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Effects of nanorod-rich ZnO on rheological, sorption isotherm, and physicochemical properties of bovine gelatin films
LWT - Food Science and Technology 58 (2014) 142e149
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Effects of nanorod-rich ZnO on rheological, sorption isotherm, and physicochemical properties of bovine gelatin films Abdorreza Mohammadi Nafchi a, *, Mahdiyeh Moradpour a, Maliheh Saeidi a, Abd Karim Alias b a b
Food Biopolymer Research Group, Food Science and Technology Department, Damghan Branch, Islamic Azad University, Damghan, Semanan, Iran Food Biopolymer Research Group, Food Technology Division, School of Industrial Technology, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 August 2013 Received in revised form 28 February 2014 Accepted 6 March 2014
The effects of nanorod-rich zinc oxide (ZnOenr) on the flow properties of bovine gelatin solution and on the sorption isotherm, antimicrobial, and physchochemical properties of gelatin films were investigated. ZnOenr was incorporated into gelatin solutions at different concentrations (0.01, 0.02, 0.03, and 0.05 g/g dried gelatin). The introduction of low ZnOenr concentrations (0.05 g/g dried gelatin) to gelatin solutions significantly increased the viscosity of the solution from 9 to 11.9 mPa s and decreased the permeability of the films to water vapor from 8.9 1011 to 1.78 1011 (g m1 s1 Pa1). Solubility in water decreased from 30% to 20%, and monolayer water content of the films decreased from 0.13 to 0.10 (g water/g dried solid), whereas their contact angle increased from 45 to 85 with increasing ZnOenr concentration from 0 to 0.05 g/g dried gelatin. The ZnOenr gelatin films had very low UV transmittance and were able to absorb more than 50% of the near-infrared spectra. These films showed excellent antimicrobial activity against Staphylococcus aureus. These properties suggest that ZnOenr can be potentially used as fillers in gelatin-based films for active packaging materials in the pharmaceutical and food packaging industries. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Bovine gelatin film Flow properties Sorption isotherm Antimicrobial activity UV-shield
1. Introduction Nanoscale science and other related technologies have been the leading technology in the past decade (Narayanamurti, 2006). The incorporation of nanoparticles into composite materials can enhance polymer properties, such as thermal, mechanical, and gas barrier (Kurian, Dasgupta, Galvin, Ziegler, & Beyer, 2006). Inorganic materials, such as metal or metal oxides, have recently attracted considerable attention because of their capability to tolerate harsh conditions (Fu et al., 2005). Among the inorganic materials, ZnO, TiO2, MgO, and CaO are safe for animals and humans and stable in harsh conditions (W. Lin et al., 2009; Stoimenov, Klinger, Marchin, & Klabunde, 2002). Bionanocomposites, which are formed by the combination of biopolymers and an inorganic material that has at least one nanometer-scale dimension, are a new generation of nanocomposites. Bionanocomposites are nanostructured hybrid materials that have emerged at the frontier of material science, * Corresponding author. Tel.: þ98 232 522 5045; fax: þ98 232 522 5039. E-mail addresses: [email protected], [email protected] (A. Mohammadi Nafchi). http://dx.doi.org/10.1016/j.lwt.2014.03.007 0023-6438/Ó 2014 Elsevier Ltd. All rights reserved.
nanotechnology, and life science (Darder, Aranda, & Ruiz-Hitzky, 2007; Ozin, Arsenault, & Cademartiri, 2009). Efforts have been exerted to develop nanobiocomposites with enhanced thermal, mechanical, and functional properties by adding matrix or nanoparticle fillers (Chen, Zhou, Yang, Gu, & Wu, 2004; Jia et al., 2006). In addition, biopolymer-based materials are known as a green technology with biodegradable and biocompatible properties and are used in pharmaceutical, food packaging, and agriculture technologies. Natural polymers are based on polysaccharides or proteins; among all biopolymers, starch and gelatin are the most studied because of their wide applications (Nafchi, Nassiri, Sheibani, Ariffin, & Karim, 2013; Park, Scott Whiteside, & Cho, 2008). The unique capacities of proteins to form network and induce plasticity and elasticity are beneficial in preparing biopolymerbased packaging materials. In this regard, inexpensive bovine gelatin containing a large number of suspended functional groups can facilitate chemical cross-linking and derivatization. Hence, bovine gelatin can be used as a basic raw material for the development of food packaging films (Voon, Bhat, Easa, Liong, & Karim, 2012). However, similar to other biopolymer films, gelatin films are highly sensitive to moisture and easily lose their dimensional
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stability at room temperature, which lead to poor mechanical and barrier properties, thereby limiting their applications as a food packaging material (Fakhoury et al., 2012; Rivero, García, & Pinotti, 2010). Nanofillers have excellent interfacial interactions on polymer branches because of their large specific surface area and high surface energy, thereby significantly enhancing polymer properties (Kovacevi c, Vrsaljko, Lu ci cBlagojevic, & Leskovac, 2008). Zinc oxide (ZnO) has been widely used as a functional filler in UV absorbers for application in cosmetics, pharmaceutical materials, pigments, and coating materials (Li et al., 2009). ZnO particles may be used to prevent infectious diseases because of the antimicrobial effects of ZnO (Li et al., 2009; X. H. Li, Xing, Li, Jiang, & Ding, 2010; Zhang, Ding, Povey, & York, 2008). The size, morphology, crystallinity, composition, and shape of particles are critical parameters of the intrinsic properties of nanoparticles (Shahrom & Abdullah, 2006; Yamamoto, 2001). O. H. Lin, Akil, and Mahmud (2009) reported that ZnO nanorods exhibit optimal UV absorption activity. Although the incorporation of ZnO nanoparticles into biopolymer films can improve several properties (Ma, Chang, Yang, & Yu, 2009; Nafchi, Alias, Mahmud, & Robal, 2012; Nafchi et al., 2013; Yu, Yang, Liu, & Ma, 2009), the hydrophobicity, barrier, and flow properties of ZnO nanorod-reinforced gelatin have not been investigated. We hypothesized that the application of low-concentration nanorod ZnO into bovine gelatin film improves film hydrophobicity and enables the fabrication of a biopolymeric film with UV-shielding and antimicrobial properties. The proposed film can be applied in packaging industries. In this study, ZnO nanorods were used as fillers to prepare gelatin/ZnO nanorod (ZnOeN) bionanocomposites. We characterized the films based on their physicochemical, antibacterial, barrier, and hydrophobicity properties. As an operational parameter, the flow behavior of the bionanocomposite solutions was also investigated. 2. Materials and methods 2.1. Materials Bovine gelatin (Type B) was purchased from Sigma Chemical Co. (St. Louis, MO, USA). Liquid sorbitol and glycerol were obtained from Liang Traco (Penang, Malaysia). ZnOenr was synthesized via the catalyst-free combust-oxidized mesh process, as described by Shahrom and Abdullah (2007). Environmental scanning electron microscopy (ESEM) results (Fig. 1) revealed that ZnOenr has a diameter of 40 nme100 nm and a length of 200 nme700 nm. Some
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microparticles were also observed. The densities of gelatin and ZnOenr were 1.369 and 5.06 g cm3, respectively to calculation of volume fraction for tortuosity estimation. For the antimicrobial assay, Staphylococcus aureus culture was obtained from the culture collection center (School of Industrial Technology, USM, Malaysia) and grown on nutrient agar slants at 4 C. 2.2. Preparation of nanobiocomposite films ZnOenr was dispersed in water at different concentrations (0.01, 0.02, 0.03, and 0.05; g/g of total solid), stirred for 1 h, and then sonicated in an ultrasonic bath (Marconi model, Unique USC 45 kHz, Piracicaba, Brazil) for 30 min to ensure complete homogenization. The solution was used to prepare the aqueous gelatin dispersion at 8 (g/L). A mixture of sorbitol and glycerol (3:1) at 0.4 (g/g total solid) was added as a plasticizer in accordance with Abdorreza, Cheng, and Karim (2011). Gelatin nanocomposites were heated to 58 2 C and the temperature was held for 1 h. The gelatin nanocomposite solutions were cooled to 45 C, and the bubbles were removed using a vacuum pump. A portion (45 g) of the dispersion was cast on Perspex plates fitted with rims around the edge to yield a 16 cm 16 cm film-forming area. The films were dried in controlled conditions in a humidity chamber (25 1 C and 50 5% relative humidity). Control films were prepared in a similar manner but without the addition of nanoparticles. The dried films were peeled and stored at 23 2 C and 50 5% RH until experimentation. The thickness of each film was measured at five locations and to the nearest 0.01 mm with a hand-held micrometer (Mitutoyo, Tokyo, Japan). All films (including the control samples) were prepared in triplicates. 2.3. Characterization 2.3.1. Flow properties of gelatin solutions Solution samples used in film preparation (w1 mL) were transferred to a rheometer (CSL100, TA Instrument, New Castel, DE, USA) set at 55 C and mounted with a 6 cm cone geometry (2 and 60 mm gap). Flow properties at 40 C were measured in continuous flow by applying shear rates in linear mode from 0 s1 to 1000 s1 for 3 min, held constant at a shear rate of 1000 s1 for 1 min, and decreased from 1000 s1 to 0 s1 in 3 min. According to previous tests and references at this temperature all gelatin dispersions in sol medium (Moraes, Carvalho, Bittante, Solorza-Feria, & Sobral, 2009). Experimental data for bovine gelatin were fitted using the Newtonian rheological model: s ¼ mg, where s is the shear stress
Fig. 1. ESEM micrograph of nanorod-rich ZnO (a) magnification ¼ 25,000, (b) magnification ¼ 50,000.
