methylcellulose biodegradable films

Journal of Food Engineering 114 (2013) 123–131

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Thermal, mechanical and water adsorption properties of corn starch–carboxymethylcellulose/methylcellulose biodegradable films E. Aytunga Arık Kibar a,⇑, Ferhunde Us b,1 a b

_ TÜBITAK, Marmara Research Center, Food Institute, PO Box 21, 41470 Gebze, Kocaeli, Turkey Hacettepe University, Faculty of Engineering, Department of Food Engineering, 06800 Beytepe, Ankara, Turkey

a r t i c l e

i n f o

Article history: Received 24 January 2012 Received in revised form 23 July 2012 Accepted 29 July 2012 Available online 4 August 2012 Keywords: Biodegradable film Starch Methylcellulose Carboxymethylcellulose Glycerol Polyethylene glycol

a b s t r a c t The objective of this study was to investigate the effect of the addition of methylcellulose and carboxymethylcellulose on the thermal, mechanical and water adsorption properties of starch-based films plasticized with glycerol or polyethylene glycol (PEG). Mechanical tests showed that as the methylcellulose and carboxymethylcellulose proportion increased, starch films became more resistant to break, resulting in higher TS values. Besides there has been a positive effect on the elasticity of starch films realized by a considerable increase in E% values. Depending on the plasticizer type, either single or dual glass transitions were seen in DSC thermograms. One glass transition temperature was observed for films plasticized with glycerol, on the contrary, dual glass transitions were detected for PEG plasticized films. This behavior was attributed to the phase separation of the PEG. In addition, the presence of an endothermic peak in the thermograms of PEG plasticized films was taken as another indicator of the phase separation. As a result, it was suggested that PEG was not as compatible as glycerol with the composite polysaccharide matrix and plasticizer type was the main factor that shaped the thermal profiles of the film samples. Water adsorption isotherm data showed that samples displayed nonlinear sorption profile which is typical for hydrophilic films. In all films tested, equilibrium moisture contents, increased almost linearly up to a aw of 0.65–0.85, beyond where a sharp increase was noted. Adsorption data was adequately fitted by BET and GAB models. Eventually, it can be concluded that film forming properties of starch can be improved by incorporation of methylcellulose and carboxymethylcellulose to the polymer matrix. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction The current global consumption of plastics is more than 200 million tones; with an annual grow of approximately 5%, which represents the largest field of application for crude oil. It emphasizes how dependent the plastic industry is on oil and consequently how the increasing of crude oil and natural gas price can have an economical influence on the plastic market. Therefore it has been becoming increasingly important to utilize alternative raw materials. Until now petrochemical-based plastics have been increasingly used as packaging materials because of their large availability at relatively low cost, good mechanical performance, good barrier to oxygen, carbon dioxide, water vapor and aroma compounds, heat sealability, and so on (Siracusa et al., 2008). But the improper disposition of the enormous volume of petroleumderived plastics in the environment has led to pollution and raised much interest in the biodegradable and renewable resources (Ma ⇑ Corresponding author. Tel.: +90 262 677 32 26; fax: +90 262 641 23 09. E-mail addresses: [email protected] (E.A. Arık Kibar), [email protected] (F. Us). 1 Tel.: +90 312 297 71 05; fax: +90 312 299 21 23. 0260-8774/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jfoodeng.2012.07.034

et al., 2008b). In addition, there has been a considerable interest in biodegradable films made from starch (Lawton, 1996). Several studies have been performed to analyze the properties of starchbased biodegradable films (Bertuzzi et al., 2007; Chang et al., 2010; Mali et al., 2005; Parra et al., 2004; Romero-Bastida et al., 2004; Talja et al., 2007; Zhang and Han, 2006a,b, 2008). Starch films generally have good barrier properties to oxygen, carbon dioxide and lipids, however they have limitations in mechanical and water vapor permeability properties (Kester and Fennema, 1986). Three common ways have been used in order to overcome these limitations: genetic modification; such as production of high amylose starch (Ryu et al., 2002), chemical modification (Parra et al., 2004) and blending with appropriate materials. Chemical or genetic modifications are useful methods to get new substances with well-defined properties but they are often time consuming and not seldom costly. On the other side blending is a well-known, efficient way to prepare new materials with improved properties (Vasile et al., 2004). Agar (Wu et al., 2009), chitosan (Bourtoom and Chinnan, 2008; Xu et al., 2005), cellulose fibers (Muller et al., 2009) cellulose crystallites (Ma et al., 2008b), pullulan (Kristo and Biliaderis, 2007), nanoclay (Almasi et al., 2010), nano-SiO2 (Tang et al., 2009) have been added to enhance film forming

