NOVELTY - A biomass saccharification mixture comprises (a) a biomass material; and (b) an enzyme composition comprising a glycosyl hydrolase family 61 enzyme having endoglucanase activity, where the biomass saccharification mixture has a lower viscosity than a biomass saccharification mixture without the glycosyl hydrolase family 61 enzyme and/or is capable of increasing the level of saccharification in the mixture as compared to the level of saccharification in a mixture having no or a lower level of glycosyl hydrolase family 61 enzyme.    USE - The biomass saccharification mixture is used for hydrolyzing a biomass material selected from seeds, grains, tubers, plant waste, byproducts of food processing or industrial processing, corn cobs, corn stover, grasses, Sorghastrum nutans, switchgrass, perennial canes, wood, wood chips, wood processing waste, sawdust, paper, paper waste, pulp, and recycled paper, potatoes, soybean, barley, rye, oats, wheat, beets, sugar cane bagasse and straw (all claimed).    ADVANTAGE - Additional ways to reduce viscosity and improve yield of desirable fermentable sugars are provided.    DETAILED DESCRIPTION - A biomass saccharification mixture comprises (a) a biomass material; and (b) an enzyme composition comprising a glycosyl hydrolase family 61 enzyme having endoglucanase activity, which is: (i) at least 65% in sequence identity to any one of SEQ ID NO: 1-29 and 148; (ii) at least 65% in sequence identity to residues 22-344 of SEQ ID NO: 27; (iii) comprises at least one amino acid sequence motifs selected from SEQ ID NOs: 84-91; (iv) comprises one or more sequence motifs selected from SEQ ID NO: 84 and 88; SEQ ID NOs: 85 and 88; SEQ ID NO: 86; SEQ ID NO: 87; SEQ ID NO: 84, 88 and 89; SEQ ID NOs: 84, 88 and 91; SEQ ID NOs: 85, 88 and 91; SEQ ID NOs: 84, 88, 89 and 91; SEQ ID NOs: 84, 88, 90 and 91; SEQ ID NOs: 85, 88, 89 and 91; and SEQ ID NOs: 85, 88, 90 and 91; (v) encoded by a polynucleotide sequence or its complement that is at least 65% sequence identity to SEQ ID NO: 30; or (vi) encoded by a polynucleotide sequence that hybridizes under high stringency conditions to SEQ ID NO: 30 or to a complement; where the biomass saccharification mixture has a lower viscosity than a biomass saccharification mixture without the glycosyl hydrolase family 61 enzyme and/or is capable of increasing the level of saccharification in the mixture as compared to the level of saccharification in a mixture having no or a lower level of glycosyl hydrolase family 61 enzyme. INDEPENDENT CLAIMS are:    (1) a method of hydrolyzing a biomass material by incubating the biomass saccharification mixture above, under conditions suitable for hydrolyzing the biomass materials in the biomass saccharification mixture and for a sufficient period of time; and    (2) a method of using the composition above to convert a biomass material into fermentable sugars in a merchant enzyme supply model or an on-site bio-refinery model.