carboxymethyl gelatin film studied by Zhu [9,29].
3.5.1. Solution rheological measurement The presence or absence of defects in thin liquid films after coating relies partly on their rheological properties [30]. Bubbles are difficult to eliminate when the edible film liquid is in gel form or has high viscosity, which prohibits the formation of a flat film. Liquids with low viscosity are prone to corrugation under the action of hot air during edible film formation, and the thickness of the formed film may be uneven [20]. The rheological properties of the membrane fluid depend on its internal structure [31]. In the present study, among the films tested, the viscosity of the pure PUL film was the largest (Fig. 5a). As BG content increased, the viscosity of the composite film liquid gradually decreased, and the viscosities of the BP2 and BP3 composite films were highly similar. The membrane fluid exhibited shear thinning behavior under static shear, so all film fluids could be considered to be non-Newtonian fluids. This result indicates that intermolecular junctions are disrupted at a rate faster than their reformation rate as the shear rate increases, resulting in a decrease in junction density and, hence, a drop in viscosity. Such findings reveal that the CMC FFS behaves as an entangled network system
3.4.2. OP measurement The results of OP testing of the composite membranes are shown in Fig. 4. The membrane-free group had a PV of 1.335 g/100, which is remarkably higher than the PV of the membrane-covered groups. This result indicates that the PUL, BG, and BP blend films could block oxygen transmission. Data analysis showed a significant difference (p b 0.05) in OP among the films. The PV of the PUL film was 1.193 g/100 g. Addition of BG gradually enhanced the OP of the composite films, and the highest OP was observed when the ratio of PUL and BG was 0.15:0.15 (BP4) and PV was 1.124 g/100 g. It indicates that addition of a certain amount of BG to the PUL film could improve its OP. This finding also confirms that BG undergoes intermolecular interactions with PUL. The
Fig. 4. The oxygen permeability of PUL film, BG film and blend films.
Fig. 5. (a) The viscosity characteristics of PUL film, BG film and blend film liquids. (b) The tan δ (G″/G′) of PUL film, BG film and blend films (G″ represents viscous modulus and G′ represents elastic modulus).
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(or a concentrated solution) [32]. The properties of the pure PUL membrane fluid were identical to those of Liu [31] but differed from those of Xiao [20]; this difference may be due to variations in the concentrations of the PUL film. Changes in the viscosity and fluid properties of the BG membrane fluid were consistent with those of the high-concentration BG solution studied by Kinnari J and Wang [33,34]. The modulus (G″) and elastic modulus (G′) characterize the viscoelasticity of a film fluid. Viscoelasticity is a state exhibited by a solution in response to some applied stress. The tangent (tan) δ (G″/G′) of the film liquids are shown in Fig. 5b. The PUL1–BP5 film liquids exhibited G′ N G″ at low vibration frequencies, which means the values are intersected when a high-frequency vibration stage is reached. This result reveals that the membrane fluid system is solid under low vibrational frequencies and may be considered elastic. The tan δ of BP6 film liquid closed to 1 in the whole scanning frequency, which expressed as viscoelasticity. For BG7 film liquid, the G″ was remarkably larger than the G′, indicating that the majority of the energy was dissipated by viscous flow, similar to that reported by de Souza on native BG [35]. The overall rheological properties of the film fluids indicate that the properties of the composite film fluid systems gradually transform from those of a solid to those of a liquid with increasing BG. Elastic behavior has been reported to be a function of the number of effective chains participating in the formation of a network structure. Increases in G′ and G″ with increasing frequency in the presence of PUL suggest that the latter is well dispersed within the PUL/BG blend solution [32]. 3.5.2. SEM The microstructure of an edible film reflects its homogeneity and compactness; indeed, an edible film with high uniformity and compactness is also believed to have relatively high barrier properties [36,37]. Fig. 6a shows that the surface of PUL1 has wrinkles, which may be caused by the movement of fluid across the membrane during drying. A crack on the flat cross-section (Fig. 6b), which may be caused by the electron beam hitting the sample, was also observed; this crack is evidence that PUL1 breaks easily. The surface and cross-section of BP2 are relatively uniform and smooth, thereby indicating that BG and PUL have good compatibility at this ratio. BP3 showed a smooth and uniform cross section; its surface was continuous but not smooth because its color was not uniform. This finding may be attributed to the decreased compatibility between BG and PUL at this ratio. The surface of BP4 was similar to that of BP3 but was not as smooth. The cross-section of BP4 was continuous and smooth. Addition of BG in the range added to BP5–BG7 caused the surface of the resulting films to become flatter and smoother, thus showing that the compatibility of BG and PUL increases gradually as the ratio of BG and PUL increases. The toothed structures observed in the cross-section of BP5, BP6, BG7 were caused by cutting. The surface of BP6 was flat but cracked, likely due to the electron beam hitting it. Overall, the BP2, BP5, and BG7 films were smooth and flat, consistent with the moisture resistance results. BP3, BP4, and BP5 showed good OP, which prevents gas permeation and reveals the presence of few microporous structures on the surface of these films. 3.5.3. FTIR The FTIR data were analyzed to identify the main functional groups of the films [38]. The infrared spectra of the composite films are shown in Fig. 7. The absorption peak in the 1800–1200 cm−1 region is characteristic of the polysaccharide. FTIR spectra showed the characteristic absorption bands of the α-anomeric and β-anomeric configurations at 850 and 890 cm−1, respectively [39]. The absorption peak at 850 cm−1 in the BG7 film is due to the purity of the BG sample, which is approximately 95%. The absorption band at 3354 cm−1 is attributed to the \\OH stretching vibrations of the polysaccharide [37]. The absorption peak of BG at 1073 cm−1 is attributed to the \\OH stretching vibrations of C6. The absorption of PUL at 1028 cm−1 is due to the stretching vibrations of C\\O\\C [38]. The absorption peaks of PUL at 1610 cm−1 and the absorption peaks of BG at 1688 cm−1 reflect the
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stretching vibrations of C\\O linked\\OH. The absorption band around 1070 cm−1 broadened and shifted to a lower wave number with the increase of PUL, indicating the gradual increase of intermolecular hydrogen indicating the gradual increase in interactions between PUL and BG. The peak at 1688 cm−1 shifted toward a lower wavenumber with increasing BG, thereby indicating a gradual increase in intermolecular hydrogen bonds between PUL and BG. These results are in agreement with the mechanical properties of the edible film.
