Protein extraction from lupin (Lupinus angustifolius L.) using combined ultrasound and microwave techniques: Impact on protein recovery, structure, and functional properties

This study presents the first application of combined ultrasound (US) and microwave (MW) techniques for the efficient extraction of lupin protein isolates (LPI) from Irish-grown narrow-leaved lupin grain, variety 'PRIMADONNA'. This variety was chosen due to its suitability for growth in the Irish temperate climate, which may influence protein extraction characteristics. By employing these emerging techniques, this research demonstrates a potential approach for improving protein recovery rates as well as enhancing the structural and functional properties of LPI. US and MW treatments significantly outperformed conventional extraction (CE), with combined US and MW treatments showing synergistic effects that significantly enhanced extraction yield and protein recovery rates of LPI (P < 0.05) within a shorter processing time (10 min) compared to 1 h CE. SDS-PAGE analyses revealed that US and MW treatment preserved the primary structure of LPI, while Fourier transform infrared spectrometer (FTIR) analyses showed that high-power MW induced significant changes in the secondary structure, transferring protein structures from highly ordered (β-sheet and α-helix) to disordered forms (random coil and β-turn). Lower power combinations (US50 + MW25) effectively improved protein solubility and reduced particle size, whereas higher power combinations (US50 + MW50, US100 + MW50) decreased solubility and increased particle size and viscosity significantly (p < 0.05). These findings underscore the potential of US and MW combinations for efficient lupin protein extraction, providing a new approach to sustainable plant extraction.