The cultivation of brown macroalgae, such as Laminaria ssp. and Saccharina latissima, has increased extensively during the last decades; according to a report by Food and Agriculture Organisation of the United Nations1 , the cultivation of brown seaweeds exceeded 16 M tons worldwide as per 2019. At the Swedish West coast, the most cultivated brown alga is S. latissima – Sugar kelp – and it is mainly produced for food purposes. The S. latissima body consists of a blade, a stipe and a holdfast. The holdfast and stipe are stiff and tough and not useful for food in its raw state; instead, they become a side stream in the production. Fermentation of the stipes and holdfasts could be a way of improving their food properties, but the traditionally used and food-safe microorganisms employed for fermentation of soybeans, cereals, and other plant-based substrates are adapted to grow on plant carbohydrates such as starch and cellulose. This could be an obstacle when attempting to use algae as a substrate, since they are mainly constituted of other carbohydrates, such as alginate, laminarin, fucoidan and mannitol. For fungi to grow on algal biomass there should be a need for enzymes that can degrade the algal carbohydrates to release sugar units to be taken up by the fungal cells, but we did not find any reports on known food-safe fungi specialized on algae. Therefore, we wanted to test different pre-treatments, with the aim to make the algal carbohydrates accessible for established and safe fungi, traditionally used for fermentation of plant material, so that they would be able to ferment the algal biomass. Our project aimed at assessing the feasibility of using side streams (stipes and holdfasts) from S. latissima as a substrate for solid-state fermentation and to make initial total protein analyses of the product. The goal was to present a proof-of-concept – a model product – for future studies of e.g. amino acid composition, nutritional value, bioavailability, sensorics and environmental impact. The long-term impact target was to enable sustainable and profitable valorization of a presently unused side stream.