Physicochemical state and in vitro digestibility of heat treated water-soluble protein from Pacific oyster (Crassostrea gigas)

Oyster water-soluble protein (OWSP) was extracted from the Pacific oyster (Crassostrea gigas) and the impact of heat treatment on the physicochemical properties, structural changes and in vitro digestibility (proteolysis using pepsin, trypsin and α-chymotrypsin) of OWSP at different temperatures (35, 45, 55, 65, 75, 85, 95, 100 °C) were studied. Sodium dodecyl sulfate polyacrylamide gel electrophoresis showed that the intensity of low molecular weight bands decreased while the intensity of high molecular weight bands increased with increased temperature, indicating the bonding of proteins. The differential scanning calorimetry curve showed that the denaturation temperature and enthalpy of OWSP were 39.3 °C and 4.42 J/g, respectively. Heating resulted in significantly increased surface hydrophobicity and electronegativity, and decreased protein solubility of OWSP (p < 0.05). However, the dynamic light scattering, confocal microscopy and Nile red fluorescence showed that protein aggregates repelled each other and dispersed more evenly in solution due to the increase of surface hydrophobicity and electronegativity at 65–95 °C. The secondary structure determined using circular dichroism spectrometer showed that the α-helix content decreased and β-sheet content increased with heating. In vitro digestion showed that the proteolysis of OWSP increased with the denaturation of proteins at lower heating temperatures (45, 55 °C), while protein aggregation and the formation of a more compact structure inhibited the digestion of OWSP at higher heating temperatures (65–100 °C).