Digestibility and structural parameters of spray-dried casein clusters under simulated gastric conditions

The digestibility of casein clusters prepared from sodium caseinate solution (plain or pH-adjusted (pH = 6.0)) was studied. The prepared solutions were spray-dried at different inlet air temperatures (150 °C and 180 °C), and the properties (i.e. encapsulation efficiency, surface hydrophobicity, and digestibility) of the resultant powders were investigated. The specimens obtained from the pH-adjusted solution had higher encapsulation efficiencies than the specimens obtained from the plain solution. A higher spray-drying temperature resulted in lower encapsulation efficiencies and higher surface hydrophobicities. Simulated gastric digestion tests were carried out to study the digestibility of the obtained casein clusters, which was analyzed in terms of reaction kinetics and structural changes during digestion. The effects of drying temperature and pH on the amount of casein digested were not significant; that is, approximately 30% of casein was digested in 120 min for all specimens. Small-angle and ultra-small-angle X-ray scattering measurements were used to analyze the structure of the obtained clusters and their changes during digestion. The results suggested that all the obtained casein clusters, with an average size of approximately 428 nm, had a rough, fractal-structured surface with many dense primary clusters. These structures changed during digestion; specifically, the cluster size increased both in the overall diameter and on the primary structure scale. The fractal characteristics changed from surface to mass fractals, and simultaneously, the cluster density decreased. The drying temperature affected the cluster size during digestion, and the trends were different in the specimens obtained from the plain and pH-adjusted solutions. These results could be useful in the design of protein-based encapsulation systems with desirable digestibility and bioavailability.