White Paper. High-efficiency cryogenic freezing for food processing.

Transcription

1 White Paper. High-efficiency cryogenic freezing for food processing.

2 02 High-efficiency cryogenic freezing for food processing. The patented CRYOLINE CW (CRYOWAVE ) IQF freezer is the first to use a rolling-wave action to keep items separate as they freeze. Abstract Implementing cryogenic solutions in the field of high-volume/high-quality food processing is important for a number of reasons. Maintaining proper food temperatures throughout the plant is essential for food preservation and food safety. These are fundamental elements for consistent food quality and the hallmark of top brands. At the same time, high-efficiency cryogenic processes using either carbon dioxide (CO 2 ) or liquid nitrogen (N 2 ) can boost productivity and streamline operations. This white paper offers an introduction to food processing utilizing cryogenic food-grade gases either carbon dioxide (CO 2 ) or nitrogen (N 2 ). It focuses primarily on inline cryogenic freezing systems, particularly in the further processing of meat, poultry and seafood; individually quick frozen (IQF) products; prepared foods; and in freezing dough products and baked goods 1. Tunnel Freezers 2. IQF Freezers 3. Immersion Freezers 4. Spiral Freezers 5. Immersion Spiral Freezers 6. Impingement Freezers 7. Cryo-Mechanical Freezing 8. Sauce and Pellet Freezers

3 High-efficiency cryogenic freezing for food processing. 03 MAPAX gases Introduction to cryogenic processing Cryogenic food processing technology has made significant advances in the past five years. When upgrading from older cryogenic systems or other freezing/chilling methods, the impact is immediate and can be dramatic. Inline and batch cryogenic processing systems can speed production or achieve higher volumes in limited space, often while improving quality. State-of-the-art cryogenic solutions can often save processors hundreds of thousands of dollars a year per line, while preserving the quality of the food product. Technically the term cryogenic pertains to temperatures below -238 ºF (-150 ºC), though it is generally used in the food industry to refer to any cold processing that uses either CO 2 or liquid N 2. At atmospheric pressure, N 2 is a liquid at -320 ºF (-196 ºC), and CO 2 is a solid at -108 ºF. The gases are stored as liquids and conveyed through insulated piping systems to the cryogenic process on the plant floor. The liquid cryogen is then injected into the freezer in order to remove the required heat from the food. In the meat, poultry and seafood industries, cryogenic processing can efficiently bring protein products to the desired equilibration temperature, and can reduce the need for holding freezers to reduce overall freezing costs at a plant. Cryogenic solutions are also used for freezing entrees and marinated products, and to individually quick freeze (IQF) small items, such as diced products. In the baking and snack food industry, cryogenic gas processes can be used before, during or after the mixing process which creates dough or batter. Tightly controlled freezing processes can protect the shape, texture and quality of foods such as frozen dough products and par-baked breads. For fully baked products, cryogenic technology can help extend shelf-life and maintain product quality all the way to the table. In both protein and baking operations, cryogenic chilling can improve the quality of downstream operations. Cryogenic systems can also supplement or boost the performance of traditional mechanical chillers and freezers. It should be noted and understood that cryogenic freezing solutions offer substantial operational benefits to the food manufacturer. Cryogenic gases freeze/chill food products quicker than traditional methods. This can equate to higher throughput, quicker changeover rates, and a reduction in floorspace needed for the equipment. Food safety Food safety is a prime concern for any food processing operation. Fresh and pre-cooked meats, pizzas, unbaked/par-baked/fully baked dough, and other chilled foods, for example, are susceptible to bacterial contamination and must be processed under strict temperature control and hygienic conditions. Although cryogenic temperatures do not kill every type of foodborne pathogen and may not kill all pathogens completely, cryogenic processing can be an important line of defense against Salmonella, E. coli and other pathogens. Hygienically-designed solutions can further reduce the risk. Additional applications Beyond cold food processing, CO 2 or N 2 gases can be used in other plant operations, including protecting dry ingredients in storage silos, and protecting packaged foods with modified-atmosphere packaging (MAP) systems. Multiple uses of these food-grade gases at a plant site can improve economies of scale.

