Optimization of extraction of phenolic content from hazelnut shell using response surface methodology

Response surface methodology was applied to predict the optimum conditions for extraction of phenolic compounds in hazelnut shell. The phenolic content in the shell extract was determined spectrophotometrically according to the Folin-Ciocalteu method and expressed as gallic acid equivalent (mg GAE g−1). Two central composite designs were used to investigate the effects of two independent variables, namely solvent composition (%) and extraction time (min), on phenolic extraction. In a first series of repeated batch extractions, the solvent consisted of different methanol percentages in distilled water at pH 4, while in a second series methanol was substituted by ethanol. The highest phenolic content (6.67mg GAE g−1 of shell) was predicted at the extraction conditions of 55.7% ethanol and 108.7min. These best conditions, obtained and applied to 13 different cultivars, showed values varying from 9.18mg GAE g−1 of shell for Barcelona to 3.00mg GAE g−1 of shell for Tonda di Giffoni.