Freezing–thawing effects on the properties of dialdehyde carboxymethyl cellulose crosslinked gelatin-MMT composite films

Food Hydrocolloids 33 (2013) 273e279

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Freezingethawing effects on the properties of dialdehyde carboxymethyl cellulose crosslinked gelatin-MMT composite films Jimin Guo a, Xinying Li b, Changdao Mu a, Hanguang Zhang a, Pan Qin a, Defu Li a, * a

Department of Pharmaceutics and Bioengineering, School of Chemical Engineering, Sichuan University, 24 Yihuan Road, South Section One, Chengdu 610065, Sichuan, China b College of Chemistry and Environment Protection Engineering, Southwest University for Nationalities, Chengdu 610041, Sichuan, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 10 September 2012 Accepted 2 April 2013

Freezingethawing is used as a new method to disperse montmorillonite (MMT) in dialdehyde carboxymethyl cellulose (DCMC) crosslinked gelatin-based films. The effects of freezingethawing on the structure and properties of gelatin-DCMC-MMT films were investigated. The data of XRD indicate that freezingethawing plays an important role in dispersing MMT into gelatin matrix and reducing the nanoparticles aggregation. The optical properties studies show that gelatin-DCMC-MMT films are very transparent and have excellent barrier properties against UV light. Freezingethawing process decreases the transparency of films at visible region due to the better dispersion of MMT. The resulting films exhibit similar total soluble matter (TSM) values. However, the films prepared by freezingethawing method have higher moisture content (MC), may be resulting from the more void volume obtained during the freezingethawing process. The water vapor permeability (WVP) measurements show that the addition of MMT decreases the WVP of the films. Moreover, the freezingethawing method can further decrease the WVP of the films. In addition, the films prepared by freezingethawing are observed with better mechanical properties and thermal stability. The results suggest that the freezingethawing method is beneficial to dispersing MMT into the gelatin matrix and raising the properties of DCMC crosslinked gelatin-MMT films. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Gelatin Edible films Dialdehyde carboxymethyl cellulose Montmorillonite Freezingethawing

1. Introduction Petroleum-based plastic materials have led to serious environmental concerns, as a result of their nondegradable and nonrenewable nature. The increasingly high oil price is another driving force for the development of substitutes for synthetic plastic. Hence there is an ever-increasing interest in biopolymers-based biodegradable materials (Sothornvit, Hong, An, & Rhim, 2010). Gelatin is an animal protein obtained by a controlled hydrolysis of the fibrous insoluble collagen, which presents in bones and skin generated as waste during animal slaughtering and processing. It is considered the most promising candidates for use as environmentally friendly packaging materials, because of their low cost, biodegradability, nontoxicity, sustainability and abundance (Carvalho et al., 2008; Patil, Mark, Apostolov, Vassileva, & Fakirov, 2000). With the appropriate film forming properties and good barriers against oxygen and aromas at low and intermediate

* Corresponding author. Tel./fax: þ86 28 8540 5221. E-mail address: [email protected] (D. Li). 0268-005X/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.foodhyd.2013.04.004

relative humidity, gelatin is suitable for use as raw materials of biodegradable packaging materials. However, gelatin-based films do not have good water vapor barrier properties and good water resistance because of intrinsically hydrophilic nature, which limit the application as packaging materials (Carvalho et al., 2008; Jongjareonrak, Benjakul, Visessanguan, Prodpran, & Tanaka, 2006; Limpisophon, Tanaka, Weng, Abe, & Osako, 2009). Moreover, brittleness and low thermal stability of gelatin films are also insufficient for food packaging applications (Bigi, Cojazzi, Panzavolta, Roveri, & Rubini, 2002; Kester & Fennema, 1989). Therefore, many researchers are working at the realm of reinforcement of gelatin. So far, many attempts have been made to modify the poor properties of gelatin films. Aldehydes (de Carvalho & Grosso, 2004), polyphenols (Gómez-Guillén, Ihl, Bifani, Silva, & Montero, 2007), transglutaminase (Chambi & Crosso, 2006) and surfactants (Andreuccetti, Carvalho, Galicia-García, Martínez-Bustos, & Grosso, 2011) are the effective crosslinked means to reinforce the properties of gelatin films. Milk protein (Barreto, Pires, & Soldi, 2003), soy protein (Guerrero, Stefani, Ruseckaite, & de la Caba, 2011), and chitosan (Pereda, Ponce, Marcovich, Ruseckaite, & Martucci, 2011) are successfully blended with gelatin to prepare composite films.

