Ultrasound coupled with weak alkali cycling-induced exchange of free sulfhydryl-disulfide bond for remodeling interfacial flexibility of flaxseed protein isolates

The limitation of plant protein adsorbed on the oil/water interface is more rigid than milk protein. Herein, the interfacial rigidity or flexibility of flaxseed protein isolate (FPI) was regulated by ultrasound coupled with weak alkali cycling (pH 7 → 10→7) treatment. The interfacial properties of FPI during the treatment were investigated by interfacial rheology, Lissajous plots, cryo-SEM, and Langmuir-Blodgett films combined AFM tests. Compared with FPI, UFPI-10 (FPI treated by ultrasound coupled with pH 10 cycling) possessed higher emulsification stability (ESI∼308.20 min), increasing by 1.74 times. Three stages of emulsifying property enhancement were obtained. First, FPI aggregates had more disulfide bond (-S-S-). After treated by ultrasound coupled with weak alkali cycling, the protein underwent a deaggregation-reassembly process, resulting in the breakage of -S-S- and the exposure of internal sulfhydryl group (-SH) as well as hydrophobic group on the protein surface. These induced the significant improve of -SH and surface hydrophobicity (H0) as well as the decrease of particle size and -S-S- (P < 0.05). Second, the above altered structural properties endowed protein faster adsorption rate onto the interface. The -SH on the surface of adsorbed proteins could convert to -S-S-, leading to form an interfacial flexible layer with high viscoelastic modulus and mechanical behavior. Third, this tight and solid-like interfacial film affirmed by the cryo-SEM and interfacial dilatational rheological results could protect emulsion droplet against flocculation, coalescence, and disproportionation, thus contributing to good emulsification stability. We conformed that the exchange between -S-S- and -SH played an important role in regulating the interfacial flexibility.