Internal mixing flights of the rotary mixer alternate in direction for gravity-driven cascading of the material back and forth, producing a 100 percent uniform blend in three minutes or less. The flights also serve to direct the beans toward the discharge port, evacuating the vessel in three to four minutes with no residual in the machine. The mixer has a stationary intake chute fed by a mechanical conveyor, and is fully enclosed, containing dust and preventing contamination of the product.