A process wherein biomass is compacted at 300 kg/m.sup.3 by a helical feeder, degassed, flooded, heated, stirred, and pre-hydrolyzed. The sugar being recovered at a first washing within the reactor with values higher than 90%. The control of the process enables one to produce cellulignin with a high specific surface, 2 m.sup.2/g (measured by BET), compared with the value of 0.4 m.sup.2/g of non-hydrolyzed biomass, maintain crystallinity of the cellulose fibers, aiming at grinding it into fine particles (f<250 mm), globulizing the lignin, exposing the cellulose to attacks by micro-organisms from cud of ruminants, fungi, bacteria or enzymes in microbiological or enzymatic fermentation.