Effect of Ultrasound Pretreatment on the Structural Characteristics of Sea Cucumber Gonad Protein Hydrolysates

The effects of different periods (0, 5, 15, 25 and 35 min) of ultrasound pretreatment at a power of 200 W on the structural characteristics of sea cucumber gonad protein hydrolysates prepared with neutral protease were investigated by ultraviolet-visible (UV-Vis) spectroscopy, Fourier transform infrared (FTIR) spectroscopy and fluorescence spectroscopy as well as determination of surface hydrophobicity and particle size distribution and microstructure observation. The results showed that the UV absorbance, β-sheet relative content, fluorescence intensity (FI), surface hydrophobicity and particle size of sea cucumber gonad protein hydrolysates initially increased and then decreased with ultrasound pretreatment time, while the opposite trend was observed for α-helix relative content. Ultrasound pretreatment for 15 min increased the UV absorbance, β-sheet relative content, FI and surface hydrophobicity of hydrolysates by 7.30%, 50.72%, 4.50% and 13.42%, respectively, and decreased the α-helix relative content by 9.98% (P < 0.05). Moreover, this treatment resulted in the smallest particle size and distribution range of hydrolysates as observed under scanning electron microscopy (SEM). Therefore, appropriate ultrasound pretreatment could promote the unfolding of sea cucumber gonad protein structure and reduce protein aggregation, and the effect of ultrasound pretreatment for 15 min was most pronounced.