- Email: [email protected]
Physical and structural characterization of edible bilayer films made with zein and corn-wheat starch
Journal of the Saudi Society of Agricultural Sciences xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Journal of the Saudi Society of Agricultural Sciences journal homepage: www.sciencedirect.com
Full length article
Physical and structural characterization of edible bilayer films made with zein and corn-wheat starch Guanjie Zuo a,c, Xiaoyong Song b,a,c, Fusheng Chen a,⇑, Zhixiang Shen a a
Henan University of Technology, Zhengzhou 450001, China North China University of Water Resources and Electric Power, Zhengzhou 450011, China c Henan Nanjie Village Group (Limited) Company, Luohe 462600, China b
a r t i c l e
i n f o
Article history: Received 17 July 2017 Revised 14 September 2017 Accepted 27 September 2017 Available online xxxx Keywords: Bilayer films Corn-wheat starch Zein Antioxidative capacity Surface and cross-sectional morphology
a b s t r a c t Edible bilayer films prepared using an additional of zein (Z) layer casted on the corn-wheat (CW) starch films were prepared and characterized in terms of colour, thickness, water content, optical, water vapour permeability (WVP), mechanical properties and antioxidative capacity. Results showed that the colour, thickness, water content, elongation at break (E), and antioxidative capacity increased with the increasing of zein addition, but WVP and tensile strength (TS) decreased. In comparison with biaxially oriented polypropylene polyethylene composite films (BOPP-PE), CW10/Z3 and CW8/Z3 even showed better antioxidative capacity and elongation at break (E). The surface and cross-section micrographs were imaged by scanning electron microscopy (SEM) and atomic force microscopy (AFM). On both of SEM and AFM images, CW10/Z1 and CW10/Z2 films exhibited smoother surface and more compacted structure than other films. Besides, CW10/Z3 and CW8/Z3 of SEM images showed the better cross-section. Therefore, CW/Z edible bilayer films could be potentially used as a new inner package material for oil bag in instant food to improve the development of food industry. Ó 2017 The Authors. Production and hosting by Elsevier B.V. on behalf of King Saud University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction Due to the reasonable price, pleasant taste, and easy preparation, instant food have increasingly satisfied nutritional requirements and met the fast-paced lifestyle (Buckley et al., 2007; Cho et al., 2010). Therefore, seasoning bags (e.g., flavor powder, oil sauce, and dried vegetable) served with the instant food are important to affect the consumer acceptance and admiration by all generations. Nowadays, synthetic petroleum-based polymers (e.g., ethylene terephthalate (PET) and polyethylene (PE)) were widely used to pack the seasonings, because of stable physicochemical properties and ideal mechanical behaviors (Choi et al., 2016). But they also bring some serious threats to human health and product quality (Cárdenas et al., 2008; Devlieghere et al., 2004; Suh et al.,
⇑ Corresponding author at: College of Food Science and Technology, Henan University of Technology, 100 Lianhua Street, Zhengzhou 450001, China. E-mail address: [email protected] (F. Chen). Peer review under responsibility of King Saud University.
Production and hosting by Elsevier
2017). Therefore, it is urgently necessary to create a novel packaging material to replace the traditional synthetic materials for seasoning bags. Biodegradable edible films developed by bio-based materials (e.g., proteins, polysaccharides, and lipids) are recently introduced. Starch-based edible films have raised considerable attention, due to its comprehensive sources, low cost, and biodegradability. But they also have own disadvantages, such as poor mechanical properties, low water blocking ability and non-thermoplasticity (Basiak et al., 2017; Danyxa et al., 2017; Das and Chowdhury, 2016). Protein-based films, such as zein films, could make up these weaknesses of starch-based films by their hydrophobic amino acids (e.g., proline, leucine, and alanine) (Wang et al., 2017; Yin et al., 2014), excellent film forming ability (Ghanbarzadeh and Oromiehi, 2008), low oxygen permeability (Fund et al., 2010) and better biocompatibility. But, poor mechanical properties (Dong et al., 2017) and expensive price of zein greatly restrict its industrial application. Therefore, it is expected that the additional zein layer on the starch films will bring synergistic effects to produce bilayer films with advanced mechanical properties and oxidative stability, in order to extend the industrial application and satisfy consumer demands. The overall objective of the present study is to design a cornwheat starch-based bilayer films with the addition of zein layer,
https://doi.org/10.1016/j.jssas.2017.09.005 1658-077X/Ó 2017 The Authors. Production and hosting by Elsevier B.V. on behalf of King Saud University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article in press as: Zuo, G., et al. Physical and structural characterization of edible bilayer films made with zein and corn-wheat starch. Journal of the Saudi Society of Agricultural Sciences (2017), https://doi.org/10.1016/j.jssas.2017.09.005
2
G. Zuo et al. / Journal of the Saudi Society of Agricultural Sciences xxx (2017) xxx–xxx
and to investigate the effect of zein addition on the colour, thickness, water content, opacity, water vapour permeability, mechanical properties, and antioxidative capacity of the films. Besides, this study will provide a new direction for developing inner packaging in oil bag. 2. Materials and methods 2.1. Materials Oxidized corn starch (C, 27.5% amylose content, Dingfeng Develop Co., Ltd. China), wheat starch (W, 29.0% amylose content, Lvyuan Starch Co., Ltd. China), and zein (Z, 91.1% protein content, Dulai iotechnology Co., Ltd. China). D-Sorbitol (98.0%, C6H14O6, 182.17 g/mol), citric acid (99.5%, C6H8O7H2O, 210.14 g/mol), carboxymethylcellulose sodium (90.0%, C8H16NaO8, 242.16 g/mol), sodium alginate (99.9%, (C6H7NaO6)n), ethyl alcohol (99.7%, CH3OH, 46.07 g/mol), polyethylene glycol-400 (99.0%, HO(CH2CH2O)nH, 400 g/mol) and glycerol (99.0%, HOCH2CHOHCH2OH, 92.09 g/mol) were supplied by Tianjing Chemical Reagent Co., Ltd. (Luoyang, China). All other chemicals (from local resources) were on analytical grade.
and glycerol (1.80 g) were added into the solution and stirred for 20 min, followed by the incubation (XMTD-7000, Beijing Yongguangming Medical Instrument Co., Ltd., Beijing, China) for 18 min at 80 °C. Through appropriate heat treating, the internal structure of zein would become loose and boost the interaction. Both polyethylene glycol-400 and glycerol are plasticizers, but the former has certain water-holding capacity. Next, the mixture was cooled down to 45–55 °C and poured onto a dried CW films within a plastic dish. During this process, the lower corn-wheat starch films would maintain its original appearance. The bilayer films forming solution was dried at room temperature for 30 min, and then heated up to 80 °C for 30 min. Films were finally removed from the plates, and conditioned to 54% relative humidity within a desiccator at room temperature for 2 d. All films were prepared in triplicate. Table 1 shows different kinds of bilayer films CW10/Z0-CW10/ Z3 were explained 10 mL of corn-wheat starch forming solution with 0 mL, 1 mL, 2 mL and 3 mL zein solution. CW8/Z3 were explained 8 mL corn-wheat starch forming solution film with 3 mL zein solution. Biaxially oriented polypropylene polyethylene composite films (BOPP-PE, Henan Nanjiecun Group Ltd., Henan, China) was a kind of commercial film. 2.3. Film colour
2.2. Preparation of bilayer films The bilayer films were designed by adding a zein (Z) layer on a dried corn-wheat starch film (CW) which showed in Table 1. In brief, the starch solution (6.00 g/100 mL) was initially prepared by dissolving corn starch and wheat starch at 2:3 (w/w) ratio in distilled water, followed by the addition of sorbitol (0.6% w/v; 0.60 g sorbitol in 100 mL distilled water), citric acid (2.5% w/v; 2.50 g citric acid in 100 mL in distilled water), and the mixture of carboxymethyl cellulose sodium and sodium alginate at 3:2 (w/w) ratio (1.4% w/v; 1.40 g mixture in 100 mL distilled water). Both sorbitol and citric acid are commonly employed as such plasticizers which can improve the films mechanical properties, harmless and even healthy. Carboxymethyl cellulose sodium and sodium alginate could enhance the barrier properties of films. The mixture was then stirring for 20 min at 40 °C with Water-bathing Constant Temperature Vibrator (THZ-82, ShanghaiHuxi Analysis Instrument Co., Ltd., Shanghai, China). Subsequently, the starch gelatinization was induced by heating the mixture at 85 °C for 40 min. Finally, the film forming solution was degassed for 30 min within a vacuum pump (DZF, Beijing Yongguangming Medical Instrument Co., Ltd., Beijing, China) at 80 °C with a vacuum degree of 0.09 Pa. The CW films layer was first formed by casting the starch solution on the plastic dishes (Diameter = 100 mm) and dried at 45 °C for 12 h. The protein solution was prepared by dissolving 6.00 g zein in 100 mL of aqueous ethanol (95%, v/v) and stirring for 3 min with magnetic stirrer. (85-2, Shanghai Huxi Analysis Instrument Co., Ltd., Shanghai, China). Both of polyethylene glycol-400 (0.90 g)
Film colour was determined based on the method described by Pereda et al. (2011) with some modifications. A WSC-S colour difference meter (Shanghai Precision & Scientific instrument Co., Ltd., Shanghai, China) was used to analyze the film colour. Whiteness (WI) of film was calculated using the following equations.
h i0:5 2 2 2 WI ¼ 100 ð100 L Þ þ a þ b
ð1Þ
where L⁄ was 0 for black and 100 for white, a⁄ value indicated red (+) to green (), and b⁄ value represent yellow (+) to blue (). Five measurements were taken on each triplicate film prepared. 2.4. Thickness, water content and opacity The thickness of bilayer films was measured by using a thickness gauge (GM280F, Shenzhen Instruments and Meters Co., Ltd., Shenzhen, China). Six thickness measurements were taken on each triplicate film prepared. The water content of film was determined gravimetrically after drying the film in an oven (101-3AB, Beijing Zhongxing Weiye Co., Ltd., Beijing, China) at 105 °C for 24 h. The measurement was performed in triplicate. According to Josiane et al. (2001), film opacity was determined by using a spectrophotometer (722S, Shanghai Instrument Electric Analytical Co., Ltd., Shanghai, China). The absorbance was measured at a wavelength of 600 nm. The opacity of the film was calculated using the following equation:
Op ¼
AbS600 L
ð2Þ
where Op was the opacity, Abs600 was the absorbance at 600 nm, and L was the film thickness (lm). Five repetitions were performed for each triplicate sample.
