ARTICLE IN PRESS Industrial Crops and Products xxx (2010) xxx xxx

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1 Industrial Crops and Products xxx (2010) xxx xxx Contents lists available at ScienceDirect Industrial Crops and Products journal homepage: Improving the barrier and mechanical properties of corn starch-based edible films: Effect of citric acid and carboxymethyl cellulose Babak Ghanbarzadeh a,, Hadi Almasi a, Ali A. Entezami b a Department of Food Science and Technology, Faculty of Agriculture, University of Tabriz, P.O. Box , Tabriz, Iran b Department of Polymer Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz, Iran article info abstract Article history: Received 28 June 2010 Received in revised form 14 October 2010 Accepted 16 October 2010 Available online xxx Keyword: Corn starch Citric acid Carboxymethyl cellulose WVP Tensile properties The films produced from pure starch are brittle and difficult to handle. Chemical modifications (e.g. crosslinking) and using a second biopolymer in the starch based composite have been studied as strategies to produce low water sensitive and relatively high strength starch based materials. A series of corn starch films with varying concentrations (0 20%, W/W) of citric acid (CA) and carboxymethyl cellulose (CMC) were produced by casting method. The effects of CA and CMC on the water vapor permeability (WVP), moisture absorption, solubility and tensile properties were investigated. The water vapor barrier property and the ultimate tensile strength (UTS) were improved significantly (p < 0.05) as the CA percentage increased from 0 to 10% (W/W). At the level of 15% (W/W) CMC, the starch films showed the lowest WVP values ( gpa 1 h 1 m 1 ) and UTS increased from 6.57 MPa for the film without CMC to MPa for that containing 20% CMC Published by Elsevier B.V. 1. Introduction Chemically synthesized polymeric films are widely used for packaging in food industry, because they are easily and inexpensively produced from uniform raw materials and are flexible as well as durable. A serious disadvantage of these films is that they are not biodegradable. The growth of environmental concerns over nonbiodegradable petrochemical-based plastics has raised interest in the use of biodegradable alternatives originating from renewable sources (Petersen et al., 1999; Weber et al., 2002). Starch is the most important polysaccharide polymer that is used to develop biodegradable films because it has capability of forming a continuous matrix and it is a renewable and abundant resource (Lourdin et al., 1995; Romero-Bastida et al., 2005; Bertuzzi et al., 2007; Talja et al., 2007). Nevertheless, starch exhibits several disadvantages such as a strong hydrophilic character (water sensitivity) and poor mechanical properties compared to conventional synthetic polymers (Averous and Boquillon, 2004), which make it unsatisfactory for some applications such as packaging purposes. Generally, many approaches are suggested to mitigate these shortcomings. One approach is the modification of starch. Chemical modification e.g. cross-linking has long been studied as a way to reduce these problems and to produce low water sensitive Corresponding author. Tel.: ; fax: addresses: Babakg1359@yahoo.com, Ghanbarzadeh@tabrizu.ac.ir (B. Ghanbarzadeh). and high strength materials (Dermigez et al., 2000; Kim and Lee, 2002). Examples of cross-linking reagents include glutaraldehyde (Ramaraj, 2007), boric acid (Yin et al., 2005), and epichlorohydrin (Sreedhar et al., 2006). However, some of these cross-linking agents always display toxicity and thus their potential applications as biomaterials are limited. To overcome these disadvantages, certain nontoxic functional additives and simple modification techniques are required to improve the mechanical properties and water resistibility of the starch films. Citric acid (CA) with one hydroxyl and three carboxyl groups exists widely in citrus fruits and pineapples, where it is the main organic acid. In this research