2.1 Mechanical Properties
(g)Wang et al., 2009, studied about the effect of pH and addition of corn oil (CO) on the properties of gelatin-based films by added CO into gelatin solution and used lactic acid or 1 M NaOH to adjust pH of the solution. Films were produced by casting and dried for 24 h at 50±5% relative humidity. Films were tested for tensile strength, puncture test, and percentage elongation at break point.
The result showed that tensile strength and puncture strength increased when rising CO content, yet were not meaningfully affected by pH. However, pH had more effects on elongation at break values than CO content. When focus on tensile strength, the result verified that CO content be more significant factor than pH on tensile strength of gelatin films. Moreover, the mixture of pH 9.5 and CO content of 47% produced the gelatin films that have the highest tensile strength value. Gelatin films that have high tensile strength value could be attributed to high crystallinity and degree of orientation of this crystallinity in the films. When increased CO addition into gelatin films showed that a more condense of film matrix, which means that a stronger of protein network was formed due to CO replace the empty rooms in the gelatin network (Bradbury and Martin, 1952 cited in Wang et al., 2009). Gelatin films that have the highest elongation at break was gained at a blend of pH 10.54 and 27.25 CO content. Gelatin films added with CO and pH (>9.0) has higher elongation at break than control films (films made from pure gelatin solutions). The puncture strength of gelatin films could develop by increasing CO amounts, whereas increase or decrease pH values from neutrality incline to increase puncture strength values. From this result indicated that pH alteration could have supported the gelatin network formation, which pH and CO content have an effects on puncture strength similar to tensile strength.
(2)Rattaya et al., 2009, had done the experiment on incorporation of seaweed (T. ornata) extract with fish skin gelatin film. Films were prepared by dissolved freeze-dried gelatins in distilled water to gained 2% (w/v) of the final protein concentration. Then adjust pHs to 9 or 10 of the solution by using 1 M NaOH. For the seaweed extracts, methanolic extract of T. ornata was dissolved with deionized water, adjusted to pHs 9 or 10 and then oxygenated for 30 min. Thereafter, 6% (w/w) (based on protein content) oxygenated extracts were added into film forming solution (freeze dried gelatin solution with added glycerol). The mixtures were then stirred and cast onto a silicone plate. The resulting films were obtained by drying at 25°C and 50% relative humidity (RH) for 24 h.
It was found that with incorporation of oxygenated seaweed extract, all gelatin films had similar tensile strength. Elongation at break of gelatin films incorporated with oxygenated seaweed extract at pHs 9 and 10 was approximately 2-fold greater than that of the gelatin film without addition of seaweed extract. The incorporation of seaweed extract could improve elongation at break of resulting film because oxidized phenolic compounds in seaweed extract might link the gelatin to gain the higher chain length.
(f)Carvalho et al., 2006, studied the effect of a chemical reticulation treatment with formaldehyde and glyoxal on the mechanical properties. The NF - native film (without modification); FMF - formaldehyde modified film and GMF - glyoxal-modified film were produced with concentrations of 3.3, 6.3, 8.8 mmole/100 mL of filmogenic solution of formaldehyde and concentrations of 6.3, 8.8, 11.3, 17.5 and 26.3 mmole/100 mL of filmogenic solution of glyoxal. All films contained 10 g of gelatin and 4.5 g of glycerol in 100g of filmogenic solution. The films were prepared by gelatin hydration technique using a mechanical shaking bath (50 °C, 15 min) and addition of glycerol and reticulant agents (formaldehyde and glyoxal) then spread on acrylic plates and dried at room temperature for 24-48 h.
The treatment with formaldehyde caused a significant increase in tensile strength for concentration above 6.3 mmoles/100 mL of filmogenic solution in relation to unmodified films and glyoxal-modified films. For the glyoxal treatment, the increase in glyoxal concentration up to 8.8 mmoles/100 mL of filmogenic solution initially caused an increase in tensile strength, however, for concentrations above 11.3 mmoles/100 mL of filmogenic solution, a slight decrease in tensile strength of the film was observed. In conclusion films treated with formaldehyde and glyoxal had higher strengths than the untreated films. Formaldehyde showed a greater effect on the strength of the film as a function of the increase in concentration of the reticulant agent in comparasion with glyoxal.
(p)Gime´nez et al., 2009, prepared film of giant squid (Dosidicus gigas) gelatin by a casting the gelatin filmogenic solutions. Giant squids were subjected to a mild-acid hydrolysis with pepsin prior to gelatin extraction (G1 gelatin). A second gelatin extraction (G2 gelatin) was performed using the collagenous residues that remained from the first extraction. Once G1 and G2 gelatin was extracted, the collagenous residues were swollen again in 0.5 M acetic acid. To form the sheets, gelatin extracts were then dried by heating at 45 °C in the air oven. Thereafter, edible films based on these gelatins were prepared. The composition of the gelatin filmogenic solutions were 4% w/v of dry gelatin in distilled water, glycerol (0.15 g/g gelatin) and sorbitol (0.15 g/g gelatin) as plasticizers. The films were prepared by casting the gelatin filmogenic solutions at 45°C for 15 to 20 mins. Then solutions were dried in hot air oven at 45 °C for 15 h.
From a puncture test the film obtained with G1 type of gelatin preparation (F-G1) were more resistant than the one with G2 type of gelatin preparation (F-G2). This is due to the structure/composition differences found between the two types of gelatins. The higher amount of high molecular weight fraction in G1 gelatin results in the higher gelatin film strength of F-G1 films. G2 was rich in low molecular weight fragments, therefore, the formation of triple helical structure was hindered. Because the renaturation ability towards triple helical structure hence the better the mechanical properties of the gelatin films depend on the content of polymers of high molecular weight (a-chain) in gelatin. F-G2 presented the lower breaking deformation (around 35%) in comparison with F-G1 (around 64%). This also attribute to the predominance of low molecular weight components that leads to less intermolecular interactions and higher molecular mobility (Sobral et al., 2001 cited in Gime´nez et al., 2009).
(3)Because blue shark skin is one of the most serious marine waste in Japan, therefore, Limpisophon and co-workers, 2009, investigated to use it for preparation of gelatin film. Gelatin was first extracted from blue shark skin and the gelatin powder was dissolved in distilled water at 60°C for 30min to obtain the film forming solution containing the protein concentration of 1, 2, and 3% (w/v). Glycerol was added into film forming solution as a plasticizer at the concentration of 50% (w/w) of protein. A 4g of film forming solution was cast onto a silicone resin plate (50×50mm) and then the solution was dried at a ventilated oven at 25±0.5°C and 50±5% relative humidity for 24h. The effect of glycerol concentration on gelatin film from a 2% protein film forming solution was also investigated, using glycerol at the range of 0, 25, and 50% (w/w) of protein.