Table 1 The bilayer films classifications.a Films
Starch form solution (mL)
CW10/Z0 CW10/Z1 CW10/Z2 CW10/Z3 CW8/Z3 BOPP-PE
10 0 10 1 10 2 10 3 8 3 Biaxially oriented polypropylene polyethylene composite films
Zein form solution (mL)
a 1 mL of protein forming solution content 0.06 g zein. And 1 mL of starch forming solution content 0.06 g corn-wheat starch.
2.5. Water vapour permeability Water vapour permeability (WVP) was determined gravimetrically based on the method described by Talja et al. (2008) with some modifications. Briefly, the film was sealed on a permeation cell (inner diameter: 45 mm, height: 25 mm), which was filled by 20.50 g of granular anhydrous calcium chloride (0% relative humidity) to ensure a consistent air gap (<6 mm). The permeation
Please cite this article in press as: Zuo, G., et al. Physical and structural characterization of edible bilayer films made with zein and corn-wheat starch. Journal of the Saudi Society of Agricultural Sciences (2017), https://doi.org/10.1016/j.jssas.2017.09.005
G. Zuo et al. / Journal of the Saudi Society of Agricultural Sciences xxx (2017) xxx–xxx
cell was then placed in a desiccator containing saturated NaCl solution (75% relative humidity). The permeation cell was weighed as a function of time until changes to the nearest 0.001 g. WVP was calculated as follows:
WVP ¼
mL AtDp
2.6. Mechanical properties Tensile strength (TS, MPa) and elongation at break (E,%) of the film were determined using a TAXT-plus texture analyzer (SMS, England) with a load cell of 1 kg (Zhong and Li, 2011). Prior to the measurement, films were cut into strips (10 50 mm) and mounted between the tensile grips (TA 96, England Stable Micro Systems). Initial grips separation was 30 mm and cross-head speed was 1.0 mm s1. The measurement was performed in triplicate. 2.7. Antioxidative capacity Oxygen permeability (OP) was represented as peroxide values (POV) of soy oil (Cho et al., 2010). The film (D = 9 cm) was sealed on a 250 mL-conical flask containing 20 g soy oil and stored at 60 °C for 10 days. The POV was then measured (AOCS, 1989). The measurement was performed in triplicate. 2.8. Scanning electron microscopy (SEM) analysis Surface and cross-sectional microscopic images of CW/Z bilayer films were obtained using a scanning electron microscope (JSM7401F, JEOL, Japan). The films specimens were cut into a chunk (5 mm 5 mm) and mounted on an aluminum stub using a double-sided tape, which was then coated with a thin gold layer (20 nm of thickness). The samples were observed on an accelerating voltage of 20 kV magnification at 2000 for cross-sectional, 2000 and 4000 for Surface, respectively. 2.9. Atomic force microscopy (AFM) analysis According to Ghanbarzadeh and Oromiehi (2008), the surface morphology of the bilayer films were determined by atomic force microscopy. The basic principle of AFM is to detect the forces between a cantilever and the sample surface (x-y plane) which is further translated to a deflection of the cantilever based on Hooke’s law and presented as a topographic image with a constant scan size (10 lm 10 lm). The captured images were analyzed using Dynamic SPM acquisition mode and Contact SPM acquisition mode (AFM, ZLAFM. Micro-Nano Equipment Co., Ltd., Shanghai, China). Finally, the roughness of the film surface was evaluated by Rq, Ra, and Z using the following formulas:
PN Ra ¼
Z¼
i¼1 Z i
N
N X Zi i¼1
N
Z
where Rq was root-mean square average of height deviations taken from the mean data plane, Ra was the average of the height deviations from a mean surface, Z was the arithmetic mean of the height (nm), Zi was the height of the data point (nm), and N was the number of the data point.
ð3Þ
where m was the poor quality of anhydrous calcium chloride before and after water absorption through the film (g), L was the thickness of the film (lm), A was the permeation area (m2), t was the time of permeation (s), and Dp was water vapour pressure difference across the film (2376.30 Pa). The measurement was performed in triplicate.
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi XN ðZi ZÞ2 Rq ¼ i¼1 N
3
ð4Þ
ð5Þ
ð6Þ
2.10. Statistical analysis Data was analyzed using SPSS software (version 17.0, Statistical Package for the Social Sciences Inc., Chicago, IL, USA). A one-way analysis of variance (ANOVA) was performed to measure statistical differences in colour, thickness, water content, opacity, water vapour permeability, mechanical properties, and oxygen permeability among the various films. Duncan’s multiple range test was used to determine the significant difference (P < .05) of means. 3. Results and discussion 3.1. The colour of CW/Z bilayer films The colour of films is primarily influenced by the ingredient composition, and can has a big impact on the consumer acceptance of packaged products. The effects of starch and zein addition on the colour of films are shown in Table 2. An analysis of variance indicated that the WI of films was significantly affected by the starch and zein addition (P < .05). Overall, because of the yellowness of zein, the WI of film was significantly decreased (P < .05) with the increase of zein addition, in which CW10/Z3 was more yellower than the other films with same amount of starch. This was also demonstrated by CW8/Z3 that was significant yellower (b⁄ of 16.89) with higher percentage of zein. Therefore, zein was the predominant ingredient to determine the WI of films. However, all bilayer films prepared in the current study (87.10 of WI) had darker colour than the BOPP-PE films (94.32 of WI). 3.2. Thickness, water content and opacity of CW/Z bilayer films Thickness, water content, and opacity of the bilayer films prepared by starch and zein solutions are shown in Table 3. The results showed that the film thickness increased significantly (P < .05) with the increase in starch and zein addition, which were over double thicker (130.27 mm) than the BOPP-PE films (60.97 mm) as commercial inner packing material. Compared with starch and polycaprolactone (234.00 mm) active bilayer films reported by Rodrigo et al. (2015), the films in our study showed a certain superiority. In particular, the film was thicken by 10 mm on every additional 1 mL of zein solution. There was no significant difference (P > .05) on the water content of the different bilayer films, except CW10/Z3 bilayer films, which had the highest water content (15.48%) (Table 3). CW10/ Z3 showed the higher value than the others, due to an enormous amount of hydrophobic groups on zein are repulsive to water but hydrophilic groups on zein are able to bind with water. And the additional polyethylene glycol-400 has both a certain waterholding and plasticized capacities. An analysis of variance indicated that only zein addition significantly (P < .05) affect the opacity of bilayer films, whereas extra starch content did not significantly influence the opacity (P > .05). Overall, the addition of zein layer generally decreased the opacity of bilayer films, except CW10/Z1 films. However, all bilayer films prepared in the current study (89.01 Abs600/mm) were more transparent than the BOPP-PE films (90.33 Abs600/mm) (Table 3). The reasons maybe that between starch and zein exhibited the better biocompatibility which were in accordance with Hosseini et al. (2013). In addition, the changes may be relative to the surface
Please cite this article in press as: Zuo, G., et al. Physical and structural characterization of edible bilayer films made with zein and corn-wheat starch. Journal of the Saudi Society of Agricultural Sciences (2017), https://doi.org/10.1016/j.jssas.2017.09.005
4
G. Zuo et al. / Journal of the Saudi Society of Agricultural Sciences xxx (2017) xxx–xxx
Table 2 Colour of CW/Z bilayer films.a,b L*
Films CW10/Z0 CW10/Z1 CW10/Z2 CW10/Z3 CW8/Z3 BOPP-PE a b
a* d
b* e
0.77 ± 0.00 1.50 ± 0.02d 2.42 ± 0.01c 2.94 ± 0.05b 3.18 ± 0.02a 0.66 ± 0.03f
96.15 ± 0.08 95.78 ± 0.10c 94.86 ± 0.23b 94.48 ± 0.06a 94.36 ± 0.06a 96.25 ± 0.03d
Whiteness b
4.65 ± 0.05 8.03 ± 0.13c 12.81 ± 0.12d 15.89 ± 0.26e 16.89 ± 0.14f 4.21 ± 0.17a
93.92 ± 0.09e 90.81 ± 0.19d 85.94 ± 0.15c 82.93 ± 0.27b 81.91 ± 0.14a 94.32 ± 0.13f
Data were shown in mean ± standard deviation. ND was shown not detection. Different superscript letters in the same column indicated significant differences (P < .05).
morphology of bilayer films that were described in SEM and AFM images.
In theory, due to the hydrophobicity of zein, the film with zein displayed less affinity to water and better water barrier property than the films without zein. This was also demonstrated by Cho et al. (2010), in which the addition of zein greatly decreased WVP by 0.33 1012 kg m/m2 s Pa in the soy protein isolatedbased films for olive oil packaging. However, the increase of zein addition only performed minor effect on the WVP of films which consist with the results of water content above. Furthermore, the higher thickness of films provides a longer pathway for the penetration of water molecule to decrease WVP of films, so CW10/Z0 (115.67 lm) had the highest of WVP. On the one hand there was a positive correlation between WVP and surface tortuosity, Pan et al. (2014) has also proved the results. On the other hand SEM images of CW10/Z0 below showed abundant holes just right explain an increase on WVP.
