solid content of the fibers fed into the extruder can be very high typically above 30 wt and even preferably above 50 wt the produced mfc will thus have increased dry content this often is beneficial in later usage of the microfibrillated cellulose if it is necessary to transport the produced mfc it is advantageous to have a high dry content in order to avoid transporting large amounts of water also if the produced mfc is added to surface of for example a paper or board web it is preferred to have high dry content in order to reduce the drying demands of the paper or board the fibers are preferably modified the modification is preferably done by addition of a modifying chemical cellulosic fibers can be modified in many different ways in order to alter the properties of the fibers i e to functionalize the fibers the fibers can for example be carboxylized oxidized or be made cationic surface modification can either be made by a direct surface reaction resulting in a modification or by indirect modification through adsorption of one or several polymers in prior art surface modification techniques such as surface deposition using e g corona flame atomic layer deposition plasma treatment or similar treatments are done in a separate process step the use of a separate modification step increases the production time and the cost for the production of modified fibers by addition of a modifying chemical to the extruder according to the invention it is possible to modify the fibers at the same time.