Microwave irradiation enhances the in vitro antifungal activity of citrus by‐product aqueous extracts against Alternaria alternata

Summary The effect of two lemon by‐product aqueous extracts at different concentrations (14, 7, 3.5 and 1 mg mL−1) was tested against the in vitro growth of Alternaria alternata. Prior to extraction, one batch of by‐product was dehydrated by freeze‐drying (untreated by‐product), while the other batch was treated by microwave irradiation in conjunction with freeze‐drying (microwave‐treated by‐product). High‐performance liquid chromatography (HPLC) was employed for the identification of individual phenolic compounds with potent antifungal activities. Both lemon by‐product aqueous extracts inhibited the mycelial growth and suppressed the spore germination of the fungus in a concentration‐dependent manner. In general, the extracts obtained from the microwave‐treated lemon by‐product displayed enhanced antifungal activity than those obtained from the untreated one. Scanning electron microscopy (SEM) revealed that both lemon by‐product extracts affected the hyphal morphology of the fungus. The antifungal activity of the extracts was attributed to their phenolic acid and ascorbic acid contents.