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Nanosized clay (Kumar, Sandeep, Alavi, Truong, & Gorga, 2010a; Kumar, Sandeep, Alavi, Truong, & Gorga, 2010b; Mascheroni, Chalier, Gontard, & Gastaldi, 2010; Sothornvit et al., 2010; Tunç & Duman, 2010) is also introduced into protein edible films to improve the mechanical properties and thermal stability. Note that dialdehyde polysaccharides whose aldehyde groups can crosslink with ε-amino groups of lysine or hydroxylysine side groups of gelatin by Schiff’s base have received an increasing attention as an ideal crosslinking agent of protein in recent years (Dawlee, Sugandhi, Balakrishnan, Labarre, & Jayakrishnan, 2005). In Martucci’s study, dialdehyde starch (DAS) has been introduced into gelatin films as an environmentally friendly crosslinking agent (Martucci & Ruseckaite, 2009). However, the polymeric nature of DAS causing some degree of phase separation in gelatin-DAS films leads to the poorer tensile strength and the lower transparent at visible light. Slightly solubility of DAS limits its application in water environment too. Dialdehyde carboxymethyl cellulose (DCMC), a new kind of dialdehyde polysaccharide which shows some properties as short chain dialdehyde and good solubility in water has advantage over DAS as a crosslinker to prepare gelatin films. Moreover, it was observed that the DCMC crosslinked gelatin edible films showed better properties of the thermal stability, mechanical properties, light barrier properties and moisture resistance (Mu, Guo, Li, Lin, & Li, 2012). Meanwhile, the evolution of biopolymer matrix with nanosized clay dispersed called bionanocomposites has blossomed (Dang, Lu, Yu, Sun, & Yuan, 2011; Huang & Netravali, 2006; Kumar et al., 2010a; Kumar et al., 2010b; Mascheroni et al., 2010). Montmorillonite (MMT), one of the most common smectite clays, naturally abundant and toxin-free, is a promising reinforcer material in food, medicine, cosmetic and healthcare recipients (Park et al., 2002). With the addition of MMT, the thermal stability and mechanical properties of the bionanocomposites can be significantly improved. However, when the clay content is high, it plays a role in poor since the strong tendency to agglomerate (Kumar et al., 2010b; Li, Zheng, Ma, & Yao, 2003; Zheng, Li, Ma, & Yao, 2002; Zheng, Li, & Yao, 2002). Thus many attempts such as ultrasonic irradiation and organic modification have been made to obtain well dispersing MMT (Martucci, Vázquez, & Ruseckaite, 2007; Ray & Okamoto, 2003). In our previous study, freezingethawing was used as a new method to prepare gelatin-MMT bionanocomposites. It was found that freezingethawing was an effective approach to exfoliate the clay for concentrations higher than 5 mass% in gelatin matrix, indicating that freezingethawing was an ideal method to prepare exfoliated gelatin-MMT bionanocomposites (Mu, Li, et al., 2012). In this paper, gelatin-DCMC-MMT composite films were prepared by freezingethawing method with MMT as the filler and DCMC as the crosslinker. The structure and properties of gelatinDCMC-MMT films were characterized with light barrier properties, water adsorption and water solubility, moisture resistance, mechanical measurements and thermogravimetric analysis (TGA). 2. Materials and methods 2.1. Materials Gelatin type B was purchased from Aladdin Reagent Database Inc, Bloom 250 (Shanghai, China). Sodium montmorillonite (MMT) was kindly supplied by Zhejiang Fenghong Clay Chemicals Co, Ltd (Anji, China). The cationic exchange capacity (CEC) of MMT was 100 mmoL/100 g. Carboxymethyl cellulose sodium (CMC), sodium periodate and glycerol were purchased from Kelong Chemical Reagent Company (Chengdu, China). The viscosity of the 2% (w/v) CMC in water was reported by the company to be 1200 mPa s and confirmed in our laboratory. The degree of substitution (DS) was

