Preparation and properties of pullulan–alginate–carboxymethylcellulose blend films

Food Research International 41 (2008) 1007–1014

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Preparation and properties of pullulan–alginate–carboxymethylcellulose blend films Qunyi Tong a,b, Qian Xiao b, Loong-Tak Lim c,* a

State Key Laboratory of Food Science and Technology, Jiangnan University, Jiangsu Wuxi 214122, China School of Food Science and Technology, Jiangnan University, Jiangsu Wuxi 214122, China c Department of Food Science, University of Guelph, Guelph, Ontario, Canada N1G 2W1 b

a r t i c l e

i n f o

Article history: Received 19 March 2008 Accepted 9 August 2008

Keywords: Edible films Pullulan Sodium alginate Carboxymethylcellulose Blend films

a b s t r a c t Pullulan, alginate, and carboxymethylcellulose (CMC) films were solvent cast from aqueous polymer solution. At 55% RH and 20 °C, their tensile strength and elongation at break were 67 MPa and 11%, 49 MPa and 5.2%, and 45 MPa and 5.8%, respectively. Pullulan films had lower water vapor permeability than alginate and CMC films (4.4  107, 9.7  107, and 1.3  106 g m/Pa h m2, respectively), but dissolved in water quicker than alginate and CMC films. By incorporating alginate and CMC into pullulan, water barrier and mechanical properties were weakened significantly. Blending pullulan with alginate or CMC up to about 17–33% (w/w total polymer) reduced film solubilization time in water. The addition of glycerol further reduced tensile strength, increased elongation at break, weakened water barrier properties, but enhanced solubilization in water. FTIR results indicated that blending pullulan with alginate and CMC resulted in weaker hydrogen bonds acting on –OH groups compared to the pure pullulan. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Edible films are thin layer of pre-formed materials which can be used as a film to separate food components, as a food wrap, as a carrier to deliver active compounds, or as package material to contain and protect food product. They are effective barrier to prevent unwanted mass transfers in foods (e.g., water vapor and oxygen transmission), thereby improving their quality and extending their shelflife. In some cases, the use of edible films also provides opportunity to simplify and/or down-gauge the synthetic secondary packaging (Chen, 1995; Diab, Biliaderis, Gerasopoulos, & Sfakiotakis, 2001; Kester & Fennema, 1986; Krochta & De Mulder-Johnston, 1997; Miller & Krochta, 1997; Seydim & Sarikus, 2006). By and large, edible films are derived from protein and polysaccharide. Because these biopolymers are hydrophilic, their mechanical and barrier properties tend to weaken when exposed to elevated relative humidity (Lim, Mine, & Tung, 1998, 1999; Yang & Paulson, 2000a). As a result, many studies aim at reducing the moisture sensitivity and enhancing the physical properties for these films, including the incorporation of hydrophobic components such as lipids into the film (McHugh & Krochta, 1994a; Perez-Gago & Krochta, 1999; Shellhammer & Krochta, 1997; Yang & Paulson, 2000b), blending with other less hydrophilic polymers (Dong, Wang, & Du, 2006; Erdohan & Turhan, 2005; Teramoto, Saitoh, Kuroiwa, Shibata, & Yosomiya, 2001; Wang, Yang, & Wang, 2003), chemical modification (Shingel, 2004; * Corresponding author. Tel.: +1 519 824 4120x56586; fax: +1 519 824 6631. E-mail address: [email protected] (L.-T. Lim). 0963-9969/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodres.2008.08.005