4 04 High-efficiency cryogenic freezing for food processing. The CRYOLINE XF Cross-flow spiral freezer offers a heat transfer rate significantly higher than a standard box spiral and can typically improve throughput by at least 10 percent. Cryogenic process advantages To better understand the advantages of cryogenic processing, it is useful to review how freezing can impact food structure. Foods freeze at different rates, and processing requirements differ. Process parameters for specific foods can be laboratory tested by some cryogenic solution providers to demonstrate freezing rates and quality benefits. Mathematical modeling may also be used for estimating the cost to freeze and gauging potential savings over current methods. Empirical testing on full-scale freezers/chillers can closely define control parameters to optimize the process, minimizing cryogen use and cycle times for dramatically lower operating costs. During cryogenic chilling or freezing, heat (measured in BTUs) is removed from foods at a high rate by both conductive and convective heat transfer processes. Conductive heat transfer occurs when a liquid cryogen directly contacts food or when foods contact cold surfaces. Convective heat transfer occurs from the circulation of cryogen around or through the food product. The design of injection nozzles, blowers and control systems can optimize flow and velocity. Freezing proteins Most meats, which can be up to 75 percent water, freeze at about 28 F. However, to properly freeze all the water inside the protein requires temperatures well below freezing, at least -8 to -22 F (-22 to -30 C). With cryogenic freezing, proteins are typically processed in the -40 to -100 F range. With traditional freezing methods, water molecules between individual muscle fibers freeze first. This is because water inside the cells contains more salt which lowers the freezing temperature, and because the water inside the cellular membrane is under higher pressure. As the temperature drops, water pressure inside the membrane increases, and water molecules may push through cellular walls and into the space between the cells. Then as ice crystals form and grow, they press back against the membrane and damage the structure of the meat. In contrast, when freezing cryogenically, water molecules are predominantly frozen before they can press through the membrane walls and move into the space between the muscle cells. Proper freezing is almost instantaneous, though the water inside the cells is still the last to freeze. However, the rapid freeze produces only microcrystals which are so small the internal structure of the muscle fiber is preserved. Freezing baked goods Finished baked goods may be high in fats or oils. With bread products, yeast or other leavening agents are worked through the dough and given time to set up prior to baking. When exposed to the elevated temperatures of a convection oven, fats emulsify and bubbles form inside the dough. The bubbles form a sponge-like structure as a bread product bakes, and hot gases rise and evaporate. The number and volume of bubbles, and how quickly the gases rise will determine the final texture. Cryogenic freezing preserves the entire structure of the baked or parbaked product including the cooked crust, or uncooked outside layer as with par-baked items. For fully frozen baked products, heat retained from the baking process is removed by cryogenic processing at a controlled rate. (If cooled too quickly, entrapped heat will come to the surface and form cracks.) Cryogenic processing can improve the mixing and handling characteristics of dry ingredients and dough as discussed in our white paper about chilling. Additionally, the high quality of dough can be preserved when frozen cryogenically.

5 High-efficiency cryogenic freezing for food processing. 05 CRYOLINE XF control panel Properties of cryogens Carbon dioxide is a liquid at high pressure (above 5.1 atmospheres). When released at normal atmospheric pressure, the liquid changes phase directly into a solid in a process known as sublimation. Unlike nitrogen, CO 2 remains as a crystallized solid up to 0 ºC (32 ºF) and then changes phase into a gas. During these phase changes, CO 2 absorbs energy from its surroundings. Sublimation always occurs below the triple point -- the temperature and pressure at which liquid/solid/gas phases exist in thermodynamic equilibrium. (See diagram below.) Nitrogen and CO 2 are extremely dense in liquid form. Liquid nitrogen (LIN) is 1/700th the volume of gaseous nitrogen. The selection of the proper cryogen depends on several factors, beginning with the type of product being frozen and current processes at the plant. In addition, the reliable sourcing and any existing or potential uses of either gas at a site can be deciding factors. CO 2 Density and Phases at atmospheric pressure unless noted Liquid Solid Gas Vapor Pressure 770. kg/m 3 (at 56 atm and 20 C) kg/m 3 ( 78.5 C) kg/m 3 (0 C) 5.73 MPa (20 C) N 2 Density and Phases Solid C Liquid g/cm 3 at boiling point ( C, F) Gas g/l (at kpa, 0 C) At atmospheric pressure, molecular nitrogen condenses (liquefies) at C (77 K) and freezes at C (63 K).

