A sample container system for use in a method for determining dietary fiber comprises a sample container and a magnetic mixer. The sample container has a floor and a rigid side-wall upstanding therefrom to together delimit a sample containing space, wherein the floor includes a porous filter portion. The magnetic mixer has at least one concentrically arranged rotor-stator, each of the at least one concentrically arranged rotor-stator including a rotor and a stator, the rotor and the stator defining therebetween an annular shear gap, the rotor configured to rotate relative to the stator, the stator including a stator body having a first plurality of openings therethrough, the rotor including a magnetic coupling and a rotor body having a second plurality of openings therethrough, the magnetic coupling configured to couple with and follow an externally generated rotating magnetic field to cause the rotor to rotate in relation to the stator.