PROCESSING FISH MEAL AND OIL 



227 



getting a meal of high nutritive quahty'^, are not widely used by the 

 fish meal industry. Heating is indirect, through high pressure steam in 

 several banks of tubes running the length of the dryer. In this type of 

 dryer it is impossible to burn the scrap since the scrap temperature can- 

 not be raised above that of the steam (335°F for 100 psi steam). However, 

 it is now known that longer exposure to temperature in the 250 to 300°F 

 range can do as much damage to protein quality as shorter exposure to 

 higher temperatures. 



It is now generally conceded that properly controlled hot air dryers can 

 produce fish meal of equal or even better quality than the steam dryer, 

 as even steam dryers can be mishandled. Increasing use of temperature 

 recorders in the exhaust gases and use of automatic controls on the 

 burners to prevent the intermittent overheating that commonly results 



AIR EXHAUST 



AIR BLOWER 



OIL BURNERS 

 FURNACE 



1 C1QQ°°| 



MATERIAL FEED 



DRYING SE 



^INSTRUMENT PANEL DISCHARGE 



Figure 16.3. Diagram of fish meal hot air dryer. 



with a variable feed rate of the press cake, are being more widely used to 

 obtain more uniform meal quality and more even moisture content of the 

 scrap. 



Curing. Herring, white fish, and some other fish meals apparently can 

 be bagged as soon as the scrap can be dried and ground. However the 

 menhaden industry, which now accounts for more than 80 per cent of 

 the fish meal produced in the United States, finds it necessary to ''cure" 

 the scrap, that is, to convert it to a stable state. When the scrap comes 

 directly from the dryers it heats rather than cools in the storage piles. 

 The exact mechanism involved in this heating is not thoroughly under- 

 stood, but polymerization and/or oxidation of the oils is certainly in- 

 volved, ^leals that are relatively high in oil, above 10 per cent, and have 

 a moisture content above 8 or 10 per cent seem more prone to excessive 

 heating. 



Antioxidants, mainly butylated hydroxy toluene (BHT), have been 

 widely used to prevent overheating and the protein quality loss that 

 often results from overheating. BHT apparently reduces the tendency to 



