PROBLEM TO BE SOLVED: To produce protein microfibrils having easy twistability, excellent in flexibility, touch and gloss and useful for a composite material, etc., by carrying out a bead impact crushing treatment of dispersed natural protein fibers and finely splitting the fibers even in the longitudinal direction parallel to the fiber axis.
SOLUTION: Natural protein fibers such as a silk yarn are treated with an aqueous solution of an alkali containing an alkaline agent or treated with an aqueous solution of a neutral salt at a pH regulated with the alkaline agent to swell the fibers and provide a state in which the fibrous structure is somewhat relaxed. The resultant fibers are dispersed in an aqueous solution and prescribed vibration and shear stress are applied to the dispersed natural protein fibers. For example, an impact crushing treatment with beads is performed under conditions of 0.2-1 mmϕ beads diameter, 5-50 min impact treating time and ≥50 Hz frequency of the beads vibration to subject the fibers to not only cutting in the transverse direction perpendicular to the fiber axis of the fibers but also fine splitting even in the longitudinal direction parallel to the fiber axis. Thereby, protein microfibrils are produced. Furthermore, the protein microfibrils are preferably contained in a polymeric substance to prepare a composite material.