Effect of Ultrasonic-Assisted Enzymatic Hydrolysis on Functional Properties and Antioxidant Activity of Eri Silkworm Pupa Protein Isolate

Abstract

Philosamia ricini (Eri silkworm) pupa protein isolate (EPI) was utilized to prepare pupa protein hydrolysate (EPIH) through enzymatic hydrolysis. Additionally, the isolate underwent ultrasonic treatment at 20 kHz to become ultrasound pretreated EPI (EPIU), which was then enzymatically hydrolyzed to obtain ultrasound pretreated protein hydrolysate (EPIUH). The physicochemical properties of these samples were investigated, including molecular weight, solubility, foaming and emulsion properties, water- and oil-holding capacity, antioxidant activity, and color. When compared to EPI (used as the control), EPIU exhibited a high degree of hydrolysis at 20 minutes ( $\text{DH}=29.24\%$ ). At a total process time of 20 minutes, the degree of hydrolysis for EPIH, EPIU, and EPIUH was found to be 13%, 29%, and 41%, respectively. SDS-PAGE analysis indicated no difference in molecular weight between EPI and EPIU (11–75 kDa). However, the molecular weight profiles of EPIH and EPIUH were reduced (8–45 kDa), resulting in changes in protein functionalities. The high DH value contributed to the enhancement of antioxidant activity, solubility, emulsion capacity, emulsion stability, and foam capacity of the protein isolate at pH 7. Furthermore, the ultrasonic pretreatment of the protein hydrolysate increased the lightness of the protein powder by reducing the enzyme activity of the polyphenol oxidase (PPO). These results suggest that ultrasonic pretreatment of the protein hydrolysate could be applied to improve the properties of Eri silkworm pupa protein for use in the food and beverage industry, such as protein-rich beverages or salad dressings.