work, CA was chosen as the additive for the following reasons. First of all, as a result of its multi-carboxylic structure, interaction could take place between the carboxyl groups of CA and the hydroxyl groups on the starch. Such an interaction would improve the water resistibility due to reducing available OH groups of starch (Borredon et al., 1997). On the other hand the carboxyl groups of CA can form stronger hydrogen bonds with the hydroxyl groups of starch molecules, so as to prevent recrystallization and retrogradation. Furthermore, because of the multi-carboxyl structure, CA may serve as a cross-linking agent and hence, it may improve the mechanical properties and water resistibility (Shi et al., 2007). And as a third point, CA is rated as nutritionally harmless since it is a nontoxic metabolic product of the body (Krebs or citric acid cycle) and it has already been approved by FDA for using in food formulations (Yang et al., 2004). Other approach to improve the functional properties of the starch films is to blend starch with other polymers. However, most /$ see front matter 2010 Published by Elsevier B.V. doi: /j.indcrop

2 2 B. Ghanbarzadeh et al. / Industrial Crops and Products xxx (2010) xxx xxx of the synthetic polymers are hydrophobic and thermodynamically immiscible with hydrophilic starch. For this reason, simple mixing will result in phase incompatibility and poor mechanical properties. Accordingly, the current research is focused on the use of biodegradable relatively hydrophilic polymers and fibers, which lead to the fabrication of highly environmental friendly biocomposites (Mohanty et al., 2002). Biocomposite materials are biodegradable polymers which their matrix reinforced with natural filler. Cellulose-based fibers are widely used as biodegradable filler (Dufresne et al., 2000; Averous et al., 2001; Lu et al., 2005). When natural fibers are mixed with starch, the mechanical properties of the resulted composite are obviously improved, because the chemical similarities of starch and plant fibers provide a good interaction (Averous et al., 2001; Lu et al., 2006). A significant improvement in water resistance is obtained by adding cellulose crystallites (Lu et al., 2005). Carboxymethyl cellulose (CMC) has no harmful effects on human health, and is used as highly effective additive to improve the product quality and processing properties in various fields of application, from foodstuffs, cosmetics and pharmaceuticals to products for the paper and textile industries (Ma et al., 2008). There are a few papers about the CMC/starch biocomposite properties (Ma et al., 2008). On the other hand, no specific studies about combined effect of CA and CMC on the functional properties of the starch films have been reported. The purpose of the present work was to evaluate the influence of the CA and CMC and the combined effects of them on the barrier and mechanical properties of the starch films. In the first step, the influence of CA on these properties was investigated and its optimum concentration was specified. In the second step, in the presence of fixed amount of CA, the influence of CMC on the barrier and mechanical properties of the starch films was investigated. 2. Materials and methods 2.1. Materials Corn starch (12% moisture) was provided by Glucosan Industry (Ghazvin, Iran). Glycerol, calcium sulfate and potassium sulfate (analytical grade) were purchased from Merck (Darmstadt, Germany). Carboxymethyl cellulose (CMC), with an average molecular weight of 41,000 (practical grade) was purchased from Caragum Parsian Corporation (Tehran, Iran). Citric acid (CA) (food grade) was procured from Tianjin Chemical Reagent Factory (Tianjin, China) Preparation of films Starch film preparation: Five grams of starch were mixed with distilled water (100 ml) and 2 ml glycerol (40%, V/W) and different CA concentrations (0, 5, 10, 15 and 20%, W/W starch) at room temperature (25 C) for 5 min. This suspension was transferred to a water bath at 90 C for 30 min, and