Fruit.

Approximately 200 ‘Nova’ mandarins (C. reticulata) and 300 ‘Unique’ tangors (C. reticulata × C. sinensis) were used in two experiments. ‘Unique’, or ‘Ortanique’, is believed to be a spontaneous hybrid of a sweet orange and a mandarin that was first found in Jamaica (Nugent et al., 1967). Fruit were obtained from Al’s Family Farms Citrus, Fort Pierce, FL. The fruit were selected for uniformity of size (≈70 and 80 mm in diameter for ‘Nova’ and ‘Unique’, respectively) and lack of defects, washed with commercial fruit cleaner (JBT 395; JBT FoodTech, Lakeland, FL), rinsed, and sanitized with 100 mg·L⁻¹ peroxyacetic acid (PAA) (Jet-Oxide; Jet Harvest Solutions, Longwood, FL) for 3 min and then air-dried at room temperature.

Coating preparations.

A conventional carnauba wax microemulsion was formulated according to Hagenmaier and Baker (1997), with a slight modification. Carnauba wax emulsion was prepared in an open reactor by heating 45 g of carnauba wax type I (Strahl & Pitsch, West Babylon, NY), 5 g oleic acid, 5 g myristic acid, 28 g ammonium hydroxide (all Sigma-Aldrich Chemical Co., St. Louis, MO), and deionized water (175 g) at 105 °C with constant mechanical stirring (800 rpm) for 3 m. Then, under mechanical stirring, the emulsion was cooled to 50 °C. The carnauba wax nano-emulsion was prepared with an oil phase composed of carnauba wax type 1 (150 g) and oleic acid (30 g) from Sigma-Aldrich Chemical Co. (St. Louis, MO). The water phase was composed of 20 g ammonium hydroxide, dimethylpolysiloxane (0.1 mL; Sigma-Aldrich Chemical Co.), and deionized water (775 g). Formulation of the nanoemulsion was accomplished by the inversion phase of the water in oil (W/O) to oil in water (O/W) system. The diameter size of the lipid micelles obtained was 44 ± 8 nm, with a narrow polydispersion index (0.28) and zeta potential of −43.8 mV (Miranda, 2015) measured by a zeta potential analyzer (Zetasizer Nano ZS; Malvern Instruments, Westborough, MA). The microemulsion, in comparison, had a larger dispersion of droplet sizes and a higher polydispersion index (PDI = 1.0) than the nanoemulsion (PDI = 0.28).

Fruit processing and coating.

This research was performed during two experiments. The first experiment tested coatings on 189 ‘Nova’ mandarin fruit with the following treatments: laboratory-made, simulated commercial carnauba wax microemulsion; a commercial shellac microemulsion (Stay Fresh 590HS, JBT FoodTech); an experimental carnauba nanoemulsion wax; and uncoated fruit. The commercial shellac microemulsion was included for a true commercial comparison of shellac resin. The second experiment was conducted with the same treatments but applied to 289 ‘Unique’ tangor fruit. Fruit were coated by spreading 1 mL of coating per fruit by hand while wearing latex gloves. Coated fruit were then dried with a heat gun (50 °C) for 60 s before analyses and storage, simulating industry drying techniques.

During the first experiment, ‘Nova’ mandarins were stored for 7 d at 20 °C and quality analyses were performed initially (0 d) and at the end of storage (7 d). During the second experiment, ‘Unique’ fruit were stored at 10 °C for 14 d, followed by a simulated marketing period of 7 d at 20 °C, which is more typical for the industry. Quality analyses were performed initially (0 d), after 14 d of cold storage (10 °C), and again after the simulated marketing period (7 d at 20 °C). The relative humidity (RH) was 80% to 85% for all storage.

Fruit quality.

Fruit were gently hand-juiced using a food service juicer at room temperature to avoid peel oil in the juice (Baldwin et al., 2012). Juice aliquots (3 mL) were immediately pipetted in crimp-capped vials for gas chromatography (GC), frozen in liquid N₂, and stored at −80 °C to optimize the volatile content. In addition, 40 mL of juice was pipetted into plastic tubes and stored at −20 °C for sugar and acid analyses. The SS content of the juice was determined by the refractive index with a digital refractometer (PR-101; Atago, Tokyo, Japan). The TA and pH were calculated from titration of 10 mL of juice with 0.1 mol·L⁻¹ NaOH to the pH 8.1 endpoint using an autotitrator (DL50; Mettler Toledo, Columbus, OH) and expressed as the percent of juice. The ratio (SS/TA) was the proportion of sugar and acid (Baldwin et al., 2012). For SS, TA, pH, and SS/TA, 15 fruit were juiced per treatment (five composite replicates of three fruit each).