The mechanical property of the film from shark skin such as tensile strength, elongation at break were evaluated. The highest tensile strength was obtained from the film of a 2% protein film forming solution. However, the tensile strength value of film from a 3% protein film forming solution was similar to that from a 2% protein film forming solution. Since an increase in the number of protein chains generally results in an increase in the number of potential intermolecular interactions (Cuq, Gontard, Cuq, & Guilbert, 1996 cited in Limpisophon et al., 2009), however, 3-dimention protein network was the ultimate with the film from the 2% protein film forming solution, so tensile strength did not increase at 3% protein film forming solution. The results also showed that elongation at break increased (61.13-74.17%) with increasing film forming solution protein concentration. These might be because the higher protein content of the film forming solution leads to a higher aggregation of protein to form the film, resulting in improved flexibility of the film (Jongjareonrak etal., 2006 A. Jongjareonrak, S. Benjakul, W. Visessanguan, T. Prodpran and M. Tanaka, Characterization of edible films from skin gelatin of brownstripe red snapper and bigeye snapper, Food Hydrocolloids 20 (4) (2006), pp. 492-501. Article | PDF (224 K) | View Record in Scopus | Cited By in Scopus (38)Jongjareonrak etal., 2006 cited in Limpisophon et al., 2009).
For the effect of glycerol concentration, it was found that tensile strength decreased from 45.90 to 23.30MPa with increasing glycerol concentration from 0 to 50%. On the other hand, elongation at break of gelatin film from shark skin increased significantly (approximately from 1.57 to 80.40%).
(Q)Jiang et al., 2010, extracted gelatin from catfish skin by thermal extraction. To improve the hydrophobic properties of resulting films, triacetin was added to the gelatin at 0, 50, 100, and 150% of the gelatin contents. The film forming solution was prepared by dissolving 1.0 g catfish skin gelatin in deionized water at 50 °C with glycerol (20% of the gelatin amount), sodium triphosphate (STP) (50% of the gelatin amount), varied amount of triacetin, and Tween 80 (added at 10% of the triacetin amount). Later, a 100 mL portion of the film forming solution was cast into a plate and dried at 25 °C and 50% relative humidity for 48 h.
The addition of triacetin resulted in a reduction in tensile strength and an increase in percent elongation of the forming film. With the addition of triacetin from 0% to 150%, tensile strength of the films decreased from 17.3 MPa to 6.0 MPa. While percent elongation had a reverse trend, an increase from 68% to 205% was observed. This is possibly due to the plasticizing effect of the triacetin. The biggest decline of tensile strength and the biggest increase of percent elongation occurred between the properties of the pure gelatin films and the 50% triacetin treatment films.
(C)Jongjareonrak et al., 2006, studied the effects of fatty acids (FA) [palmitic acid (PA) and stearic acid (SA)] and their sucrose esters (FASE) on the properties of films from bigeye snapper and brownstripe red snapper skin gelatins. Gelatin films were prepared by mixing gelatin powder with distilled water to obtain the film forming solution containing the protein concentration of 3% (w/v). Glycerol as the plasticizer was added into film forming solution at the concentration of 50% of protein. In addition, 10 mM EDTA was added into film forming solution to reduce the degradation of bigeye snapper gelatin caused by heat-activate proteinase. The film forming solution of skin gelatins was completely dissolved with occasional stirring at 70 ◦C for 30 min in water bath. Then film forming solution was casted onto a silicone resin plate and dried at 25±0.5 ◦C and 50±5% relative humidity with a ventilated oven for 24 h. The effects of FA (PA or SA) and FASE [palmitic acid sucrose ester (PASE) or stearic acid sucrose ester (SASE)] on film properties were investigated by adding various amounts of FA or FASE (25, 50, 75, and 100% substitution of glycerol) into incubated film forming solution.
It was found that brownstripe red snapper skin gelatin films generally have greater tensile strength and elongation at break than those of bigeye snapper skin gelatin film. Possibly the different compositions in particular amino acid composition and size of protein chains between both gelatins are the reasons (Jongjareonrak et al., Paschoalick et al., Muyonga et al., 2004 cited in Jongjareonrak et al., 2006). With the addition of FA, tensile strength of films generally decreased. Both PA or SA might partially reduce the cross-linking of protein molecules via hydrogen bonds or hydrophobic interactions because generally FA lacks the structural integrity of protein films, leading to the decrease in tensile strength (Krochta, Gontard et al., 1995 cited in Jongjareonrak et al., 2006). While tensile strength of films gradually increased with increasing FASE amount. This is because FASE is an emulsifier contained both hydrophilic and hydrophobic character in the molecules (Soultani et al., 2003 cited in Jongjareonrak et al., 2006). Therefore, the cooperation of the FASE into film forming solution leads to the intermolecular interaction between proteins and FASE, which is possibly occurred, via the hydrophilic head of FASE. This results in the structural integrity between gelatin and sucrose ester molecules and consequently the increase in tensile strength. From the result, the chain length of FA or FASE somehow showed the effect on the mechanical property of resulting film. The film added with SA or SASE exhibit the greater increase in tensile strength of films in comparison to those containing PA or PASE. Possibly the longer the FA or FASE chain dispersed in film forming solution, the greater the interaction with gelatin in the fashion that strengthened the film network. In addition, significant increase in elongation at break was observed in the film of either FA or FASE at a level of 25% substitution. Similar effects on tensile strength and elongation at break of the resulting film by the addition of FA or FASE were observed in both bigeye snapper and brownstripe red snapper skin gelatin films.
(B)The effects of chemical and enzymatic modifications on properties of gelatin-based films were studied by Calvaho et al., 2004. NF-native film; EMF-enzyme modified film (transglutaminase Activa, TGSâ); FMF-formaldehyde modified film and GMF-glyoxal modified film were prepared from filmogenic solution contained 10% of gelatin, 4.5% of glycerol/100 g of solution and the following cross-linking agents. The transglutaminase concentration was fixed at 10 U/g of protein in the filmogenic solution; the formaldehyde concentration was 8.8 mmol/100 ml of filmogenic solution and the glyoxal concentration 26.5 mmol/100 ml of filmogenic solution. Firstly, the gelatin was hydrated (25 °C, 60 min), dissolved in water with stirring (50 °C, 15 min) followed by addition of the glycerol and subsequently of the cross-linking agents. In case of the enzyme the filmogenic solution was heated for enzyme inactivation at 85 °C for 10 min. Thereafter, the filmogenic solutions were poured on to acrylic plates and dried at room temperature for 24 to 48 h.
