NOVELTY - Hydrolysis of biomass comprises (a) contacting the biomass with an enzyme-composition containing at least one enzyme from the class of hydrolases in a vessel; (b) eliminating a part of the biomass; (c) subjecting the part of biomass to a filtration and removing the permeate; (d) back-feeding of at least part of the filtrated biomass to the vessel; and (e) optionally adding an amount of liquid corresponding to the amount removed by filtration, where the step (b) and (c) are carried out in distinct circuits.    USE - The process is useful for hydrolysis of biomass, which is cellulose, hemicelluloses and/or lignin- containing biomass; and sugar-beet, sugar-cane, straw, maize, wood and/or oilseed. The process is useful for producing producing protein-containing product and saccharide-containing permeate-product, which is useful for producing organic compounds, preferably alcohols, organic acids, biopolymers, antibiotics, amino acids, caprolactams, polysaccharides, organic solvents, biofuels, aminoglucans, nucleotides/nucleosides, vitamins, biosurfactants and/or enzymes. The protein-containing product is useful for the production of a fermentation medium (all claimed).    ADVANTAGE - The process is an energy-efficient and economical process for hydrolysis of biomass and provides high amount of monomeric and dimeric sugars and also other valuable components, with a reduced amount of hydrolytic enzymes.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (1) a saccharide-containing permeate-product produced by the process;    (2) a protein-containing product produced by the process; and    (3) a process for producing organic compounds from the saccharide-containing permeate-product, comprising contacting the saccharide-containing permeate-product with at least one microorganism of yeast, bacteria and/or fungi capable of converting the permeate-product to at least one organic compound, fermenting the saccharide-containing permeate-product, and separating the organic compounds.