Curative activity against citrus postharvest green mold of composite hydroxypropyl methylcellulose-beeswax edible coatings with zeolites containing Ag-nanoparticles

Two types of commercial Na-zeolites, linde type A (LTA) and faujasite (FAU) were prepared with different percentage of silver that was incorporated by ion exchange using solutions of AgNO3. The physico-chemical properties of the samples were established using different techniques such as atomic emission spectroscopy with inductively coupled plasma (ICP-OES), X-ray diffraction (XRD), electronic microscopy (SEM and EDX), and temperature-programmed reduction (TPR). The curative activity of these materials was first evaluated as water dispersions in in vivo primary screenings by placing a drop (30 µL) in a rind wound of 'Valencia' oranges that had been inoculated 24 h before with a conidial suspension of Penicillium digitatum. Selected Ag-zeolites were incorporated as antifungal ingredients at different concentrations into composite hydroxypropyl methylcellulose (HPMC)-beeswax (BW) edible emulsions prepared with 8% total solid content with stearic acid as emulsifier and glycerol as plasticizer. Depending on the experiment, these coatings were applied 24 h after fungal inoculation to 'Valencia' oranges as drops in rind wounds or covering the entire fruit. Controls included inoculated but untreated fruit and inoculated fruit treated with HPMC-BW without Ag-zeolites. After incubation of treated fruit at 20°C for 7 days, coatings amended with Ag-zeolites reduced the incidence and severity of green mold by more than 80%, showing high potential for effective disease control. Curative activity depended on Ag concentration and FAU zeolites were more effective than LTA zeolites, probably because of their larger pore apertures and higher Si/Al ratio that favored silver diffusion. However, coatings with the highest silver content were phytotoxic causing dark blemishes on the fruit rind. Further work is needed to optimize non-phytotoxic coatings with the highest curative activity