w0.90 according to Fourier transform infrared (FTIR) analysis. Periodate and glycerol were of analytical grade. 2.2. Preparation of dialdehyde carboxymethyl cellulose Dialdehyde carboxymethyl cellulose (DCMC) was prepared similar to the conventional procedure (Li, Wu, Mu, & Lin, 2011). About 1.0 g CMC was dissolved in 20 mL distilled water in the flask which was immersed in a DF-101S temperature controlled water bath with a magnetic stirrer (Shanxi Taikang Biotech., China). Then, 10 mL periodate solution (0.11 g/mL) was added to the CMC solution under stirring. The pH was adjusted to 3.0 with 1 M sulfuric acid solution. After the mixture was stirred in the dark at 35  C for 4 h, the oxidized product, referred to DCMC was precipitated by pouring the solution into a large amount of ethanol. It was then recovered and cross-washed with distilled water and ethanol until all iodic compounds were removed. The product was dried at 37  C to constant weight for the subsequent use. 2.3. Preparation of gelatin-DCMC-MMT edible films Gelatin solution (10%, w/v) was prepared by dissolving gelatin powder in distilled water for 30 min and then heated at 50  C for 30 min under continuous stirring. MMT powder was dispersed into distilled water under stirring for 12 h at room temperature to produce a 1% (w/v) solution. Then certain volume gelatin and MMT solution was mixed and stirred at 50  C for 1 h. The achieved mixture was divided into two parts: One was frozen at 20  C for 24 h and subsequently thawed to obtain freezing gelatin-MMT solution. The rest gelatin-MMT solution was not processed. Then glycerol as plasticizer at concentration of 30 wt% (based on dry gelatin weight) and DCMC solution (1%, w/v) as crosslinking agent at concentration of 5 wt% (based on dry gelatin weight) were added. Thereafter, the two solutions stirred at 50  C for 1 h and then spread on 20  20 cm plexiglass plates to obtain composite films of around 0.3 mm in thickness. The spread films were dried at 25  C and 50% relative humidity for three days. The freezing gelatin-DCMC-MMT samples with 0, 0.5/100, 1/100, 2/100 and 5/100 MMT content (based on dry gelatin weight) were named FG-0MMT, FG-0.5MMT, FG-1.0MMT, FG-2.0MMT and FG-5.0MMT, respectively. The unfrozen samples with 0, 0.5/100,1/100, 2/100 and 5/100 MMT content (based on dry gelatin weight) were named G-0MMT, G-0.5MMT, G-1.0MMT, G-2.0MMT and G-5.0MMT, respectively. 2.4. X-ray diffraction (XRD) The XRD profiles were obtained using an 18 KW rotating anode X-ray diffractometer (MXPAHF, Japan) with a fixed CuKa radiation of 0.154 nm. The diffraction angle was scanned at a rate of 2 /min. 2.5. Light barrier properties and transparency The ultraviolet and visible light barrier properties of the film were measured using an UVeVISeNIR spectrophotometer (UV3600, Shimadzu Co., Kyoto, Japan) at selected wavelengths from 200 to 800 nm following the ASTM method D 1746e92 with slight modifications (Hamaguchi, Weng, & Tanaka, 2007). The transparency of the films was calculated by the following equation:

Transparency ¼ logT=x where T is transmission (%) at each wavelength; x is film thickness (mm). According to the equation, high transparency indicates opaque and low transparent.

