Gastric aggregation of micellar casein powders induced by high hydrostatic pressure: Effect of serum Ca²⁺ level

Micellar casein (MC) has a unique gastric colloidal behavior in response to Ca²⁺ cross-linking, and its aggregation properties are closely related to pepsin and gastric acid. In this study, MC with different levels of colloidal calcium phosphate (CCP) was obtained by high hydrostatic pressure (HHP) at different pressures, followed by spray drying to obtain the powders. Different amounts of calcium chloride (exogenous Ca²⁺) were added to MC powders prior to in vitro simulated digestion to investigate the effect of exogenous serum Ca²⁺ levels on the aggregation behavior and the structure change of curds generated in gastric tract. The results revealed that HHP induced the emergence of more Ca²⁺-binding sites, thus Ca²⁺ was more likely to bind to MC matrix with low CCP levels. Meanwhile, high serum Ca²⁺ level provided more opportunities to form aggregates. The Highest pressure (500 MPa) with the highest Ca²⁺ level (5 mM) caused the lowest solubility aggregates, which were only 30% at the end of gastric digestion (120 min), half of the control sample (0 MPa with 0.15 mM Ca²⁺). The results of wide-angle X-ray scattering / small-angle X-ray scattering suggested that both pepsin and gastric acid-induced aggregation via Ca²⁺ as a bridge. For pepsin, Ca²⁺ cross-linked between para-κ-casein; For gastric acid, Ca²⁺ recombined phosphorylation sites and caused cross-linking of casein subunits.