Teramoto et al., 2001; Xiao, Weng, & Zhang, 2002), and judicious use of plasticizer in the film formulation (Chick & Ustunol, 1998; McHugh & Krochta, 1994b). In some applications, rapid solubilization of film in water is desirable for certain products, such as mouth refresher film and dissolvable sachet for containment of pre-measured quantity of food ingredients. Here, moisture barrier and mechanical properties may not be critical since these films are protected by an outer secondary packaging. Pullulan films are highly water-soluble, colorless, tasteless, odorless, transparent, flexible, possess low permeability to oil and oxygen, and heat-sealable. These properties make them an ideal material for edible films and coatings, as well as a biodegradable and water-soluble packaging material (Deshpande, Rale, & Lynch, 1992; Singh, Saini, & Kennedy, 2008; Yuen, 1974). Pullulan is an exocellular homopolysaccharide produced by a fungus Aureobasidium pullulans (Fig. 1). It is a linear mix of a-D-glucan consisting mainly of maltotriose repeating units interconnected by a(1 ? 6) linkages (Saha & Zeikus, 1989). The regular alternation of a-(1 ? 4) and a-(1 ? 6) bonds results in distinctive structural flexibility and enhanced water-solubility (Leathers, 1993). Despite the many potential applications of pullulan, extensive use of this biopolymer is hampered by its high cost. One of the strategies to reduce the cost of pullulan-based films is to blend them with other compatible polymers that are abundant and lower in cost, such as sodium alginate and carboxymethylcellulose (CMC). Alginate is the water-soluble salt of alginic acid, a naturally occurring non-toxic polysaccharide found in brown algae (Al-Musa, Abu Fara, & Badwan, 1999; Rubio & Ghaly, 1994). It contains two uronic

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Q. Tong et al. / Food Research International 41 (2008) 1007–1014

Fig. 1. Structures of pullulan, alginate, and CMC polymers. Adapted from Trabelsi, Albouy, Imperor-Clerc, Guillot, and Langevin (2007), Gombotz and Wee (1998) and Teramoto et al. (2001).

acids, namely b-(1–4)-linked D-mannuronic acid (M) and a-(1–4) linked L-guluronic acid (G). The polysaccharide can exist in homopolymeric M–M and G–G forms, and an alternating sequence of M– G blocks (Dong et al., 2006; Yotsuyanagi, Yoshioka, Segi, & Ikeda, 1991). CMC is another water-soluble polymer. The semi-synthetic derivative of cellulose is produced by partial substitution of the 2, 3, and 6 hydroxyl groups of cellulose by carboxymethyl groups. CMC polymers are made up of linear b-(1 ? 4)-linked glycanes which exhibit polyelectrolyte characteristic due to the presence of weakly acidic groups (Chakraborty, Chakraborty, & Ghosh, 2006; Just & Majewicz, 1985). Because both alginate and CMC are water-soluble, they are compatible with pullulan. Moreover, the formation of hydrogen bonds between the COO groups of alginate and CMC with the – OH groups of pullulan may synergistically enhance the material properties of the resulting film. While there are numerous studies reported on the material properties for the respective pullulan, alginate, and CMC films (Kawahara et al., 2003; Lee, Chan, Dolzhenko, & Heng, 2006; Park, Weller, Vergano, & Testin, 1993), information on the composite films based on these polymers are lacking. In order to exploit the cost-reduction of pullulan film through blending with alginate and CMC, it is important to characterize their blends to ensure that the materials properties remain acceptable. Against this backdrop, the objectives of this study are to develop composite edible films by blending pullulan, alginate, and CMC, and to characterize their barrier and physical properties. 2. Materials and methods 2.1. Materials Pullulan PI20 (MW 200,000 Da) and CMC (FVH9-A) were donated by Hayashibara Biochemical Laboratory (Japan) and by Suz-

hou Elifa Chemical Company Ltd. (China), respectively. Sodium alginate was purchased from Sinopharm Group Chemical Reagent Co. Ltd. (China). 2.2. Preparation of blend films Pullulan, alginate, and/or CMC were mixed and dissolved in distilled water using a magnetic stirrer to form film-forming solutions of various blend weight ratios. All polymer solutions were prepared based on 6 g total polymer weight dissolved in 200 mL of distilled water. Selected films were plasticized by incorporating 1.5 g of glycerol into the polymer solution. This level of glycerol was chosen to produce flexible films without imparting excessive surface stickiness. To remove the entrapped air bubbles during mixing, the film-forming solutions were deaerated under vacuum with the aid of a vacuum pump (SHZ-D(III), Gongyi City Yuhua Instruments Co. Ltd., China). The polymer solutions were then cast onto leveled glass plates fitted with rims around the edge, followed by drying in an electrical blast drying chest (Shanghai Sanfa Scientific Instruments Co. Ltd., China) at 60 °C for 20 h. The resulting films were peeled off from the glass plate and conditioned at 20 ± 1 °C and 55 ± 1% RH prior to further testing. Film samples were tested in triplicate. 2.3. Conditioning All films were conditioned according to ASTM D618-05 (ASTM, 2005). Films for water vapor permeability (WVP) and mechanical testing were conditioned at 55 ± 1% RH and 20 ± 1 °C by placing them in a desiccator containing a saturated solution of Mg(NO3)2  6H2O for 72 h or longer. For all other tests, films were kept in plastic bags after peeling and stored in a desiccator maintained at 0% RH and 20 ± 1 °C until use.