agitated by magnetic stirrer (500 rpm). After cooling, about 50 ml of the sample was poured into a teflon casting tray (176 cm 2 area and 7.5 cm diameter) resulting in films with 0.08 ± 0.01 mm thickness, measured with an Alton M hand-held micrometer (China) having a sensitivity of 0.01 mm and then dried at 60 C in an oven to cast the films. Starch/CMC film preparation: Starch suspension containing 2 ml glycerol and 10% (W/W) CA was prepared as described above. CMC (0, 5, 10, 15 and 20%, W/W starch) was solubilized in 75 ml of water at 75 C for 10 min. CMC and starch solutions were mixed together and stirred at 75 C for 10 min. Dispersions were then cooled at 40 C and mixed gently for 20 min to release all air bubbles. Then, about 70 ml of the sample was poured into a teflon casting tray and dried at 60 C in an oven to cast the films Water vapor permeability (WVP) WVP tests were carried out by ASTM method E96-95 (1995) with some modifications (Mali et al., 2006). Special cups, with an average diameter of 2 cm and a depth of 4.5 cm were utilized to determine WVP of films. Films were cut into discs with a diameter slightly larger than the diameter of the cup. After placing 3 g of anhydrous CaSO 4 in each cup, they were covered with edible films of varying composition. RH 0 was maintained using anhydrous CaSO 4 in the cup. Each cup was placed in a desiccators containing saturated K 2 SO 4 solution in a small beaker at the bottom. A small amount of solid K 2 SO 4 was left at the bottom of the saturated solution to ensure that the solution remained saturated at all times. Saturated K 2 SO 4 solution in the desiccator provides a constant RH of 97% at 25 C. The desiccator was kept in an incubator at 25.0 ± 0.1 C. In first, cups were weighed every 2 h (along one day) and then measurement carried out every 12 h for 2 days and water vapor transport was determined by the weight gain of the cup. Changes in the weight of the cup were recorded as a function of time. Slopes were calculated by linear regression (weight change vs. time). The water vapor transmission rate (WVTR) was defined as the slope (g/h) divided by the transfer area (m 2 ). WVP (g. Pa 1 h 1 m 1 ) was calculated as: WVP = WVTR P(R 1 R 2 ) X (1) where P is the saturation vapor pressure of water (Pa) at the test temperature (25 C), R 1 is the RH in the desiccator, R 2, the RH in the cup and X is the film thickness (m). Under these conditions, the driving force is Pa. All measurements were performed in three replicates Moisture absorption Moisture absorption was measured according to the method of Angles and Dufrense (2000). The dried sheets of 20 mm 20 mm were first conditioned at 0% RH (CaSO 4 ) for 24 h. After weighing, they were conditioned in a desiccator containing Ca(NO 3 ) 2 saturated solution at C to ensure a relative humidity of 55%. The samples were weighed at desired intervals until the equilibrium state was reached. The moisture absorption of the samples was calculated as follows: moisture absorption (%) = W t W (2) W 0 where W t and W 0 are the weights of the sample after t time at 55% RH and the initial weight of the sample, respectively. All measurements were performed in three replicates Solubility in water Solubility in water was defined as the percentage of the dry matter of film which is solubilized after 24 h immersion in water (Shi et al., 2007). Film specimens were kept in a desiccator containing dry calcium sulfate till they reached constant weight. Then, about 500 mg of each film were immersed in beakers containing 50 ml of distilled water at 23 C for 24 h with periodical gentle manual agitation. The Films were removed from the water and were placed back in the desiccator until they reached a constant weight to obtain the final dry weight of the film. The percentage of the total soluble matter (%TSM) of the films was calculated using the following equation: initial dry wt final dry wt %TSM = 100 (3) initial dry wt TSM tests for each type of film were carried out in three replicates.