From the mechanical tests of the chemically and enzymatically modified gelatin based films, the treatment with formaldehyde significantly increased tensile strength (approximately 60%) as compared to the other films prepared. This is because formaldehyde is a low molecular weight molecule and could easily migrate between the protein chains and form new covalent bonds with the Lys, Cys and His amino acid groups of the proteins (Gallieta, Gioia, Guilbert, & Cuq, 1998 cited in Calvaho et al., 2004). A reduction in the elongation was observed for the enzyme modified film. This is due to the introduced cross-linkages in the polymeric matrix (Babin & Dickinson, 2001 cited in Calvaho et al., 2004).
(e)Jongjareonrak et al., 2006, investigated the effects of protein concentration, plasticizer levels, and proteinase inhibitors on the properties of gelatin based films from fish skin gelatin of brownstripe red snapper (Lutjanus vitta) and bigeye snapper (Priacanthus macracanthus). The gelatin films were prepared by dissolving gelatin powder in distilled water to obtain the film forming solution with the protein concentration of 1-4% (w/v). Glycerol was used as a plasticizer (25% of protein concentration). Film forming solution was cast onto a silicone resin plate and dried at 25±5 °C and 50±5% relative humidity for 24 h in a ventilated oven. To study the effect of plasticizer concentrations on fish gelatin film, various concentrations of glycerol (25, 50, 75% of protein) was used. To investigate the effect of proteinase inhibitors on the protein degradation, soybean trypsin inhibitor (0.01 and 0.1 mM) and EDTA (10 and 20 mM) were added into film forming solution.
From the film preparation, it was found that the film forming solution with 1% protein content result in film that is too thin to peel off from the casting plate. On the other hand, the film forming solution with 4% protein content had too high viscosity and could not be used for film casting. For the film with protein concentration of 2 and 3%, the later exhibited the higher tensile strength. This possibly because the higher protein content in film forming solution results in the higher aggregation of protein to form the film, consequently improved its mechanical properties. Elongation at break of films from bigeye snapper skin gelatin increase when the protein content increased from 2 to 3%. However, for the film from brownstripe red snapper skin gelatin, no change in elongation at break was observed.
For the effect of plasticizer levels on the properties of fish skin gelatin films, tensile strength of films was found to be generally decreased with increasing glycerol concentrations from 25 to 75%. It is because glycerol has the relatively small molecule with hydrophilic characteristic, which could be easily inserted between protein chains and form hydrogen bonds with amide group and amino acid side chains of proteins (Gontard, Guilbert, & Cuq, 1993 cited in Jongjareonrak et al., 2006). The direct interactions and the proximity between protein chains in the gelatin film network were, therefore, reduced by the addition of glycerol. Elongation at break of films largely increased with increasing glycerol from 25 to 75% of protein. This is because the presence of plasticizer causes a decrease in intermolecular interaction and also increases the mobility of macromolecules (Gontard et al., 1993 cited in Jongjareonrak et al., 2006). Furthermore, with increasing the plasticizer concentration, the moisture content of films increases because of its high hygroscopic character, leading to the reduction of the forces between the adjacent macromolecules (Sobral, Menegalli, & Guilbert, 1999 cited in Jongjareonrak et al., 2006). However, with addition of proteinase inhibitors (EDTA) no improvement in tensile strength of the gelatin films was observed. However, 2-folds increment in elongation at break was obtained with the added 10 mM EDTA gelatin films. It is because EDTA inhibited the proteolysis process subsequently, leading to an increase in mechanical properties of film.
(R)Chambi et al., 2006, studied the cross-linking effects by transglutaminase on the casein, gelatin and casein-gelatin blend edible films. 7% (w/w) of gelatin and casein aqueous solutions were prepared by dispersing them in distilled water at room temperature. The solution was then heated under constant stirring for the dissolution. For the casein-gelatin blend film, to obtain films with the desired composition of casein-gelatin ratio (100:0, 75:25, 50:50, 25:75 and 0:100), different volumes of the casein and gelatin solutions were gently mixed. The pH of the mixture was adjusted to neutral and the temperature to 50 °C followed by the addition of glycerol (25 g/100 g of dry protein), subsequently the enzyme (10 U/g of protein, according Carvalho & Grosso, 2004 cited in Chambi et al., 2006). The mixture solution was incubated at 50 °C for 15 min and then heated at 85 °C for 10 min to inactivate the transglutaminase. Thereafter, the solutions were poured onto acrylic plates and dried at room temperature for 24 h.
From the mechanical property test, the tensile strength of the films produced from casein, gelatin and casein-gelatin were extensively different and altered as a function of the gelatin and casein concentrations in the mixture. Gelatin film exhibited the higher tensile strength than casein film approximately 4 times. Therefore, the higher the concentration of gelatin composition in the casein-gelatin blend film, the stronger the film. The difference in tensile strength from casein and gelatin appear to be related to the organizational level of the protein network. Caseins are generally classified as non-ordinate proteins containing approximately 63% structures of random or twisted conformations (Siew, Heilmann, Easteal, & Cooney, 1999 cited in Chambi et al., 2006). Therefore, the films obtained from this protein have a less organized matrix. Unlike casein, protein structures in gelatin can renature during the gelling and film forming process (Achet & He, 1995 cited in Chambi et al., 2006), to form a protein with high degree of organization. Consequently, gelatin films possess a better-organized network as compared to film produced from casein. The increase in chain organization probably optimizes molecular packing, resulting in the films with higher mechanical and barrier properties (Siew et al., 1999 cited in Chambi et al., 2006). However, it was found that in all film, casein-gelatin mixture, the tensile strength was independent of the action of the enzyme transglutaminase. No significant difference in tensile strength was observed between the films with and without enzyme modification. However, for the elongation it can be seen that blend film of casein-gelatin mixture exhibited greater elongation than films obtained from either of the proteins individually. Probably the synergetic interactions of both proteins results in greater elongation for the films obtained with mixture from different proportions of casein and gelatin (Howell, 1995 cited in Chambi et al., 2006). The interactions between casein and gelatin probably increase the mobility of the polymers chains as compared to the one protein component film. In the presence of transglutaminase the film presented higher values of elongation than those produced without the enzyme. Furthermore, enzymatic action significantly increased the elongation of the blend films containing different proportions of casein and gelatin. The mixture of casein-gelatin in composition of (75:25) produced the greatest synergistic effect on the film properties.
