Food processing, practically by definition, 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. This article will explore 7 points to consider about freezing equipment for food processing and other process applications.
1. Process Cooling Is Predominantly about Time and Temperature
Most of us know process cooling is about time and temperature from simple experience —either with food and our home refrigerators and freezers, or from weather changes and being outdoors. Time comes in as such: all things being equal, a 70°F (21°C) product subjected to a 0°F (-17°C) 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 70°F product, if placed in the freezer rather than the refrigerator, will reduce in temperature at a faster rate.
For 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.
Food products typically are chilled or frozen with reasonable dimensions for individual consumption.
2. Process Cooling Is about Convection and Conduction
Convection and conduction also play roles in process cooling. Regarding convection, suppose you stand outside on a 50°F (10°C) day. You will get chilled much faster if there is a breeze than if the air is still. Regarding conduction, exposure to a 50°F 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 than air.
In the case of 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 of liquid nitrogen, greatly increases conduction for more effective cooling.
By depressing the operating temperature, 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 the purposeful and meaningful temperature change as part of the process.
3. Consider Practical and Financial Limitations to Cooling Rates
So far, it seems like 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 — with little or no improvement to heat transfer.
Process cooling also is limited by the ability of the product to internally conduct heat. Again, you likely know this from real-world experiences. For instance, 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. Or, 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. Most Products Require Time to Equilibrate
As few foods or materials have perfect internal conduction, when using an environment that is below the desired temperature for cooling, the material 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.
Contact with carbon dioxide snow or direct impingement of liquid nitrogen greatly increases conduction for more effective cooling.
5. Most Freezing Equipment Is Designed for Food
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.
Yet, chilling non-food items is still about time and temperature. In other words, at a given low temperature, it takes a certain number 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 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 useful 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 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. Contraction of one part and expansion of the other can ease assembly of interference fit parts.
Food is typically 60 lb/ft3, so the conveyor system or racks are designed around that value, plus or minus.
7. Cooling or Freezing Can Improve Further Processing
Freezing or chilling can have a positive impact on machine-ability. We see this 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 shredding for mixing or recycling.
In conclusion, time and temperature (and convection and conduction) can be used to positively impact your process. From empanadas to machined polymers, freezers for chilling or freezing carry a myriad of novel and interesting uses in industry. PC