3 B. Ghanbarzadeh et al. / Industrial Crops and Products xxx (2010) xxx xxx 3 Fig. 1. WVP of the starch films as a function of CA content. Different letters show significantly difference (p < 0.05) Tensile properties Ultimate tensile strength (UTS) and strain at break (SB) of the films were determined at 21 ± 1 C using a tensile tester (Zwick/Roell model FR010, Germany) according to ASTM standard method D (ASTM D882-91, 1996). Three film specimens, 8cm 0.5 cm dumbbelly forms, were cut from each of film samples and were mounted between the grips of the machine. The initial grip separation and cross-head speed were set to 50 mm and 5 mm/min, respectively Statistical analysis Statistics on a completely randomized design were performed with the analysis of variance (ANOVA) procedure in SPSS (Version 11.5, SPSS Inc., Chicago, IL) software. Duncan s multiple range test (p < 0.05) was used to detect differences among mean values of films properties. 3. Results and discussion 3.1. Effect of citric acid Water vapor permeability (WVP) In food packaging applications, film should to avoid or at least decrease moisture transfer between food and surrounding atmosphere; water vapor permeability should be as low as possible. As shown in Fig. 1, the WVP of the plasticized starch film without CA (control sample) was gpa 1 h 1 m 1 and adding CA at all concentrations, caused to significantly decrease in the WVP of the films. The films containing 10% (W/W) CA showed significantly (p < 0.05) the lowest WVP value ( gpa 1 h 1 m 1 ). This could be attributed to the hydrophilic OH groups substitution with hydrophobic ester groups. On the other hand, the addition of CA probably introduced a tortuous path for water molecules to pass through. When the CA content of the films enhanced from 10% to 20% (W/W), the WVP values increased significantly from to gpa 1 h 1 m 1. This might be attributed to the plasticization effect of excessive CA. The residual-free CA in the biopolymer matrix may act as a plasticizer. Indeed, increasing CA content causes the increase in the interchain and spaces chain mobility due to inclusion of residual-free CA molecules between the polymer chains which in turn promotes water vapor diffusivity through the film and hence, it accelerates the water vapor transmission. This is in agreement with Ma et al. (2009) who investigated the effect of CA-modified starch on the WVP of the thermoplastic starch and reported that when filler content increased above 8% (W/W), the WVP values decreased gradually. Fig. 2. Moisture absorption of the starch films as a function of CA content Moisture absorption Because of the strong hydrophilicity of starch molecules, a pure starch film showed high water absorbency (31.67% is saturation limit). Incorporation of various amounts of CA into the starch matrix was able to reduce the water absorption of these films as shown in Fig. 2. Because of the multi-carboxyl structure of CA, it may act as a cross-linking agent. Cross-linking of starch macromolecules reinforces the intermolecular binding by introducing covalent bonds that supplement natural intermolecular hydrogen bonds so as to improve the water resistibility (Krumova et al., 2000). Approximately 8.5% reduction in total water absorption was achieved by a 5% CA addition in the film, and it was further reduced to more than 23% when the CA content reached to 10%. However, continuing to increase CA percentage in the film did not achieve any further improvement in water absorption. When the CA content of the films reached to 20%, the water uptake increased to 33.97%. This could be attributed to the plasticization effect of CA, as was previously mentioned Solubility in water The water solubility of the starch films as a function of CA content is depicted in Fig. 3. Incorporation of CA into the starch matrix (until 10%, W/W CA) was able to decrease the water solubility of the starch films. The %TSM was 26.64% for the control sample and decreased significantly (p < 0.05) to 23.76% for the films containing 10% (W/W) CA. The carboxyl groups of CA can form strong hydrogen bonds with the hydroxyl groups on starch, thus improving the interactions between the molecules and decreasing the water sensitivity (Angles and Dufrense, 2000). Increased concentrations of CA were able to increase the solubility of the film due to its plasticization properties. The highest concentration of CA (20%) induced the highest solubility (36.56%). Excessive CA probably could not inter- Fig. 3. Solubility of the starch films as a function of CA content. Different letters show significantly difference (p < 0.05).