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(Pa), and g is the shear rate (s1) (Wulansari, Mitchell, Blanshard, & Paterson, 1998).
Germany) equipped with an Oxford INCA 400 energy dispersive device.
2.3.2. Moisture sorption isotherm The moisture sorption isotherm of the films at 25 C was studied using the method described by Bertuzzi, Castro Vidaurre, Armada, and Gottifredi (2007). The moisture content at equilibrium (g absorbed water/g dry film) was measured in triplicates for each relative humidity. The experimental sorption data were fitted using the GAB equation (van den Berg, 1984).
2.3.9. Antimicrobial assay An antimicrobial activity test of the films was carried out using the agar diffusion method described by Maizura et al. (2007). The antimicrobial effects of the films were determined by the inhibition zone against S. aureus on solid media.
W ¼
wm CKaw ð1 Kaw Þð1 Kaw þ CKaw Þ
where wm, K, and C are the GAB parameters, W is the moisture content (dry basis), and aw is water activity. 2.3.3. Solubility in water The solubility of the nanocomposite gelatin films in water was determined following the method described by Maizura, Fazilah, Norziah, and Karim (2007) at room temperature. 2.3.4. Water vapor permeability (WVP) WVP tests of the films were carried out following the modified method (Yu et al., 2009) of ASTM standard E96-05 (ASTM, 2005) at 25 1 C. 2.3.5. Contact-angle measurements Water contact-angle measurements were performed on a static contact-angle meter (CAM-PLUS, Tantec, Germany) to determine the hydrophobicity of the nanocomposite gelatin films. Data presented are the means of ten independent determinations at different sites. The contact angles on each film surface were immediately after the addition of 1 mL of deionized water by using the Sessile Drop Half-AngleÔ Tangent line method.
2.4. Statistical analysis ANOVA and Tukey’s Post Hoc tests were performed to compare the means of the physical and antimicrobial properties of the gelatin nanocomposite films at 5% significance level. Statistical analyses were conducted using GraphPad Prism 6 (GraphPad Software Inc., La Jolla, USA). Curve fitting for GAB and polynomial sorption isotherm was evaluated by non-linear regression by using the solver module in Microsoft ExcelÒ 2010. 3. Results and discussion 3.1. Flow properties of gelatin nanocomposite solutions Viscosity and flow properties of fluids are directly related to the interaction of the fluid layers. The factors that increase this interaction would also increase viscosity. The symmetric relationship of viscosity during increasing and decreasing shear rates (Fig. 2(a)) indicates that the solutions in this study have non-thixotropic properties. Fig. 2(b) shows a Newtonian behavior or independency of the viscosity of the gelatin solution in different shear rates from 0 s1 to 1000 s1, held at 1000 s1, and finally from 1000 s1 to 0 s1. Table 1 lists the dynamic viscosity of gelatin solutions with
2.3.6. UVevisible transmission spectra The UVevisible (UVevis) transmission spectra of the bionanocomposites films were obtained from 190 nm to 1100 nm by using a UVevis spectrophotometer model UV-1650PC (Shimadzu, Tokyo, Japan). A blank glass plate was used as the reference. 2.3.7. Color measurement Film samples in five replicated were measured using a colorimeter (Minolta CM-3500D; Minolta Co. Ltd., Osaka, Japan) for both transmission and reflection colorimetry. The instrument was calibrated with zero transmittance calibration plate CM-A100 and air as fully transmittance prior to use in transmission mode. A large size aperture was used, and the color parameters (L*, a*, b*, C*, hab) corresponding to the uniform color space CIELAB were obtained via the computerized system by using the Spectra Magic software version 2.11 (Minolta Cyberchrom Inc., Osaka, Japan). The L value is the psychometric lightness (darkelight) and corresponds to black (L ¼ 0) and white (L ¼ 100), and the a and b values correspond to psychometric chromaticity. A positive a value represents red, whereas a negative value denotes green. A positive b value corresponds to yellow, whereas a negative value indicates blue. C* and hab represent the chroma and hue of the color, respectively. 2.3.8. Film surface morphology The conditioned gelatin nanocomposite samples were vacuumcoated with gold for field emission scanning electron microscopy (FESEM). The surface microstructure of the films was investigated using a Leo Supra 50 VP FESEM (Carl-Ziess. SMT, Oberkochen,
Fig. 2. (a) Variation of viscosity as a function of time and shear rate, (b) shear stress as a function of shear rate for bovine gelatin (Solid lines) and 0.05 (g/g dried gelatin) ZnOenr/bovine gelatin (Dashed lines) solution at 40 C.