(S)Calvaho et al., 2009, studied the effect of the hydrolysis degree (HD) and the concentration (CPVA) of two types of poly(vinyl alcohol) (PVA) and of the type (glycerol and sorbitol) and the concentration (CP) of plasticizers on physical properties of blends films based on gelatin and PVA. The films were produced by casting 2 g of gelatin and PVA in 100 g film forming solution. The gelatin solution was prepared by hydrated the gelatin (1 g/100 g of solution) for 30 min, and then dissolved at 55 °C (Sobral et al., 2001 cited in Calvaho et al., 2009). The plasticizer was then added and the solution was kept at 55 °C for 30 min. For the PVA solution, PVA (1 g/100 g of solution) was dissolved in distilled water at 90 °C (Chiellini et al., 2001 cited in Calvaho et al., 2009).
For the mechanical properties of film by puncture tests, it was found that plasticizer type (TP) and concentration (CP) exhibited a significant influence on the puncture force of the film. Films plasticized with sorbitol showed the higher puncture force value than those of the films plasticized with glycerol. Furthermore, the increase in concentration of plasticizer results in a considerably reduction in puncture force of the film. However, no significant influence was observed on the puncture force of the film by the effect of hydrolysis degree (HD) and concentration of PVA. It was noted that the film based on blends of gelatin and PVA produced in this work were stronger to puncture than films based on pure gelatin. In opposite to puncture force, the puncture deformation had an inverse behavior concerning the type and concentration of plasticizer.
In conclusion, the most important effect on the physical properties of the films was that of the plasticizer type and concentration. The effect of hydrolysis degree and concentration of PVA was not significant. Nevertheless, increase in CPVA caused an important increase in the puncture deformation regardless to CP.
(T)The effect of antioxidant extract from two different murta ecotypes leaves (Ugni molinae Turcz) on the properties of edible films made from tuna-fish skin gelatin was investigated by Go´ mez-Guille´n (2007). The two antioxidants (Soloyo Grande ‘‘SG'' and Soloyo Chico ‘‘SC'') were extracted from the leaves and analysed for their antioxidant capacity. The aqueous extracts were obtained by putting 1.5 g cut up leaves in 20mL distilled water, then heating at 35 °C for 20 min and filtering. Gelatin filmogenic solutions were prepared using gelatin extracted from approximately 1-month frozen cleaned tuna-fish (Thunnus tynnus) skins at a protein concentration of 2 g in 100mL distilled water, glycerol (0.25 g/g protein) as plasticizer. In the film incorporated with antioxidant, aqueous extracts from murta (Ugni molinae TURCZ) leaves were added in a proportion 1:1 v/v of gelatin solution+glycerol/extract. After mixing, filmogenic solutions were left at 35 °C for one hour and then filtered through a glass fibre filter. Thereafter, cleaned filmogenic solutions were poured on plates and were dried at 42 °C in a ventilated oven for 18 to 20 h.
Puncture force and puncture deformation values of the tuna-fish skin gelatin based film were evaluated. The incorporation of SG antioxidant into the gelatin films results in no significant difference in puncture force and puncture deformation when compare to the control film. However, puncture force and puncture deformation of the gelatin films were found to be significantly decreased incorporation with SC antioxidant. From the analysis, SC extract shows a higher antioxidant capacity than SG extract. This difference affects puncture properties of tuna-fish (Thunnus tynnus) gelatin-based films and could be attributed to both quantitative and qualitative in polyphenols content in each antioxidant extracted. In addition, the reductions in puncture force and puncture deformation possibly due to the alteration of the plasticizer and gelatin ratio in the murta extracts added films, hence, the slightly lower protein content films.
(i)Fish gelatin films added with gellan and k-carrageenan to improve properties was prepared by Pranato et al., 2007. Fish gelatin films were prepared by dissolving granule of fish gelatin into distilled water for concentration of 5g/100 ml to obtain the film forming solutions. Thereafter, added Gellan and k-carrageenan into gelatin solution to final concentration of 1 and 2 g/100 g gelatin granule. After that, the gelatin film solutions were casted onto teflon-coated glass plates and then dried at room temperature for 24 h.
Film prepared from pure fish gelatin exhibited tensile strength of 101.23 MPa and elongation at break of 5.8%. The addition of both polysaccharides (Gellan and k-carrageenan) had a positive effect on tensile strength of the fish gelatin film. From the mechanical properties test, the film with added gellan 1 g/100 g showed the highest increase in tensile strength (109.76 MPa). This possibly due to gellan can form networks with the gelatin molecule (via anionic domain of gellan and cationic domain of gelatin), leading to a strengthening effect in the films. While, the addition of k-carrageenan at both 1 and 2g/100 g resulted in a slight increase in tensile strength (103.63-104.48 MPa). It is because k-carrageenan can form polyelectrolyte complex with positive charge of gelatin, therefore, strengthening the film structure although the effect was less than that of gellan (Haung et al., 2004 cited in Pranato et al., 2007). However, the addition of k-carrageenan at 2g/100 g resulted in the highest increase in elongation at break in the fish gelatin films (6.81%).
(j)Sazedul Hoque et al., 2010, studied effects of heat treatment of film-forming solution on the properties of film from cuttlefish (Sepia pharaonis) skin gelatin. Film forming solutions were prepared by mixed squid gelatin powder with distilled water to attain the protein concentration of 3% (w/v). Glycerol as a plasticizer at a concentration of 25% of protein was added. Afterward, incubated film forming solution at different temperatures (40, 50, 60, 70, 80 and 90 °C) for 30 min in water bath. Film forming solution with and without heating was cast onto a silicone resin plate and then dried at the temperature of 25 ± 0.5 °C and 50 ± 5% relative humidity for 24 h.
