Mechanical properties, hydrophilicity and water activity of starch-gum films: effect of additives and deacetylated xanthan gum

Food Hydrocolloids 19 (2005) 341–349 www.elsevier.com/locate/foodhyd

Mechanical properties, hydrophilicity and water activity of starch-gum films: effect of additives and deacetylated xanthan gum P. Veiga-Santosa,*, L.M. Oliveirab, M.P. Ceredac, A.J. Alvesd, A.R.P. Scamparinia a

DCA-FEA-UNICAMP, P.O. Box 6121, 13081-970, Campinas, SP, Brazil b CETEA-ITAL, P.O. Box 139, 13.073-001, Campinas, SP, Brazil c FCA-UNESP, P.O. Box 237, 18.603-970, Botucatu, SP, Brazil d 3M, P.O. Box 123, Sumare´, SP, Brazil Received 22 April 2004; revised 29 July 2004; accepted 30 July 2004

Abstract The effect of deacetylated xanthan gum, additives (sucrose, soybean oil, sodium phosphate and propylene glycol) and pH modifications on mechanical properties, hydrophilicity and water activity of cassava starch–xanthan gum films has been studied. Sucrose addition resulted in the highest effect observed on cassava starch films elongation at break. The deacetylated xanthan gum had higher effect on elongation at break when comparing to the acetylated gum, although both gums presented an inferior effect in relation to the obtained with sucrose. However, when comparing to the control and PVC films, lower tensile strength resistance values were observed when adding sucrose. Increased water activity was observed for films added with sucrose, thus, increasing the material biodegradation. Sucrose and deacetylated xanthan gum addition resulted in a slight hydrophilicity increase. q 2004 Elsevier Ltd. All rights reserved. Keywords: Biofilms; Mechanical properties; Water activity; Hydrophilicity

1. Introduction Starch is considered one of the most promising natural polymers for packaging application (Guilbert, Gontard, & Gossis, 1996; Krochta & Mulder-Johnston, 1997) because of their low cost, renewability and biodegradability (Ave´rous, Fringant, & Moro, 2001). Unfortunately, there are some strong limitations for developing starch-based products, due to its poor mechanical properties and high moisture sensitivity (Martin, Schwach, Ave´rous, & Couturier, 2001). To overcome these weaknesses, association with additives (Coupland, Shaw, Monahan, O’riordan, & O’sullivan 2000; Garcia, Martino, & Zaritzki, 2000) and different process parameters (Gontard, Guilbert, & Cuq, 1992) are recommended. Although, starch materials elongation at break and tensile strength rather values up to * Corresponding author. Address: UNICAMP, Department of Food Science, R. Maetro Torquato Amore, 332, apto 72A, 05622050, SP, Brazil. Tel.: C55-1937-882151; fax: C55-1937-882153. E-mail address: [email protected] (P. Veiga-Santos). 0268-005X/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodhyd.2004.07.006

6% and 40–50 MPa, respectively (Lourdin, Coignard, Bizot, & Colonna, 1997), justifying the continuance of searching for additives and process parameters that may increase starch films feasibility. Hydrocolloid increases starch systems viscosity (Baird & Pettitt, 1991) and decreases retrogradation rate (Yoshimura, Takaya, & Nishinari, 1999). Starch and hydrocolloids (such as xanthan gum) simultaneous utilization in films can modify the material mechanical resistance. Xanthan gum is the most important commercial microbial hydrocolloid and exhibits synergism interaction with starch (Katzbauer, 1997). Is largely used as thickening, stabilizer (Katzbauer, 1997) and structuring agent in different technology areas (Capron, Brigand, & Muller, 1997). Its primary structure consists of a linear bK1,4 linked D-glucose chain substituted on every second glucose residues by charged trisaccharide side chain containing a glucuronic acid residue between two mannose residues. The inner mannose residue is normally acetylated at C(6) (Pettitt, 1982), located on the periphery of the helix, preventing aggregation. Removal of these groups, allows the polymer chain to associate