A. Mohammadi Nafchi et al. / LWT - Food Science and Technology 58 (2014) 142e149 Table 1 Dynamic viscosity for 8 (w/w) of gelatin based nanocomposite solutions at 40 C. ZnO nanorod (g/g dried gelatin)
m (mPa s)
0.00 0.01 0.02 0.03 0.05
9.1 10.6 10.9 11.9 11.9
0.2c 0.3b 0.5b 0.4a 0.9a
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Table 2 GAB parameters for sorption isotherm of bovine gelatin/ZnOenr composite films at 25 C. ZnO nanorod (g/g dried gelatin)
wm
C
K
E (%)
0.00 0.01 0.02 0.03 0.05
0.13555 0.12102 0.11016 0.10662 0.10756
2.07764 2.00608 2.08078 2.18891 2.10085
0.86005 0.87854 0.89584 0.89929 0.89705
6.01 4.51 6.67 8.93 4.38
Values are mean (n ¼ 5) SD. Different letters represent the significance difference in 5% level of probability among gelatin solutions.
and without ZnOenr as support. The viscosity of the gelatin solutions increased upon the addition of ZnOeN. This result can be attributed to the fact that ZnO nanorods function as fillers that eliminate the free volume in the network, which contributes to the viscous property of gelatin (Nafchi et al., 2012; Rao, 2005). An increase in viscosity after the incorporation of ZnOenr during the gelatinization of other biopolymers has also been previously reported (Ma et al., 2009; Nafchi et al., 2012; Sawai, 2003; Wulansari et al., 1998; Yu et al., 2009).
films can be classified as type II. The parameter K in the GAB equation is a factor used to correct the properties of multilayer molecules in bulk liquid. According to Müller et al. (2011), an increased K value for bionanocomposites in comparison with C indicates a reduction in sorption energy for the absolute values of the multilayers. The monolayer factor (wm) for the films in this study is consistent with those of previous studies because the addition of nanoparticles decreased the hydrophilic behavior of the biopolymer films (Blahovec, 2004; Müller et al., 2011; Nafchi et al., 2012, 2013; Zeppa, Gouanvé, & Espuche, 2009).
3.2. GAB and polynomial moisture sorption isotherm
3.3. Solubility in water
The theoretical sorption isotherm curves fitted with the GAB model and experimental data for bovine gelatin films at 25 C are presented in Fig. 3. The GAB parameters are listed in Table 2. Fig. 3 shows that in all ranges of aw (0.1e0.9), the gelatin films incorporated with ZnOenr exhibited less equilibrium water content compared with the control films. This observation may be attributed to the interaction among the plasticizer, biopolymer matrix, and ZnOeN, which reduced the hydroxyl groups available for interaction with water and led to a less hygroscopic matrix. Müller, Laurindo, and Yamashita (2011) reported that ionedipole interactions occur among ZnO, water, and/or plasticizer, specifically between the zinc and the hydroxyl groups of the plasticizer and water. The percentage of mean relative deviation modulus (E) for the gelatin bionanocomposite films was lower than 10% (Table 2) (Masclaux, Gouanvé, & Espuche, 2010), which indicates that the GAB model gave a good fit for the sorption isotherm. Based on the Brunauer, Emmett, and Teller classification, films with 0 K 1 and C > 2 are type II and those with 0 K 1 and 0 C 2 are type III (Blahovec, 2004). Therefore, the bovine gelatin nanocomposite
The solubility of the ZnOenr bovine gelatin films is presented in Table 3. The introduction of ZnOenr to bovine gelatin matrix significantly decreased the solubility of the biocomposites. This finding may be attributed to the interactions between ZnO and gelatin in the biopolymer film structure. Studies have reported that increasing the nanoparticle (ZnO) content of films results in the formation of more hydrogen bonds in the ZnO and the matrix components (Nafchi et al., 2012; Tunç & Duman, 2010). Thus, free water molecules do not interact as strongly with nanocomposite films compared with composite films alone. These results are consistent with the sorption isotherm and previous reports on bionanocomposites (Müller et al., 2011; Nafchi et al., 2013; Tunç & Duman, 2010; Wulansari et al., 1998). 3.4. Hydrophobic effects of ZnOeN One method of comparing the hydrophobicity of material surfaces is the determination of the contact angle of a water droplet on the surfaces. A high contact angle indicates greater hydrophobicity
Fig. 3. GAB moisture sorption isotherm for bovine gelatin films (solid line) and 0.05 (g/g dried gelatin) ZnOenr incorporated bovine gelatin films (dashed line) at 25 C and respective practical data.
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Table 3 Water solubility gelatin nanocomposite films. ZnO nanorod (g/g dried gelatin)
Water solubility (%)
0.00 0.01 0.02 0.03 0.05
30.10 27.66 26.50 25.20 21.72
1.23a 1.20b 0.94bc 1.34c 2.43d
Values are mean (n ¼ 5) SD. Different letters represent the significance difference in 5% level of probability among gelatin films.
of the surface and vice versa. The contact angle increased with increasing ZnOenr content of the gelatin films (Fig. 4), which indicates that the tendency to absorb water decreased and that the surfaces became more hydrophobic. These findings concur with the findings of water solubility and moisture sorption isotherm investigations detailed above. The knowledge of exact surface topography in measuring the contact angle is critical (Hiemenz & Rajagopalan, 1997) but in this study the reported contact angle is a preliminary report and just for comparing the effects of ZnOenr on hydrophobicity of starch films. 3.5. UVevisible transmission spectra Fig. 5 shows the light transmission of the bovine gelatin nanocomposites. Control films in the UV range (290 nme400 nm) showed a very high transmittance. The addition of ZnOenr decreased UV transmission to almost 0%. By contrast, Yu et al. (2009) recently reported that the addition of 4% nanoeZnO to starch film resulted in the transmission of 3.4% UV light. Likewise, the transmission of visible to IR (>400 nm) spectra decreased by more than 50% after the addition of ZnOenr. The different behavior of ZnOenr in the present study can be attributed to the morphology of particles because the optimum shape for UV absorption is nanorod (O. H. Lin et al., 2009). The absorption peak appeared at 375 nm for gelatin/ZnO nanorod composite. The absorption peaks of these bionanocomposites exhibited an obvious blue-shift phenomenon. The blue shift is an emission of ZnOenr biocomposites that can be attributed to the increased band gap of ZnOenr as a result of the quantum confinement effect (Subramani et al., 2007). These findings suggest that biopolymer matrices reinforced with ZnOenr can be used as UV-shielding films and heat insulators in
Fig. 5. UVevis transmission spectra for gelatin nanocomposite films at 25 C (a: 0.0, b: 0.01, c: 0.02, d: 0.03, e: 0.05 g/g dried gelatin).
the packaging industry. In the same research, the introduction of nanorod ZnO in sago starch matrix caused a decrease in UV transmission, but the effects of ZnOenr on starch are more pronounced than on gelatin (Nafchi et al., 2012). 3.6. Color characteristics The color characteristic parameters for pure matrix and ZnO nanorod-incorporated bovine gelatin nanocomposites are summarized in Table 4. The lightness (L*) significantly decreased with increasing nano-ZnO content. The films turned into light yellow and red, which suggests significant increases in a* and b*. The chroma (C*) was increased by increasing the nanoparticles; this result is in agreement with the color change. The same results were reported for the effects of ZnOenr on sago starch nanocomposites (Nafchi et al., 2012). 3.7. Permeability to water vapor The results of WVP studies are presented in Table 5. The significant decrease in WVP after the addition of ZnOenr may be attributed to the greater water resistance of ZnOenr compared with the biocomposite matrix. The incorporation of these nanorods to the matrix introduces a tortuous pathway for water vapor molecules to pass through (Yu et al., 2009). The reduction in permeability in the gelatin films incorporated with ZnOenr can be described based on Nielsen’s (1967) simple model of tortuosity. This model proposes that each layer of filler particle is perpendicularly oriented to the diffusion pathway, which indicates that water vapor should travel in a longer diffusive path for the permeability coefficient to decrease. The relationship between tortuosity and permeability given by Nielsen’s (1967) model is
Pc 1 fs ¼ s Pm
Fig. 4. Water contact angle on bovine gelatin films surfaces as a function of ZnOenr content. The bars show mean (n ¼ 8) SD. Different letters on the bars represent the significant difference at 5% level of probability.
