Optimization of the extraction of phenolic compounds from purple corn cob (Zea mays L.) by sequential extraction using supercritical carbon dioxide, ethanol and water as solvents

Abstract

Purple corn cob (Zea mays L.) extracts were obtained by sequential extraction in fixed bed, using GRAS solvents (Generally Recognized as Safe) ordered by increasing polarity, as follows: supercritical carbon dioxide (scCO2) (nonpolar) in a first step, ethanol (polarity 5.2) in a second step, and water (polarity 9.0) in a third step at the same conditions of temperature and pressure. CCRD (Central Composite Rotatable Design) was used to study the effect of pressure (259–541bar) and temperature (36–64°C) on the following parameters: overall yield (X0) of each extraction step; yield and content of total monomeric anthocyanins (TMA), total phenolics (TP), and total flavonoids (TF); antioxidant activity (AA) expressed as effective concentration EC50/DPPH responsible for 50% decrease in the initial activity; color by CIELAB L* a* b* system. Quantification of specific anthocyanins was performed by HPLC. The process was optimized using response surface methodology (RSM), and the best responses were: X0 of 15.4% in the 3rd step at 65°C and 440bar; TMA of 64mg cyanidin-3-glucoside per gram extract in the 2nd step at 45°C and 420bar; TF of 93.7mg catechin equivalents per gram extract obtained in the 3rd step at 50°C and 400bar; TP of 389mg gallic acid equivalents per gram extract, and AA (EC50/DPPH) of 21μg/mL in the 2nd step at 65°C and 450bar. The color of the extracts was affected by the type of solvent, and the highest cyanidin 3-glycoside concentration was observed in the 2nd extraction step, with values of 26–38mg per gram extract, determined by HPLC.