Examples Of Microwave Processing Applications Tempering

Conventional frozen food thawing methods are extremely inefficient taking many hours to several days depending on the product. The reason for this is that the thermal conductivity of the frozen product is about three times that of the thawed product. Thus as the surface thaws, heat is more slowly conducted toward the center.

Complete thawing of frozen foods by means of microwave energy also has its problems. It has been pointed out that as the frozen products approached 0°C, microwave penetration decreases rapidly and care has to be taken to prevent runaway heating (9). Furthermore, if frozen materials are held at ambient temperature too long before being exposed to microwave energy most of the energy will be absorbed at the surface and thawing to the core will not be possible.

Fortunately it is not necessary in most cases to completely thaw frozen materials. The tempering process stops short of thawing by several degrees; thus tempering has come to mean "raising the temperature of frozen food to a higher temperature that is still below the freezing point, but at which temperature the product is firm but no longer hard." Properly tempered frozen food is easily sliced, diced, separated, or otherwise handled for further processing.

Some data on the energy required to freeze foods are also pertinent to thawing foods and clearly illustrate the benefits of stopping the process prior to complete thawing (10). From Table 3 it can be seen that the energy required to raise the temperature of frozen beef from —17.7 to — 4.4°C is about one-half that required to raise the temperature to — 2.2°C. Thus, where possible, tempering should be terminated at the lowest acceptable temperature. It should also be noted that microwave tempering is uniform throughout the frozen mass being tempered.

Microwave tempering is one of the more economical processes in terms of energy consumption. The cost of tempering in terms of energy and magnetron replacement costs is less than $0.01/lb. Other factors that contribute to the return on investment include improved yield and quality, space and labor savings, simplified production scheduling, and increased plant productivity.

Most microwave tempering equipment today is operated at 915 MHz at power levels varying from 25 to at least 150 kW and are continuous. Batch units are available for smaller food-processing operations and usually operate with 40 kW of power at 915 MHz. Continuous equipment also is available at 2,450 MHz and varies in power from 33 to 132 kW. The microwave penetration at this frequency is less than at 915 MHz and requires the use of a refrigerated air circulation system to prevent surface thawing so that internal tempering can be accomplished. There are approximately three hundred microwave tempering systems in operation worldwide, with about one hundred eighty systems in the United States. These systems average 100 kW of microwave power. Annual tonnage of products tempered in such equipment is on the order of 5 billion lb.

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