Structural and functional evolution of whey protein isolate-dextran Maillard conjugates during vacuum thermal...

Herein, a novel dry-heating method was developed for spray-dried substrate to prepare whey protein isolate-dextran (WPI-DX) Maillard conjugates via vacuum thermal reaction. The degree of grafting progressively increased up to 40.07 % after a 4-h thermal reaction at 100 °C. Furthermore, multispectral analyses were conducted to evaluate the evolution of the grafted products during the vacuum thermal reaction. UV-Vis and fluorescence spectroscopy confirmed the continuous progression of the Maillard reaction, while Fourier transform infrared spectroscopy corroborated the covalent conjugation and the structural alterations of the protein. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis results further provide evidence of the formation of high-molecular-weight glycation products. Circular dichroism spectra revealed a decrease in α-helical and β-sheet content, accompanied by an increase in random coil structures, indicating a comparatively relaxed protein conformation. The loosening of tertiary structure was further corroborated by intrinsic-fluorescence attenuation and a λ_max redshift of ~1.2 nm (from 329.6 to 330.8 nm), which was indicative of a more hydrophilic microenvironment. Moreover, scanning electron microscopy analysis revealed a more open microstructure during the vacuum thermal reaction. Functionally, a decline in surface hydrophobicity and an increase in solubility were observed during the vacuum thermal reaction. Interfacial performance markedly improved: the emulsifying activity index rose from 22.99 to 34.45 (+49.84 %) and the emulsifying stability index increased from 18.19 to 33.51 (+84.22 %) relative to the spray-dried control. Overall, these findings demonstrated that the vacuum thermal reaction of spray-dried substrates is an effective approach that provided new insights into the preparation of the Maillard conjugates.