Are aspen sterols and steryl esters changed structurally by kraft pulping and bleaching

Abstract

Steryl esters and sterols are a major portion of the total lipids in aspen wood, and they contribute to pitch deposit problems during pulping. The composition of steryl esters and sterols was compared in (a) aspen wood and (b) unbleached and EoDED-bleached aspen pulp from a kraft mill. The fractions were isolated from acetone extracts by solid-phase extraction (SPE) and were analyzed by gas chromatography (GC) and gas chromatography-mass spectrocospy (CG-MS). GC analysis after alkaline hydrolysis of the steryl ester fraction in aspen wood showed five major components : the sterols α 1 and β-sitosterol and the triterpene alcohols α- and β-amyrin and lupeol. Of these, α 1 , β-sitosterol, and β-amyrin were also present as free sterols in the wood. A comparison of GC profiles of steryl esters/waxes in aspen wood and unbleached pulp indicated that kraft cooking caused no structural changes in the components of this fraction in contrast, during bleaching, α, and β-sitosterat, both originally in free form and as steryl esters in wood, were completely converted to oxidized products, white, β-amyrin was not significantly oxidized. Oxidation of the sterols and unsaturated fatty acids typically occurred at or near double bonds, creating hydroxyl or carbonyl groups. Application : steryl ester levels in aspen kraft pulps are reduced during bleaching. They are converted to free sterols, triterpene alcohols, and fatty acids and their derivatives. These compounds remain present in the pulp in significant amounts. Therefore, improving the removal of steryl esters from aspen wood before pulping will lower the extractives content and reduce pitch problems.