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Mechanical and barrier properties of biodegradable soy protein


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LWT 40 (2007) 232–238 www.elsevier.com/locate/lwt

Mechanical and barrier properties of biodegradable soy protein isolate-based ?lms coated with polylactic acid
Jong-Whan Rhima,?, Jun Ho Leeb, Perry K.W. Ngc
a

Department of Food Engineering, Mokpo National University, 61 Dorimri, Chungkyemyon, Muangun, Chonnam 534-729, Republic of Korea b Division of Food, Biological and Chemical Engineering, Daegu University, Gyeongsan, Gyeongbuk 712-714, Republic of Korea c Department of Food Science and Human Nutrition, Michigan State University, East Lansing, MI 48824-1224, USA Received 23 July 2005; received in revised form 18 October 2005; accepted 20 October 2005

Abstract Polylactic acid (PLA)-coated soy protein isolate (SPI) ?lms were prepared by dipping SPI ?lm into PLA solution. The effects of coating on improvements in mechanical and water barrier properties of the ?lm were tested by measuring selected ?lm properties such as tensile strength (TS), elongation at break (E), water vapor permeability (WVP), and water solubility (WS). TS of SPI ?lms increased from 2.870.3 up to 17.472.1 MPa, depending on the PLA concentration of the coating solution, without sacri?cing the ?lm’s extensibility. In contrast, the extensibility of SPI ?lm coated with solution containing more than 2 g PLA/100 ml solvent, increased. WVP of PLA-coated SPI ?lms decreased from 20 to 60 fold, depending on the concentration of PLA coating solution. Water resistance of SPI ?lms was greatly improved as demonstrated by the dramatic decrease in WS for PLA-coated ?lms. The improvement in water barrier properties was mainly attributed to the hydrophobicity of PLA. r 2005 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved.
Keywords: Soy protein isolate; Polylactic acid; Biodegradable; Coating ?lm

1. Introduction The development of plastics or packaging ?lms based on biopolymers has attracted attention and renewed interest due to their environmentally friendly nature and their potential use in the food and packaging industry (Debeaufort, Quezada-Gallo, & Voilley, 1998; Kester & Fennema, 1986; Krochta & De Mulder-Johnston, 1997; Petersen et al., 1999; Weber, Haugaard, Festersen, & Bertelsen, 2002). As one of the candidates for such biopolymers, soy protein, the major component of the soybean (30–45 g/100 g), is abundant, renewable, and biodegradable, making it attractive feedstock for bioplastics. A new generation of soy-based environmentally friendly plastics and their potential use as alternative resources for bioplastics to be used in packaging applications, has been extensively studied (Cunningham, Ogale, Dawson, & Acton, 2000; Ly, Johnson, & Jane, 1998; Paetau, Chen, & Jane, 1994a,
?Corresponding author. Tel.: +82 61 450 2423; fax: +82 61 454 1521.

E-mail address: jwrhim@mokpo.ac.kr (J.-W. Rhim).

1994b; Sue, Wang, & Jane, 1997; Swain, Biswal, Nanda, & Nayak, 2004; Zhang, Mungara, & Jane, 2001). However, there are some limitations to the application of soy protein ?lm for packaging due to its poor mechanical properties and high sensitivity to moisture (Gennadios, McHugh, Weller, & Krochta, 1994). Numerous studies have concentrated on improving mechanical and water barrier properties of soy protein-based ?lms through physical, chemical, and enzymatic treatments, or compositing with hydrophobic materials (Brandenburg, Weller, & Testin, 1993; Gennadios, Ghorpade, Weller, & Hanna, 1996; Rhim, 2004; Rhim, Gennadios, Weller, Cezeirat, & Hanna, 1998; Rhim, Lee, & Kwak, 2005; Rhim & Weller, 2000; Stuchell & Krochta, 1994). Though previously reported methods indicated a signi?cant improvement in ?lm properties, the moisture barrier property of soy proteinbased ?lms has not yet been fully addressed. Another strategy to overcome the problem of low water resistance of soy protein is to associate it with a moisture resistant polymer, while maintaining the overall biodegradability of the product. Generally, association between