where s is tortuosity, fs is the volume fraction of ZnOenr, and Pc and Pm are the permeability coefficients of the nanocomposite and base matrix, respectively. Table 5 presents the tortuosity value for each bovine gelatin film matrix. The introduction of 0.02 (g/g dried gelatin) ZnOenr to the gelatin matrix doubled the tortuosity and consequently doubled the path that water vapor needs to traverse from the film through the matrix. The results showed that the permeability coefficient to water vapor significantly decreased
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Table 4 Colorimetric parameter for transmittance (T) and reflectance (R) of gelatin nanocomposite. a*
b*
hab
ZnO nanorod (g/g dried gelatin)
L*
C*
T T T T T
0.00 0.01 0.02 0.03 0.05
95.63 95.30 88.87 84.17 74.39
0.31A 0.28A 0.84B 1.02C 0.99D
0.31 0.36 0.03 0.67 1.47
0.03C 0.03C 0.00C 0.08B 0.01A
3.51 3.79 11.67 14.99 19.38
0.11D 0.10D 0.44C 0.22B 0.40A
3.52 3.81 11.67 15.00 19.44
0.14D 0.11D 0.21C 0.46B 0.37A
95.09 95.41 90.14 87.46 85.67
0.84A 0.44A 0.87B 0.41C 0.21D
R R R R R
0.00 0.01 0.02 0.03 0.05
90.47 89.70 88.13 88.05 87.65
1.02a 1.24a 0.95a 0.88a 0.91a
0.98 1.02 1.00 0.61 0.08
0.03c 0.04c 0.04c 0.08b 0.00a
6.78 7.58 17.94 19.76 21.84
0.51d 0.66d 0.52c 0.23b 0.34a
6.85 7.65 17.96 19.77 21.84
0.36d 0.07d 0.36c 0.21b 0.14a
98.20 97.63 93.21 91.77 90.20
1.21a 1.03b 0.23b 1.04bc 0.88c
Values are mean (n ¼ 5) SD. Different letters in each column represent the significance difference in 5% level of probability among transmittance (capital) or reflectance (small). Table 5 Water vapor permeability (WVP), and tortuosity value of bovine gelatin nanocomposites. ZnOenr (g/g dried gelatin)
WVP 1011 [g m1 s1 Pa1]
0.00 0.01 0.02 0.03 0.05
8.90 7.19 4.08 2.70 1.78
0.31a 0.29b 0.87c 0.48d 0.34e
WVPcomposite/WVPmatrix
s
1 0.81 0.46 0.30 0.20
1 1.25 2.21 3.37 5.11
Values are mean (n ¼ 5) SD. Different letters in WVP column values represent significant difference at 5% level of probability among bovine gelatin films. s: Tortuosity value from Nielson’s equation.
from 8.90 to 4.08 1011 (g m1 s1 Pa1) after the addition of 0.02 (g/g dried gelatin) ZnOenr to the gelatin matrix. The addition of 0.05 (g/g dried gelatin) ZnOenr to the bovine gelatin matrix
decreased the WVP to 1.78 1011 (g m1 s1 Pa1). Addition of ZnOenr significantly increased hydrophobicity of the film surface as indicated by contact angles and reduced solubility of the films from 30 to 20%. Thus the changes in permeability of ZnOenr incorporated film can be not only affected by diffusivities, but also by solubility. Nafchi et al. (2012), Yu et al. (2009), and Zeppa et al. (2009) reported similar results for gas permeation after the incorporation of nanoparticles.
3.8. Film morphology Fig. 6(a) and (b) show the FESEM of the bovine gelatin film surface at magnifications of 10,000 and 20,000, respectively. Fig. 6(c) and (d) show the surface morphology of the bovine gelatin supported by ZnOenr at different magnifications. ZnOenr maintained its rod shape even after introduction to the biopolymeric matrix.
Fig. 6. FESEM micrograph for bovine gelatin film surface with magnification (a) 10,000, (b) 20,000 and 0.05 (g/g dried gelatin) ZnOenr incorporated gelatin film surface with magnification (c) 5000, and (d) 20,000 (small cracks on the films are due to gold coating process, some crystal can see on the films maybe crystallization of sorbitol).
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Fig. 7. Effects of ZnOenr contents on antimicrobial activity bovine gelatin nanocomposite films against S. Aureus. Inhibition zone ¼ total inhibition area total film area. The bars show mean (n ¼ 5) SD. Different letters on the bars represent the significant difference at 5% level of probability.
3.9. Antimicrobial assay The antimicrobial effects of ZnOenr on the bovine gelatin composite film against S. aureus growth were investigated. The inhibition zone of the nano-incorporated films was significantly increased with increasing ZnOenr content (>0.02 (g/g dried gelatin)), which suggests (Fig. 7) that bovine gelatin film supported by ZnOenr can be used as an active packaging against microorganisms. The excellent antimicrobial activity of ZnO nanoparticles against S. aureus and Escherichia coli and the corresponding mechanism of action have also been demonstrated by other researchers (Li et al., 2009; X. H. Li et al., 2010; Nafchi et al., 2012; Zhang et al., 2008). The mechanisms of the antibacterial behavior of ZnO have been categorized as chemical and/or physical interaction between ZnO particles and the cell envelope of the microorganism. Zn2þ can penetrate through the cell wall of the microorganism and react with interior components, thereby affecting cell viability. Another possible mode of action is the generation of H2O2 in the presence of ZnO particles (Zhang et al., 2010). In the present study, the nanorods can function as needles that easily penetrate through the cell wall. Other researchers also reported the same results for ZnOenr against S. aureus or E. coli (Nafchi et al., 2013; Sawai, 2003). 4. Conclusion In this study, nanorod-rich ZnO was introduced to the bovine gelatin matrix to fabricate new bionanocomposites. The specific morphology of ZnOenr is crucial in the basic properties of bovine gelatin films. After incorporation of low levels of the filler (w0.02 (g/g dried gelatin)), significant differences were observed in biocomposite film properties, especially in WVP, UV shielding, and antimicrobial activity. Zinc is one of the essential trace elements under strict regulation in the human body. Therefore, bionanocomposites based on ZnOenr may have potential applications in the medical, pharmaceutical, and food packaging industries. References Abdorreza, M. N., Cheng, L. H., & Karim, A. A. (2011). Effects of plasticizers on thermal properties and heat sealability of sago starch films. Food Hydrocolloids, 25(1), 56e60. http://dx.doi.org/10.1016/j.foodhyd.2010.05.005.
