Kinetic modelling for polyphenol extraction from onion (Allium cepa) solid wastes using acidified water/ethanol mixture

Abstract

Previous reports highlighted the onion solid wastes as abundant, residual material that might contain a significant load of antioxidant polyphenols. Although there have been studies pertaining to polyphenol recovery from onion wastes, the effect of temperature has not been adequately addressed. In this line, this study was undertaken with the aim of establishing a correlation between the extraction yield in total polyphenols and the extraction temperature, using acidified aqueous ethanol as the solvent system. Extraction of polyphenols from onion solid wastes was found to obey 2nd-order kinetics. On such a basis, the yield in total polyphenols at saturation could be very effectively determined and correlated with temperature using non-linear regression. The results indicated that the extraction yield at saturation is highly correlated with temperature, following a quadratic function. The extract obtained at optimal temperature (40 °C) had a total polyphenol yield of 21.10 mg gallic acid equivalents per gram of dry weight, and it was further analysed by liquid chromatography-mass spectroscopy to characterise its major constituents. The polyphenols detected were quercetin glucosides, as well as quercetin oxidation derivatives, including certain degradation products and dimers. The outcome of this study outlined that temperatures above 40 °C are rather not favourable for polyphenol extraction from onion solid wastes, as suggested by the model established through kinetics. The extract obtained under optimal conditions contained peculiar polyphenolic composition, not encountered in any other food processing residue.