Green Ultrasound-Assisted Extraction of Onion Polyphenols Using a Choline Chloride-Urea Deep Eutectic Solvent:...

Area of Science:

• Green Chemistry
• Food Science
• Analytical Chemistry

Background:

• Deep eutectic solvents (DES) are emerging as sustainable extraction solvents.
• Their analytical performance and compatibility with antioxidant assays are not well-characterized.
• Polyphenols are valuable antioxidants found in plants like onions.

Purpose of the Study:

• To compare the efficiency of a choline chloride-urea-water DES with ethanol for polyphenol extraction from onions.
• To evaluate different extraction methods (vortex, ultrasound-assisted extraction [UAE], stirring) under DES conditions.
• To assess the compatibility of DES extracts with common antioxidant assays (FRAP, ABTS).

Main Methods:

• Extraction of polyphenols from onion bulbs and peels using DES and 70% ethanol.
• Optimization of extraction conditions through comparative evaluation of vortex, UAE, and stirring.
• Quantification of total phenolic content (TPC) and identification of specific polyphenols using UHPLC-MS/MS.
• Correlation analysis between TPC and antioxidant activity (FRAP, ABTS assays).

Main Results:

• Solvent composition was the primary factor influencing TPC; UAE-DES significantly enhanced recovery (up to 2.2-fold) and reduced time (<5 min).
• DES showed solvent-dependent selectivity, favoring phenolic acids, while ethanol favored flavonols.
• FRAP assay showed good correlation with TPC, but the ABTS assay was incompatible with DES extracts.
• Storage led to a significant reduction in TPC (45-75%) but maintained cultivar ranking.

Conclusions:

• UAE-DES provides a rapid and efficient method for polyphenol extraction from onions.
• Solvent choice impacts polyphenol profile selectivity.
• Validation of antioxidant assays is crucial when using non-conventional solvents like DES.