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properties of starch. These studies have demonstrated that mechanical and permeability properties of starch films could be improved in some cases. In the present article we have reinforced the starch film by mixing with methylcellulose and carboxymethylcellulose. Those cellulose ethers have no harmful effects on human health, and are used as highly effective additive to improve the product and processing properties in various fields of application, from foodstuffs, cosmetics and pharmaceuticals to products for the paper and textile industries (Feller and Wilt, 1990). Methylcellulose has been widely used to prepare edible films and documented in several publications (Debeaufort and Voilley, 1997; Donhowe and Fennema, 1993a,b; Turhan and Sahbaz, 2004). Carboxymethylcellulose is an anionic linear polysaccharide derived from cellulose. It is an important industrial polymer with a wide range of applications (Biswal and Singh, 2004). Plasticizers are added to polymers to reduce brittleness, since they increase the free volume between polymer chains, decreasing intermolecular forces and thus increasing flexibility and extensibility of polymers (Romero-Bastida et al., 2005). Many researchers studied the effects of various polyols on starch-based films (Yang and Paulson, 2000; Zhang and Han, 2006a,b, 2008). The most preferred polyols were glycerol, sorbitol, and PEG (Mali et al., 2002; Mchugh et al., 1993). In the presented study glycerol and PEG have been used as plasticizers. There are some studies about the carboxymethylcellulose and/ or methylcellulose starch composite films in the literature. Peressini et al. (2003) have investigated the rheological properties of starch–methylcellulose blends and exhibited the compatibility of two polysaccharides in the film forming dispersionsIn their subsequent work, starch–methylcellulose–lipid film has been developed and the influence of deposition process of film-forming dispersion on the shelf-life of dry bakery food has been examined (Bravin et al., 2006). Ma et al. (2008a) have studied the thermoplastic starch/cellulose derivatives as potential biodegradable packaging materials. They have proposed that the introduction of carboxymethylcellulose and methylcellulose increased the glass transition temperature and improved the tensile stress and elongation at break, as well as the barrier property against water vapor. In a recent study of Tongdeesoontorn et al. (2011) mechanical properties of CMC reinforced cassava starch films have been investigated. They have reported that addition of CMC to the cassava starch films has increased tensile strength and reduced elongation at break (Tongdeesoontorn et al., 2011). However, there is negligible data available about the physicochemical properties of corn starch–carboxymethylcellulose and corn starch–methylcellulose based films. Thus the objective of this study is to determine the effect of blending level and plasticizer type on the physicochemical properties of carboxymethylcellulose and methylcellulose–corn starch composite films and investigate the potential usage as biodegradable packaging material.

2. Materials and methods 2.1. Materials Normal corn starch (Unmodified regular corn starch containing approximately 73% amylopectin and 27% amylose) and methylcellulose (Molecular weight of 41,000 and degree of substitution of 1.5–1.9) were purchased from Sigma Chemical CO. (St. Luis, Missouri, USA). Carboxymethylcellulose, with a molecular weight of 90,000 and degree of substitution of 0.7 was purchased from Acros Organics (Geel, Belgium). Analytical grade glycerol (GLI; Merck; Darmstadt, Germany) and polyethylene glycol 400 (PEG; Merck; Hohenbrunn, Germany) were used as plasticizer.

