Ultrasonically assisted antioxidant extraction from grape stalks and olive leaves

Grape stalks and olive leaves present high amount of phenolic compounds with antioxidant properties. The extraction of these compounds may be considered a way to increase in value both agro-food by-products. Ultrasound is widely applied in extraction due to its effects (cavitation, microstirring or sponge effect) over the process. The goal of this work was to address the application of ultrasound on the antioxidant extraction of olive leaves and grape stalk. For that purpose, the extraction of antioxidant compounds from grape stalks and olive leaves, previously dried at 100 °C, were carried out using a ethanolic solution (80 % v/v) at 60 °C. Extractions were carried out with (US; 30 kHz; 600W)) and agitation (AG) without ultrasound application. In the AG experiments, the solution was agitated with a stirrer. Samples were obtained at different extraction time (10, 30, 60, 120, 180, 240, 360, 480 and 1440 min) and their antioxidant capacity was measured using FRAP method. The Naik model was used to model the extraction kinetics, being identified the antioxidant capacity of extracts at the equilibrium (Y _eq) and the initial velocity of extraction (Y _eq/B). For grape stalks, the antioxidant capacity of extracts at the equilibrium (Y _eq) and the initial velocity of extraction (Y _eq/B) were higher in AG experiments than in US experiments. In the olive leaves extractions, the Y _eq/B was of the same order for both treatments but Y _eq was significantly higher for US experiments. The different influence of ultrasound for both by-products can be explained from their different geometry and structure.