0023-6438/$30.00 r 2005 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2005.10.002

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polymers can be achieved through blending, laminating or coating with other polymers with desirable properties. Though blending is an easier and effective way to prepare multiphase polymeric materials, poor interfacial adhesion between the polymer phases causes problems when applying the method (Wang, Shogren, & Carriere, 2000). On the other hand, laminated ?lms can be prepared with fewer compatibility problems than experienced in the preparation of blend ?lms, but require multiple steps to prepare. One novel approach is coating a biopolymer ?lm with biodegradable hydrophobic polymers, to exploit full advantages of both polymers, with fewer processing steps. One of the most probable candidates for this purpose is polylactic acid (PLA) (Drumright, Gruber, & Henton, 2000; Garlotta, 2001). PLA is synthesized from lactic acid, which is derived from renewable resources such as corn or sugar beets (Garlotta, 2001), and is a thermoplastic with high-strength and hydrophobic properties. It is also completely biodegradable and therefore perfectly safe for the environment. The main objective of the present study was to improve mechanical and barrier properties of soy protein isolatebased ?lms by coating them with PLA, and to determine some selected properties including tensile strength (TS), elongation at break (E), water vapor permeability (WVP), and water solubility (WS) of the ?lms.

2.2.1. PLA coating PLA solutions were prepared by dissolving 1–4 or 5 g of PLA (w/v) in 100 ml of chloroform. SPI ?lms were dipped into the prescribed chloroform solution for 2 min, and drained of excess solution by using small clamps to hang the ?lms in an upright position from a horizontal string (mounted between two laboratory bench stands), where they were allowed to dry inside a vented hood at ambient conditions. For comparison, PLA ?lm was also prepared by dissolving 5 g of PLA in 100 ml of chloroform and casting it onto Te?on-coated glass plates (see preparation of ?lms), dried in a hood at ambient conditions (Rhim, Mohanty, Singh, & Ng, 2006), and peeled off the plate. For the triplicate measurement of ?lm properties, three sheets of each type of ?lm were prepared. 2.2.2. Film thickness and conditioning All ?lm samples were cut into seven 2.54 ? 15 cm, one 7 ? 7 cm, and ten 2 ? 2 cm-sized pieces for the measurement of TS and E, WVP, and WS, respectively. Film thickness was measured to the nearest 0.01 mm using a hand-held micrometer (Dial Thickness gauge 7301, Mitutoyo, Japan). Five thickness measurements were made along the length of the rectangular ?lm strip and the mean value was used in the TS calculation for that sample. Similarly, ?ve measurements were taken on each WVP specimen, one at the center and four around the perimeter, and the mean value was used in calculating WVP for that ?lm sample. All ?lm samples were preconditioned for at least 48 h in a constant temperature humidity chamber set at 25 1C and 50% RH before testing. 2.2.3. Transparency Transparency of PLA-coated ?lms was determined by measuring the percent transmittance at 660 nm using a UV/ Visible spectrometer (Lamda 25, Perkin Elmer Instruments, Norwalk, CT, USA). Transmittance was measured on all four sides of the ?lm specimens measuring 7 ? 7 cm in size. Triplicate measurements were performed with individually prepared ?lm samples and the average value was reported. 2.2.4. Tensile properties TS and E of each ?lm were determined with an Instron Universal Testing Machine (Model 5565, Instron Engineering Corporation, Canton, MA, USA) according to ASTM Method D 882-88 (1995a). Rectangular specimens of 2.54 ? 15 cm were cut using a precision double blade cutter (model LB.02/A, Metrotec, S.A., San Sebastian, Spain). Initial grip separation was set at 50 mm and the cross-head speed was set at 50 mm/min. TS was calculated by dividing the maximum load to break the ?lm by the cross-sectional area of the ?lm and E was calculated by dividing ?lm elongation at rupture by the initial gauge length of the specimen and multiplying by 100. Each type of ?lm was prepared and evaluated in triplicate (one individually prepared ?lm was considered one replicate or

