Chlorophyllin and sunlight against Penicillium digitatum: exploring Photodynamic Inactivation as a green postharvest technology in citriculture

Green mold, induced by the fungal phytopathogen Penicillium digitatum, is one of the major causes of postharvest losses in citriculture. To minimize mold infections oranges are treated harshly with fungicides, edible coatings, or physical treatment, leading to evolving resistance, low consumer acceptance, or reduced crop quality, respectively. Photodynamic Inactivation (PDI) might represent an ecofriendly alternative for treatment of P. digitatum spoilage, especially if based on natural photosensitizers. Here, we introduce PDI using three formulations consisting of different concentrations of the natural photosensitizer sodium magnesium chlorophyllin (Chl), Na₂EDTA as cell-wall permeabilizing agent and a surfactant for postharvest treatment of P. digitatum. As experimental model systems (i) mycelial spheres in liquid suspension, (ii) fungal spores or (iii) a newly developed experimental setup using orange peel plugs are employed. Illumination was done by an LED device with a main wavelength of 395 nm (106 J cm^-2). The lowest concentrated photosensitizer formulation (219 µM Chl) effectively photokilled samples of model systems (i) and (ii) with 100% and 62.5% dead samples, respectively. Orange peel plugs of model system (iii) were best disinfected using the mid-concentrated formulation (475 µM Chl, 70% dead samples). Additionally, model systems (ii) and (iii) were treated with the mid-concentrated formulation and illuminated by sunlight. Eradication of P. digitatum liquid spore culture (ii) was enhanced when illuminating with sunlight (300 J cm^-2). Further, a complete disinfection of orange peel plugs (iii, 100% dead samples) was achieved with sunlight (300 J cm^-2). To evaluate the antioxidant scavenging activity post-PDI treatment with LED light (395 nm, 106 J cm^-2) a DPPH assay was performed on model system (iii). The treatment with the mid- and low-concentrated Chl formulations and LED light showed little to no change in DPPH scavenging activity when compared to the not-illuminated controls. Concisely, with this study we demonstrate that PDI using Chl-based photosensitizer formulations has an in vitro antifungal effect against P. digitatum, without altering the antioxidant scavenging activity of the fruit. Different model systems, to mimic the different stages of green mold infection, were effectively treated with Chl and sunlight.