Microwave-assisted extraction (MAE) was investigated for recovering of total phenolic compounds from dried waste grape skins using a domestic microwave oven. Influence of vessel geometry, irradiation cycles, irradiation power and time was investigated. The results in terms of phenolics yield, antioxidant capacity and energy consumption were compared with a reference solid–liquid extraction (SLE) carried out for 2 h at 60°C. Equivalent yield of total phenolics as in SLE was achieved with a MAE extraction time of 1,033 sec (corresponding to 83 sec of irradiation at 900 W, 83% saving in extraction time compared to SLE and with a 70% energetic efficiency). Pre-maceration of samples and solvent pre-heating are proposed for large-scale industrial processes to enhance phenolics extraction and process efficiency.

Exploitation of grape waste material is scarce. One of the main issues to deal with is its high moisture content, as it causes spoilage and degradation of valuable compounds. In order to assess this limitation, four different Vitis vinifera waste grape skins from the juice industry were dehydrated at 60, 90 and 100°C. Characterisation of dehydrated waste grape skins (DWGS) was firstly done in wine model solution to evaluate the release of colour, phenolic and volatile compounds. Colour of DWGS-60°C solutions was similar to the control one (freeze-drying) in terms of red and yellow components regardless of the variety. Samples of Garnacha Tintorera, Bobal and AMIX dehydrated at 60°C showed the highest content of low molecular weight phenolic compounds. Bobal DWGS-60°C released the highest concentration of volatiles. An assay by direct addition of Bobal DWGS-60°C into a white wine demonstrated the potential of these residues for producing rosé wines.