J. Guo et al. / Food Hydrocolloids 33 (2013) 273e279

275

2.6. Moisture content and water solubilityy

EB ¼ ðL=25Þ  100

Moisture content (MC) and total soluble matter (TSM) was determined according to the method proposed elsewhere (Rhim, Gennadios, Weller, Cezeirat, & Hanna, 1998). The samples (2  2 cm2) from each type of film were weighed (mh) (0.0001 g) and subsequently dried in an air-circulating oven at 105  C for 24 h. Then films were recovered and reweighed (0.0001 g) to determine their initial dry matter (m0). MC was calculated by the following equation:

where L is film elongation at the moment of rupture (mm); 25 is initial grip length (mm) of samples. In each film type, nine samples were tested.

where mh is the initial weights (g) of samples; m0 is the weights (g) of initial dry matter. Afterward, the samples were directly immersed in a 50 mL beaker containing 30 mL of distilled water with sodium azide (0.02%) to prevent the microbial growth. After 24 h of storage in environmental chamber at 25  C, the samples were recovered, gently rinsed with distilled water and dried in an air-circulating oven at 105  C until reaching constant weight (mf). TSM was calculated by the following equation:

TSMð%Þ ¼



 m0  mf =m0  100

where m0 is the weights (g) of initial dry matter; mf is the weight (g) of the remnant un-soluble matter. The measurements were repeated three times for each type of film, and an average was taken as the result. 2.7. Water vapor permeability measurement Water vapor permeability (WVP) was determined gravimetrically on the basis of the ASTM E96e92 method reported by Gontard, Guilbert, and Cuq (1992). The film was sealed on the top of a glass permeating cup containing distilled water (100% RH; 2337 Pa vapor pressure at 20  C), which was placed in a desiccator at 20  C and 0% RH containing allochroic silicagel (0 Pa water vapor pressure). The cup was weighed at 1 h intervals over 10 h periods. The WVP of the film was calculated by the following equation:

WVP ¼ w  x  A1  t 1  ðP2  P1 Þ1 where w is the weight gain (g); x is film thickness (m); A is the area of exposed film (m2); t is time of gain (s); and (P2P1) is the vapor pressure differential across the film (Pa). This entire procedure was repeated three times for each film type.

TGA measurements were performed on a Netzsch TG 209F1 instrument. The measurements were running from 40  C up to 800  C at a heating rate of 10  C/min and under nitrogen atmosphere in order to avoid thermoeoxidative reactions. 2.10. Statistical analyses Statistics on a completely randomized design were performed with the analysis of variance (ANOVA) procedure using SPSS version 17.0 software. Duncan’s multiple range tests were used to compare the difference among mean values of films’ properties at the level of 0.05. 3. Results and discussion 3.1. X-ray diffraction (XRD) Fig. 1 shows the low-angle XRD patterns of gelatin-DCMC-MMT edible films. It is well-known that the partially crystalline gelatin shows a characteristic peak 2 at 2q z 7e8 , which is due to the triple-helix structure in collagen and also in renatured gelatin (Ghoshal, Mattea, Denner, & Stapf, 2010). Another relatively sharp peak 1 is observed at 2q z 4.50 , which is owing to the aggregated MMT. Fig. 1A shows that the peak 1 in G-1.0MMT is very low, while it is raised to higher level in G-5.0MMT. The low peak intensity observed for G-1.0MMT is caused by the low MMT concentration and an increase in this diffraction peak intensity is due to the

2

A

TS ¼ Fmax =A where Fmax is maximum load (N) needed to pull the sample apart; A is cross-sectional area (m2) of the samples. EB (%) was calculated by following equation:

1

G-5.0MMT

2.8. Tensile strength and elongation at break measurement

4

6

8

10

2 / deg 2

B Intensity

Tensile strength (TS) and elongation at break (EB) of the films were determined using a servo control universal testing machine (AI-7000S, Gotech Testing Machines Inc., Taiwan) according to the standard testing method ASTM D882-97 (Limpisophon et al., 2009). The measurements were made at 25  C and 50% RH in a controlled room. Two rectangular strips (width 5 mm; length 50 mm) were prepared from each film to determine their mechanical properties. Initial grip separation and mechanical crosshead speed were set at 25 mm and 100 mm/min, respectively. TS (MPa) was calculated by the following equation:

G-1.0MMT

Intensity

MCð%Þ ¼ ðmh  m0 =m0 Þ  100

2.9. Thermogravimetric analysis (TGA)

FG-1.0MMT

1

FG-5.0MMT

4

6

8

2

deg

10

Fig. 1. XRD patterns of gelatin-DCMC-MMT edible films with different MMT content with or without freezingethawing process.