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(Morris, Gothard, Hember, Manning, & Robinson, 1996). The utilization of deacetylated xanthan gum as additive could, therefore, affect biofilms structure. Chen and Nusinovitch (2000, 2001) have already introduced xanthan gum into wax-based coatings, however, xanthan gum application on hydrophilic films have not been investigated. Neither the potential of deacetylated xanthan gum has been explored as additive for biodegradable materials. Sucrose had demonstrated a higher plasticizing efficacy when comparing to sorbitol and glycerol (Arvanitoyannis, Psomiadou, & Nakayama, 1996). Thus, demonstrating that sucrose, which has a low coast when comparing to polyols, can also be a good alternative for increasing mechanical resistance of biodegradable films. Although propylene glycol is also considered a plasticizer (Lacroix, Jobin, Mezgheni, Srour, & Boileau, 1998), its addition in low concentrations may prevent possible sucrose crystallization due to its humectant character, justifying the attempt in investigating such polyol effect on starch-gum films physical properties. Mechanical properties can also be improved by phosphate (Mezgheni, Vachon, & Lacroix, 2000) and propylene glycol addition. Moisture sensitivity of hydrophilic films may be reduced by addition of hydrophobic materials (Garcia et al., 2000; Yang & Paulson, 2000). However, when waxes and long chain saturated fatty acids and fatty alcohols are utilized, a reduction on the optic properties of the films may be observed (Yang & Paulson, 2000). In attempt to avoid or diminished optical interferences due to lipids addition, the utilization of unsaturated lipid additives such as soybean oil would be more recommended. With the aim to evaluate the simultaneous and isolate effect of deacetylated xanthan gum, additives (sucrose, propylene glycol, sodium phosphate and soybean oil) and pH modifications on cassava starch films; tensile strength, elongation at break, contact angle measurement and water activity were investigated.

2. Material and methods 2.1. Materials Commercial cassava starch (Flor de Lotus, SP, Brazil), acetylated xanthan gum-rodigel (Rhodia, SP, Brazil), deacetylated xanthan gum (CP Kelco S.A., SP, Brazil), soybean oil (Cargill Agricola S.A., SP, Brazil) and polyvinyl cloride (obtained at Campinas local market, SP, Brazil). Analytically pure sucrose, propylene glycol and monobasic sodium phosphate (Synth, SP, Brazil). 2.2. Sample preparation Cassava starch (3–5% w/w) was blended with water and the additives: acetylated or deacetylated xanthan gum

(0–1% w/w), sucrose (0–2% w/w), propylene glycol (0–1% w/w), sodium phosphate (0–0.2% w/w) and soybean oil (0-0.06% w/w), considering the total weight of the film forming solution. Medium pH was adjusted (4–8) with 50% citric acid solution or 5% sodium hydroxide solution, heated to 75 8C with constant stirring and placed under vacuum (30 min), to remove bubbles that could become pinholes after film drying. The films were prepared according to the casting technique, by dehydrating 30 g of the film forming solution over Petri plastic dishes, under renewable circulated air (308G 2 8C). Films containing 5% cassava starch (no additives or pH adjustment), served as the control. Samples were storage (23 8C, 75% RH) for at least 4 days prior to testing. Since starch films have a hydrophilic character (Ave´rous, Moro, Dole, & Fringant, 2000), 75% relative humidity was chosen to conditioning the experimental films in order to evaluate the material performance on high content moisture environment. Although the relative humidity around 50% is more often utilized (Ave´rous et al., 2001; Gontard et al., 1992) to preconditioning biodegradable films, 75% RH is also utilized (Ruotsalainen, Heina¨ma¨ki, Guo, Laitinen, & Yliruusi, 2003; Trezza & Krochta, 2000; Wu, Sakabe, & Isobe, 2003). 2.3. Mechanical properties Films tensile strength and elongation at break were measured using an Instron Universal Testing Instrument (model 5500R) operated according to ASTM standard method D882-00 (ASTM, 2001). Tested filmstrips (8! 2.5 cm) were cut from each preconditioned (75% RH, 23 8C) sample and mounted between the grips of the machine. The thickness of each specimen was measured in four points along its length with a flat and parallel surfaces Mitutoyo digital micrometer, (JP). Initial grip separation and crosshead speed were set to 50 mm and 12.5 mm/min, respectively. The maximum load (kgf) and extension (mm) were recorded and used to calculate the tensile strength (MPa) and elongation at break. Ten specimens were tested for each sample. The experimental design samples tensile strength (DT%) and elongation at break (DE%) increase or decrease size in relation to the control were calculated by Eqs. (1) and (2), respectively.    sample tensile strength DT Z !100 K 100 (1) control tensile strength  DE Z