6 06 High-efficiency cryogenic freezing for food processing. Cold processing with carbon dioxide or nitrogen With mechanical freezing and chilling methods, a coolant, typically anhydrous ammonia, expands as it circulates through coils inside the compartment to remove heat, and then returns back to the condenser to be re-chilled. While ammonia offers favorable heat transfer properties, it is toxic and can corrode metals. In contrast, CO 2 and N 2 are inert gases. Cryogenic freezers also operate at colder temperatures than mechanical freezers, and freeze food faster, preserving food quality. With inline cryogenic processing, high volumes of food move at a controlled rate through a flow of cryogen. Depending on the process, the cryogen that contacts the food product may be in liquid, solid or gaseous form. Improving a plant s productive capacity is often a key consideration as an operation grows. Inline cryogenic processing is well suited to high-value products and prepared foods, including cookie dough, marinated poultry and individually quick frozen (IQF) meats, seafood and vegetables. Because of the high freezing efficiency of new cryogenic equipment, a single freezer can sometimes do the work of two existing freezers, or help fold two or more production lines into one continuous flow. Hygienic design New cryogenic freezers may include hygienic design features such as seamless construction, sloped surfaces for drainage, and complete access to conveyor belts and other internal components for maintenance. Hygienically-designed freezers have grown in importance with the increased attention being placed on plant hygiene and food safety. Controls Standard controls for inline cryogenic processing include belt speed, dwell time and cryogenic flow parameters, which can be pre-set to enable rapid change-overs between different products or recipes. Worker safety Safety training should be considered a normal part of any new installation. Because of the extremely cold temperatures of cryogenic processes, equipment operators must use protective eyewear and gloves rated for use with cryogens. Prolonged exposure to high levels of inert gas can be harmful or fatal, so proper operating and maintenance procedures must be followed at all times. Always consult equipment manuals and manufacturer recommendations before placing any new system into service. Exhaust venting State-of-the-art cryogenic processing systems are designed to utilize the cryogen as safely and efficiently as possible. The nature of the process requires direct venting systems to capture the gases and exhaust them to the outside of the plant. Freezers and chillers may include shrouds at inlet and exit ports, enclosures and plenums to contain gases during a process cycle, and hoods with blowers to convey gases to roof vents. Work stations, particularly in confined areas, must also be properly ventilated to ensure a safe work environment. Shipping and storage Bulk gases are shipped by tanker truck and delivered into dedicated cryogenic storage tanks. Because the liquids are stored under pressure, they freely flow from storage tanks to low-pressure injection points. High-purity food-grade gases are often produced at ISO-certified plants. The gases are stored as liquids and conveyed on-demand through appropriate cryogenic piping systems to the cryogenic process on the plant floor. The liquid cryogen is injected inside a chiller/freezer with specialized injectors in order to facilitate freezing/chilling of the food.