2. Materials and methods 2.1. Materials Soy protein isolate (Pro-FAM 646, Archer Daniels Midland, Decatur, IL, USA), minimum 90% protein content on a dry basis, was used without further treatment. Poly L-lactide (PLLA, Biomers L9000) was obtained from Biomer Inc. (Krailling, Germany) and dried in a vacuum oven at 60 1C for 24 h before use. Chloroform and glycerin of reagent grade were purchased from J.T. Baker (Mallinkrodt Baker, Inc., Phillipsbury. NJ, USA).

2.2. Preparation of ?lms SPI ?lms were prepared according to the method of Brandenburg et al. (1993). Five grams of SPI was dissolved in a constantly stirred mixture of distilled water (100 ml) and glycerin (2.5 g). The solution pH was adjusted to 1070.1 with 1 M sodium hydroxide solution. The ?lm solutions were heated for 20 min at 90 1C in a constant temperature water bath to denature the soy protein, and then cast onto a leveled Te?ons protective overlay (ColeParmer Instrument Co., Chicago, IL, USA) mounted on a glass plate (24 cm ? 30 cm) framed on four sides. The castings were dried at ambient conditions (E23 1C) for about 20 h and peeled from the plates.

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one experimental unit). For each ?lm sample, TS and E measurements were made on seven specimens taken from the same ?lm, and reported as the mean for the replicate. 2.2.5. Water vapor permeability (WVP) WVP (g m/m2 s Pa) was calculated as WVP ? ?WVTR ? L?=Dp, where WVTR was the measured water vapor transmission rate (g/m2 s) through a ?lm, L was the mean ?lm thickness (m) and Dp was the partial water vapor pressure difference (Pa) between the two sides of the ?lm. WVTR was determined gravimetrically with a modi?ed procedure in accordance with ASTM Method E 96-95 (1995b) using a cup containing water as described by Gennadios, Weller, and Gooding (1994). The ?lm-covered cups were placed in an environmental chamber set at 25 1C and 50% RH. A fan was operating within the chamber creating an air velocity of 198 m/min over the surface of the cups in order to remove the permeating water. Because of the low water vapor resistance of protein-based ?lms, actual RH values at the ?lm undersides during testing were lower than the theoretical value of 100%. Actual RH values at the ?lm undersides and ?lm WVP values were calculated after accounting for the resistance of the stagnant air layer between the ?lm undersides and the surface of the water in the cups. WVP values were corrected according to the method of Gennadios et al. (1994). 2.2.6. Water solubility (WS) WS of each ?lm was determined as the percentage of ?lm dry matter solubilized after 24 h immersion in distilled water (Rhim et al., 1998). Three randomly selected 2 ? 2 cm samples from each type of ?lm were ?rst dried at 105 1C for 24 h to determine the weight of the initial dry matter. An additional three pieces of weighed ?lm were placed in a 50ml beaker containing 30 ml of distilled water. Beakers were covered with Para?lm (American National Can, Greenwich, CT, USA) and stored in an environmental chamber at 25 1C for 24 h with occasional, gentle stirring. Undissolved dry matter was determined by removing the ?lm pieces from the beakers, gently rinsing them with distilled water, and then oven drying the rinsed ?lms (105 1C, 24 h). 2.2.7. Thermal analysis Thermal analyses of the SPI, PLA, and PLA-coated SPI ?lms were performed on a Differential Scanning Calorimeter (DSC Q100, TA Instruments, New Castle, USA) ? using the method of Martin and Averous (2001). For each ?lm type, about 5 mg of ?lm sample was sealed in an aluminum pan and heated from 25 to 100 1C at a rate of 10 1C/min, held at that temperature for 1 min, then cooled to ?100 1C with liquid nitrogen (cooling rate of 25 1C/min) before a second heating scan to 200 1C at a 10 1C/min scan rate. A nitrogen ?ow (60 ml/min) was maintained throughout the test. The glass transition temperature (Tg), melting temperature (Tm), and apparent enthalpy of fusion (DHf)

