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

Abstract

Coatings are generally applied to fruit as microemulsions, but nanoemulsions are still experimental. ‘Nova’ mandarins (Citrus reticulata) were coated with shellac or carnauba (Copernica cerifera) microemulsions or an experimental carnauba nanoemulsion; these were compared with an uncoated control during storage for 7 days at 20 °C. Coatings were also tested on ‘Unique’ tangors (C. reticulata × C. sinensis) stored for 14 days at 10 °C followed by a simulated marketing period of 7 days at 20 °C. Fruit quality evaluations included weight loss, gloss, soluble solids (SS), titratable acidity (TA), pH, SS/TA ratio, internal CO2, O2, fruit juice ethanol, and other aroma volatile content. Sensory visual shine and tangerine (C. reticulata) flavor rank tests after storage were conducted, followed by an off-flavor rating. The carnauba waxes resulted in less weight loss compared with the uncoated control and shellac coating during both experiments. There were no differences in gloss measurements of ‘Nova’ mandarins; however, shellac-coated fruit ranked highest for shine in a sensory test. For ‘Unique’ tangors, initially, shellac showed the highest gloss (shine) measurement; however, at the end of storage, the nanoemulsion exhibited the highest gloss, although it was not different from that of the microemulsion. Similarly, after storage, the nanoemulsion ranked highest for visual shine, although it was not different from that of the microemulsion. There were only minor differences in SS, TA, pH, and SS/TA among treatments. The internal CO2 gas concentration and juice ethanol content generally increased and internal O2 decreased during storage. The highest levels of CO2 and ethanol were found for the shellac treatment, as was the lowest O2, indicating anaerobic respiration. There were only minor differences among the other coating treatments; however, they were only sometimes different from those of the control, which generally had the highest O2, lowest CO2, and lowest ethanol. Shellac and the carnauba microemulsion also altered the volatile profile more than the control and the nanoemulsion did, especially for ‘Unique’ tangors. For ‘Unique’ tangors, the control and 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 gloss, and caused less ethanol production than shellac, with the nanoemulsion exhibiting higher gloss measurements, less modifications of the atmosphere and volatile profile, and, consequently, better flavor compared with the microemulsion.

Nanoemulsions as Edible Coatings: A Potential Strategy for Fresh Fruits and Vegetables Preservation

“…It is appropriate on a laboratory scale to avoid solution contaminations and to minimize waste of experimental coating solutions during screenings. However, a negative aspect consists of the non-homogeneous film thickness formed on the entire fruit surface [18,35].…”

“…Currently, edible coatings are used as a strategy to increase the shelf life and postharvest quality of many fresh fruits and vegetables during storage [17,18]. Edible coatings are defined as thin layers applied on the fruit surface, forming clear films produced from food-grade materials and adding to, or as a substitute for, the waxes naturally present on the fruit surface.…”

“…The coating efficiency depends on the coating thickness formed on the fruit surface, since there is a negative correlation between thickness and coating permeability [32]. Another important point is related to low permeability coatings, based on resins such as shellac, for example, which can restrict gas exchange almost entirely, leading to the accumulation of CO 2 within the fruit, and the production of compounds resulting from the fermentation process that can cause off-flavor, such as acetaldehyde and ethanol, thus affecting fruit quality [18,33].…”

“…Nanotechnology has been successfully used to produce plant-based waxes nanoemulsions, such as carnauba wax [18] and candelilla wax [78] without the addition of morpholine. Wax-based nanoemulsions can have improved barrier properties due to the small size of the droplets, promoting greater homogeneity compared to conventional emulsions, greater transparency, improved physico-chemical properties (optical, mechanical, and barrier) and greater stability in comparison with conventional emulsions [18,54,78].…”

“…Nanotechnology has been successfully used to produce plant-based waxes nanoemulsions, such as carnauba wax [18] and candelilla wax [78] without the addition of morpholine. Wax-based nanoemulsions can have improved barrier properties due to the small size of the droplets, promoting greater homogeneity compared to conventional emulsions, greater transparency, improved physico-chemical properties (optical, mechanical, and barrier) and greater stability in comparison with conventional emulsions [18,54,78]. In addition, these nanoemulsions can be used for the development of nanocomposite coatings, in combina-tion with hydrocolloid components (polysaccharides and proteins) in order to improve the water barrier properties of these compounds and minimize the impact of the incorporation of lipid compounds in the matrix hydrocolloids [29].…”