232 FISHERY INDUSTRIAL PRODUCTS 



as a preservative. In the Lassen process^, acid is added to the raw stick- 

 water and serves also as a protein coagulant, precipitating some of the 

 solids that are in part responsible for high viscosity and also freeing oil 

 held in suspension. 



Several types of evaporators are used for fish stickwater. Large installa- 

 lations in the United States have used continuous vertical, triple-effect 

 vacuum evaporators almost universally. Steam jet ejectors are used to 

 obtain the desired vacuum, and usually a horizontal preheater is used. 

 These installations have high capacity and are practically automatic, once 

 control valves are set for a given lot of stickwater, so that the plant opera- 

 tion can be handled by one experienced evaporator man. Liitial cost is 

 fairly high and shutdowns are not infrequent, with scale in the tubes or 

 corrosion of the tubes both common sources of trouble. 



Another type of evaporator that has been used with some success is the 

 Vincent hot air evaporator. This works on the same principle as a spray 

 dryer, the stickwater being fed into a turbine rotor in the center of the 

 top of the conical chamber with the hot gas from the burner being intro- 

 duced tangentially. Part of the water flashes off (solids content of feed is 

 usually from 5 to 8 per cent), and the thickened material is thrown to 

 the side walls from which it runs down to a sump. It is not usually prac- 

 tical to take the stickwater down to 50 per cent solids in one step, so that 

 the sump contents are pumped to an identical or somewhat smafler second 

 stage evaporator or to temporary storage for a later second run through 

 the same unit. 



The main objection to this type of installation is the fact that the 

 solubles pick up undesirable contaminants from the heating gases. This 

 may be only fine carbon from incomplete combustion of the fuel oil, which 

 gives the product an abnormal greenish or gray color. However, high 

 sulfur fuel oils are occasionally found and may result in high sulfur dioxide 

 in the solubles, which occasionally will have a quite acrid or sharp odor, 

 and other unidentified impurities may be acquired in this same manner. 



Stickwater from small reduction operations connected with canneries 

 may be batch-evaporated in steam-jacketed kettles in locations where 

 this material cannot be discarded because of pollution restrictions. The 

 capacity of this type of equipment is very low compared to the three-stage 

 vacuum evaporators. 



Full Meals. Solubles may be returned to the scrap in proportions 

 approximating the original relative content of dry solids in the press cake 

 and press liquors, forming a product known as full meal or whole meal. 

 Since neither the chemical composition nor nutritive value of the mixture 

 is sufficiently different for the whole meal to be readily distinguished from 

 the straight meal, except by determination of the soluble protein, these 

 products may not be separately identified as marketed. Consequently, 