were determined from the second heating scans. The ?rst scan was meant to discard the thermal history of the samples. 2.3. Statistical analysis The measurements of TS, E, WVP, and WS were triplicated with individually prepared ?lms as the replicated experimental units. Statistics on a completely randomized design were determined using the general linear models procedure in the SAS program (SAS, 1990). Duncan’s multiple range tests were conducted to determine the signi?cant (Po0:05) differences between each type of ?lm. 3. Results and discussion 3.1. Apparent ?lm properties SPI ?lms were homogeneously coated on both sides by being dipped into a speci?ed solution of PLA. The surfaces of coated ?lms were more smooth and glossy compared with those of uncoated SPI ?lms. With multi-layer ?lms, delamination of each component layer is frequently observed when the component layers are incompatible ? with each other (Lawton, 1997; Martin, Schwach, Averous, & Couturier, 2001). In such cases, compatibilizers are usually used for improvement in adhesion of polymer ?lms (Lawton, 1997; Zhang & Sun, 2004). However, in the present study, the PLA layers were so tightly adhered to the SPI ?lm matrix that manual separation of each layer was not possible. This indicates the SPI was quite compatible with the PLA layer. Table 1 shows the results for thickness, moisture content, and percent transmittance of SPI ?lms coated with PLA. Mean thickness for control SPI and PLA ?lms were 78.872.2 and 89.572.7 mm, respectively. The thickness for PLA-coated SPI ?lms did not change signi?cantly (P40:05) when ?lms were coated with solutions containing up to 2 g PLA/100 ml solvent, but it increased with an increase of over 3 g PLA/100 ml solvent concentration in

Table 1 Apparent properties of soy protein isolate (SPI) ?lms coated with polylactic acid (PLA) solution of varying concentrations Film SPI SPI/(1 g SPI/(2 g SPI/(3 g SPI/(4 g SPI/(5 g PLA Thickness (mm) 78.872.2a 76.973.7a 76.474.3a 84.571.6b 86.272.8b 87.572.7b 89.572.7b DM (%) 75.571.2a 78.772.0b 78.871.4b 79.570.3b 81.970.7c 86.970.1d 87.370.1d T (%) 91.370.4a 92.870.3b 92.570.2b 93.370.3bc 94.170.1c 94.370.3c 95.170.1d

PLA/100 ml PLA/100 ml PLA/100 ml PLA/100 ml PLA/100 ml

solvent) solvent) solvent) solvent) solvent)

Means of three replicates 7standard deviation. Any two means in the same column followed by the same letter are not signi?cantly different (P40.05) by Duncan’s multiple range test. DM: dry matter; T: transmittance of the ?lm determined at 660 nm.

