Nanostructural changes in Polysaccharide-Casein Gel-Like structures upon in vitro gastrointestinal digestion

This work reports on the nanostructural changes taking place during the in vitro gastrointestinal digestion of polysaccharide-casein gel-like structures through the use of small angle X-ray scattering (SAXS). The results indicated that during the gastric phase, the hydrolysis of casein led to a swelling of the micellar structure, yielding peptide clusters. The presence of sulphated polysaccharides such as agar and kappa-carrageenan was seen to limit the hydrolysis of casein during the gastric phase, hence decreasing the size of the formed clusters. After the intestinal phase, the produced peptidic fragments appeared to interact with the bile salts present in the digestion medium, yielding a mixture of bile salt lamellae/micelles and vesicular structures. However, in the presence of polysaccharides, which can interact with bile salts, the formation of vesicular structures was limited. Interestingly, the inclusion of casein within hybrid gel-like structures led to the formation of strong polysaccharideprotein interactions, especially in the case of kappa-carrageenan. As a result, in some of the formulations, polysaccharide-peptide complexes were released towards the liquid medium, which formed larger vesicular structures. This was related to the greater protective effect of these particular gel-like structures. Furthermore, kappa-carrageenan hindered the formation of bile salt lamellae/micelles. These results are of high relevance to understand the intestinal transport mechanism of the digestion products from protein-based ingredients and will allow a rational design of novel products with optimum nutritional and functional properties.