NOVELTY - Method (M1) of producing fermentative alcohol product, involves (a) obtaining a biomass feedstock slurry comprising fermentable carbon source, undissolved solids, and water, (b) separating at least a portion of the undissolved solids from the slurry to produce an aqueous solution comprising fermentable carbon source and a wet cake co-product comprising solids, and (c) adding the aqueous solution to a fermentation broth comprising recombinant microorganisms in a fermentation vessel to obtain fermentative product with improved biomass processing productivity.    USE - The method is useful for producing fermentative product chosen from methanol, ethanol, propanol, butanol, pentanol, and their isomers, preferably butanol, more preferably isobutanol (all claimed) as fuel additive, feedstock chemical in plastic industry, and food-grade extractant in food and flavor industry.    ADVANTAGE - The biomass processing productivity and alcohol production are improved. The method enables production of butanol with high yield, and is environmentally-friendly and cost-effective.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) method (M2) of producing butanol, involves liquefying the feedstock to obtain the feedstock slurry, separating the undissolved solids from the feedstock slurry to obtain aqueous solution, wet cake and an oil phase, contacting the aqueous solution with the fermentation broth in a fermentor, fermenting the oligosaccharides in the fermentor to produce butanol, and performing in situ removal of the butanol from the fermentation broth as the butanol is produced, where removal of the undissolved solids from the feedstock slurry increases the efficiency of the butanol production;    (2) system for producing butanol, comprising a liquefaction vessel configured to liquefy a feedstock to obtain feedstock slurry, where the liquefaction vessel comprises an inlet for receiving the feedstock, and an outlet for discharging a feedstock slurry, and the feedstock slurry comprises sugar and undissolved solids, a centrifuge configured to remove the undissolved solids from the feedstock slurry to obtain aqueous solution comprising the sugar and wet cake, where the centrifuge comprises an inlet for receiving the feedstock slurry, a first outlet for discharging the aqueous solution, and a second outlet for discharging the wet cake, and a fermentor configured to ferment the aqueous solution to produce butanol, where the fermentor comprises a first inlet for receiving the aqueous solution, a second inlet for receiving extractant, a first outlet for discharging the extractant rich with butanol, and a second outlet for discharging fermentation broth; and    (3) composition comprising (in wt.%): (a) crude protein (20-35, preferably 25-31), crude fat (1-20, preferably 6-10), triglycerides (0-5 or 4-8), fatty acids (4-10 or 0-2), and fatty acid isobutyl esters (2-6 or 1-3), or (b) crude protein (26-34), crude fat (15-25), triglycerides (12-20), fatty acids (1-2), fatty acid isobutyl esters (2-4), lysine (1-2), neutral detergent fiber (11-23), and acid detergent fiber (5-11).