  sample elongation !100 K 100 control elongation

(2)

Preconditioned (75% RH, 23 8C) polyvinyl chloride (PVC) filmstrips (8!2.5 cm) were also evaluated in the same conditions and used as reference for mechanical comparison parameters.

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2.4. Moisture content and water activity Moisture content of each preconditioned sample (75% RH, 23 8C) was determined after 24 h of drying at 105 8C (Pouplin, Redl, & Gontard, 1999). Water activity measurements were performed with a DECAGON CX-2T, AQUALAB (USA) apparatus. Pure water (aw 1.000G0.003%) and KCl (aw 0.843G0.003%) were used as standard for equipment calibration. Preconditioned (75% RH, 23 8C) sample (4 cm2) were cut from the center of the films and evaluated in triplicate. The experimental design samples water activity (Daw%) increase or decrease size in relation to the control was calculated according to Eq. (3).    sample aw Daw Z !100 K 100 (3) control aw 2.5. Contact angle measurements The contact angle measurements were determined using a VCA optima surface analysis systems equipment AST products, Inc., USA to evaluate samples hydrophilic feature (Ave´rous & Fringant, 2001). The evolution of the water drop (3 ml) shape on the surface of a 4 cm2 sample was recorded (in duplicate) to determine the initial contact angle and the material absorption kinetic (during 5 min) (Veiga, Alves, Sine´zio, Scamparini, & Cereda, 2003). The initial contact angle values just after deposition indicate the material hydrophilicity behavior. In order to illustrate the kinetic of absorption, the slope at the origin (8/s) was also calculated by linear regression from the points of the contact angle versus time (Ave´rous et al., 2000). 2.6. Statistical analysis Two blocks of a 27K3 experimental factorial design were performed in order to select additives and process parameters that significatively (p!0.05) influenced cassava starch films mechanical properties, hydrophilicity and water activity. The blocks were designed with three central points, totalizing 19 experiments per block, differing only for one variable (xanthan gum, deacetylated or not). The other variables were cassava starch, sucrose, soybean oil, sodium phosphate, propylene glycol and pH (Table 1).

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The independent variables significant (p!0.05) influence was evaluated by the pareto chart of standardized effect, considering the ANOVA pure error. The experimental data were generated and analyzed using the software Statistica for Windows, version 5.0.

3. Results and discussion The experimental designs demonstrated in Table 2 were used to estimate the main effects of independent variables. 3.1. Mechanical properties It is well known that the selection of additives and process parameters is of importance since it can strongly affect film tensile strength and elongation at break. Therefore, few additives have been investigated to improve mechanical performance of cassava starch films. The additives and pH tested in this study have affected cassava starch films tensile strength and elongation at break (Figs. 1 and 2, respectively). As can clearly be observed by Figs. 1 and 2, the experimental samples added with sucrose (X9–X19 and DX9–DX19) presented a very different behavior from the other films, independent of xanthan gum deacetylation. Results shown on Fig. 2. have also indicated that films with 2% sucrose addition (X9–X16 and DX9–DX16) presented higher elongation at break (54.23–120.62%) when comparing to films added with 1% sucrose (X17–X19 and DX17– DX19) (32.19–46.59%). Therefore, indicating that increasing sucrose addition increases cassava starch film elongation at break. Sucrose addition can increase elongation at break until 2900% when comparing to the control. The pareto chart of standardized effect comproved that cassava starch films elongation at break is positively (p!0.05) affected by sucrose addition for both studied blocks. The preferential hydration of sugars molecules (Stansell, 1995) could limit plasticizing effect of sucrose on starch films. However, at the concentration added in this study, sucrose have demonstrated efficacy as plasticizing agent. Arvanitoyannis et al. (1996), have also observed plasticizing effect for sucrose, mentioning an even more pronounced

