Freezing preserves raw or prepared foods to allow them to be stored for an extended period. Placing food in a frozen state slows decomposition by converting residual moisture to ice, inhibiting the growth of most bacterial species, and locking in important sensory attributes like taste, color and essential nutrients.
On an industrial scale, two options typically are used for food freezing:
- Mechanical freezing systems utilize refrigerants to cool coils that lower the circulating air temperature. That cold circulating air is used remove heat from the food products.
- Cryogenic systems remove heat through direct impingement with the food product. Cryogenic systems use either liquid nitrogen gas or liquefied carbon dioxide as well as circulating gas vapors to extract heat quickly from the food products.
Freezing or chilling is all about time, temperature, convection and conduction. Using descriptions from familiar home-life scenarios can help explain the concepts.
Time. All things being equal, a 70°F (21°C) product subjected to a 0°F (-18°C) freezer overnight will likely be frozen by morning. The same product in that freezer for just one hour will likely only see a few degrees of temperature reduction.
Temperature. A 70°F product, if placed in the freezer at 0°F, will cool faster than the same product placed in a refrigerator set at 37°F. The greater the temperature differential, the faster the freezing process.
Convection. Standing outside on a 50°F (10°C) day with no wind is cool but not uncomfortable, and it can be tolerated for a period. Add a convective breeze, and the result is a colder experience. The air movement cools the body faster, making it colder in a shorter amount of time.
Conduction. Exposure to a 50°F day with slight air movement would not likely result in hypothermia. By contrast, a person submerged in a 50°F pool of circulating water would quickly experience hypothermia. The reason? Water has a much greater conduction capability than air.
An efficient tunnel freezer allows for enough length to provide a proper heat exchange between the products to be frozen and the cryogen injected.
Cryogenic tunnel freezers take advantage of all these variables — time, temperature, convection and conduction — to great effect.
Cryogenic heat removal is fast. Quicker freezing generates smaller ice crystals, which yields a better-maintained cellular structure in the food. A typical cryogenic freezer system will have an approximate operating range from -80 to -170°F (-62 to -112°C), establishing a substantial temperature differential compared to a 45°F (7.2°C) product ready to be frozen.
To get the most efficient cryogenic freezing system, it is necessary to use as much of the available energy as possible from each pound of cryogen. The energy available in the cryogen will depend on a variety of factors; however, the most important variables are:
- The cryogen’s storage pressure.
- The exhaust vapor temperature of the freezer.
Carbon dioxide (CO2) heat extraction comes from the direct contact of solid CO2 snow with the food product surface. When released into a freezer, liquid cryogenic carbon dioxide is roughly 80 percent solid (dry-ice snow) and 20 percent carbon dioxide vapor. Much of the heat removal through conduction from the food is achieved during the sublimation of the solid CO2. The remaining heat removal in the freezing step is by convection. It is achieved by circulation of the CO2 vapor within the tunnel or spiral freezer.
For a nitrogen system, about 50 percent of the refrigeration capacity is obtained by conduction during the vaporization of the liquid, and 50 percent from convection by the cold vapors.
This flighted freezer is designed for IQF production.
Understanding the refrigeration capacity of the cryogen helps inform the choice of freezing tunnel or spiral configuration that will work best. Efficient utilization of the cryogen is determined by an analysis of the system and the composition of the food being frozen. In cryogenic systems, a calculation also must be made to determine the final temperature of the frozen product. This allowance for equilibration of the food product after the cryogen has been delivered is essential to achieving the precise, targeted temperature. Different food stuffs continue to freeze (equilibrate) at different rates
Cryogenic Freezer Systems
Many standard and special designs of cryogenic freezer systems are available to meet the needs of virtually any application. In some cases, the design is practical to meet specific requirements — product dwell times or capacities, product size and shape, product composition and other production features.
Tunnel Freezer Systems. An efficient tunnel freezer allows for enough length to provide a proper heat exchange between the products to be frozen and the cryogen injected. At the same time, it maintains an exhaust temperature as close as possible to the products’ required equilibrated core temperature.
The most basic cryogenic tunnel freezer is a linear tunnel freezer, which consists of a single, linear conveyor. The length and width of the conveyer belt can be created in specific increments to provide proper belt loading and dwell time to achieve the desired freezing rate.
Flighted-Belt Tunnel Systems. Flighted freezers are configured with multiple, ramped conveyors to promote individually quick frozen (IQF) freezing of portioned products. It does so by continuously tumbling the products from ramp to ramp. By exposing all sides of the bulk products to direct impingement, the flighted freezer design produces a consistent IQF product.
Vertical Spiral Freezer Systems. Spiral freezers allow for extended belt lengths but utilize a smaller amount of floor space by stacking the belt in a spiral path rather than a straight run. Controlling the time and temperature are identical to other freezing orientations. The convective cooling of cryogen directly above each tier is enhanced by the convective flow of vapor through the center-cage core of the spiral and directed across the frozen product at each tier.
Regardless of the freezer configuration, in an industrial food-processing operation, a typical installation consists of three primary components:
- A cryogen storage vessel.
- The cryogenic tunnel freezer.
- The piping between the vessel and the freezer.
Spiral freezers allow for extended belt lengths but utilize a smaller amount of floor space by stacking the belt in a vertical path.
Liquid CO2 is stored in an insulated vessel at a pressure of approximately 300 psig. A compressor typically is used to maintain that pressure for the CO2 to remain in the liquid phase. At lower pressures, CO2 exists only in its solid phase (dry-ice snow) and its gaseous, vapor phase. (As mentioned, it is the cold gas and CO2 snow that provide the energy for a liquid CO2 cryogenic freezer.) The tank pressure and control system are used to move the liquid CO2 into the plant and into the freezer system rather than a pump. The piping system into the plant must maintain the tank pressure and be suitable for that pressure.
By contrast, liquid nitrogen typically is stored in a vacuum-insulated vessel at relatively low pressures (35 psig) and is continuously boiling at a very low rate. The vessels tend to be tall and slender because the boiling rate is a function of the surface area of the liquid column. This configuration stems from the vapor pressure of liquid nitrogen being extraordinarily high.
Again, for the liquid nitrogen system, the tank pressure and control system allow for the liquid nitrogen to be pushed through the system piping and into the tunnel freezer without a pump. A typical piping system uses vacuum-jacketed piping made from stainless steel because carbon steel becomes brittle at liquid nitrogen temperatures (-321°F boiling point). A vacuum-jacketed piping system continues the insulation of the vacuum-jacketed tank all the way into the cryogenic freezer in place on the production floor.
Cryogenic freezing on an industrial scale produces frozen product rapidly and handles production rates from 1,000 pounds per hour to more than 10,000 pounds per hour. This on-demand flexibility provides processors with the ability to change production to meet current demands, process many products on the same line, and produce high quality frozen food. PC