Abstract: Ultrasonic frequency is one of the major ultrasonic parameters that affects protein enzymatic hydrolysis. In this study, we therefore investigated the effect of ultrasonic pretreatment with different working modes (single-, dual- and triplefrequency ultrasonic) on the degree of papain-catalyzed hydrolysis of lotus seed protein and the angiotensin converting enzyme (ACE) inhibitory activity of the resulting hydolsates. Twenty, 20/35 and 20/35/50 kHz were found to be the optimal conditions for single-, dual- and triple-frequency ultrasonic treatment, respectively. Moreover, the effect of ultrasonic pretreatment with different working modes on the surface hydrophobicity and amino acid composition of the enzymatic hydrolysates and we also characterized their structure by intrinsic fluorescence spectroscopy. The results showed that the surface hydrophobicity and the number of hydrophobic amino acids with the increase of ultrasonic frequency, primarily because ultrasonic treatment resulted in protein unfolding and consequently exposure of hydrophobic groups buried within the protein molecule. Furthermore, ultrasonic treatment increased the fluorescence intensity of lotus seed protein and the highest fluorescence intensity was obtained with triple-frequency mode followed by dual- and single-frequency modes. The maximum absorption wavelength of the enzymatic hydrolysates showed a slight red shift. The above findings are of great significance for studying the enzymatic hydrolysis of lotus seed protein to prepare ACE inhibitory peptides.

Key words: lotus seed protein, ultrasound pretreatment with different working modes, angiotensin converting enzyme inhibitory rate, fluorescence spectroscopy, structure characteristics