Investigating the Impact of Ultrasound Pretreatment on the Physicochemical, Structural, and Antioxidant Properties of Lupin Protein Hydrolysates

In our study, the effects of high-power (400 W) and low-frequency (20 kHz) ultrasound pretreatment applied for different durations (5, 10, and 20 min) and proteases (alcalase and flavourzyme) on the physicochemical, structural, and antioxidant properties of lupin protein hydrolysates (LPH) were examined. Changes in structural characteristics of the isolates and the resulting hydrolysates were measured using circular dichroism (CD), Fourier-transform infrared (FTIR), and UV–Vis spectroscopic techniques. Antioxidant activities of the LPH were assessed using 2,2-diphenyl-1picrylhydrazyl (DPPH) and 2,2-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) assays. Physicochemical properties including the yield of hydrolysate, relative crystallinity, and surface charge were also evaluated. FTIR analysis revealed that ultrasound pretreatment altered the secondary structure of the proteins as indicated by the movement of absorption peaks to higher wavelength. Also, CD studies showed a reduction in α-helix and increase in the β-sheet component of LPH samples after ultrasound pre-treatment except hydrolysates prepared using flavourzyme. The particle size, zeta potential, and relative crystallinity of the hydrolysates were significantly affected by ultrasonication time and enzyme type. Also, ultrasonication had a noticeable impact on the antioxidant activity as maximum DPPH activity was recorded after 10 min sonication in hydrolysates prepared using alcalase and flavourzyme. In addition, yield of hydrolysate increased significantly (p < 0.05) after ultrasonication pretreatment. Principal component analysis also produced a two-principal component model that explained 65.33% of total data variation. It may be suggested that application of ultrasound treatment to lupin protein could enhance the various characteristics including physicochemical and antioxidant activities of its hydrolysate.