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Preparation and properties of glycerol plasticized-pea starch/zinc oxide-starch bionanocomposites. Carbohydrate Polymers, 75(3), 472e478. http://dx.doi.org/10.1016/j.carbpol.2008.08.007. Maizura, M., Fazilah, A., Norziah, M. H., & Karim, A. A. (2007). Antibacterial activity and mechanical properties of partially hydrolyzed sago starchealginate edible film containing lemongrass oil. Journal of Food Science, 72(6), C324eC330. http://dx.doi.org/10.1111/j.1750-3841.2007.00427.x. Masclaux, C., Gouanvé, F., & Espuche, E. (2010). Experimental and modelling studies of transport in starch nanocomposite films as affected by relative humidity. Journal of Membrane Science, 363(1e2), 221e231. http://dx.doi.org/10.1016/ j.memsci.2010.07.032. Moraes, I. C. F., Carvalho, R. A., Bittante, A. M. Q. B., Solorza-Feria, J., & Sobral, P. J. A. (2009). Film forming solutions based on gelatin and poly(vinyl alcohol) blends: thermal and rheological characterizations. Journal of Food Engineering, 95(4), 588e596. http://dx.doi.org/10.1016/j.jfoodeng.2009.06.023. Müller, C. M. O., Laurindo, J. B., & Yamashita, F. (2011). Effect of nanoclay incorporation method on mechanical and water vapor barrier properties of starchbased films. Industrial Crops and Products, 33(3), 605e610. http://dx.doi.org/ 10.1016/j.indcrop.2010.12.021. Nafchi, A. M., Alias, A. K., Mahmud, S., & Robal, M. (2012). Antimicrobial, rheological, and physicochemical properties of sago starch films filled with nanorod-rich zinc oxide. Journal of Food Engineering, 113(4), 511e519. http://dx.doi.org/ 10.1016/j.jfoodeng.2012.07.017. Nafchi, A. M., Nassiri, R., Sheibani, S., Ariffin, F., & Karim, A. A. (2013). Preparation and characterization of bionanocomposite films filled with nanorod-rich zinc oxide. Carbohydrate Polymers, 96(1), 233e239. http://dx.doi.org/10.1016/ j.carbpol.2013.03.055. Narayanamurti, V. (2006). 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Effects of nanorod-rich ZnO on rheological, sorption isotherm, and physicochemical properties of bovine gelatin films Abdorreza Mohammadi Nafchi a, *, Mahdiyeh Moradpour a, Maliheh Saeidi a, Abd Karim Alias b a b
Food Biopolymer Research Group, Food Science and Technology Department, Damghan Branch, Islamic Azad University, Damghan, Semanan, Iran Food Biopolymer Research Group, Food Technology Division, School of Industrial Technology, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 August 2013 Received in revised form 28 February 2014 Accepted 6 March 2014
The effects of nanorod-rich zinc oxide (ZnOenr) on the flow properties of bovine gelatin solution and on the sorption isotherm, antimicrobial, and physchochemical properties of gelatin films were investigated. ZnOenr was incorporated into gelatin solutions at different concentrations (0.01, 0.02, 0.03, and 0.05 g/g dried gelatin). The introduction of low ZnOenr concentrations (0.05 g/g dried gelatin) to gelatin solutions significantly increased the viscosity of the solution from 9 to 11.9 mPa s and decreased the permeability of the films to water vapor from 8.9 1011 to 1.78 1011 (g m1 s1 Pa1). Solubility in water decreased from 30% to 20%, and monolayer water content of the films decreased from 0.13 to 0.10 (g water/g dried solid), whereas their contact angle increased from 45 to 85 with increasing ZnOenr concentration from 0 to 0.05 g/g dried gelatin. The ZnOenr gelatin films had very low UV transmittance and were able to absorb more than 50% of the near-infrared spectra. These films showed excellent antimicrobial activity against Staphylococcus aureus. These properties suggest that ZnOenr can be potentially used as fillers in gelatin-based films for active packaging materials in the pharmaceutical and food packaging industries. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Bovine gelatin film Flow properties Sorption isotherm Antimicrobial activity UV-shield
1. Introduction Nanoscale science and other related technologies have been the leading technology in the past decade (Narayanamurti, 2006). The incorporation of nanoparticles into composite materials can enhance polymer properties, such as thermal, mechanical, and gas barrier (Kurian, Dasgupta, Galvin, Ziegler, & Beyer, 2006). Inorganic materials, such as metal or metal oxides, have recently attracted considerable attention because of their capability to tolerate harsh conditions (Fu et al., 2005). Among the inorganic materials, ZnO, TiO2, MgO, and CaO are safe for animals and humans and stable in harsh conditions (W. Lin et al., 2009; Stoimenov, Klinger, Marchin, & Klabunde, 2002). Bionanocomposites, which are formed by the combination of biopolymers and an inorganic material that has at least one nanometer-scale dimension, are a new generation of nanocomposites. Bionanocomposites are nanostructured hybrid materials that have emerged at the frontier of material science, * Corresponding author. Tel.: þ98 232 522 5045; fax: þ98 232 522 5039. E-mail addresses: [email protected], [email protected] (A. Mohammadi Nafchi). http://dx.doi.org/10.1016/j.lwt.2014.03.007 0023-6438/Ó 2014 Elsevier Ltd. All rights reserved.
nanotechnology, and life science (Darder, Aranda, & Ruiz-Hitzky, 2007; Ozin, Arsenault, & Cademartiri, 2009). Efforts have been exerted to develop nanobiocomposites with enhanced thermal, mechanical, and functional properties by adding matrix or nanoparticle fillers (Chen, Zhou, Yang, Gu, & Wu, 2004; Jia et al., 2006). In addition, biopolymer-based materials are known as a green technology with biodegradable and biocompatible properties and are used in pharmaceutical, food packaging, and agriculture technologies. Natural polymers are based on polysaccharides or proteins; among all biopolymers, starch and gelatin are the most studied because of their wide applications (Nafchi, Nassiri, Sheibani, Ariffin, & Karim, 2013; Park, Scott Whiteside, & Cho, 2008). The unique capacities of proteins to form network and induce plasticity and elasticity are beneficial in preparing biopolymerbased packaging materials. In this regard, inexpensive bovine gelatin containing a large number of suspended functional groups can facilitate chemical cross-linking and derivatization. Hence, bovine gelatin can be used as a basic raw material for the development of food packaging films (Voon, Bhat, Easa, Liong, & Karim, 2012). However, similar to other biopolymer films, gelatin films are highly sensitive to moisture and easily lose their dimensional
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stability at room temperature, which lead to poor mechanical and barrier properties, thereby limiting their applications as a food packaging material (Fakhoury et al., 2012; Rivero, García, & Pinotti, 2010). Nanofillers have excellent interfacial interactions on polymer branches because of their large specific surface area and high surface energy, thereby significantly enhancing polymer properties (Kovacevi c, Vrsaljko, Lu ci cBlagojevic, & Leskovac, 2008). Zinc oxide (ZnO) has been widely used as a functional filler in UV absorbers for application in cosmetics, pharmaceutical materials, pigments, and coating materials (Li et al., 2009). ZnO particles may be used to prevent infectious diseases because of the antimicrobial effects of ZnO (Li et al., 2009; X. H. Li, Xing, Li, Jiang, & Ding, 2010; Zhang, Ding, Povey, & York, 2008). The size, morphology, crystallinity, composition, and shape of particles are critical parameters of the intrinsic properties of nanoparticles (Shahrom & Abdullah, 2006; Yamamoto, 2001). O. H. Lin, Akil, and Mahmud (2009) reported that ZnO nanorods exhibit optimal UV absorption activity. Although the incorporation of ZnO nanoparticles into biopolymer films can improve several properties (Ma, Chang, Yang, & Yu, 2009; Nafchi, Alias, Mahmud, & Robal, 2012; Nafchi et al., 2013; Yu, Yang, Liu, & Ma, 2009), the hydrophobicity, barrier, and flow properties of ZnO nanorod-reinforced gelatin have not been investigated. We hypothesized that the application of low-concentration nanorod ZnO into bovine gelatin film improves film hydrophobicity and enables the fabrication of a biopolymeric film with UV-shielding and antimicrobial properties. The proposed film can be applied in packaging industries. In this study, ZnO nanorods were used as fillers to prepare gelatin/ZnO nanorod (ZnOeN) bionanocomposites. We characterized the films based on their physicochemical, antibacterial, barrier, and hydrophobicity properties. As an operational parameter, the flow behavior of the bionanocomposite solutions was also investigated. 2. Materials and methods 2.1. Materials Bovine gelatin (Type B) was purchased from Sigma Chemical Co. (St. Louis, MO, USA). Liquid sorbitol and glycerol were obtained from Liang Traco (Penang, Malaysia). ZnOenr was synthesized via the catalyst-free combust-oxidized mesh process, as described by Shahrom and Abdullah (2007). Environmental scanning electron microscopy (ESEM) results (Fig. 1) revealed that ZnOenr has a diameter of 40 nme100 nm and a length of 200 nme700 nm. Some