3.3. Water vapour permeability
3.4. Mechanical properties
High water vapour permeability of edible films is not desirable for the industrial applications and would resulted in food degenerating easily. An analysis of variance exhibited that the addition of zein significantly (P < .05) lower the WVP of films, whereas the additional of zein was not significant (P > .05). However, all bilayer films prepared in the current study had significantly (P < .05) lower water barrier property than the BOPP-PE films. Overall, CW10/Z0 had the highest WVP (8.89 1011 g/m s Pa). The bilayer films with same amount of starch but increased zein addition had similar water barrier property (5.96 1011 g/m s Pa of WVP). And the WVP of the film with same amount of zein was gently decreased (about 0.25 1011 g/m s Pa) with the increase of starch addition (Fig. 1).
The tensile strength (TS, MPa) and elongation at break (E,%) of the bilayer films were presented in Fig. 2. It showed that when changing the addition of zein forming solution, TS and E of bilayer films expressed the significant difference (P < .05), but the effect of starch addition from CW10/Z3 and CW8/Z3 on TS and E was not significant (P > .05). Overall, TS data were declined and E data were improved as zein level increased. On another side, different amount of starch in the films with same zein addition resulted in the similar TS (9.39 MPa) and E (77.14%) of films. All bilayer films prepared in the current study exhibited poorer mechanical properties than the BOPP-PE films (84.56 MPa of TS and 42.54% of E). Several reasons could contribute to the results. The accounts maybe that glycerol and polyethylene glycol-400 have better
Fig. 1. Water vapour permeability of the CW/Z bilayer films. Different letters indicated significant difference (P < .05). Abbreviations: corn-wheat (CW), zein (Z), biaxially oriented polypropylene polyethylene composite films (BOPP-PE), and water vapour permeability (WVP).
Fig. 2. The mechanical properties of CW/Z bilayer films. Different letters indicated significant difference (P < .05). Abbreviations: corn-wheat (CW), zein (Z), biaxially oriented polypropylene polyethylene composite films (BOPP-PE), tensile strength (TS, MPa), and elongation at break (E,%).
Table 3 Thickness, water content and opacity of CW/Z bilayer films.a,b Films
Thickness (mm)
Water content (%)
Opacity (Abs600/mm)
CW10/Z0 CW10/Z1 CW10/Z2 CW10/Z3 CW8/Z3 BOPP-PE
115.67 ± 1.53b 126.67 ± 1.53bc 135.33 ± 9.29cd 147.00 ± 0.81d 126.67 ± 3.21bc 60.97 ± 0.75a
11.80 ± 0.33a 11.41 ± 0.26a 12.95 ± 0.57a 15.48 ± 1.22b 12.94 ± 0.21a ND
89.73 ± 0.06c 90.30 ± 0.20d 88.93 ± 0.15b 87.97 ± 0.15a 88.13 ± 0.06a 90.33 ± 0.06d
a
Data were shown in mean ± standard deviation. ND was shown not detection. Different superscript letters in the same column indicated significant differences (P < .05). b
Please cite this article in press as: Zuo, G., et al. Physical and structural characterization of edible bilayer films made with zein and corn-wheat starch. Journal of the Saudi Society of Agricultural Sciences (2017), https://doi.org/10.1016/j.jssas.2017.09.005
G. Zuo et al. / Journal of the Saudi Society of Agricultural Sciences xxx (2017) xxx–xxx
plasticizing properties than sorbitol and citric acid. Therefore, the increase of glycerol and polyethylene glycol-400 addition will make the films with better flexibility. Besides, the water content which was act as a bridge in Table 2 also explained this phenomenon. Namely, with the additional zein increased, E showed bigger and TS decreased which perhaps that the enough zein forming solution has already achieved saturated beyond itself, leading to uneven distribution of zein on the starch film will create molecular space on the film structure to produce more stretchable films (Corradini et al., 2014). Although TS value was far from BOPP-PE films, it could meet the needs of the inner packing (Cho et al., 2010). Meanwhile, the E value was higher than that of commercial film.
3.5. Oxidative stability of soybean oil packaged in the CW/Z bilayer films Lipid oxidation generally results in the loss of nutritional value and develops off-flavors to negatively impact consumer acceptance, so the antioxidative capacity of biodegradable edible film is an important feature to study. The peroxide value of soybean oil packed by the CW/Z bilayer films during storage were shown in Fig. 3. An analysis of variance presented that the antioxidative capacity of the bilayer films was significantly improved with the increase of zein addition (P < .05), whereas the starch addition did not obviously affect the peroxide value of soybean oil packed by the bilayer films (P > .05). Overall, the CW10/Z0 film showed the lowest antioxidative capacity with the highest POV (231.90 ± 3.24 meq/kg), whereas CW10/Z3 films exhibited the best antioxidative capacity with the lowest POV (94.69 ± 0.29 meq/kg) value. Moreover, the CW/Z bilayer films showed the better oxygen barrier properties than monolayer film, because the cross-linking reaction between starches and zein can product films with compacted structure, to further decrease oxygen permeability; the aromatic amino acids in zein are able to scavenge the free radicals by the aromatic rings; the sulfur-containing amino acids in zein can inhibit oxidation by chelating metal ions. In the current study, the CW/Z bilayer films had better antioxidative capacity (98.95 meq/kg oil of POV) than the BOPP-PE films (219.32 meq/kg of POV), therefore, they are able to potentially replace the synthetic petroleum-based materials used for oil sauce packaging in the industry.
Fig. 3. The peroxide values of CW/Z bilayer films. Different letters indicated significant difference (P < .05). Abbreviations: corn-wheat (CW), zein (Z), biaxially oriented polypropylene polyethylene composite films (BOPP-PE), and peroxide values (POV).
5
3.6. SEM images The microstructure of the bilayer films is mainly affected by the structural arrangement of different components in the initial dispersion. Fig. 4 showed the SEM micrographs of the surface and cross-section of the films. Without zein the film prepared with corn-wheat starch (only) had rough surface with insoluble starch granules (Fig. 4, a1) and intensive and thinner cross-section (Fig. 4, a3). CW10/Z1 films were characterized by the smooth surface and the highly compacted cross-section (Fig. 4, b1 and b3). This explained better tensile strength in comparison with other bilayer films (Fig. 2). However, the bilayer films with increased zein content (CW10/Z2) had relative loss cross-sectional structure with thicker zein layer (Fig. 4, c3), which could illustrate the improvement of elongation at break of the films (Fig. 2). Both of CW10/Z3 and CW8/Z3 films had the relative harsh surface in comparing with CW10/Z1 and CW10/Z2 films (Fig. 4), because of saturated dissolution of zein in 95% ethanol and the movement of glycerol and polyethylene glycol-400 to the films surface. Besides, the increased porosity and more coagulated structure with more protein aggregates on the cross-sectional structure of CW10/Z3 and CW8/Z3 films (Fig. 4, d3 and e3) predicted the lower tensile strength of films (Fig. 2). The phenomenon found by SEM was consistent with the results of opacity (Table 3). CW10/Z1 showed the highest opacity (90.30 ± 0.20 Abs600/mm), and the SEM of CW10/ Z1 also showed relatively dense and homogeneity. Hosseini et al. (2013) reported that there were a certain relationship between the appearance and opacity. 3.7. AFM images Atomic force microscopy (AFM) technique is a powerful tool to provide qualitative and quantitative information in nano-scale to further investigate the surface morphology of vary films (Herrmann et al., 2004). Fig. 5 showed bi and tri-dimensional AFM images to indicate the morphology (qualitative parameter) and the roughness (quantitative parameter) of different bilayer films. The CW films without zein had the highest bulges (Fig. 5, a2). The bilayer films incorporated with different additional zein showed relatively smooth and continuous matrix with less pores and cracks (Fig. 5, b1, b2, c1, and c2). However, films surface roughness increased with increasing zein addition (Fig. 5, d1, d2, e1, and e2). Therefore, moderate addition of zein increased the smoothness of film surface, but it was counter-productive to be too excess. The roughness on the film surface can affect its mechanical and physical properties. Ra, Rq and Z values of different films are shown in Table 4. The Rq values of CW10/Z0, CW10/Z1, CW10/ Z2, CW10/Z3 and CW8/Z3 were 5.177 nm, 0.568 nm, 3.878 nm, 4.586 nm and 5.836 nm, respectively, so both CW10/Z0 and CW8/Z3 had rougher surface than the others. Meanwhile, when only 0.06 g of zein was added, CW10/Z1 films exhibited the least roughness of surface, whereas any additional zein reduced smoothness of film surface. Generally, the surface topography of bilayer films with certain additional zein bears many cracks, voids, and wrinkles (Shi et al., 2012). S, L, and D values calculated based AFM images are shown in Table 5, in which CW10/Z0 had the biggest particle size (S = 261136.415 nm2, L = 2562.967 nm, D = 576.619 nm) among all the films. Variation trend of particle sizes was always consistent with the roughness (Table 3). According to AFM images, the addition of zein can increase the smoothness of film surface. Besides, both SEM and AFM analysis results may related to WVP values. In general, films have the same substrate, and the additional zein increased, as Ra and Rq rose, WVP values would reduce (Pan et al., 2014). During the film formation, there were a few of interactions between molecules. E.g., protein-protein interactions between zein molecules, hydrogen
Please cite this article in press as: Zuo, G., et al. Physical and structural characterization of edible bilayer films made with zein and corn-wheat starch. Journal of the Saudi Society of Agricultural Sciences (2017), https://doi.org/10.1016/j.jssas.2017.09.005
6
G. Zuo et al. / Journal of the Saudi Society of Agricultural Sciences xxx (2017) xxx–xxx
Fig. 4. SEM micrographs of the surface and cross-section of the bilayer films. (a1, a2 and a3 showed CW10/Z0; b1, b2 and b3 showed CW10/Z1; c1, c2 and c3 showed CW10/Z2; d1, d2 and d3 showed CW10/Z3; e1, e2 and e3 showed CW8/Z3). a1, a2, b1, b2, c1, c2, d1, d2, e1, and e2 showed the surface morphology, a3, b3, c3, d3 and e3 showed the crosssection morphology of the studied films.
bonding between starch/zein molecules and water molecule, van der Waals forces, and electrostatic interactions between polysaccharides and proteins (Wang et al., 2008). Polar amino acids were transferred to the top of the films, but relative non-polar amino acids would turn to the other side (Shi et al., 2009).
