Cultivation of white-rot fungi on brewer’s spent grain by solid-state fermentation. Photo: Freja Manø Busk Karlsen

PhD defence about the potential of using brewer’s spent grain in fish feed

Friday 09 Dec 22


Peter Vilhelm Skov
Associate Professor
DTU Aqua
+45 35 88 32 63

Part of SureAqua project

Freja Manø Busk Karlsen's PhD project is part of Sure Aqua—Nordic Centre for Sustainable and Resilient Aquatic Production funded by NordForsk.

Learn more about SureAqua

On 20 December, Freja Manø Busk Karlsen will defend her PhD thesis. It is possible to attend the defence at DTU in Hirtshals or online.

Fishmeal and soybean meal constitute the most common protein sources in feed used in aquaculture. Both ingredients are characterized by a high content of protein and an ideal amino acid composition. Yet, increasing demands and competition for resources have driven up the prices of these protein sources which has increased the need for implementing sustainable alternatives. 

PhD Student Freja Manø Busk Karlsen, DTU Aqua, has investigated whether brewer’s spent grain can be valorized to make it suitable as a protein source in fish feed. Brewer’s spent grain is the most abundant by-product from beer production and represents a potential protein source for feed in aquaculture due to its low market value, immense availability, and an initial protein content of 20-30% dry matter. 

However, inclusion of unrefined brewer’s spent grain in aquafeeds is hindered by its high levels of lignin, cellulose and hemicellulose which impair the protein digestion in fish. Furthermore, the protein content of unrefined spent grain is too low to meet the protein requirement of most fish species. 

Freja Manø Busk Karlsen has evaluated different refinement strategies for brewer’s spent grain that may allow for its incorporation in aquafeeds. She has used different biotechnological and chemical methods, including protein extraction and microbial fermentation, to remove or modify lignin, cellulose and hemicellulose and increase the protein content of the spent grain. 

In her thesis, Freja Manø Busk Karlsen concludes that her experiments show that it is possible to develop promising methods for processing brewer’s spent grain into a product that can be used in aquafeeds. Next step would be to optimize and upscale the methods to make them applicable at an industrial scale.  

About the defence

Freja Manø Busk Karlsen, DTU Aqua, will defend her PhD thesis "Evaluation of chemical and biological refinement strategies for transforming brewer’s spent grain into a high-quality protein source for fish feed” on Tuesday 20 December 2022, 13:00 on Zoom and at DTU Aqua in Hirtshals (find link and adress below).


  • Principal supervisor: Associate Professor Peter Vilhelm Skov, DTU Aqua
  • Co-supervisor: Senior Researcher Ivar Lund, DTU Aqua


  • Senior Researcher Anne Johanne Tang Dalsgaard, DTU Aqua (chair)
  • Senior Research Scientist Elisabete Coelho, University of Aveiro, Portugal
  • Senior Research Scientist Florbela Carvalheiro, National Laboratory of Energy and Geology, Portugal

Chairperson at defence

  • Researcher Carlos Letelier-Gordo, DTU Aqua
Learn more

A copy of the thesis is available for reading at DTU Aqua. Please contact PhD Secretary Karin Stubgaard,

How på participate in the defence


Everybody is welcome to attend Freja Manø Busk Karlsen's PhD defence at DTU Aqua, North Sea Science Park, Willemoesvej 2, 9850 Hirtshals.


The defence can be followed online on Zoom using this link:

Please, enter the meeting 10 minutes prior to the defence proceedings are scheduled to start. All participants are muted per default, but we ask you to double check that your microphone is turned off at all times. There will be instructions regarding the proceedings in the beginning of the defence.


Tuesday 20 December 2022, 13:00-16:00.