Table 1 Independent variables, coded and real values for the acetylated (X) and deacetylated (DX) gum factorial experiment blocks Symbols

A B C D E F G

Variables

Levels

Block-X (%)

Block-DX (%)

K1

0

C1

Sodium phosphate pH Cassava starch Sucrose Propylene glycol Soybean oil Acetylated gum

Sodium phosphate pH Cassava starch Sucrose Propylene glycol Soybean oil Deacetylated gum

0.00 4.00 3.00 0.00 0.00 0.00 0.00

0.35 6.00 4.00 1.00 0.25 0.03 0.05

0.70 8.00 5.00 2.00 0.50 0.06 0.10

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Table 2 27-3 factorial design with coded values for the acetylated (X) and deacetylated (DX) xanthan gum factorial experiment blocks Assays

Variables

Acetylated gum block

Deacetylated gum block

A

B

C

D

E

F

G

X-1 X-2 X-3 X-4 X-5 X-6 X-7 X-8 X-9 X-10 X-11 X-12 X-13 X-14 X-15 X-16 X-17(c)a X-18(c)a X-19(c)a

DX-1 DX-2 DX-3 DX-4 DX-5 DX-6 DX-7 DX-8 DX-9 DX-10 DX-11 DX-12 DX-13 DX-14 DX-15 DX-16 DX-17(c)a DX-18(c)a DX-19(c)a

K1 (0.00) C1 (0.70) K1 (0.00) C1 (0.70) K1 (0.00) C1 (0.70) K1 (0.00) C1 (0.70) K1 (0.00) C1 (0.70) K1 (0.00) C1 (0.70) K1 (0.00) C1 (0.70) K1 (0.00) C1 (0.70) 0 (0.35) 0 (0.35) 0 (0.35)

K1 (4.0) K1 (4.0) C1 (8.0) C1 (8.0) K1 (4.0) K1 (4.0) C1 (8.0) C1 (8.0) K1 (4.0) K1 (4.0) C1 (8.0) C1 (8.0) K1 (4.0) K1 (4.0) C1 (8.0) C1 (8.0) 0 (6.0) 0 (6.0) 0 (6.0)

K1 (3.0) K1 (3.0) K1 (3.0) K1 (3.0) C1 (5.0) C1 (5.0) C1 (5.0) C1 (5.0) K1 (3.0) K1 (3.0) K1 (3.0) K1 (3.0) C1 (5.0) C1 (5.0) C1 (5.0) C1 (5.0) 0 (4.0) 0 (4.0) 0 (4.0)

K1 (0.0) K1 (0.0) K1 (0.0) K1 (0.0) K1 (0.0) K1 (0.0) K1 (0.0) K1 (0.0) C1 (2.0) C1 (2.0) C1 (2.0) C1 (2.0) C1 (2.0) C1 (2.0) C1 (2.0) C1 (2.0) 0 (1.0) 0 (1.0) 0 (1.0)

K1 (0.00) C1 (0.50) C1 (0.50) K1 (0.00) C1 (0.50) K1 (0.00) K1 (0.00) C1 (0.50) K1 (0.00) C1 (0.50) C1 (0.50) K1 (0.00) C1 (0.50) K1 (0.00) K1 (0.00) C1 (0.50) 0 (0.25) 0 (0.25) 0 (0.25)

K1 (0.00) K1 (0.00) C1 (0.06) C1 (0.06) C1 (0.06) C1 (0.06) K1 (0.00) K1 (0.00) C1 (0.06) C1 (0.06) K1 (0.00) K1 (0.00) K1 (0.00) K1 (0.00) C1 (0.06) C1 (0.06) 0 (0.03) 0 (0.03) 0 (0.03)

