Sustainable Protein Extraction from Sugar Beet Leaves: A Comparative Study of Combined Techniques and Structural Properties

Sugar beet (Beta vulgaris L.) is one of the most widely cultivated industrial crops worldwide, generating significant amounts of leaf biomass as a by-product. Rich in protein, these leaves offer potential for sustainable protein recovery. This study investigated protein extraction from sugar beet leaves using various combined techniques and evaluated the properties of the resulting protein concentrates. Among the methods, ultrasound-assisted extraction (U-AIP) yielded the highest protein content (61.75%), indicating its effectiveness in enhancing recovery. The ultrasound–enzyme-assisted method (UE-AIP) resulted in the highest bulk density, while U-AIP had the lowest, reflecting variations in porosity and compactness. Color analysis revealed intense coloration in enzyme-assisted samples (E-AIP), while UE-AIP appeared darkest, demonstrating how extraction techniques influence pigment retention. FTIR analysis confirmed characteristic protein peaks and indicated the presence of cellulose and its derivatives, particularly in enzyme- and ultrasound-assisted extracts, suggesting interactions between protein and non-protein compounds. SEM analysis showed that combined methods led to more porous and fragmented microstructures than alkali extraction/isoelectric precipitation. Overall, ultrasound-assisted extraction significantly enhanced protein yield and modified structural characteristics such as bulk density and porosity. Spectroscopic and microscopic findings supported the presence of compositional interactions, affecting the functionality and structure of the resulting protein isolates.