Quantitative Analysis of the Reaction between Gliadin and Citric Acid under Weak Acidic and Weak Alkaline Conditions

Gliadin was reacted with citric acid under weak acidic and weak alkaline conditions in both wet and dry states and the reaction mechanism was studied. The low morphological stability in an aqueous environment and inferior mechanical properties have restricted the applications of plant proteins, although these materials possess a unique structure, biocompatibility and biodegradability. Carboxylic acids such as citric acid are inexpensive and nontoxic chemicals and are preferred for crosslinking proteins and cellulose to improve the desired properties of the materials. In this study, gliadin was chosen as a model of plant proteins because it contained relatively more amine side groups that can react with a carboxylic acid crosslinker than other plant proteins, e.g. zein and soy proteins. In order to avoid using toxic chemicals and experiencing strength loss and/or yellowing of the crosslinked materials, alkaline catalyzed crosslinking of gliadin powders in aqueous citric acid solutions at low temperatures was employed. However, previous research only provided limited evidences, such as improvement in mechanical properties to support the presence of the acylation reaction. To explore the reaction mechanism, titration method was used to investigate the influences of pH, citric acid concentration and reaction temperature on both carboxyl and amine group changes during the reaction. The kinetic parameters of both reaction states have been obtained at different temperatures. Additionally, to further improve the crosslinking degree, a dry state crosslinking of gliadin films with citric acid was also studied. A relationship between the mechanical properties and the crosslinking degree has also been developed.