Role of Disulfide Bonds on the Microstructure and Mechanical Properties of Acid-Induced Casein Gels

In this study, acid gels prepared from micellar casein powder with identical heat treatment and different levels of N-ethylmaleimide blocking of free thiol groups were compared with an untreated reference sample. Heat treatment promotes the formation of disulfide bridges between whey proteins and casein, resulting in covalent networks. Microscopic examination showed that all samples formed spherical aggregates, alt-hough the aggregate size was smaller in the thiol-blocked gels. These resulted in dense, finely dispersed networks stabilized by physical interactions. Under shear, the gels with N-ethylmaleimide showed the highest G' and G'' values, which increased with in-creasing thiol-blocker concentration. In compression tests, the stress increased in all samples in two linear phases, the first increase being reversible-elastic and the second becoming unstable after a maximum. The penetration depth of the elastic region de-creased with increasing blocker concentration. Additionally, both the slope of the elastic region (parabolic trend) and the sharpness of the transition to plastic deformation (point-symmetric trend) showed characteristic changes. These patterns reflect the shift from covalently to physically dominated network structures. The findings demonstrate that targeted blocking of thiol groups enables systematic modulation of gel micro-structure and rheological behavior under shear and compressive load.