2.2. Film preparation and casting Film-forming solutions were prepared with different blending levels of carboxymethylcellulose/corn starch (CMC/CS) and methylcellulose/corn starch (MC/CS) to study the roles of these components on the physical properties of the composite films. The blends of CMC/CS and MC/CS (0:100; 20:80; 40:60; 60:40; 80:20; 100:0) and the plasticizer content (50% w/w on dry basis) were established according to the preliminary tests. In each formulation, the weight of dry matter was maintained at a constant value of 1.5 g per 100 mL water. Film-forming dispersions were obtained by the dispersion and solubilization of CMC and MC in 50 mL of water at room temperature and at 95 °C, respectively. CS was gelatinized in 50 mL of water at 95 °C for 45 min in the presence of the plasticizer. When the CS/plasticizer solution temperature was around 50 °C, solution was added to the CMC or MC solution. Then the mixture was homogenized using an Ultra Turrax T25 (Ika Labortechnick, Staufen, Germany) for 2 min at 13,000 rpm, followed by 2 min at 11,000 rpm. In order to remove air bubbles, the solutions were placed in an ultrasonic water bath (Elma LC 30 H, Singen, Germany) for 30 min and finally, solutions were allowed to stabilize at room temperature overnight. Films were cast by pouring 30 mL of solution onto the 85 mm internal diameter Petri dishes and dried in a climatic room with controlled conditions (25 °C and 45% RH) for at least 3 days. Thickness of films was determined using a digital micrometer (Mitutoyo, Manufacturing Co. Ltd., Japan, 0.001 mm accuracy). Reported thickness values were the mean value of five measurements for each film sample. 2.3. Differential scanning calorimetry (DSC) analysis DSC experiments were carried out using TA Q20 model DSC apparatus (TA Instruments, USA). The calorimeter was calibrated with indium (melting point = 156.6 °C, DH = 28.5 J/g). The DSC runs were operated under nitrogen gas atmosphere (30 mL/min) and an empty pan was used as the reference. The film samples, approximately 3 mg, were hermetically sealed in aluminum pans after equilibration over P2O5 for 10 days. The pans were heated from 90 °C to 100 °C at the scanning rate of 10 °C/min. The DSC thermograms were evaluated to characterize the onset, peak and end temperatures and the enthalpy changes of the phase transitions. The glass transition temperature was determined by taking the first derivative of the thermograms. Glass transition was analyzed for the onset, mid, end points and the midpoint temperatures were reported as glass transition temperatures of the samples. 2.4. Mechanical properties A TA Plus Texture Analyzer (Lloyd Instruments, West Sussex, England) was used to determine the tensile strength and percentage of elongation at break. Film specimens were tested as suggested by ASTM D683M (ASTM, 1993). All film strips were equilibrated for ten days to 52 ± 2% RH in a cabinet using saturated magnesium nitrate solution at room temperature (25 ± 1 °C). At least 10 replications of each test sample were run. Tensile strength (MPa) was calculated by dividing maximum load by cross-sectional area of the film. Per cent elongation at break was expressed as percentage of change of the original length of a specimen between grips at break. 2.5. Moisture adsorption isotherm Moisture adsorption isotherm of films was determined at 25 °C for aw varying from 0.11 to 0.92 using saturated salt solutions (Merck; Darmstadt, Germany) in desiccators [LiCl, aw 0.11; CH3COOK, aw 0.22; MgCl2, aw 0.33; K2CO3, aw 0.43; Mg(NO3)2, aw

E.A. Arık Kibar, F. Us / Journal of Food Engineering 114 (2013) 123–131

0.52; NaNO2, aw 0.64; NaCl, aw 0.75; KCl, aw 0.84; KNO3, aw 0.92]. Film samples were dried over P2O5 at 25 °C for 10 days prior to adsorption analysis. Samples were checked at certain time intervals to ensure saturation. Equilibrium was judged to have attained when the difference between two consecutive sample weightings was less than 1 mg/g dry solid ( 40 days). Moisture content was determined in the equilibrated samples as the difference in weight before and after drying in an oven at 130 °C for 1 h (AACC, 1995). Water activity was evaluated at 25 °C by means of an AquaLab Series CX2 model instrument (Decagon Devices, Inc., Washington, USA). Equilibrium moisture content (Xe) was expressed as grams per 100 g of dry solid. 2.6. Sorption models A number of sorption isotherm models that have been reported in the literature. In the present study BET (Labuza, 1968) and GAB (Zhang and Han, 2008) models were used for fitting the sorption data.

X Caw ¼ X m ½ð1  aw Þð1  aw þ Caw Þ X Ckaw ¼ The GAB model : X m ½ð1  kaw Þð1  kaw þ Ckaw Þ

The BET model :

ð1Þ ð2Þ

In Eqs (1) and (2), Xm; the monolayer moisture content, C; a constant related to thermal effects and k; the GAB constant related to the properties of multilayer water molecules with respect to bulk liquid. The sorption data was analyzed according to the models and the corresponding constants were determined. The goodness of fit of each model was computed in terms of coefficient of regression, R2 and root mean square error per cent (%RMS) values, as

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  2 uX u n ðXoi X pi Þ u X oi t %RMS ¼ 100 i¼1 n

ð3Þ

where Xoi is the observed equilibrium moisture content, Xpi is the predicted equilibrium moisture content and n is the number of observations. The isotherm equation with a %RMS value of less than or equal to 10 was considered to be a good fit (Yanniotis et al., 1990). 2.7. Statistical analysis Experimental data was subjected to one-way analysis of variance (ANOVA), using SPSS version 11.5 (SPSS Inc., USA). Treatment means were tested separately for least significant difference (LSD) test. Nonlinear curve fitting was performed by using Origin 7.0 software (Northampton, USA).

