Solvent Dependency of Sorghum Bran Phytochemicals Acting as Potential Antioxidants and Antibacterial Agents

Abstract

Research background. Sorghum bran, although considered as an agricultural waste, is an abundant source of various bioactive compounds. These bioactive compounds are specific to extraction with particular solvents and therefore ionic liquid and three different conventional solvents, viz., anhydrous methanol, acidified methanol and water were employed in this work. Experimental approach. To evaluate the phytochemicals in the different sorghum bran extracts, total phenol content, flavonoids, condensed tannins and anthocyanins were determined as per standard protocols. LC/MS/MS analysis of extracts was also performed for their phenolic profiling. The antioxidant activity of the extracts was estimated via three assays: DPPH, ABTS and CUPRAC. The antibacterial activity against two most opportunistic food borne pathogens: Escherichia coli and Staphylococcus aureus were measured by agar well diffusion assay and minimum inhibitory concentration (MIC) was determined by serial dilution method. Results and conclusions. Following similar process of extraction, ionic liquid extract of sorghum bran exhibited highest yield (14.9±0.7 %) which indicated to the various possible interactions like Van der Waals forces, H-bonding, hydrophobic and cation-π which ionic liquid can offer in contrast to other conventional solvents, though total phenol content was observed to be only (7.41±0.73 mg GAE/g sample dry weight (DW)). The hydrophobicity of the ionic liquid also helped in efficient extraction of condensed tannins (63.15±2.13 mg CE /g sample DW) which resulted in significant antioxidant activity of the ionic liquid extract (DPPH: 85.22±1.21 µmol/g sample DW; ABTS: 100.75±0.93 µmol/g sample DW; CUPRAC: 63.24±1.86 µmol/g sample DW). An interesting revelation reported in this work is the inability of DPPH assay to evaluate antioxidant activity in acidic environment. The anhydrous methanolic extract of sorghum bran displayed pH sensitivity making the extract beneficial for certain applications. Qualitative analysis of extracts revealed greater number of phenolic compounds to be present in methanol and distilled water extracts. Moreover, various derivatives of apigenin and luteolin were also observed in all the four extracts. In addition, the acidified methanol extract of the sorghum bran exhibited antimicrobial property at a concentration of 12 mg/mL. A larger inhibition zone was observed against Escherichia coli in comparison to Staphylococcus aureus while the MIC against these two bacteria was found to be 2.2 and 1.1 mg/mL, respectively. Novelty and scientific contribution. This paper presents the first information on the application of ionic liquids as extracting phase for sorghum bran polyphenols and the quantification of such extracts. As evident from the study, each solvent has its own role in extraction of bioactive compounds. This work also proves that sorghum bran imparts antibacterial activity against foodborne pathogens.