Nano- and Micro- Carnauba Wax Emulsions Versus Shellac Protective Coatings on Postharvest Citrus Quality

Coatings are generally applied to fruits as microemulsions, whereas nanoemulsions are still experimental. 'Nova' mandarins were coated with shellac and carnauba microemulsions, and an experimental carnauba nanoemulsion for comparison with uncoated control during storage at 20 °C for seven days. Coatings were also tested on 'Unique' tangors stored for 14 days at 10 °C followed by a simulated marketing period of seven 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 as well as fruit juice ethanol and other aroma volatile content. Sensory shine and tangerine flavor rank tests after storage were conducted as well as an off-flavor rating. The carnauba waxes resulted in the less weight loss compared to the uncoated control and shellac coating. There were no differences for gloss measurements for 'Nova' mandarins, however, shellac-coated fruit ranked highest for shine in a sensory test. For 'Unique' tangors initially shellac showed the highest gloss, but at the end of storage the nanoemulsion exhibited the highest gloss measurement, although not different from the microemulsion. Similarly, after storage the nanoemulsion ranked highest for shine, although not different from the microemulsion. There were only minor differences for SS, TA, pH and SS/TA among treatments. CO2 and ethanol generally increased and O2 decreased during storage, with the highest levels of CO2 and ethanol found for shellac treatment along with the lowest O2, indicating anaerobic respiration. There were only minor differences among the other coating treatments, although sometimes different from control, which generally had the highest O2, lowest CO2 and lowest ethanol. Shellac and the carnauba microemulsion also altered the volatile profile more than did control or the nanoemulsion, especially for 'Unique' tangors. For 'Unique' tangors, the control and the nanoemulsion ranked highest for tangerine flavor and had the least off-flavor at the end of storage. Among the coatings tested, the carnauba emulsions demonstrated less water loss, imparted more sustainable shine and caused less ethanol production than did shellac. Between the micro- and nanoemulsions, the microemulsion presented a better water barrier, but the nanoemulsion exhibited more shine, less modification of the atmosphere and volatile profile and consequently better flavor.