Q. Tong et al. / Food Research International 41 (2008) 1007–1014

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2.4. Mechanical properties measurement

2.7. Water-solubility

Tensile strength (TS) and percentage elongation at break (EAB) were determined using a texture analyzer (TA-XT2i, Stable Micro Systems, Surrey, UK), operated according to the ASTM standard method D882-02 (ASTM, 2002). Three rectangular strip specimens (width 25.4 mm, length 80 mm) were cut from each film for tensile testing. Initial grip separation and cross-head speed were set at 60 mm and 1 mm/s, respectively. Film thickness was measured using a micrometer (Chengdu measure and knife Co. Ltd., China) to the nearest 0.001 mm at nine randomly selected film locations. Average thickness of the film strip was used to estimate the cross-sectional area of the sample. TS (MPa) was calculated by dividing the maximum load (N) by the cross-sectional area (m2):

To evaluate the solubility of film in water, specimens of 2 cm  2 cm were cut and dropped into a beaker containing 200 mL of distilled water (20 °C) agitated continuously with a magnetic stirrer. Concurrently, a stopwatch was started and the presence of the film was monitored visually. When the film dissolved completely, the stopwatch was stopped. The time taken for the film to dissolve was reported as the solubilization time.

TS ¼

P ðb  dÞ

ð1Þ

where P is the maximum load (N), b is the width of sample (mm), and d is the film thickness (mm). Percentage EAB was calculated as follows:

EAB ¼

ðL  L0 Þ  100 L0

ð2Þ

where L0 is length of sample before deformation and L is the length of sample at failure. 2.5. Water vapor permeability

2.8. FTIR analysis FTIR spectra of the films were recorded in attenuated total reflection (ATR) mode using a FTIR spectrometer (Shimadzu Corporation, Tokyo, Japan). The films were applied directly onto the KRS5 crystal (ATRMax II, Pike Technologies, Madison, WI) and scanned from 700 to 4000 cm1 at 4 cm1 resolution. Each spectrum represented an average of 50 consecutive scans. 2.9. Statistical analysis Analysis of variance and Duncan multiple-range tests were conducted using SAS program (Statistical Analysis System Inst. Inc., Cary, NC, USA) at p < 0.05. Each experiment was replicated three times. 3. Results and discussion 3.1. Mechanical properties

Water vapor transmission rate (WVTR) of the film specimens was measured according to modified ASTM E96 method (ASTM, 1993; Ou, Kwok, & Kang, 2004). Glass cups with diameter of 3 cm and depth of 4 cm were used. To maintain 0% RH in the cup headspace, 3 g of dried CaCl2 were added to the cup, followed by sealing the film over the rim of the cup by applying molten paraffin. The cups were placed in hermetically sealed jars maintained at 20 °C and 100% RH. The RH was maintained by placing 1000 mL of water in the bottom of the jar. The cups were weighed every 12 h for 1 week. The amount of water permeated through the films was determined from the weight gain of the cups. WVTR and water vapor permeability (WVP) were calculated according to:

WVTR ¼ Dw=Dt  A

ð3Þ

WVP ¼ WVTR  L=Dp;