143
microparticles were also observed. The densities of gelatin and ZnOenr were 1.369 and 5.06 g cm3, respectively to calculation of volume fraction for tortuosity estimation. For the antimicrobial assay, Staphylococcus aureus culture was obtained from the culture collection center (School of Industrial Technology, USM, Malaysia) and grown on nutrient agar slants at 4 C. 2.2. Preparation of nanobiocomposite films ZnOenr was dispersed in water at different concentrations (0.01, 0.02, 0.03, and 0.05; g/g of total solid), stirred for 1 h, and then sonicated in an ultrasonic bath (Marconi model, Unique USC 45 kHz, Piracicaba, Brazil) for 30 min to ensure complete homogenization. The solution was used to prepare the aqueous gelatin dispersion at 8 (g/L). A mixture of sorbitol and glycerol (3:1) at 0.4 (g/g total solid) was added as a plasticizer in accordance with Abdorreza, Cheng, and Karim (2011). Gelatin nanocomposites were heated to 58 2 C and the temperature was held for 1 h. The gelatin nanocomposite solutions were cooled to 45 C, and the bubbles were removed using a vacuum pump. A portion (45 g) of the dispersion was cast on Perspex plates fitted with rims around the edge to yield a 16 cm 16 cm film-forming area. The films were dried in controlled conditions in a humidity chamber (25 1 C and 50 5% relative humidity). Control films were prepared in a similar manner but without the addition of nanoparticles. The dried films were peeled and stored at 23 2 C and 50 5% RH until experimentation. The thickness of each film was measured at five locations and to the nearest 0.01 mm with a hand-held micrometer (Mitutoyo, Tokyo, Japan). All films (including the control samples) were prepared in triplicates. 2.3. Characterization 2.3.1. Flow properties of gelatin solutions Solution samples used in film preparation (w1 mL) were transferred to a rheometer (CSL100, TA Instrument, New Castel, DE, USA) set at 55 C and mounted with a 6 cm cone geometry (2 and 60 mm gap). Flow properties at 40 C were measured in continuous flow by applying shear rates in linear mode from 0 s1 to 1000 s1 for 3 min, held constant at a shear rate of 1000 s1 for 1 min, and decreased from 1000 s1 to 0 s1 in 3 min. According to previous tests and references at this temperature all gelatin dispersions in sol medium (Moraes, Carvalho, Bittante, Solorza-Feria, & Sobral, 2009). Experimental data for bovine gelatin were fitted using the Newtonian rheological model: s ¼ mg, where s is the shear stress
Fig. 1. ESEM micrograph of nanorod-rich ZnO (a) magnification ¼ 25,000, (b) magnification ¼ 50,000.
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(Pa), and g is the shear rate (s1) (Wulansari, Mitchell, Blanshard, & Paterson, 1998).
Germany) equipped with an Oxford INCA 400 energy dispersive device.
2.3.2. Moisture sorption isotherm The moisture sorption isotherm of the films at 25 C was studied using the method described by Bertuzzi, Castro Vidaurre, Armada, and Gottifredi (2007). The moisture content at equilibrium (g absorbed water/g dry film) was measured in triplicates for each relative humidity. The experimental sorption data were fitted using the GAB equation (van den Berg, 1984).
2.3.9. Antimicrobial assay An antimicrobial activity test of the films was carried out using the agar diffusion method described by Maizura et al. (2007). The antimicrobial effects of the films were determined by the inhibition zone against S. aureus on solid media.
W ¼
wm CKaw ð1 Kaw Þð1 Kaw þ CKaw Þ
where wm, K, and C are the GAB parameters, W is the moisture content (dry basis), and aw is water activity. 2.3.3. Solubility in water The solubility of the nanocomposite gelatin films in water was determined following the method described by Maizura, Fazilah, Norziah, and Karim (2007) at room temperature. 2.3.4. Water vapor permeability (WVP) WVP tests of the films were carried out following the modified method (Yu et al., 2009) of ASTM standard E96-05 (ASTM, 2005) at 25 1 C. 2.3.5. Contact-angle measurements Water contact-angle measurements were performed on a static contact-angle meter (CAM-PLUS, Tantec, Germany) to determine the hydrophobicity of the nanocomposite gelatin films. Data presented are the means of ten independent determinations at different sites. The contact angles on each film surface were immediately after the addition of 1 mL of deionized water by using the Sessile Drop Half-AngleÔ Tangent line method.
2.4. Statistical analysis ANOVA and Tukey’s Post Hoc tests were performed to compare the means of the physical and antimicrobial properties of the gelatin nanocomposite films at 5% significance level. Statistical analyses were conducted using GraphPad Prism 6 (GraphPad Software Inc., La Jolla, USA). Curve fitting for GAB and polynomial sorption isotherm was evaluated by non-linear regression by using the solver module in Microsoft ExcelÒ 2010. 3. Results and discussion 3.1. Flow properties of gelatin nanocomposite solutions Viscosity and flow properties of fluids are directly related to the interaction of the fluid layers. The factors that increase this interaction would also increase viscosity. The symmetric relationship of viscosity during increasing and decreasing shear rates (Fig. 2(a)) indicates that the solutions in this study have non-thixotropic properties. Fig. 2(b) shows a Newtonian behavior or independency of the viscosity of the gelatin solution in different shear rates from 0 s1 to 1000 s1, held at 1000 s1, and finally from 1000 s1 to 0 s1. Table 1 lists the dynamic viscosity of gelatin solutions with
2.3.6. UVevisible transmission spectra The UVevisible (UVevis) transmission spectra of the bionanocomposites films were obtained from 190 nm to 1100 nm by using a UVevis spectrophotometer model UV-1650PC (Shimadzu, Tokyo, Japan). A blank glass plate was used as the reference. 2.3.7. Color measurement Film samples in five replicated were measured using a colorimeter (Minolta CM-3500D; Minolta Co. Ltd., Osaka, Japan) for both transmission and reflection colorimetry. The instrument was calibrated with zero transmittance calibration plate CM-A100 and air as fully transmittance prior to use in transmission mode. A large size aperture was used, and the color parameters (L*, a*, b*, C*, hab) corresponding to the uniform color space CIELAB were obtained via the computerized system by using the Spectra Magic software version 2.11 (Minolta Cyberchrom Inc., Osaka, Japan). The L value is the psychometric lightness (darkelight) and corresponds to black (L ¼ 0) and white (L ¼ 100), and the a and b values correspond to psychometric chromaticity. A positive a value represents red, whereas a negative value denotes green. A positive b value corresponds to yellow, whereas a negative value indicates blue. C* and hab represent the chroma and hue of the color, respectively. 2.3.8. Film surface morphology The conditioned gelatin nanocomposite samples were vacuumcoated with gold for field emission scanning electron microscopy (FESEM). The surface microstructure of the films was investigated using a Leo Supra 50 VP FESEM (Carl-Ziess. SMT, Oberkochen,
Fig. 2. (a) Variation of viscosity as a function of time and shear rate, (b) shear stress as a function of shear rate for bovine gelatin (Solid lines) and 0.05 (g/g dried gelatin) ZnOenr/bovine gelatin (Dashed lines) solution at 40 C.
