222 FISHERY INDUSTRIAL PRODUCTS 



small quantities of fish must be handled, or for species such as ground 

 fish, shark, or others with a low-oil content (less than 3 per cent), the dry 

 process is more practical. 



The dry process is usually a batch rather than a continuous process. 

 The fish is placed in a steam- jacketed vessel that serves both as cooker 

 and dryer. A typical installation consists of a cylinder about 5 feet in 

 diameter and 20 feet long that holds about 4 tons of fish. The vessel is 

 fitted with a paddle-type stirrer that is rotated slowly but continuously 

 during the rendering operation, which may take 6 hours. The dried scrap 

 may contain more oil than is desirable. If so, excess oil is removed in a 

 hydraulic press, also a batch operation, before the scrap is ground and 

 sacked. 



Solvent Processes. Solvent extraction has been used in the past when 

 oil was the product primarily desired, such as in the preparation of vita- 

 min oils from fish livers. It has also been tried in the past on batch-dried 

 salmon meal and on a small scale with other raw material, but solvent 

 and equipment costs have not made these operations economical. How- 

 ever, there is at the present time a great deal of interest in solvent proc- 

 esses for preparing so-called ''fish flours," more accurately termed concen- 

 trated fish protein or fish-protein concentrates, for human consumption^ ^ 



The various modifications in this process will, therefore, be reviewed 

 briefly. The major differences are related to the starting material, which 

 may be either raw fish, cooked and pressed fish, or dried scrap or meal. 



Raw fish can be handled in two ways: by an azeotropic process using 

 solvents that are non-miscible with water or by direct extraction, in 

 which case the solvent must remove both water and fat. If press cake is 

 used the same processes are applicable, but the operation is somewhat 

 simplified by the prior removal in pressing of much of the Avater and oil 

 as well as some possibly objectionable nitrogenous constituents. If dry 

 scrap or meal is used, most of the fat can be removed by any of the 

 hydrocarbon-type solvents or chlorinated solvents, but complete removal 

 of lipids is difficult and involves multiple extraction using one of the 

 alcohols as well as the afore-mentioned type of solvent. 



Azeotropic extraction is dependent on the fact that when both water 

 and solvent are present in a mixture the vapors distilling from the mix- 

 ture contain both substances in proportions dependent upon their partial 

 pressures. Dichloroethane or hexane are suitable solvents, the former 

 being used in the Vio-Bin process^- ^'^. 



As the vapors are condensed, the condensate separates into two layers 

 and the solvent layer can be immediately pumped back into the process 

 vessel to carry off more water. The mixed boiling point is lower than 

 that of either constituent but approaches the boiling point of the solvent 



