A tunnel freezer comprises an elongate tunnel which is inclined to the horizontal and which can be rotated about its longitudinal axis. The inside of the tunnel is provided with a weir which retains a pool of liquid nitorgen between the weir and the upper end of the tunnel. Particles to be frozen are introduced into the tunnel on a conveyor and drop into the pool where they freeze substantially without aggregation. Rotation of the tunnel carries the particles out of the liquid nitrogen and through the remainder of the tunnel to the outlet end of the tunnel. The frozen particles and the nitrogen vapour from the tunnel pass through a common outlet which is provided with a pivoting plate which restricts the flow through the common outlet so that the tunnel operates at or slightly above atmospheric pressure thereby inhibiting ingress of moist air into the tunnel. Movement of particles through the tunnel is facilitated by lifting bars which form part of a liner which can be readily removed and replaced as desired. In a second embodiment the tunnel is provided with a spiral ribbon which forms a multiplicity of pools extending along the tunnel. This embodiment is particularly useful where the supply of frozen particles is variable.