Research Article Received: 1 October 2013
Revised: 17 December 2013
Accepted article published: 15 January 2014
Published online in Wiley Online Library:
(wileyonlinelibrary.com) DOI 10.1002/jsfa.6570
Barrier, structural and mechanical properties of bovine gelatin–chitosan blend films related to biopolymer interactions Nasreddine Benbettaïeb,a,b Mia Kurek,b Salwa Bornazc and Frédéric Debeaufortb,d* Abstract BACKGROUND: The increased use of synthetic packaging films has led to a high ecological problem due to their total non-biodegradability. Thus, there is a vital need to develop renewable and environmentally friendly bio-based polymeric materials. Films and coatings made from polysaccharide polymers, particularly chitosans and gelatins have good gas barrier properties and are envisaged more and more for applications in the biomedical and food fields, as well as for packaging. In this study a casting method was used to develop an edible plasticised film from chitosan and gelatin. Aiming to develop a blend film with enhanced properties, the effects of mixing chitosan (CS) and gelatin (G) in different proportions (CS:G, 75:25, 50:50, 25:75, w/w) on functional and physico-chemical properties have been studied. RESULTS: Mean film thickness increased linearly (R2 = 0.999) with surface density of the film forming solution. An enhancement of mechanical properties by increasing the tensile strength (38.7 ± 11 MPa for pure chitosan and 76.8 ± 9 MPa for pure gelatin film) was also observed in blends, due to gelatin content. When the gelatin content in blend films was increased an improvement of both water vapour barrier properties [(4 ± 0.3) × 10−10 g m−1 s−1 Pa−1 for pure chitosan and (2.5 ± 0.14) × 10−10 g m−1 s−1 Pa−1 for pure gelatin, at 70% RH gradient] and oxygen barrier properties ((822.62 ± 90.24) × 10−12 g m−1 s−1 Pa−1 for blend film chitosan:gelatin (25:75 w/w) and (296.67 ± 18.76) × 10−12 g m−1 s−1 Pa−1 for pure gelatin was observed. Fourier transform infrared spectra of blend films showed a shift in the peak positions related to the amide groups (amide-I and amide-III) indicating interactions between biopolymers. CONCLUSIONS: Addition of gelatin in chitosan induced greater functional properties (mechanical, barrier) due to chemical interactions, suggesting an inter-penetrated network. © 2014 Society of Chemical Industry Keywords: water vapour permeability; mechanical properties; oxygen permeability; FTIR; biopolymer interactions
INTRODUCTION Many developing countries face serious problems in managing and minimising the quantity of solid waste. The amount of waste generated annually increases in proportion with the increase in population and urbanisation. Most solid waste is from plastic packaging. The increased use of synthetic packaging films has led to a severe ecological problem due to their total non-biodegradability. Thus, there is a vital need to develop renewable and environmentally friendly bio-based polymeric materials.1 In the last decade, the use of natural biodegradable polymers has been extensive as they offer many advantages over synthetic or non-biodegradable polymers for topical coating applications on foodstuffs. Biopolymer-based packaging materials originating from naturally renewable resources such as polysaccharides, proteins, and lipids have become one of the main preoccupations of research teams.2 While their biodegradability and environmental compatibility are guaranteed, edible biopolymers offer the best opportunities for supplementing the nutritional value of foods.3 A number of polysaccharides, including alginate, carrageenan, chitosan, cellulose derivates, plant gum, starch and pectin, have been used as the key ingredient for the preparation of edible films.4,5 J Sci Food Agric (2014)
In general, they form reasonably resistant films, and their barrier properties against oxygen and organic vapours are good under low relative humidity (RH) conditions (Tg => rubbery
50%CS-50%G T ~0.8 Tg
20 75%CS-25%G T >Tg => rubbery
0
0
10
20
30
40
50
60
70
Strain (%)
Figure 5. Typical stress–strain curves from tensile test of control [chitosan (CS) and gelatin (G)] films and three blended ones (75CS:25G, 50CS:50G and 25CS:75G) equilibrated at 50% relative humidity (RH) and 25∘ C. T is the temperature of mechanical measurements (20∘ C) and T g is the theoretical glass temperature from differential scanning calorimetry (DSC) analysis.