ð4Þ

where WVTR is in g/h m2, Dw/Dt is rate of water gain in g/h, A is the exposed area of the film in m2, L is the mean thickness of film specimens in m, and Dp is the difference in partial water vapor pressure between the two sides of film specimens in Pa. The water vapor pressure on the high-stream side of the film was 2.34 kPa (i.e., saturated water vapor pressure at 20 °C), while the low-stream side is assumed to be zero. 2.6. Scanning electron microscopy (SEM) A scanning electron microscope (SEM) (Model S-570 SEM, Hitachi Ltd., Tokyo, Japan) was used to examine the representative regions of film surface and cross-section. Films were conditioned in a desiccator at 20 ± 1 °C before measurement. Samples were attached to double-sided adhesive tape and then mounted on the specimen holder. The samples were sputter coated with 10 lm thickness of gold under vacuum. The sputtered coated film samples were scanned with an accelerating beam voltage of 10 kV.

TS measures film strength, whereas EAB is an indicator of toughness and stretch-ability prior to breakage. These parameters dictate the end-use handling properties and mechanical performance of the films. TS and EAB data for pullulan, alginate, CMC and their blends are summarized in Figs. 2 and 3. TS and EAB values for pullulan films were 67 MPa and 11%, respectively. In contrast, alginate and CMC films had weaker TS and lower EAB, at 49 and 45 MPa, and 5.2% and 5.8%, respectively. The greater flexibility of pullulan may be caused by the presence of a-(1 ? 6) glycosidic linkages which are absent in CMC and alginate polymer chains (Shingel, 2004). In the absence of glycerol, the tensile strength for pullulan–alginate blend films decreased from 67 to 49 MPa as the alginate content increased (Fig. 2). Similar decrease was observed for pullulan–CMC films when the CMC content increased (from 67 to 45 MPa). It is noteworthy that the reduction in TS was not monotonous for both blended films; below 50% blend ratio, the incorporation of alginate or CMC to pullulan did not result in significant changes on TS. These data showed that alginate and CMC can be added to pullulan, up to about 50% (w/w) level, without significantly affecting the mechanical properties of the resulting composite film. In contrast, the TS and EAB values for the three-component films were not significantly different (p > 0.05) for all the three blend ratios tested (Figs. 4 and 5), even when alginate + CMC was increased to 67% content (2:2:2 pullulan:alginate:CMC). On average, the TS and EAB were 67 MPa and 10%, respectively, which were comparable to the pure pullulan films. This suggested that pullulan films blended with alginate + CMC were stronger than those blended with alginate or CMC alone. Incorporation of glycerol markedly reduced TS and increased EAB for all films tested. As shown in Figs. 2 and 3 (data points indicated by arrows), glycerol had a greater plasticization effect on pullulan compared to alginate and CMC. The addition of 1.5 g glycerol caused a dramatic TS drop from 67 to 24 MPa for pure pullulan film, but less change was observed for alginate (from 49 to 37 MPa)

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Q. Tong et al. / Food Research International 41 (2008) 1007–1014 60 Tensile Strength Elongation

50

70

70 40 60 30 50

Elongation, %

Tensile strength, MPa

80

20 40 10

30

60 50 40 30 20 10 0

0

20 0

10

20

30

40

50

60

70

80

90

No glycerol +1.5 g glycerol

80

Tensile strength, MPa

90

Pul+Alg+CMC (2g 2g 2g)

100

Pul+Alg+CMC (3g 1g 2g)

Pul+Alg+CMC (3g 2g 1g)

Alginate content in pullulan, % wt/wt Fig. 2. Tensile strength and elongation of films prepared from pullulan, alginate, and their blends in the absence of glycerol plasticizer. Polymer solutions were prepared from 6 g total polymer weight dissolved in 200 mL of distilled water. Arrows indicate tensile properties of pullulan and alginate films plasticized with 1.5 g of glycerol.

Tensile Strength Elongation

80

60

50

50

40

70 40 60 30 50

Elongation, %

Tensile strength, MPa

60

20 40

Elongation, %

90

Fig. 4. Tensile strength of three-component films, showing the effect of glycerol for three film formulations.

