Antimicrobial Activity and Minimum Inhibitory Concentrations of Purple Corn Phytochemical Extracts and the Role of Solvent Residues

Purple corn pericarp, a low-value corn processing byproduct, is an extremely rich source of diverse polyphenolic compounds with total anthocyanins (TA), total phenolics (TP), and condensed tannins (CT) concentrations of 125.8, 306.6, and 632.1 g/kg of pericarp, respectively. No previous studies have evaluated the highly concentrated liquid phytochemical extracts from purple corn pericarp and corresponding freeze-dried powders for their antimicrobial properties and analyzed the effects of any residual solvent. The antimicrobial activities of phytochemical extracts were evaluated using the microwell dilution method against two Gram-positive and two Gram-negative bacteria. Minimum inhibitory concentrations (MIC) were determined using the Clinical and Laboratory Standards Institute (CLSI) method. Importantly, residual ethanol concentrations ranging between 62.5 and 79.1 µL/mL were detected in liquid phytochemical extracts by HPLC with a RID detector despite using a rotavapor step. Following this discovery, the extracts were freeze-dried to remove any residual ethanol prior to the antimicrobial analysis. Concentrated ethanolic extracts exhibited antimicrobial effects with the MIC values of 200 µg/mL against Escherichia coli, Enterococcus faecium, and Listeria monocytogenes, and 300 µg/mL against Salmonella Typhimurium. No ethanol residues were detected in the freeze-dried extracts. UHPLC-MS analysis identified 55 polyphenols, comprising 30 flavonoids, 11 anthocyanins, and 10 phenolic acids. The relative abundance of condensed tannins, epicatechin, and caffeic and ferulic acids may have contributed to the observed antimicrobial effects. Future antimicrobial studies must take into consideration any residual solvents in the prepared extracts.