Which casein in sodium caseinate is most resistant to in vitro digestion? Effect of emulsification and enzymatic structuring

We investigated the resistance of individual constituent casein epitopes (αS₁-, αS₂-, β- and κ-CN) in food-grade milk protein sodium caseinate (NaCN) to simulated human gastro-duodenal digestion. The influence of NaCN adsorption to the surface of oil-in-water emulsion droplets and the effect of crosslinking of the protein with enzyme transglutaminase (TG) on the proteolysis were studied by indirect ELISA. TG crosslinking rendered fragments of casein molecules significantly resistant to digestion. However, it depended on the type of casein and whether NaCN was presented in solution or emulsion. The crosslinking was found to considerably hinder the digestion of several amino acid regions in one of the major caseins of NaCN, β-CN. For αS₁- and αS₂-CN, only limited resistance to digestive enzymes was observed after NaCN had been crosslinked in solution but not (or to a limited extent) in emulsion. κ-CN proved to be the least resistant to the enzymatic hydrolysis regardless of the TG treatment. Our work shows for the first time how the digestibility of individual components of important food-grade protein ingredients can differ in a complex, colloidal food system. It also shows an example of how the digestibility can be modulated by chemical and physical structuring.