A tunnel freezer is a cryogenic device used to rapidly cool food products. It works by blowing cold air over the product to ensure that the cell structure of the food remains intact and that nutrients, vitamins, and flavors are preserved.
Most tunnel freezers use liquid nitrogen or carbon dioxide as the cooling refrigerant. The choice of one or the other typically depends on the water content, weight, shape, and fragility specifications of the product to be frozen. Liquid nitrogen is the preferred freezer refrigerant for most manufacturers because it does not inhibit microbial growth and offers higher refrigeration efficiency than carbon dioxide.
The typical tunnel freezer consists of an insulated enclosure, which keeps the food and refrigerant at the same temperature; a conveyor belt that moves the food through the system; an injection system to introduce the cryogenic liquid, usually nitrogen or carbon dioxide; ventilators to distribute the cold air to the product; and a drain to discharge the condensation. The insulated enclosure also prevents the cryogenic refrigerant from being blown off of the food, which would decrease the freezing speed.
Both liquid nitrogen and carbon dioxide offer the same refrigeration efficiency in a tunnel freezer, but they have different physical properties that affect their performance and the way in which they are applied to a product. Liquid nitrogen is injected as a fine spray into the tunnel and travels with the product as a very cold mist of vapor. It removes heat from the product by sublimation (turning directly from a solid into a gas).
Carbon dioxide, on the other hand, is piped to the tunnel as a high-pressure liquid but instantly expands into a mixture of liquid and tiny particles of dry ice solid called “dry ice snow.” The dry ice snow rides with the food and removes heat from it through sublimation. The remaining vapor, which is -109F, does not contribute to the refrigeration of the product, but provides considerable cooling benefit from the latent heat of vapor sublimation (246 BTUs per pound of solid CO2).
Another type of tunnel freezer, called a fluidized bed tunnel freezer or an impingement tunnel freezer, uses a flat conveyor belt to move the product through a bath of liquid nitrogen. This type of freezer offers the fastest heat transfer rate available in tunnel freezers, but it does not allow as much product to be frozen at once and requires a higher level of maintenance.
Other freezer types include immersion tunnels in which the food product is lowered into a bath of liquid nitrogen, and spiral tunnels in which the product is conveyed through a rotating bed of ice crystals that freeze it. In these systems, the resulting products have improved texture and color and require less post-freezing processing than do other foods that are frozen in conventional tunnel freezers.