Over the last several decades, aquaculture has gained importance since wild capture can no longer meet the global demand for fish. Therefore, this market has grown exponentially, and with it, the demand for fish feed. In particular feed with high amounts of fat has gained relevance, since it allows for many species (e.g. salmon, trout) the most efficient growth and, thus, farming.
Extrusion processing is often used to produce fish feed pellets. Within the heated barrel containing rotating screws, the raw materials are mixed, heated, and sheared along the extruder. The combined thermal and mechanical stresses applied to the material lead to its plasticization and cooking. Due to the temperature and pressure gradients between the melt and the room conditions, the material can expand after it leaves the die and is pelletized through a rotating knife. Although addition of oil is desired, it has a negative influence on the cooking process during extrusion, as its lubricating properties reduce the specific thermal and mechanical energy input and, consequently, the degree of cook. In general, a lower degree of cook has a negative influence on the product quality of the feed pellets, i.e. decreased water stability and hardness. Therefore, to cook the material sufficiently and to achieve the aimed product properties, manufacturers avoid adding high oil levels during extrusion. Instead, manufacturers add oil in a separate coating step post-extrusion. This additional step makes the production of aquatic feed more complicated and expensive. Additionally, if fat does not bind properly to the pellets fat leakage can occur, whereby the fat drains out of the pellet, consequently, leading to increased fat oxidation. The addition of additives, especially silica, would serve to resolve these issues while achieving high fat levels in high quality fish feed pellets without a coating step. Since the increase of fat leads to a decrease in the matrix viscosity, the mechanical energy input is expected to decrease leading to a lower degree of cook. As silica is known for its water and oil binding capacity as well as for its ability to control rheology and therefore to increase viscosity, its addition is expected to counteract this effect leading to an enhanced gelatinization during the extrusion process. This strategy will be introduced and the relationship between process conditions, addition of different oil and silica concentrations and resulting product properties (among others pellet hardness, pellet abrasion and fat content) will be evaluated.