TECHNOLOGY FOCUS - BIOLOGY - Preferred Process: The hydrolysate (H1) obtained in the reactor 1 outlet is sent to an ultra filtration membrane for extracting cellulases. The majority of the hydrolysate flux (H2) obtained from the second reactor, preferably the entire hydrolysate flux (H2) obtained from the second reactor, is reinjected into the first reactor during the conversion phase. The production of sweet juice is carried out in a continuous or discontinuous mode. The production of the sweet juice is carried out in a continuous mode, the hydrolysate (H2) is separated into at least two flux (H2a) and (H2b), the flux (H2b) is introduced continuously into a contactor and the flux (H2b) is reinjected into the first reactor. The flux (H2a) is contacted with new lignocellulosic substrate in equipment. The new substrate impregnated with cellulases obtained from the contactor is separated from the hydrolysate (H3) before sending into the first reactor. The production of sweet juice is carried out in a batch mode, comprising a first step consisting of emptying the reactors inside which the hydrolysate flux (H2) obtained from the second reactor is sent into the contactor where hydrolysate flux is contacted with new lignocellulosic substrate and a second step consisting of restarting by introducing the new substrate impregnated with cellulases in the first reactor. The reaction mixture contained in the reactor, during the complete emptying of the first reactor, is sent to a filter medium from which the solid residue (R) is separated, then towards the means for separation in outlet from which a liquid fraction (H4) is obtained constituted of a sweet juice, optionally sent towards the second reactor. The liquid fraction (H4) is totally or partly reintroduced in the first reactor during the starting phase of the enzymatic hydrolysis by reintroducing fresh cellulases and new substrate. The liquid fraction (H4) is sent to the contactor to impregnate the new substrate. In the process, the operating conditions of the temperature and/or the pH of the reactors and are different. Preferred Components: The device comprises at least a filtering medium used during the phase of complete draining of the hydrolysis reactor, at least one pipe for extracting the solid residue R, at least one pipe for extraction of H4 flux from produced sweet juice and optionally at least one pipe for the introduction of H4 flux in the second reactor. The second reactor comprising beta -glucosidases is a moving bed reactor or agitated slurry or a fixed bed reactor. The filtering medium is a grid or a grade bed installed in the second reactor. EXAMPLE - No suitable example given.