J. Guo et al. / Food Hydrocolloids 33 (2013) 273e279

Light transmission in the UV and visible light range from 200 nm to 800 nm, and transparency at 280 nm and 600 nm of the gelatinDCMC-MMT films are shown in Table 1. As the previous reports, the low transmission of UV light at 200 nm and 280 nm for all films shows the excellent barrier properties against UV light (Benjakul, Artharn, & Prodpran, 2008; Hamaguchi et al., 2007; Leerahawong, Arii, Tanaka, & Osako, 2011). The high UV barrier properties of protein films are considered to prevent lipid oxidation in the food system, owing to their high content of aromatic amino acids which absorb UV light (Coupland & McClements, 1996). However, in previous study, transparency of gelatin-DCMC films at 280 nm and 600 nm is w2.6 times and w5.5 times as that of G-0MMT, respectively (Mu, Guo, et al., 2012). This result might be due to the restriction of the polymer chain stretch when films prepared using casting techniques. Moreover, the transparency of the gelatinDCMC-MMT films is very low at 600 nm and slightly increases with the addition of clay, which demonstrates that gelatin-DCMCMMT films are very transparent. Generally, it is known that the optical property of a well-developed film is not significantly changed when the clay platelets with about 1 nm thick are well dispersed through the polymer matrix, since such clay platelets with sizes less than the wavelength of visible light do not hinder light’s passage (Ogata, Jimenez, Kawai, & Ogihara, 1997). In addition, the slightly increase of the transparency depending on the amount of clay added may be due to the aggregation of nanoparticles which obstruct the transmission of light. Similar to the present study, at higher concentrations of nanoclay, a pronounced number of aggregated nanoparticles were observed in the PET nanocomposite though the X-ray diffraction (Litchfield & Baird, 2008). It is worth noting that the transparency of three FG-MMT films at visible region is significant lower than those of G-MMT films with the same MMT content. The result suggests that freezingethawing method is beneficial to dispersing MMT into the Table 1 Light transmission and transparency of different gelatin-DCMC-MMT edible films with different MMT contents with or without freezingethawing process. Sample

Light transmission at different wavelengths (%) Transparency* 200 nm

280 nm

600 nm

800 nm

280 nm 600 nm

G-0MMT G-0.5MMT G-1.0MMT G-2.0MMT G-5.0MMT FG-0MMT FG-0.5MMT FG-1.0MMT FG-2.0MMT FG-5.0MMT

0.115 0.076 0.080 0.092 0.104 0.078 0.070 0.056 0.052 0.058

0.665 0.669 0.596 0.551 0.948 0.784 0.477 0.311 0.490 0.631

88.902 86.757 85.901 84.718 84.287 87.840 88.111 87.548 86.552 84.692

91.547 89.805 88.531 88.207 87.134 90.884 91.656 90.770 90.144 88.094

6.60b 7.42c 7.50c 7.06d 6.98d 6.02a 6.83bd 7.60c 8.25e 7.10d

0.16a 0.21b 0.22b 0.23b 0.26c 0.16a 0.16a 0.17a 0.22b 0.23b

*Different letters in the same column indicate the significant differences (p < 0.05).

20 10

G -0 FG MM -0 T M M T G -0 F G .5 M -0 M T .5 M M T G -1 .0 M FG M -1 .0 T M M T G -2 .0 FG M M -2 .0 T M M T G -5 .0 FG M -5 M T .0 M M T

0

Fig. 2. The moisture content (MC) of gelatin-DCMC-MMT edible films with different MMT content with or without freezingethawing process.

