Fish Products and Byproducts Technology

Unit 9 - Fish meal and fish oil

9.5.1. Wet reduction/rendering process
Wet rendering is a system which leaves a high amount of moisture in the product, until, or if, it is to be dried. This process used primarily for fatty fish (menhaden, herring, pilchards, anchovy, sardine etc., which are caught specifically for fish meal production) or offal contains high fat content, where simultaneous production of fish mea and fish oil is envisaged. This process is continuous rather than batch process and is particularly suitable for large scale operation.

The essential steps are as follows:

• Grinding or hashing of large fish
• Cooking and heating usually with steam
• Pressing to squeeze out water and oil

• The liquid portion is known as press liquor and is passed through ascreen to remove solid particle of fish which are then return to the solid portion called press cake which contains about 50-55% moisture and 3-4% oil

• Fluffing out of press cake
• Drying the press cake to a moisture level of 10%
• Grinding and packing the dried meal

Press liquor cam be treated, after screening to remove solids in a number of different ways; generally the liquid is heated and centrifuged to remove the suspended solid particles and the oil. The oil may then be further refined and polished while the solids are returned to the meal plant for drying. The liquor or stick water can be concentrated by evaporation of the water to about 50% solids. The concentrated liquor can be sold separately as fish soluble. If the fish soluble is added to the press cake and dried then the end product is called as whole meal.

Advantages and disadvantages of Wet reduction process