Effect of isolation procedure on molecular weight distribution and monosaccharide composition of cotton stalk lignins

Abstract

Neutral detergent fiber (NDF) of cotton stalks (CS) was ball milled for 7, 14, 21 and 28 days in a porcelain rotary ball mill, hydrolyzed by a cellulase for 4 days and lignin extracted from it by either dioxane:H 2O or 1 M NaOH. The effects of ball-milling duration (BMD) and extraction procedure on yield, molecular weight distribution and carbohydrate content of lignins were investigated in this study. The dioxane system extracted 25% of the permanganate lignin following 28 days of BM; the major increment, from 10.6 to 20% was observed between the 7th and 14th days of BM. The alkali system reached its yield peak at 14 days of BM at which time 56% of the permanganate lignin had been extracted. The molecular weight distribution pattern of the dioxane lignins (DL) was constant irrespective of BMD. The alkali system had probably targeted larger structures of lignin since the proportion of 23–123 kDa molecules in alkali lignins (AL) was two to three times higher than in DLs. The 14 day BM AL contained the largest proportion of 23–123 kDa lignins, had the highest “weight average molecular weight” (MWt) (17.3 kDa) and was the most dispersive preparation. Both (DL and AL) preparations were low in carbohydrates (5–6 g per 100 g lignin) with xylose and uronic acid as the main monosaccharide components. The high level of uronic acid in the AL preparation indicates that these isolates originated at least partly from the middle lamella area.