Choosing a Method for Quick Chilling or Freezing
Understand the differences among tunnel coolers, spiral chillers and blast chambers used to freeze and chill food products.
Chilling a chicken breast requires a different system than freezing a ready-to-eat meal or quick chilling a ham in order to maintain product integrity and safety. An error can be disastrous to the consumer — and the processor. When choosing an appropriate refrigeration method and equipment to do the job, one must understand how the characteristics of the product and the packaging affect the rate at which the product will give up its heat. This is accomplished by examining the amount of energy and time required to chill or freeze the product.
Physical properties of the product might include the thickness, mass and composition of the product. The thicker the product, the longer it takes to give up its internal heat. Also, the higher the fat content, the longer it takes to give up its heat. Conversely, the higher the water content, the less time it takes to give up its heat. (In other words, water gives up its heat more quickly than fat.) If a chill or freeze can be accomplished with very low temperatures, very quickly, product yield is increased. Also, the freezing temperature of a product ranges from a high of 32°F (0°C) and below, depending on characteristics such as water, sodium and fat content.
Generally speaking, a product’s rate of heat loss is proportional to the difference in temperatures between the product and its surroundings. However, due to the internal characteristics, shape and diameter of a product, the actual rate of heat loss will be limited to a maximum rate. This is true regardless of how cold the surrounding temperature gets or how fast the air flows around the product.
When considering packaging needs, it is important to know if the product is chilled before packaging or after packaging. Also, at what stage of the packaging process is it chilled? Product chilled later in the packaging process takes longer to chill. In addition, products packaged using a vacuum-packaging method — which will help increase yield — will need slightly more chill time. Packaged product with an air barrier — which acts as an insulator — may increase the chill time considerably. Add a carton to this package, and the time needed to chill increases even more.
If a product is case packed in a corrugated box, the cardboard also will act as an insulator. When the box is palletized, additional time is required. Airflow around those boxes — and in the right places — is essential. Freezer spacers often are used to allow for airflow, but each time the package is layered, cooling time is increased. Tight packing should be avoided.
In food processing, three primary methods of refrigeration are used for chilling of solid foods such as meat, poultry, seafood, ready-to-eat meals and breads. They include tunnel cooling, spiral chilling and blast chambers. The qualities of the product, facility space available and budget all must be considered. The associated needs will help pare down the options. Equipment manufacturers should know the capabilities and limits of their systems. Equipment can be customized if necessary. Processors may also make arrangements to test a product on equipment if questions exist. The chilling and freezing process is not an exact science, so adjustments often need to be made to the design of the chilling equipment.
Tunnel Cooling Foods
A cooling tunnel houses a belt conveyor that moves through the tunnel with product on the belt. As the product moves in the tunnel, it can be cryogenically chilled by spraying liquid refrigerant (liquid nitrogen/carbon dioxide) into the tunnel. As an alternative, it can be cooled using a mechanical refrigeration system that will utilize coils in the tunnel filled with ammonia or halocarbon refrigerant.
Some tunnels systems are designed for one pass through the tunnel; others are intended for multiple passes of the belt. This continuous cooling method is effective for quick freezing small, thin products such as patties, particularly if the product is hot.
Tunnels typically are limited by space constraints. Products that require longer cooling times will require longer tunnels. The cost of operation for a cryogenic tunnel is driven by the cost of the liquid nitrogen or carbon dioxide. The cost of operation for a mechanically cooled tunnel is primarily the cost of electric for operating the refrigeration compressors.
Spiral Chilling and Freezing Foods
Spiral chilling or freezing utilizes a belt conveyor to transport product through a refrigerated enclosure, rotating in a helical manner while being blasted by cold air. The cooling of the enclosure can be accomplished using either cryogenics or mechanical cooling of coils.
A spiral cooler or freezer typically is used when a tunnel will not provide adequate dwell time for cooling or freezing a product. (A longer belt is possible with a spiral.) Spiral chilling also is preferred for medium-sized products such as sausages. Mechanical cooling often is favored over cryogenic cooling if a spiral chiller is operating on a daily basis because the cost of operation is generally less. Product yield also is improved for unpackaged products using this method because the initial blast of chilled air will help to crust-freeze the outside of the product and help seal in moisture.
Spiral chillers require a smaller footprint than tunnel chillers and freezers. As with the tunnel freezer, this cooling method requires low temperature but high velocity.
Using a Blast Chamber to Freeze Foods
A blast chamber is a room or enclosure in which high velocity air flows around the product, which typically is placed in the chamber on pans that slide into racks. Some products, instead of being placed onto pans, can be hung on rods that are then hung on the rack (sometimes called trees). Such a system allows the product to be separated so air can flow freely around the product. The racks then are rolled into the blast cell.
One advantage of a blast chamber is that product may be kept in the space for as long as it takes to chill the product, and that time depends on the product. Blast chambers are best utilized when larger, solid foods that are heavier or larger in diameter need to be chilled. Blast chambers are a batch process rather than the continuous process used in a tunnel or spiral cooler. As one engineer explains, “It takes 12 hours to chill a ham no matter how fast you try to do it.” In other words, no matter how much you increase the air velocity or decrease the cooling temperature, it is not going to cool any faster.
Achieving proper airflow is the challenge with a blast chamber. Air must move around the products as consistently and evenly as possible. If airflow is not balanced — reaching only some of the product rather than the entire batch — only those products effectively exposed to the airflow will freeze or chill. The size of the chamber also is a factor. If the chamber is too large, airflow will be compromised, and it will be more difficult to direct it through the product racks.
In conclusion, every product has a unique time-and-temperature cooling relationship. Prior to selecting any equipment, this relationship should be tested by the equipment manufacturer. The time it takes to chill or freeze a product can be surprising. Freezing is particularly time-consuming because the time it takes to pass the freezing point of a product — the latent heat of fusion — is often the longest part of the process.