Traditional cryogenic freezing is used for improving frozen product quality, reducing capital expenditures and saving plant space in frozen-food plants. It is used for almost every conceivable food product that is processed today, from shrimp to ice cream cake.
The major attribute of cryogenic refrigerants is that they are extremely cold. The two practical cryogenic refrigerants are liquid nitrogen, which has a temperature of -320oF (-195oC), and solid carbon dioxide (dry ice), which has a temperature of -110oF (-79oC) when used for freezing food.
There are several basic types of cryogenic freezers, including tunnels and spirals. A tunnel freezer is the simplest continuous cryogenic freezer. It consists of an insulated tunnel with a conveyor belt traveling through it that continuously moves the food product to be frozen. Spiral freezers generally are used for higher production rates than tunnels and have a relatively small footprint due to the spiral configuration of the conveyor inside the large insulated box. In both of these, cryogens are injected via nozzles or orifices and the system is temperature controlled (usually at an operating temperature warmer than that of the actual cryogen temperature). Fans or blowers create convection that removes heat rapidly. Variable belt-speed control determines the dwell/residence time in the freezer.
IQF FreezingIndividual quick freezing or individually quick frozen (IQF) can be accomplished with cryogenic freezers. IQF means that each unit of the food product - peas, for example - is frozen individually rather than being frozen together in one whole clump (bulk). The advantage of IQF is that the product can be portioned and poured while still frozen. If the product is block frozen, the entire block must be thawed to be poured or portioned. With IQF, one only needs to use the amount that is actually required.
IQF Cryogenic CoatingMany consumers have become very busy in the new fast-paced economy - too busy, in fact, to prepare home-cooked meals on a day-to-day basis. In light of this phenomenon, the food industry has responded with solutions to the consumer's desire to have a "home-cooked meal" when time does not allow for the preparation required. Many home meal replacement (HMR) and convenience foods of high quality and appeal have been created, especially by the frozen food sector and, in particular, with the use of IQF products. This category of food puts added value to the basic food products, making it beneficial to both the producer and the consumer.
In Europe in the mid-1990s, a new frozen product market appeared: coated IQF products. Coated IQF products consist of IQF pieces individually enrobed in a frozen sauce coating. Typical examples were paella and Cantonese rice. These products initially appeared in Northern Europe and were made possible by incorporating cryogenic freezing into the coating process. Cryogenic freezing rapidly freezes the sauce layer onto the IQF pieces, forming a hard crust that subsequently prevents the pieces from sticking together.
To meet the convenience needs (pourability, portionability, etc.) of complete dishes with sauce and similarly incorporated items, this new process of cryogenic IQF coating - also called "cryogenic coating" or "IQF coating" - was created.
A coated product is an IQF piece of food that is encapsulated individually in frozen sauce. When using broccoli in Hollandaise sauce for example (figure 1), each frozen floweret of broccoli, typically already cooked, would be coated completely with the frozen Hollandaise sauce yet will remain an individual piece not frozen in sauce to other pieces. Figure 2 shows cross-sectional pieces of a diced product with the layer of sauce around them surrounded by the remainder of the food mixture, some of which is uncoated product.
The Cryogenic IQF Coating ProcessCryogenic IQF coating is a batch system for individually coating precooked, deep-frozen products with sauce in a rotating drum. The process involves sequentially injecting a cryogenic fluid (liquid nitrogen or liquid carbon dioxide) and a coating sauce. This sequential injection is accomplished in equipment similar to a tumbler (figure 3), which is loaded with the IQF products to be coated. Cryogenic freezing is essential for the coating, which cannot be achieved with mechanical refrigeration because it operates at temperatures too warm to freeze the sauce quickly enough.
The process stages are carried out successively as follows (figure 4):
- The individual products are fed into the coating equipment already deep-frozen.
- These deep-frozen products (0oF [-18oC]) are mixed with the sauce. The sauce is relatively warm so that as a liquid, it can coat the IQF products.
- A surface-crusting effect appears as soon as the sauce is applied to the deep-frozen product. A fine film of sauce begins to freeze on the outer surface of the product.
- The cryogenic fluid then is injected into the equipment, deep-freezing the sauce to re-establish the coated product temperature.
Steps two through four can be repeated one to several times, depending on the desired amount of sauce coating. The resulting products are now individually deep-frozen. Each frozen piece is coated with frozen sauce, and the product is ready for packaging, storage, shipment and use.
A host of coated IQF products currently is being produced, including cauliflower in cheese sauce, stir-fried vegetables, rotini with marinara sauce, chili con carne, Swedish meatballs, beef stew, fried rice and Seafood Newburg.
Preparation of the IQF coated product is easy and convenient due to its pourable, portionable nature. The end user -- either the home consumer or the food service operator -- would pour the exact amount of product to be heated into a bowl or onto a plate, and the unused portion is put back in the storage freezer for later use. The result after heating, usually in a microwave oven, is product bathed in the correct proportion of sauce and ready to eat. Figure 5 shows what a frozen-coated product looks like while still frozen -- in this case, rotini pasta in a white sauce with uncoated ham strips.
All coated products in their final form are unique: The combination of product and sauce, including the level of sauce coating, is distinct to the processor's specifications. In consideration of this, and to ensure optimal results in terms of appearance and coating ratio, product trials should be done. Test results not only establish the qualitative aspects of the product, they also are used to determine equipment size, how many injection sequences to carry out, optimum viscosity of the sauce and other parameters such as cryogenic consumption, rotational speed and operating temperature. Small test units are available for this purpose.
Cryogenic or IQF coated products are part of a growing market of convenient frozen foods that can be sold at a higher margin than uncoated IQF products. With the advent of cryogenic equipment that can facilitate the process, more and more processors will now be able to add value to the products they present to the market place.