3.5.4. DSC The endothermic peaks (Tg) of a polymer blend can reveal useful information related to the miscibility of its constituent components [40]. In the current study, the DSC curves of all films were comparable, which indicates that no chemical reaction occurs between PUL and BG, as shown in Fig. 8. The Tg values of BP3 and BP4, at 134.95 and 135.03 °C, respectively, were the highest among the composite films. This result indicates that the two films have good thermal stability and compatibility and that hydrogen bonds are formed between the molecules, consistent with their OP results. Among the films, the Tg of BP6 was the lowest, indicating unsatisfactory compatibility between PUL and BG in this film. This result may be due to stronger hydrogen bonding interactions between BG molecules than between BG and PUL molecules when the PUL:BG ratio is 0.05:0.25, which causes microscopic phase separation, consistent with the mechanical properties of the composite membrane. The Tg of the PUL film obtained in this experiment was higher than that obtained by Sakata [41] and Xiao [40], likely due to differences in the sample amounts and heating rates employed.
3.5.5. X-ray diffraction (XRD) XRD is used to assess the apparent crystallite sizes of polymers, determine crystallization processes and kinetics, characterize the fine structure of materials, and explore interactions between molecules. A completely incompatible polymer blend system will show separate crystalline regions owing to the absence of or weak interactions, and the resulting XRD patterns could be expressed as a proportional diffraction spectrum of each component. The XRD results are shown in Fig. 9. The crystal peaks of all edible films are observed at approximately 20° and demonstrate an amorphous structure, thus revealing the formation of a dense network between molecules [28]. This finding is consistent with the SEM results and similar to those reported by Xiao, Zhang, and Trovatti [5,42,43].
4. Conclusion BP films were successfully prepared with different ratios of PUL and BG by using the solvent casting method. The optimum PUL:BG mixing ratio of 0.15:0.15 yielded good optical transparency, reduced the WVP, and enhanced tensile properties, all of which are essential properties required for packaging applications. The experiments showed that the thickness of the BP composite films increases with increasing BG content. The overall rheological properties of the film fluids indicate that the properties of the composite film fluid systems gradually transform from those of a solid to those of a liquid with increasing BG. The microstructure of the final edible film indicated that BG and PUL have good compatibility at some ratio. Addition of increasing amounts of BG to the films increased the mechanical properties of the films, likely due to the formation of hydrogen bonds between PUL and BG, confirmed by the FTIR data. All of the films could block the permeation of grease. The BP3, BP4, and BP5 films revealed the best barrier properties and could be used as cling film. However, the sealing strength of these films requires further improvement if they are to be used as packaging materials.
Please cite this article as: J. Chang, W. Li, Q. Liu, et al., Preparation, properties, and structural characterization of β-glucan/pullulan blend films, https://doi.org/10.1016/j.ijbiomac.2019.08.208
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Fig. 6. (a) SEM micrographs of the surface of PUL film, BG film and blend films. (b) SEM micrographs of the cross-section of PUL film, BG film and blend films.
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J. Chang et al. / International Journal of Biological Macromolecules 140 (2019) xxx
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Acknowledgment This study was supported by the Chinese National Science Foundation (Grant No. 31771925) and China Central Government Guide the Development of Local Science and Technology Special Funds (2017 No. 88). References
Fig. 7. FTIR spectra of PUL film, BG film and blend films at wavenumbers between 400 and 4000 cm−1.
Fig. 8. DSC curves of PUL film, BG film and blend films.
Fig. 9. X-ray diffraction patterns of PUL film, BG film and blend films.
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