Methods for reducing contamination during enzymatic hydrolysis of biomass-derived cellulose

A process for producing a fermentation product from cellulosic biomass, said process comprising:

• (a) in a digestor, fractionating a feedstock comprising lignocellulosic biomass in the presence of a solvent for lignin, a hydrolysis catalyst, and water, to produce a liquor containing hemicellulose, cellulose-rich solids, and lignin;
• (b) substantially separating said cellulose-rich solids from said liquor;
• (c) hydrolyzing said hemicellulose contained in said liquor to produce fermentable hemicellulosic monomers;
• (d) enzymatically converting cellulose-rich solids to glucose, comprising exposing said cellulose-rich solids to (i) cellulase enzymes, to hydrolyze cellulose to glucose;
• and (ii) an external sulfur-containing compound, to deter bacterial and/or yeast contamination during cellulose hydrolysis;
• and
• (e) fermenting at least a portion of said glucose and optionally said fermentable hemicellulosic monomers to a fermentation product.