K1 C1 K1 C1 C1 K1 C1 K1 C1 K1 C1 K1 K1 C1 K1 C1 0 0 0

a

(0.00) (0.10) (0.00) (0.10) (0.10) (0.00) (0.10) (0.00) (0.10) (0.00) (0.10) (0.00) (0.00) (0.10) (0.00) (0.10) (0.05) (0.05) (0.05)

Central points.

influence that observed when utilizing polyols. Plasticizer agents reduce intermolecular forces and increase polymer chains mobility, thereby improving films flexibility and extensibility (Coupland et al., 2000). Although xanthan gum presented little influence on elongation at break measurements (Fig. 2), the pareto chart of standardized effect have indicated a significant (p!0.05) negative effect when using both gums, deacetylated or not. The xanthan gum effect on film elongation at break may be explained by a synergism interaction between the gum

and the cassava starch (Katzbauer, 1997), preventing amylose-amylose interactions (Navarro, Martino, & Zaritzky, 1997). The more unfolded network could present a weaker interaction forces, resulting in films with lower elongation at break performance. Allonce and Doublier (1991) and Annable, Fitton, Harris, Phillips, and Williams (1994), also observed weak gel network when adding gums to starch dispersion due to a binary hydrocolloid phase separation, which could also be an explanation to the lower elongation at break observed in this study.

Fig. 1. Tensile strength measurements obtained for the acetylated (X) and deacetylated (DX) xanthan gum statistical experiments, control (C) and polyvinyl chloride films (PVC).

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Fig. 2. Elongation at break measurements obtained for the acetylated (X) and deacetylated (DX) xanthan gum statistical experiments, control (C) and polyvinyl chloride films (PVC).

The deacetylated xanthan gum effect can be observed comparing both blocks central point (X17–X19 and DX17–DX19) measurement results. Once the only difference between them is the xanthan gum deacetylation, observing their elongation at break measurements results (Fig. 2), it was noticed an increase in relation to the control (DE) for the deacetylated gum (DX) block when comparing to the acetylated gum (X) block (w988 and w778%, respectively). Removing the acetylated groups, allows the xanthan gum chain to associate, increasing its viscoelasticity (Morris et al., 1996), which when interacting with the starch network, may result in higher elongation at break behavior compared to the starchacetylated gum films. When comparing to the PVC films tensile strength resistance (19.30 MPa), only the films added with sucrose resulted in materials with lower resistance (2.02–8.43 MPa) performance. The pareto chart of standardized effect also has indicated a significant (p!0.05) decrease in films tensile strength (90%) in relation to the control. According to Coupland et al. (2000), an increase in plasticizer content can result in lower tensile strength resistance. Although the film forming sucrose concentration is low (!2%), after the casting step, the final sucrose concentration increases significantly with the water removing, what could explain the lower tensile strength results observed for samples X9–X19 and DX9–DX19 (K79.55 to K91.81 and K77.37 to K3.37%, respectively). The pareto chart of standardized effect have also indicated significant (p!0.05) negative influence of soybean oil and propylene glycol addition on tensile strength for both blocks.

The negative effect on tensile strength observed when adding propylene glycol can be explained by its plasticizing character (Kim, Ko, & Park, 2002). The great amount of plasticizer, caused by the simultaneous utilization of propylene glycol and sucrose, could result in excessive interaction among the plasticizers and the film network (Arvanitoyannis et al., 1996). Therefore, leading to a weak network, with low tensile strength resistance. Also the plasticizing character (Garcia et al., 2000) of soybean oil may explain the observed negative effect on tensile strength. Another explanation could be due to the fact that lipids form inclusion compounds with amylose (Belitz & Grosh, 1999), which could result in an opened network, lowering the films tensile strength resistance. Lower mechanical resistance when using lipid additives was also reported by Yang and Paulson (2000). When comparing the control with a PVC film analyzed by the same conditions, the control presented higher tensile strength resistance (59.41%) (Fig. 1), but lower elongation at break (K4833.08%) (Fig. 2). Such results indicated that the elongation at break is the most disadvantageous mechanical characteristics of the cassava starch films and efforts should be made to increase such property. Sucrose addition have resulted in lower tensile strength resistance values when comparing to control (until 95%) and PVC films (until 89%) (Fig. 1). However, it was also noticed a great sucrose effect increasing materials elongation at break % when comparing to the control (until 2900%) (Fig. 2). Such results have indicated that sucrose utilization as additive increased the mechanical feasibility of starch films by increasing their elongation at break, justifying the continuance of studying this additive.