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3. Results and discussion 3.1. Thermal properties and glass transition temperature In order to improve the processability of polymer films, incorporation of a plasticizer is required. Plasticizers reduce intermolecular forces and increase the mobility of polymer chains. In this way, plasticizers decrease the glass transition temperature of these materials and improve their flexibility (Mali et al., 2005). Glass transition temperatures of plasticizers used in this study were detected as 78.6 ± 2.1 °C and 65.0 ± 1.3 °C for glycerol and PEG, respectively. Moreover, a melting endotherm of PEG was detected at 6.50 ± 0.05 °C which had a melting enthalpy of 107.1 ± 2.2 J/g (Fig 1). Similar results have been reported in the literature. Averous et al. (2000) and Buera et al. (1999) have measured the glass transition temperature of glycerol by DSC technique and reported as 78 °C and 77 °C, respectively. The glass transition temperature of PEG has been reported as 67 °C (Feldstein, 2001) and 70 °C (Feldstein et al., 2003). Feldstein et al. (2003) have reported also a fusion endotherm of PEG at 6 °C with a melting enthalpy of 118 J/g (Feldstein et al., 2003). In our study, depending on the plasticizer type, DSC thermograms of biodegradable films have showed either single or dual glass transitions. One glass transition has been observed for films plasticized with glycerol which had a midpoint temperature between 76.5 °C and 38.1 °C (Fig 2). The presence of one single glass transition temperature for multiple polymer blends may be attributed to their similar plasticization behavior in the presence of plasticizers (Arvanitoyannis and Biliaderis, 1999). As previously reported by Bizot et al. (1997) polysaccharides (starch, pullulan, dextran, phytoglycogen, fructoslated amylose and amylopectin), widely differing in their molecular structure, branching and conformation of glycosidic linkages, exhibited parallel trends in the glass transition temperature-moisture content plots, indicative of similar plasticization responses. It is unlikely, therefore, to expect multiple glass transitions of composite polysaccharide matrices because of their similar plasticization behavior in the presence of plasticizers. As a result, the observed single glass transition for all glycerol plasticized films was attributed to the whole polymer matrix. Similarly Arvanitoyannis and Biliaderis (1999) have reported only one glass transition temperature for methylcellulose–soluble starch–glycerol blends. On the contrary of glycerol plasticized films, two glass transitions have been detected for films plasticized with PEG. The ‘‘upper’’ and ‘‘lower’’ glass transitions have been observed at temperatures between (46 °C) to (55 °C) and (80 °C) to (67 °C), respectively (Fig. 3). Dual glass transitions in DSC thermograms are typical of a phase separated system (Feldstein et al., 2003; Forssell et al., 1997). As suggested by the earlier observations,

Fig. 1. DSC thermograms of (a) glycerol and (b) polyethylene glycol. (The glass transition temperatures were pointed out on the insert view.)

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Fig. 2. DSC thermograms of glycerol plasticized (a) methylcellulose–corn starch and (b) carboxymethylcellulose–corn starch-based films. (The first derivatives of heat flow curves were given on the right side of DSC curves.) (Glass transition temperatures (Tg) were pointed out on the first derivatives of heat flow curves.) (Cellulose ether: corn starch blending ratios were given next to each curve.)

Fig. 3. DSC thermograms of PEG plasticized (a) methylcellulose–corn starch and (b) carboxymethylcellulose–corn starch-based films. (The first derivatives of heat flow curves were given on the right side of DSC curves.) (Glass transition temperatures (Tg) were pointed out on the first derivatives of heat flow curves.) (Cellulose ether: corn starch blending ratios were given next to each curve.)

composite films have showed only one glass transition in the presence of glycerol. Therefore, dual glass transitions in PEG plasticized films seemed to be due to the phase separation of PEG. This phenomenon has been previously reported for poly(N-vinyl pyrrolidone)-PEG blends, starch–glycerol–water mixtures and amylose–

amylopectin films by Feldstein et al. (2003), Forssell et al. (1997) and Myllarinen et al. (2002), respectively. In these studies, the phase separation of the plasticizer has been reported and the upper transition has been attributed to a polymer-rich phase, whereas the lower transition has been due to the existence of

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E.A. Arık Kibar, F. Us / Journal of Food Engineering 114 (2013) 123–131 Table 1 Thermal properties of methylcellulose–corn starch and carboxymethylcellulose–corn starch films. Blending ratio