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the coating solution. Ghorpade, Weller, and Hanna (1997) also found a similar trend of thickness increase in PLAcoated wheat gluten ?lms. Dry matter content for control SPI ?lms was 75.571.2 g/ 100 g, while that of PLA ?lms was 87.370.1 g/100 g. It is important to recognize that the remaining material in the matrices of SPI ?lms (24.5 g water/100 g) is water moisture, but that in the PLA ?lms is the dissolving solvent, i.e., chloroform. Rhim et al. (2006) showed the presence of solvent in dried solvent-cast PLA ?lms through TGA (thermogravimetric analysis). Dry matter for PLA-coated ?lms also increased with an increase in the PLA concentration. Consequently, the moisture and the solvent (chloroform) in the ?lms made in the present study are expected to act as plasticizers and to affect other ?lm properties. Transmittance (T) of control SPI and PLA ?lms were 91.370.4% and 95.170.1%, respectively. T of the PLA-coated SPI ?lms increased from 92.870.3% for coating with 1 g PLA/100 ml solvent solution to 94.370.3% for coating with 5 g PLA/100 ml solvent solution. Generally, transmittance through multilayer ?lms decreases when component layers are not compatible with each other. Increase in T of SPI ?lms coated with PLA is indirect evidence for the compatibility of the two polymers. 3.2. Mechanical properties Table 2 shows the results for TS and E for SPI ?lms coated with PLA. TS for the PLA-coated SPI ?lms increased dramatically from 2.870.3 MPa for the uncoated SPI ?lms up to 17.472.1 MPa for coating with 5 g PLA/ 100 ml solvent solution. Note that TS increased more than three times that of the uncoated SPI ?lm even when coated with a solution containing a concentration as low as 1 g PLA/100 ml solvent. TS also increased with increases in the PLA concentration of the coating solution. The TS values of SPI ?lms coated with 5 g PLA/100 ml solvent solution (17.472.1 MPa) were similar to those of PLA ?lm itself (17.270.5 MPa). These values are comparable to those of widely used plastic ?lms such as LDPE and HDPE, for
Table 2 Tensile strength (TS) and elongation at break (E) of soy protein isolate (SPI) ?lms coated with polylactic acid (PLA) solution of varying concentrations Film SPI SPI/(1 g SPI/(2 g SPI/(3 g SPI/(4 g SPI/(5 g PLA TS (MPa) 2.870.3a 8.571.1b 10.971.0c 11.570.5d 14.271.6e 17.472.1e 17.270.5e E (%) 165.7715.0b 82.675.1a 176.079.7bc 218.3730.7d 349.9716.3e 207.6734.6cd 203.4720.8cd

PLA/100 ml PLA/100 ml PLA/100 ml PLA/100 ml PLA/100 ml

solvent) solvent) solvent) solvent) solvent)

Means of three replicates7standard deviation. Any two means in the same column followed by the same letter are not signi?cantly different (P40:05) by Duncan’s multiple range test.

which TS values are known to be 13 and 26 MPa, respectively (Salamie, 1986). Ghorpade et al. (1997) reported slightly different results with PLA-coated wheat gluten ?lms. They found that TS increased for wheat gluten ?lms with up to 1 g PLA/100 ml solvent concentration in coating solutions, then decreased with increasing PLA concentration above 1 g PLA/100 ml solvent. Mean TS values ranged from 3.09 MPa for the control wheat gluten ?lms to 4.18 MPa for 1 g PLA/100 ml solvent concentration in the coating solution. They explained that such a difference might be due to the different thicknesses of the interior wheat gluten layer, caused by technical dif?culties in control of wheat gluten ?lm thickness. The main difference in the effect on TS of coating with PLA between wheat gluten and SPI ?lms seems to be caused by the difference in compatibility of the component polymer matrices, and indicates that SPI is more compatible with PLA than wheat gluten. Mean values for the percentage E, a measure of a ?lm’s extensibility, for the SPI ?lms coated with PLA are also presented in Table 2. Except for ?lms coated with 1 or 2 g PLA/100 ml solvent solution, ?lms coated with PLA showed a signi?cant increase in elongation over uncoated SPI ?lms (165.7715.0%). However, E decreased for ?lms coated with 1 g PLA/100 ml solvent concentration in the coating solution. The direct contact of the coating solution seems to cause destabilization of the SPI ?lm matrix by dissolving it, especially when low concentration of PLA solution (1 g PLA/100 ml) has been used. Then E values steadily increased as PLA concentration in the coating solution increased up to 4 g PLA/100 ml solvent, to reach a value of 349.9716.3%. This value of elongation is comparable with that of polyethylene ?lm (Salamie, 1986). Further increase in concentration of PLA (5 g PLA/100 ml solvent) in the coating solution resulted in a decrease in E to the level of PLA ?lm. Ghorpade et al. (1997) also found an increase in elongation of wheat gluten ?lms coated with PLA. They explained the increase in elongation of ?lms coated with solutions of higher PLA concentrations as being due to effects of the PLA itself. However, their explanation does not agree with the fact that PLA is a very brittle polymer (Lawton, 1997; Martin et al., 2001). Rhim et al. (2006) showed that mechanical properties of solvent casting PLA ?lm (TS: 16.671.0 MPa; E: 203.4720.8%) were quite different from those of thermo-compression PLA ?lms (TS: 44.072.2 MPa; E: 3.070.1%). They explained that such differences in mechanical properties were caused by the plasticization effect of the solvent (chloroform) trapped in the PLA ?lm matrix. It is also well known that water plasticizes hydrophilic ?lms and improves ?lm extensibility (Banker, 1966; Gontard, Guilbert, & Cuq, 1993). In the case of SPI ?lms coated with 3 or 4 g PLA/100 ml solvent solution, both water in the SPI ?lm layer and solvent in the PLA coating layer seemed to work in tandem as plasticizers to result in increased ?lm extensibility. Generally, PLA is known to be brittle, with TS and E values of

