As a fermentable carbohydrate, cellulose differs from other carbohydrates generally used as a substrate for fermentation. Cellulose is insoluble and is polymerized as 1-4, в-glucosidic linkage. Each cellulose molecule is an unbranched polymer of 15 to 10,000 D-glucose units. Hydrolysis of crys­talline cellulose is a rate-controlling step in the conversion of biomass to ethanol, because aqueous enzyme solutions have difficulty acting on insol­uble, impermeable, highly structured cellulose. Therefore, making soluble enzymes act on insoluble cellulose is one of the principal challenges in pro­cess development of cellulosic ethanol.

Cellulose needs to be efficiently solubilized such that an entry can be made into cellular metabolic pathways. Solubilization is brought about by enzy­matic hydrolysis catalyzed by a cellulase system of certain bacteria and fungi. Cellulase is a class of enzymes produced primarily by fungi, bacteria, and protozoans, that catalyze the hydrolysis of cellulose, that is, cellulolysis, as described below.