Effect of lignin structure on enzymatic hydrolysis of plant residues

Biochemical conversion of lignocellulose into high value and energy-intensive products necessitates pretreatments that enhance enzymatic hydrolysis of lignocellulosic carbohydrates. This thesis investigated structural changes in lignin during various analytical and industrially relevant treatments of crop residues. The objective was to elucidate the effect of lignin structure on enzymatic digestibility of cellulose. Fractionation of lignin during sequential alkaline treatments of maize stem was studied using thioacidolysis. The developed fractionation procedure enabled isolation of two structurally distinct lignin-carbohydrate fractions which comprised approximately half of the total lignin. Based on these results, the distribution of lignin structural units within the plant cell walls was identified. The effect of ball milling treatment of maize stem on lignin structure and on enzymatic carbohydrate conversion was assessed. Aryl ether linkages of lignin were not cleaved by the milling which decreased crystallinity and led to high enzymatic digestibility of cellulose. Despite the long duration of the milling only minor hydrolysis of arabinoxylan occurred. This resistance was interpreted to result from interlinkages between lignin and arabinoxylan, as revealed using the isolated lignin-carbohydrate fractions as reference materials. A spectrophotometric method was developed for determination of lignin surface area in solid-state. The method was based on quantitative determination of binding of a cationic dye to acidic hydroxyl groups of lignin in neutral aqueous suspension. About one half of the total acidic hydroxyl groups of wheat straw alkali lignin was accessible to the dye, a phenomenon interpreted as a reflection of the three-dimensional structure of lignin. The effect of increasing autohydrolysis severity on structure of wheat straw lignin was determined. Chemical and chromatographic analyses confirmed that lignin was cleaved during both autohydrolysis and the subsequent high intensity extraction. It was observed that lignin surface area decreased as a function of autohydrolysis severity. An underlying mechanism was proposed to include acid-catalysed hydrolysis of lignin-carbohydrate network which caused densification of lignin. One of the main results of the thesis was the inverse correlation found between lignin surface area and enzymatic digestibility of cellulose. Utilisation of this new information should lead to setting reduction of lignin surface area as an objective of lignocellulose pretreatment processes.