Quebec provincial police say the fire broke out at Meunerie Côté-Paquette in the municipality of L'Ange-Gardien at around 3:10 a.m. on Monday.
About 40 firefighters were called to the scene and 15 homes in the surrounding area had to be evacuated.
High winds and frigid temperatures made it difficult to put out the flames, firefighters said.
The building, made mostly of wood, was still burning at 7 a.m.
Meunerie Côté-Paquette produces feed for animals.