Soy Protein Isolate/Poly(ethylene oxide) Films

Cereal Chem. 72(6):559-563 Films were prepared by casting and drying alkaline aqueous filmelongation at break (E). TS values ranged from 1.4 to 3.9 MPa and E forming solutions of soy protein isolate (SPI). Four additional types of values ranged from 83 to 152%. Water vapor permeability of the films films were made by combining SPI with poly(ethy1ene oxide) (PEO) in ranged from 3.0 x to 4.0 x glm.sec.Pa. Scanning electron SPI to PEO ratios of 19: 1,9: 1,4: 1, and 1.5: 1 (wlw). Glycerin was added micrographs of film cross sections showed an increase in the inferior to all film-forming solutions as a plasticizer at 60% of total solid weight. texture with increasing amounts of PEO in the films. Addition of PEO decreased film tensile strength (TS) and increased There is a growing concern over nondegradable plastic in the municipal solid waste (MSW) stream in the United States, as well as around the world. Plastic materials account for =7% of the MSW, this number is expected to grow to > lo% by the beginning of the next century (Thayer 1990). Plastic materials are indestructible in nature and resist rapid degradation. As a result, interest exists in developing degradable materials for single use items, such as utensils, garbage and grocery bags, plates, planting pots, and mulches, as an alternative to petroleum-based plastic products. Incorporation of biopolymers such as starches and proteins has shown promise in terms of enhancing degradation of plastic materials (Griffin 1974). Plant proteins have the ability to form films that can be used in edible packaging applications. Edible films from biopolymers have been extensively reviewed by Kester and Fennema (1986), Guilbert (1986), Krochta (1992), and Gennadios et al (1994a). In general, protein films have poor moisture barrier properties due to the hydrophilic nature of their amino acids. Recent studies have concentrated on improving protein film mechanical and barrier properties (Brandenburg et al 1993, Gennadios et al 1993a, Shih 1994, Stuchell and Krochta 1994). Approaches more or less successfully employed to improve properties of soy protein films include treatment with alkali (Brandenburg et a1 1993), alkylation with sodium alginate (Shih 1994), treatment with propylene glycol alginate (Shih 1994), and enzymatic treatment with horseradish peroxidase (Stuchell and Krochta 1994). Several studies reported on incorporation of biopolymers, such as starch and protein, into extrusion blown polyethylene films (Otey et a1 1974, 1977, 1980, 1987; Otey and Westhoff 1984; Ghorpade and Hanna 1993; Park et a1 1993). Otey et al (1977, 1980, 1987) and Otey and Westhoff (1984) prepared starch-based compostible films containing polyethylene (ethylene-co-acrylic acid) for agricultural mulches. Dennenberg et al (1978) demonstrated the biodegradability by Aspergillus niger of a starch graft polymethylacrylate copolymer that exhibited excellent tensile properties. Park et a1 (1 993) reported characteristics of zein-filled polyethylene compostible films, while Ghorpade and Hanna (1993) studied properties of extrusion-blown soy protein isolate1 'Journal Series 11067, Agricultural Research Division, Institute of Agriculture and Natural Resources, University of Nebraska-Lincoln, Presented in pan at the AACC Annual Meeting, Nashville, TN, 1994. 2Corresponding author. Department of Biological Systems Engineering, University of Nebraska-Lincoln, L. W. Chase Hall, Lincoln, NE 68583-0726. E-mail: [email protected] 3Assistant professor, research associate, professor, and associate professor, respectively, Department of Biological Systems Engineering, University of Nebraska-Lincoln. O 1995 American Association of Cereal Chemists, Inc polyethylene oxidellow density polyethylene films. Films combining protein with synthetic plastics show potential for production of compostible plastic materials. Degradation of protein by microorganisms can be easily achieved, thus rendering the remaining synthetic polymer vulnerable to photoo r thermal degradation. This study was conducted with the objective of forming cast films from soy protein isolate (SPI) with various amount of poly(ethylene oxide) (PEO) and determining the effects of PEO addition on mechanical and barrier film properties. PEO was used in this study because of its high water solublity and to study interactions with protein molecules. MATERIALS AND M E T H O D S Reagents SPI (ARPRO 1100) was obtained from Archer Daniels Midland Corp. (Decatur, IL) and stored at 4OC before use. PEO was purchased from Scientific Polymer Products, Inc., (Ontario, NY). Glycerin was purchased from Fisher Scientific (Pittsburgh, PA). Film Formation Film-forming