With heat treatment at different temperature from 40 to 90 °C of film forming solution, the film prepared with heat treatment at 60 °C showed the highest tensile strength of 9.66 MPa. This is because the heat treatment at appropriate temperature resulted in the more stretched or unfolded gelatin molecules leading to higher inter-chain interaction via hydrogen bonding of the network structure of the gelatin films. As a result, the film with improved mechanical property was obtained. However, with further increase in heating temperatures of film forming solution from 70 to 90 °C, the gradually decrease in tensile strength of the gelatin film was observed. This might be due to the degradation of gelatin molecule at higher temperature. This leads to the shorter gelatin chains, therefore, the lower inter connection of gelatin molecules formed resulting in a deterioration of the film performance (Shiku et al., 2004 cited in Sazedul Hoque et al., 2010). For elongation at break of the film, as the temperature of heat treatment increased from 40 to 60 °C, there was a noticeably decrease in elongation at break observed. This decrease in elongation at break coincides with the increased in tensile strength of the gelatin film. This is because as increasing temperature the gelatin molecule became more stretched or unfolded which results in more interaction of the gelatin network, hence, the increase in tensile strength. Nevertheless, this brings the losses in flexibility as evidenced by the decreased elongation at break of the gelatin film. On the other hand, at temperature above 60 °C from 70 to 90 °C, the increase in elongation at break of the films was observed. This is possibly due to the degradation of gelatin molecules, causing the shorter chains network. As a result, the lower interaction of the network between gelatin molecules was obtained, yielding the film with higher elongation at break (Go´ mez-Guille´n et al., 2009 cited in Sazedul Hoque et al., 2010). In conclusion, heat treatment of film forming solution had the direct impact on the properties of cattle fish gelatin film.
(4)The blends film of gelatin and five different types (varying degree of hydrolysis) of PVA [poly(vinyl alcohol)], with and without a plasticizer were prepared by Maria et al., 2008. The effect of the degree of hydrolysis of the PVA and the glycerol concentration on the film properties were investigated. The films were produced from a mixture of gelatin and PVA solutions. Gelatin solution was prepared by hydrated the gelatin 1 g in 100 g of solution for 30 min, and then dissolved at 55 °C (Sobral et al., 2001 cited in Maria et al., 2008) using a thermostatic bath. After that, added glycerol and the solution held at 55 °C for a more 30 min. To prepare PVA solution, 1 g of the PVA in 100 g of solution was homogenized in distilled water and then dissolved at 90 °C (Chiellini et al., 2001a cited in Maria et al., 2008). To produce film forming solutions with 2 g macromolecules/100 g film forming solution, gelatin and PVA solutions were mixed together with magnetic stirring for 15 min at room temperature. Different plasticizer concentrations of 0, 25 and 45 g glycerol/100 g macromolecules were added into the film forming solution. These solutions were poured in acrylic plates and then dried in ventilated oven at 30 °C for 24 h.
From the tensile test, it was found that there is no particular relationship between the mechanical properties of the gelatin and PVA blends film and the type of PVA used (different degree of hydrolysis of PVA). The blend film prepared with gelatin and PVA Celvolâ 418 (degree of hydrolysis = 91.8%) without plasticizer exhibited the highest tensile resistance (tensile strength = 38 MPa). However, this study found the closer dependence of the film properties with the glycerol concentration. The increase in concentration of glycerol as plasticizer caused a reduction in resistance (tensile strength) and Young's modulus (stiffness) and an increase in elongation at break (flexibility) of the film. This can be explained as a result of typical plasticization phenomenon. It is because with the increase in plasticizer concentration, the chain mobility increases, consequently (Chiellini et al., 2001a cited in Maria et al., 2008).
(K)The effects of plasticizers (four types of polyols: glycerol—GLY, propylene glycol—PPG, di- DTG and ethylene glycol—ETG) and their concentrations on the mechanical properties of gelatin-based films were investigated by Vanin et al., 2005. Five different concentrations: 10, 15, 20, 25, and 30 g plasticizer/100 g of gelatin were varied. The films were prepared by dissolved 2 g of gelatin in 100 mL of water and added varying amount of plasticizer in 100 g gelatin. Then the pH of the solutions was adjusted to neutral. These film forming solution were applied on plexiglass plate and dried. Plasticizers used in this study were the glycerol (C3H8O3, molecular weight, MW=92 g/gmol), the ethylene glycol (C2H6O2, MW=62 g/gmol), the diethylene glycol (C4H10O3, MW=106 g/gmol) and the propylene glycol (C3H8O2, MW=76 g/gmol).
According to the mechanical properties test, it was found that the GLY showed the greatest plasticizing effects and efficiency, exhibiting the lower value of puncture force and the higher value of puncture deformation than those of the gelatin based films containing other plasticizers, PPG, ETG, DTG. In this work, it is found that the theory of chain length of plasticizer could not explain the behavior of the film obtained. Because the higher effect and plasticizing effect belongs to the film added with GLY, which has the molecular weight larger than ETG and PPG. In addition, this work found that the mechanical properties of the film presented a relationship with its glass transition temperature (Viroben et al., 2000 cited in Vanin et al., 2005).
(L)A nanoclay composite film based on warm water fish gelatin were produced by Bae et al., 2009. The effects of treatment with transglutaminase on the mechanical properties of the composite film were investigated. The nano clay solution were prepared by first dissolving 8 g of sorbitol in 100 ml of 50 °C deionized water and stirred for 30 min. Then 2 g (5% w/w) of clay were added and the solutions were stirred for further 30 min at 50 ± 5 °C. After that, the solution was then sonified in order to intercalate and exfoliate the clay and plasticizers. For the gelatin solution, 40 g of gelatin were dissolved in 100 ml of 60 °C in deionized water and stirred for 2 h. Separately, microbial transglutaminase (MTGase) powder 800 mg was dissolved in 5 ml of distilled water and then mixed until all powder was well dispersed in solution. Thereafter, the MTGase solution was then added into the previously prepared gelatin solution. MTGase was then deactivated by heating the solution at 100 °C for 15 min (Kutemeyer, Froeck, Werlein, & Watkinson, 2005 cited in Bae et al., 2009). After the deactivation, the clay solution was mixed into gelatin solution by droplets and gently stirred for 24 h at 35 ± 5 °C. To cast the film approximately 35 ml of the prepared solution were poured onto a glass plate and dried.
In this study, with the treatment of MTGase it was found that the tensile strength and elongation at break of the nanoclay gelatin based composite film was not significantly different as compared to the controlled film. It was found that MTGase treatment created cross-linking network and increased molecular weight, but decreased tensile strength and elongation at break of the nanoclay composite films. This probably related to the effect of the formation of gelatin structure and the intercalation of nanoclay in the composite film. The cross-linking by MTGase might lead to the randomly unwound of gelatin molecules, which likely to cause the hinder information of helix structure of gelatin and result in a significant reduction in nanoclay intercalation. Furthermore, the increase in molecular weight by cross-linking also causes a steric hindrance in nanoclay intercalation. The result of this study, however, is in contrast with the theory. Because it is well known that an increase in degree of cross-linking in polymer matrices generally increases the rigidity of polymer network and also increases the molecular weight. This normally leads to an increase in the tensile strength and a decrease in elongation at break of films due to a reduction in the molecular chain mobility.
