A process for simultaneous saccharification and fermentation of a cellulosic solids fraction extracted from a lignocellulosic feedstock. The viscosity of the cellulosic solids fraction is reduced by intermixing with a liquid carbohydrate stream. A suitable liquid carbohydrate stream is a de-lignified liquids fraction that was previously separated from the solids fraction during processing of the lignocellulosic feedstock. Alternatively, the viscosity of the solids fraction may be reduced by commingling with a liquid carbohydrate stream comprising one or more monosaccharides. The reduced-viscosity cellulosic solids fraction is then commingled with a fermentative microbial inoculant and a cellulosic biomass-degrading enzyme composition. The commingled mixture is maintained in a pressurized reaction vessel under elevated temperatures to enable simultaneous enzymatic hydrolysis of the cellulosic solids to monosaccharides and fermentation of the monosaccharides to produce an ethanolic beer. The ethanolic beer is distillable for recovery of fuel-grade ethanol and a stillage that may be further processed.