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The effects of extraction conditions on ultrasonic-assisted extraction (UAE) of polyphenolic compounds from okra leaves (OKL) were investigated. Dried OKL powder was extracted using different conditions, including ethanol concentration (20-100%), sample-to-solvent ratio (1:10-1:40 g/mL), UAE temperature (10-70°C) and time (15-75 min); and their effects on total phenolic (TPC) and flavonoid (TFC) contents, as well as the 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical scavenging activities, were examined via a single-factor design. The principal component analysis (PCA) was useful in indicating the optimal extraction parameters and aided a clearer understanding of correlative relationships between the extraction variables. The results showed all extraction conditions to have significant effects on OKL polyphenolics yield and antioxidant activities. The extraction conditions of 60% ethanol, 1:40 g/mL solid-to-solvent, at 70°C for 30 min, favored OKL polyphenolics recovery and antioxidant stability. At this optimal condition, the values for OKL extract yield, TPC, TFC, DPPH, and ABTS were 31.50%, 13.21 mg GAE/g, 29.57 mg RE/g, 16.86 mg AAE/g and 31.90 mg AAE/g, respectively. PCA result substantiated the results obtained for optimal extraction conditions and informed a close correlation between OKL polyphenolics and antioxidant activity. Moreover, chlorogenic acid, p-coumaric, and catechin were identified as the major polyphenols in OKL. Conclusively, optimal UAE conditions for OKL polyphenols were obtained, and this could provide a basis for the utilization of OKL as a potential functional material.

Effect of extraction conditions on ultrasonic-assisted extraction of polyphenolic compounds from okra (Abelmoschus esculentus L.) leaves