and the slopes were in the elastic region defining the Young’s modulus. At strains >8%, the stress increased slowly until failure occurred. The chitosan film was more deformable than the gelatin film. Blending chitosan and gelatin enhanced the stiffness. Hosseini et al.64 showed that the addition of CS to gelatin films produced more flexible films suggesting that CS takes part in the weakening or reduction of the number of hydrogen bonds, acting as a plasticiser. These results were opposite of those reported by other authors.34 They indicated that chitosan film was tough and hard whereas gelatin–chitosan films were softer and more flexible. The mechanical parameters, tensile strength (TS), Young’s modulus (Y mod ) and elongation at break (E bp ) are summarised in Table 3. TS, Y mod and E bp of chitosan films were 38.73 ± 11.00 MPa, 5.00 ± 0.75 MPa and 65.00 ± 17.20%, respectively. These values are higher than those reported by Pereda et al.65 (TS = 8.41 MPa and E bp = 19.55%), but TS is in the same range as reported by Oguzlu and Tihminlioglu.36 Mechanical properties of chitosan or composite films containing chitosan depend on numerous parameters: the molecular mass of the polymer, the pH of the FFS, the deacetylation degree of the chitosan, the drying conditions, the solubilisation method, and the water content17,22,57,66 Thus, the comparison with the literature data for tensile tests gives opposite interpretations. In contrast to chitosan, gelatin film showed significantly higher (P < 0.05) TS value (76.79 ± 9.91 MPa) and Y mod (18.82 ± 1.50 MPa), but reduced E bp (5.9 ± 2.2%). Thus, gelatin films were mechanically stronger but less deformable than chitosan. Indeed, Gontard and Gorris,19 Park20 and Cuq et al.21 showed that the mechanical properties of protein-based films are generally better than those of polysaccharide-based films. In blend film formulations, the addition of gelatin significantly (P < 0.05) increased TS and Y mod , whereas E bp substantially decreased (Table 3). This can be explained by the anti-plasticisation effect of added gelatin. The film exhibited a very high Y mod , a high TS and a lower E bp at room
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Table 3. Mechanical properties, water solubility and oxygen permeability of chitosan films (100/0), gelatin films (0/100) and chitosan–gelatin (CS-G) blend films
CS-G film (w/w)
Tensile strength (MPa)
100/0 75/25 50/50 25/75 0/100
38.734 ± 11.297a,b 28.066 ± 6.855a 43.188 ± 3.507b 43.306 ± 10.902b 76.794 ± 9.913c
Young’s modulus (MPa) 5.00 ± 0.75a 5.68 ± 0.28a 7.69 ± 0.58b 12.97 ± 1.17c 18.82 ± 1.50d
Elongation at break (%) 65.0 ± 17.2a 41.0 ± 7.7b 49.8 ± 4.7b 12.7 ± 6.6c 5.9 ± 2.2c
Film solubility (%) 33.97 ± 0.86a 33.09 ± 1.19a 37.15 ± 1.84b 37.84 ± 0.77b 51.64 ± 1.79c
Oxygen permeability (10−12 g m−1 s−1 Pa−1 ) ND 822.62 ± 90.24a 943.21 ± 50.51b 486.21 ± 18.45c 296.67 ± 18.76d
Mechanical properties (tensile strength, Young’s modulus, and elongation at break) were determined at 25∘ C and 50% relative humidity. The film solubility was determined after immersion in water for 24 h. Oxygen permeability was measured at 57% relative humidity and 25∘ C. Values are mean ± standard deviation. Means with the same letter in the same column are not significantly different at P < 0.05. ND, not determined.
temperature, which are typical properties of glassy materials.14 The enhancement of the strength by increasing the gelatin proportion in blend CS-G film might be explained by the formation of a denser matrix. Therefore, a more stable network due to the attractive interactions between CS and G in blend films could have been formed. The loss in E bp that was observed for films with higher gelatin content means that the stretchability of blend films could be affected when a sufficient amount of G is added. These results were not in accordance with the work of Pereda et al.34 who reported that chitosan films showed substantially higher TS and reduced E bp values, when compared with gelatin films. However, there are some examples where opposite behaviour was reported for chitosan–bovine gelatin blend64 and gelatin–chitosan blend.67 From the shape of stress–strain curves, in chitosan-containing films a large extension to failure was observed (Fig. 5). This film behaviour was attributed to the fact that with a high proportion of chitosan in blend films, the polymer deforms homogeneously by a viscous process that gives a large extension to failure. Cheng et al.67 showed that the mechanical properties of chitosan were improved by blending it with gelatin. This is due to the interactions between gelatin and chitosan produced by both electrostatic and hydrogen bonding. Therefore, an increment in the TS value could be linked to the