Antifungal action and inhibitory mechanism of polymethoxylated flavones from Citrus reticulata Blanco peel against Aspergillus niger

Polymethoxylated flavones (PMFs), isolated from the peels of Citrus reticulata Blanco, were identified and quantitated as tangeretin (TAN) (33.87%), nobiletin (NOB) (20.98%), 5-demethylnobiletin (3.52%), tetramethyl-o-scutellarein (1.61%), tetramethyl-o-isoscutellarein (1.23%), pentamethoxyflavone (1.08%) and sinensetin (0.35%). PMFs are promising natural antimicrobial compounds with potential applications in the food industry. The antifungal effects of PMFs on Aspergillus niger (A. niger) were evaluated by microbroth dilution assay and growth curve determination. The minimum inhibition concentration (MIC) of PMFs extract and of TAN against A. niger was determined to be 0.12 mg/mL and 1.5 mg/mL respectively. PMFs affected the permeability of cytomembrane, resulting in instant increased flux of K+ and increased relative electrical conductivity. PMFs also dose-dependently reduced the chitin production. These results suggest that the antifungal effects of PMFs could be explained by the permeability change of cytomembrane and the fragility of cell walls caused by chitin inhibition.