Processing of Onion Skin Extracts with Quercetin-Molecularly Imprinted Adsorbents Working at a Wide Range of Water Content

Abstract

A simple adsorption process allowing a high retention of polyphenols contained in extracts of onion skin with ethanol/water volume ratio going up to 80/20 is described. We show that the straightforward processing of the extracts is possible, even at low water content, by using quercetin-molecularly imprinted (Q-MIP) adsorbents synthesized with 4-vinylpyridine (4VP) as the functional monomer. The favorable interactions between the pyridyl functional groups of 4VP and the polyphenols, as well as the improved binding site accessibility introduced by molecular imprinting, are at the source for the good performance observed with the Q-MIPs. The usefulness of the Q-MIPs in onion skin polyphenols purification, fractionation and concentration is demonstrated with few sorption/desorption steps and considering sonicated, Soxhlet and supercritical CO2 extracts. Polyphenol retention of c.a. 88% is possible with Q-MIPs (7% with non-tailored adsorbents) when directly processing ethanol/water 80/20 extracts. Protocatechuic acid and other very hydrophilic molecules (such as simple sugars) were readily removed from the extracts, leaving fractions containing mostly quercetin and quercetin derivatives. Polyphenol recovery higher than 90% (measured with quercetin) and concentration factors up to 34 times were observed with the Q-MIPs.