Optimization of extraction of stinging nettle leaf phenolic compounds using response surface methodology

A full three level factorial design was implemented for optimization of extraction parameters in order to maximize total phenolic (TP) yield from stinging nettle leaf. Factors considered were percentage of methanol in solvent (X1: 50-100% methanol) and extraction time (X2: 30-90 min), and maceration was used as extraction method. A second-order polynomial model was applied for fitting experimental data and predicting the response, and this mathematical model produced a satisfactory fit (R2 = 0.993, p ˂ 0.01). The optimal extraction conditions were 54% aqueous methanol and 38 min extraction time, while maximal theoretical TP yield was 8.9 mg GAE/g DW. Solvent composition significantly affected extraction efficiency causing decrease of TP yield with increase of methanol percentage. On the other hand, extraction time did not influence significantly efficiency of extraction. Using LC/MS and HPLC analysis we detected and quantified three most abundant phenolic compounds: 2-O-caffeoyl malic acid, chlorogenic acid, and rutin. Comparison between maceration and ultrasound-assisted extraction (UAE) obtained extracts based on TP content as well as antiradical activity and HPLC results, showed that UAE have a better extraction capability affecting yield and time of extraction. Of all tested extracts, 54% aqueous methanolic extracts obtained with UAE and 38 min extraction time had the highest TP content.