(a)The composite film based on gelatin with the addition of hydrocolloids and lipids were produced by Bertan et al., 2005. The effect of fatty acids and ‘Brazilian elemi' on the composite film properties was investigated. The film forming solution was prepared by hydrating 10 g gelatin in 100 ml distilled water at 25 ± 1 °C for 1 h, then heating for 10 min at 90 °C to complete the dissolution (Solution A). After that, 15% (w/w) dry gelatin of triacetin as plasticizer was added to Solution A with continuous stirring, until it well mixed (Solution B). For the composite film formation with fatty acids, 10% (w/w of dry gelatin) of stearic or palmitic or the 1:1 (w/w) blend of stearic and palmitic acids (ac) were mixed with Solution B at 45 °C for 30 min. The composite films formation with elemi were prepared by added elemi to Solution B, which compose of the blend of stearic and palmitic acids, magnetic stirring was used for mixed the solutions and then kept at 45 ± 2 °C for 30 min. These solutions were then poured onto a Plexiglass plate and dried at 25 ± 1 °C for 24 h.
From the tensile test, the addition of fatty acid (stearic and palmitic acids) causes a reduction in the tensile strength of the prepared gelatin film. This is due to the characteristic of the lipids, which affects the interaction within the structural matrix of the protein. It was also found that the addition of elemi or hydrophobic substance causes the proportional decrease in tensile strength and increase in elongation at break of the film. In this way, elemi acts as plasticizer causing plasticizing effect on the film properties.
(h)Bao et al., 2009, studied the effect of tea polyphenol-loaded chitosan nanoparticles (TPCN) on antioxidant activity and mechanical properties of gelatin film. Gelatin was extracted from channel catfish skin. Film forming solution was prepared by mixing gelatin with distilled water 100 mL with 80 g/L a protein concentration and then 1.33 g of glycerol as plasticizer was added. The blend was maintained in a water bath for 20 min at 60 °C until perfectly dissolved. Thereafter, 5 mL of film forming solution was cast onto a plexiglass plate and then dried in a circulated air oven for 20 h at 20 ± 0.5 °C. For the composite film formation with nanoparticles, 70 mL of film forming solution (containing 8 g of protein and 1.33 g of glycerol) were added to 30 mL of empty chitosan (CTS) - tripolyphosphate (TPP) [E-CTS-TPP], tea polyphenols (TP)-CTS-TPP [TP-CTS-TPP] and dialysed TP-CTS-TPP (D-TP-CTS-TPP) suspensions separately.
In all cases, the tensile strength and elongation at break of channel catfish skin gelatin films were decreased with the incorporation of TPCN. However, no marked difference in tensile strength between films incorporated with E-CTS-TPP (E-CTSTPP F) and films incorporated with D-TP-CTS-TPP (D-TP-CTS-TPP F). It was also found that no significant differences in elongation at break were observed among the control film and other films containing TPCN. It is suggested that with the addition of CTS, the interaction of CTS with gelatin molecules is established leading to hindrance in the processing of gelatin film formation. This results in a decrease in triple-helix content of gelatin molecules obtained in the film and, consequently, a decline in its mechanical properties (Bigi et al., 2004 cited in Bao et al., 2009). For the CTS-TPP nanosystem, it has larger size than the CTS molecules, therefore, this system might obstruct the development of an orderly and close structure in the process of gelatin film formation. Thus, the protein-protein interactions within the gelatin network was weakened, hence, leading to the lower tensile strength and elongation at break of the film incorporated with CTS-TPP nanosystem (Gómez-Guillén, Cao et al., 2007 cited in Bao et al., 2009).
(1)The effect of incorporation of antioxidant borage extract into films based on sole skin gelatin (a commercial fish gelatin) were studied by Gómez-Estaca et al., 2009. The film forming solution was prepared by dissolved 4 g in 50 mL of gelatin with distilled water to obtained final concentration in the film forming solution of 4 g in 100 mL, and then a blend of 0.15 g in g gelatin sorbitol and 0.15 g/g gelatin glycerol as plasticizer was added. Thereafter, dissolved gelatin was incorporated with borage extract at a ratio 1:1 then mixed again. For the film forming solution without borage extract was obtained by dissolved 4 g of the gelatin into distilled water 75 mL and ethanol 25 mL, then added the blend of plasticizers (0.15 g in g gelatin sorbitol and 0.15 g/g gelatin glycerol). Afterward, all blends were maintained at 40 °C for 15 min in order to gain a well mix and then 40 ml of film forming solution were cast onto plates and dried at 45 °C for 15 h in an oven.
From the film formation, both gelatin films from sole and catfish skins were flexible, transparent and completely soluble in water. The catfish gelatin film exhibited the higher breaking force value and markedly lower the breaking deformation than those of the sole gelatin film. This differences in mechanical properties of both films supposedly due to the differences in the physico-chemical of the characteristics gelatin, especially the amino acid composition (a greatly species-specific characteristic of gelatin), and the molecular weight distribution (mostly determined by the processing condition of gelatin). Due to the higher imino acid content and the higher degree of hydroxylation in the catfish gelatin, this promotes inter and intra chain interaction of the gelatin network and possibly contribute to a strengthening effect of the films. However, with addition of borage extract a significant decrease in the breaking force was observed in both gelatin films of sole and catfish skins. This might be because the soluble polyphenolic fraction in borage extract weakens the protein-protein interaction of the film network leading to the weaker films. In addition, all gelatin films with or without antioxidant extracts can be considered as amorphous films due to the plasticizing effects of the added glycerol or sorbitol and the moisture content (Gómez-Estaca et al., 2008 cited in Gómez-Estaca et al. 2009). It can be conclude that the incorporation of borage extract (antioxidant) increased the antioxidant power of gelatin films. However, the minor decrease in mechanical properties (i.e. breaking force) of the gelatin films was obtained.
(m)Giménez et al., 2009, studied on the improvement of the antioxidant properties of squid skin gelatin films by the addition of hydrolysates from squid gelatin. Gelatin was extracted from giant squid (Dosidicus gigas) in distilled water at 60 °C for 18 h. Once gelatin (G1) was extracted, the residues were again extracted with the same condition to obtained a second gelatin extracted (G2). Thereafter, gelatins (G1 and G2) were submitted to enzymatic hydrolysis by Alcalase at pH 8, 50 °C for 3 h with ratio of 1:20 (enzyme:substrate). The products from the enzymatic hydrolysis were gelatin hydrolysates HG1 (from gelatin G1) and HG2 (from gelatin G2) were freeze-dried and stored at -80 °C. To prepare the control gelatin filmogenic solution, gelatin G1 or G2 were dissolved in phosphate buffer 10 mM (4%, w/v) at 45 °C for 15-20 min, then a mixture of glycerol (0.15 g/g gelatin) and sorbitol (0.15 g/g gelatin) used as plasticizers was added. The filmogenic solution was then cast on plexiglass plates and dried in hot air oven at 45 °C for 15 h. For the film containing gelatin hydrolysates HG1 and HG2, the filmogenic solution was prepared in the similar way as the control gelatin filmogenic solution but gelatin was replaced by hydrolysed gelatin.
