Weak Organic Acids and Inhibitors of pH Homeostasis Suppress Growth of Penicillium Infesting Litchi Fruits

To prevent the rapid postharvest browning, litchi fruits can be fumigated with SO2, followed by a hydrochloric acid (HCl) dip to restore their red colour. Most fungal decay agents are sensitive to SO2 fumigation or acidification but the treated fruits are highly sensitive to infection by Penicillium commune and closely related species. The present study examined whether the addition of weak organic acids or pH homeostasis inhibitors, alone and in combination, could effectively control fungal growth under the physiological conditions that are relevant to litchi fruits. It was found that the fungus can proliferate in culture at pH > 2.5 and is capable of either alkalizing or acidifying the medium, depending on the initial pH and the extent of growth inhibition. Propionic, sorbic and decanoic acids progressively inhibited fungal growth at decreasing pH, presumably by inducing acid stress. Likewise, the inhibitors of pH homeostasis, omeprazole and erythrosin B, inhibited fungal growth, and their efficacy was significantly increased at lower pH and in combination with weak organic acids. Litchi fruits treated with sorbic acid in the presence of HCl were significantly less prone to decay by Penicillium. This study shows that it is possible to utilize low pH as a lever to prevent fungal development.