4. Conclusion The bilayer films based on corn-wheat starch incorporated with zein were developed, and the effect of starch and zein addition on the physical properties (e.g., colour, water content, thickness, and
Please cite this article in press as: Zuo, G., et al. Physical and structural characterization of edible bilayer films made with zein and corn-wheat starch. Journal of the Saudi Society of Agricultural Sciences (2017), https://doi.org/10.1016/j.jssas.2017.09.005
G. Zuo et al. / Journal of the Saudi Society of Agricultural Sciences xxx (2017) xxx–xxx
7
Fig. 5. Typical AFM images and Ra and Rq of the bilayer films. (a1 and a2 showed CW10/Z0; b1 and b2 showed CW10/Z1; c1 and c2 showed CW10/Z2; d1 and d2 showed CW10/ Z3; e1 and e2 showed CW8/Z3. a1, b1, c1, d1 and e1 showed a two-dimensional structure of AFM images, a2, b2, c2, d2 and e2 showed a three-dimensional structure of AFM images.
Please cite this article in press as: Zuo, G., et al. Physical and structural characterization of edible bilayer films made with zein and corn-wheat starch. Journal of the Saudi Society of Agricultural Sciences (2017), https://doi.org/10.1016/j.jssas.2017.09.005
8
G. Zuo et al. / Journal of the Saudi Society of Agricultural Sciences xxx (2017) xxx–xxx
Table 4 Comparison of Ra, Rq and Z values obtained from AFM images of different films.a Film
Ra (nm)
Rq (nm)
Z (nm)
CW10/Z0 CW10/Z1 CW10/Z2 CW10/Z3 CW8/Z3
4.062 0.408 2.070 2.915 3.463
5.177 0.568 3.878 4.585 5.836
48.305 7.246 26.668 59.374 51.324
a Ra the average roughness. Rq the root-mean-square roughness. Z arithmetic mean of height.
Table 5 Particle sizes obtained from AFM images of different films.a Film
S/nm2
L/nm
D/nm
CW10/Z0 CW10/Z1 CW10/Z2 CW10/Z3 CW8/Z3
261136.415 147918.003 154965.552 124496.236 134722.226
2562.967 1887.222 1717.272 1605.441 1671.428
576.619 433.976 444.194 398.138 414.166
a S the average value of area; L the average value of circumference; D the average value of diameter.
opacity), water vapour permeability, antioxidative capacity, and mechanical properties of films were studied. As the increase of zein addition, the opacity, WVP, and TS were decreased, whereas the water content, E and antioxidative capacity increased. The CW/Z bilayer films even exhibited outstanding E and antioxidative capacity in comparison with the BOPP-PE films, which indicated that the film developed in the current study can be potentially used in the food industry. SEM and AFM images showed that CW10/Z1 and CW/10/Z2 had smoother surface and highly compacted structure than the other bilayer films, CW10/Z3 and CW8/Z3 showed the better cross-section among all the studied films. So, the addition of zein exerted an influence on the appearance and physical properties of films. Based on these findings, the CW/Z bilayer films show promise as a potential packaging material for dry vegetables, oil sauces, and seasoning during the production of instant food. Acknowledgments This research was funded by the Postdoctoral Science Foundation of China (2015M582184) and National Natural Science Foundational of China (21376064, 21676073, 31301586). References AOCS, 1989. AOCS Official and Recommended Methods of the American Oil Chemists Society. American Oil Chemists Society, Champaign. Basiak, E., Lenart, A., Debeaufort, F., 2017. Effect of starch type on the physicochemical properties of edible films. Int. J. Biol. Macromol. 98, 348–356. Buckley, M., Cowan, C., McCarthy, M., 2007. The convenience food market in Great Britain: convenience food lifestyle (CFL) segments. Appetite 49, 600–617.
Cárdenas, F.C., Giannuzzi, L., Zaritzky, N.E., 2008. Mathematical modelling of microbial growth in ground beef from Argentina. Effect of lactic acid addition, temperature and packaging film. Meat Sci. 79, 509–520. Cho, S.Y., Lee, S.Y., Rhee, C., 2010. Edible oxygen barrier bilayer film pouches from corn zein and soy protein isolate for olive oil packaging. LWT – Food Sci. Technol. 43, 1234–1239. Choi, W.S., Singh, S., Lee, Y.S., 2016. Characterization of edible film containing essential oils in hydroxypropyl methylcellulose and its effect on quality attributes of ‘Formosa’ plum (Prunus salicina L.). LWT – Food Sci. Technol. 70, 213–222. Corradini, E., Curti, P.S., Meniqueti, A.B., Martins, A.F., Rubira, A.F., Muniz, E.C., 2014. Recent advances in food-packing, pharmaceutical and biomedical applications of zein and zein-based materials. Int. J. Mol. Sci. 15, 22438–22470. Danyxa, P.H., Carolina, M.J., Alex, L.C., Silvia, Goyanes, 2017. Edible cassava starch films carrying rosemary antioxidant extracts for potential use as active food packaging. Food Hydrocolloid. 63, 488–495. Das, M., Chowdhury, T., 2016. Heat sealing property of starch based self-supporting edible films. Food Pack. Shelf Life 9, 64–68. Devlieghere, F., Vermeiren, L., Debevere, J., 2004. New technologies: possibilities and limitations. Int. Dairy J. 14, 273–285. Dong, S., Gao, A., Xu, H., Chen, Y., 2017. Effects of dielectric barrier discharges (DBD) cold plasma treatment on physicochemical and structural properties of zein powders. Food Bioprocess Tech. 10, 434–444. Fund, T., Isa, D.A., Banu, O., 2010. Water vapor and oxygen-barrier performance of corn–zein coated polypropylene films. J. Food Eng. 96, 342–347. Ghanbarzadeh, B., Oromiehi, A.R., 2008. Biodegradable biocomposite films based on whey protein and zein: barrier, mechanical properties and AFM analysis. Int. J. Biol. Macromol. 43, 209–215. Herrmann, P.S.P., Yoshida, C.M.P., Antunes, A.T., Marcondes, J.A., 2004. Surface evaluation of whey protein films by atomic force microscopy and water vapour permeability analysis. Pack. Technol. Sci. 17, 267–273. Hosseini, S.F., Rezaei, M., Zandi, M., Ghavi, F.F., 2013. Preparation and functional properties of fish gelatin–chitosan blend edible films. Food Chem. 136, 1490– 1495. Josiane, I.M., Bauduin, G., Boutevin, B., Gontard, N., 2001. New plasticizers for wheat gluten films. Eur. Polym. J. 37, 1533–1541. Pan, H.Y., Jiang, B., Chen, J., 2014. Blend-modification of soy protein/lauric acid edible films using polysaccharides. Food Chem. 151, 1–6. Pereda, M., Ponce, A.G., Marcovich, N.E., Ruseckaite, R.A., Martuccite, J.F., 2011. Chitosan-gelatin composites and bi-layer films with potential antimicrobial activity. Food Hydrocolloid. 25, 1372–1381. Rodrigo, O.T., Morey, I., Talens, P., Chiralt, A., 2015. Active bilayer films of thermoplastic starch and polycaprolactone obtained by compression molding Rodrigo. Carbohyd. Polym. 127, 282–290. Shi, K., Kokini, J.L., Huang, Q.R., 2009. Engineering zein films with controlled surface morphology and hydrophillicity. J. Agr. Food Chem. 57, 2186–2192. Shi, K., Yu, H.L., Rao, S.L., Lee, T.C., 2012. Improved mechanical property and water resistance of zein films by plasticization with tributyl citrate. J. Agr. Food Chem. 60, 5988–5993. Suh, J.H., Niu, Y.S., Hung, W.L., Ho, C.T., Wang, Y., 2017. Lipidomic analysis for carbonyl species derived from fish oil using liquid chromatography–tandem mass spectrometry. Talanta 168, 31–42. Talja, R.A., Helen, H., Roos, Y.H., Jouppila, K., 2008. Effect of type and content of binary polyol mixtures on physical and mechanical properties of starch-based edible films. Carbohyd. Polym. 71, 269–276. Wang, Q., Xian, W.J., Li, S.F., Liu, C., Padua, G.W., 2008. Topography and biocompatibility of patterned hydrophobic/hydrophilic zein layers. Acta Biomater. 4, 844–851. Wang, X.J., Zheng, X.Q., Liu, X.L., Kopparapu, N.K., Cong, W.S., Deng, Y.P., 2017. Preparation of glycosylated zein and retarding effect on lipid oxidation of ground pork. Food Chem. 227, 335–341. Yin, Y.C., Yin, S.W., Yang, X.Q., Tang, C.H., Wen, S.H., Chen, Z., Xiao, B.J., Wu, L.Y., 2014. Surface modification of sodium caseinate films by zein coatings. Food Hydrocolloid. 36, 1–8. Zhong, Y., Li, Y., 2011. Effects of surfactants on the functional and structural properties of kudzu (Pueraria lobata) starch/ascorbic acid films. Carbohyd. Polym. 85, 622–628.