formation of a more stable network due to attractive interactions between CS and G in blend film. However, these are only assumptions and these authors did not display or measure interactions really involved in their systems. Pranoto et al.68 reported that there was an optimum level for the interaction between polysaccharides and gelatin where gelatin presents the major and dominant phase in the film system they used. The increase of the mechanical strength with an increasing gelatin proportion may be an important advantage for blend films in some applications. Film solubility The film solubility in water is an important property of edible films or biodegradable films. Their potential applications may require a low water solubility to enhance both product integrity and water resistance.69 Generally, a higher solubility would indicate lower water resistance.70 In this study, the solubility of chitosan films in water is 34% (Table 3). This is in accordance with the values reported by Hosseini et al.64 Pure bovine gelatin films showed significantly higher solubility values (52%) than chitosan films. This was considerably lower than those reported by Hosseini
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et al.64 for fish gelatin films (around 64%). Furthermore, in previous works reported by Pereda et al.,34 Gennadios48 and Martucci and Ruseckaite,71 it was found that plasticised gelatin films were completely soluble in water. In contrast, plasticised chitosan films were slightly soluble in water (e.g. about 16.2% after 24 h). Blended CS-G films were significantly less soluble than gelatin but slightly more soluble than chitosan film (P < 0.05).This indicated that the obtained values did not respond to a simple mixture but could result from interactions between both biopolymers, such as electrostatic forces and hydrogen bonding, as expected by Taravel and Domard.29 But the effect of gelatin on the solubility of blend films is effective since 50% (w/w) proportion of gelatin content. In blend films when the proportion of gelatin was more than 50%, the solubility was significantly improved. These results pointed out that there is a molecular miscibility between both biopolymers for a higher amount of gelatin added in the mixture. Fourier transform infrared spectroscopy of chitosan–gelatin blend films Table 4 and Fig. 6 show FTIR spectra of G, CS and CS:G blend films. The position of relevant peaks in the spectrum of CS films was similar to those described by other authors.34,72 – 74 The characteristic bands appeared at 1636 cm−1 (amide-I band of the acetyl group), 1570 cm−1 (NH3 + absorption band, partially over imposed with amide-II band) and the broad absorption band at 3100–3500 cm−1 due to N—H and hydrogen bonded O—H stretch vibrations. Peaks between 900 and 1150 cm−1 were assigned to the C2 position of pyranose rings and amino groups, respectively.75 The spectrum of gelatin films displayed relevant peaks arisen from an amide C&dbond;O stretching/hydrogen bonding (around 3000 cm−1 ) coupled with COO at 1636 cm−1 (amide-I), bending vibration of N—H groups and stretching vibrations of C—N groups at 1550 cm−1 (amide-II) and the in-plane vibrations of C—N and N—H groups of bound amide or vibrations of CH2 groups of glycine at 1239 cm−1 (amide-III).48,72,76,77 The same absorption peaks were reported by Liu et al.78 for gelatin films made using tilapia fish skin. The spectra of the blend films exhibited the characteristic peaks of both G and CS. The intensity of characteristic peaks changed depending on the blend formulation. However, only shifts in the peak position of the amide-I (𝜈 CO ) and amide-III (𝛿 CN ,𝛿 NH ) groups were observed. Thus it indicated that the obtained spectra result from the molecular interactions between the biopolymers, and do not correspond to a simple mixture or
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Properties of gelatin-chitosan films
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Table 4. Specific spectral bands and their attributed functions from Fourier transform infrared analysis for of chitosan films (CS), gelatin films (G) and chitosan–gelatin (CS-G) blend films Frequency (cm−1 )
Groups
CS
𝜈 OH 𝜈 NH amide A and free water 𝜈 CH 𝜈 CO amide-I 𝛿 NH amide-II 𝛿 OH , 𝛿 CH , 𝛿 CH3 𝛿 CN , 𝛿 NH amide-III Pyranose cycle bond Pyranose ring and amino groups
3100–3600 2850–2960 1630–1660 1540–1570 1325–1410 1204–1310 1167–1033 900–1150
3267 2930 1636 1559 1410 1257 1152 927
75CS:25G 3300 2931 1642 1559 1409 1246 1152 927
50CS:50G 3298 2931 1653 1559 1408 1244 1152 925
25CS:75G 3299 2938 1653 1559 1407 1241 1153 923
G 3315 2938 1646 1558 1403 1240 1043 922
Results in bold type are shifted peaks. Measurements are given as the mean of three scans.
