Red pepper seed protein isolate: Pulsed electric field assisted modification to improve its structural and functional properties

This study investigated the effects of pulsed electric field (PEF) treatment (nominal 5, 10, 15, 20, and 25 kV/cm) on protein isolates extracted from red pepper seeds (RPS). Based on oscilloscope measurements, the actual field strengths corresponding to these nominal settings were 0.51, 0.49, 0.74, 1.00, and 1.25 kV/cm, respectively. It showed that PEF15 (0.74 kV/cm) and PEF20 (1 kV/cm) partially unfolded the RPS protein isolate structures by disrupting non-covalent interactions, revealing hydrophobic residues and bound -SH groups to the aqueous solution. PEF treatment significantly increased the free -SH content by 4.52 μmol/g and surface hydrophobicity as compared to the control sample. Also, the PEF treatments significantly dissociated from the large RPS protein isolate aggregates, reducing the average particle size from 608.8 ± 4 nm to 504.1 ± 2.8 nm, thereby increasing solubility up to 39% and reducing turbidity. The redistribution of polar residues and exposure of charged groups increased electrostatic repulsion, increasing the zeta potential up to -55.56 and stabilizing RPS protein isolates more effectively. Structural analyses via Fourier Transform infrared Spectroscopy (FTIR), X-ray Diffraction (XRD), and Scanning Electron Microscopy (SEM) verified these results by indicating secondary structure modifications, shifts in crystallinity, and microstructural reorganization. Ultimately, these molecular-level changes enabled the PEF-treated sample to exhibit improved emulsifying stability index, emulsifying activity index, foaming stability, and foaming characteristics compared to a control sample. Overall, PEF processing makes the RPS protein isolates more suitable for food formulations and other functional applications in the food industry.