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Previous work utilizing sucrose as additive (o to 6%), at similar film thickness (0.09 mm), has also observed a significant increase on elongation at break (from 3.2 to 89.0%) (Cherian, Gennadios, Weller, & Chinachoti, 1995). Arvanitoyannis et al. (1996), which also investigated sucrose addition (from 6 to 26%) to starch films, at similar film thickness (0.1 mm), have observed lower elongation at break (from 2.9 to 3.7%) when comparing to the experimental samples investigated in this study (32.74– 120.62%). The different elongation at break performance observed by Arvanitoyannis et al. (1996) when comparing to the experimental samples and with the results obtained by Cherian et al. (1995) may be explained by an excessive plasticizer addition due to the sucrose incorporation (26%). Arvanitoyannis et al. (1996) also observed higher tensile strength (24.2–8.4 MPa) when comparing to the experimental samples investigated in this study and with the results obtained by Cherian et al. (1995) (2.02–7.74 and 4.2– 3.8 MPa, respectively). The high tensile strength values obtained, may resulted from the high starch content of their films (!89%), when comparing to the experimental films (!5%) or Cherian et al. (1995) films (!15%). Among independent variables investigated in this study, sucrose and cassava starch concentration have affected (p!0.05) the material thickness, which ranged from 0.04 to 0.12 mm (Table 3). However, no correlation was observed between the mechanical properties results and the materials thickness, indicating that, for the conditions utilized in this

study, thickness did not affect, or have a little effect, on films mechanical properties. The experimental films formulated without sucrose presented similar tensile strength (from 19.51 to 34.28 MPa) and elongation at break (3.80–5.26%) when comparing starch films (without additives) conditioned at 50% RH (44.7 MPa and 3.9%, respectively) (Arvanitoyannis et al., 1996). Such results indicate that the high relative humidity utilized in this study (75% RH) did not have a great impact on such characteristics. 3.2. Moisture content and water activity The experimental films moisture content (between 13.56 and 18.58%) was very close to the control moisture content (13.51%) and was not affected (p!0.05) by any additive, pH modification or cassava starch concentration. Although, the determination of moisture content is one of the most frequent analysis in food research, the sole value of water content does not inform about the nature of water; if it is ‘bound’ or ‘free’, ‘inhert’ or occluded’ (Mathlouthi, 2001). Water activity, on the other side, is a critical factor affecting the shelf life of the product (Yang et al., 2000). Has been regarded as the most important parameter controlling the behavior or intermediate and low moisture food during processing and storage (Anese, Shtylla, Torreggiani, & Maltini, 1996), indicating to be a more important parameter to be evaluated.

Table 3 Thickness (Tk) and Water activity (aw) measurement values, increase size of water activity in relation to the control (Daw%), initial contact angle (CA) and slope (8/s) for the acetylated (X) and deacetylated (DX) xanthan gum blocks assays

Acetylated gum Block (X) a

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 (c)b 18 (c)b 19 (c)b Control R2d a b c d

Deacetylated gum Block (DX)

Tk

aw

Daw

CA (8)

Slope

Tka

aw

Daw

CA (8)

Slope

0.04 0.04 0.05 0.05 0.09 0.09 0.07 0.08 0.08 0.07 0.06 0.08 0.12 0.10 0.11 0.11 0.08 0.08 0.08 0.07