CS
75%CS-25%G
25%CS-75%G
CONCLUSIONS
50%CS-50%G
G
105
Transmittance (%)
100 95 90 85 80 4000 3600 3200 2800 2400 2000 1600 1200
800
Wavenumber (cm-1)
Figure 6. Fourier transform infrared (FTIR) spectra of chitosan, gelatin and their different blends films equilibrated at 50% relative humidity (RH).
just a position of these networks in the blend structure (Table 4). The absorbance of amide-II and amide-III bands increases slightly while increasing the proportion of chitosan. According to Hosseini et al.64 this might be related to amino and carbonyl moieties. These authors suggested that these groups interact mainly through electrostatic interactions confirming the formation of a soluble polyelectrolyte complex. Liu et al.78 noticed that a decrease in vibrational wavelength (1649 cm−1 , 1542 cm−1 and 1234 cm−1 associated with amide-I, amide-II and amide-III peaks, respectively) and a broadening of the OH and NH vibration bands are indicative of a hydrogen bonding interaction between both biopolymers forming the final film. Xie et al.79 observed identical behaviour. In previous work, Cheng et al.67 also found that incorporation of gelatin generated small modifications in the spectrum of chitosan, suggesting that during the formation of the polyelectrolyte complex hydrogen bonding occurs between chitosan and gelatin molecules. In general, the simple analysis of specific spectra is confirmed not be a good method of determining molecular interactions between these biopolymers. But Sionkowska et al.,28 Yin et al.72 and Liu et al.78 developed an easy and pertinent way to make up the presence of specific molecular interactions consisting in comparison of the predicted FTIR spectrum as the weighted sum of the experimental spectra of the individual G and CS films. J Sci Food Agric (2014)
In this study, the effect of the film thicknesses and the RH gradient on the water permeability of blend films (25CS:75G, w/w) was studied. Results showed that the mean film thickness increases linearly with the amount of solids of the film-forming solution poured (R2 = 0.999). In blend films for the same film thickness, the higher relative humidity leads to the higher water vapour permeation. The improvement of chitosan film properties was investigated by its association with bovine gelatin. Therefore, the attempt to develop composite–blended edible films using polysaccharide and protein mixtures showed that gelatin could be an advantageous component that contributes to the improvement of the barrier and the mechanical properties of chitosan films. Gelatin can interact with chitosan via strong inter-molecular bonds during film formation resulting in a compact structure that enhances mechanical and barrier properties of the blended films. The gelatin incorporation of different proportions to chitosan film-forming solution increased tensile strength and Young’s modulus, and decreased the oxygen and water vapour permeability of the final films. From FTIR analysis, the obtained spectra showed a shift in peak position of the amide-I and amide-III groups, indicating some hydrogen interactions between both biopolymers. Increasing the water solubility by adding gelatin is considered a disadvantageous and critical factor in some applications of food packaging. In spite of this the adequate mixture of blend films was referred to the film 25CS:75G (w/w) with good mechanical and barrier properties as well as fairly weak solubility. This can give a promising utility and great advantage for their use in packaging or coating to improve food quality. Further investigation on foodstuff is needed to check the advantage of the produced coatings when applied on real food. In addition, these films could be used for the incorporation of antimicrobial agents for the creation of controlled release systems for food preservation.
ACKNOWLEDGEMENTS The authors gratefully acknowledge the CNSTN Direction and the Ministry of Higher Education and Scientific Research in Tunisia for the financial support of this project. The authors wish to thank the colleagues from the PAM-PAPC Laboratory for precious collaboration and help. The authors wish to thank Prof. J.P. Gay for English improvement.
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66 Caner C, Vergano PJ and Wiles JL, Chitosan film mechanical and permeation properties as affected by acid, plasticizer, and storage. J Food Sci 63:1049–1053 (1998). 67 Cheng M, Deng J, Yang F, Gong Y, Zhao N and Zhang X, Study on physical properties and nerve cell affinity of composite films from chitosan and gelatin solutions. Biomaterials 24:2871–2880 (2003). 68 Pranoto Y, Lee CM and Park HJ, Characterizations of fish gelatin films added with gellan and k-carrageenan. LWT – Food Sci Technol 40:766–774 (2007). 69 Turhan KN and Sahbaz F, Water vapor permeability, tensile properties and solubility of methylcellulose-based edible films. J Food Eng 61:459–466 (2004). 70 Bourtoom T and Chinnan MS, Preparation and properties of rice starch–chitosan blend biodegradable film. LWT – Food Sci Technol 41:1633–1641 (2008). 71 Martucci JF and Ruseckaite RA, Tensile properties, barrier properties and biodegradation in soil of compression emolded gelatine starch dialdehyde films. J Appl Polym Sci 112:2166–2178 (2009). 72 Yin Y, Li Z, Sun Y and Yao K, A preliminary study on chitosan/gelatine polyelectrolyte complex formation. J Mater Sci Lett 40:4649–4652 (2005). 73 Abugoch LE, Tapia C, Villamán MC, Yazdani-Pedram M and Díaz-Dosque M, Characterization of quinoa protein–chitosan blend edible films. Food Hydrocolloids 25:879–886 (2011). 