Tg (°C)i

Methylcellulose:corn starch Plasticizer:glycerol

100:0 80:20 60:40 40:60 20:80 0:100

76.5 ± 0.2g 74.6 ± 0.3f 73.5 ± 0.6f 67.2 ± 0.3e 63.8 ± 0.2d 62.5 ± 0.5d

Carboxymethylcellulose:corn starch Plasticizer:glycerol

100:0 80:20 60:40 40:60 20:80 0:100

38.4 ± 0.9a 38.1 ± 0.7a 47.6 ± 0.2b 57.2 ± 1.2c 63.7 ± 0.1d 62.5 ± 0.5d

Blending ratio

Tg,1 (°C)i

Tg,2 (°C)i

Tp (°C)ii

Methylcellulose:corn starch Plasticizer:PEG

100:0 80:20 60:40 40:60 20:80 0:100

67.0 ± 0.2a 69.2 ± 0.2b 71.1 ± 1.0b 74.7 ± 0.4c nd nd

46.3 ± 0.5c 46.7 ± 0.5c 47.8 ± 0.1b 48.7 ± 0.1b nd nd

6.6 ± 1.1e 5.0 ± 0.8de 3.1 ± 0.9d 3.0 ± 0.8d nd nd

0.78 ± 0.03f 4.29 ± 0.50e 6.61 ± 0.63d 7.68 ± 1.20cd nd nd

Carboxymethylcellulose:corn starch Plasticizer:PEG

100:0 80:20 60:40 40:60 20:80 0:100

80.2 ± 0.2d 79.9 ± 0.2d 78.9 ± 0.8d 75.4 ± 0.4c 75.9 ± 1.2c nd

55.4 ± 1.1a 48.5 ± 0.6b 49.0 ± 2.1b 50.1 ± 2.2b 54.0 ± 1.3a nd

4.6 ± 0.4a 2.8 ± 0.2ab 2.3 ± 0.7bc 2.1 ± 0.3bc 0.6 ± 0.3c

21.98 ± 0.66a 11.50 ± 1.69b 14.90 ± 2.93b 14.53 ± 3.01b 13.41 ± 2.68bc nd

nd

DH (J/g)ii

nd, not determined; All values shown are means ± standard deviations. Data with the same letter (a–f) within a column are not statistically different at a (p < 0.05) level. i Tg: Glass transition temperature, Tg,1 and Tg,2: ‘‘upper’’ and ‘‘lower’’ glass transition temperatures, respectively, ii Tp; DH: peak temperature and enthalpy of melting endotherm, respectively.

Fig. 4. Mechanical properties of methylcellulose–corn starch and carboxymethylcellulose–corn starch composite films. (For each property mean values in the same graph with different letters are not statistically different at a (p < 0.05) level.)

plasticizer-rich microdomains. Moreover, we have found an endothermic peak between 6.6 °C to 4.6 °C in DSC thermograms of films plasticized with PEG (Fig. 3). The presence of a fusion endotherm of PEG in the film structure could also be considered as another evidence for partial phase separation of PEG. Plasticizing activity of polyols has been related to various factors including plasticizer molecular size and 3-dimensional compatibility between the plasticizers and polymers (Zhang and Han, 2006a). When the differences between the phase separation behaviors of glycerol and PEG were taken into consideration, it could be concluded that glycerol was a more compatible plasticizer than PEG with the polysaccharide matrix. This result could be due to PEG’s bigger molecular size that could reduce its plasticizing efficiency. Compared to the PEG, glycerol’s smaller size facilitates its penetration into the polymer matrix. Zhang and Hang (2006a) have obtained the same conclusion in their study where the plas-

ticization effect of various polyols in starch films has been investigated. They have suggested that sorbitol is a larger molecule when compared to glycerol, as a result it has a limited accessibility to the high-density junction zones of the polymer matrix. On the other hand, better plasticizing efficiency of glycerol should be evaluated in detail. Because in our study polymer matrix contains three types of polysaccharides and their individual compatibility with the plasticizer might be different. The glass transition temperatures of composite film samples have been evaluated in order to compare this property (Table 1). Plasticizers decrease the glass transition temperature, as a result; observing lower glass transition temperature at the same plasticizer content, indicates a better compatibility of the polymer matrix with the plasticizer. When the glass transition temperatures of glycerol plasticized films have been considered, the lowest glass transition temperature has been detected as 76.5 °C for the methylcellulose

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Fig. 5. Moisture adsorption isotherms of methylcellulose–corn starch and carboxymethylcellulose–corn starch films at 25 °C. (Corn starch: methylcellulose and corn starch: carboxymethylcellulose blending ratios were given in data labels.)

