Ultrasound-assisted Extraction of Polyphenols from Red-grape (Vitis vinifera) Residues
Red-grape (Vitis vinifera) residues contain large amounts of polyphenols, especially anthocyanins which act as antioxidants. Red-grape seeds and peel were separated from the pulp and exposed to pre-treatments with ultrasound at 35kHz over 15, 30, 45, 60, 75 y 90min as an alternative to improve the level of polyphenols extraction. Right after the pre-treatment with ultrasound, samples were subjected to an extraction operation by methanol/water (96% v/v) at 60°C over 90min, according to similar conditions previously optimized by Pinelo, M (2004). Both total extractive yield (dry basis) and total polyphenols content were determined for all the extracts obtained from the treated samples, and compared against an extract obtained from a non treated sample. Results showed that at the studied frequency (35kHz), there is a significant effect (α=0.05) of the ultrasound exposure time on both total extractive yield and total polyphenols content. When compared to non treated samples, the exposure to ultrasound at 45min and above showed to significantly (α=0.05) improve the extraction operation by increasing in more than 25% the total extractive yield, except at 75min. Results also showed that ultrasound exposure of the samples at 45, 75 and 90min significantly increased (α=0.05) the total polyphenols content of the extracts in about 65, 50 and 140%, respectively. The effect of ultrasound on the antioxidant capacity of the extracts was also evaluated. Results indicated that ultrasound at 35 kHz decreases the antioxidant capacity of the extracts in about 50%.