Improvements in baseline production rates are achieved by reducing the amount of time it takes to remove heat from a product. Additional benefits of cryogenic freezing include a marked increase in product yield due to less dehydration. Fast heat transfer allows products to retain moisture normally lost during mechanical freezing. Microbial growth and related oxidative reactions are reduced due to the short freezing time. This improves product safety and minimizes product degradation. Better texture retention also is shown to exist with cryogenic systems.
Whether to use carbon dioxide or liquid nitrogen depends on a number of factors that must be determined for each operation. Both carbon dioxide and liquid nitrogen systems employ conveyors to transport food products. Product type will determine type of cryogen as well as product loading and dwell time.
Carbon Dioxide vs. Liquid NitrogenCarbon dioxide is a consumable refrigerant that is sprayed directly onto the product being frozen. When carbon dioxide expands through the spray nozzle, it changes to approximately equal parts (by weight) of solid and vapor. The combined effect of the vapor, solid "snow" and internal distribution mechanisms creates a blizzard within the freezer.
As solid carbon dioxide particles contact a food surface, the solid changes to vapor, which draws heat out of the product. This system provides approximately 85% of its chilling from the sublimation of the solid carbon dioxide. The remaining 15% of the cooling is a result of the cold vapor. To obtain the maximum refrigeration benefit, this system injects carbon dioxide throughout the length of the freezer.
Nitrogen, also a consumable refrigerant, is sprayed in a freezing system as liquid and vapor. As droplets touch the product surface, the liquid changes to vapor, which extracts heat from the food. Vapor distribution through the freezer creates a wind chill effect that increases the freezing rate.
Liquid nitrogen provides approximately 50% of its refrigeration from liquid vaporization and 50% from cold vapor. To ensure efficient and economical use of nitrogen, it is critical that the system contains a vapor heat exchange area. Typically, liquid nitrogen is injected in a single zone and cold vapors are directed to the ends of the freezer.
Cryogenic System DesignThe standard tunnel freezer is considered the workhorse of the food industry. Product moves through the length of the tunnel on a continuous conveyor belt and is in direct contact with the cryogen. Conveyor speed and cryogen injection can be adjusted for maximum processing flexibility. This design allows maximum heat transfer from the product. In addition, because each system can be calibrated easily, time in the tunnel is calculated precisely for each product.
Alternative system designs are flighted and spiral freezing. A flighted system allows individually quick frozen (IQF) food products to gently tumble as they freeze. The tumbling provides cryogen contact over all surfaces, ensuring that pieces remain separate. Spiral systems meet the unique operational needs of limited floor space or the need to process delicate products. Products move through the freezer on a continuous spiral conveyor belt, which allows high production capability in a small footprint.
The use of cryogenic systems to chill and cool products has distinct benefits. Quick chilling of raw food products dramatically affects food safety concerns by slowing or stalling the growth of spoilage organisms and allowing the food product to reach a safe holding temperature quickly.
In the poultry industry, cryogenic cooling with carbon dioxide is emerging as a choice over traditional water chill baths. With a carbon dioxide system, poultry carcasses never contact another bird while traveling through the system on the shackle. This eliminates cross contamination, reduces labor costs associated with unshackling and reshackling birds and provides additional savings by reducing water consumption.
Food operations that prepare product for further processing also may benefit from cryogenic chilling. Uniform temperature throughout large transport bins or totes is difficult to achieve by traditional chilling methods. Application of solid carbon dioxide "snow" via specially designed equipment ensures consistent temperature throughout the product, thereby improving product quality during and after shipping.
Cryogenic systems also can extend shelf life. For example, an egg cooling technology was developed to address food safety issues. Using direct impingement of carbon dioxide on a fresh, warm (over 100oF [38oC]) egg reduces it to an ideal holding temperature of 45oF (7oC) within minutes. Mechanical refrigeration can take up to six to seven days to provide the same cooling. The quick cooling provided by the cryogenic system impedes bacteria growth. In addition, the processor can meet government temperature mandates immediately. Overall, shelf life is extended up to 60 days.
In today's industry, change occurs at a mind-boggling rate. With new demands and needs continually being identified, cryogenic freezing is evolving and helping the food industry move toward a more productive and safer production process.