film and the highest one at 38.1 °C for the carboxymethylcellulose film. In addition, the glass transitions have increased from 74.6 °C to 63.8 °C as the methylcellulose proportion has decreased for the composite film samples. On the contrary, the glass transition temperatures decreased from 38.1 °C to 63.7 °C as the carboxymethylcellulose proportion has decreased. This result indicated that glycerol had its best plasticizing effect on methylcellulose and it decreased in the sequence of methylcellulose, starch and carboxymethylcellulose, respectively. The thermal properties of fusion endotherm detected in PEG plasticized samples have been given in Table 1. If the melting enthalpy of this endotherm has been considered as corresponding to the relative amount of phase separated PEG, it could be said that the compatibility of PEG increased as the proportion of methylcellulose increased, while that of carboxymethylcellulose decreased in the formulation. That was also obvious when the peak temperatures of the melting endotherm have been examined. The peak temperatures increased in the similar trend with the melting enthalpies (Table 1). The proximity of the peak temperature to that of pure PEG (Fig. 1) could be considered as an indirect evidence of the interaction between the polymeric matrix and the PEG molecules. Therefore it may be suggested that as the interaction declines, the peak temperature of fusion endotherm diverges from that of pure PEG. Eventually it could be said that type of the plasticizer is the main factor that affects the thermal profiles. In this context, it may be concluded that glycerol is highly efficient plasticizer that is compatible with all of the polymers considered in this study. On the contrary, PEG has been compatible only with the methylcellulose portion. 3.2. Tensile properties Mechanical properties of films have been characterized by the tensile strength (TS) and elongation% (E%) values and high values are generally required, which are the indicators of the film’s strength and flexibility. Mechanically, starch, methylcellulose and

carboxymethylcellulose films have behaved differently, which can be seen easily in Fig 4. Methylcellulose and carboxymethylcellulose films have showed higher TS and E% values than starch film. In this case, it is expected that mechanical properties of starch film can be improved by incorporation of methylcellulose and carboxymethylcellulose into the film formulation. TS and E% values of carboxymethylcellulose–starch blend films have been determined between 3.6–24.1 MPa and 2.5–136.1%, respectively. TS values of composite films have increased as the carboxymethylcellulose level has increased. These results are consistent with corn starch films (Ghanbarzadeh et al., 2010), cassava starch films (Tongdeesoontorn et al., 2011) and pea starch films (Ma et al., 2008a) in which TS has improved as the concentration of added carboxymethylcellulose has been increased. Furthermore, Tongdeesoontorn et al. (2011) reported that the increase in the TS of cassava starch–carboxymethylcellulose films could be attributed to the formation of intermolecular interaction between the hydroxyl group of starch and carboxyl group of carboxymethylcellulose. The flexibility of composite films has been also affected by the blending level. As it is shown in the Fig. 4 there has been a synergistic effect between carboxymethylcellulose and starch on the elasticity by a considerable increase in E% values for glycerol plasticized films. Indeed the highest E% values among all samples in this study have been measured for glycerol plasticized carboxymethylcellulose–starch blend films. This synergistic effect could be attributed to the carboxymethylcellulose–starch interaction which has been also reported in the literature. Aguirre-Cruz et al. (2005) notified that carboxymethylcellulose has increased the viscosity of corn starch, and this was mainly due to the three-dimensional network formed by the carboxymethylcellulose–starch association. Lee et al. (2002) have also reported the interaction of carboxymethylcellulose and potato starch and they have proposed a mechanism in order to explain this interaction: carboxymethylcellulose associates with swollen starch or leached amylose chains. However, this synergistic effect on E% has not been observed for PEG plasticized carboxymethylcellulose–starch blend films,

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besides significantly lower E% values have been measured. Perhaps this could be related to the phase separation behavior of PEG, which has induced the crystallization and reduced the amount of plasticizing portion of PEG. As a result this might decrease the carboxymethylcellulose–starch association in polymer network and thus allowed changes in elongation properties. Similarly for the methylcellulose–starch blend films, the TS and E% values increased as the methylcellulose level increased. TS of methylcellulose–starch blend films have been determined to be 2.5–28.4 MPa and E% measured between and 8.8–109.7%. It is obvious that incorporation of methylcellulose has improved both the mechanical strength and flexibility of starch films (Fig. 4). When the mechanical test results were inspected with respect to the plasticizer type, an interesting trend has been detected. In the formulations where methylcellulose proportion exceeded 60%, the films plasticized by PEG have been stronger than those plasticized by glycerol. On the contrary, when the starch portion dominated the formulation, better tensile properties have been measured for the glycerol plasticized formulations than that of PEG plasticized films. It could be associated with the difference in the compatibility of the plasticizers; that is, glycerol was miscible with both starch and methylcellulose while PEG was compatible only with the methylcellulose fraction. If the tensile test results obtained in this study have been compared to the synthetic polymers, they had comparable TS values with low and high density polyethylene, which have been reported between 10–20 MPa and 16–41 MPa, respectively (Cuq et al., 1995) and also E% values were better than cellophane and cellulose acetate which have been reported between 15–25% and 15–70%, respectively (Briston, 1986; Cuq et al., 1995). Eventually it might be concluded that incorporation of methylcellulose and carboxymethylcellulose into the starch films could be a potential solution to the classical problem encountered with this kind of films and thus widen the application of starch films in food packaging. 3.3. Moisture adsorption isotherm Water acts as a good plasticizer in most hydrophilic films and water adsorption of hydrophilic films depends on the environmen-

