Mechanical and Barrier Properties of Cross-Linked Soy and Whey Protein Based Films S. F. Sabato, † B. Ouattara, ‡ H. Yu, ‡ G. D’Aprano, § C. Le Tien, ‡,# M. A. Mateescu, # and M. Lacroix* ,‡ Canadian Irradiation Centre (CIC), Research Center in Microbiology and Biotechnology, INRS-Institut Armand-Frappier, 531 Boulevard Des Prairies, Laval, Que ´bec H7V 1B7, Canada; Departamento de Aplicac ¸ o ˜es Nucleares, IPEN-CNEN/SP, Travessa R. 400, 05508-900 Sa ˜ o Paulo, SP, Brazil; Sabex Inc., 145 Jules-Leger, Boucherville, Que ´bec J4B 7K8, Canada; and Department of Chemistry and Biochemistry, Universite ´ du Que ´bec a ` Montre ´al, C.P. 8888, Succursale A Centre Ville, Montre ´al, Que ´bec H3C 3P8, Canada Sterilized biofilms based on soy protein isolate (SPI, S system) and a 1:1 mixture of SPI and whey protein isolate (WPI, SW system) were achieved through the formation of cross-links by means of γ-irradiation combined with thermal treatments. The effect of the incorporation of carboxymeth- ylcellulose (CMC) and poly(vinyl alcohol) was also examined. γ-Irradiation combined with thermal treatment improved significantly the mechanical properties, namely, puncture strength and puncture deformation, for all types of films. Irradiated formulations that contain CMC behave more similarly as elastomers. CMC showed also significant improvements of the barrier properties, namely, water vapor permeability, for irradiated films of the S system and for non-irradiated films of the SW system. Keywords: γ-Irradiation; cross-linking; soy protein; whey protein; carboxymethylcellulose; biofilms; poly(vinyl alcohol) INTRODUCTION The past decade has seen considerable interest in the development of protein-based biodegradable edible films and coatings due to their application in the food industry as substitute for traditional plastic films (1). Many publications have revealed their effectiveness to prevent quality changes in processed food and thus to enhance shelf life by serving as selective barriers to moisture transfer, oxygen uptake, and loss of volatile flavors and aromas, as well as lipid oxidation (1, 2). Polysaccharides, proteins, and lipids or a combination of any of these macromolecules were investigated as film-forming agents (2, 3). Soy protein isolate (SPI) is a mixture of proteins with different molecular properties. Most soy proteins, ∼90%, are storage proteins, viz., globulins. Recent reports pointed out the use of SPI to develop edible and biodegradable films (4, 5). Films produced from SPI were found to be excellent oxygen barriers (6, 7). Whey proteins and caseinates are milk proteins, which have also been extensively studied owing to their excellent nutritional value and their numerous functional properties, which are important for the formation of edible films (1, 8). WPC are concentrated whey (∼70% proteins), whereas WPI are whey isolate (∼90% proteins). Edible films based on whey proteins were reported to be flavorless, tasteless, and flexible, and, depending on the formulation, they varied from transparent to translucent (1). All of these characteristics make them suitable for applications in food sciences and technology. Protein films exhibit poor water vapor barrier proper- ties due to the hydrophilic nature of their amino acid groups (2, 9). Recent studies have concentrated on improving protein film mechanical and barrier proper- ties (10-12). The increase of cohesion between protein polypeptide chains was thought to be effective for the improvement of the barrier properties of the films. For instance, the cross-linking of proteins by means of chemical, enzymatic, or physical treatments was re- ported to improve the permeability as well as the mechanical properties. Attempts have been made to stabilize native proteins by inducing the formation of hydrogen, electrostatic, and covalent bonds (13). For example, improvements in protein functionality by cross-linking the soybean 11S protein fraction and whey protein isolate using guinea pig liver transglutaminase has been reported by Yildirim et al. (14). However, high production cost and limited availability of transglutami- nase have limited its potential use in food systems. Electrostatic complexes between proteins and acidic polysaccharides, such as alginate, pectate, and car- boxymethylcellulose (CMC), are also interesting mech- anisms. However, these complexes are very unstable due to their sensitivity to pH changes (15, 16). Heat treatment is well-known to generate cross-links in some proteins, such as soy (10) and whey (17). Indeed, heating favors soy protein cross-linking by disrupting the protein structure and exposing sulfhydryl and hydrophobic groups (18, 19). Sulfhydryl groups con- tained in 11S protein were reported to be responsible for the formation of disulfide linkages, resulting thus in the formation of a three-dimensional network (18, 19). * Address correspondence to this author at INRS-Institut Armand-Frappier, Building 22, CIC, 531 boul. des Prairies, Laval, PQ, Canada H7V 1B7 [telephone (450) 687-5010, ext. 4489; fax (450) 687-5792; email monique.lacroix@ inrs-iaf.uquebec.ca]. † IPEN-CNEN/SP. ‡ INRS-Institut Armand-Frappier. § Sabex Inc. # Universite ´ du Que ´bec a ` Montre ´al. 1397 J. Agric. Food Chem. 2001, 49, 1397-1403 10.1021/jf0005925 CCC: $20.00 © 2001 American Chemical Society Published on Web 02/17/2001