Abstract
While ultrasound-assisted protein extraction is well-studied, the application of ultrasound in both the extraction and precipitation stages has not been explored. This study explored the ultrasound (U)-assisted alkaline extraction (AE) at pH 9.5 and ultrasound-assisted isoelectric precipitation (UIP) at pH 3.5 on the rice bran protein yield, functionality, and nutritional quality. The UAE and conventional precipitation (UAE-IP) yielded the highest protein yield of 65.7 %, while conventional extraction yielded 38.9 %. FTIR showed that UAE-IP increased β-sheets by 96.8 % while AE-UIP decreased the α-helices by 42.4 %. Although UAE-IP showed no significant change in surface hydrophobicity, UAE-UIP decreased it by 34.7 % to the control. Consequently, functional properties such as solubility, emulsifying, and foaming capacities were significantly improved by 5 %, 0.05, and 3 % respectively. AE-UIP exhibited the highest solubility at pH 7, while UAE-IP showed better solubility at acidic and alkaline pHs. UAE-UIP demonstrated superior emulsion and foaming properties. Ultrasound application enhanced protein digestibility, with AE-UIP showing the highest digestibility of 96.91 %. However, dual-stage ultrasound reduced protein digestibility and subsequently affected the amino acid score (AAS). The amino acid profile varied with extraction methods, with AE-IP showing the highest levels of essential amino acids. AE-IP and UAE-IP showed no significant difference in PDCAAS (protein digestibility-corrected amino acid score) and AAS, whereas AE-UIP and UAE-UIP reduced them to 0.9 and 0.6, respectively. In summary, while ultrasound-assisted extraction enhances protein yield and certain properties, ultrasound-assisted precipitation shows great promise in further enhancing mass yield and functionality, which underscores the importance of a dual-stage ultrasound.