NOVELTY - Method (M1) for producing a fluidized composition from a solid biomass feedstock, comprises: transporting the solid biomass feedstock from (at or near) a proximal end of an extruder or conveyor reaction vessel to a distal end of the reaction vessel; introducing counter flow of a hydrolysis fluid from at or near the distal end of the reaction vessel to the proximal end of the reaction vessel under conditions sufficient to provide the fluidized composition comprising monomeric carbohydrate and/or oligomeric carbohydrate from the solid biomass feedstock; and obtaining the fluidized composition.    USE - The method is useful for producing the fluidized composition.    ADVANTAGE - The method: coverts at least 40 wt.%, preferably at least 90 wt.% of the solid biomass feedstock into the fluidized composition (claimed); provides rapid hydrolysis and fluidization of cellulosic materials; is energy efficient and has smaller footprint than batch processes; increases the relative fluid velocity; involves non-enzymatic hydrolysis of polycarbohydrates; utilizes acetone that is cost-effective and is easily available; minimizes the residence time of mobilized products in the reactor; and provides the fluidized composition with increased yield.    DETAILED DESCRIPTION - Method (M1) for producing a fluidized composition comprising a monomeric carbohydrate and/or oligomeric carbohydrate from a solid biomass feedstock comprising a polycarbohydrate, comprises: transporting the solid biomass feedstock from (at or near) a proximal end of an extruder or conveyor reaction vessel to a distal end of the reaction vessel, where at least a portion of the solid biomass feedstock is comminuted within the reaction vessel; introducing counter flow of a hydrolysis fluid from at or near the distal end of the reaction vessel to the proximal end of the reaction vessel under conditions sufficient to provide the fluidized composition comprising monomeric carbohydrate and/or oligomeric carbohydrate from the solid biomass feedstock; and obtaining the fluidized composition at or near the proximal end of the reaction vessel, where the initial hydrolysis fluid comprises at least 5 vol.% of water, acid and an organic solvent, and the mean residence time of the solid biomass feedstock in the reaction vessel is less than or equal to 60 minutes. INDEPENDENT CLAIMS are also included for:    (1) a method (M2) for producing the fluidized composition from a solid biomass feedstock comprising cellulose using a counter current reaction vessel that comprises a proximal end comprising an inlet port and an outlet port, and a distal end comprising an inlet port, comprising: introducing the solid biomass feedstock into the reaction vessel through the proximal end inlet port; transporting the solid biomass feedstock to the distal end of the reaction vessel; comminuting at least a portion of the solid biomass feedstock within the reaction vessel; and introducing a hydrolysis fluid into the reaction vessel through the distal end inlet port and flowing the hydrolysis fluid towards the proximal end outlet port of the reaction vessel under conditions sufficient to produce the fluidized composition from the biomass feedstock, where the hydrolysis fluid comprises water, acid and an organic solvent, and the amount of water in the hydrolysis fluid is at least 5 vol.% of the hydrolysis fluid; and    (2) a method (M3) for producing a fluidized carbohydrate composition from a solid biomass feedstock comprising cellulose using a counter current reaction vessel that comprises a proximal end comprising an inlet port and an outlet port and a distal end comprising an inlet port and an outlet port, comprising: introducing the solid biomass feedstock into the reaction vessel through the proximal end inlet port; transporting the introduced solid biomass feedstock to the distal end outlet port of the reaction vessel with a mean residence time of less than or equal to 60 minutes; comminuting at least a portion of the solid biomass feedstock within the reaction vessel; introducing a hydrolysis fluid into the reaction vessel through the distal end inlet port; and flowing the hydrolysis fluid towards the proximal end outlet port of the reaction vessel under conditions sufficient to produce the fluidized carbohydrate composition from the solid biomass feedstock, where the hydrolysis fluid comprises water, acid and an organic solvent.