No glycerol +1.5 g glycerol

30

20

10

10

30

0 Pul+Alg+CMC (2g 2g 2g)

Pul+Alg+CMC (3g 1g 2g)

Pul+Alg+CMC (3g 2g 1g)

0

20 0

10

20

30

40

50

60

70

80

90

100

CMC content in pullulan, % wt/wt Fig. 3. Tensile strength and elongation of films prepared from pullulan, CMC, and their blends in the absence of glycerol plasticizer. Polymer solutions were prepared from 6 g total polymer weight dissolved in 200 mL of distilled water. Arrows indicate tensile properties of pullulan and CMC films plasticized with 1.5 g of glycerol.

and CMC (from 45 to 55 MPa). The corresponding increases of EAB for pullulan, alginate, and CMC were from 11 to 53%, from 5 to 33%, and from 6 to 39%, respectively. Similar trends were observed for the three-component films. Here, TS dropped from 67 to 29 MPa and EAB increased from 10% to 45% when 1.5 g glycerol was incorporated (Figs. 4 and 5). The plasticization effects of glycerol in biopolymers have been well-documented in the literature (Chen & Zhang, 2005; Lim et al., 1998, 1999; McHugh & Krochta, 1994b; Wang et al., 2003). In general, it is believed that glycerol enhanced molecular mobility by acting as a lubricant between the polymer chains. Although this plasticization effect may be useful to impart film flexibility, the amount of glycerol added should be minimized since the glycerol-plasticized films tend to be sticky which may present end-use challenges. 3.2. Water vapor permeability (WVP) The WVP data for pure and blended films are summarized in Fig. 6. As shown, pullulan film had lower WVP (4.4  107 g m/

Fig. 5. Elongation at break of three-component films, showing the effect of glycerol for three film formulations.

Pa h m2) compared to alginate (9.7  107 g m/Pa h m2) and CMC (1.3  106 g m/Pa h m2) films. Adding alginate or CMC to pullulan resulted in an increased WVP of the resulting composite films, probably due to the increased free-volume of the composite matrix caused by the bulkier anionic side groups of CMC and alginate. The addition of 1.5 g of glycerol resulted in a dramatic increase in WVP values for all films tested (Fig. 6; data pointed with arrows). This can be attributed to the increased film hydrophilicity caused by the added polyol, and the enhanced polymer chain mobility due to the plasticization effect of glycerol which increased the diffusivity of water molecules in the film matrix. Among the three tested films, pullulan had the greatest increase in WVP when plasticized with glycerol, suggesting that pullulan was more sensitive to the plasticization effect of glycerol than alginate and CMC. This agreed with the tensile data which showed that glycerol-plasticized pullulan film was the weakest (lowest TS and highest EAB) among the three films (Figs. 2 and 3). For the three-component films, all the three blend ratios tested had similar WVP values before and after plasticization with glycerol (Fig. 7). Overall, these films also had weaker water barrier properties than the two-component films (Fig. 6). Addition of 1.5 g of glycerol to the three-component films caused a significant increase of WVP from 1.3  106 to 4.2  106 g m/Pa h m2.

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Q. Tong et al. / Food Research International 41 (2008) 1007–1014 300

5.00E-06

Alginate blends

4.50E-06 250

4.00E-06

Solubilization time, s

3.50E-06

WVP, g.m/Pa.h.m

2

1.50E-06

CMC blends 1.20E-06

9.00E-07

200 CMC blends 150

100

Alginate blends 50

6.00E-07 0 0

3.00E-07 0

10

20

30

40

50

60

70

80

90

10

20

30

40

50

60

70

80

90

100

Alginate or CMC content in pullulan, % wt/wt

100

Alginate or CMC content in pullulan, % wt/wt Fig. 6. Water vapor permeability (WVP) values of films prepared from pullulan, CMC, and their blends in the absence of glycerol. Polymer solutions were prepared from 6 g total polymer weight dissolved in 200 mL of distilled water. Arrows indicate tensile properties of pullulan and CMC films plasticized with 1.5 g of glycerol.

Fig. 8. Effects of alginate and CMC on solubilization time for pullulan in the absence of glycerol. Polymer solutions were prepared from 6 g total polymer weight dissolved in 200 mL of distilled water. Arrows indicate water-solubility of pullulan and CMC films plasticized with 1.5 g of glycerol.

