Impact of ball milling on maize (Zea mays L.) stem structural components and on enzymatic hydrolysis of carbohydrates

• Ball milling of maize (Zea mays L.) stem was studied as a function of milling time.
• Cellulose crystallinity decreased and hemicelluloses depolymerized.
• Milling time ≥3 h increased enzymatic cellulose-to-glucose conversion up to 90%.
• Arabinoxylan-rich fraction associated to lignin resisted enzymatic conversion.
• Lignin chemical structure was not modified by ball milling or enzymatic hydrolysis.

The impact of ball milling on structural components of maize (Zea mays L.) stem was investigated in relation to enzymatic hydrolysis of carbohydrates. Ball milling of extractive-free maize stem material was carried out with different milling times up to 12 h. Carbohydrate conversion from ball-milled maize stem material with cellulolytic preparation Onozuka R-10 increased to 79% with increasing ball milling time up to 4 h, but did not thereafter increase further. Ball milling caused drastic depolymerization of hemicelluloses as revealed by increasing amounts of monosaccharides released in absence of enzymes. Infrared spectroscopy showed that cellulose crystallinity decreased within the first 6 h milling thereafter reaching a plateau. Thioacidolysis of solid residue fractions obtained after treatment of ball-milled samples with Onozuka R-10 suggested that no extensive degradation of lignin occurred during the milling. The effect of lignin structure on enzymatic hydrolysis of associated carbohydrates was further studied independently of cellulose crystallinity. Two lignin-carbohydrate fractions comprising hemicellulose and lignin with either high or low amount of aryl ether linkages were used as model materials. Lignins abundant in aryl ether linkages appeared more detrimental than condensed lignins to enzymatic hydrolysis of associated carbohydrates.

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