Establishment of a sonotrode extraction of antioxidant phenolic compounds from avocado leaf agri-food residues

Abstract

This study aimed to optimize ultrasonic-assisted extraction of phenolic compounds from avocado leaves using a sonotrode. A Box-Behnken design with response surface methodology (RSM) was employed to optimize factors such as amplitude, extraction time, and solvent composition to maximize phenolic content and in-vitro antioxidant activity (DPPH and FRAP assays) in ‘Hass’ avocado leaves. Optimal extraction conditions were 60% amplitude, 10 minutes, and 50% ethanol/water (v/v), which resulted in a 47-52% increase in total phenolic content and antioxidant activities compared to ultrasonic bath extraction. These conditions were applied to both ‘Hass’ and ‘Fuerte’ cultivars, yielding 37 phenolic compounds and 14 proanthocyanidins, determined by HPLC-ESI-TOF-MS and HPLC-FLD, respectively. Compounds such as p-coumaric acid glucoside, octylgallate, 3-feruloylquinic acid, and kaempferol 3-O-(2"-O-hexosyl)-hexoside-7-O-rhamnoside were identified for the first time in avocado leaves by HPLC-MS. The 'Fuerte' avocado leaves exhibited 1.7 times higher sum of phenolic compounds, twice the sum of proanthocyanidins, and 1.3-1.6 times higher antioxidant activities (DPPH and FRAP) compared to 'Hass' leaves. This higher phenolic content in ‘Fuerte’ correlated with its stronger antioxidant power. Additionally, very strong positive correlations were observed between antioxidant activities and both the sum of flavonoids (r > 0.971 for DPPH, r > 0.996 for FRAP) and proanthocyanidins (r > 0.924 for DPPH, r > 0.937 for FRAP). These findings emphasize the key role of flavonoids and proanthocyanidins, particularly quercetin derivatives and proanthocyanidin dimers and trimers, which are most concentrated in avocado leaves and likely drive their strong antioxidant properties, indicating their potential for nutraceutical use.