In vitro protein digestibility assessment in edible seaweeds: Methodological approach and species comparison

Seaweeds are promising sustainable alternative protein sources, however, their complex cell wall structures may limit protein bioaccessibility. This study evaluated the in vitro protein digestibility of three seaweeds, Ulva lacinulata (green), Porphyra dioica (red), and Saccharina latissima (brown), applying the INFOGEST protocol specifically adapted for seaweed matrices. The influence of preservation methods (freezing vs freeze-drying) was also assessed. Compositional and multi-scale structural characterization of the seaweeds before and after digestion was performed to understand their digestive behaviour. Protein digestibility differed significantly among species, with Porphyra dioica showing the highest values (45-60%), followed by Saccharina latissima (20-35%) and Ulva lacinulata (10-30%). These differences were linked to variations in polysaccharide composition and cell wall architecture, which influenced enzyme accessibility. Freeze-drying generally reduced digestibility, likely due to processing-induced structural changes that may reduce protein accessibility. Additionally, digestibility values were strongly affected by the formulation of the protein-free blanks, highlighting the importance of matrix-specific controls in the in vitro protocol. Overall, this work provides new insights into the relationship between seaweed composition and structure, and protein bioaccessibility, contributing to a more accurate evaluation of seaweeds as alternative protein sources and supporting the development of tailored processing strategies to enhance their nutritional value.