74 Kurek M, Brachais CH, Nguimjeu CM, Bonnotte A, Voilley A, Galic K, et al., Structure and thermal properties of a chitosan coated polyethylene bilayer film. Polym Degrad Stab 97:1232–1240 (2012). 75 Lima CGA, de Oliveira RS, Figueiro SD, Wehmann CF, Goes JC and Sombra ASB, DC conductivity and dielectric permittivity of collagene chitosan films. Mater Chem Phys 99:284–288 (2006). 76 Nur Hanani ZA, McNumara M, Ross YH and Kerry JP, Effect of plasticizer content on the functional proporties of extruded gelatine based composite films. J Food Hydrocolloids 31:264–269 (2013). 77 Tongnuanchan P, Benjakul S and Prodpran T, Properties and antioxidant activity of fish skin gelatin film incorporated with citrus essential oils. J Food Chem 134:1571–1579 (2012). 78 Liu Z, Ge X, Lu Y, Dong S, Zhao Y and Zeng M, Effects of chitosan molecular weight and degree of deacetylation on the properties of gelatine-based films. Food Hydrocolloids 26:311–317 (2012). 79 Xie YL, Zhou HM and Qian HF, Effect of addition of peach gum on physicochemical properties of gelatin-based microcapsule. J Food Biochem 30:302–312 (2006).
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Revised: 17 December 2013
Accepted article published: 15 January 2014
Published online in Wiley Online Library:
(wileyonlinelibrary.com) DOI 10.1002/jsfa.6570
Barrier, structural and mechanical properties of bovine gelatin–chitosan blend films related to biopolymer interactions Nasreddine Benbettaïeb,a,b Mia Kurek,b Salwa Bornazc and Frédéric Debeaufortb,d* Abstract BACKGROUND: The increased use of synthetic packaging films has led to a high ecological problem due to their total non-biodegradability. Thus, there is a vital need to develop renewable and environmentally friendly bio-based polymeric materials. Films and coatings made from polysaccharide polymers, particularly chitosans and gelatins have good gas barrier properties and are envisaged more and more for applications in the biomedical and food fields, as well as for packaging. In this study a casting method was used to develop an edible plasticised film from chitosan and gelatin. Aiming to develop a blend film with enhanced properties, the effects of mixing chitosan (CS) and gelatin (G) in different proportions (CS:G, 75:25, 50:50, 25:75, w/w) on functional and physico-chemical properties have been studied. RESULTS: Mean film thickness increased linearly (R2 = 0.999) with surface density of the film forming solution. An enhancement of mechanical properties by increasing the tensile strength (38.7 ± 11 MPa for pure chitosan and 76.8 ± 9 MPa for pure gelatin film) was also observed in blends, due to gelatin content. When the gelatin content in blend films was increased an improvement of both water vapour barrier properties [(4 ± 0.3) × 10−10 g m−1 s−1 Pa−1 for pure chitosan and (2.5 ± 0.14) × 10−10 g m−1 s−1 Pa−1 for pure gelatin, at 70% RH gradient] and oxygen barrier properties ((822.62 ± 90.24) × 10−12 g m−1 s−1 Pa−1 for blend film chitosan:gelatin (25:75 w/w) and (296.67 ± 18.76) × 10−12 g m−1 s−1 Pa−1 for pure gelatin was observed. Fourier transform infrared spectra of blend films showed a shift in the peak positions related to the amide groups (amide-I and amide-III) indicating interactions between biopolymers. CONCLUSIONS: Addition of gelatin in chitosan induced greater functional properties (mechanical, barrier) due to chemical interactions, suggesting an inter-penetrated network. © 2014 Society of Chemical Industry Keywords: water vapour permeability; mechanical properties; oxygen permeability; FTIR; biopolymer interactions
INTRODUCTION Many developing countries face serious problems in managing and minimising the quantity of solid waste. The amount of waste generated annually increases in proportion with the increase in population and urbanisation. Most solid waste is from plastic packaging. The increased use of synthetic packaging films has led to a severe ecological problem due to their total non-biodegradability. Thus, there is a vital need to develop renewable and environmentally friendly bio-based polymeric materials.1 In the last decade, the use of natural biodegradable polymers has been extensive as they offer many advantages over synthetic or non-biodegradable polymers for topical coating applications on foodstuffs. Biopolymer-based packaging materials originating from naturally renewable resources such as polysaccharides, proteins, and lipids have become one of the main preoccupations of research teams.2 While their biodegradability and environmental compatibility are guaranteed, edible biopolymers offer the best opportunities for supplementing the nutritional value of foods.3 A number of polysaccharides, including alginate, carrageenan, chitosan, cellulose derivates, plant gum, starch and pectin, have been used as the key ingredient for the preparation of edible films.4,5 J Sci Food Agric (2014)
In general, they form reasonably resistant films, and their barrier properties against oxygen and organic vapours are good under low relative humidity (RH) conditions (Tg => rubbery
50%CS-50%G T ~0.8 Tg
20 75%CS-25%G T >Tg => rubbery
0
0
10
20
30
40
50
60
70
Strain (%)
Figure 5. Typical stress–strain curves from tensile test of control [chitosan (CS) and gelatin (G)] films and three blended ones (75CS:25G, 50CS:50G and 25CS:75G) equilibrated at 50% relative humidity (RH) and 25∘ C. T is the temperature of mechanical measurements (20∘ C) and T g is the theoretical glass temperature from differential scanning calorimetry (DSC) analysis.