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45.6–61.4 MPa and 3.1–5.8%, respectively, for PLA ?lms prepared without plasticizer (Garlotta, 2001). However, the PLA ?lms prepared in the present study were more resilient (203.4720.8%) with lower TS (17.270.5 MPa). This is most likely due to the plasticizing effect of the solvent retained in the solvent-cast PLA ?lms (Rhim et al., 2006). 3.3. Water vapor permeability (WVP) and water solubility (WS) The WVP values, along with the calculated actual RH conditions at the undersides of ?lms during testing, and the WS values for the different types of SPI ?lms are shown in Table 3. WVP of the uncoated SPI ?lms was 268.30721.50 ? 10?14 kg m/m2 s Pa, which was in agreement with reported values for SPI ?lms of similar compositions (Brandenburg et al., 1993). Coating with PLA improved water vapor barrier properties of the SPI ?lms dramatically as evidenced by decreases in WVP values. The water vapor barrier improved by 20- to 60-fold, compared it to control SPI ?lms with increasing PLA concentrations of coating solution. WVP of SPI ?lms decreased to the lowest value 4.4470.17 ? 10?14 kg m/m2 s Pa with PLA coating of 5 g PLA/100 ml solvent solution, which was comparable to that of PLA ?lms. WVP values of hydrophilic protein ?lms such as wheat gluten and SPI ?lms are known to be 3 or 4 orders of magnitude higher than those of widely used plastic ?lms (Gennadios et al., 1994). The WVP (in kg m/m2 s Pa) for various polymeric ?lms are documented in the literature (Briston, 1988) as: polyvinylidene chloride (0.7–2.4 ? 10?16); high-density polyethylene (2.4 ? 10?16); low-density polyethylene (7.3– 9.7 ? 10?16); cast polypropylene (4.9 ? 10?16); ethylene vinyl acetate (2.4–4.9 ? 10?15); polyester (1.2–1.52.4 ? 10?15); and cellulose acetate (0.5–1.6 ? 10?14). Results from the present study indicate that the water vapor barrier of the SPI ?lm coated with more than 2 g PLA/ 100 ml solvent solution is comparable to that of cellulose acetate ?lms. Ghorpade et al. (1997) also found a similar result of WVP values with PLA-coated wheat gluten ?lms. They also reported that WVP of wheat gluten ?lms