A. Mohammadi Nafchi et al. / LWT - Food Science and Technology 58 (2014) 142e149 Table 1 Dynamic viscosity for 8 (w/w) of gelatin based nanocomposite solutions at 40 C. ZnO nanorod (g/g dried gelatin)
m (mPa s)
0.00 0.01 0.02 0.03 0.05
9.1 10.6 10.9 11.9 11.9
0.2c 0.3b 0.5b 0.4a 0.9a
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Table 2 GAB parameters for sorption isotherm of bovine gelatin/ZnOenr composite films at 25 C. ZnO nanorod (g/g dried gelatin)
wm
C
K
E (%)
0.00 0.01 0.02 0.03 0.05
0.13555 0.12102 0.11016 0.10662 0.10756
2.07764 2.00608 2.08078 2.18891 2.10085
0.86005 0.87854 0.89584 0.89929 0.89705
6.01 4.51 6.67 8.93 4.38
Values are mean (n ¼ 5) SD. Different letters represent the significance difference in 5% level of probability among gelatin solutions.
and without ZnOenr as support. The viscosity of the gelatin solutions increased upon the addition of ZnOeN. This result can be attributed to the fact that ZnO nanorods function as fillers that eliminate the free volume in the network, which contributes to the viscous property of gelatin (Nafchi et al., 2012; Rao, 2005). An increase in viscosity after the incorporation of ZnOenr during the gelatinization of other biopolymers has also been previously reported (Ma et al., 2009; Nafchi et al., 2012; Sawai, 2003; Wulansari et al., 1998; Yu et al., 2009).
films can be classified as type II. The parameter K in the GAB equation is a factor used to correct the properties of multilayer molecules in bulk liquid. According to Müller et al. (2011), an increased K value for bionanocomposites in comparison with C indicates a reduction in sorption energy for the absolute values of the multilayers. The monolayer factor (wm) for the films in this study is consistent with those of previous studies because the addition of nanoparticles decreased the hydrophilic behavior of the biopolymer films (Blahovec, 2004; Müller et al., 2011; Nafchi et al., 2012, 2013; Zeppa, Gouanvé, & Espuche, 2009).
3.2. GAB and polynomial moisture sorption isotherm
3.3. Solubility in water
The theoretical sorption isotherm curves fitted with the GAB model and experimental data for bovine gelatin films at 25 C are presented in Fig. 3. The GAB parameters are listed in Table 2. Fig. 3 shows that in all ranges of aw (0.1e0.9), the gelatin films incorporated with ZnOenr exhibited less equilibrium water content compared with the control films. This observation may be attributed to the interaction among the plasticizer, biopolymer matrix, and ZnOeN, which reduced the hydroxyl groups available for interaction with water and led to a less hygroscopic matrix. Müller, Laurindo, and Yamashita (2011) reported that ionedipole interactions occur among ZnO, water, and/or plasticizer, specifically between the zinc and the hydroxyl groups of the plasticizer and water. The percentage of mean relative deviation modulus (E) for the gelatin bionanocomposite films was lower than 10% (Table 2) (Masclaux, Gouanvé, & Espuche, 2010), which indicates that the GAB model gave a good fit for the sorption isotherm. Based on the Brunauer, Emmett, and Teller classification, films with 0 K 1 and C > 2 are type II and those with 0 K 1 and 0 C 2 are type III (Blahovec, 2004). Therefore, the bovine gelatin nanocomposite
The solubility of the ZnOenr bovine gelatin films is presented in Table 3. The introduction of ZnOenr to bovine gelatin matrix significantly decreased the solubility of the biocomposites. This finding may be attributed to the interactions between ZnO and gelatin in the biopolymer film structure. Studies have reported that increasing the nanoparticle (ZnO) content of films results in the formation of more hydrogen bonds in the ZnO and the matrix components (Nafchi et al., 2012; Tunç & Duman, 2010). Thus, free water molecules do not interact as strongly with nanocomposite films compared with composite films alone. These results are consistent with the sorption isotherm and previous reports on bionanocomposites (Müller et al., 2011; Nafchi et al., 2013; Tunç & Duman, 2010; Wulansari et al., 1998). 3.4. Hydrophobic effects of ZnOeN One method of comparing the hydrophobicity of material surfaces is the determination of the contact angle of a water droplet on the surfaces. A high contact angle indicates greater hydrophobicity
Fig. 3. GAB moisture sorption isotherm for bovine gelatin films (solid line) and 0.05 (g/g dried gelatin) ZnOenr incorporated bovine gelatin films (dashed line) at 25 C and respective practical data.
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Table 3 Water solubility gelatin nanocomposite films. ZnO nanorod (g/g dried gelatin)
Water solubility (%)
0.00 0.01 0.02 0.03 0.05
30.10 27.66 26.50 25.20 21.72
1.23a 1.20b 0.94bc 1.34c 2.43d
Values are mean (n ¼ 5) SD. Different letters represent the significance difference in 5% level of probability among gelatin films.
of the surface and vice versa. The contact angle increased with increasing ZnOenr content of the gelatin films (Fig. 4), which indicates that the tendency to absorb water decreased and that the surfaces became more hydrophobic. These findings concur with the findings of water solubility and moisture sorption isotherm investigations detailed above. The knowledge of exact surface topography in measuring the contact angle is critical (Hiemenz & Rajagopalan, 1997) but in this study the reported contact angle is a preliminary report and just for comparing the effects of ZnOenr on hydrophobicity of starch films. 3.5. UVevisible transmission spectra Fig. 5 shows the light transmission of the bovine gelatin nanocomposites. Control films in the UV range (290 nme400 nm) showed a very high transmittance. The addition of ZnOenr decreased UV transmission to almost 0%. By contrast, Yu et al. (2009) recently reported that the addition of 4% nanoeZnO to starch film resulted in the transmission of 3.4% UV light. Likewise, the transmission of visible to IR (>400 nm) spectra decreased by more than 50% after the addition of ZnOenr. The different behavior of ZnOenr in the present study can be attributed to the morphology of particles because the optimum shape for UV absorption is nanorod (O. H. Lin et al., 2009). The absorption peak appeared at 375 nm for gelatin/ZnO nanorod composite. The absorption peaks of these bionanocomposites exhibited an obvious blue-shift phenomenon. The blue shift is an emission of ZnOenr biocomposites that can be attributed to the increased band gap of ZnOenr as a result of the quantum confinement effect (Subramani et al., 2007). These findings suggest that biopolymer matrices reinforced with ZnOenr can be used as UV-shielding films and heat insulators in
Fig. 5. UVevis transmission spectra for gelatin nanocomposite films at 25 C (a: 0.0, b: 0.01, c: 0.02, d: 0.03, e: 0.05 g/g dried gelatin).
