Coumarins in the Flavedo of Citrus limon Varieties-Ethanol and Natural Deep Eutectic Solvent Extractions With HPLC-DAD Quantification.

Introduction: Peels are an abundant but still underutilized waste product in the Citrus fruit industry. They contain coumarins with antiadipogenic potential that could be promising targets in new valorization strategies for Citrus peels.

Objectives: In this study, these coumarins, that is, citropten, bergamottin, and 5-geranyloxy-7-methoxycoumarin (5G7MC), were investigated in Citrus limon peels of different commercial varieties by HPLC-DAD after extraction with ethanol and choline chloride-based natural deep eutectic solvents (NADES) as alternative extraction agents in green natural product extraction.

Results: Target coumarins were almost exclusively found in the flavedo (exocarp) peel layer and in whole peel samples, but not in the albedo (mesocarp) layer. Flavedo samples from varieties "Verdelli" (ES) and "Lisbon" (ZA) showed particularly high concentrations in ethanolic extracts. When applying NADES extraction, best results for citropten were achieved with malonic acid, 1,2-propanediol, and citric acid as hydrogen bond donors (HBDs), with higher yields than the reference solvent ethanol. For bergamottin and 5G7MC, promising results were obtained for 1,2-propanediol as HBD, though lower yields than with ethanol. The possible chlorination of coumarin epoxides byakangelicol and oxypeucedanin was recognized for acidic NADES in LC-MS analysis.

Conclusion: Overall, C. limon peels proved to be a relevant starting material for valorization through coumarin extraction with ethanol and NADES.