Statistical optimization of ultrasound assisted extraction of free and bound phenolic acids, antioxidant and antibacterial activities and UPLC–MS/MS characterization from two varieties of Eleusine coracana

Phenolic acids are an important class of secondary metabolites present in millets possessing diverse bioactivities. They maximally exist in bound form and their extraction is an important aspect for their nutritional and therapeutic uses. This study compares amount of total phenolics in seeds and microgreens followed by the optimization of ultrasound assisted extraction (UAE) of free and bound phenolics from two varieties (VL-324 and VL-352) of Eleusine coracana (Finger millet) using response surface methodology. A Box–Behnken design for recovery of total phenolic content (TPC), DPPH and ABTS radical scavenging was used. The optimized conditions for extracting free phenolics from VL-324 and VL-352 varieties were 50 min, 45.11% methanol and 10 ml/g solvent to matter (S/M) ratio, and 60 min, 49.86% methanol and 16.69 ml/g S/M, whereas bound phenolics were 60 min, 1.65 M NaOH and 26.38 ml/g S/M, and 97 min, 1 M NaOH and 26.53 ml/g S/M, respectively. Free TPC was comparable in both varieties whereas, antioxidant activity was higher in VL-352 extract. Bound TPC and antioxidant activity were higher in VL-352 variety. UPLC–MS/MS revealed ferulic acid was the most abundant phenolic acid in free and bound forms. Comparing with conventional solvent extraction (CSE) revealed UAE yielded higher TPC and excellent antioxidant activity. Bound phenolics extracts displayed substantial antibacterial activity against opportunistic bacteria (Staphylococcus aureus and Cronobacter sakazakii) with lower MIC (0.125 µg/ml) against Gram-negative bacteria with more complex cell walls. These optimized conditions help in enriching phenolics with higher antioxidant and antibacterial potential. Graphical abstract: [Figure not available: see fulltext.]