180

No glycerol +1.5 g glycerol

Solubilization time, s

4.5E-06 4.0E-06

WVP, g.m/Pa.h.m

2

3.5E-06 3.0E-06 2.5E-06 2.0E-06

No glycerol +1.5 g glycerol

160 140 120 100 80 60 40

1.5E-06

20

1.0E-06

0 Pul+Alg+CMC (2g 2g 2g)

5.0E-07 0.0E+00

Pul+Alg+CMC (2g 2g 2g)

Pul+Alg+CMC (3g 1g 2g)

Pul+Alg+CMC (3g 2g 1g)

Fig. 7. Water vapor permeability (WVP) values of three-component films, showing the effect of glycerol for three film formulations.

3.3. Water-solubility of films The effects of alginate and CMC on the solubility of pullulan in water are summarized in Fig. 8. The solubility data showed that pullulan films dissolved much faster in water than alginate and CMC films. When alginate or CMC was added to pullulan at 17– 33% level, alginate and CMC enhanced the solubility of the composite film. The quicker dissolution behavior could be caused by the more open structure induced by the added alginate or CMC, making the composite film matrices more accessible to water compared to the pure pullulan film. However, as the concentration of alginate or CMC increased beyond the 33% level, the time taken to achieve complete solubilization increased significantly. It is interesting to note that the added alginate and CMC had less effect on increasing the solubility time for the three-component films compared to the two-component counterparts. For instance, when a combined 50% of alginate and CMC was added to the formulation (3 g + 1 g + 2 g and 3 g + 1 g + 2 g pullulan + alginate + CMC), the

Pul+Alg+CMC (3g 1g 2g)

Pul+Alg+CMC (3g 2g 1g)

Fig. 9. Time taken for complete solubilization in water for three-component composite films, showing the effect of glycerol for three film formulations.

solubilization time was around 120 s (Fig. 9). In contrast, when either alginate or CMC was blended at 50% level with pullulan, the solubilization time was extended to about 150 s (Fig. 8). Thus, the three-component formulation appeared to be more effective for maintaining the solubility of edible films, with minimal impact on mechanical properties (Figs. 2–4). Nevertheless, the WVP values for the three-component films (Fig. 7) were generally higher than the two-component films (Fig. 6), which must be taken into consideration in end-use applications where water barrier are important. The addition of plasticizers also decreased the solubilization time significantly for all films tested (data points indicated by arrows in Figs. 8 and 9). This is consistent with the greater WVP values for glycerol-plasticized films than un-plasticized counterparts (Figs. 6 and 7). The decreased solubilization time is probably due to the increased film hydrophilicity caused by the hydroxyl groups from the added glycerol. As glycerol is highly water-soluble, its extraction from the film into water can also resulted in a more open polymer matrix which facilitated the diffusion of water into the film, thereby enhancing the solubility of film.

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Fig. 10. Surface and cross-section morphologies of pullulan, alginate, CMC, and blended films.

3.4. Scanning electron microscopy

the films showed different morphologies depending on the film types. The pullulan film showed a compact structure with occasional pores in the film matrix which may be due to the entrapped air bubbles. Alginate exhibited rougher matrix morphologies. CMC showed similar compact morphologies as pullulan, except that clumps were evident in the micrographs which were visually dis-

Fig. 10 shows the representative electron scanning micrographs of outer surface and cross-section for different film specimens. Overall, the surface morphologies for all films showed continuous structures without cracks or pores. However, the cross-sections of 3.8 3.6

