Chilling and freezing are two of the most common food preservation methods available to food producers. Practically by definition, all food production involves the addition of heat that then must be removed before the product is further distributed. Some products like soup are canned and heat sterilized, and they must be cooled before leaving the factory. Other products like cereal are manufactured using heat and must be cooled before packaging. Still other products like chilled or frozen grocery items are brought to their respective temperatures in the factory. Then, they are stored, transported and displayed prior to purchase under those same conditions.
The removal of heat in each of these examples can be undertaken with different methods. For instance, canned soups might use water cooling while cereal might use ambient cooling. Shredded cheese might be cooled with refrigeration while freshly picked fruits are flash frozen. Multiple methods also may be used to achieve the final desired temperature.
It is important to note that freezing and cooling are quite energy intensive. You may have observed this yourself in your home: typically, heating your home during the winter tends to be easier and less expensive than cooling it in the summer. The same holds true for industrial processes. The ability to create and use extremely cold temperatures in process cooling applications is not nearly as great as our ability to create and use hot temperatures in process heating applications. The temperature differential available for cooling or freezing is small, difficult and expensive. As such, it is imprudent to cool or freeze any faster or further than is necessary. In pursuit of proper freezing, therefore, employing multiple methods to achieve the desired final temperature provide many benefits. These methods may fall into several categories.
For some foods, how they are cooled or frozen has little impact on the final product, so food processors can focus their attention simply on energy efficiency. For instance, for many breakfast cereals, their toasty goodness comes from high heat processes meant to impart flavor. Upon completion of the toasting, the products are far too warm to be packaged in plastic films and must be cooled. The cereal’s piece size and moisture-content level as well as a material-handling system’s flowing movement and long duration of conveying from toasting to packaging can provide quite a bit of ambient cooling for a breakfast cereal. In such applications during warmer months — when the ambient cooling is not quite enough — perhaps supplemental fans are employed.
This is a scenario where energy efficiency takes the lead. The cereal is relatively hot compared to the ambient conditions. The desired temperature is not cold or cool (simply 85°F [29.4°C]), and there is ample time available to allow for ambient cooling to do the trick. If fans are employed along with conveyors and product movement, a multi-method approach is applied to cooling. There is no need to use refrigeration to achieve the final temperature.
As another example, consider brownies, which are dense, moist and very hot when leaving the oven. In this example, suppose the exit of the oven is near the packaging line, and the brownies need to cool prior to packaging. A cooling tunnel will be used, either mechanical refrigeration or cryogenic, depending upon the space available.
Even in this instance, a multi-method approach for cooling makes good sense. When the brownies exit the oven, they are steaming hot. Any ambient or fan cooling that can be squeezed between the oven and the cooling tunnel will be of benefit. Much of the steam can be dissipated before it is trapped in the tunnel, and the temperature differential between the brownies and the room can be put to good use, thereby lessening the work that the more expensive cooling tunnel must do. In this instance, it is more efficient to use the large temperature difference available between the room and the brownies prior to continuing to a more expensive means.
Other multi-method approaches also are available for improving energy efficiency. For instance, if multiple SKUs are being frozen on the same production line, perhaps some are more heavily loaded on the belt leading to larger freezing equipment. It may be better to use a smaller spiral freezer supplemented with a cryogenic tunnel. Using the cryogenic tunnel only when needed for the heavier product leads to a greater operational efficiency than running a larger-than-required freezer for all sizes of product.
For some products, how the cooling or freezing occurs can be important to the final product quality. Freezing strawberries or scallops too slowly causes cell damage and results in a mushy finished product. Freezing ice cream too slowly produces ice crystals and results in “crunchy” ice cream. Freezing some filled products too quickly can cause the shell or outside layer to crack.
Suppose you are freezing raw, marinated chicken breasts for retail sale. As a result, it is desirable to maintain a uniform appearance. A smooth, flat side can be achieved using a cryogenic plate-belt or super-contact freezer. Yet, while these freezers are an optimal solution for freezing or setting one side of the chicken breast, it is not economical to use them for the full freeze. Once the bottom is set, another freezer of a different style should be employed. In this case, a multi-method approach to freezing is advised. In addition to a uniform appearance, using a cryogenic plate-belt or super-contact freezer improves yield by preventing adhesion and embrittlement of small pieces of food that break free and are lost to waste.
Other multi-method approaches can be used to achieve proper chilling or freezing in pursuit of product quality. For instance, a quick cryogenic freeze immediately prior to a mechanical or cryogenic spiral freezer can be employed to secure moisture in a cooked product to lessen dehydration losses. A cryogenic, flighted-belt freezer can be used to quickly create individually quick frozen (IQF) products such as shrimp or vegetables prior to their entering a spiral freezer. If such products entered a spiral freezer directly, without an IQF assist, they would freeze together into clumps.
In food manufacturing, a production line can run out of freezing capacity quickly. The demand for production grows or a formulation changes, requiring more refrigeration capacity. Large new spiral freezers are expensive and can require multiple months of downtime for installation. A quick and easy fix is to move to a multi-method freeze. Adding a cryogenic system prior to a spiral freezer can increase overall capacity along with yield savings (through moisture retention) and coil downtime reduction. Delivery and installation times typically are lower than purchasing a new spiral freezer.
For batch chilling or freezing of smoked or cooked products, the initial heat upon loading takes quite a while to dissipate even with blast chilling available. A cryogenic coil can be used to rapidly return the batch chiller environment to one with a useful temperature differential, so that the blast chiller can return to cooling the product and not the environment. This multi-method approach only uses the cryogenic coil to return the batch chiller to a useful temperature, potentially shortening the chill time so that an additional batch can be chilled each day.
Some products are more easily manufactured if a multi-method freezing system is put into place. One example is frozen, par-fried, sauced chicken wings, which are in high demand. Sauce pickup is difficult on a par-fried wing. The sauce warms up and drips off prior to being frozen, and much of the dripping taking place in the spiral freezer. Based upon what has been presented, it might immediately come to mind to do a quick cryogenic freeze after the sauce step to secure the sauce to the wings prior to their entering the spiral freezer. Testing has shown, however, that a better multi-method approach is to use the spiral freezer to fully freeze the par-fried wings before applying sauce. A cryogenic freezer is used after the sauce is applied.
Several benefits result from this approach. First, the cost of using the cryogenic freezer is isolated to only freezing the sauce. If the cryogenic freezer were before the spiral freezer, some cryogenic energy would need to be used to start to freeze the par-fried wings underneath. Second, cleanup can be isolated to the sauce applicator and the cryogenic freezer when changing flavors. Inevitably, some amount of sauce coat the spiral freezer. By pushing the sauce application downstream of the spiral freezer, no sauce enters the spiral. Third, pickup of the sauce can be increased dramatically. Applying sauce to a frozen substrate (the wing) is much easier than applying sauce to a hot or warm wing. PC
Chris Johnson is the business development director for food with Linde. The Burr Ridge, Ill.-based company can be reached at 630-320-4640 or visit lindeus.com.