tal relative humidity (van Soest et al., 1995). The moisture adsorption isotherm data of films have been displayed in Fig. 5. In general, the moisture adsorption isotherms of films displayed sigmoid shaped curvatures. In all films tested equilibrium moisture contents, Xe, (g/100 g dry solid) has increased almost linearly up to a aw of 0.65–0.85, beyond a sharp increase has been noted. That type of nonlinear sorption profile is typical for hydrophilic films (de la Cruz et al., 2001; Turhan and Sahbaz, 2004). The sorption levels have been determined within the range of high amylose corn starch (Bader and Goritz, 1994; Bertuzzi et al., 2007) and cellulosic films (de la Cruz et al., 2001; Turhan and Sahbaz, 2004). Carboxymethylcellulose and methylcellulose are hydrophilic polymers; as expected, incorporation of carboxymethylcellulose and methylcellulose have not decreased the moisture adsorption of starch films, in fact slightly increased the adsorption capacity (Fig. 5). This could be due to the etheric groups on the cellulose ethers. The repeating side groups on the polymer chains could have led to the higher moisture adsorption capacity by increasing the intermolecular distance between the polymer chains, and hence facilitates the penetration of water into the polymer matrix. In starch films, plasticizers are generally more hygroscopic than starch. Thus, the difference in the water adsorption capacity of starch films is mostly dependent on the type of the plasticizers when the starch content remains constant (Zhang and Han, 2006a). In line with this context, when a comparison has been carried out with respect to the plasticizer type, moisture contents have been higher in films containing glycerol at constant starch/ cellulose ether blending ratios (Fig. 5). This could be related to the better plasticizing efficiency of glycerol than PEG as shown in the DSC results previously. Glycerol had a better influence on decreasing the attractive forces between the polymer chains, increased the free volume and segmental motions, hence water molecules entered more easily and higher moisture contents resulted. Similar results were reported in the literature. Zhang and Han (2006a, 2008) have noted that glycerol-plasticized starch films contained significantly higher level of moisture than the other polyols plasticized film and it has been attributed to the high polarity of glycerol. They have suggested that glycerol is acting like a ‘‘water holding agent’’ and therefore entrapped large amount of

Table 2 Monolayer moisture contents (Xm), coefficient of regression, (R2) and root mean square% (RMS%) values of GAB and BET models for moisture adsorption isotherms of methylcellulose–corn starch and carboxymethylcellulose–corn starch composite films. Sample

Methylcellulose:corn starch

Plasticizer

Glycerol

Model (aw)

BET (0.1–0.4)

GAB (0.1–0.9)

Polyethylene glycol

BET (0.1–0.4)

GAB (0.1–0.9)

Carboxymethylcellulose:corn starch

Glycerol

BET (0.1–0.4)

GAB (0.1–0.9)

Polyethylene glycol

BET (0.1–0.4)

GAB (0.1–0.9)