gelatin matrix and reducing the nanoparticles aggregation, which is agree with the result of X-ray diffraction. 3.3. Moisture content and water solubility The moisture content (MC) of gelatin-DCMC-MMT edible films with different MMT content is observed in Fig. 2. MC is a parameter related to the total void volume occupied by water molecules in the network microstructure of the film (Jiang, Li, Chai, & Leng, 2010). The MC values of G-MMT and FG-MMT exhibit the similar equilibrium moisture content comparing with those of chitosan-gelatin films (Pereda et al., 2011). Fig. 2 shows that the MMT content has no significant effects (p > 0.05) on MC values of the G-MMT and FGMMT films. FG-MMT films, however, have significant higher MC values than those of G-MMT films at the same MMT loading. The reason might be that the thawing of ice crystallites introduced by the freezing method has provided more void volume during the drying process. The total soluble matter (TSM) of gelatin-DCMC-MMT edible films with different MMT content is observed in Fig. 3. Generally, plasticized-gelatin films are completely soluble in water as well as the gelatin-MMT bionanocomposites (Martucci & Ruseckaite, 2009, 2010). However, gelatin-DCMC-MMT films are slightly soluble in water (about 24% after 24 h) in consistency with the observation of gelatin-DAS films whose TSM are about 30%. This finding was previously shown that an increase in crosslinking in collagenous material provoked a decrease in the water binding capacity, which would have led to a decrease in solubility of the modified films

25 20 15 10

G -0 FG MM -0 T M M T G -0 . 5 FG M -0 M .5 T M M T G -1 .0 FG M M -1 .0 T M M G T -2 . 0 FG M -2 M T .0 M M T G -5 F G .0 M -5 M T .0 M M T

3.2. Light transmission and transparency

30 MC (%)

increase in MMT concentration (Martucci et al., 2007). In addition, some exfoliation structures resulting from the lower clay content might be formed in G-1.0MMT, which will also decrease the peak intensity (Ray & Okamoto, 2003). It is noteworthy that the diffraction peak 1 in FG-5.0MMT changes dramatically in comparison with G-5.0MMT. The absence of diffraction peak in FG-5.0MMT indicates the formation of exfoliation structure. From this, the freezingethawing process might play an important role in dispersing MMT into the polymer matrix and reducing the nanoparticles aggregation. It has been reported that freezing can enforce phase separation, ice growth and changes in pH, which may be beneficial for exfoliation of nanoclay in bionanocomposites (Mu, Li, et al., 2012).

TSM (%)

276

Fig. 3. The total soluble matter (TSM) of gelatin-DCMC-MMT edible films with different MMT content with or without freezingethawing process.

J. Guo et al. / Food Hydrocolloids 33 (2013) 273e279

277

(Patil, Dalev, Mark, Vassileva, & Fakirov, 2000; de Carvalho & Grosso, 2004). The soluble fraction in gelatin-DCMC-MMT films should be principally attributed to the loss of low molar mass polypeptide chains that can not be crosslinked by DCMC and to the exudation of glycerol out of the films (Martucci & Ruseckaite, 2009). As expected, hydrogen bonding interactions between glycerol, clay and gelatin matrix in the films are less effective in preventing water solubility than crosslinking due to DCMC (Martucci & Ruseckaite, 2010).

significant lower than that of G-2.0MMT. As the influence of MMT concentration on WVP depends on the dispersion of nanoclay, best performances are commonly observed with the exfoliated structures. The freezingethawing is an effective mean to prepare exfoliated gelatin-MMT bionanocomposites. So the well exfoliated structure of FG-MMT films might be one of the contributors to induce lower WVP values.

3.4. Water vapor permeability measurement

Table 2 shows the tensile strength (TS) and elongation at break (EB) of gelatin-DCMC-MMT edible films with different MMT content. The data show that with the addition of MMT the mechanical strength of G-MMT has a significant improvement: the TS of G-0MMT and G-1.0MMT are 22.82 MPa and 27.02 MPa, respectively. This result suggests that uniform dispersion of the nanosized clay particles produces an ultra-high interfacial interaction and ionic bonds between the nanoclay and host polymer (Alexandre & Dubois, 2000; Pandey & Singh, 2005; Pavlidou & Papaspyrides, 2008). However, the elongation at break of nanocomposite films does not show any significant change as Rhim and others’ reports (Rhim, Lee, & Hong, 2011). Furthermore, Table 2 shows that the tensile strength of G-MMT films begins to decrease when the MMT content is over 1 wt%. The same phenomenon is also observed in FG-MMT films when the MMT content is over 2 wt%. The result should be due to the poor dispersion of clay and the aggregation of gelatin chains induced by excess clay addition (Zheng, Li, Ma, et al., 2002). At the same time, Table 2 shows that the FG-0MMT exhibits a TS of 25.38 MPa, which is higher than that (22.82 MPa) of G-0MMT. And the TS of two FG-MMT films (FG-2.0MMT and FG-5.0MMT) are significant higher than those of G-MMT films at the same MMT content. It has been reported that freezingethawing method has the advantages for gelatin molecules to renature into triple-helix. The higher crystallinity (triple-helix content) of FG-MMT films gives higher TS (Mu, Li, et al., 2012; Payne, Mccormick, & Francis, 1999). An additional factor to impact TS is the good dispersion of MMT. Due to freezingethawing processing, the FG-MMT films present exfoliated structures, which should be another reason to induce high TS.

