Extraction and Quantification of Bioactive Phenolic Compounds in Olive Oil by Acid Hydrolysis Method

Phenolic compounds are responsible for healthy, nutritional, and sensory quality of olive oil. The present work deals with the optimization of a method for performing direct acid hydrolysis and extraction of the bound forms of hydroxytyrosol (Htyr) and tyrosol (Tyr) from olive oil. The proposed method was simpler and less time-consuming and required minimum sample pre-treatment steps, compared to liquid–liquid extraction and solid phase extraction (SPE) methods. The hydrolysis and extraction conditions were optimized in terms of extracting solvent, acid type and concentration, temperature, and mixing time. The maximum phenolic compound (Htyr and Tyr) contents were obtained when the olive oil sample was treated with 2.0 M H 2 SO 4 and water as solvent for 4 h at 75 °C. The method showed satisfactory linearity ( R 2 > 0.99), high precision (%RSD ˂ 3.0%), and high recovery (> 94.0%) for Tyr and Htyr. The limits of quantification (LOQ) were 0.56 and 0.69 mg/L for Tyr and Htyr, respectively. Some commercial olive oil samples from Jordanian olive cultivars such as Nabali Baladi, Nabali Mohasen, K18, and Rumi were analyzed using the developed method. The results obtained varied depending on the type of cultivar, and highest value of total phenolic compounds (421.70 mg/kg) was reported for K18 variety that harvested at early time. Furthermore, phenolic compounds including phenols, secoiridoids, flavones, and lignans were determined using SPE method. The results presented statistically significant differences ( p < 0.05) between SPE and acid hydrolysis methods. Graphical Abstract