Evaluation of Polyphenol Profile from Citrus Peel Obtained by Natural Deep Eutectic Solvent/Ultrasound Extraction

Abstract

Citrus fruits are widely consumed worldwide; however, one of their primary uses is juice production, resulting in over 40 million tons of agro-industrial waste. Citrus peel is the main agro-industrial by-product in citrus production. In recent years, secondary metabolites of interest, mainly polyphenols such as hesperidin, have been identified in citrus peels. Currently, green alternatives like natural deep eutectic solvents (NADES) based on choline chloride and glucose (Glu), combined with ultrasound-assisted extraction, are studied to obtain polyphenol-rich extracts with potential health applications. This study aims to evaluate the effect of: (1) molar ratios (MR) of 1:0.5, 1:1 or 1:2 mol/mol of choline chloride (ChCl):glucose (Glu); (2) the percentage of added water (WA: 50, 60 or 70%) to NADES; and (3) different citrus peels of Citrus aurantium (bitter orange), Citrus sinensis (sweet orange), and Citrus limon (lemon) used for extraction, on polyphenol profiles, total polyphenol content (TPC), and antioxidant capacity (Ax) of the extracts. The extracts were analyzed using ultra-performance liquid chromatography (UPLC) and evaluated using the Folin–Ciocalteu method for TPC and DPPH assay for quantifying AC. A factorial experimental design 33 was implemented. The extract obtained with an MR of 1:1 (ChCl:Glu) from Citrus aurantium peel exhibited the highest concentration of hesperidin (2003.37 ± 10.91 mg/100 g dry mass), whereas an MR of 1:2 (ChCl:Glu) exhibited the highest concentration of neohesperidin (1045.94 ± 1.27 mg/100 g dry mass), both using 60% WA. This extract also showed the highest antioxidant capacity, achieving 100% inhibition. On the other hand, the highest concentration of total phenolic content (TPC) (96.23 ± 0.83 mg GAE/100 g dry mass) was obtained using C. aurantium peel with an MR of 1:0.5 (ChCl:Glu) and 60% WA. The extracts also presented high concentrations of rutin and catechin. These findings highlight the potential of revalorizing citrus peels, particularly Citrus aurantium, and their extracts obtained with NADES for possible health applications.