From this same juice, ethanol and other volatile compounds were quantified by GC (6890; Agilent Technologies, Santa Clara, CA) using a standard curve with authentic standards (Baldwin et al., 2012). Temperatures of the oven, injector, and detector were 70, 250, and 250 °C, respectively. For quantification of ethanol and other aroma volatiles, 3 mL of fruit juice was transferred to a 10-mL crimp-capped vial, rapidly frozen in liquid N₂, and then stored at −80 °C. Frozen samples were later thawed under running tap water and inserted in a Gerstel multipurpose auto sampler for head space injection (3 mL) in the gas chromatograph equipped with a polar column (Stabilwax; Restek Corp., Bellefonte, PA), a nonpolar low-bleed column (HP-5; Agilent Technologies) and a flame ionization detector. The gas flow rates for He, H₂, and air were 10, 35, and 350 mL·min⁻¹, respectively. Temperatures of the oven, injector, and detector were 90, 200, and 250 °C, respectively. In addition to ethanol, common citrus volatiles were analyzed, including the alcohols hexanol, cis-3-hexenol, trans-2-hexenol, methanol, 2-methylpropanol, α-terpineol, and linalool; aldehydes acetaldehyde, decanal, and hexanal; esters methybutanoate, ethylbutanoate, ethylacetate, ethylhexanoate, and ethyl-3-hydroxyhexanoate; and terpenoids valencene, limonene, myrcene, α-pinene, sabinene, and γ-terpinene. Volatile compound identification was confirmed using solid phase microextraction (SPME) (50/30 μm DVB/CAR/PDMS; Supelco, Bellefonte, PA) and GC-mass spectrometry (MS), as reported by Wang et al. (2015). The instrument and column for SPME injection were performed using GC-MS (6890 GC + 5973N MS, Agilent Technologies) with a nonpolar column (0.25 mm × 60 m, 0.50-μm-thick film thickness, DB-5, Agilent Technologies).

Internal gases, CO₂ and O_2, from 10 fruit samples per treatment were evaluated by withdrawing a 10-mL internal fruit gas sample from the fruit columella using a syringe while the fruit was submerged in room temperature water. These samples were analyzed using a gas chromatograph (Hewlett Packard 5890A, Agilent Technologies) fitted with a CTR column (Cole-Parmer, Vernon Hills, IL) and a thermal conductivity detector. The gas flow rates for helium and air were 80 and 350 mL·m⁻¹, respectively. Temperatures of the oven, injector, and detector were 70, 250, and 250 °C, respectively. Weight loss of 10 fruit per treatment was measured during storage, and results were expressed as a percentage of initial weight.

Gloss measurements.

A reflectometer (micro-TRI-gloss; BYK-Gardner, Colombia, MD) was used to evaluate gloss units, which are an indication of visual shine for coating formulations on test paper (140 × 254 mm) and on the fruit. For test papers, an aliquot of 0.5 mL of each coating was spread on a 0.05-mm-thick polished paper (Leneta Co., Mahwah, NJ) with a 0.1-mm castor (BYK-Gardner) with a speed of 1 cm·s⁻¹. The control consisted of uncoated paper. Five papers per treatment were used, and measurements were repeated five times per paper at different positions. The reflectance was measured at angles of 20° for test papers (Bai et al., 2003a) and 60° for fruit (Bai et al., 2003b) according to the manufacturer’s instructions because the paper is flat and the fruit is round. For coated fruit, a case with a circular 19-mm-diameter orifice was attached to the equipment to accommodate the round fruit shape. For gloss measurements, 10 fruit per treatment were used and measurements were performed twice at opposite points of the equatorial region of the fruit.

Sensory visual shine analyses.

