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.

Development of ionic liquid plasticized high‐tensile starch‐protein‐sorghum bran composite films with antimicrobial activity

“…SBE‐SP‐IL showed antimicrobial activity (21 ± 0.15 mm) against E. coli via disk diffusion method however, the same was not observed in the case of S. aureus (Figure 4). Although SBE in films exhibited decreased antimicrobial capacity compared to its native counterpart, this observation is in tandem with our earlier report, wherein SBE was found to have greater sensitivity against E. coli (22.3 mm) than S. aureus (15.6 mm) 39 . This may be attributed to the lowered release of SBE from the film matrix when compared to the extract itself along with varying sensitivity in between the two gram‐negative microbial strains, as well as their interaction with the bioactive compounds in the extract 39 .…”

“…The antioxidant activity of the film samples (2 cm 2 ) was measured by two different assays, 39 namely: DPPH scavenging activity and ABTS radical cation decolorization assay.…”

“…Sorghum bran extract (SBE) was prepared in acidified methanol solvent 39 wherein supernatants were collected following overnight incubation at À20 C. Thereafter, the collected supernatants were stored at À20 C for further analysis. Qualitative analysis (LC/MS/MS) of sorghum bran extracts identified diferulic acid, coumaric acid, and 6-C-pentosyl-8-C-hexosyl apigenin as a few of the major compounds.…”

“…were found to be the most common alternatives to phthalates as plasticizers for biopolymers 9–12 . Ionic liquids (ILs), yet another alternative, are gradually gaining momentum in the scientific world due to their plasticization effects and reduced leaching properties 9,10,13–16 . Thermoplastic starch (TPS) has been plasticized with 1‐butyl‐3‐methyl imidazolium chloride ([bmim]Cl) wherein the IL plasticized TPS had higher elongation at break and less hygroscopic than TPS plasticized with glycerol 17 .…”

“…As far as their incorporation in films is concerned, sorghum brans have been used as a reinforcing material for low‐density polyethylene films that also provided enhanced thermal stability 38 . Recently, antioxidant and antimicrobial activity of sorghum bran has been reported which was observed to be the result of specificity towards various solvents 39 …”