Antimicrobial and antioxidant activities of defatted Moringa oleifera seed meal extract obtained by ultrasound-assisted extraction and application as a natural antimicrobial coating for raw chicken sausages

Moringa oleifera is a medicinal plant, and its seed oil is known to have high antioxidant properties. The byproduct obtained after oil extraction is called Moringa oleifera seed meal (MoSM), whose potential bioactivity has not been sufficiently documented. Therefore, this study was undertaken with the aim of identifying Moringa oleifera seed meal extract (ME) as a natural antibacterial and antioxidant alternative. First, the effect of solvent extraction of MoSM with 60% (v/v) ethanol combined with ultrasound (UAE) was determined at the sample-to-solvent ratio of 1: 10, with different time and power combinations of UAE. The lowest minimum inhibitory concentration (MIC) exhibited by ME at 200 W and 20 min (ME_200/20) against both Escherichia coli among the gram-negative bacteria and Bacillus cereus among the gram-positive bacteria was 3.12 mg/g. The leakage of intracellular constituents as well as the damage of the B. cereus cells after being treated with ME were confirmed by measuring the absorbance of the supernatant at 260 and 280 nm and observation of the cells under a scanning electron microscope. Moreover, the highest total phenolic content was also recorded with ME_200/20 at 460 mg gallic acid equivalent/g of extract, and the subsequent significantly higher (p < 0.05) antioxidant activity was determined to be 11.18 and 9.20 mg acetic acid equivalent/g of extract by 2,2-diphenyl-1-picrylhydrazyl and ferric reducing antioxidant power assays, respectively. The high-performance liquid chromatography results showed that gallic acid and vanillic acid were predominantly present in the extract, followed by quercetin, sinapic acid and chlorogenic acid. Interestingly, caffeic acid appeared in the sample treated with UAE only. The extract was finally applied with a chitosan-based coating to evaluate the antimicrobial activity against E. coli and B. cereus on raw chicken sausages in the concentration range of 1-5%. Incorporating 3% extract could inactivate B. cereus and E. coli for a 1.4 log reduction on day 3 and a 1.78 log reduction on day 6 of storage and, further, could significantly suppress the growth of both bacteria until day 9. Thus, this study confirmed the strong antibacterial and antioxidant properties of ME and its potential use in real food systems as a natural food antibacterial agent.

Keywords: Challenge test; Foodborne pathogen inhibition; Oil industry byproduct; Polyphenols; Real food system; Scanning electron microscopy.