and the slopes were in the elastic region defining the Young’s modulus. At strains >8%, the stress increased slowly until failure occurred. The chitosan film was more deformable than the gelatin film. Blending chitosan and gelatin enhanced the stiffness. Hosseini et al.64 showed that the addition of CS to gelatin films produced more flexible films suggesting that CS takes part in the weakening or reduction of the number of hydrogen bonds, acting as a plasticiser. These results were opposite of those reported by other authors.34 They indicated that chitosan film was tough and hard whereas gelatin–chitosan films were softer and more flexible. The mechanical parameters, tensile strength (TS), Young’s modulus (Y mod ) and elongation at break (E bp ) are summarised in Table 3. TS, Y mod and E bp of chitosan films were 38.73 ± 11.00 MPa, 5.00 ± 0.75 MPa and 65.00 ± 17.20%, respectively. These values are higher than those reported by Pereda et al.65 (TS = 8.41 MPa and E bp = 19.55%), but TS is in the same range as reported by Oguzlu and Tihminlioglu.36 Mechanical properties of chitosan or composite films containing chitosan depend on numerous parameters: the molecular mass of the polymer, the pH of the FFS, the deacetylation degree of the chitosan, the drying conditions, the solubilisation method, and the water content17,22,57,66 Thus, the comparison with the literature data for tensile tests gives opposite interpretations. In contrast to chitosan, gelatin film showed significantly higher (P < 0.05) TS value (76.79 ± 9.91 MPa) and Y mod (18.82 ± 1.50 MPa), but reduced E bp (5.9 ± 2.2%). Thus, gelatin films were mechanically stronger but less deformable than chitosan. Indeed, Gontard and Gorris,19 Park20 and Cuq et al.21 showed that the mechanical properties of protein-based films are generally better than those of polysaccharide-based films. In blend film formulations, the addition of gelatin significantly (P < 0.05) increased TS and Y mod , whereas E bp substantially decreased (Table 3). This can be explained by the anti-plasticisation effect of added gelatin. The film exhibited a very high Y mod , a high TS and a lower E bp at room
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Table 3. Mechanical properties, water solubility and oxygen permeability of chitosan films (100/0), gelatin films (0/100) and chitosan–gelatin (CS-G) blend films
CS-G film (w/w)
Tensile strength (MPa)
100/0 75/25 50/50 25/75 0/100
38.734 ± 11.297a,b 28.066 ± 6.855a 43.188 ± 3.507b 43.306 ± 10.902b 76.794 ± 9.913c
Young’s modulus (MPa) 5.00 ± 0.75a 5.68 ± 0.28a 7.69 ± 0.58b 12.97 ± 1.17c 18.82 ± 1.50d
Elongation at break (%) 65.0 ± 17.2a 41.0 ± 7.7b 49.8 ± 4.7b 12.7 ± 6.6c 5.9 ± 2.2c
Film solubility (%) 33.97 ± 0.86a 33.09 ± 1.19a 37.15 ± 1.84b 37.84 ± 0.77b 51.64 ± 1.79c
Oxygen permeability (10−12 g m−1 s−1 Pa−1 ) ND 822.62 ± 90.24a 943.21 ± 50.51b 486.21 ± 18.45c 296.67 ± 18.76d
Mechanical properties (tensile strength, Young’s modulus, and elongation at break) were determined at 25∘ C and 50% relative humidity. The film solubility was determined after immersion in water for 24 h. Oxygen permeability was measured at 57% relative humidity and 25∘ C. Values are mean ± standard deviation. Means with the same letter in the same column are not significantly different at P < 0.05. ND, not determined.
