Antifungal activity of extracts and essential oil of ginger in nanostructured carnauba wax coatings to postharvest conservation of tangerine and papaya

Objective: The aim of the present study was the evaluation of the antifungal activity of ginger alcoholic extracts and essential oils against common post-harvest phytopathogens, as isolated compounds or in association to carnauba wax nanoemulsion. In coating format these composites are able to preserve quality and slow down natural or induced decay on tangerines and papayas. Formulation based in carnauba wax nanoemulsions and their association with hydroxypropyl methylcellulose (HPMC), as a neutral film forming adjuvant, and ginger essential oil (GEO) incorporation, were applied as protective coatings and evaluated under several conditions of storage. Methodology: Alcoholic extracts (GEs) and essential oils (GEOs) were extracted from ginger rhizomes. Antifungal activities of GEs and GEOs were evaluated in vitro against P. digitatum, P. expansum, F. solani and A. alternata. Two experiments were performed with citrus (Nova’ mandarins). These fruits were coated with shellac and carnauba microemulsions, as well as nanoparticulated carnauba (nanoemulsions) and compared with uncoated control fruits after 7 days storage at 20 °C. Coatings were also tested on ‘Unique’ tangors stored for 14 days at 10 °C followed by a simulated marketing period of 7 days at 20 ºC. Fruit quality evaluation included weight loss, gloss, soluble solids (SS), titratable acidity (TA), pH, SS/TA ratio, internal CO2 and O2, ethanol, and a sensory shine rank test after storage at 20°C for 7 days. For papaya postharvest assays, the formulation was prepared in a concentration of 9% and 18% (w/v) carnauba wax nanoemulsion. HPMC was used as inert coating control and GEO (at 3% v/v) tested as active compounds against fungal infestation. The fruits were stored and evaluated in two separate experiments. The first conducted after 6 days at 22 °C and 9 days at 13 °C followed by 5 days at room temperature to simulate marketing conditions. In the second assay papayas were stored for 5 days at 22 °C, and 10 days at 16 °C before simulated marketing condition of 3 days at 22 °C. Post-harvest analyzes and protective action towards reducing natural diseases severity and inhibit of fungal spread on samples inoculated with C. gloeosporioides, were performed along with sensory evaluations. Results: GEOs showed a more effectiveness than GEs and at minimum inhibitory concentration (MIC) lower for oils than those measured to alcoholic extracts. The GEs did not show minimum fungicidal concentration (MCF) against the tested fungi even at the highest concentration tested (6%) in broth medium. GEOs on in vitro conditions showed a better antifungal activity than GEs, indicating that they are more appropriate for incorporation into edible coatings, aiming phytopathogenic postharvest microorganism’s control. When in vitro assessment, the GEO showed a better antifungal activity than the in vivo conducted tests. The P. digitatum fungus was the most sensible to ginger compounds (with lower MIC), followed by A. alternata, F. solani and P. expansum, in this order. These required higher concentrations to attain mycelial inhibition. For tangerines, conventional and nanoemulsion carnauba wax resulted in inferior weight loss compared to control and shellac. There were no differences for gloss measurements for ‘Nova’ mandarins, however, shellac-coated fruit ranked highest for shine in visual sensory assessment. For ‘Unique’ tangors, initially, the shellac coating promoted the highest gloss (shine), but at the end of storage, the nanoemulsion exhibited the highest gloss measurement, although not different from the microemulsion. CO2 and ethanol generally increased and O2 decreased internally during storage for all treatments. The highest levels of CO2 and ethanol were measured in shellac treated samples along with the lowest O2, indicating internal fermentation, with no differences among the other treatments. Shellac and the carnauba microemulsion also altered the volatile profile more intensely than control or nanoemulsion coating, especially for ‘Unique’ tangors. For papaya, all coatings provided some protection, with variations on the effects due to storage conditions and between formulations compositions. Under cold storage, most of analyzes did not resulted in any statistically significant differences among samples. At room temperature and after simulated market conditions the differences become more evident. The nanoemulsions were able to a better maintaining of papaya quality over storage and market conditions. Higher reductions on loss of firmness, color alterations and respiration rate were recorded, with positive action in delaying maturity. No changes in attributes as sweetness, sourness, papaya flavor or the presence of off-flavors related to internal fermentation were reported in the sensory test. The GEO presented some effect in reducing natural diseases on papaya skin, particularly when associated with carnauba, although no action was observed in inhibiting C. gloeosporioides fungal growth after inoculation. Conclusion: GEOs proved to be the most indicated for use as active agents for application in post-harvest fungal control since, in general, they exhibited higher antifungal activity than the alcoholic extracts. The combination of nanoemulsion and GEO into a coating was more effective in vitro experiment than in vivo. For tangerine assays among the coatings tested, the carnauba emulsions resulted in lesser water loss, imparted more sustainable shine, and caused less ethanol production than did shellac. The carnauba nanoemulsion coatings also exhibited higher shine and fewer modifications of the atmosphere and volatiles profile, and consequently assuring a better flavor compared to the microemulsion. Papayas coated with carnauba wax nanoemulsions had the quality preserved over storage and market conditions. The incorporations of GEO into coatings, promoted a reduction of natural diseases on papaya skin, mainly when combined with carnauba. (AU)