1078

3.4 1599

3.2 3

1150

1412

1152 1593

2.6

1412

1321

3292

Absorbance

1603

2.2

1148

1410

3262

0P-0A-6C

1589

1.8

1412 1321

3296

1.4

1599

0P-6A-0C 1408

1.2 1

3273

1078

0.8 0.6

3P-3A-0C

1078

2.4

1.6

3P-0A-3C

3281

2.8

2

2P-2A-2C

1078

6P-0A-0C

1150 3314

1412

1607

0.4 0.2

3292

2909

0 3700 3450 3200 2950 2700

1600

1600

1413

1500

1400

1150

1322

1300

1200

1079

1100

850 816

1000

900

800

700

Wavenumber, cm-1 cm-1 Wavenumber, Fig. 11. FTIR spectra of pullulan, alginate, CMC, and blended films. 2P–2A–2C: 2 g pullulan + 2 g alginate + 2 g CMC; 3P–0A–3C: 3 g pullulan + 0 g alginate + 2 g CMC; 3P–3A– 0C: 3 g pullulan + 3 g alginate + 0 g CMC; 0P–0A–6C: 0 g pullulan + 0 g alginate + 6 g CMC; 0P–6A–0C: 0 g pullulan + 6 g alginate + 0 g CMC; 6P–0A–0C: 6 g pullulan + 0 g alginate + 0 g CMC. Spectra are offset for clarity.

Q. Tong et al. / Food Research International 41 (2008) 1007–1014

cernable as tiny gel-like particles. The micrograph for the threecomponent film was homogeneous without signs of phase separation between the components, indicating that the three polymers are physically compatible with each other. This agreed with the visual appearance of the three-component films that they are transparent with a light yellow tint inherent to alginate. Note that the multiple ridges on the micrograph for the blended film were likely the artefacts created due to cutting of film specimen. 3.5. FTIR analysis FTIR spectra for pullulan, alginate, and CMC films showed two peaks around 2909 cm1 due to C–H stretching (Fig. 11). The broad band located around 3293 cm1 coexisted in all films was caused by O–H stretching and intermolecular/intramolecular hydrogen bonds. The O–H stretching for pullulan, alginate, and CMC films occurred at 3314, 3273, and 3296 cm1, respectively. By blending pullulan with alginate and CMC, the O–H band of the resulted films shifted to lower wavenumbers (3262 and 3292 cm1 for pullulan– alginate and pullulan–CMC, respectively). This shift indicated that the hydrogen bonds acting on the –OH groups for the blends were weaker compared to the pure polymers films. Unique spectral features were observed among the different films. Sodium alginate and CMC films exhibited peaks around 1600 and 1413 cm1, which have been assigned to antisymmetric and symmetric vibrations for COO group, respectively (Gunzler & Gremlich, 2002; Xiao et al., 2002). These peaks were absent for pullulan film which is in accordance with its polymer structure. The interaction of pullulan with alginate and CMC was elucidated by the shift in wavenumber for these two peaks. Adding pullulan caused the COO band to shift from 1599 to 1603 cm1, and from 1589 to 1593 cm1 for alginate and CMC, respectively. This provided an evidence that the antisymmetric and symmetric vibrations of C@O and C–O bonds were enhanced, probably due to the disruption of intermolecular hydrogen bonds present originally between the carboxylic groups caused by added pullulan. The symmetric COO stretching around 1413 cm1 showed a similar trend, although the wavenumber shifts were less obvious. CMC also showed a unique band around 1322 cm1. Although this peak has been assigned to –OH bending vibration (Biswal & Singh, 2004; Pushpamalar, Langford, Ahmad, & Lim, 2006), the origin of this peak was unclear since this band was absent for both alginate and CMC. Furthermore, the wavenumber for this band was not affected by the addition of pullulan. In general, the peak near 1150 cm1 is believed to be originated from glycosidic linkages in polysaccharides (Kacurakova, Capek, Sasinkova, Wellner, & Ebringerova, 2000; Nikonenko, Buslov, Sushko, & Zhbankov, 2000; Sekkal, Dincq, Legrand, & Huvenne, 1995). Sekkal et al. (1995) assigned this band to antisymmetric a-(1 ? 4) stretching mode of the glycosidic linkage. This appeared to be consistent with our observations, specifically for pullulan which showed an absorbance at 1150 cm1, probably due to its a-(1 ? 4) glycosidic linkages. However, closer examination of IR spectrum for CMC (Fig. 11) revealed that CMC also absorbed IR energy at this wavenumber, although at a much weaker intensity than pullulan. This suggested that 1150 cm1 band might not be solely caused by the a-(1 ? 4) glycosidic linkage, since CMC adopted a different linkage (i.e., b-(1 ? 4)) between the sugar monomer units. Furthermore, the wavenumber of this band for pullulan shifted from 1150 to 1148 cm1 after blending with alginate, indicating that other vibration modes may be at play. Based on the proposal by Bose and Polavarapu (2000) that 1150 cm1 is related to exocyclic C–O stretching vibrations, and our observation that this band was not detected in alginate, we speculated that this band may be caused by the C–OH group at the C6 position. This assignment seems to be reasonable since there is a larger propor-

