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 CO 2 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.

Enrichment of Quercetin from Winemaking Residual Diatomaceous Earth via a Tailor-Made Imprinted Adsorbent

“…For instance, using protic solvents such as ethanol and water would decrease the strength of hydrogen bonding. On the other hand, other less H-bonding-competitive solvents such as acetonitrile would increase the observed imprinting effect (see references [ 23 , 30 , 31 ]). However, in the context of winemaking valorization for food, cosmetics, and pharmaceutical applications, we are avoiding the use of this kind of toxic solvent.…”

“…In fraction six, kaempferol was enriched the most by a factor of 10.6, whereas the enrichment of quercetin, myricetin and their glycosides was the highest in the seventh fraction. This discrepancy of enrichment factors can be attributed to the material affinity to each flavonoid, which is advantageous from a practical point of view if one desires to integrate the MIP in a continuous process of adsorption/desorption as demonstrated in the team’s previous works [ 30 , 31 ].…”

Hybrid cellulose-poly(4-vinylpyridine) adsorbents produced via ATRP and their application to target polyphenols in winemaking, olive oil production and almond processing residues

“…Indeed, the pyridyl group behaves as a proton acceptor and is also a good ligand for multiple metal ions, giving rise to complexation [7]. Due to these specific features, adsorbents based on 4VP also present a strong binding capacity for many kinds of polyphenols, as shown in our previous works [16][17][18]. Comparison of the results here presented with MCC-P(4VP) and MCC-PAN adsorbents also confirm the stronger interaction of the pyridyl group with polyphenols comparatively to the nitrile group (the binding stabilization provided by the conjugated system in the pyridyl moiety plays as important role).…”

“…Besides the MCC-P(4VP) and MCC-PAN synthetized adsorbents, the native MCC particles (20 μm, cotton linters) and different commercial resins often used for polyphenols targeting were included in the testing runs. Moreover, 4VP and AN based adsorbents, synthesized via FRP in the framework of our previous researches [16][17][18], were also used with grape pomace extracts, olive mill wastewater and almond extracts/ blanching wastewater.…”

“…The linear gradient method was used, starting with 100% of solvent A and ending with 100% of solvent B. Previous results of our research with plant extracts analysis through LC-DAD-ESI-MS n [16][17][18] were also considered in the present work for polyphenols identification.…”