Comparative structural and functional characterization of Chlamydomonas reinhardtii proteins extracted by alkaline and ultrasound-assisted enzymatic methods

Microalgae are anticipated to be one of the most promising sources of new foods in addressing both current and long-term food security challenges. This study focuses on Chlamydomonas reinhardtii as the subject of research. We compared the effects of alkaline extraction (AE) and ultrasound-assisted enzymatic extraction (UAEE) on the extraction of C. reinhardtii proteins (CRP), examining their structural and functional properties. AE achieved a protein extraction rate of 41.24 % ± 0.18 % with a purity of 71.18 % ± 0.36 %, while UAEE yielded a slightly higher extraction rate of 48.92 % ± 0.3 % but with lower purity at 28.57 % ± 0.81 %. AE-CRP displayed a broader variety of protein subunits and a complete amino acid profile, with an amino acid content of 578.78 ± 0.10 mg/g. Both methods produced CRP with low bulk density and a porous, lamellar-like structure. However, AE-CRP outperformed UAEE-CRP, as well as commercially available whey proteins (WP) and soy protein isolates (SPI), in solubility, water absorption, oil absorption, foaming ability, and emulsification. AE-CRP's superiority stems from its predominantly disordered and flexible structure, a higher proportion of β-turns in its secondary structure, and greater fluorescence intensity, which better preserves the natural configuration of CRP. While UAEE enhances mass transfer and extraction efficiency, it disrupts protein cross-linking, resulting in increased rigidity and exposed hydrophobic sites in UAEE-CRP. Consequently, AE-CRP outperforms UAEE-CRP in various functional properties, highlighting its potential as a novel commercial alternative protein source.