Response surface parallel optimization of extraction of total phenolics from separate white and red grape skin mixtures with microwave-assisted and conventional thermal methods

Microwave-assisted extraction (MAE) was compared with conventional thermal extraction (CTE) to find optimal conditions to maximize total phenolic (TP) content and colour CIELAB C*, separately from white and red skin mixtures of six common grape cultivars each. Five factors were modelled in preliminary and advanced screenings and response surface (RS) optimization followed by the model validation. Optimized and practically acceptable levels of the factors were identified. For white skin those levels were: 60% ethanol content, 6.6 mL/g grape skins liquid to solid ratio, 70 °C temperature, pH 1.5, time of extraction 4 min and 60 min for MAE and CTE, respectively. Correspondingly, for red skin mixtures 70% ethanol content, 6.6 mL/g grape skins liquid to solid ratio, 60 °C temperature, pH 1.5, time of extraction 4 min and 60 min for MAE and CTE. The model validation experiment confirmed the reproducibility of extraction at the levels identified. Efficiency of TP extraction was comparable between MAE and CTE for both white and red skins. The colour difference between MAE and CTE was noticeable for white skins only. This research presents a novel comparison of two rival industrial methods in application to phenolics extraction from grape skins, when both methods are optimized to extract maximum TP from the same biological material and with the same solvent.