Natural Deep Eutectic Solvent Assisted Extraction of Freshwater Algae Spirogyra: Kinetics, Anti-Obesity Activity, and Multivariate Correlation Analysis

Spirogyra is a filamentous freshwater green alga that has been identified as an excellent source of phenolics, with potential applications in food and nutraceuticals. In this study, the effectiveness of l-proline:lactic acid natural deep eutectic solvents (NADES) with 15% and 20% water (w/w) in the extraction of phenolics from Spirogyra, as well as their antioxidant activity and inhibition of digestive enzymes, was studied. The extraction process's kinetics were modeled using a two-site kinetics model with degradation factors, which accounted for fast desorption and diffusion-based extraction. The 20% hydrated NADES system had higher extraction efficiency compared to other systems investigated, with the highest total phenolics (77.37 mg GAE/g), total flavonoids (63.56 mg QE/g), DPPH radical scavenging activity (75.03%), α-amylase inhibition (62.81%), and lipase inhibition (66.91%) at 80 min extraction time. However, compared to the 20% hydrated combination (0.6514–0.7857), the 15% hydrated mixture showed a more consistent kinetic performance with higher R2 values (0.9518–0.9949). Regression analysis and correlation analysis both showed a strong positive association between the bioactive components and the related functions. In conclusion, higher extraction efficiency and bioactivities were made possible by moderate hydration of NADES, whereas more consistent extraction kinetics were made possible by low hydration.