Applying Direct Yellow 11 to a modified Simons’ staining assay

For quantification of overall fiber accessibility of lignocellulosic substrates, Direct Yellow 11 (C.I. 40000) is a suitable alternative to the discontinued Pylam Products’ dye Direct Orange 15 (C.I. 40002/40003). In this study we present a side-by-side comparison between the two azo-stilbene dyes. We characterize individual dye fractions and provide equations to determine individual concentrations. We present a modified Simons’ staining protocol incorporating the high molecular weight fraction of Direct Yellow 11. We perform tests on lignin, cellulosic, and lignocellulosic materials. In all tests, the two dyes perform similarly and satisfy many accessibility measurement criteria. We demonstrate that the adsorption of Direct Yellow 11 onto a substrate correlates with that substrate’s propensity for enzymatic hydrolysis. We confirm this correlation on a series of organic solvent pretreatments and on a series of lignocellulosic substrates. Finally, we outline the inherent limitations of performing adsorption experiments with Direct Yellow 11 and other high molecular weight dyes.