Please cite this article in press as: Zuo, G., et al. Physical and structural characterization of edible bilayer films made with zein and corn-wheat starch. Journal of the Saudi Society of Agricultural Sciences (2017), https://doi.org/10.1016/j.jssas.2017.09.005
Contents lists available at ScienceDirect
Journal of the Saudi Society of Agricultural Sciences journal homepage: www.sciencedirect.com
Full length article
Physical and structural characterization of edible bilayer films made with zein and corn-wheat starch Guanjie Zuo a,c, Xiaoyong Song b,a,c, Fusheng Chen a,⇑, Zhixiang Shen a a
Henan University of Technology, Zhengzhou 450001, China North China University of Water Resources and Electric Power, Zhengzhou 450011, China c Henan Nanjie Village Group (Limited) Company, Luohe 462600, China b
a r t i c l e
i n f o
Article history: Received 17 July 2017 Revised 14 September 2017 Accepted 27 September 2017 Available online xxxx Keywords: Bilayer films Corn-wheat starch Zein Antioxidative capacity Surface and cross-sectional morphology
a b s t r a c t Edible bilayer films prepared using an additional of zein (Z) layer casted on the corn-wheat (CW) starch films were prepared and characterized in terms of colour, thickness, water content, optical, water vapour permeability (WVP), mechanical properties and antioxidative capacity. Results showed that the colour, thickness, water content, elongation at break (E), and antioxidative capacity increased with the increasing of zein addition, but WVP and tensile strength (TS) decreased. In comparison with biaxially oriented polypropylene polyethylene composite films (BOPP-PE), CW10/Z3 and CW8/Z3 even showed better antioxidative capacity and elongation at break (E). The surface and cross-section micrographs were imaged by scanning electron microscopy (SEM) and atomic force microscopy (AFM). On both of SEM and AFM images, CW10/Z1 and CW10/Z2 films exhibited smoother surface and more compacted structure than other films. Besides, CW10/Z3 and CW8/Z3 of SEM images showed the better cross-section. Therefore, CW/Z edible bilayer films could be potentially used as a new inner package material for oil bag in instant food to improve the development of food industry. Ó 2017 The Authors. Production and hosting by Elsevier B.V. on behalf of King Saud University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction Due to the reasonable price, pleasant taste, and easy preparation, instant food have increasingly satisfied nutritional requirements and met the fast-paced lifestyle (Buckley et al., 2007; Cho et al., 2010). Therefore, seasoning bags (e.g., flavor powder, oil sauce, and dried vegetable) served with the instant food are important to affect the consumer acceptance and admiration by all generations. Nowadays, synthetic petroleum-based polymers (e.g., ethylene terephthalate (PET) and polyethylene (PE)) were widely used to pack the seasonings, because of stable physicochemical properties and ideal mechanical behaviors (Choi et al., 2016). But they also bring some serious threats to human health and product quality (Cárdenas et al., 2008; Devlieghere et al., 2004; Suh et al.,
⇑ Corresponding author at: College of Food Science and Technology, Henan University of Technology, 100 Lianhua Street, Zhengzhou 450001, China. E-mail address: [email protected] (F. Chen). Peer review under responsibility of King Saud University.
Production and hosting by Elsevier
2017). Therefore, it is urgently necessary to create a novel packaging material to replace the traditional synthetic materials for seasoning bags. Biodegradable edible films developed by bio-based materials (e.g., proteins, polysaccharides, and lipids) are recently introduced. Starch-based edible films have raised considerable attention, due to its comprehensive sources, low cost, and biodegradability. But they also have own disadvantages, such as poor mechanical properties, low water blocking ability and non-thermoplasticity (Basiak et al., 2017; Danyxa et al., 2017; Das and Chowdhury, 2016). Protein-based films, such as zein films, could make up these weaknesses of starch-based films by their hydrophobic amino acids (e.g., proline, leucine, and alanine) (Wang et al., 2017; Yin et al., 2014), excellent film forming ability (Ghanbarzadeh and Oromiehi, 2008), low oxygen permeability (Fund et al., 2010) and better biocompatibility. But, poor mechanical properties (Dong et al., 2017) and expensive price of zein greatly restrict its industrial application. Therefore, it is expected that the additional zein layer on the starch films will bring synergistic effects to produce bilayer films with advanced mechanical properties and oxidative stability, in order to extend the industrial application and satisfy consumer demands. The overall objective of the present study is to design a cornwheat starch-based bilayer films with the addition of zein layer,
https://doi.org/10.1016/j.jssas.2017.09.005 1658-077X/Ó 2017 The Authors. Production and hosting by Elsevier B.V. on behalf of King Saud University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article in press as: Zuo, G., et al. Physical and structural characterization of edible bilayer films made with zein and corn-wheat starch. Journal of the Saudi Society of Agricultural Sciences (2017), https://doi.org/10.1016/j.jssas.2017.09.005
2
G. Zuo et al. / Journal of the Saudi Society of Agricultural Sciences xxx (2017) xxx–xxx
and to investigate the effect of zein addition on the colour, thickness, water content, opacity, water vapour permeability, mechanical properties, and antioxidative capacity of the films. Besides, this study will provide a new direction for developing inner packaging in oil bag. 2. Materials and methods 2.1. Materials Oxidized corn starch (C, 27.5% amylose content, Dingfeng Develop Co., Ltd. China), wheat starch (W, 29.0% amylose content, Lvyuan Starch Co., Ltd. China), and zein (Z, 91.1% protein content, Dulai iotechnology Co., Ltd. China). D-Sorbitol (98.0%, C6H14O6, 182.17 g/mol), citric acid (99.5%, C6H8O7H2O, 210.14 g/mol), carboxymethylcellulose sodium (90.0%, C8H16NaO8, 242.16 g/mol), sodium alginate (99.9%, (C6H7NaO6)n), ethyl alcohol (99.7%, CH3OH, 46.07 g/mol), polyethylene glycol-400 (99.0%, HO(CH2CH2O)nH, 400 g/mol) and glycerol (99.0%, HOCH2CHOHCH2OH, 92.09 g/mol) were supplied by Tianjing Chemical Reagent Co., Ltd. (Luoyang, China). All other chemicals (from local resources) were on analytical grade.
and glycerol (1.80 g) were added into the solution and stirred for 20 min, followed by the incubation (XMTD-7000, Beijing Yongguangming Medical Instrument Co., Ltd., Beijing, China) for 18 min at 80 °C. Through appropriate heat treating, the internal structure of zein would become loose and boost the interaction. Both polyethylene glycol-400 and glycerol are plasticizers, but the former has certain water-holding capacity. Next, the mixture was cooled down to 45–55 °C and poured onto a dried CW films within a plastic dish. During this process, the lower corn-wheat starch films would maintain its original appearance. The bilayer films forming solution was dried at room temperature for 30 min, and then heated up to 80 °C for 30 min. Films were finally removed from the plates, and conditioned to 54% relative humidity within a desiccator at room temperature for 2 d. All films were prepared in triplicate. Table 1 shows different kinds of bilayer films CW10/Z0-CW10/ Z3 were explained 10 mL of corn-wheat starch forming solution with 0 mL, 1 mL, 2 mL and 3 mL zein solution. CW8/Z3 were explained 8 mL corn-wheat starch forming solution film with 3 mL zein solution. Biaxially oriented polypropylene polyethylene composite films (BOPP-PE, Henan Nanjiecun Group Ltd., Henan, China) was a kind of commercial film. 2.3. Film colour
2.2. Preparation of bilayer films The bilayer films were designed by adding a zein (Z) layer on a dried corn-wheat starch film (CW) which showed in Table 1. In brief, the starch solution (6.00 g/100 mL) was initially prepared by dissolving corn starch and wheat starch at 2:3 (w/w) ratio in distilled water, followed by the addition of sorbitol (0.6% w/v; 0.60 g sorbitol in 100 mL distilled water), citric acid (2.5% w/v; 2.50 g citric acid in 100 mL in distilled water), and the mixture of carboxymethyl cellulose sodium and sodium alginate at 3:2 (w/w) ratio (1.4% w/v; 1.40 g mixture in 100 mL distilled water). Both sorbitol and citric acid are commonly employed as such plasticizers which can improve the films mechanical properties, harmless and even healthy. Carboxymethyl cellulose sodium and sodium alginate could enhance the barrier properties of films. The mixture was then stirring for 20 min at 40 °C with Water-bathing Constant Temperature Vibrator (THZ-82, ShanghaiHuxi Analysis Instrument Co., Ltd., Shanghai, China). Subsequently, the starch gelatinization was induced by heating the mixture at 85 °C for 40 min. Finally, the film forming solution was degassed for 30 min within a vacuum pump (DZF, Beijing Yongguangming Medical Instrument Co., Ltd., Beijing, China) at 80 °C with a vacuum degree of 0.09 Pa. The CW films layer was first formed by casting the starch solution on the plastic dishes (Diameter = 100 mm) and dried at 45 °C for 12 h. The protein solution was prepared by dissolving 6.00 g zein in 100 mL of aqueous ethanol (95%, v/v) and stirring for 3 min with magnetic stirrer. (85-2, Shanghai Huxi Analysis Instrument Co., Ltd., Shanghai, China). Both of polyethylene glycol-400 (0.90 g)
Film colour was determined based on the method described by Pereda et al. (2011) with some modifications. A WSC-S colour difference meter (Shanghai Precision & Scientific instrument Co., Ltd., Shanghai, China) was used to analyze the film colour. Whiteness (WI) of film was calculated using the following equations.
h i0:5 2 2 2 WI ¼ 100 ð100 L Þ þ a þ b
ð1Þ
where L⁄ was 0 for black and 100 for white, a⁄ value indicated red (+) to green (), and b⁄ value represent yellow (+) to blue (). Five measurements were taken on each triplicate film prepared. 2.4. Thickness, water content and opacity The thickness of bilayer films was measured by using a thickness gauge (GM280F, Shenzhen Instruments and Meters Co., Ltd., Shenzhen, China). Six thickness measurements were taken on each triplicate film prepared. The water content of film was determined gravimetrically after drying the film in an oven (101-3AB, Beijing Zhongxing Weiye Co., Ltd., Beijing, China) at 105 °C for 24 h. The measurement was performed in triplicate. According to Josiane et al. (2001), film opacity was determined by using a spectrophotometer (722S, Shanghai Instrument Electric Analytical Co., Ltd., Shanghai, China). The absorbance was measured at a wavelength of 600 nm. The opacity of the film was calculated using the following equation:
Op ¼
AbS600 L
ð2Þ
where Op was the opacity, Abs600 was the absorbance at 600 nm, and L was the film thickness (lm). Five repetitions were performed for each triplicate sample.