4 4 B. Ghanbarzadeh et al. / Industrial Crops and Products xxx (2010) xxx xxx Fig. 5. WVP of the starch films as a function of CMC content. Different letters show significantly difference (p < 0.05). Fig. 4. Tensile properties of the starch films as a function of CA content, (a) stress vs. strain curves, (b) ultimate tensile strength and (c) strain at break. act with starch molecules. Therefore, might interact with water and interrupting the network by hydrogen bonds, reducing the cohesiveness of the starch matrix and increasing its solubility in water. The UTS and SB as the function of CA concentration are shown in Fig. 4b and c. By increasing CA content, the UTS of the films increased and SB decreased. At the level of 10% (W/W) CA, the starch films showed the highest UTS and lowest SB values (6.57 MPa and 66.18%, respectively). This could be attributed to the crosslinking caused by the small quantity of CA. As well as, CA can probably hydrolyze branched chains of starch molecule that induce formation of highly linear structure which in turn allow forming more hydrogen bonds between the starch chains and increasing the tensile strength in the resulted films. As the CA percentage was increased, the residual CA in the blends probably played a role as the plasticizer and reduced the interactions among the macromolecules, which in turn resulted in the decrease of the UTS and increase of the SB. With the increase of the CA concentration from 10% to 20%, the UTS reduced significantly (p < 0.05) from 6.57 to 1.80 MPa, however, the SB increased noticeably (p < 0.05) from 66.18% to 80.67%. While, CA at the concentrations more than 15%, probably weakened structure of film exceedingly which in turn caused the decrease in SB of films. Ning et al. (2007) reported similar results for the effect of CA on the mechanical properties of thermoplastic starch/linear lowdensity polyethylene blends. However, there are some reports that differ from our findings. For example, Shi et al. (2008) noticed that increasing CA concentration (from 5% to 30%) in polyvinyl alcohol/starch films, the UTS did not reduce significantly, however, the elongation at break increased noticeably. On the other hand, Ma et al. (2009) showed only cross-linking agent effects (increasing UTS and reducing SB) of CA at 9% concentration, on the mechanical properties of the thermoplastic pea starch. According to the mechanical and water vapor barrier results, 10% (W/W) CA, was the optimum concentration of CA to act as a cross-linking agent in the starch matrix and was selected to evaluate CMC effects on the aforementioned properties Tensile properties Mechanical properties resulted from the tensile test are shown in Fig. 4. The cross-linking agents and the plasticizers often have the contrary effects on the tensile properties. Generally, the tensile strength increased and the strain at break decreased as the percentage of cross-linking agent increased. The results are often opposite when the plasticizers are increased (Sreedhar et al., 2005). The CA acted both as a cross-linking agent and a plasticizer in the starch films; and these different functions of CA were observed varying the concentration of CA added. As shown in Fig. 4, the mechanical properties of the starch films were improved by increasing CA content to 10%, W/W. Stress strain curves showed the transition from ductile to plastic material behavior when CA concentration was increased to level of 15% and 20% Effect of CMC Water vapor permeability (WVP) WVP of the biocomposite films diminished with the increase of CMC content as can be seen in Fig. 5. The WVP was gpa 1 h 1 m 1 for the control sample (0%, W/W CMC and 10% CA) and decreased significantly to and gpa 1 h 1 m 1 for the films containing 10 and 15% (W/W) CMC, respectively. The film containing 15% (W/W) CMC has significantly (p < 0.05) the lowest WVP value. However, when the CMC content reached to 20%, W/W, films maintained their WVP. Reduction of WVP with increasing CMC content results in an improvement of the functional properties of these biocomposites, considering the hydrophilic characteristics of the matrix. The

