Microwave-assisted and conventional phenolic and colour extraction from grape skins of commercial white and red cultivars at veraison and harvest

Phenolics and colourants in grape skin vary among cultivars. Their extraction under efficient and environmentally friendly methods is of high industry importance. Grape berry skins from six white and six red commercial Australian cultivars were collected at veraison and harvest over a single vintage. Phenolics and colourants were extracted from the skins of individual cultivars under microwave-assisted (MAE) and conventional thermal extraction (CTE) methods. The settings for each method, previously identified, were fixed at levels maximising total phenolic (TP) extraction from the mixture of those same white skins at veraison and harvest and, unconnectedly, from the mixture of the red skins at veraison and harvest. For white and red grapes, the current study quantifies the variation in TP and colour (CIELAB chroma C∗), which can be attributed to the joint effects of method of extraction, cultivar and time of berry development. The efficiencies of the extraction methods are compared for each cultivar. For each method, the average TP content achieved across corresponding individual skin collections and the TP extractions from their mixtures are well aligned for red cultivars and with a few exceptions for white cultivars. The results are of interest to researchers in phenolic and colour extraction, oenology and industrial post-winemaking waste management.