Table 1 The bilayer films classifications.a Films
Starch form solution (mL)
CW10/Z0 CW10/Z1 CW10/Z2 CW10/Z3 CW8/Z3 BOPP-PE
10 0 10 1 10 2 10 3 8 3 Biaxially oriented polypropylene polyethylene composite films
Zein form solution (mL)
a 1 mL of protein forming solution content 0.06 g zein. And 1 mL of starch forming solution content 0.06 g corn-wheat starch.
2.5. Water vapour permeability Water vapour permeability (WVP) was determined gravimetrically based on the method described by Talja et al. (2008) with some modifications. Briefly, the film was sealed on a permeation cell (inner diameter: 45 mm, height: 25 mm), which was filled by 20.50 g of granular anhydrous calcium chloride (0% relative humidity) to ensure a consistent air gap (<6 mm). The permeation
Please cite this article in press as: Zuo, G., et al. Physical and structural characterization of edible bilayer films made with zein and corn-wheat starch. Journal of the Saudi Society of Agricultural Sciences (2017), https://doi.org/10.1016/j.jssas.2017.09.005
G. Zuo et al. / Journal of the Saudi Society of Agricultural Sciences xxx (2017) xxx–xxx
cell was then placed in a desiccator containing saturated NaCl solution (75% relative humidity). The permeation cell was weighed as a function of time until changes to the nearest 0.001 g. WVP was calculated as follows:
WVP ¼
mL AtDp
2.6. Mechanical properties Tensile strength (TS, MPa) and elongation at break (E,%) of the film were determined using a TAXT-plus texture analyzer (SMS, England) with a load cell of 1 kg (Zhong and Li, 2011). Prior to the measurement, films were cut into strips (10 50 mm) and mounted between the tensile grips (TA 96, England Stable Micro Systems). Initial grips separation was 30 mm and cross-head speed was 1.0 mm s1. The measurement was performed in triplicate. 2.7. Antioxidative capacity Oxygen permeability (OP) was represented as peroxide values (POV) of soy oil (Cho et al., 2010). The film (D = 9 cm) was sealed on a 250 mL-conical flask containing 20 g soy oil and stored at 60 °C for 10 days. The POV was then measured (AOCS, 1989). The measurement was performed in triplicate. 2.8. Scanning electron microscopy (SEM) analysis Surface and cross-sectional microscopic images of CW/Z bilayer films were obtained using a scanning electron microscope (JSM7401F, JEOL, Japan). The films specimens were cut into a chunk (5 mm 5 mm) and mounted on an aluminum stub using a double-sided tape, which was then coated with a thin gold layer (20 nm of thickness). The samples were observed on an accelerating voltage of 20 kV magnification at 2000 for cross-sectional, 2000 and 4000 for Surface, respectively. 2.9. Atomic force microscopy (AFM) analysis According to Ghanbarzadeh and Oromiehi (2008), the surface morphology of the bilayer films were determined by atomic force microscopy. The basic principle of AFM is to detect the forces between a cantilever and the sample surface (x-y plane) which is further translated to a deflection of the cantilever based on Hooke’s law and presented as a topographic image with a constant scan size (10 lm 10 lm). The captured images were analyzed using Dynamic SPM acquisition mode and Contact SPM acquisition mode (AFM, ZLAFM. Micro-Nano Equipment Co., Ltd., Shanghai, China). Finally, the roughness of the film surface was evaluated by Rq, Ra, and Z using the following formulas:
PN Ra ¼
Z¼
i¼1 Z i
N
N X Zi i¼1
N
Z
where Rq was root-mean square average of height deviations taken from the mean data plane, Ra was the average of the height deviations from a mean surface, Z was the arithmetic mean of the height (nm), Zi was the height of the data point (nm), and N was the number of the data point.
ð3Þ
where m was the poor quality of anhydrous calcium chloride before and after water absorption through the film (g), L was the thickness of the film (lm), A was the permeation area (m2), t was the time of permeation (s), and Dp was water vapour pressure difference across the film (2376.30 Pa). The measurement was performed in triplicate.
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi XN ðZi ZÞ2 Rq ¼ i¼1 N
3
ð4Þ
ð5Þ
ð6Þ
2.10. Statistical analysis Data was analyzed using SPSS software (version 17.0, Statistical Package for the Social Sciences Inc., Chicago, IL, USA). A one-way analysis of variance (ANOVA) was performed to measure statistical differences in colour, thickness, water content, opacity, water vapour permeability, mechanical properties, and oxygen permeability among the various films. Duncan’s multiple range test was used to determine the significant difference (P < .05) of means. 3. Results and discussion 3.1. The colour of CW/Z bilayer films The colour of films is primarily influenced by the ingredient composition, and can has a big impact on the consumer acceptance of packaged products. The effects of starch and zein addition on the colour of films are shown in Table 2. An analysis of variance indicated that the WI of films was significantly affected by the starch and zein addition (P < .05). Overall, because of the yellowness of zein, the WI of film was significantly decreased (P < .05) with the increase of zein addition, in which CW10/Z3 was more yellower than the other films with same amount of starch. This was also demonstrated by CW8/Z3 that was significant yellower (b⁄ of 16.89) with higher percentage of zein. Therefore, zein was the predominant ingredient to determine the WI of films. However, all bilayer films prepared in the current study (87.10 of WI) had darker colour than the BOPP-PE films (94.32 of WI). 3.2. Thickness, water content and opacity of CW/Z bilayer films Thickness, water content, and opacity of the bilayer films prepared by starch and zein solutions are shown in Table 3. The results showed that the film thickness increased significantly (P < .05) with the increase in starch and zein addition, which were over double thicker (130.27 mm) than the BOPP-PE films (60.97 mm) as commercial inner packing material. Compared with starch and polycaprolactone (234.00 mm) active bilayer films reported by Rodrigo et al. (2015), the films in our study showed a certain superiority. In particular, the film was thicken by 10 mm on every additional 1 mL of zein solution. There was no significant difference (P > .05) on the water content of the different bilayer films, except CW10/Z3 bilayer films, which had the highest water content (15.48%) (Table 3). CW10/ Z3 showed the higher value than the others, due to an enormous amount of hydrophobic groups on zein are repulsive to water but hydrophilic groups on zein are able to bind with water. And the additional polyethylene glycol-400 has both a certain waterholding and plasticized capacities. An analysis of variance indicated that only zein addition significantly (P < .05) affect the opacity of bilayer films, whereas extra starch content did not significantly influence the opacity (P > .05). Overall, the addition of zein layer generally decreased the opacity of bilayer films, except CW10/Z1 films. However, all bilayer films prepared in the current study (89.01 Abs600/mm) were more transparent than the BOPP-PE films (90.33 Abs600/mm) (Table 3). The reasons maybe that between starch and zein exhibited the better biocompatibility which were in accordance with Hosseini et al. (2013). In addition, the changes may be relative to the surface
Please cite this article in press as: Zuo, G., et al. Physical and structural characterization of edible bilayer films made with zein and corn-wheat starch. Journal of the Saudi Society of Agricultural Sciences (2017), https://doi.org/10.1016/j.jssas.2017.09.005
4
G. Zuo et al. / Journal of the Saudi Society of Agricultural Sciences xxx (2017) xxx–xxx
Table 2 Colour of CW/Z bilayer films.a,b L*
Films CW10/Z0 CW10/Z1 CW10/Z2 CW10/Z3 CW8/Z3 BOPP-PE a b
a* d
b* e
0.77 ± 0.00 1.50 ± 0.02d 2.42 ± 0.01c 2.94 ± 0.05b 3.18 ± 0.02a 0.66 ± 0.03f
96.15 ± 0.08 95.78 ± 0.10c 94.86 ± 0.23b 94.48 ± 0.06a 94.36 ± 0.06a 96.25 ± 0.03d
Whiteness b
4.65 ± 0.05 8.03 ± 0.13c 12.81 ± 0.12d 15.89 ± 0.26e 16.89 ± 0.14f 4.21 ± 0.17a
93.92 ± 0.09e 90.81 ± 0.19d 85.94 ± 0.15c 82.93 ± 0.27b 81.91 ± 0.14a 94.32 ± 0.13f
Data were shown in mean ± standard deviation. ND was shown not detection. Different superscript letters in the same column indicated significant differences (P < .05).
morphology of bilayer films that were described in SEM and AFM images.
In theory, due to the hydrophobicity of zein, the film with zein displayed less affinity to water and better water barrier property than the films without zein. This was also demonstrated by Cho et al. (2010), in which the addition of zein greatly decreased WVP by 0.33 1012 kg m/m2 s Pa in the soy protein isolatedbased films for olive oil packaging. However, the increase of zein addition only performed minor effect on the WVP of films which consist with the results of water content above. Furthermore, the higher thickness of films provides a longer pathway for the penetration of water molecule to decrease WVP of films, so CW10/Z0 (115.67 lm) had the highest of WVP. On the one hand there was a positive correlation between WVP and surface tortuosity, Pan et al. (2014) has also proved the results. On the other hand SEM images of CW10/Z0 below showed abundant holes just right explain an increase on WVP.
