Comparative Study of Novel Methods for Olive Leaf Phenolic Compound Extraction Using NADES as Solvents

Natural deep eutectic solvents (NADES) composed of choline chloride with maltose (CMA), glycerol (CGL), citric (CCA) and lactic acid (CLA) combined with microwave (MAE), ultrasound (UAE), homogenate (HAE) and high hydrostatic pressure (HHPAE)-assisted extraction methods were applied to recover and compare olive leaf phenolic compounds. The resultant extracts were evaluated for their total phenol content (TPC), phenolic profile and antioxidant activity and compared with those of water and ethanol:water 70% v/v extracts. HAE was proven to be the most efficient method for the recovery of olive leaf phenolic compounds. The highest TPC (55.12 +/- 1.08 mg GAE/g d.w.) was found in CCA extracts after HAE at 60 degrees C and 12,000 rpm, and the maximum antioxidant activity (3.32 +/- 0.39 g d.w./g DPPH) was found in CGL extracts after UAE at 60 degrees C for 30 min. The TPCs of ethanol extracts were found to be higher than those of NADES extracts in most cases. The predominant phenolic compounds in the extracts were oleuropein, hydrohytyrosol and rutin.

Automated summary

Natural deep eutectic solvents (NADES) combined with microwave-assisted extraction (MAE), ultrasound-assisted extraction (UAE), homogenate-assisted extraction (HAE), and high hydrostatic pressure-assisted extraction (HHPAE) were used to recover olive leaf phenolic compounds. The extracts were evaluated for their total phenol content (TPC), phenolic profile, and antioxidant activity, and compared with water and ethanol-water 70% extracts. HAE was found to be the most efficient method, and the highest TPC (55.12 +/- 1.08 mg GAE/g d.w.) was obtained from CCA extracts after HAE at 60 degrees C and 12,000 rpm. The highest antioxidant activity (3.32 +/- 0.39 g d.w./g DPPH) was found in CGL extracts after UAE at 60 degrees C for 30 min. Ethanol extracts generally had higher TPCs than NADES extracts. The main phenolic compounds in the extracts were oleuropein, hydrohytyrosol, and rutin.