Optimized ceramic membrane-based method for efficient and acid-free rice protein preparation from alkaline extracts

Rice protein, stands out as a high-quality plant-based protein, suitable for dietary supplementation and food processing. Traditional methods, involving alkaline extraction and acid precipitation, are challenged by the extensive use of chemicals and the difficulty in desalination. This study proposed a novel separation method that utilizes ceramic membranes for direct alkali filtration, eliminating the formation of salts by acid neutralization. The membrane pore size and the operating parameters such as transmembrane pressure, cross-flow velocity, and pH were optimized to enable a high flux (>80 L m⁻²·h⁻¹) and favorable protein rejection (>95 %) while permitting the removal of excess alkali. Additionally, the effects of non-protein substances, e.g., starch, present in the alkaline extract on the separation performance were explored. Subsequently, an optimized strategy for starch removal was proposed, involving the concentration of the alkali extract using an ultrafiltration membrane, followed by enzymatic hydrolysis of starch and diafiltration to eliminate the residuals of starch hydrolysates and alkali. Under stabilization of protein particles by carboxymethyl cellulose, rice protein with a uniform particle size of 25 nm and a purity of over 80 % was produced. The direct removal of alkali from alkali extracts of rice by ceramic membranes presents a promising strategy for producing rice protein with high purity and reduced aggregation degree, enabling better functionalities such as solubility than the conventional alkali-extraction and acid-precipitation method.