A fermentation apparatus (10) with a fermentation vessel (12) having a low speed stirrer, the stirrer comprising a shaft (14) entering the vessel through an aperture (16) in the bottom of the vessel, one or more stirring elements (18) being mounted on the shaft for rotation therewith within the vessel; a seal (20) being provided to seal the gap between the shaft and aperture, the seal being located in a cylindrical housing attached to the bottom of the vessel, the seal having first and second seal assemblies (30, 50) spaced axially of one another to define a chamber (64) therebetween, an inlet (66) opening into said chamber for connection of the chamber to a supply of processing gas, the first seal assembly (30) being located between the fermentation vessel and the chamber comprising a seat (32) mounted on the shaft for rotation therewith and a mating ring mounted (36) in fixed rotational relationship but moveably axially of the housing, a sealing face (44) of the mating ring being resiliently biased into engagement with a sealing face (42) of the seat, the sealing face of one of the mating ring or the seat having grooves (68) , which will provide hydrodynamic separation of the sealing faces upon rotation of the shaft .