Polyphenols in Sugar Beet Leaves: Composition, Variability, and Valorization Opportunities

Sugar beet (root) is primarily used by industry as a raw material for sugar production, and its large-scale cultivation is closely linked to the sugar industry. Currently, sugar beet leaf (SBL) is not processed and is typically left on the field as green fertilizer after mechanical harvesting. This represents an underutilized biomass stream with potential bioactive compounds. The aim of this study was to evaluate the distribution of polyphenol and proteins in the leaf blade and petioles of different sugar beet cultivars harvested at various time points. Total polyphenols were quantified using vitexin as a reference standard, and the phenolic profile of methanolic extracts was characterized using complementary HPLC-DAD and LC-MS methods. The protein content in leaf blades ranged from 19% to 29% (dry weight) and was significantly influenced by cultivar and harvest date. Petioles contained significantly lower protein content, ranging from 4.9% to 9.5% (dry weight). The total polyphenol content (TPC) varied with cultivar and harvest time, ranging from 7.8 to 11.0 mg/g DW in leaf blades and from 0.8 to 2.7 mg/g DW in petioles. Leaf blades also contained substantially higher concentrations of vitexin derivatives (mean 7.4 ± 2.3 mg/g DW) than petioles (1.1 ± 0.6 mg/g DW). The percentage contribution of vitexin derivatives to TPC was high in both tissues (>70%) and decreased with later harvest dates. The results provide a detailed characterization of polyphenolic and protein distribution in blades and petioles of sugar beet leaves and can support further evaluation of their potential use in value-added applications.