Biorefinery approach for the development of an integrated extraction and purification process for the recovery of pectin derived compounds and flavonoids from onion peels

This work aims the valorisation of onion peels by recovering high added value compounds (flavonoids and pectin derived compounds (PDC)) in a three stages water-based cascade process, using subcritical water (SubW) hydrolysis and diafiltration using ceramic tubular membranes. After quercetin and derivatives were recovered in a hydroalcoholic extraction process, the onion peels were subjected to a SubW process that yielded three different hydrolysates (severity factors in the range 2.88-3.29). These hydrolysates resulted to be complex multicomponent mixtures with variable shares of partially hydrolysed PDC, degradation products (HMF, furfural, organic acids) and free monosaccharides. The 50 kDa membrane effectively removed more than 98 % of the impurities present in the hydrolysates after a dilution volume of 3.5. In order to mitigate the observed membrane fouling, a pre-treatment step via centrifugation was used to remove colloidal matter besides colour which allowed the use of the 5 kDa membrane to obtain a retentate that contained over 80 % of the initial PDC with a final purity of 82 % after a dilution volume of only 2.0.