7 Things to Know about Freezing & Freezing Equipment
May 18, 2023
Temperature-reduction equipment for food processing and other non-food applications is available to remove processing heat and to freeze, chill or cool products. Food processing often involves the use of heat. Once the processing transformation has taken place, the food will need to be cooled. There are plenty of reasons to cool products and processes, and many temperature reduction equipment systems are available in myriad configurations effective at removing processing heat. Here are seven things to consider about freezing equipment for food processing and other process applications.
1 - Cooling Time and Temperature
Most of us know process cooling is about time and temperature from the simple experience with food and home refrigeration. Time is a factor. All things being equal, a 70°F (21°C) product subjected to 0°F (-17°C) in a freezer overnight will likely be frozen while the same product in that freezer for an hour will likely only see a few degrees of temperature reduction. Temperature comes in where that same 70°F product, if placed in a 0°F (-17°C) freezer rather than the 37°F (3°C) refrigerator section will reduce in temperature at a faster rate.
For larger scale process cooling, the operating temperature of the equipment frequently can be substantially lower than the desired final temperature of the material being cooled. By depressing the operating temperature, the food can be cooled much faster than expecting the product to simply reach the target storage temperature. This segregates process cooling from chilled storage. Process cooling is a purposeful and meaningful temperature change as part of the process. By contrast, any temperature change imparted by a storage cooler or freezer generally stems from an unintended rise in temperature during the accumulated time for transportation, general product exposure without temperature control or under chilling during the initial process cooling step
2 - Convection and Conduction
Process cooling is also about convention and conduction. Convection is easily explained with commonplace experiences. When you stand outside on a 50°F (10°C) day, you will get chilled much faster it there is a breeze than if the air is still. Regarding conduction, exposure to a 50°F (10°C) day with slight air movement would not likely result in hypothermia. If you were exposed to 50°F slightly moving water, however, you would likely develop hypothermia. This is because water has much greater conduction capability than air.
Applying these same concepts to production sized process cooling, the heat transfer through convection by air is improved with higher velocities of the air in and around the products. If product temperature is being lowered through circulated chilled air, there is not much to be done to improve conduction. If, however, water or another liquid could be used, conductive heat transfer could be greatly improved. Cryogenics use this feature to great advantage. Contact with carbon dioxide snow, or direct impingement with cryogenic liquid nitrogen, greatly increases conduction for more effective cooling.
3 - Practical & Financial Limitations
Decreasing temperature while increasing convection and conduction will speed the cooling rate indefinitely. From a financial standpoint, however, continuing to decrease the operating temperature becomes more and more expensive. Mechanically, adding higher horsepower fans to improve convection can begin to add heat to the system.
Process cooling also is limited by the ability of the product to internally conduct heat. For example, baked potatoes can stay hot enough to burn your mouth for quite some time while something like toast barely stays warm long enough to melt the butter. Think about the example from the heat-removal side. For a product with low internal conduction, once the surface of the product reaches the temperature of the freezer, heat transfer is no longer taking place. The product continues to conduct heat internally, incrementally warming the surface temperature, but this internal conduction begins to limit the cooling rate.
The overall size of the portion is determined by height, width and depth. Minimizing one of the three dimensions of a product can improve the impact of freezer temperature reduction, increase convection and optimize conduction.
4 - Products Require Time to Equilibrate
Few foods or materials have perfect internal conduction. When using an environment that is below the desired temperature for cooling, the product will complete its cooling cycle while there are still temperature gradients within the material. If the time, temperature, convection and conduction are established properly, the material being cooled will complete the cooling cycle when the proper number of BTUs have been removed - not necessarily when the material has a perfectly uniform temperature throughout.
As a very simple example, if a 30°F (-1.1°C) chicken breast is placed in a -15°F (-26°C) freezer, and the desired final temperature is 0°F (-17°C), there will be gradient within the chicken from about -15°F at the surface to perhaps 30°F in the very center. Because the total mass of the "outside" of the chicken breast is greater than the mass of the "inside," the outside's subzero temperature and higher mass will equilibrate over time with the warmer and smaller mass of the inside. This results in the desired equilibrated temperature of 0°F, provided enough total BTUs have been removed. As long as these temperature gradients within the product do not adversely impact the product in the short term, the product will eventually reach an equilibrated temperature.
5 - Freezing Equipment Design
When we talk about freezing, the food industry typically is referring to the change of state of water from a liquid to a solid. Many other materials and foods do not change state when subjected to cold temperatures. Other materials may become firmer or more brittle. For instance, fats can crystallize. With such materials in an industrial environment, cooling may simply allow for further processing without waiting long periods of time.
At a given low temperature, it takes a certain number of minutes to get from initial temperature (also known as Ti, or temperature in) to final temperature (also known as To, or temperature out). Most food freezing systems, however, are designed with food products and food temperatures in mind. Product weights, temperatures and dimensions for food items are narrow when compared with all products that may need to be cooled.
Food products have a mass that is typically 60 lb/ft3 so the conveyor system or racks are designed around that value, plus or minus. Food is typically fully frozen at 0°F, ±10°F (5.5°C), and a freezer's typical operating temperature - even in cryogenic applications - rarely drops below -180°F (-118°C). Food products typically are chilled or frozen with reasonable dimensions for individual consumption. Most non-food processes routinely exceed these rather tight use ranges. So, special consideration must be made for products that are very different from the general properties of food items.
6 - Contraction must be Considered
Some food and other non-food products can react poorly to an exterior that is frozen too quickly, resulting in stress and pressure on the inside. Empanadas are a good example of a food product where care must be taken for freezing. The tender, relatively dry empanada shell freezes very quickly under almost any freezing condition while the warm, relatively wet and dense filling takes some time to freeze. This can lead to cracked shells - an undesirable outcome. This brings us back to time and temperature. By dialing the temperature settings and exposure times properly, empanadas freeze beautifully.
Alternatively, dissimilar materials cooled together may create challenges. The expansion and contraction of different materials is typically... different! For example, cooling products with a coating, metal-backed polymers or assemblies can present specific challenges. Fortunately, they typically can be overcome. Contraction also can be helpful. If you are trying to remove or separate parts, coatings or assemblies, low temperatures from freezers can greatly improve productivity. As materials or products contract, they frequently pull away from molds or fixtures, allowing for ease of extraction.
7 - Improve Further Processing
Freezing or chilling can have a positive impact on machine-ability. This is seen in the food industry for spices, herbs and other products that will be ground or shredded (machined). Spices are chilled to keep the flavors from wafting away. Fresh herbs are frozen for shredding prior to drying to maintain flavors and speed dehydration. Meats are chilled to keep important fats from melting.
Polymers may benefit from chilling for machining or forming. Metals may be less prone to galling if chilled before machining. Embrittlement may improve grinding or pulverizing for mixing or recycling.