THE PRESERVATION OF FISH BY SMOKING AND DRYING 421 



nets that would be economically feasible on a commercial scale. Because of the 

 military need for products that would keep well and at the same time require a 

 minimum of space researchers were assigned during both world wars to study 

 the problems of dehydration. While the primary demand was military, it was 

 hoped that a suitable product could be developed that would be acceptable to the 

 domestic markets in normal times. 



Much progress has been made and many problems solved. Information is now 

 available on the proper pretreatment of the raw material. Correct temperature 

 and humidity in relation to the time element have been worked out, and more 

 is known about how to pack and store the finished products. However, even in 

 the light of the most recent experiments successful storage is difficult to attain 

 and the cost of the dehydrated product is almost too high for commercial produc- 

 tion for the domestic market (Stansby, 1945). 



Dehydration Processes. A convenient definition of dehydration as applied to fish 

 as well as other foods is "drying in an atmosphere of controlled temperature and 

 humidity to a definite end product in a given time" (Cutting and Reay, 1944). 

 In order to accomplish this a variety of drying methods have been used, such as 

 vacuum-drying, vacuum-drying combined with freezing temperatures, and spray- 

 drying. More orthodox methods, including cabinet driers or tunnel driers, of both 

 the conveyor and stationary type, have also been used. The more elaborate proc- 

 esses of drying, such as in vacuo, have been found impractical for fish products. 

 Since fish drying can be carried out under a fairly wide range of conditions, 

 complicated processes and equipment are not so essential as with some other 

 types of food products. The most important factors were found to be preliminary 

 preparation of the raw material and storage of the finished product. 



In all the processes so far devised it has been demonstrated that the fish must 

 be precooked in order to obtain a satisfactory rate of drying. In addition it was 

 learned that the removal of moisture could be accelerated by mincing or grinding 

 the precooked material. According to Stansby (1945) the dehydration of raw 

 fish proceeded at a rate of only one-third that of cooked fish. In addition rehydra- 

 tion was slower and only half as much water was absorbed. Canadian investigators 

 (Young and Sidaway, 1943) recommend a further step in preliminary prepara- 

 tion. After cooking, the fish is skinned and boned; while the flesh is still warm, it 

 is pressed at approximately 100 pounds per square inch to remove the expres- 

 sible oil and water. The pressed cake is then ground or minced before spreading 

 on screens for drying. It appears that this extra step not only speeds up the 

 dehydration process, but aids materially in assuring satisfactory keeping qualities. 

 The expressed oil and water, which contains some dissolved protein matter, 

 represents a nutritional loss unless recovered and utilized as a by-product. 



Processes for precooking are variable and range from 22 to 30 minutes at 

 212° F (100° C) to 7 to 10 minutes at 5- to 10-pounds pressure. The temperatures 

 to be used in the dehydration process also cover a wide range. It is generally 

 agreed that the temperature should start high and be gradually reduced toward 

 the end of the drying process. As the product becomes more concentrated, a high 

 temperature in the final stages would scorch and darken it. Temperatures as high 

 as 185° F (85° C) to 205° F (96.1° C) have been shown to do no harm in the 

 initial stages. The final drying should, however, be carried out in temperatures 

 ranging from 145° F (62.8° C) to 158° F (70° C). Although the relative hu- 