Parameter

Blending ratio 100:0

80:20

60:40

40:60

20:80

0:100

Xm R2 RMS% Xm R2 RMS% Xm R2 RMS% Xm R2 RMS%

15.7 0.977 4.05 16.3 0.995 5.07 12.3 0.979 3.60 13.1 0.996 5.96

21.4 0.984 2.66 20.0 0.990 6.45 12.6 0.951 4.79 13.1 0.997 5.33

21.4 0.979 2.58 20.8 0.991 6.28 11.5 0.967 4.05 13.4 0.995 7.31

17.5 0.968 4.26 17.8 0.987 8.54 10.4 0.961 5.04 13.4 0.988 11.54

17.6 0.980 3.20 18.8 0.986 8.88 – – – – – –

18.9 0.976 3.43 22.0 0.990 7.52 – – – – – –

Xm R2 RMS% Xm R2 RMS% Xm R2 RMS% Xm R2 RMS%

15.7 0.977 4.05 16.3 0.995 5.07 12.6 0.983 3.10 14.6 0.997 6.60

19.8 0.956 4.39 19.0 0.990 5.44 9.3 0.966 6.97 13.8 0.993 12.12

19.1 0.951 4.34 18.7 0.996 4.11 9.4 0.989 3.68 13.0 0.993 9.70

17.3 0.962 3.43 17.4 0.999 2.56 10.7 0.917 12.81 15.8 0.990 13.25

21.0 0.928 6.07 20.2 0.996 5.09 9.9 0.936 10.22 15.1 0.992 12.84

21.1 0.984 2.97 19.6 0.999 2.15 – – – – – –

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water molecules inside the starch polymer network (Zhang and Han, 2008). Also, Talja et al. (2007) have reported that glycerol increased the moisture adsorption of starch films, and this was related to the lower molecular weight of the glycerol. Table 2 shows the estimated parameters and goodness of fit of BET and GAB models to experimental data of films between the aw ranges of 0.1–0.4 and 0.1–0.9, respectively. Among the sorption isotherm models that have found in the literature, the GAB model has received the most attention in practical applications. It is regarded as reliable in modeling sorption data for many food materials for almost the entire sorption isotherm (Biliaderis et al., 1999). The BET model could also be used to provide the estimates of the monolayer value. However, the BET model does not take into account the effect of water on structural change of the films. When dissolution or swelling of the films occur, the BET model is not useful in providing insight into the sorption process. Therefore, the BET model is usually restricted to a narrow aw range where the change in film structure occurs hardly (Mathlouthi, 2001; Zhang and Han, 2008). The monolayer water content values of glycerol plasticized films have been determined to be 15.7–21.4 g/100 g dry solid and 16.3–22.1 g/100 g dry solid as predicted by BET and GAB equations, respectively. Similarly monolayer moisture values of PEG plasticized films ranged from 10.4 to 13.4 g/100 g dry solid and from 9.3 to 15.8 g/100 g dry solid as predicted by BET and GAB models, respectively. These values have been comparable with monolayer water contents reported for high amylose corn starch films (Bertuzzi et al., 2007), potato starch films (Talja et al., 2007), pea starch films (Zhang and Han, 2008) and methyl and ethyl cellulose based films (de la Cruz et al., 2001). The monolayer moisture contents of glycerol plasticized films have been higher than that of PEG plasticized films (Table 2). The value of the monolayer moisture content is of particular interest, since it indicates the amount of water that is strongly adsorbed to specific sites at the surface. In other words monolayer value can be used to express the number of active sites available to the water adsorption (Inchuen et al., 2009). Therefore, it could be proposed that incorporation of glycerol into the structure led to an increase in the number of available sorption sites. This result is not surprising when the molecular weight of glycerol has been taken into consideration. Due to its smaller weight, at constant plasticizer content, glycerol had larger number of hydroxyl groups compared to the PEG. In the literature many authors reported that glycerol increased the monolayer moisture content of films compared to other plasticizers (Cho and Rhee, 2002; Mali et al., 2005; Martelli et al., 2006). It could be concluded that the presence of cellulose ethers in the starch film has increased hygroscopic characteristics. 4. Conclusions In this study carboxymethylcellulose–corn starch and methylcellulose–corn starch biodegradable blend films have been prepared and characterized and the following conclusions have been derived; (i) In the DSC thermograms the glycerol plasticized blend films have showed one glass transition while, PEG plasticized films, two step transitions, which suggested two glass transitions of a phase separated system. When this behavior has been taken into consideration, it has been suggested that PEG was not as compatible as glycerol with the composite polysaccharide matrix. (ii) Composite films have been more elastic and resistant to break when compared to starch-based film. Thus, addition of methylcellulose and carboxymethylcellulose to starch-

based films could be a potential solution to the classical problem encountered with this kind of films. (iii) The moisture adsorption isotherms have showed the moisture adsorption capacity of the films have increased in the presence of glycerol and also it has increased as the methylcellulose and carboxymethylcellulose level in the formulation has been increased. Mathematical fitting of adsorption data to BET and GAB models have given the monolayer values and an opportunity to assess the amount of available sorption sites on the composite polymer matrix. (iv) Eventually, it can be concluded that film forming properties of starch can be improved by incorporation of methylcellulose and carboxymethylcellulose to the polymer matrix.

Acknowledgments _ The authors thank to TÜBITAK (Project number: TBAG107T899) and Hacettepe University Research Centre Office (Project number: 010 T02 604 001) for providing funds in the form of a research project. The authors also wish to thank to Prof. Dr. Piotr P. Lewicki (recently passed away) in the Faculty of Food Sciences, Warsaw University of Life Sciences for his contribution to the intellectual content.

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