Microwave-Assisted Extraction of Lupin Proteins: A Sustainable Approach Using Food-Grade Solvents

In recent years, the search for alternative protein sources and sustainable extraction processes has gained attention. Lupin proteins stand out due to their high protein content (29.5–55%) and versatile functionalities. Conventional extraction methods often rely on hexane for lipid removal and chemical reagents for protein extraction and precipitation (NaOH, HCl), raising safety concerns and requiring additional purification steps. This study investigated the use of food-grade solvents for producing lupin protein isolates. Lipid removal was performed using supercritical CO₂, which extracted 75% of the lipids (mainly unsaturated fatty acids), followed by protein extraction in an alkaline medium (pH 9) using Na₂CO₃, enhanced by microwave-assisted extraction (MAE). Protein precipitation was performed at pH 4.5 using citric acid. The MAE process was conducted at three power levels (50, 150 and 250 W) and five extraction times (1, 3, 4, 6 and 8 min). Protein isolates were analyzed for protein content, amino acid profile, structural integrity (SDS-PAGE, FTIR), and functional properties (foaming and emulsifying). The obtained protein isolates contained 86–93% protein, with MAE improving protein yield by up to 9% compared to conventional method, while maintaining the amino acid profile. FTIR revealed structural modifications at higher power and longer times (250 W, 8 min), and SDS-PAGE showed reduced band intensity under these conditions. Solubility increased by 140% with lower power and extended extraction times. MAE also significantly enhanced emulsion stability over six days, reducing droplet size (d₃₂) by 118% (50 W, 1 min) and improving foam formation and stability compared to non-microwaved samples. These findings demonstrate the potential of combining MAE with food-grade solvents as a sustainable and efficient strategy for producing lupin protein isolates.