Ultrasound-assisted extraction Vs Ultrasound-assisted precipitation: Impact on yield, functional properties, and nutritional profile of rice bran proteins

Justification: While ultrasound-assisted extraction (UAE) is extensively studied, protein-fiber coagulation during precipitation often reduces purity and functionality. This study hypothesizes that applying ultrasound during precipitation disrupts these complexes through cavitation, enhancing protein reorganization, functionality, and overall quality.

Objective: The objective of this study is to evaluate and compare the impact of ultrasound-assisted extraction (UAE) and ultrasound-assisted precipitation (UIP) on the yield, functional properties, and nutritional profile of rice bran protein.

Methods: Defatted rice bran protein was extracted using alkaline extraction (AE) and ultrasound-assisted alkaline extraction (UAE) under optimized conditions (pH 9.5, 400W, 5 minutes) using an ultrasound horn. Protein isolates were precipitated at pH 3.5 using isoelectric precipitation (IP) and ultrasound-assisted isoelectric precipitation (UIP) using a sonication bath. The combinations included AE-IP, UAE-IP, AE-UIP, and UAE-UIP, representing different extraction and precipitation methods. Proteins were freeze-dried and evaluated for functional, structural, and digestibility properties using FT-IR, SDS-PAGE, particle size analysis, and PDCAAS.

Result: The UAE-IP method exhibited the highest protein yield of 65.7%, while conventional extraction yielded 38.9%. The UIP method induced substantial structural modifications, increased β-sheets by 96.8%, and decreased the α-helices by 42.4%. UAE-IP showed no significant change in surface hydrophobicity, while UAE-UIP decreased by 34.7% compared to the AE-IP. Consequently, ultrasound-assisted precipitation showed significantly higher solubility at pH 7. The highest solubility was 20.6 mg/mL by AE-UIP at pH 7. UAE-UIP demonstrated superior emulsion stability and foaming properties. Ultrasound application enhanced protein digestibility, with AE-UIP showing the highest digestibility of 96.91%. However, excessive ultrasound could potentially reduce digestibility. The amino acid profile varied with extraction methods, with AE-IP showing the highest levels of essential amino acids. UAE-IP and UAE-UIP maintained PDCAAS, while AE-UIP lowered it to 0.9.

Significance: This research opens up new opportunities for developing ultrasound-assisted precipitation to improve the functionality and nutritional quality of protein.