0.576 0.722 0.886 0.614 0.621 0.599 0.632 0.535 0.703 0.751 0.721 0.759 0.806 0.745 0.739 0.748 0.761 0.742 0.701 0.507 0.82

13.54 42.47 74.69 21.10 22.49 18.15 24.65 5.52 38.72 48.13 42.27 49.70 58.97 46.94 45.83 47.53 50.10 46.35 38.26 nac nac

45.90 71.60 71.20 75.80 73.30 70.70 65.80 76.10 77.70 57.70 76.70 80.10 64.00 38.20 61.30 64.40 50.60 70.00 59.10 70.10 0.29

K9.56 K7.86 K8.55 K10.97 K11.20 K11.75 K12.56 K12.85 K5.57 K3.33 K7.81 K9.72 K5.97 K3.84 K9.13 K9.02 K7.11 K9.72 K9.00 K4.79 0.99

0.04 0.04 0.05 0.06 0.10 0.09 0.08 0.08 0.07 0.08 0.07 0.09 0.12 0.10 0.11 0.11 0.08 0.08 0.09

0.590 0.667 0.859 0.566 0.772 0.605 0.563 0.529 0.702 0.751 0.726 0.748 0.811 0.758 0.733 0.773 0.708 0.669 0.749

16.37 31.56 69.43 11.64 52.27 19.33 11.05 4.34 38.46 48.13 43.20 47.53 59.96 49.51 44.58 52.47 39.64 31.95 47.73

69.80 47.60 71.20 68.90 57.00 70.70 61.40 76.10 58.70 57.70 50.20 80.10 64.00 43.40 61.30 64.50 60.10 45.30 58.30

K8.20 K8.67 K12.82 K11.09 K11.66 K11.75 K9.43 K12.85 K4.40 K3.33 K2.52 K5.72 K5.97 K3.47 K5.13 K2.20 K8.07 K8.69 K8.45

0.94

nac

0.31

0.80

Tk: thickness (mm). (c): Central points. na: Not applicable. R2: Coefficient of determination.

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Extending the shelf life of perishable products consist in preventing its degradation by biochemical reactions and/or microbial growth. Such degradation mechanisms can generally be inhibited by low water activity (aw) contents (Mathlouthi, 2001). Samples have presented aw content between 0.53 and 0.89 (Table 3), thus being considered intermediate moisture products, which are susceptible to microorganism growth and chemical reactions (Belitz & Grosch, 1999). Considering the biodegradable nature of the material and the soybean oil addition, microorganism growth and lipid oxidation may have the most concerning effect on material shelf life. The insaturation on soybean oil molecules combined with the films aw contents (above 0.6%), indicates that the material is susceptible to a fast rate of lipid oxidation (Belitz & Grosch 1999; Nawar, 1996). Microbial growth can be expected, however molds growth is more likely to be observed. Most molds optimum aw range growth is between 0.80 and 0.87% (Fennema & Tannenbaum, 1996). Thus, in general, the sucrose content films (samples X9–X19 and DX9–DX19) are more susceptible for microbial, especially molds, growth during storage. Only sucrose had significant (p!0.05) effect on films water activity for both studied blocks. Although sucrose is known as a water activity depressor (Mathlouthi, 2001), the opposite was observed in this study. Sucrose increased the material aw (Table 2). A possible explanation is the plasticizing effect of sucrose on the film network structure (Mitchell, 1998), mainly related to the creation of highly mobile regions, which allow a even more pronounced water uptake (Cheryan et al., 1995). No correlation was observed between the water activity measurements and the materials thickness (Table 3). Such result indicates that, for the conditions utilized in this study, thickness did not affect films water activity. 3.3. Contact angle measurements The films hydrophilicity was quantitatively illustrated by measurements of the water droplet absorption kinetics given by the contact angle slope at the origin (8/min) (Table 3). All samples presented a rapidly absorption, probably due to the hydrophilic nature of the cassava starch film and additives (with exception of soybean oil) (Ave´rous et al., 2000). Soybean oil, which has a hydrophobic character, did not influenced (p!0.05) water absorption during time. Such result can be explained by the low soybean oil concentration (!0.06%) used in attempt to completely incorporate the lipid additive to the film network. Thus, indicating that soybean oil concentrations lower than 0.06% is not enough to affect cassava starch films hydrophilicity. The pareto chart of standardized effect, have indicated that no additive or pH modification have affected (p!0.05) the material initial contact angle. However, the R2 (coefficient of determination) for both analyzed blocks were very low (0.29 and 0.31) to trustfully predict the main

