Cryogenic Freezing & Chilling
Today's foods must meet consumers' ever-changing demands for variety, convenience and preparation time savings. For this reason, food processors increasingly are relying on the efficiency and versatility of cryogenics for freezing and chilling operations. This is especially true for freezing because a cryogenic freezer can be adapted specifically to each food product while maintaining modular flexibility. Formulations, shapes, food types, handling concerns and product safety are all taken into account to achieve the highest product quality while the product temperature is rapidly and significantly reduced.
When compared to mechanical freezers, cryogenic freezers can improve baseline production rates by reducing the amount of time required to remove heat from a product. They also can provide a marked increase in product yield due to less product dehydration, as the quick heat transfer allows the product to retain more of the moisture normally lost during mechanical freezing. Microbial growth and related oxidative reactions are reduced due to the short freezing time, thereby improving product safety and minimizing product degradation. Cryogenic systems form smaller internal ice crystals than are formed by mechanical systems while freezing the product. This results in better texture retention because cryogenic freezing does not adversely affect cell structure to the extent found with the larger ice crystal formation associated with mechanical freezing. Additionally, cryogenic systems can help decrease labor costs through reduced product handling and quicker cleanup, better inventory management control due to consistent production rates, and preservation of valuable floor space due to the smaller size and modular format of the equipment.
How Do Cryogenic Freezing Systems Work?Cryogenic systems use direct impingement with a cryogen -- either liquid nitrogen or carbon dioxide (CO2) -- to rapidly remove heat from the products being processed.
CO2 is a consumable refrigerant that is sprayed directly onto the product. When CO2 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 the solid CO2 particles contact the food surface, the solid changes to a vapor and draws heat out of the product. The carbon dioxide system obtains approximately 85 percent of its refrigeration from the sublimation of the solid CO2; the remaining 15 percent of the cooling is from the cold vapor. To provide the maximum refrigeration benefit, CO2-based systems typically inject CO2 throughout the length of the freezer.
In nitrogen-based systems, nitrogen, also a consumable, refrigerant is sprayed into the system as both a liquid and a vapor. As the liquid nitrogen droplets touch the product's surface, the liquid changes to a vapor that extracts heat from the food. A simultaneous distribution of nitrogen vapor throughout the freezer creates a wind-chill effect that increases the rate of freezing. Liquid nitrogen systems obtain about 50 percent of their refrigeration from the vaporization of the liquid and the other 50 percent from the cold vapor. To ensure efficient and economical use of nitrogen, the system must contain a vapor heat exchange area. In standard cryogenic freezers, liquid nitrogen is injected into a single zone, and the cold vapors are directed to the ends of the freezer to completely envelope the food product.
Whether carbon dioxide or liquid nitrogen is the correct choice for the application depends on a number of factors. Both systems use conveyors to transport the food product continuously through the operation, thereby providing increased production rates compared to mechanical freezers. Parameters such as product loading and dwell time depend on the product being processed, and the spent cryogen vapor is removed from the operation through simple exhaust venting.
Selecting the Right Cryogenic SystemThe standard tunnel freezer is 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. The conveyor speed and cryogen injection can be adjusted quickly for maximum processing flexibility. This design allows for the maximum heat transfer from the product. In addition, because each freezer system can be calibrated specifically for the food product being processed, the amount of time each product spends in the tunnel is precise and repeatable, which keeps the production rate on pace.
Advances in cryogenic refrigeration have led to the development of designs that take full advantage of the cooling power of the cold vapor in the freezer. These systems allow for a more efficient extraction of all the cooling power available in the cryogen before venting the spent gas, which provides as much as a 10 percent reduction in the amount of cryogen needed to freeze or to cool product to the target temperature. The increased efficiencies allow for up to 33 percent less floor space for equipment providing the same level of product throughput.
Other system designs include flighted, spiral and immersion freezing systems. Flighted systems gently tumble individually quick frozen (IQF) food products as they freeze, thereby allowing cryogen contact over all surfaces and ensuring that the pieces remain separate. Spiral systems, which were created for operations that have limited floor space or that process more delicate products, move items through the freezer on a continuous spiral conveyor belt and allow a high production capacity in a small footprint. Nitrogen immersion freezers, which also have small floor space requirements, immerse products in a nitrogen bath prior to full freezing. Such systems often are used to provide as a pre-chill boost to production-strapped systems.
Whether used alone or in conjunction with existing mechanical systems, a cryogenic freezer can provide the flexibility to expand existing freezing systems, improve food quality and increase throughput.
Cryogenic ChillingThe use of cryogenic systems to quickly chill products also has distinct benefits. Quick chilling of raw food products dramatically reduces food safety concerns by slowing or stalling the growth of spoilage organisms and by allowing the products to reach a safe holding temperature quickly. In the poultry industry, for example, cryogenic cooling with liquid CO2, paired with continuous cooling equipment, is an emerging new technology that can replace traditional water chill baths, water ice or dry ice. Chilling the poultry whole or in pieces (boned or bone-in) in a continuous system quickly removes heat from the raw product for further processing. Quality improvements, labor savings and reduced CO2 infiltration are some of the benefits the cryogenic chilling methods can provide compared to conventional methods.
For food operations that prepare products for further processing, achieving a uniform temperature throughout large transport bins or totes can be difficult with traditional chilling methods. Installing a cryogenic system that allows for a quick, uniform chill throughout the product reduces rework, returns and product rejections.
As with freezing, cryogenic cooling offers increased cooling speed, improved quality, extended shelf life and cost savings. Whether the product is vegetables, cooked meat, fresh seafood or raw poultry, cooling with cryogenic gas is improving product handling and food safety.
Food processing is just one industry that continually encounters new challenges. Fortunately, the flexibility of cryogenic freezing and chilling allows these technologies to provide solutions to many of the changing requirements. As new demands and needs are identified, cryogenics will continue to evolve to satisfy these needs and help the food industry move toward safer, more efficient processes. PCE