The over-pasteurization of a product in a tunnel pasteurizer can result when the normal movement of the product through the pasteurizer is delayed. Over-pasteurization is prevented by cooling the product in the pasteurizing zone to reduce its temperature to a temperature at which no appreciable pasteurization occurs. The temperature of the product may be returned to pasteurizing temperatures when the delay no longer exists. An improved tunnel pasteurizer is described for preventing over-pasteurization of spraying the product with cool water to reduce its temperature. The preferred pasteurizer also includes an automatic level control system for the regenerative paired zones, an external water tank to recover and reuse energy and water usually wasted and a closed cooling system which provides for cooling of the pasteurizer water without the risk of cross-contamination of the cooling source and the pasteurizer.