decreased exponentially with the increases in the PLA concentration in the coating solutions. The calculated RH values at the inner surface of ?lm samples, mounted on the water vapor transmission rate measuring cup, increased with increasing concentration of PLA coating solution. When coating with solution as low as 1 g PLA/100 ml solvent concentration, calculated RH values came close to the theoretical value, 100% RH. This indicates that it is not necessary to account for resistance of the stagnant air layer between the ?lm and water surface in the water vapor transmission rate measuring cups (Gennadios et al., 1994). Ghorpade et al. (1997) also observed the increase in actual RH at the underside of the ?lms with wheat gluten ?lms coated with PLA. As they explained, the increase in actual RH at the underside of ?lm means an increase in RH gradient across the ?lm layer. Consequently, expected WVP values for PLA-coated ?lms would most likely have been even lower if equal RH gradients conditions had been applied across the various ?lm samples. The observed increase in water vapor barrier properties of SPI ?lms coated with PLA was attributed to the hydrophobicity of PLA. Results for WS, which indicates a resistance of ?lm against water, also supports the fact that the water barrier property of SPI ?lms increases by PLA coating. Measuring WS of SPI ?lms was not possible because they disintegrated within 20 min of immersion into water. However, ?lms coated with PLA maintained their shape even after immersion in water for 24 h. WS of SPI ?lms coated with 1 g PLA/100 ml solvent coating solution was 81.074.4% and it decreased exponentially as concentrations of PLA increased. SPI ?lms coated with 5 g PLA/100 ml solvent solution became almost insoluble in water. SPI ?lms coated with lower concentration levels of PLA solution had appreciable amounts of solids dissolved. The higher WS values for SPI ?lms coated with low concentration PLA coating solution were attributed to the solubilization of the SPI layer between PLA coating layers. Most of the absorbed water probably entered through from the cut edges of the ?lm sample due to the absence of a PLA protection layer where ?lm was cut for WS measurement

Table 3 Water vapor permeability (WVP) and water solubility (WS) of soy protein isolate (SPI) ?lms coated with polylactic acid (PLA) solution of varying concentrations Film SPI SPI/(1 g SPI/(2 g SPI/(3 g SPI/(4 g SPI/(5 g PLA WVP ( ? 10?14 kg m/m2 s Pa) 268.30721.50e 12.2071.25d 7.6070.14c 5.6870.11b 4.7470.25a 4.4470.17a 4.6670.15a RH inside cup (%) 72.270.3a 96.770.3b 97.870.1c 98.270.03d 98.570.03e 98.770.03e 98.770.1e WS (%) nd 81.074.3e 57.472.0d 39.870.9c 19.676.6b 0.170.1a 0.070.0a

PLA/100 ml PLA/100 ml PLA/100 ml PLA/100 ml PLA/100 ml

solvent) solvent) solvent) solvent) solvent)

Means of three replicates7standard deviation. Any two means in the same column followed by the same letter are not signi?cantly different (P40:05) by Duncan’s multiple range test. RH: relative humidity; actual value underneath the ?lm covering the WVP measuring cup; nd: not determined due to disintegration of ?lm samples during immersion in water.

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(2 ? 2 cm size). However, water resistance of SPI ?lms was greatly improved by coating with a PLA solution of higher concentration. High water resistance of a ?lm is one of the most important properties from a food packaging point of view, especially for high water activity foods or foods coming into contact with high-humidity environments during transportation and storage. Again, the improvement in water resistance of the ?lm samples was attributed to the hydrophobicity of PLA. 3.4. Thermal properties Thermal properties of SPI ?lms coated with PLA as well as of control SPI and PLA ?lms were investigated using DSC and are shown in Fig. 1 and Table 4. Only three thermograms for SPI, SPI/(3 g PLA/100 ml solvent), and PLA ?lms are shown in Fig. 1. The ?rst endothermic peak (around 5 1C) in all the thermograms may be attributed to the melting of ice. Thermal properties of SPI ?lms coated with PLA indicated that they were re?ecting properties of both component materials. The glass transition tempera1

