NOVELTY - Simultaneous saccharification and fermentation (SSF) comprises culturing a complete fermentation medium comprising at least one fermenting microorganism, at least one xylan-containing biomass, at least one cellulase, at least one hemicellulase, and at least one inverting or retaining beta -xylosidase, for a period and under conditions suitable for producing a fermentation product or alkyl- beta -xylopyranoside (AXP).    USE - The methods are used for SSF, for producing a cellulase preparation, for biomass saccharification, and for producing a fermentation product and AXP compound (all claimed).    ADVANTAGE - The present invention provides inverting beta-xylosidase enzymes to reduce byproduct formation and increase the yield of fermentation products, as well as, retaining beta-xylosidase enzymes to improve production of AXP compounds, in a simultaneous saccharification and fermentation reactions. It provides methods and compositions for improving the efficiency of SSF reactions and increasing the yield of biofuels such as bioethanol.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are: (1) a Trichoderma reesei cell, which has been engineered such that the native beta -xylosidase gene has been deleted or that it has no detectable beta -xylosidase activity, or such that its native beta - xylosidase gene has been overexpressed or that it expresses a foreign beta - xylosidase gene; (2) a method for producing a cellulase preparation by culturing the T. reesei cell of (1) under conditions that result in the production of the cellulase preparation; (3) a cellulase preparation obtained from performing the method above; (4) a culture broth produced by culturing the T. reesei cell of (1); (5) a method of biomass saccharification by contacting the biomass with the cellulase preparation of (3); (6) a composition comprising at least one fermenting microorganism, at least one xylan-containing biomass, at least one cellulase, at least one hemicellulase, and at least one inverting or retaining beta -xylosidase; (7) a method of producing a fermentation product by culturing the composition of (6) for a period of time; and (8) a method of producing an AXP compound by culturing the composition of (5) for a period of time.