Method for preparation of decontaminated natural rubber, that comprises a decontamination step in which wet natural rubber coagulum with a humidity level greater than 12% by weight passes through a system comprising an extruder and a filter installed at the extruder outlet, the extruder comprising a grooved sheath, a screw arranged in the sheath and comprising a hub and a thread which extends radially outwardly relative to the hub, a grooved sheath having an axially symmetrical structure comprising in its thickness grooves (211, 212) opening on the inner surface of the sheath, each groove (211, 212) comprising - a bottom face delimited by two bottom ridges (132, 133), - two side faces (122, 123) extending towards the inside from the bottom face, each being delimited by a bottom ridge (132, 133) and a ridge (112, 113) intersecting the inner surface of the sheath, the process being characterized in that the sheath is such that, in any plane perpendicular to the axis of the sheath, R expressed in mm being the distance between the sheath centre and the inner surface of the sheath: a. the ratio (total length of groove openings expressed in mm)/(2πR), designated A, is at least equal to 0.25 and at most equal to 0.9; b. the ratio (number of grooves/2R), designated B, is greater than or equal to 0.1; c. the ratio (depth of grooves expressed in mm)/, designated C, is greater than 0.02, and d. the plane passing through the two bottom ridges (132, 133) forming, with each plane passing through a bottom ridge (132, 133) and an intersection ridge (112, 113) delimiting a side face (122, 123), an angle δ at least equal to 60° and at most equal to 90°. the extruder comprising a grooved sheath, a screw arranged in the sheath and comprising a hub and a thread which extends radially outwardly relative to the hub, a grooved sheath having an axially symmetrical structure comprising in its thickness grooves (211, 212) opening on the inner surface of the sheath, each groove (211, 212) comprising - a bottom face delimited by two bottom ridges (132, 133), - two side faces (122, 123) extending towards the inside from the bottom face, each being delimited by a bottom ridge (132, 133) and a ridge (112, 113) intersecting the inner surface of the sheath, - a bottom face delimited by two bottom ridges (132, 133), - two side faces (122, 123) extending towards the inside from the bottom face, each being delimited by a bottom ridge (132, 133) and a ridge (112, 113) intersecting the inner surface of the sheath, the process being characterized in that the sheath is such that, in any plane perpendicular to the axis of the sheath, R expressed in mm being the distance between the sheath centre and the inner surface of the sheath: a. the ratio (total length of groove openings expressed in mm)/(2πR), designated A, is at least equal to 0.25 and at most equal to 0.9; b. the ratio (number of grooves/2R), designated B, is greater than or equal to 0.1; c. the ratio (depth of grooves expressed in mm)/, designated C, is greater than 0.02, and d. the plane passing through the two bottom ridges (132, 133) forming, with each plane passing through a bottom ridge (132, 133) and an intersection ridge (112, 113) delimiting a side face (122, 123), an angle δ at least equal to 60° and at most equal to 90°. a. the ratio (total length of groove openings expressed in mm)/(2πR), designated A, is at least equal to 0.25 and at most equal to 0.9; b. the ratio (number of grooves/2R), designated B, is greater than or equal to 0.1; c. the ratio (depth of grooves expressed in mm)/, designated C, is greater than 0.02, and d. the plane passing through the two bottom ridges (132, 133) forming, with each plane passing through a bottom ridge (132, 133) and an intersection ridge (112, 113) delimiting a side face (122, 123), an angle δ at least equal to 60° and at most equal to 90°.