Investigating the impact of different dietary fibres on the gastrointestinal digestion of food proteins

Polysaccharides such as agar, alginate, carrageenan, and cellulose, are highly relevant to the food industry, since they can be naturally found in alternative food sources such as seaweeds, as well as used as functional ingredients in multiple food products. However, their impact on the gastrointestinal digestion of proteins is not yet fully understood. This study investigated the impact of these dietary fibres on the digestion of two model food proteins: casein and whey protein isolate (WPI). In vitro gastrointestinal digestions were carried out, and the resulting digesta were evaluated in terms of digestibility, rheological behaviour, microstructure, and nanostructural organization. Our results showed that adding dietary fibres, especially at higher concentrations (75%), markedly decreased protein digestibility; however, the action mechanism is different depending on the polysaccharide type. Among the tested polysaccharides, alginate and cellulose produced the lowest digestibilities, which in the case of cellulose was mostly driven by limited accessibility of the enzymes towards the protein substrate, while in the case of alginate was due to a combination of strong protein-polysaccharide interactions and increased medium viscosity. SAXS analyses also revealed that the nanoassembly process of the digestion products was strongly affected by the presence of polysaccharides, with protein-polysaccharide interactions hindering the formation of bile salts lamellar structures. These findings provide fundamental insights into how dietary fibres modulate protein digestion, highlighting their relevance in the rational design of functional food systems with improved nutritional and structural properties. • Dietary fibres significantly modulate protein digestibility in model food systems. • Alginate and cellulose had the strongest effect in reducing casein proteolysis. • Carrageenan and cellulose reduce proteolysis through barrier mechanisms. • Agar and alginate reduce digestibility through protein-polysaccharide interactions. • Dietary fibres affect the nanoassembly process of the digestion products.