PROCESSIXG FISH MEAL AXD OIL 223 



as the water content approaches zero, indicatmg completion of the cook 

 in a batch process. The azeotropic principle has also been adopted to a 

 continuous process". 



After the water is removed, the fat phase remains in the vessel, dis- 

 solved in the residual solvent which then is filtered off. The residue is 

 washed once or twice with clean solvent, then heated in a steam-jacketed 

 vessel under vacuum to remove as much of the residual solvent as possible. 

 As a final step, the vessel is sparged with dry steam to remove the last of 

 the solvent from the fish protein, which is then ground and bagged. 



In this, as in any solvent extraction process, economical operation de- 

 pends on maximum recovery of the relatively expensive solvent, which 

 requires that the entire system be vapor-tight and that solvent residue in 

 both meal and oil be reduced to a minimum. Solvent is distilled from the 

 solvent-oil mixture, and the oil is then further refined. This is usually 

 needed because solvent extraction processes remove almost all the con- 

 stituents responsible for color and odor of the raw material and these 

 stay in the oil. 



Isopropanol has been used for direct extraction of raw fish. The process 

 is basically simple, the coarsely ground fish being mixed with the hot sol- 

 vent, the solvent phase then being removed by filtration and/or by cen- 

 trifugation. The residue is then treated with successive batches of fresh 

 solvent until water and oil are reduced to the desired point. As in the 

 azeotropic process, the solvent is costly and a high degree of recovery is 

 necessary for the operation to be economically feasible. Recovery is some- 

 what more difficult because the water content of the solvent requires the 

 use of fractionation techniques. 



Digestion Processes. In the past, whole fish or wastes have been di- 

 gested with acids, alkalies, enzymes, or various combinations of these re- 

 agents. In this process, proteins are broken down into the more soluble 

 polypeptides or amino acids and the resulting liciuid has been concen- 

 trated in evaporators yielding a product called homogenized condensed 

 fish. i\Iore recently, digested fish has been mixed with soy bean meal or 

 other vegetable carriers. This mixture can be readily dried to yield a 

 product with a balanced protein which is also enriched with vitamins 

 and unknown growth factors. Enzymes are preferable for digestion proc- 

 esses as strong acid or alkali treatment results in destruction of some of 

 the amino acids. The enzymes may be added or whole fish may be chopped 

 and digestion obtained with the autolytic enzymes present in the digestive 

 tract. In this process the mixture is heated to a point that will retard 

 bacterial decomposition but still permit enzyme activity. 



Recent interest in a digestion process is derived from the possibility 

 that the liquid, filtered and perhaps centrifuged to remove undigested 