In this study, with increasing the content of gelatin hydrolysates in the gelatin films, a decrease in mechanical resistance (puncture force) and an increase in flexibility (puncture deformation) were observed. This is possibly because gelatin hydrolysates as small peptide molecules could easily insert in the protein network and form hydrogen bonding with the gelatin chain. As a result, the gelatin chain-chain interactions were intervened, resulting in a decrease in the density of inter molecular interaction and an increase in the free volume between the gelatin chains. Moreover, with increasing the content of low molecular weight fragments of gelatin hydrolysates, the renaturation of gelatin chains into helix coil structure that occurs during the film formation process of the gelatin films might be interfered. This finally leads to a decrease in mechanical resistance of the gelatin films (Arvanitoyannis, Nakayama, & Aiba, 1998 cited in Giménez et al., 2009).
(n)Biodegradable films based on blends of gelatin and poly (vinyl alcohol) (PVA), were prepared by Silva et al., 2008. The effect PVA types and concentration on the mechanical of the films were investigated. Firstly, the gelatin and PVA solutions were prepared separately. For the gelatin solution (solution A), gelatin was hydrated in water at room temperature and then dissolved at 55 °C in a water bath. On the other hand, the PVA solution (solution B) was prepared by dissolved PVA in water at 95 °C for 30 min with stirring condition. Secondly, to obtain the desired film forming solution of the gelatin and PVA mixture, solution A and solution B were mixed together in the desired ratio for a concentration of 2 g of macromolecules in 100 g of film forming solution. The mixture was then homogenized by stirring at room temperature for 15 min. Then the film forming solution was poured on plexiglass plates and dried at 30 °C and relative humidity of 55-65% in ventilated oven for 24-28 h. For the first part of this work, 23.1 g PVA in 100 g of macromolecules was used. For the second part, 9.1; 16.7; 23.1; 28.6 and 33.3 g PVA in 100 g of macromolecules were used.
From the puncture and tensile test, puncture force, puncture deformation, tensile strength, elongation at break and elastic modulus of the film were evaluated. According to puncture test, the films prepared with PVA of a higher degree of hydrolysis (Celvolâ425, Celvolâ350 and Celvolâ125) had a higher mechanical resistance than those prepared with a low degree of hydrolysis (Celvolâ504 and Celvolâ418). However, it was found that the degree of hydrolysis had no effect on the puncture deformation of the films prepared. In relation to the tensile test, the film prepared with PVA Celvolâ418 exhibited the highest tensile strength of 82.3 MPa with elastic modulus of 27.4 MPa/% and elongation at break of 5.2%. Nevertheless, it was also found that the elongation at break of the film was not effected by the type of PVA used, as observed in the puncture test. In conclusion, this study found no logical relationship between different PVA types (varied degree of hydrolysis) and the mechanical properties of the film based on blends of gelatin and PVA.
For the effects of the PVA concentration, it was found that the resistance to puncture of the films decreased with increasing the PVA concentration of the blends. However, the puncture deformation of the films was not effected by the concentration of PVA. In line with the puncture test results, the tensile resistance and rigidity of the blend films decreased linearly with increasing PVA concentration up to 33.3 g PVA in 100 g of macromolecules in the blends.
(o)The mechanical properties of tuna-skin and bovine-hide gelatin films with added aqueous oregano or rosemary extracts (two different concentrations) were studied by Gómez-Estaca and co-workers (2009). The freeze-dried oregano (Origanum vulgare) and rosemary (Rosmarinus officinalis) were extracted with distilled water by continuous stirring in a warm water bath at 45 °C for 10 min. The aqueous extract obtained was determined for the total phenolic content afterwards. For the film formation, the film forming solutions were prepared by dissolving gelatin made from tuna skins or bovine-hide at concentration of 4 g in 100 mL of distilled water. After that, a mixture of sorbital (0.15 g/g gelatin) and glycerol (0.15 g/g gelatin) was added. The film with added oregano (OE) and rosemary (RE) extracts were prepared with different compositions including batch O-L (theoretical phenol content of 130 mg caffeic acid/mL film forming solution) using the proportion of 6.25 mL OE/100 mL film forming solution, batch O-H (theoretical phenol content of 520 mg caffeic acid/mL film forming solution) using the proportion of 25 mL OE/100 mL film forming solution, batch R-L (theoretical phenol content of 83 mg caffeic acid/mL film forming solution) using the proportion of 12.5 mL RE/100 mL film forming solution and lastly, batch R-H (theoretical phenol content of 665 mg caffeic acid/mL film forming solution) using the proportion of 100 mL RE/100 mL film forming solution. Before film casting, all mixtures of film forming solution were warmed and stirred at 40 °C for 15 min to obtain a good mixing. The mixtures were then casted using an amount of 40 mL on square plates and dried at 45 °C for 15 h in a forced-air oven to produce the gelatin based films.
From the puncture test of the films, the breaking force and the breaking deformation values were recorded. The breaking force value of the control film and the films with added plant extracts were similar or slightly decrease for some of the latter films. Similarly, it was found no differences or a slight decrease in breaking deformation values when compare between the control film and the film with added plant extracts, except for the tuna-skin gelatin film with the more concentrated rosemary extract (batch R-H), which had a significant reduction breaking deformation value. It can be explained that the addition of polyphenolic antioxidants especially higher-molecular-weight polyphenols results in a weakening of the interactions that stabilize the protein matrix (Orliac, Rouilly, Silvestre, and Rigal, 2002 cited in Gómez-Estaca et al., 2009). However, from the analysis the phenolic composition of the aqueous oregano and rosemary extracts had a predominant of low molecular weight compounds. As a result, a slight reduction in tensile strength and elongation at break of the films with added plant extracts were observed in this work.
