A mill assembly for processing product comprises a mill housing assembly, at least first and second port assemblies defining an exterior inlet opening and an exterior outlet opening, at least one collection assembly, an air guide assembly, and a grinding assembly. In one form, a feed/collection ratio of the exterior inlet opening to the exterior outlet opening is at least approximately 1.5:1. In another form, seals are formed between the port assemblies and the mill housing assembly. In another form, the mill assembly operates in a first mode and a second mode. In the first mode, a ring plug plate and a wall plug plate are detached from the mill housing. In the second mode, the ring plug plate and the wall plug plate are attached to the mill housing to prevent air from flowing out of the third housing port.