solutions were prepared by slowly adding 5 g of SPI to constantly stirred mixtures of 100 ml of distilled water and 3 g of glycerin. Glycerin was added as a plasticizer to overcome film brittleness and to obtain free-standing films. Solution pH was adjusted to 11.0 k 0.1 with I N sodium hydroxide. The solutions were incubated for 30 min in a 70°C constant temperature water bath. Solutions were strained through cheese cloth (grade 40, Fisher Scientific) upon removal from the water bath and cast on Teflon-coated glass plates. Films were peeled from the plates after drying at ambient temperature for =30 hr. Four additional types of films were made by combining SPI with PEO in ratios of 19: 1, 9: 1, 4: 1, and 1.5: 1 (wlw). These films are hereafter referred as 5, 10, 20, and 40%, respectively, based upon PEO concentration. Moisture Content Film moisture content (MC) was measured after conditioning films in an environmental chamber at 25OC and 50% rh for three days. These conditions were similar to those for conditioning film specimens before tensile testing. Samples of 400-500 mg were weighed in aluminum dishes and dried for 24 hr in an air-circulating oven at 105°C. MC was calculated in duplicate for each type of film as percentage weight lost during drying and reported on a wet basis. Tensile Strength a n d Elongation a t Break Films were conditioned at 50% rh and 25°C for three days before testing. A universal testing instrument (model 5566, InVol. 72, No. 6, 1995 559 stron Engineering Corp., Canton, MA) was used to determine tensile strength (TS) and elongation at break (E), according to ASTM Method D 882-88 (ASTM 1989). Film specimens (2.54 cm wide x 10 cm long) were cut. Five thickness measurements were taken along each specimen with a hand-held micrometer (B. C. Ames Co., Waltham, MA); the mean of the five measurements was used in TS calculations. The initial grip separation and crosshead speed were set at 5 cm and 50 cdmin, respectively. TS was calculated by dividing the maximum (peak) load necessary to pull the specimen apart by the original cross-sectional area of the specimen. E was calculated by dividing film elongation at rupture by the initial gauge length of the specimen and multiplying by 100. TS and E determinations for each type of film were replicated four times with individually prepared films as the replicated experimental units and six sampling units (specimens) tested from each film replicate. Water Vapor Permeability Film specimens (7 x 7 cm) were cut. Five thickness measurements were taken on each specimen: one at the center and four around the perimeter. The mean value was used as the specimen thickness in water vapor permeability (WVP) calculations. Before testing, all film specimens were conditioned at 2S°C and 50% rh for two days. Four individually cast film specimens were tested from each type of film. WVP (g.rn/m2 .set Pa) was calculated as: WVP = (WVTR x L)lAp ( 1 ) where WVTR was the measured water vapor transmission rate (g/m2.sec) through a film specimen, L was the mean film thickness (m), and p was the partial water vapor pressure difference (Pa) across the two sides of the film specimen. WVTR was determined gravimetrically using a modified ASTM Method E 96-80 (ASTM 1989). Film specimens were mounted on poly(methy1 methacrylate) cups filled with distilled water up to 1 cm from the film underside. Design of the cups was described by Gennadios et a1 (1994b). The cups were placed in an environmental chamber set at 25°C and 50% rh. A fan was operated within the chamber creating an air velocity of 198 d m i n over the surface of the cups to remove the permeating water vapor. Weights of the cups were recorded six times at 1 hr intervals. Steady state was reached after 1 hr. Slopes of the steady state (linear) portion of weight loss versus time curves were used to estimate WVTR. Because of the low water vapor resistance of protein-based films, actual rh values at the film undersides during testing were lower than the theoretical value of 100%. Actual rh values at the film undersides and film WVP values were calculated after accounting for the resistance of the stagnant air layer between the film undersides and the water surface in the cups (McHugh et a1 1993, Gennadios et a1 1994b). The mean of the initial and the final stagnant air gap heights was used in the calculations. Color Color values of the SPI-PEO films were measured (CR-300 Minolta Chroma Meter, Minolta Camera Co., Ltd., Osaka, Japan). This instrument is a tristimulus color analyzer with an 8-mm diameter measuring area. Film specimens were placed on a white standard plate (calibration plate CR-A43) and the HunterLab color scale was used to measure color: L = 0 (black) to L = 100