2.4

2

WVP (10 gm / m sPa)

The effects of MMT content on WVP properties for gelatinDCMC-MMT films are given in Fig. 4. In general, the high WVP is one of the major limitations in using protein-based films as food packaging materials due to the inherent hydrophilicity of proteins. Therefore, reduction in WVP is desirable for potential applications in food packaging. The WVP values of gelatin-DCMC-MMT films are in the range of 1.61$1010 g m/m2 s Pa to 2.36$1010 g m/m2 s Pa, which is comparable to the WVP (2.33$1010 g m/m2 s Pa) of gelatin-DCMC films containing 30 wt% glycerol and 5 wt% DCMC (Mu, Guo, et al., 2012) and lower than that reported for extruded and compressed molded plasticized gelatin films (Park, Whiteside, & Cho, 2008). When compared with control (Fig. 4), the WVP values of gelatin-DCMC-MMT films decrease significantly with the addition of MMT. The decrease in WVP with blended nanoclay has been frequently observed with various types of polymers such as polyimide (Yano, Usuki, & Okada, 1997), thermoplastic starch (Park, Lee, Park, Cho, & Ha, 2003), and chitosan (Rhim, Hong, Park, & Ng, 2006). Sothornvit et al. (2010) had shown that the WVP of polymer/clay films decreased significantly when the clay content increased or when clay with higher aspect ratios was used. The reduction in WVP by nanoclay should be attributed to the presence of water vapor impermeable silicate layers with large aspect ratios dispersed in the polymer matrix. These dispersed impermeable silicate layers force water vapor to travel through the film following a tortuous path through the polymer matrix surrounding the particles, thereby increasing the effective path length for diffusion (Yano et al., 1997; Rhim et al., 2006). However, when the MMT content in G-MMT is over 1/100, the WVP values increase. This result might arise from the poor dispersion of MMT and the aggregation of gelatin chains induced by excess MMT addition, which had been reported in previous studies (Casariego et al., 2009). The same phenomenon is observed in FG-MMT composite films, but then the WVP values increase when the MMT content is over 2/100. Moreover, Fig. 4 shows that the WVP value of FG-2.0MMT is

-10

1.6 0.8

G -0 FG MM -0 T M M T G -0 F G .5 M M -0 .5 T M M T G -1 .0 FG M -1 M T .0 M M T G -2 .0 FG M -2 M T .0 M M T G -5 .0 FG M -5 M T .0 M M T

0.0

Fig. 4. The water vapor permeability (WVP) of gelatin-DCMC-MMT edible films with different MMT content with or without freezingethawing process.

3.5. Tensile strength and elongation at break measurement

3.6. Thermogravimetric analysis (TGA) Thermogravimetric analysis (TGA) is used to obtain information on the thermal stability of the gelatin-DCMC-MMT films. The normalized mass loss evolutions with temperature of films at different MMT contents are shown in Fig. 5. There are 3 main steps of thermal degradation of the films in the temperature range of 40e 800  C. The temperature range for the first step of thermal Table 2 The tensile strength (TS) and elongation at break (EB) of gelatin-DCMC-MMT edible films with different MMT content with or without freezingethawing processa. Sample

Tensile strength(MPa)