ture (Tg) of each PLA-coated ?lm showed one distinctive value between the Tg values of control SPI and PLA ?lms. The Tg of PLA ?lms (29.5 1C) was lower than the reported values (55–60 1C) (Drumright et al., 2000), and may be due to the plasticizing effect of chloroform solvent remaining in the ?lm (about 10 g/100 g), as indicated by Rhim et al. (2006). A melting temperature peak for control SPI ?lms was not detected, but all the PLA-coated ?lms demonstrated melting peaks. Melting temperature (Tm) and the apparent enthalpy of fusion (DHf) of PLA-coated ?lms were affected by the presence of PLA. Tm of the PLAcoated ?lms was around 164.7–166.4 1C, which was close to that of control PLA ?lm (167.3 1C). DHf of the ?lms increased with the increase in concentration of PLA. Generally, Tg is used as one of the most important criteria for the compatibility of a polymer blend. It is known that usually only one Tg will appear in DSC thermograms at an intermediate temperature compared to that of the Tg value of each component polymer (Shuai, He, Asakawa, & Inoue, 2001). In this study, all the PLA-coated SPI ?lms showed a single Tg value between those of the control SPI and PLA ?lms. This indicates that both SPI and PLA are fairly compatible for making coated ?lms. 4. Conclusions

0 Heat Flow (W/g)

-1

-2

-3 -100

-50

0

50 150 100 Temperature (οC)

200

250

In conclusion, ?exible PLA-coated SPI ?lms were prepared through a simple coating method without addition of any compatibilizer or chemical modi?cation of ?lm surfaces. Mechanical and water barrier properties of PLA-coated SPI ?lms were greatly improved up to the levels of LDPE and cellulose acetate ?lms, respectively. The PLA-coated SPI ?lms may be suitable for applications in packaging for foods with high water activity or use under high RH conditions. Acknowledgments

Fig. 1. Second heating run differential scanning calorimetry (DSC) thermographs of control soy protein isolate (SPI) ?lm ( ), SPI ?lm coated with a 3 g PLA/100 ml solvent solution ( ), and control PLA ?lm ( ).

This work was supported from the Basic Research Program of the Korea Science and Engineering Foundation (KOSEF; R01-2003-000-10389-0) and is greatly acknowledged. References
ASTM. (1995a). Standard test methods for tensile properties of thin plastic sheeting. Standard D882-88. In , Annual book of ASTM standards, Vol. 8.01 (pp. 182–190). Philadelphia, PA, USA: American Society for Testing and Materials. ASTM. (1995b). Standard test methods for water vapor transmission of materials. Standard E96-95. In , Annual book of ASTM standards, Vol. 4.06 (pp. 697–704). Philadelphia, PA, USA: American Society for Testing and Materials. Banker, G. S. (1966). Film coating theory and practice. Journal of Pharmaceutical Science, 55, 81–89. Brandenburg, A. H., Weller, C. L., & Testin, R. F. (1993). Edible ?lms and coatings from soy protein. Journal of Food Science, 58, 1086–1089. Briston, J. H. (1988). Plastic ?lms. New York: Wiley.

Table 4 Differential scanning calorimetry (DSC) measurement results of soy protein isolate (SPI) ?lms coated with polylactic acid (PLA) solution of varying concentrations Film SPI SPI/(1 g SPI/(2 g SPI/(3 g SPI/(4 g SPI/(5 g PLA Tg (1C) 117.7 111.3 103.3 99.4 91.8 90.8 29.5 Tm (1C) nd 165.0 166.4 165.8 166.1 164.7 167.3 DHf (J/g) nd 0.04 0.35 0.37 0.59 2.24 24.64

PLA/100 ml PLA/100 ml PLA/100 ml PLA/100 ml PLA/100 ml

solvent) solvent) solvent) solvent) solvent)

Tg: glass transition temperature; Tm: melting temperature; DHf: apparent enthalpy of fusion; nd: not detected.

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