Fish Products and Byproducts Technology

Unit 9 - Fish meal and fish oil

9.5. Processing Method
To understand the principle of fish meal and oil manufacture, it is necessary to consider the raw material as composed of three major fractions:

• Solid (fat free dry matter)
• Oil
• Water

The purpose of process is to separate these fractions from each other as completely as possible with the least possible expense and under conditions rendering the best possible product. There are several ways of making fish meal from raw fish; the simplest is to let the fish dry in the sun. This method is still used, in some parts of the world where processing plants are not available, but the product is poor in comparison with ones made by modern methods. Almost all fish meal is made by cooking, pressing, drying and grinding the fish in machinery designed for the purpose. Although the process is simple in principle, considerable skill and experience are necessary to obtain a high yield of high quality product, and to make the plant efficient. There are two methods (wet reduction and dry reduction) and many minor variations of commercial fish meal production. A typical process is shown diagrammatically in figure 1. Common steps to all methods of practical importance are as follows:

• Heating, which coagulates the protein, ruptures the fat depots and liberates oil and physico-chemically bound water;
• Pressing (or occasional centrifugation), which removes a large fraction of the liquids from the mass;
• Separation of the liquid into oil and water (stickwater). This step may be omitted if the oil content of the fish is less than 3%;
• Evaporation of the stickwater into a concentrate (fish solubles);
• Drying of the solid material (presscake) plus added solubles, which removes sufficient water from the wet material to form a stable meal,
• Grinding the dried material to the desired particle size.