Abstract: The invention relates to a method of maintaining the pressure difference in a heat treatment plant such as, for example, a pasteurizer for liquid foods. In a temporary production disruption, the insufficiently treated product is caused to recycle by a return stroke. The pressure difference is maintained in that the already heat-treated product is enclosed in a conduit on the downstream side (14) of the plant, and that the space for the product in the conduit is restricted. The space for the product is restricted either in that a food gas is led into the conduit, or alternatively that the product conduit (29) is provided with an expansive portion (27). The expansive portion (27) consists of a pipe length (28) with an inner elastic wall (30) which expands by means of a gas or a liquid.