Formaldehyde-free Crosslinking Of Plant Proteins

This thesis reports a novel crosslinking method with carboxylic acids for renewable plant protein fibers and a systematic study with citric acid for new gliadin fibers. In addition, systematic studies of crosslinking new wheat gluten fibers and gliadin fibers with glutaraldehyde respectively are discussed. Recently there is an increasing interest in using renewable and biodegradable materials to replace limited petroleum-derived raw materials, because the price of petroleum oil is rising, and the consumption is causing more social and environmental problems as well. Renewable plant proteins including wheat gluten, gliadin, soybean, and zein proteins are abundantly available now at low prices from industrial byproducts; therefore cheap plant protein fibers have been developed to meet environmental and social needs. However, the mechanical properties of these plant protein fibers are weak compared to natural protein fibers, such as wool. It is necessary to strengthen them for various industrial applications. Of physical, chemical and enzymatic modifications chemical crosslinking is the most common method to improve the tenacity of protein fibers. Formaldehyde used to be the most effective chemical to crosslink various materials, but it is carcinogenic. Glutaraldehyde is one of the best-known formaldehyde-free agents for crosslinking materials including proteins, but it is said to be cytotoxic. Carboxylic acids are the preferred aldehyde containing crosslinkers for crosslinking proteins and cellulose because of their low toxicity and cost. Carboxylic acids are used to crosslink cellulose and protein materials in the presence of toxic phosphorus- containing catalysts and the process is cured at elevated temperatures, (150 - 185℃) but catalysts containing phosphorus cause significant shade changes in dyed fabrics because of their reductive nature, and the effluents containing phosphorus cause eutrophication in rivers and lakes. In this study, gliadin, soybean, and zein fibers have been crosslinked with malic acid, citric acid (CA), and butanetetracarboxylic acid (BTCA) to improve the tenacity of fibers without phosphorus-containing catalysts and high temperatures. In addition, current knowledge in carboxylic acids’ crosslinking of proteins and cellulose suggests that the use of carboxylic acids with at least three carboxylic groups is necessary. We report malic acid as a new chemical to crosslink plant protein without phosphorus containing catalysts and at low temperatures.Though the chemistry of a crosslinking reaction is very complex and is largely still unexplained, it is possible and more important to control the extent of the crosslinking reactions. For that reason, kinetic data are needed for various industrial applications, and also in case of over crosslinking. This research shows a kinetic study between the mechanical properties of crosslinked gliadin fibers and the crosslinking conditions using glutaraldehyde and citric acid respectively. It shows that the reaction between glutaraldehyde and gliadin or gluten fibers respectively follows a pseudo 0.6 order and the reaction between citric acid and gliadin fibers follows a pseudo 1.2 order. This research also shows that the activation energy of the reaction is highly dependent on pH and therefore alkali acts as a catalyst to the reaction. It also shows changes in the molecular weight of the crosslinked proteins and the stability in water. This research shows that the stability of the crosslinked gliadin fibers in water can apparently be better than that of the control.