Effects of ultrasonic, microwave, and combined ultrasonic‐microwave pretreatments on the enzymatic hydrolysis process and protein hydrolysate properties obtained from Chinese sturgeon ( Acipenser sinensis )

Degree of hydrolysis (DH), yield, amino acid profile, protein solubility, and antioxidant activity of Chinese sturgeon protein hydrolysates, as influenced by thermal pretreatment, ultrasonic (US), microwave (MW), and combined US-microwave pretreatments were investigated. Initially, the samples were subjected to thermal pretreatments in order to measure their effect on DH, which increased at 55°C. The DH recorded 7.63, 5.55, and 6.02% for US, MW, and combined US-MW pretreatment (US + MW), respectively, at the optimal time (8 min). The enzymatic hydrolysis (EN) of pretreated samples increased the DH to 19.41, 14.18, and 16.91% for US + EN, MW + EN, and US + MW + EN, respectively. The US + EN treatment was most effective for obtaining higher DH and yield, which were 19.41% and 18.62%, respectively. The use of US + EN also resulted in an increase in the percentage of molecular weights (≤1,000 Da), amino acid content and protein solubility, which reached 89.24, 80.08, and 98.58%, respectively. While, US + MW + EN pretreatment has achieved the highest antioxidant activities by IC₅₀ of 1,1-diphenyl-2-picrylhydrazyl and 2,2-Azinobis (3-ehtylbenzothiazoli- 6-sulfnic acid), which were 3.01 and 1.85 mg/ml, respectively, in addition to the reducing power assay, which was 0.528 at a protein concentration of 5 mg/ml. Therefore, US and combined US-MW techniques can play a promising role in the production of protein hydrolysates and the improvement of their antioxidant properties. PRACTICAL APPLICATIONS: Nowadays, interest in Chinese sturgeon production has increased as a promising source of protein and antioxidant peptides. The optimal thermal pretreatment can be used to enhance the degree of hydrolysis. The results indicated that the use of ultrasound as a pretreatment enhanced the degree of hydrolysis, which could be useful in the preparation of protein hydrolysate with higher yields. The use of combined US-MW significantly improved the antioxidant properties of the protein hydrolysate. The combined US-MW technique is a novel method for obtaining valuable peptides and protein hydrolysates that can be applied as antioxidant constituents in the food products.