Abstract: A retort system for thin walled, generally flat containers filled with a food product or the like including a pressure vessel having one or more batches of flat containers therein disposed within a heat treatment tunnel. A heat transfer liquid is circulated through the tunnel parallel to the flat sidewalls of the containers at a rate sufficient to transfer heat between the fluid and the containers without incurring a large change in temperature of the liquid between the inlet and outlet of the tunnel for assuring uniform cooking of the contents of the containers. During a cooking cycle the liquid is reheated while being returned externally of the tunnel from the discharge end of the tunnel to the inlet end of the tunnel. The tunnel is partially defined by the side walls of nesting containers supporting trays, which trays may be rotated for an agitating cook or may be stationary for a still cook.