temperature, which are typical properties of glassy materials.14 The enhancement of the strength by increasing the gelatin proportion in blend CS-G film might be explained by the formation of a denser matrix. Therefore, a more stable network due to the attractive interactions between CS and G in blend films could have been formed. The loss in E bp that was observed for films with higher gelatin content means that the stretchability of blend films could be affected when a sufficient amount of G is added. These results were not in accordance with the work of Pereda et al.34 who reported that chitosan films showed substantially higher TS and reduced E bp values, when compared with gelatin films. However, there are some examples where opposite behaviour was reported for chitosan–bovine gelatin blend64 and gelatin–chitosan blend.67 From the shape of stress–strain curves, in chitosan-containing films a large extension to failure was observed (Fig. 5). This film behaviour was attributed to the fact that with a high proportion of chitosan in blend films, the polymer deforms homogeneously by a viscous process that gives a large extension to failure. Cheng et al.67 showed that the mechanical properties of chitosan were improved by blending it with gelatin. This is due to the interactions between gelatin and chitosan produced by both electrostatic and hydrogen bonding. Therefore, an increment in the TS value could be linked to the formation of a more stable network due to attractive interactions between CS and G in blend film. However, these are only assumptions and these authors did not display or measure interactions really involved in their systems. Pranoto et al.68 reported that there was an optimum level for the interaction between polysaccharides and gelatin where gelatin presents the major and dominant phase in the film system they used. The increase of the mechanical strength with an increasing gelatin proportion may be an important advantage for blend films in some applications. Film solubility The film solubility in water is an important property of edible films or biodegradable films. Their potential applications may require a low water solubility to enhance both product integrity and water resistance.69 Generally, a higher solubility would indicate lower water resistance.70 In this study, the solubility of chitosan films in water is 34% (Table 3). This is in accordance with the values reported by Hosseini et al.64 Pure bovine gelatin films showed significantly higher solubility values (52%) than chitosan films. This was considerably lower than those reported by Hosseini
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et al.64 for fish gelatin films (around 64%). Furthermore, in previous works reported by Pereda et al.,34 Gennadios48 and Martucci and Ruseckaite,71 it was found that plasticised gelatin films were completely soluble in water. In contrast, plasticised chitosan films were slightly soluble in water (e.g. about 16.2% after 24 h). Blended CS-G films were significantly less soluble than gelatin but slightly more soluble than chitosan film (P < 0.05).This indicated that the obtained values did not respond to a simple mixture but could result from interactions between both biopolymers, such as electrostatic forces and hydrogen bonding, as expected by Taravel and Domard.29 But the effect of gelatin on the solubility of blend films is effective since 50% (w/w) proportion of gelatin content. In blend films when the proportion of gelatin was more than 50%, the solubility was significantly improved. These results pointed out that there is a molecular miscibility between both biopolymers for a higher amount of gelatin added in the mixture. Fourier transform infrared spectroscopy of chitosan–gelatin blend films Table 4 and Fig. 6 show FTIR spectra of G, CS and CS:G blend films. The position of relevant peaks in the spectrum of CS films was similar to those described by other authors.34,72 – 74 The characteristic bands appeared at 1636 cm−1 (amide-I band of the acetyl group), 1570 cm−1 (NH3 + absorption band, partially over imposed with amide-II band) and the broad absorption band at 3100–3500 cm−1 due to N—H and hydrogen bonded O—H stretch vibrations. Peaks between 900 and 1150 cm−1 were assigned to the C2 position of pyranose rings and amino groups, respectively.75 The spectrum of gelatin films displayed relevant peaks arisen from an amide C&dbond;O stretching/hydrogen bonding (around 3000 cm−1 ) coupled with COO at 1636 cm−1 (amide-I), bending vibration of N—H groups and stretching vibrations of C—N groups at 1550 cm−1 (amide-II) and the in-plane vibrations of C—N and N—H groups of bound amide or vibrations of CH2 groups of glycine at 1239 cm−1 (amide-III).48,72,76,77 The same absorption peaks were reported by Liu et al.78 for gelatin films made using tilapia fish skin. The spectra of the blend films exhibited the characteristic peaks of both G and CS. The intensity of characteristic peaks changed depending on the blend formulation. However, only shifts in the peak position of the amide-I (𝜈 CO ) and amide-III (𝛿 CN ,𝛿 NH ) groups were observed. Thus it indicated that the obtained spectra result from the molecular interactions between the biopolymers, and do not correspond to a simple mixture or
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Properties of gelatin-chitosan films
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Table 4. Specific spectral bands and their attributed functions from Fourier transform infrared analysis for of chitosan films (CS), gelatin films (G) and chitosan–gelatin (CS-G) blend films Frequency (cm−1 )
Groups
CS
𝜈 OH 𝜈 NH amide A and free water 𝜈 CH 𝜈 CO amide-I 𝛿 NH amide-II 𝛿 OH , 𝛿 CH , 𝛿 CH3 𝛿 CN , 𝛿 NH amide-III Pyranose cycle bond Pyranose ring and amino groups
3100–3600 2850–2960 1630–1660 1540–1570 1325–1410 1204–1310 1167–1033 900–1150
3267 2930 1636 1559 1410 1257 1152 927
75CS:25G 3300 2931 1642 1559 1409 1246 1152 927
50CS:50G 3298 2931 1653 1559 1408 1244 1152 925
25CS:75G 3299 2938 1653 1559 1407 1241 1153 923
G 3315 2938 1646 1558 1403 1240 1043 922
Results in bold type are shifted peaks. Measurements are given as the mean of three scans.