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tion of C–OH group at the C6 position for pullulan compared to the substituted CMC. Another band which was unique to pullulan was located at 1079 cm1. Shingel (2002) observed that this band was much stronger in pullulan than dextran, and speculated that it was due to C–OH stretching at the C6 position for pullulan, based on the fact that all C6 atoms for dextran participated in the formation of the glycosidic C6–O–C1 linkages (Shingel, 2002). This band was absent in the spectra for alginate and CMC which is consistent with Shingel’s observation. It is noteworthy that the wavenumber for this pullulan band remained at 1079 cm1 after blending with alginate and CMC, indicating the added polymers did not affect vibration behavior for the C–OH group. 4. Conclusion Overall, pullulan has higher tensile strengths, longer elongation at break, better water barrier properties, and shorter solubilization time in water, than both alginate and CMC. Adding either alginate or CMC to pullulan, especially beyond 50% (w/w) level, significantly compromised the mechanical performance and water-solubility of the resulting composite films. However, blending pullulan with alginate + CMC to form three-component films allowed the incorporation of a greater proportion of filler polymers with lesser performance losses. The incorporation of glycerol decreased tensile strength, increased elongation at break, weakened water barrier properties, and enhanced water-solubility of all films tested. Based on the homogeneous microstructures observed in SEM analysis and the wavenumber shifts detected from FTIR investigation, it can be concluded that both alginate and CMC are compatible with pullulan. Through judicious blending of alginate and CMC with pullulan, and the use of glycerol as a plasticizer, composite films of optimal mechanical, barrier and water-solubility can be achieved for various end-use applications. Considering that alginate and CMC are less costly than pullulan, these biopolymers may be useful to reduce the cost of pullulanbased edible films. Acknowledgements The authors gratefully acknowledge the financial support from National High Technology Research and Development Program of China (863) (No. 2007AA10Z362). This research was also supported by the China Scholarship Council. References ASTM E96. (1993). Standard test method for water vapor transmission of materials. Philadelphia: American Society for Testing and Materials. ASTM D882-02. (2002). Standard test method for tensile properties of thin plastic sheeting. Philadelphia: American Society for Testing and Materials. ASTM D618-05. (2005). Standard practice for conditioning plastics and electrical insulating materials for testing. Philadelphia: American Society for Testing and Materials. Biswal, D. R., & Singh, R. P. (2004). Characterization of carboxymethyl cellulose and polyacrylamide graft copolymer. Carbohydrate Polymers, 57, 379–387. Bose, P. K., & Polavarapu, P. L. (2000). Evidence for covalent binding between copper ions and cyclodextrin cavity: A vibrational circular dichroism study. Carbohydrate Research, 323, 63–72. Chakraborty, T., Chakraborty, I., & Ghosh, S. (2006). Sodium carboxymethylcellulose–CTAB interaction: A detailed thermodynamic study of polymer–surfactant interaction with opposite charges. Langmuir, 22, 9905–9913. Chen, H. (1995). Functional properties and applications of edible films made of milk proteins. Journal of Dairy Science, 78, 2563–2583. Chen, P., & Zhang, L. (2005). New evidence of glass transitions and microstructures of soy protein plasticized with glycerol. Macromolecular Bioscience, 5, 237–245. Chick, J., & Ustunol, Z. (1998). Mechanical and barrier properties of lactic acid and rennet-precipitated casein-based edible films. Journal of Food Science, 63, 1024–1027. Diab, T., Biliaderis, C. G., Gerasopoulos, D., & Sfakiotakis, E. (2001). Physicochemical properties and application of pullulan edible films and coatings in fruit preservation. Journal of the Science of Food and Agriculture, 81, 988–1000.

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