FISHERY BULLETIN: VOL. 71, NO. I 



meal" containing 1 part condensed solubles to 

 3.5 parts of fish meal. This recombination of 

 the two fish products is a logical approach as it 

 is really equivalent to the original fish minus a 

 major part of the original oil. Numerous other 

 feedstuffs, such as soybean meal, alfalfa meal, 

 or wheat middlings are also used to produce a 

 free-flowing dry product consisting of one-half 

 carrier and one-half dried condensed solubles. 



PROCESS METHOD 



Condensed fish solubles is one of two by- 

 products produced by the wet reduction of whole 

 fish to fish meal. Briefly, the production of con- 

 densed fish solubles by the "wet reduction" pro- 

 cess involves cooking whole fish (e.g., menhaden) 

 and subjecting the cooked fish to screw-type 

 presses. The liquid fraction which passes 

 through the screens of the presses is collected 

 and separated by first passing through a vibrat- 

 ing screen to remove suspended material. The 

 remaining liquor is either pumped into settling 

 tanks for gravity separation or, more commonly, 

 centrifuged to separate the oil and aqueous por- 

 tion. Usually the centrifugation system consists 

 of a three-phase centrifuge designed to separate 

 oil, water, and suspended material. The aqueous 

 portion in these centrifuges is collected in large 

 storage tanks. During this storage process usu- 

 ally a quantity of sulfuric acid, or occasionally 

 phosphoric acid, is added to the stickwater until 

 a pH of 4.5 is reached (normal pH 6-7.0). The 

 storage tanks are heated by steam coils to a tem- 

 perature of approximately 150°F for 30 min to 

 coagulate the proteins. The suspended and co- 

 agulated proteins are removed by centrifuges, 

 and the free oil is removed by oil-separating cen- 

 trifuges. The solid materials are dried on press 

 cake and the liquid is called "stickwater." Table 

 1 shows the composition of several samples of 

 stickwater produced in 1970. These samples con- 

 tained an average of about l^/r total solids and 

 about 5% protein. The amino acid profile in- 

 dicates that this protein is gelatinous in nature 

 because of the relatively high glycine, proline, 

 and alanine content. 



Stickwater is then concentrated by means of 

 vacuum evaporation, generally operated in units 



of three or more. Thus, the terms "triple effect" 

 or "quadruple eflFect" are applied to the evap- 

 oration procedure. The evaporators may be hor- 

 izontal or vertical, depending on the position of 

 the steam tubes used for heat. Each evaporator 

 consists of a series of tubes, usually vertical, hold- 

 ing the liquor and surrounded by steam. The 

 tubes passing through the evaporator allow the 

 vaporized water to escape at the top, and the 

 partially evaporated product passes to the next 

 unit at a lower position in the evaporator. 



These evaporators are subjected to reduced 

 pressure. The first unit is usually heated by low 

 pressure exhaust steam from the boilers and is 

 generally maintained at about 24 inches of mer- 

 cury pressure (or under a slight vacuum). As 

 the liquor reaches a predetermined concentra- 

 tion, it is drawn into the second unit. The vapors 

 from the boiling liquor in the first unit are passed 

 through the steam tubes of the second, and the 

 liquor continues to boil by increasing the vac- 

 uum to about 28 inches of mercury. 



In the third stage the process is either com- 

 pleted under a vacuum of about 30 inches of 

 mercury or continued depending on the number 



of stages employed. When the liquor reaches 

 approximately 50% solids, the process is fin- 

 ished and the solubles are stored. Thus, during 

 the process the original material containing 

 about 93% water is concentrated by a factor of 

 at least seven. Other methods of condensing fish 

 solubles have been and are used but not exten- 

 sively enough to warrant further discussion here. 



Table 1. — Chemical composition of Atlantic menhaden 



stickwater.' 



^ Values are averages of data from three analyses. 



256 



