Water Vapor Permeability and Mechanical Properties of Grain Protein-Based Films as Effected by Mixtures of Polyethylene Glycol amd Glycerin Plasticizers

Grain protein-based films containing mixtures of glycerin and polyethylene glycol (PEG) as plasticizers were prepared and evaluated for water vapor permeability (WVP), tensile strength (TS), and elongation (E). Changes in mechanical properties during storage were also studied. The PEG produced opposite trends in E of wheat gluten and corn-zein films. The TS of wheat gluten films increased and E of the films decreased as the ratio of glycerin/PEG decreased. Corn-zein films containing only glycerin were very brittle (E of 4%), and E of the films improved to 94% when the ratio ofmL PEGIg protein was 0.39. The WVP of both films decreased as the ratio of glycerin/PEG was decreased. Also, WVP of both films increased as the total amount of plasticizer added to the films increased. Mixtures of glycerin and PEG as plasticizer are less fugitive than glycerin alone in grain protein-based films and can reduce the deterioration of mechanical properties during storage. Keywords, Plasticizers, Corn-zein, Wheat gluten. Protein-based films have been investigated for potential applications as wrapping and coating materials for food products. Protein-based films have been made from grain proteins (cereals and oilseeds) and animal proteins (casein, gelatin, keratin, etc.) (Guilbert, 1986). The development history, property evaluations, and application trials of protein-based films have been reviewed by Kester and Fennema (1986), Guilbert (1986), Gennadios and Weller (1990) and Park (1991). Protein-based films and coatings have been used as gas and grease barriers and for mechanical protection in several food systems as a means of extending shelf life and minimizing quality changes during marketing (Aydt et al., 1991; Gennadios et al., 1993b; Guilbert, 1986; Park and Chinnan, 1993; Park et al., 1992). Soybean films such as "Yuba" in Japan, "Tou-Fu-Pi" in China, and "Fu Chok" in Malaysia have been used traditionally as edible wraps for containing meat and vegetable mixes which are fried in oil (Snyder and Kwon, 1987). Cosier (1958) used corn-zein as an oxygen barrier coating to prevent oxidation in confectionery products. Park et al. (1993a) reported that Article has been reviewed and approved for publication by the Food and Process Engineering Inst, of ASAE. This research was supported in part by USDA National Research hiitiative Competitive Grant, No. 92-37500-8208. Technical Contribution No. 94-3514 of the South Carolina Agricultural Experiment Station, Clemson University, Clemson, S.C. The authors are Hyun Jin Park, ASAE Member Engineer, Assistant Professor, Dept. of Food Engineering, Mokpo National University, Mokpo, Korea (former Research Associate/Assistant Professor of the Dept. of Agricultural and Biological Engineering, Clemson University); Joe M. Bunn, ASAE Fellow Engineer, Professor, Dept. of Agriculmral and Biological Engineering, Clemson University, Clemson, S.C; Curtis L. Weller, ASAE Member, Assistant Professor, Dept. of Biological Systems Engineering, University of Nebraska, Lincoln (former Associate Professor, Dept. of Agricultural and Biological Engineering, Clemson University); Peter J. Vergano, ASAE Member, Associate Professor, Dept. of Food Science, and Robert F. Testin, Associate Professor, Dept. of Food Science and Dept. of Agricultural and Biological Engineering, Clemson University, Clemson, S.C. corn-zein provides a good oxygen barrier on tomatoes, resulting in doubling of their shelf life. Trezza and Vergano (1993) found zein-coated paper to have a high potential as a wrapping material in quick service restaurants. Krochta et al. (1988) developed casein coatings which can be used for precut fruits and vegetables to extend the marketing period. Barrier and mechanical properties of protein-based films are affected by factors such as concentration and selection of plasticizer, pH of solvent, salts, and other additives. Gennadios et al. (1993a) found that the addition of mineral oil and keratin to film-forming solutions reduced water vapor permeability in protein-based films. Gennadios et al. (1992) also reported that the tensile strength of com zein and wheat gluten films varied significantly with conditioning at different combinations of relative humidity (23 to 75%) and temperature (5 to 45° C). Gontard et al. (1992) reported that pH and ethanol concentration had strong interactive effects on film opacity, water solubility, and water vapor permeability. The concentration of plasticizer greatly affects mechanical properties of grain protein-based films. Flexibility is improved, but puncture strength and water vapor and oxygen barrier properties are decreased as plasticizer concentration is increased in wheat gluten films (Gontard et al., 1993; Park et al., 1992). In these studies, only one plasticizer, glycerin, was used in the protein-based films. Even glycerin that is well dispersed in protein film solutions migrates from the bulk to the surface of the film matrix because of binding limitations between protein molecules and glycerin. Excess glycerin can migrate through the film matrix. The migration speed of glycerin depends on the types of functional groups, polarity, and structure of the film matrix. For example, the surface of new com zein films is transparent, but it has a greasy appearance within a few hours because of glycerin migration to the surface. The loss of flexibility of the film is an another indicator of glycerin migration to the surface (Park et al., 1992). Also, glycerin could migrate slowly from the film matrix of wheat gluten during storage if the protein molecules of wheat gluten VOL. 37(4): 1281-1285 Transactions of the ASAE © 1994 American Society of Agricultural Engineers 0001-2351 / 94/ 3704-1281 1281 have a limited capability for binding with glycerin. The lowering of elongation values of wheat gluten film during storage is an indicator of glycerin migration. The structure and molecular size of glycerin (MW, 92) allows more rapid diffusion than polyethylene glycol (PEG) (MW, 400) because PEG as a plasticizer has longer carbon chains (number of carbons in chain « 16) than glycerin (number of carbons in chain = 3). Loss of flexibility in protein-based films during storage is a major limitation for their use as a wrap or coating for food products. However, no evidence has been found of attempts to use other plasticizers to overcome loss of elongation properties in protein-based films. The objectives of this study were to: • Determine the effect of glycerin and PEG mixtures as plasticizers on water vapor permeability and mechanical properties (tensile strength and elongation) of grain protein-based films. • Evaluate the effectiveness of PEG and glycerin mixtures in reducing elongation during storage. MATERIALS AND METHODS