3.3. Water vapour permeability
3.4. Mechanical properties
High water vapour permeability of edible films is not desirable for the industrial applications and would resulted in food degenerating easily. An analysis of variance exhibited that the addition of zein significantly (P < .05) lower the WVP of films, whereas the additional of zein was not significant (P > .05). However, all bilayer films prepared in the current study had significantly (P < .05) lower water barrier property than the BOPP-PE films. Overall, CW10/Z0 had the highest WVP (8.89 1011 g/m s Pa). The bilayer films with same amount of starch but increased zein addition had similar water barrier property (5.96 1011 g/m s Pa of WVP). And the WVP of the film with same amount of zein was gently decreased (about 0.25 1011 g/m s Pa) with the increase of starch addition (Fig. 1).
The tensile strength (TS, MPa) and elongation at break (E,%) of the bilayer films were presented in Fig. 2. It showed that when changing the addition of zein forming solution, TS and E of bilayer films expressed the significant difference (P < .05), but the effect of starch addition from CW10/Z3 and CW8/Z3 on TS and E was not significant (P > .05). Overall, TS data were declined and E data were improved as zein level increased. On another side, different amount of starch in the films with same zein addition resulted in the similar TS (9.39 MPa) and E (77.14%) of films. All bilayer films prepared in the current study exhibited poorer mechanical properties than the BOPP-PE films (84.56 MPa of TS and 42.54% of E). Several reasons could contribute to the results. The accounts maybe that glycerol and polyethylene glycol-400 have better
Fig. 1. Water vapour permeability of the CW/Z bilayer films. Different letters indicated significant difference (P < .05). Abbreviations: corn-wheat (CW), zein (Z), biaxially oriented polypropylene polyethylene composite films (BOPP-PE), and water vapour permeability (WVP).
Fig. 2. The mechanical properties of CW/Z bilayer films. Different letters indicated significant difference (P < .05). Abbreviations: corn-wheat (CW), zein (Z), biaxially oriented polypropylene polyethylene composite films (BOPP-PE), tensile strength (TS, MPa), and elongation at break (E,%).
Table 3 Thickness, water content and opacity of CW/Z bilayer films.a,b Films
Thickness (mm)
Water content (%)
Opacity (Abs600/mm)
CW10/Z0 CW10/Z1 CW10/Z2 CW10/Z3 CW8/Z3 BOPP-PE
115.67 ± 1.53b 126.67 ± 1.53bc 135.33 ± 9.29cd 147.00 ± 0.81d 126.67 ± 3.21bc 60.97 ± 0.75a
11.80 ± 0.33a 11.41 ± 0.26a 12.95 ± 0.57a 15.48 ± 1.22b 12.94 ± 0.21a ND
89.73 ± 0.06c 90.30 ± 0.20d 88.93 ± 0.15b 87.97 ± 0.15a 88.13 ± 0.06a 90.33 ± 0.06d
a
Data were shown in mean ± standard deviation. ND was shown not detection. Different superscript letters in the same column indicated significant differences (P < .05). b
Please cite this article in press as: Zuo, G., et al. Physical and structural characterization of edible bilayer films made with zein and corn-wheat starch. Journal of the Saudi Society of Agricultural Sciences (2017), https://doi.org/10.1016/j.jssas.2017.09.005
G. Zuo et al. / Journal of the Saudi Society of Agricultural Sciences xxx (2017) xxx–xxx
plasticizing properties than sorbitol and citric acid. Therefore, the increase of glycerol and polyethylene glycol-400 addition will make the films with better flexibility. Besides, the water content which was act as a bridge in Table 2 also explained this phenomenon. Namely, with the additional zein increased, E showed bigger and TS decreased which perhaps that the enough zein forming solution has already achieved saturated beyond itself, leading to uneven distribution of zein on the starch film will create molecular space on the film structure to produce more stretchable films (Corradini et al., 2014). Although TS value was far from BOPP-PE films, it could meet the needs of the inner packing (Cho et al., 2010). Meanwhile, the E value was higher than that of commercial film.
3.5. Oxidative stability of soybean oil packaged in the CW/Z bilayer films Lipid oxidation generally results in the loss of nutritional value and develops off-flavors to negatively impact consumer acceptance, so the antioxidative capacity of biodegradable edible film is an important feature to study. The peroxide value of soybean oil packed by the CW/Z bilayer films during storage were shown in Fig. 3. An analysis of variance presented that the antioxidative capacity of the bilayer films was significantly improved with the increase of zein addition (P < .05), whereas the starch addition did not obviously affect the peroxide value of soybean oil packed by the bilayer films (P > .05). Overall, the CW10/Z0 film showed the lowest antioxidative capacity with the highest POV (231.90 ± 3.24 meq/kg), whereas CW10/Z3 films exhibited the best antioxidative capacity with the lowest POV (94.69 ± 0.29 meq/kg) value. Moreover, the CW/Z bilayer films showed the better oxygen barrier properties than monolayer film, because the cross-linking reaction between starches and zein can product films with compacted structure, to further decrease oxygen permeability; the aromatic amino acids in zein are able to scavenge the free radicals by the aromatic rings; the sulfur-containing amino acids in zein can inhibit oxidation by chelating metal ions. In the current study, the CW/Z bilayer films had better antioxidative capacity (98.95 meq/kg oil of POV) than the BOPP-PE films (219.32 meq/kg of POV), therefore, they are able to potentially replace the synthetic petroleum-based materials used for oil sauce packaging in the industry.
Fig. 3. The peroxide values of CW/Z bilayer films. Different letters indicated significant difference (P < .05). Abbreviations: corn-wheat (CW), zein (Z), biaxially oriented polypropylene polyethylene composite films (BOPP-PE), and peroxide values (POV).
5
3.6. SEM images The microstructure of the bilayer films is mainly affected by the structural arrangement of different components in the initial dispersion. Fig. 4 showed the SEM micrographs of the surface and cross-section of the films. Without zein the film prepared with corn-wheat starch (only) had rough surface with insoluble starch granules (Fig. 4, a1) and intensive and thinner cross-section (Fig. 4, a3). CW10/Z1 films were characterized by the smooth surface and the highly compacted cross-section (Fig. 4, b1 and b3). This explained better tensile strength in comparison with other bilayer films (Fig. 2). However, the bilayer films with increased zein content (CW10/Z2) had relative loss cross-sectional structure with thicker zein layer (Fig. 4, c3), which could illustrate the improvement of elongation at break of the films (Fig. 2). Both of CW10/Z3 and CW8/Z3 films had the relative harsh surface in comparing with CW10/Z1 and CW10/Z2 films (Fig. 4), because of saturated dissolution of zein in 95% ethanol and the movement of glycerol and polyethylene glycol-400 to the films surface. Besides, the increased porosity and more coagulated structure with more protein aggregates on the cross-sectional structure of CW10/Z3 and CW8/Z3 films (Fig. 4, d3 and e3) predicted the lower tensile strength of films (Fig. 2). The phenomenon found by SEM was consistent with the results of opacity (Table 3). CW10/Z1 showed the highest opacity (90.30 ± 0.20 Abs600/mm), and the SEM of CW10/ Z1 also showed relatively dense and homogeneity. Hosseini et al. (2013) reported that there were a certain relationship between the appearance and opacity. 3.7. AFM images Atomic force microscopy (AFM) technique is a powerful tool to provide qualitative and quantitative information in nano-scale to further investigate the surface morphology of vary films (Herrmann et al., 2004). Fig. 5 showed bi and tri-dimensional AFM images to indicate the morphology (qualitative parameter) and the roughness (quantitative parameter) of different bilayer films. The CW films without zein had the highest bulges (Fig. 5, a2). The bilayer films incorporated with different additional zein showed relatively smooth and continuous matrix with less pores and cracks (Fig. 5, b1, b2, c1, and c2). However, films surface roughness increased with increasing zein addition (Fig. 5, d1, d2, e1, and e2). Therefore, moderate addition of zein increased the smoothness of film surface, but it was counter-productive to be too excess. The roughness on the film surface can affect its mechanical and physical properties. Ra, Rq and Z values of different films are shown in Table 4. The Rq values of CW10/Z0, CW10/Z1, CW10/ Z2, CW10/Z3 and CW8/Z3 were 5.177 nm, 0.568 nm, 3.878 nm, 4.586 nm and 5.836 nm, respectively, so both CW10/Z0 and CW8/Z3 had rougher surface than the others. Meanwhile, when only 0.06 g of zein was added, CW10/Z1 films exhibited the least roughness of surface, whereas any additional zein reduced smoothness of film surface. Generally, the surface topography of bilayer films with certain additional zein bears many cracks, voids, and wrinkles (Shi et al., 2012). S, L, and D values calculated based AFM images are shown in Table 5, in which CW10/Z0 had the biggest particle size (S = 261136.415 nm2, L = 2562.967 nm, D = 576.619 nm) among all the films. Variation trend of particle sizes was always consistent with the roughness (Table 3). According to AFM images, the addition of zein can increase the smoothness of film surface. Besides, both SEM and AFM analysis results may related to WVP values. In general, films have the same substrate, and the additional zein increased, as Ra and Rq rose, WVP values would reduce (Pan et al., 2014). During the film formation, there were a few of interactions between molecules. E.g., protein-protein interactions between zein molecules, hydrogen
Please cite this article in press as: Zuo, G., et al. Physical and structural characterization of edible bilayer films made with zein and corn-wheat starch. Journal of the Saudi Society of Agricultural Sciences (2017), https://doi.org/10.1016/j.jssas.2017.09.005
6
G. Zuo et al. / Journal of the Saudi Society of Agricultural Sciences xxx (2017) xxx–xxx
Fig. 4. SEM micrographs of the surface and cross-section of the bilayer films. (a1, a2 and a3 showed CW10/Z0; b1, b2 and b3 showed CW10/Z1; c1, c2 and c3 showed CW10/Z2; d1, d2 and d3 showed CW10/Z3; e1, e2 and e3 showed CW8/Z3). a1, a2, b1, b2, c1, c2, d1, d2, e1, and e2 showed the surface morphology, a3, b3, c3, d3 and e3 showed the crosssection morphology of the studied films.
bonding between starch/zein molecules and water molecule, van der Waals forces, and electrostatic interactions between polysaccharides and proteins (Wang et al., 2008). Polar amino acids were transferred to the top of the films, but relative non-polar amino acids would turn to the other side (Shi et al., 2009).