5 B. Ghanbarzadeh et al. / Industrial Crops and Products xxx (2010) xxx xxx 5 Fig. 6. Moisture absorption of the starch films as a function of CMC content. Fig. 7. Solubility of the starch films as a function of CMC content. Different letters show significantly difference (p < 0.05). decrease of WVP by incorporation of fibers is in agreement with results usually reported for biocomposites which are studied for packaging applications (Arvanitoyannis and Biliaderis, 1999; Ma et al., 2008; Fama et al., 2009). Water resistance of CMC biopolymer is better than starch one (Ma et al., 2008). This could be attributed to the highly crystalline and hydrophobic character of the cellulose fibers in comparison to starch polymer. The addition of CMC could introduce a tortuous path for water molecule to pass through (Kristo and Biliaderis, 2007). At a low content of filler, CMC probably disperse well in the starch matrix, and blocks the water vapor transmission. However, additional amounts of CMC might congregate which decreased the effective contents of the CMC and facilitates the water vapor permeation through the film matrix. Fig. 8a show an improvement of the mechanical strength with the increase of CMC content. This is in agreement with Ma et al. (2008) who reported that a significant improvement of water resistance and mechanical properties of starch films was achieved by adding relatively small amounts of CMC (till 10%). The UTS and SB as the Moisture absorption Fig. 6 shows the moisture absorption of the starch films as a function of the CMC content. Comparing Figs. 2 and 6 can be observed that the moisture absorption of starch CMC biocomoposites was less than that of starch film, at an environmental relative humidity of 55%. These results indicated that the addition of CMC improve the water resistance of the cross-linked starch matrix. The reason could be that the starch is able to form hydrogen bonds with the hydroxyl and carboxyl groups of the CMC macromolecules and this strong structure could reduce the diffusion of water molecules in the material. With the increase of the CMC contents from 15% to 20%, the moisture absorption increased slightly from 20% to 21.33%. This result indicates that the combination of cellulosic fiber with starch improves water resistance to a certain degree since, as already known, CMC is hydrophilic material, although its hydrophilicity is lower than that of starch. Similar results were obtained by other researchers who observed a reduction in the water absorbed by biocomposites made with the addition of cellulosic fiber (Ban et al., 2006), hemicellulose (Gaspar et al., 2005) and jute strands (Vilaseca et al., 2007) to starch matrix Solubility in water The water solubility of the starch/cmc films as a function of CMC content is shown in Fig. 7. Addition of CMC, in all concentrations, decreased the water solubility of starch films. The %TSM was 23.76% for the samples without CMC, which decreased to 21.33% and 20.13% for the films containing 5 and 10% (W/W) CMC, respectively. However, a significant (p < 0.05) decrease in solubility observed at the high levels of CMC. At the level of 20% (W/W) CMC, the starch films showed the lowest %TSM values at 17.65% Tensile properties Fig. 8 shows the relationships between CMC content and the tensile properties of the biocomposite films. The curves reported in Fig. 8. Tensile properties of the cross-linked starch films as a function of CMC content, (a) stress vs. strain curves, (b) ultimate tensile strength and (c) strain at break.

6 6 B. Ghanbarzadeh et al. / Industrial Crops and Products xxx (2010) xxx xxx function of CMC concentration are shown in Fig. 8b and c. It was observed an important increase of two and threefold in the UTS when 15 and 20% (W/W) of CMC was added to the starch, respectively. With the increase of the CMC concentration from 0% to 20%, the UTS increased significantly (p < 0.05) from 6.57 to MPa. This was due to the interfacial interaction between the matrix and filler due to the chemical similarity (polysaccharide structure) of starch film and CMC. It was interesting that with the increase of the CMC concentration (from 0% to 20%), the UTS increased significantly (p < 0.05), however, the SB did not reduce considerably (it reduced from 66.18% to 58.49%). It seems that, CMC could improve the films strength without significant depressing effect on flexibility. The convenient cross-linking agents (ex. borax) (Sreedhar et al., 2005) and plasticizers (polyols) (Arvanitoyannis et al., 1997) often do not show the double function at relatively high concentrations. On