Optimizing Extraction and HPLC-ESI-QTOF-MS/MS Analysis of Bound Phenolics From Okra (Abelmoschus esculentus) and...

Okra (Abelmoschus esculentus) is rich in phenolic compounds and shows various bioactivities. Free phenolic compounds in okra were identified, while the knowledge about its bound phenolics remains unclear. Consequently, this study aimed to optimize the alkaline extraction for okra pulp bound phenolics (OPBP) and okra seed bound phenolics (OSBP). The maximum total phenolic content (TPC) of OPBP was 7.77 mg GAE/g DW under the optimal conditions of liquid-solid ratio 60:1, 60 min, and 60°C. The maximum TPC of OSBP was 83.03 mg GAE/g DW under the optimal conditions of liquid-solid ratio 80:1, 40 min, and 70°C. Then 14 phenolic compounds in OPBP and 12 phenolic compounds in OSBP were identified by using HPLC-ESI-QTOF-MS/MS, among which nine other compounds including formononetin were reported in okra for the first time. Furthermore, the ferric reducing antioxidant power (FRAP) values were 4.30 and 5.18 mmol Fe²⁺/g for OPBP and OSBP, respectively. The IC₅₀ values of diammonium 2,2'-azino-bis (3-ethylbenzothiazoline-6-sulfonate) (ABTS) radical scavenging activity were 3.34 for OPBP and 3.69 μg/mL for OSBP. Interestingly, OPBP exhibited stronger antibacterial activity against Escherichia coli and Vibrio parahaemolyticus, while OSBP was more effective in inhibiting the growth of Staphylococcus aureus and Listeria monocytogenes. These results provide valuable insights for assessing the nutritional and health benefits of okra.