|Abstract: ||The increased demand for seafood, due to both population growth and an increased health awareness, has resulted in an increased demand for ﬁsh feed in aquaculture production. This since wild ﬁsh capturing cannot fulﬁll the demand in a sustainable way. Traditionally, ﬁshmeal has been the major protein ingredient in ﬁsh feed, providing the ﬁsh with the essential amino acids needed for optimal growth. However, the production of ﬁshmeal has decreased rapidly the past decades. Instead, plant based protein sources has partly replaced ﬁshmeal. The issues with this replacement is the diﬀerent amino acid composition and the presence of antinutritional substances, resulting in decreased growth rates.
One alternative that is gaining interest is the use of single cell protein (SCP) as a potential protein ingredient in ﬁsh feed. SCP consists of microorganisms such as yeast, bacteria, algae and ﬁlamentous fungi. SCP has several advantages, such as similar amino acid composition to ﬁshmeal, short generation time, no requirement to arable land and the ability to grow on various substrates. The ﬁlamentous fungus Paecilomyces variotii was selected in this project due to its high protein content and ability to grow on residual streams from the forest indutry.
Since the proteins in SCP are intracellular, cell wall degradation can result in higher digestibility. Further, if the polysaccharides in the cell wall are decreased, the protein content of the SCP would increase, resulting in a ﬁsh feed ingredient of higher quality. To achieve this, the following cell wall degradation methods were investigated: High Pressure Homogenization (HPH), enzymatic lysis with b-glucanase and a combination of the two methods. After the diﬀerent treatments, pH-shifting was performed in order to precipitate the proteins. The pellets obtained were analyzed for protein content. Further, the carbohydrate concentration was determined in the supernatant to investigate the eﬀect of the enzyme, and samples from the pellets were studied in a light microscope to study the cell wall degradation. Further, extrusion was investigated as a possible treatment method, and both the physicochemical and structural properties of the extruded samples were investigated. Growth and survival of Artemia was evaluated in a feeding trial.
The results showed that HPH resulted in the highest degree of cell wall degradation. The pH-shifting resulted in protein concentrate with a higher protein content (about 50 %) than the untreated biomass (35-40 %), with a yield of 53.81 %. When analyzing the physicochemical and structural properties of extruded samples, it was concluded that extrusion at 115 °C, with a screw speed of 120 rpm and with a moisture content of 40 % resulted in samples which held together well, both in the micro- and macrostructure, and had the lowest water solubility index (WSI) among the extruded samples. Finally, extrusion appeared to increase growth and survival in Artemia during the feeding trial, but no signiﬁcant diﬀerence was found.|