CS
75%CS-25%G
25%CS-75%G
CONCLUSIONS
50%CS-50%G
G
105
Transmittance (%)
100 95 90 85 80 4000 3600 3200 2800 2400 2000 1600 1200
800
Wavenumber (cm-1)
Figure 6. Fourier transform infrared (FTIR) spectra of chitosan, gelatin and their different blends films equilibrated at 50% relative humidity (RH).
just a position of these networks in the blend structure (Table 4). The absorbance of amide-II and amide-III bands increases slightly while increasing the proportion of chitosan. According to Hosseini et al.64 this might be related to amino and carbonyl moieties. These authors suggested that these groups interact mainly through electrostatic interactions confirming the formation of a soluble polyelectrolyte complex. Liu et al.78 noticed that a decrease in vibrational wavelength (1649 cm−1 , 1542 cm−1 and 1234 cm−1 associated with amide-I, amide-II and amide-III peaks, respectively) and a broadening of the OH and NH vibration bands are indicative of a hydrogen bonding interaction between both biopolymers forming the final film. Xie et al.79 observed identical behaviour. In previous work, Cheng et al.67 also found that incorporation of gelatin generated small modifications in the spectrum of chitosan, suggesting that during the formation of the polyelectrolyte complex hydrogen bonding occurs between chitosan and gelatin molecules. In general, the simple analysis of specific spectra is confirmed not be a good method of determining molecular interactions between these biopolymers. But Sionkowska et al.,28 Yin et al.72 and Liu et al.78 developed an easy and pertinent way to make up the presence of specific molecular interactions consisting in comparison of the predicted FTIR spectrum as the weighted sum of the experimental spectra of the individual G and CS films. J Sci Food Agric (2014)
In this study, the effect of the film thicknesses and the RH gradient on the water permeability of blend films (25CS:75G, w/w) was studied. Results showed that the mean film thickness increases linearly with the amount of solids of the film-forming solution poured (R2 = 0.999). In blend films for the same film thickness, the higher relative humidity leads to the higher water vapour permeation. The improvement of chitosan film properties was investigated by its association with bovine gelatin. Therefore, the attempt to develop composite–blended edible films using polysaccharide and protein mixtures showed that gelatin could be an advantageous component that contributes to the improvement of the barrier and the mechanical properties of chitosan films. Gelatin can interact with chitosan via strong inter-molecular bonds during film formation resulting in a compact structure that enhances mechanical and barrier properties of the blended films. The gelatin incorporation of different proportions to chitosan film-forming solution increased tensile strength and Young’s modulus, and decreased the oxygen and water vapour permeability of the final films. From FTIR analysis, the obtained spectra showed a shift in peak position of the amide-I and amide-III groups, indicating some hydrogen interactions between both biopolymers. Increasing the water solubility by adding gelatin is considered a disadvantageous and critical factor in some applications of food packaging. In spite of this the adequate mixture of blend films was referred to the film 25CS:75G (w/w) with good mechanical and barrier properties as well as fairly weak solubility. This can give a promising utility and great advantage for their use in packaging or coating to improve food quality. Further investigation on foodstuff is needed to check the advantage of the produced coatings when applied on real food. In addition, these films could be used for the incorporation of antimicrobial agents for the creation of controlled release systems for food preservation.
ACKNOWLEDGEMENTS The authors gratefully acknowledge the CNSTN Direction and the Ministry of Higher Education and Scientific Research in Tunisia for the financial support of this project. The authors wish to thank the colleagues from the PAM-PAPC Laboratory for precious collaboration and help. The authors wish to thank Prof. J.P. Gay for English improvement.
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