Ultrasound-assisted modification to improve the red pepper seed protein isolate structural, functional, and...

The study aims to extract protein from red pepper seeds and analyze the effect of ultrasound (US) on red pepper seed protein isolates (RPSPI) at pH 7 and 9. Circular dichroism (CD) spectroscopy analyzed structural changes in RPSPI, revealing that the increase in US treatment duration from 5 to 20 min was associated with decreased β-turn and α-helix while improving the β-sheet and random coil content. It demonstrates that US treatment stimulated protein unfolding and significantly disrupted distinct types of H bonds in the RPSPI, especially those involved in forming β-turns and α-helix. US treatment significantly improved the free sulfhydryl groups, revealing modified tertiary protein structures. The dissociation of large RPSPI aggregates into smaller ones significantly reduced the average particle size in aqueous suspensions. Ultimately, these changes enabled US-treated RPSPI samples to exhibit increased water solubility, emulsifying activity index, emulsifying stability index, water holding capacity, oil holding capacity, foaming characteristic, and foaming stability at pH 7 and 9, with maximum improvement of 25 %, 49.4 %, 45.11 %, 65.1 %, 46.2 %, 71.1 %, and 81.8 %, respectively. US treatments significantly increased the DPPH, ABTS, and OH scavenging ability (%) of RPSPI at pH 9, with a maximum improvement of 58 %, 33 %, and 36 %, respectively. Moreover, the principal component analysis demonstrated that the structural changes correlated with its functional and antioxidant properties.