7 High-efficiency cryogenic freezing for food processing. 07 Cryogenic freezing solutions 1. Tunnel freezers 2. IQF freezers With tunnel freezing, food products are loaded onto a straight conveyor belt. The amount of food that may be stacked on the belt will depend on the mass and type of food, and the height of the freezer inlet. The height of the opening may be adjustable but the average opening is approximately 4 inches. The goal is to maximize the cryogen within the freezer while minimizing the amount of room air being introduced into the freezer. As food moves into the freezer, the cryogen and circulation fans are used to remove heat from the product. The design of the cryogen injection and the circulation of the cryogenic gases within the freezer contribute to heat transfer and cryogenic efficiency. High-speed blowers may direct and enhance heat transfer. Some conveyors permit gas flow through the belt for better convection throughout the freezer. Food may move to another area for further processing before final packaging and transport, or moved to a holding freezer for final equilibration or full freezing. For many foods, a crust freeze is all that is necessary. Proteins such as deli meats, for example, may travel to a slicing area, where a precise crust freeze will improve slicing quality and minimize product losses and unnecessary downtime. New modular designs permits additional cryogenic freezer modules to be added in order to quickly expand production capacity. Individually quick freezing with a cryogen locks in the moisture, shape and freshness of small food items, keeping them separate as they freeze. While flighted and triple-pass freezers have been the primary method for quick freezing food for more than two decades, they are now being replaced with newer, more efficient freezing methods. One new approach to individually quick freezing in a cryogenic tunnel is a proprietary IQF freezer known as the CRYOLINE CW (CRYOWAVE ). The patented design is the first to use a rolling-wave action which keeps IQF products separate as they freeze. The wave action is fully adjustable so foods can be gently tumbled to maximize heat transfer efficiency as they move through the cryogenic tunnel. This is in contrast to older-style triple-pass and IQF flighted freezers, which drop food products multiple times from one conveyor belt to the next as they freeze. The CRYOWAVE, utilizes either CO 2 or N 2 and can reduce or eliminate the belt sticking and CO 2 snow carryover associated with these older technologies. For cleaning and maintenance, the entire top of the hygienicallydesigned CRYOWAVE lifts upward to provide ready access to the processing area inside. Cleaning liquids flow out through a center drain along the bottom of the tunnel. This hygienic feature is also available on other new tunnel freezer designs.

8 08 High-efficiency cryogenic freezing for food processing. 3. Immersion freezers 4. Spiral freezers Cryogenic immersion is the fastest method of freezing. Food products are loaded onto a conveyor belt and submersed in a bath of liquid nitrogen (LIN). Because the LIN bath is approximately -320ºF, the outside surface of the food hardens instantly, and further heat is drawn from the inside of the product. Small items, such as shrimp and strawberries, can freeze solid in seconds utilizing this technology. While different food products freeze at different rates, it is not necessary for products to remain immersed until they are fully frozen. Once removed from the LIN, the products will continue to freeze and equilibrate. Heat will be drawn from the inside of the product over time until the entire product is at one unified temperature. To maximize cryogen efficiency, the final freeze for many food products is applied by a subsequent freezer, such as a cryogenic tunnel or spiral, or mechanical freezing system. When used as an initial freezing process, cryogenic immersion freezers can quickly improve overall freezing throughput. Immersion time is calculated based on the heat transfer rate of the equipment, the overall food volume, the type, shape and weight of the food item, as well as the temperature of the product coming in, and the desired temperature on exit. An immersion freezer can be especially effective for marinated proteins such as chicken breasts. The crust freeze can seal in moisture and improve yield pickup on sauces or marinades, before the protein is fully frozen at a precise rate in a secondary system. With cryogenic spiral freezing, foods travel on a conveyor belt that wraps around a drum through the freezer. Freezers are on average approximately 10 to 12 ft. high. The spiral belt design accommodates high volumes of food products in less floor space, while applying a controlled freezing rate to the food product. Foods can be loaded from either the top or bottom of the spiral (bottom is typically standard) depending on the production layout. Spiral freezing is especially important to processing high production volumes for products such as bakery items, pizzas, cooked poultry, or even trayed products & entrees (which include a variety of foods, from sauces to vegetables, which freeze at different rates). Most spiral freezers have a box frame. However, new cylindrical designs offer multiple thermodynamic advantages. A cylindrical chamber is about one-third the volume of a box of the same height, so higher volumes of foods can be processed, or the same volume processed in less space. The cylindrical shape also reduces steady-state losses from surrounding sidewall materials. The flow and circulation of cryogen is critical to freezing efficiency. Box spirals are prone to temperature differences across the width of the belt. Properly engineered airflow inside the freezer will maximize heat transfer efficiency and help lower operating costs. The patented CRYOLINE XF (Cross-flow) spiral from Linde is a state-of-the art hygienic design that offers a heat transfer rate significantly higher than a standard box spiral and can typically improve throughput by 10 percent or more.