the other hand, some researchers who examined the effect of cellulosic fibers on the mechanical properties of the starch films, reported different results from these work. The increasing UTS and reducing SB, are the common results reported as result of the incorporation of cellulosic fiber (Ban et al., 2006), hemicelluloses (Gaspar et al., 2005), jute strands (Vilaseca et al., 2007), Manila hemp fiber (Ochi, 2006) and vegetable fiber (Fama et al., 2009) to the starch matrix. Our different results may be attributed to the cross-linking effect of CA. In the presence of CA, the reinforcement effect of CMC is probably weakened, and it cannot interact tightly with starch matrix. Therefore, due to the creation of limited interactions, tensile strength of films increased maintaining their flexibility. 4. Conclusions Multi-carboxyl structure of CA could induce interaction between CA and starch which in turn improved the barrier properties, moisture absorption, solubility of films and UTS of the resulted films. However, increasing CA content probably causes the increase in chain mobility, interchain spaces and voids due to inclusion of residual-free CA molecules between the polymer chains which in turn promotes the water vapor transmission and SB values and decrease the UTS values. Generally, the effect of CA on the physical properties of starch films depends on its concentration. The addition of CMC improved the moisture resistance of the composites. By the addition of 15% (W/W) CMC, WVP, the moisture absorption and solubility of the resulted films decreased. Adding CMC (20%, W/W) could increase the UTS of the films without decreasing appreciably in SB Values. In the presence of CA, reinforcement effect of CMC was weakened probably due to poorly interact with starch matrix. At the level of 10% (W/W) CA, the UTS of the composite films also increased, however, when higher amount of CA was added (from 10 to 20%, W/W), the UTS decreased from 6.57 to 1.80 MPa and the SB increased from 66% to 80%. In the second step, CA content was fixed at 10% (W/W) and the effects of CMC levels on the properties of resulted polymer blends were investigated. At the level of 15% (W/W) CMC, the blend films showed the lowest WVP values. References Angles, M.N., Dufrense, A., Plasticized starch/tunicin whiskers nanocomposites. 1. Structural analysis. Macromolecules 33, Arvanitoyannis, I., Biliaderis, C.G., Physical properties of polyol-plasticized edible blends made of methyl cellulose and soluble starch. Carbohyd. Polym. 38, Arvanitoyannis, I., Kolokuris, I., Nakayama, A., Yamamoto, N., Aiba, S., Physico-chemical studies of chitosan poly (vinyl alcohol) blends plasticized with sorbitol and sucrose. Carbohyd. Polym. 34, ASTM E96-95, Standard Test Methods for Water Vapor Transmission of Material, Annual Book of ASTM. American Society for Testing and Materials, Philadelphia, PA. ASTM D882-91, Standard Test Methods for Tensile Properties of Thin Plastic Sheeting, Annual Book of ASTM. American Society for Testing and Materials, Philadelphia, PA. Averous, L., Boquillon, N., Biocomposites based on plasticized starch: thermal and mechanical behaviours. Carbohyd. Polym. 56, Averous, L., Fringant, C., Moro, L., Plasticized starch cellulose interactions in polysaccharide composites. Polymer 42, Ban, W., Song, J., Argyropoulos, D.S., Lucia, L.A., Improving the physical and chemical functionality of starch-derived films with biopolymers. J. Appl. Polym. Sci. 100, Bertuzzi, M.A., Armada, M., Gottifredi, J.C., Physicochemical characterization of starch based films. J. Food Eng. 82, Borredon, E., Bikiaris, D., Prinos, J., Panayiotou, C., Properties of fatty-acid esters of starch and their blends with LDPE. J. Appl. Polym. Sci. 65, Dermigez, P.D., Elvira, C., Mano, E.J.F., Cunha, S.H.A.M., Piskin, E., Reis, R.L., Chemical modification of starch based biodegradable polymeric blends: effects on water uptake, degradation behaviour and mechanical properties. Polym. Degrad. Stabil. 70, Dufresne, A., Dupeyre, D., Vignon, M.R., Cellulose microfibrils from potato tuber cells: processing and characterization of starch cellulose microfibril composites. J. Appl. Polym. Sci. 76, Fama, L., Gerschenson, L., Goyanes, S., Starch-vegetable fibre composites to protect food products. Carbohyd. Polym. 