Towards a Better Understanding and Control of Factors Affecting the Physical Quality of Fish Feed

Sammanfattning: Popular Abstract in English Fish are an excellent source of healthy protein and fat in the human nutrition. However, the fisheries cannot supply enough of fish to satisfy the demand of a growing world’s population. Sustainable aquaculture can help feeding the growing world. This business is growing every year, and together with it the fish farms and fish feed factories are growing. This created the need for longer transportation systems, which could deliver the fish feed to diverse destinations. Fish feed is most commonly produced in an extrusion process, where raw materials (after mixing under high pressure and high temperature) are formed into small cylindrical particles named pellets,. These, are later infused with oil in order to increase the amount of energy (calories) available for fish. These feed pellets need to be strong enough, to remain unscathed after all the handling necessary to transport the feed both within the factory and during the delivery to fish. A lot of research has been done on finding the optimal nutritional composition of the feed ingredients that will support good growth of healthy fish. Now, it is time to explore more the factors influencing physical quality of extruded fish feed. In order to evaluate physical stability of feed pellets, reliable methods are needed. A part of this work was to optimize the method used in other pelletizing industries, so-called Holmen tester, in order to make it practical for the high fat extruded fish feed. This method is supposed to mimic the various stresses pellets undergo during e.g. pneumatic transportation. We have observed that when the method is properly standardised, the results obtained are very repeatable and reliable. Second part of this study was to evaluate the influence of different raw materials on the physical properties of feed, immediately after the production, as well as after three weeks of storage. Five feeds that were supposed to be as close to commercial product as possible, with varying inclusion of sunflower meal, starch and moisture were produced in the pilot plant and tested for both physical, as chemical stability. During three weeks, no changes in the chemical composition of the feeds were observed. However, most of the feeds showed significant aging in terms of physical quality. In fact, the decrease in the physical quality during three weeks of storage was so big that the feed would not be accepted by the quality criteria of the commercial feed factory. Only the feed with the high sunflower meal, starch and moisture content retained its good physical quality over the test period. Third part of this study focused on the effect the intensity of handling may have on the physical quality of pellets with varying amounts of sunflower meal, starch and moisture. In this case, also storage stability was tested. It was observed, that in most of the cases, intensive handling negatively affected the physical quality of feed not only after the production, but also resulted in faster aging. The feeds with higher moisture content were tolerating the intensive handling better than the ones with lower moisture. The negative effect of intensive transportation and storage time can be overcome by specific feed composition. The feed that did not show any aging despite intensive handling was the one with the high sunflower meal, starch and moisture content. The reason for the observed aging of feed during storage might be fat migration. The liquid oil could migrate through the structure of a pellet; soaking from the pores to the matrix of solid structure, eventually making it more vulnerable to breakage. The fourth part of this study was to interpret the effect of enzymatic modifications on the physical quality of the fish feed. The first kind of modification evaluated was the crosslinking of proteins. With the aim to improve the durability and water stability of pellets, a special enzyme, called transglutaminase, which creates new linkages between protein molecules was applied. It was observed, that transglutaminase can be effective in the fish feed production application. Moreover, the effect of the enzyme was dose-dependent. The second kind of enzymatic modification involved slight starch hydrolysis. Starch is not very well digested by salmon and is used in the fish feed mainly for its binding properties. In this study, small amount of the amylase enzyme was added to the raw material mix (just before the extrusion), foreseeing that it would be inactivated by the elevated temperature and pressure. Despite the very short-reaction time, amylase had a significant influence on the durability of the pellets. Probably, the enzyme dosage was still too high for this application, and can be further optimized in following studies.