Influence of sonication pre-treatment on physicochemical, structural, and antioxidant characteristics of bighead carp (Hypophthalmichthys nobilis) protein hydrolysate obtained using Protamex

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This study investigates the effect of pre-treatment with ultrasonication (US) on the degree of hydrolysis (HD), structural characteristics, thermal stability, and antioxidant properties in bighead carp protein hydrolysate produced using Protamex. The bighead carp meat was pretreated with US for 0, 7.5, and 15 min and then hydrolyzed with Protamex. Compared to the control (0 min US; HD of 22.4%, hydrolysate yield of 17.3%, total amino acids of 79.54 g/100g, and IC 50 of 3.0 mg/mL), the optimized US pretreatment (7.5 min) led to an increase in the HD to 30.0%, the hydrolysate yield to 26.0%, and total amino acids to 85.12 g/100g. The 7.5-min pretreatment led to an improvement in ABTS radical scavenging activity to an IC 50 of 2.5 mg/mL while displaying solubilities up to 94% (pH 8.0). On the other hand, the ultrasound pre-treatments resulted in noticeable differences in molecular weight distribution when compared to the control (which showed a low molecular weight peptide fraction below 1000 Da of 94.18%), where the proportion of low molecular weight peptides (<1,000 Da) increased up to approximately 95% following the 15-minute US pre-treatment. Circular dichroism spectra showed a change in protein secondary structure in the sonicated samples, reflecting an increase in α-helix and unordered fractions due to acoustic cavitation and peptide unfolding, which is fully consistent with the ATR-FTIR and DSC results. The results indicated that fractions derived from sonicated hydrolysates of bighead carp could serve as effective natural antioxidants, presenting a promising option to synthetic additives in both pharmaceutical and food industries.