G-0MMT G-0.5MMT G-1.0MMT G-2.0MMT G-5.0MMT

22.82 24.69 27.02 21.82 17.78

    

3.07b 1.24bc 2.36c 1.71b 0.65a

Elongation at break(%) 55.04 49.09 45.72 42.51 51.88

    

5.33b 4.75 ab 3.82a 5.70a 3.66 ab

FG-0MMT FG-0.5MMT FG-1.0MMT FG-2.0MMT FG-5.0MMT

25.38 27.78 29.84 35.05 27.61

    

3.67bc 3.58c 2.50c 3.41d 3.12c

58.19 46.87 59.71 61.27 43.47

    

5.75b 3.04a 4.07b 6.87b 3.68a

Different letters in the same column indicate the significant differences (p < 0.05). a Data are shown as means  SD.

278

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TG / %

90

MMT content. The well exfoliated structure of FG-MMT films should be the contributors to induce high thermal stability. Agglomerated clay particles may not significantly affect the thermal stability of the polymer matrix (Pramoda, Liu, Liu, He, & Sue, 2003).

G-5.0MMT G-2.0MMT G-1.0MMT G-0.5MMT G-0MMT

60

4. Conclusion

30

0

200 400 600 o Temperature / C

TG / %

90

800

FG-5.0MMT FG-2.0MMT FG-1.0MMT FG-0.5MMT FG-0MMT

60 30

0

200

400

600

800

In this paper, freezingethawing is used as a new method to disperse MMT in DCMC crosslinked gelatin films. The effects of freezingethawing on the structure and properties of gelatinDCMC-MMT films are studied. The data of XRD indicate that the freezingethawing process plays an important role in dispersing MMT into the polymer matrix and reducing the nanoparticles aggregation. The resulting films have flexible and transparent appearance, excellent barrier properties against UV light and similar TSM values. However, the films prepared by freezinge thawing method have a little higher moisture content and lower WVP values. Moreover, the thermal stability and mechanical properties of the films prepared by freezingethawing method are improved due to the better dispersion of MMT. The results indicate that freezing/thawing is an effective mean to prepare exfoliated gelatin-DCMC-MMT films.

o

Temperature / C

Acknowledgment

Fig. 5. TGA curves of gelatin-DCMC-MMT edible films with different MMT content with or without freezingethawing process.

degradation is 40e250  C, which corresponds to the loss of low molecular mass compounds, mainly adsorbed and bounded water. The second and main stage, in the range of 250e500  C, is mainly related to the degradation of gelatin chains. The higher temperature step which exceeds 510  C can be attributed to the decomposition of more thermally stable structure (Martucci et al., 2007; Tunc et al., 2007). The temperature of 50% weight loss and the residue at 700  C of the thermal degradation of G-MMT and FG-MMT films are given in Table 3. The results show that the addition of MMT produces a delay in mass loss in the temperature range of gelatin chains degradation (250e500  C). This improvement in thermal stability of the films is mainly attributed to thermal resistance of MMT and the nanodispersion of MMT sheets in the gelatin matrix. It is known that gelatin and MMT could exert a stronger interaction in composite, and MMT acts as physical crosslinking sites to retard the thermal decomposition of gelatin to a certain extent. In addition, nanodispersive MMT sheets have an excellent barrier property in preventing the release of degraded gelatin fragments. As a result, the thermal degradation of gelatin could be delayed and the thermal stability of composite is improved (Rao, 2007). Note that the FGMMT films have higher temperature of 50% weight loss and higher residue at 700  C than those of G-MMT films with the same Table 3 The temperature of 50% weight loss and the residue at 700  C of the thermal degradation of gelatin-DCMC-MMT edible films with different MMT content with or without freezingethawing process. Samples

T50 ( C)

Residue at 700  C (%)

G-0MMT G-0.5MMT G-1.0MMT G-2.0MMT G-5.0MMT FG-0MMT FG-0.5MMT FG-1.0MMT FG-2.0MMT FG-5.0MMT

312.7 314.7 319.2 324.6 328.5 313.6 316.5 321.3 327.7 330.5

14.5 18.0 18.8 20.3 22.3 14.8 18.5 19.7 22.1 23.7

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