(U)The mechanical properties of soybean-protein isolate (SPI) and cod gelatin blend films were studied by Denavi et al., 2009. Film forming solutions were prepared by dissolved 4% (w/v) of SPI and/or gelatin (w/v) in distilled water with different ratios of SPI to gelatin (0:100, 25:75, 50:50, 75:25 and 100:0 [w/w]). To prepare SPI solution, the dry proteins were dissolved in distilled water at room temperature. For the gelatin solution, dissolving the gelatin in distilled water at 60 °C in a water bath. Thereafter, 1.5% a mixture of 0.75% glycerol with 0.75% sorbitol was added as plasticizer. When the solutions completely dissolved, the pH was adjusted to 10.5 using 2N NaOH and then the gelatin and SPI solutions were blended at the different ratio stated above. Afterward, 40 mL of each ratio film forming solution were poured onto plexiglass plates and dried in a ventilated oven at 45 °C for 18-20 h.
From the puncture test, gelatin films exhibited a 1.8-fold higher breaking force and a more than 10-fold greater deformation than those of the SPI films. For the composite films, the puncture deformation, therefore, decreases with increasing SPI composition in the films. However, it was found that the composite films of 50S:50G and 25S:75G films showed the higher breaking force than those of the films made from gelatin or SPI alone. This synergistic effect on the mechanical properties of the composite films is probably induced by a certain degree of cross-linking between the proteins of both gelatin and SPI.
(v)The effects of sunflower oil addition into cod gelatin films were investigated by Pérez-Mateor et al., 2009. The film forming solution was prepared by dissolving 4% (w/v) of gelatin powder with distilled water and heated at 60 °C for 2 h in a water bath, subsequently added plasticizers, which are 0.75% glycerol and 0.75% sorbitol (w/v). Afterward, film forming solution was incorporated with the different amounts of sunflower oil (0%, 0.3%, 0.6%, and 1% w/v). Finally, film forming solution 40 mL were poured onto plexiglass plates and then dried in the ventilated oven at 45 °C for 18-20 h. To study the stability of films, the remaining films were stored for 1 month at 22 °C and 58% relative humidity in the desiccators.
In the present study, the puncture force and puncture deformation of cod gelatin films was found to decrease with the addition of sunflower oil. As increasing the amount of oil in the film both puncture force and puncture deformation were decreased. In addition, after storage time of 30 days at 22 °C, 58% relative humidity, the cod skin gelatin-based films with added sunflower oil showed a significant decrease in their puncture force value as well as the film deformation value.
(w)By using different kinds of plasticizers, the effects of plasticizer composition, size and shape on the mechanical properties of gelatin films were studied by Cao et al., 2009. The film forming solution was prepared by dissolving 12% (w/w) gelatin with distilled water in a water bath at 50 °C for 20 min. To study the different kind of plasticizers, oligosaccharides, which are sucrose, and some organic acids such as oleic acid, citric acid, tartaric acid and malic acid (MA) were added to gelatin. For studying the effects of plasticizer composition, polyethylene glycols (PEG) as plasticizer with different molecular weights (300, 400, 600, 800, 1500, 4000, 10 000, 20 000) were mixed into gelatin films. The incorporation process of all solutions was maintained at 50 °C for 20 min until dissolution. Thereupon, the solution were poured onto a polyester plate and dried at room temperature.
This work conclude that with the addition of oligosaccharide (sucrose) and some organic acids such as oleic acid (OA), citric acid (CA), malic acid (MA), tartaric acid (TA) to gelatin films, only MA could improve the ductility or flexibility of gelatin film. For the addition of polyethylene glycol (PEG) with different molecular weights (300, 400, 600, 800, 1500, 4000, 10 000, 20 000), it was found that PEG had a plasticizing effect on gelatin film. The lower the molecular weight of PEG, the better the plasticizing effect was obtained in the gelatin film. In addition, Mannitol and sorbitol result in a more flexible gelatin film. Moreover, it was also found that ethylene glycol (EG), diethylene glycol (DEG), triethylene glycol (TEG) series and ethanolamine (EA), diethanolamine (DEA), triethanolamine (TEA) series has a plasticizing effect on the gelatin films, hence, improved the films flexibility.
(6)Rivero et al., 2009, developed composite, bi-layer and laminated biodegradable films based on gelatin and chitosan. Gelatins of two different sources; bovine gelatin type B provided by Gelico (Belo Horizonte, Brazil) (G1) and commercial one (G2), were utilized for solution preparation. Gelatin solutions (G1, G2) were prepared, with a concentration of 7.5% (w/w) using gelatin powder (G1, G2). The powders were hydrated with distilled water for 8 h and then heated to 60 °C to obtain a completely dissolved gelatin solution. For Chitosan (CH) solution, commercial CH from crab shell (with 85% degree of deacetylation) was dissolved in 1% (v/v) acetic acid solution to prepare 1% (w/w) CH solution. Glycerol was used as plasticizer (P) for the film forming solution at concentration of 0.75% (w/w). Both gelatin films with addition of glycerol (GP) and without (G) were prepared. The composite films were prepared by blending gelatin and CH solutions in proportion of 50:50 (w/w). Thereafter, the film forming solutions were poured onto acrylic plates and then dried at 37 °C in an oven. Both composite films of gelatin chitosan with addition of glycerol (GCHP) and without (GCH) were prepared. The coating technique (a two-step procedure) was used to form bi-layer films. In this preparation, the pre-formed gelatin based films were coated with a CH solution. Finally, the bi-layer formulation was dried in the oven at 37 °C. Three systems of bi-layer films of gelatin and chitosan were prepared: G1-CH, G1P-CH and G1CH-CH (one layer of CH and other of G1, G1P or G1CH). On the other hand, laminated films were formed by combining two individual films together: G1 + CH, G1P + CH, and G1CH + CH (one film of CH and other of G1, G1P, or G1CH).
From the tensile test, for all of the unplasticized films, chitosan film (CH) exhibited the highest tensile strength value of 95 MPa and elongation at break of 3.68%. The gelatin films G1 and G2 showed lower tensile strength values around 59.5 and 60 MPa and elongation at break approximately 2.2 and 2.8%, respectively. With addition of plasticizer (glycerol), the gelatin films exhibited higher elongation at break and lower tensile strength. This is because plasticizer obstructs chain interaction between gelatin molecules causing an increase in film flexibility. Among the 3 systems, composite, bi-layer and laminated films, bi-layer system exhibited the highest mechanical performance with the chitosan addition. This bi-layer film possesses the high tensile strength value for G1CH-CH of 77.2 MPa. Nevertheless, elongation at break values of composite, bi-layer and laminated films were in the similar range and not significantly difference from the gelatin films, ranging between 2.2% and 5.7%. In laminated films, two individual rupture peaks of chitosan and gelatin components were observed in some systems such as G1 + CH and G1P + CH.