Chemistry and Physical Properties of Melt-Processed and Solution-Cross-Linked Corn Zein DAVID J. SESSA,* ,‡ ABDELLATIF MOHAMED, § AND JEFFREY A. BYARS § Plant Polymer Research Unit, Cereal Products and Food Science Research, National Center for Agricultural Utilization Research, United States Department of Agriculture, Agricultural Research Service, 1815 North University Street, Peoria, Illinois 61604 Corn zein was cross-linked with glutaraldehyde (GDA) using glacial acetic acid (HAc) as catalyst. The objectives are to evaluate the swelling characteristics of GDA cross-linked zein gels in water, ethanol, and their combinations. Similar formulations, upon solvent evaporation, form films. The mechanical properties of the films are compared to compression molded tensile bars from GDA melt-processed zein as a second objective. Chemistry of the cross-linking reaction was based on the aldehyde binding characteristics defined by use of fluorescence spectroscopy; sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) to demonstrate the cross-linking reaction; FTIR to observe absorption differences of the cross-linked product; differential scanning calorimetry, dynamic mechanical analysis and thermogravimetric analysis to assess thermal properties; and the use of Instron Universal Testing Machine to evaluate mechanical properties. A reaction mechanism for acid catalyzed GDA cross-linking of zein is proposed. Thermal and mechanical properties of tensile bars cut from either film or formed by compression molding were similar, where both showed increased tensile strengths, ductility and stiffness when compared with unmodified controls. Samples that were reacted with 8% GDA by weight based on weight of zein from either process retained their integrity when tensile bars from each were subjected to boiling water for 10 min or soaking in either water or HAc for 24 h. The melt-processed, cross- linked zein is a more environmentally friendly method that would eliminate the need for HAc recovery. KEYWORDS: Cross-linking; proteins; reaction chemistry; mechanical properties; thermal properties INTRODUCTION Maize R-zeins belong to a class of hydrophobic proteins known as prolamins. They are a group of major storage proteins in corn as well as in corn gluten meal, a coproduct of the ethanol industry generated from wet-milled corn. These proteins are made up of two microheterogenous groupings within apparent molecular weights of 23.8 and 26.7 kDa, respectively (1). Aqueous ethanol and aqueous acetone or acetonyl acetone are generally used to dissolve zein for preparing films and coatings from zein because zein is insoluble in water. This solubility characteristic is attributed to its high content of nonpolar amino acids (2). Recently, Sessa et al. (3) published research on the properties of films from a Japanese white zein reacted with glutaraldehyde (GDA). In that publication, those investigators demonstrated the positive attributes of using glacial acetic acid (HAc) as an excellent solvent for generating their poured films, where HAc also was used to catalyze the GDA cross-linking zein. The focus of that investigation was to assess the structure property correlations of their GDA-modified zein films by thermal and rheological measurements. In that investigation a concentration of 4% or higher GDA, based on weight of zein, was found to generate films as well as organo-gels that were insoluble in solvents common for zein. Based on these findings, a criterion for assessing GDA cross-linked zein will be its insolubility in HAc. In order to tailor zein proteins modified with GDA for potentially new uses in the pharmaceutical, medical, plastics, and coatings industries, a basic understanding of the chem- istry of the reaction is needed. One objective is to evaluate the swelling characteristics of GDA cross-linked zein gels formed in a hermetically sealed system. Similar formulations, open to the air, form films. Because a wet process based on solvent/carrier evaporation of dispersed or solubilized proteins poses environmental, health, and safety concerns with the use of HAc as well as the need to recover solvent, a dry process based on the thermoplastic properties of zein will be evaluated for the cross-linking of zein with GDA as a second objective. Mechanical properties and thermal proper- * Corresponding author. Tel.: 1-309-681-6351; fax: 1-309-681-6686; e-mail: David.Sessa@ars.usda.gov. ‡ Plant Polymer Research Unit. § Cereal Products and Food Science Research. J. Agric. Food Chem. 2008, 56, 7067–7075 7067 10.1021/jf800712k This article not subject to U.S. Copyright. Published 2008 by the American Chemical Society Published on Web 07/18/2008