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effects of the independent variables (Table 3). Thus, the results should be analyzed with caution. Ave´rous et al. (2001), also observed a large dispersion for starch materials initial contact angle measurements, attributed to a variation on the surface smoothness and hydrophilicity, which could also be an explanation for the deviation observe in this study. Other researches investigating the effect of additives on starch films have presented similar initial contact angle (39.9–73.68) (Demirgo¨z, Elvira, Mano, Cunha, Piskin, & Reis, 2000), (32–718) (Ave´rous, Faiconnier, Moro, & and Fringant, 1999) when comparing to the experimental measurements obtained (38.208 to 80–108) (Table 3). The pareto chart of standardized effect, considering ANOVA pure error, have indicated that, for the acetylated gum block (X), increasing sucrose concentration increases (p!0.05) the water absorption kinetic (slope). For the deacetylated xanthan gum block (DX), water absorption kinetic increased (p!0.05) with sucrose and deacetylated xanthan gum addition. The sucrose addition effect can be observed when comparing the water absorption behavior of samples added with sucrose (X9–X19 and DX9–DX19) with the other studied films (Table 3). Plasticizer addition reduces chain-to-chain interactions (Sothornvit & Krochta, 2001). Such reducing interactions could result in opened structures, which may be an explanation for the hydrophilicity increase effect when using sucrose. Similar water absorption values (K2.02 to K4.208/s) were observed for starch films added with glycerol as plasticizer (Ave´rous et al., 2000) when comparing to the experimental films added with sucrose (K2.20 to K9.13 8/ s) (Table 3). However, sucrose concentration may result in films with higher water absorption kinetic performance when comparing to films added with glycerol. The deacetylated xanthan gum resulted on films with a slight higher water absorption behavior when comparing to the acetylated gum, as can be observed comparing the slope values of the acetylated block central point films (X17–X19) to the deacetylated block samples (DX17–DX19) (Table 2). The associative effect of xanthan gum deacetylation (Morris et al., 1996) could increased the water absorption capacity of the xanthan gum molecule, which could explain the positive effect of the deacetylated xanthan gum on the water absorption kinetic of the film. No correlation was observed between the initial contact angle or the slope at the origin and the materials thickness (Table 3). Such result indicates that, for the conditions utilized in this study, thickness did not affect films hydrophilicity or water absorption behavior.

4. Conclusions Although xanthan gum deacetylation had resulted in significant effects on films mechanical and water absorption

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kinetic properties, xanthan gum (deacetylated or not) did not present a great impact on the final materials when comparing to the control. Among the additives investigated, only sucrose addition has strongly affected the studied characteristics. Although a tensile strength decrease (until 94%), and water activity and hydrophilicity increase was observed when using sucrose, the surprising increase (until 2900%) on materials elongation at break in relation to the control justify the continuance of studding such additive. The possibility of increasing starch films elongation at break characteristics and its low cost indicates that sucrose may be an interesting approach to increase the performance of friendly packaging materials.

Acknowledgements The authors gratefully acknowledge CNPq and FAEPUNICAMP for the financial support and Ms Joyce I. S. Florecio for water activity determinations. We also acknowledge the Brazilian Packaging Technology Center (CETEA-ITAL) for the Instron Universal Testing Machine access and 3M Brazil for the contact angle VCA optima equipment access. The author acknowledge Rhodia-Brazil; CP Kelco-Brazil S.A and Flor de Lotus-Brazil for the donation of the Rodigel acetylated xanthan gum, the deacetylated xanthan gum and the commercial cassava starch, respectively.

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