arrangement shown in fig1 of the type having a high pressure tap and having through filling pockets that can be rotated from a filling position shown with solid lines on the pocket to an emptying position shown with dashed lines on the pocket the sawdust mixture can when the sluice is in the filling position flow through the pocket where the sawdust mixture expels the cooking fluid that is present in the pocket and out to the pump p this pump returns the expelled cooking fluid under pressure to a pocket that is in the emptying position the high pressure tap 6 provides good insulation between the pressurised part of the system and the part at atmospheric pressure but it can be eliminated completely in a simple embodiment of the system being replaced by one or two pumps arranged in series only when the sawdust mixture has been carried to the cooking vessel is steam mp st added to the top of the cooking vessel 5 such that the sawdust that lies above the level of the cooking fluid is heated to its full cooking temperature within the interval 130 160 u00b0 c in the steam phase of the cooking vessel the level of cooking fluid in the cooking vessel is regulated by withdrawal of cooking fluid appropriately from the bottom of the cooking vessel preferably as is shown through a withdrawal strainer in the form.