Effects of sweeping frequency ultrasound treatment on enzymatic preparations of ACE-inhibitory peptides from zein

In this study, the effect of sweeping frequency ultrasound (SFU) treatment on the degree of hydrolysis of zein and the angiotensin I-converting enzyme (ACE)-inhibitory activity of its hydrolysates were investigated. The mechanism through which ultrasonic pretreatment releases peptides with ACE-inhibitory activity was also studied by fluorescence, circular dichroism (CD), scanning electron microscopy (SEM), and atomic force microscopy (AFM). Compared to the control, SFU and fixed frequency ultrasound (FFU) increased the degree of zein hydrolysis by approximately 11.5 %. Sweeping frequency ultrasound pretreatment increased ACE-inhibitory activity of zein hydrolysates by 12.3–116.7 % over the control. At 40 ± 2 kHz, SFU-treated zein hydrolysates had 42.9 % ACE-inhibitory activity, representing an increase of 116.7 % over the control. The fluorescence intensity of SFU- and FFU-treated zein was weaker than in untreated zein, indicating that more Phe, Trp, and Tyr residues were exposed outside the polypeptide chains. CD spectra show that SFU treatment resulted in increase in the α-helix content by 3.4 %, and β-sheet, β-turns, and random coils content by 24.4 %. Analyses of microstructure by SEM and AFM revealed that ultrasonic pretreatment ruptured the fine meshwork structure of zein resulting in the appearance of several micro-holes. We conclude that the SFU pretreatment for 40 ± 2 kHz can remarkably raise the degree of zein hydrolysis and ACE-inhibitory activity of the hydrolysates by altering the second structure of zein and rupturing the smooth surface of protein.