4. Conclusion The bilayer films based on corn-wheat starch incorporated with zein were developed, and the effect of starch and zein addition on the physical properties (e.g., colour, water content, thickness, and
Please cite this article in press as: Zuo, G., et al. Physical and structural characterization of edible bilayer films made with zein and corn-wheat starch. Journal of the Saudi Society of Agricultural Sciences (2017), https://doi.org/10.1016/j.jssas.2017.09.005
G. Zuo et al. / Journal of the Saudi Society of Agricultural Sciences xxx (2017) xxx–xxx
7
Fig. 5. Typical AFM images and Ra and Rq of the bilayer films. (a1 and a2 showed CW10/Z0; b1 and b2 showed CW10/Z1; c1 and c2 showed CW10/Z2; d1 and d2 showed CW10/ Z3; e1 and e2 showed CW8/Z3. a1, b1, c1, d1 and e1 showed a two-dimensional structure of AFM images, a2, b2, c2, d2 and e2 showed a three-dimensional structure of AFM images.
Please cite this article in press as: Zuo, G., et al. Physical and structural characterization of edible bilayer films made with zein and corn-wheat starch. Journal of the Saudi Society of Agricultural Sciences (2017), https://doi.org/10.1016/j.jssas.2017.09.005
8
G. Zuo et al. / Journal of the Saudi Society of Agricultural Sciences xxx (2017) xxx–xxx
Table 4 Comparison of Ra, Rq and Z values obtained from AFM images of different films.a Film
Ra (nm)
Rq (nm)
Z (nm)
CW10/Z0 CW10/Z1 CW10/Z2 CW10/Z3 CW8/Z3
4.062 0.408 2.070 2.915 3.463
5.177 0.568 3.878 4.585 5.836
48.305 7.246 26.668 59.374 51.324
a Ra the average roughness. Rq the root-mean-square roughness. Z arithmetic mean of height.
Table 5 Particle sizes obtained from AFM images of different films.a Film
S/nm2
L/nm
D/nm
CW10/Z0 CW10/Z1 CW10/Z2 CW10/Z3 CW8/Z3
261136.415 147918.003 154965.552 124496.236 134722.226
2562.967 1887.222 1717.272 1605.441 1671.428
576.619 433.976 444.194 398.138 414.166
a S the average value of area; L the average value of circumference; D the average value of diameter.
opacity), water vapour permeability, antioxidative capacity, and mechanical properties of films were studied. As the increase of zein addition, the opacity, WVP, and TS were decreased, whereas the water content, E and antioxidative capacity increased. The CW/Z bilayer films even exhibited outstanding E and antioxidative capacity in comparison with the BOPP-PE films, which indicated that the film developed in the current study can be potentially used in the food industry. SEM and AFM images showed that CW10/Z1 and CW/10/Z2 had smoother surface and highly compacted structure than the other bilayer films, CW10/Z3 and CW8/Z3 showed the better cross-section among all the studied films. So, the addition of zein exerted an influence on the appearance and physical properties of films. Based on these findings, the CW/Z bilayer films show promise as a potential packaging material for dry vegetables, oil sauces, and seasoning during the production of instant food. Acknowledgments This research was funded by the Postdoctoral Science Foundation of China (2015M582184) and National Natural Science Foundational of China (21376064, 21676073, 31301586). References AOCS, 1989. AOCS Official and Recommended Methods of the American Oil Chemists Society. American Oil Chemists Society, Champaign. Basiak, E., Lenart, A., Debeaufort, F., 2017. Effect of starch type on the physicochemical properties of edible films. Int. J. Biol. Macromol. 98, 348–356. Buckley, M., Cowan, C., McCarthy, M., 2007. The convenience food market in Great Britain: convenience food lifestyle (CFL) segments. Appetite 49, 600–617.
Cárdenas, F.C., Giannuzzi, L., Zaritzky, N.E., 2008. Mathematical modelling of microbial growth in ground beef from Argentina. Effect of lactic acid addition, temperature and packaging film. Meat Sci. 79, 509–520. Cho, S.Y., Lee, S.Y., Rhee, C., 2010. Edible oxygen barrier bilayer film pouches from corn zein and soy protein isolate for olive oil packaging. LWT – Food Sci. Technol. 43, 1234–1239. Choi, W.S., Singh, S., Lee, Y.S., 2016. Characterization of edible film containing essential oils in hydroxypropyl methylcellulose and its effect on quality attributes of ‘Formosa’ plum (Prunus salicina L.). LWT – Food Sci. Technol. 70, 213–222. Corradini, E., Curti, P.S., Meniqueti, A.B., Martins, A.F., Rubira, A.F., Muniz, E.C., 2014. Recent advances in food-packing, pharmaceutical and biomedical applications of zein and zein-based materials. Int. J. Mol. Sci. 15, 22438–22470. Danyxa, P.H., Carolina, M.J., Alex, L.C., Silvia, Goyanes, 2017. Edible cassava starch films carrying rosemary antioxidant extracts for potential use as active food packaging. Food Hydrocolloid. 63, 488–495. Das, M., Chowdhury, T., 2016. Heat sealing property of starch based self-supporting edible films. Food Pack. Shelf Life 9, 64–68. Devlieghere, F., Vermeiren, L., Debevere, J., 2004. New technologies: possibilities and limitations. Int. Dairy J. 14, 273–285. Dong, S., Gao, A., Xu, H., Chen, Y., 2017. Effects of dielectric barrier discharges (DBD) cold plasma treatment on physicochemical and structural properties of zein powders. Food Bioprocess Tech. 10, 434–444. Fund, T., Isa, D.A., Banu, O., 2010. Water vapor and oxygen-barrier performance of corn–zein coated polypropylene films. J. Food Eng. 96, 342–347. Ghanbarzadeh, B., Oromiehi, A.R., 2008. Biodegradable biocomposite films based on whey protein and zein: barrier, mechanical properties and AFM analysis. Int. J. Biol. Macromol. 43, 209–215. Herrmann, P.S.P., Yoshida, C.M.P., Antunes, A.T., Marcondes, J.A., 2004. Surface evaluation of whey protein films by atomic force microscopy and water vapour permeability analysis. Pack. Technol. Sci. 17, 267–273. Hosseini, S.F., Rezaei, M., Zandi, M., Ghavi, F.F., 2013. Preparation and functional properties of fish gelatin–chitosan blend edible films. Food Chem. 136, 1490– 1495. Josiane, I.M., Bauduin, G., Boutevin, B., Gontard, N., 2001. New plasticizers for wheat gluten films. Eur. Polym. J. 37, 1533–1541. Pan, H.Y., Jiang, B., Chen, J., 2014. Blend-modification of soy protein/lauric acid edible films using polysaccharides. Food Chem. 151, 1–6. Pereda, M., Ponce, A.G., Marcovich, N.E., Ruseckaite, R.A., Martuccite, J.F., 2011. Chitosan-gelatin composites and bi-layer films with potential antimicrobial activity. Food Hydrocolloid. 25, 1372–1381. Rodrigo, O.T., Morey, I., Talens, P., Chiralt, A., 2015. Active bilayer films of thermoplastic starch and polycaprolactone obtained by compression molding Rodrigo. Carbohyd. Polym. 127, 282–290. Shi, K., Kokini, J.L., Huang, Q.R., 2009. Engineering zein films with controlled surface morphology and hydrophillicity. J. Agr. Food Chem. 57, 2186–2192. Shi, K., Yu, H.L., Rao, S.L., Lee, T.C., 2012. Improved mechanical property and water resistance of zein films by plasticization with tributyl citrate. J. Agr. Food Chem. 60, 5988–5993. Suh, J.H., Niu, Y.S., Hung, W.L., Ho, C.T., Wang, Y., 2017. Lipidomic analysis for carbonyl species derived from fish oil using liquid chromatography–tandem mass spectrometry. Talanta 168, 31–42. Talja, R.A., Helen, H., Roos, Y.H., Jouppila, K., 2008. Effect of type and content of binary polyol mixtures on physical and mechanical properties of starch-based edible films. Carbohyd. Polym. 71, 269–276. Wang, Q., Xian, W.J., Li, S.F., Liu, C., Padua, G.W., 2008. Topography and biocompatibility of patterned hydrophobic/hydrophilic zein layers. Acta Biomater. 4, 844–851. Wang, X.J., Zheng, X.Q., Liu, X.L., Kopparapu, N.K., Cong, W.S., Deng, Y.P., 2017. Preparation of glycosylated zein and retarding effect on lipid oxidation of ground pork. Food Chem. 227, 335–341. Yin, Y.C., Yin, S.W., Yang, X.Q., Tang, C.H., Wen, S.H., Chen, Z., Xiao, B.J., Wu, L.Y., 2014. Surface modification of sodium caseinate films by zein coatings. Food Hydrocolloid. 36, 1–8. Zhong, Y., Li, Y., 2011. Effects of surfactants on the functional and structural properties of kudzu (Pueraria lobata) starch/ascorbic acid films. Carbohyd. Polym. 85, 622–628.
Please cite this article in press as: Zuo, G., et al. Physical and structural characterization of edible bilayer films made with zein and corn-wheat starch. Journal of the Saudi Society of Agricultural Sciences (2017), https://doi.org/10.1016/j.jssas.2017.09.005