the packaging industry. In the same research, the introduction of nanorod ZnO in sago starch matrix caused a decrease in UV transmission, but the effects of ZnOenr on starch are more pronounced than on gelatin (Nafchi et al., 2012). 3.6. Color characteristics The color characteristic parameters for pure matrix and ZnO nanorod-incorporated bovine gelatin nanocomposites are summarized in Table 4. The lightness (L*) significantly decreased with increasing nano-ZnO content. The films turned into light yellow and red, which suggests significant increases in a* and b*. The chroma (C*) was increased by increasing the nanoparticles; this result is in agreement with the color change. The same results were reported for the effects of ZnOenr on sago starch nanocomposites (Nafchi et al., 2012). 3.7. Permeability to water vapor The results of WVP studies are presented in Table 5. The significant decrease in WVP after the addition of ZnOenr may be attributed to the greater water resistance of ZnOenr compared with the biocomposite matrix. The incorporation of these nanorods to the matrix introduces a tortuous pathway for water vapor molecules to pass through (Yu et al., 2009). The reduction in permeability in the gelatin films incorporated with ZnOenr can be described based on Nielsen’s (1967) simple model of tortuosity. This model proposes that each layer of filler particle is perpendicularly oriented to the diffusion pathway, which indicates that water vapor should travel in a longer diffusive path for the permeability coefficient to decrease. The relationship between tortuosity and permeability given by Nielsen’s (1967) model is
Pc 1 fs ¼ s Pm
Fig. 4. Water contact angle on bovine gelatin films surfaces as a function of ZnOenr content. The bars show mean (n ¼ 8) SD. Different letters on the bars represent the significant difference at 5% level of probability.
where s is tortuosity, fs is the volume fraction of ZnOenr, and Pc and Pm are the permeability coefficients of the nanocomposite and base matrix, respectively. Table 5 presents the tortuosity value for each bovine gelatin film matrix. The introduction of 0.02 (g/g dried gelatin) ZnOenr to the gelatin matrix doubled the tortuosity and consequently doubled the path that water vapor needs to traverse from the film through the matrix. The results showed that the permeability coefficient to water vapor significantly decreased
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Table 4 Colorimetric parameter for transmittance (T) and reflectance (R) of gelatin nanocomposite. a*
b*
hab
ZnO nanorod (g/g dried gelatin)
L*
C*
T T T T T
0.00 0.01 0.02 0.03 0.05
95.63 95.30 88.87 84.17 74.39
0.31A 0.28A 0.84B 1.02C 0.99D
0.31 0.36 0.03 0.67 1.47
0.03C 0.03C 0.00C 0.08B 0.01A
3.51 3.79 11.67 14.99 19.38
0.11D 0.10D 0.44C 0.22B 0.40A
3.52 3.81 11.67 15.00 19.44
0.14D 0.11D 0.21C 0.46B 0.37A
95.09 95.41 90.14 87.46 85.67
0.84A 0.44A 0.87B 0.41C 0.21D
R R R R R
0.00 0.01 0.02 0.03 0.05
90.47 89.70 88.13 88.05 87.65
1.02a 1.24a 0.95a 0.88a 0.91a
0.98 1.02 1.00 0.61 0.08
0.03c 0.04c 0.04c 0.08b 0.00a
6.78 7.58 17.94 19.76 21.84
0.51d 0.66d 0.52c 0.23b 0.34a
6.85 7.65 17.96 19.77 21.84
0.36d 0.07d 0.36c 0.21b 0.14a
98.20 97.63 93.21 91.77 90.20
1.21a 1.03b 0.23b 1.04bc 0.88c
Values are mean (n ¼ 5) SD. Different letters in each column represent the significance difference in 5% level of probability among transmittance (capital) or reflectance (small). Table 5 Water vapor permeability (WVP), and tortuosity value of bovine gelatin nanocomposites. ZnOenr (g/g dried gelatin)
WVP 1011 [g m1 s1 Pa1]
0.00 0.01 0.02 0.03 0.05
8.90 7.19 4.08 2.70 1.78
0.31a 0.29b 0.87c 0.48d 0.34e
WVPcomposite/WVPmatrix
s
1 0.81 0.46 0.30 0.20
1 1.25 2.21 3.37 5.11
Values are mean (n ¼ 5) SD. Different letters in WVP column values represent significant difference at 5% level of probability among bovine gelatin films. s: Tortuosity value from Nielson’s equation.
from 8.90 to 4.08 1011 (g m1 s1 Pa1) after the addition of 0.02 (g/g dried gelatin) ZnOenr to the gelatin matrix. The addition of 0.05 (g/g dried gelatin) ZnOenr to the bovine gelatin matrix
decreased the WVP to 1.78 1011 (g m1 s1 Pa1). Addition of ZnOenr significantly increased hydrophobicity of the film surface as indicated by contact angles and reduced solubility of the films from 30 to 20%. Thus the changes in permeability of ZnOenr incorporated film can be not only affected by diffusivities, but also by solubility. Nafchi et al. (2012), Yu et al. (2009), and Zeppa et al. (2009) reported similar results for gas permeation after the incorporation of nanoparticles.
3.8. Film morphology Fig. 6(a) and (b) show the FESEM of the bovine gelatin film surface at magnifications of 10,000 and 20,000, respectively. Fig. 6(c) and (d) show the surface morphology of the bovine gelatin supported by ZnOenr at different magnifications. ZnOenr maintained its rod shape even after introduction to the biopolymeric matrix.
Fig. 6. FESEM micrograph for bovine gelatin film surface with magnification (a) 10,000, (b) 20,000 and 0.05 (g/g dried gelatin) ZnOenr incorporated gelatin film surface with magnification (c) 5000, and (d) 20,000 (small cracks on the films are due to gold coating process, some crystal can see on the films maybe crystallization of sorbitol).
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Fig. 7. Effects of ZnOenr contents on antimicrobial activity bovine gelatin nanocomposite films against S. Aureus. Inhibition zone ¼ total inhibition area total film area. The bars show mean (n ¼ 5) SD. Different letters on the bars represent the significant difference at 5% level of probability.
3.9. Antimicrobial assay The antimicrobial effects of ZnOenr on the bovine gelatin composite film against S. aureus growth were investigated. The inhibition zone of the nano-incorporated films was significantly increased with increasing ZnOenr content (>0.02 (g/g dried gelatin)), which suggests (Fig. 7) that bovine gelatin film supported by ZnOenr can be used as an active packaging against microorganisms. The excellent antimicrobial activity of ZnO nanoparticles against S. aureus and Escherichia coli and the corresponding mechanism of action have also been demonstrated by other researchers (Li et al., 2009; X. H. Li et al., 2010; Nafchi et al., 2012; Zhang et al., 2008). The mechanisms of the antibacterial behavior of ZnO have been categorized as chemical and/or physical interaction between ZnO particles and the cell envelope of the microorganism. Zn2þ can penetrate through the cell wall of the microorganism and react with interior components, thereby affecting cell viability. Another possible mode of action is the generation of H2O2 in the presence of ZnO particles (Zhang et al., 2010). In the present study, the nanorods can function as needles that easily penetrate through the cell wall. Other researchers also reported the same results for ZnOenr against S. aureus or E. coli (Nafchi et al., 2013; Sawai, 2003). 4. Conclusion In this study, nanorod-rich ZnO was introduced to the bovine gelatin matrix to fabricate new bionanocomposites. The specific morphology of ZnOenr is crucial in the basic properties of bovine gelatin films. After incorporation of low levels of the filler (w0.02 (g/g dried gelatin)), significant differences were observed in biocomposite film properties, especially in WVP, UV shielding, and antimicrobial activity. Zinc is one of the essential trace elements under strict regulation in the human body. Therefore, bionanocomposites based on ZnOenr may have potential applications in the medical, pharmaceutical, and food packaging industries. References Abdorreza, M. N., Cheng, L. H., & Karim, A. A. (2011). Effects of plasticizers on thermal properties and heat sealability of sago starch films. Food Hydrocolloids, 25(1), 56e60. http://dx.doi.org/10.1016/j.foodhyd.2010.05.005.
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