Seaweed as a sustainable source of food protein: maximizing seaweed protein content, protein recovery, and nutritional quality

Sammanfattning: This thesis focuses on utilizing seaweed, such as Saccharina latissima and Ulva fenestrata , as sustainable food protein sources to complement terrestrial protein currently limited by land and water supply. While seaweed holds promise, its protein content is lower than pulses and antinutrients reduce protein nutritional quality. Additionally, S. latissima often contains excessive iodine, necessitating post-harvest blanching. We aimed to produce protein-rich seaweed using food-process waters as nutrient sources; assess how blanching parameters impact downstream pH-shift-based protein extraction; create an efficient extraction method targeting aqueous-soluble and lipophilic proteins; and evaluate the influence of extraction on protein nutritional quality after in vitro digestion. When food-process waters, mostly herring-derived, were added separately to the cultivation media of tank-cultivated U. fenestrata , protein content increased 2.4-fold compared to seawater media, reaching 24% per dry weight (dw). Growth rates generally remained unaffected and S. latissima was incompatible with this new nutrient loop. Blanching sea-cultivated S. latissima at 45 or 80 °C for 2 minutes was equally effective at reducing iodine. However, biomass blanched at 45 °C provided higher protein extraction yields (23% vs. 14%). Iodine was still the limiting element for the daily adult consumption of extracts from blanched biomasses (0.5 g dw), though higher than extracts from crude biomass (0.1 g). Employing 0.1-0.5% aqueous Triton X-114 during protein extraction from U. fenestrata followed by alkaline extraction provided a 3.3-fold increase in extraction yields (23%) compared to two alkaline extraction cycles. In both protocols, proteins were concentrated via isoelectric precipitation. It was confirmed that Triton disintegrated membranes, targeting also lipophilic proteins. Digestibility of pH-shift extracts from U. fenestrata increased from 28% for crude biomass to 36%. Extraction also raised amino acid accessibility from 57% to 73%. When using the Caco-2 cell model, amino acids from U. fenestrata and extracts thereof were as bioavailable as casein. Altogether, we raised seaweed protein content by recycling nutrients currently lost during food processing, improved protein extraction yields, and proved that extracts have higher digestibility than crude seaweed. Based on theoretical estimations, seaweed can offer a modest contribution to sustainable food systems, though this relies on scaling up seaweed production volumes.