9 High-efficiency cryogenic freezing for food processing Immersion spiral freezers 6. Impingement freezers An immersion spiral freezer combines the advantages of both immersion and spiral freezing methods using nitrogen in both stages. Immersion spirals are especially useful for proteins such as seafood and cooked poultry items to be fully frozen. The immersion bath crust freezes the product for high-moisture retention and seals seasonings to the outer layer. High-volumes of protein then proceed on the conveyor belt to be fully frozen within the spiral stage in one continuous process. The space between stages is insulated and enclosed to trap the cold gases. State-of-the-art immersion spiral freezers are engineered to maximize the cryogenic efficiency by utilizing the BTU s in both the liquid and gaseous phases of the nitrogen. Both the cold nitrogen gas and the food product flow from the nitrogen bath to the spiral in order to capture the maximum BTUs from the cryogen. This includes special blower systems that capture gases from the bath headspace and circulate them into the spiral freezer stage. Immersion spiral freezers are high capacity freezing systems, achieving production rates up to 20,000 lbs/hour. Impingement freezing is highly efficient and ideal for marinated poultry, dinner entrees, pizzas, cookie dough and many other high-quality foods. Better moisture retention translates to lower drip losses of protein products and more consistent package weight. It also can mean higher product quality, flavor retention, and texture profile of many bakery and prepared food items. Linde engineered a proprietary, space-saving design that reduces dehydration losses by a factor of two or three compared with standard cryogenic tunnel freezers. The Linde impingement freezer offers the highest freezing capacity per square ft. of any cryogenic freezer on the market, and is about 60 percent smaller than a cryogenic tunnel freezer with the same throughput. It handles the nitrogen in a proprietary way as it streams with the product flow along the length of the freezer -- which maximizes cryogenic efficiency.

10 10 High-efficiency cryogenic freezing for food processing. 7. Cryo-mechanical freezing systems 8. Sauce and pellet freezers Cryogenic Production Mechanical Cryogenic capacity increase Cryogenic peak shave capacity Mechanical system capacity Time As production demands increase at a plant, existing mechanical freezing systems can outgrow their capacity. A cryogenic system can be added to boost capacity of the existing systems at a customer s facility. This solution allows a customer to increase production in a limited space without making a major investment in an entirely new system. With the Linde CryoAssist system, a high-efficiency cryogenic freezer is installed before or after a mechanical freezer to improve throughput and performance. By sharing the freezing task with a cryogenic freezer, the overall process can benefit from lower operating costs, higher quality, and increased production rates. Furthermore, dehydration and product losses are typically reduced 1 to 3%. Loading and unloading freezer trays into freezer cabinets is labor intensive -- and waiting for sauces, purees and other liquids to cool or freeze in mechanical freezers can create production bottlenecks. Stateof-the-art pellet freezers use cryogenic gases and in-line processing to boost productivity and consistency. The CRYOLINE PE Sauce Pellet Freezer from Linde features a patented cryogenic design to boost efficiency for fast turnaround. The freezer produces individual sauce pellets in a regular size with high repeatability using an automated, in-line process. The dual cryomechanical system is often used to improve downstream handling and processing. Placement of a CryoAssist system in front of a mechanical freezer, pressing machine or other processing equipment helps to prevent belt sticking and product clumping, increases yield, and minimize rejected, reworked, and downgraded products. While spiral and tunnel freezers can be used in cryomechanical applications, immersion and impingement freezers are preferable, because of their rapid crust freezing rates, compact size, quick installation, ease of use and process versatility.

11 High-efficiency cryogenic freezing for food processing. 11 Sources: International Journal of Food Engineering. Volume 6, Issue 1, ISSN (Online) , DOI: / , January North, Mike F and Simon J. Lovatt. Handbook of Frozen Food Processing and Packaging: Second Edition CRCPress (2011). pp PJ Fellows, Food Processing Technology, 2nd edition, Woodhead Publishing Ltd. and CRC Press (2002). Sommers CH, Rajkowski KT, Sheen S, Samer C, Bender E (2011). The Effect of Cryogenic Freezing Followed by Gamma Radiation on the Survival of Salmonella spp. on Frozen Shrimp. Journal of Food Process Technology S doi: / s

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