75, Gaspar, M., Benko, Zs, Dogossy, G., Reczey, K., Czigany, T., Reducing water absorption in compostable starch-based plastics. Polym. Degrad. Stabil. 90, Kim, M., Lee, S.-J., Characteristics of crosslinked potato starch and starch-filled linear low-density polyethylene films. Carbohyd. Polym. 50, Kristo, E., Biliaderis, C.G., Physical properties of starch nanocrystal reinforced films. Carbohyd. Polym. 29 (1), Krumova, M., Lopez, D., Benavente, R., Mijangos, C., Perena, J.M., Effect of crosslinking on the mechanical and thermal properties of poly(vinyl alcohol). Polymer 41, Lourdin, D., Valle, G.D., Colonna, P., Influence of amylose content on starch films and foams. Carbohyd. Polym. 27 (4), Lu, Y.S., Weng, L.H., Cao, X.D., Biocomposites of plasticized starch reinforced with cellulose crystallites from cottonseed linter. Macromol. Biosci. 5, Lu, Y.S., Weng, L.H., Cao, X.D., Morphological, thermal and mechanical properties of ramie crystallites-reinforced plasticized starch biocomposites. Carbohyd. Polym. 63, Ma, X., Chang, P.R., Yu, J., Properties of biodegradable thermoplastic pea starch/carboxymethyl cellulose and pea starch/microcrystalline cellulose composites. Carbohyd. Polym. 72, Ma, X., Chang, P.R., Yu, J., Mark Stumborg, M., Properties of biodegradable citric acid-modified granular starch/thermoplastic pea starch composites. Carbohyd. Polym. 75, 1 8. Mali, S., Grossmann, M.V.E., Garcia, M.A., Martino, M.N., Zaritzky, N.E., Effects of controlled storage on thermal, mechanical and barrier properties of plasticized films from different starch sources. J. Food Eng. 75, Mohanty, A.K., Misra, M., Drzal, L.T., Sustainable bio-composites from renewable resources: opportunities and challenges in the green materials world. J. Polym. Environ. 10 (1/2), Ning, W., Jiugao, Y., Xiaofei, M., Ying, W., The influence of citric acid on the properties of thermoplastic starch/linear low-density polyethylene blends. Carbohyd. Polym. 67, Ochi, S., Development of high strength biodegradable composites using Manila hemp fiber and starch-based biodegradable resin. Composites: A 37, Petersen, K., Nielsen, P.V., Bertelsen, G., Lawther, M., Olsen, M.B., Nilsson, N.H., Potential of biobased materials for food packaging. Trends Food Sci. Technol. 10, Ramaraj, B., Crosslinked poly (vinyl alcohol) and starch composite films. II. Physicomechanical, thermal properties and swelling studies. J. Appl. Polym. Sci. 103, Romero-Bastida, C.A., Bello-Perez, L.A., Garcia, M.A., Martino, M.N., Solorza-Feria, J., Zarintzky, N.E., Physicochemical and microstructural characterization of films prepared by thermal and cold gelatinization from non-conventional sources of starches. Carbohyd. Polym. 60, Shi, R., Bi, J., Zhang, Z., Zhu, A., Chen, D., Zhou, X., Zhang, L., Wei Tian, W., The effect of citric acid on the structural properties and cytotoxicity of the polyvinyl alcohol/starch films when molding at high temperature. Carbohyd. Polym. 74, Shi, R., Zhang, Z.Z., Liu, Q.Y., Han, Y.M., Zhang, L.Q., Chen, D.F., Characterization of citric acid/glycerol co-plasticized thermoplastic starch prepared by melt blending. Carbohyd. Polym. 69, Sreedhar, B., Chattopadhyay, D.K., Karunakar, M.S.H., Sastry, A.R.K., Thermal and surface characterization of plasticized starch polyvinyl alcohol blends crosslinked with epichlorohydrin. J. Appl. Polym. Sci. 101, Sreedhar, B., Sairam, M., Chattopadhyay, D.K., Syamala Rathnam, P.A., Mohan Rao, D.V., Thermal, mechanical, and surface characterization of starchpoly(vinyl alcohol) blends and borax-crosslinked films. J. Appl. Polym. Sci. 96,

7 B. Ghanbarzadeh et al. / Industrial Crops and Products xxx (2010) xxx xxx 7 Talja, R.A., Helén, H., Roos, Y.H., Jouppila, K., Effect of various polyols and polyol contents on physical and mechanical properties of potato starch-based films. Carbohyd. Polym. 67, Vilaseca, F., Mendez, J.A., Pelach, A., Llop, M., Canigueral, N., Girones, J., Turon, X., Mutje, P., Composite materials derived from biodegradable starch polymer and jute strands. Process Biochem. 42, Weber, C.J., Haugaard, V., Festersen, R., Bertelsen, G., Production and application of biobased packaging materials for the food industry. Food Addit. Contam. 19, Yang, J., Webb, A., Ameer, G., Novel citric acid-based biodegradable elastomers for tissue engineering. Adv. Mater. 16, Yin, Y., Li, J., Liu, Y., Li, Z., Starch crosslinked with poly(vinyl alcohol) by boric acid. J. Appl. Polym. Sci. 96,