Coated fruit were visually evaluated for shine by 25 and 19 panelists for ‘Nova’ and ‘Unique’ fruit, respectively, for comparison with the gloss units measured with the reflectometer. Panelists consisted of laboratory staff familiar with the assessment of citrus products. For ‘Nova’, two groups of four fruit were presented, with each group containing all four treatments, and panelists were asked to pair fruit by appearance (tetrad test) (Ennis, 2012). Then, panelists were asked to rank the pairs for shine in decreasing order of shine intensity (most shine to least shine). For ‘Unique’ tangors, panelists were also instructed to squeeze fruit between the middle finger and thumb to rate firmness/hardness of the fruit using a 10-point scale (0 soft to 10 hard). During both tests, each fruit was presented with a three-digit randomized code on a tray under daylight illumination.

Sensory flavor evaluation.

‘Unique’ fruit were juiced as described previously and frozen at 20 °C. On the day of the panel, the juice was thawed overnight at 5 °C, and 40 mL of juice was poured in 120-mL cups with lids (Solo, Urbana, IL). Coded juice samples were presented to panelists at 14 ± 1 °C, following the William’s design. An additional cup of control juice was presented and labeled as “warm up.” Panelists were instructed to first taste the “warm up” sample to familiarize their taste buds with the juice (O’Mahony et al., 1988). Then, they were instructed to take a sip from each coded cup and rank the samples for overall flavor from best to worst. They were allowed to taste as many times as necessary. Finally, a question was asked about any off-flavor perceived in each sample. Panelists were asked to rate off-flavor from 1 (no off-flavor) to 10 (extreme off-flavor) and to describe the off-flavor if perceived. Tasting was performed in isolated booths under red lighting. Data were recorded using Compusense Cloud (Compusense, Guelph, ON, Canada). Twenty-four panelists participated in a flavor evaluation of ‘Unique’ juice.

Statistical analyses.

Physical and chemical parameters were analyzed using a univariate parametric analysis of variance (ANOVA) at each storage time, a multiple comparison Duncan test or Games-Howell (according to homogeneity of variance assumed or not), and a t test for paired samples. The significance level for all analyzes was 5%. IBM SPSS Statistics (subscription trial free version; IBM Corp., Armonk, NY) and R (R Core Team, Vienna, Austria) software were used. Ranked sensory data were analyzed using a critical absolute rank sum differences table at P < 0.05 (Newell and MacFarlane, 1987). Tetrad test (Ennis, 2012) data were analyzed using an approximation equation for tetrad (Z-test) at P = 0.05. A comparison of sensory firmness scores was performed using a nonparametric ANOVA and multiple comparison of Kruskal-Wallis because of the ordinal level of the variables and four independent samples in the experiment (P < 0.05). Off-flavor ratings were analyzed by the ANOVA in a mixed model with random panelists using a statistical package (Senpaq version 5.01; Qi Statistics, Reading, UK). Principal component analysis (PCA) was performed using a multivariate statistical package (JMP version 13; SAS Institute, Cary, NC) to test the separation among coating treatments and storage times based on the volatile compounds.

Weight loss.

A major function of coatings on fruit is to retard water loss, which is measured as weight loss. Microemulsion (conventional) and nanoemulsion (experimental) carnauba wax coatings resulted in less weight loss compared with control and shellac-coated fruit, which were not different from each other, during the first experiment with ‘Nova’ mandarins (Fig. 1A). The microemulsion fared slightly better than the nanoemulsion for retarding weight (water) loss; therefore, perhaps, the more numerous and smaller lipid micelles were more permeable to water vapor than the larger lipid micelles of the microemulsion. For the second experiment with ‘Unique’ tangors, fruit weight loss was minimal after 14 d at 10 °C (less than 2%) (Fig. 1B), with the control showing the most weight loss, but it was not different from that of the shellac. At the end of storage (14 d at 10 °C + 7 d at 20 °C), the microemulsion followed by the nanoemulsion coating exhibited less weight loss when compared with uncoated fruit, with the shellac coating being intermediate between control and the waxes.

View Full Size

Nanoemulsions and microemulsions of carnauba wax that help to slow water/weight loss from citrus fruit compared with a commercial shellac microemulsion. (A) Weight loss of coated and uncoated ‘Nova’ mandarins and (B) weight loss of ‘Unique’ tangors (10 fruit/treatment). For each storage period, columns with different letters are significantly different according to Duncan’s test (P < 0.05).

Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 146, 1; 10.21273/JASHS04972-20

• Download Figure
• Download figure as PowerPoint slide

Carnauba wax nanoemulsions and microemulsions are less permeable to water vapor than shellac due to its lipophilic nature. The solid lipid structure is dense and restricts water diffusion (Morillon et al., 2002); therefore, the results are not surprising. Several studies have reported that carnauba-based coatings decrease water loss (Jo et al., 2014; Kim et al., 2014).

Sugars and acids.

Sweet and sour tastes, attributed to sugars and acids, are important to fruit flavor quality. There were minor statistical, but no practical, differences in the SS, TA, pH, and ratio among treatments (Fig. 2). For ‘Nova’ mandarin coated fruit (Fig. 2A, C, E, and G), there were no differences in the SS between coatings (Fig. 2A); however, shellac had the highest TA and lowest pH (not different from the microemulsion), which resulted in the lowest SS/TA (not different from control) (Fig. 2C, E, and G). Generally, although not tested statistically, there was a slight increase in the SS, pH, and SS/TA and a decrease in the TA (except shellac for TA and pH and microemulsion for pH) after 7 d at 20 °C. Similar results were reported by Obenland et al. (2011). The authors associated the higher SS/TA with the slightly superior flavor for mandarin fruit stored at 8 °C for up to 6 weeks and 1 week at 20 °C.

View Full Size

Nanoemulsions and microemulsions of carnauba wax compared with a commercial shellac microemulsion to determine effects on citrus fruit sugar and acid measurements. (A, B) Soluble solids (SS), (C, D) titratable acidity (TA), (E, F) pH, and (G, H) SS/TA ratio values for ‘Nova’ mandarin and Unique’ tangors, respectively, that were uncoated and coated with different coatings (five composite replicates of three fruit each). Columns with different letters are significantly different within each storage period according to Duncan’s test (P < 0.05).

Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 146, 1; 10.21273/JASHS04972-20

• Download Figure
• Download figure as PowerPoint slide

For ‘Unique’ tangors, there were also no substantial differences in the SS, TA, pH, or ratio among treatments (Fig. 2B, D, F, and H). However, the pH and SS/TA ratio for uncoated fruit were statistically the lowest among the treatments after 14 d at 10 °C (not different from shellac or nanoemulsion for SS/TA) (Fig. 2F and H, respectively), and there were no differences after an additional 7 d at 20 °C. Similarly, no significant differences in SS or TA were detected for different storage temperatures or during the storage period for ‘Valencia’ oranges coated with shellac, a cellulose-based coating, or uncoated fruit (Baldwin et al., 1995b).

Internal gases.

Fruit coatings can result in a modified fruit internal atmosphere that can affect flavor. The volatile ethanol is discussed here because it is affected by O₂ levels and is an indicator of anaerobic respiration and the resulting off-flavor. For fruit in these two experiments, internal CO₂ and juice ethanol generally increased and internal O₂ decreased during storage (Fig. 3). For ‘Nova’ mandarins, the highest levels of CO₂ and ethanol and the lowest O₂ level were found with the shellac treatment, suggesting anaerobic respiration. The lowest levels of CO₂ and ethanol and the highest O₂ level were found in uncoated control fruit. The nano- and microemulsion carnauba wax coatings were intermediate and not different from each other (Fig. 3A–C). High ethanol has been directly linked to off-flavor and altered flavor (Baldwin et al., 1995b; Hagenmaier, 2000, 2002; Hagenmaier and Goodner, 2002; Ke and Kader, 1990). Hagenmaier (2002) coated several mandarin hybrids (eight cultivars) with wax (polyethylene and candelilla) and resin (shellac) and reported that a sensory taste panel found that fruit coated with the low gas permeability coating (shellac) had less fresh flavor compared with those coated with higher gas permeability coatings (polyethylene and candelilla waxes). The author demonstrated that the mandarin flavor may be affected when the internal CO₂ is higher than 14%, the internal O₂ is lower than 4%, and the juice ethanol content is higher than 1500 µL·L⁻¹ after 7 d storage at 20 °C (see dashed lines in Fig. 3A–F).

View Full Size

Nanoemulsions and microemulsions of carnauba wax compared with a commercial shellac microemulsion to determine citrus fruit internal atmosphere modifications in (A, D) internal CO₂ for ‘Nova’ mandarins and ‘Unique’ tangors, respectively, (B, E) internal O₂ (10 fruit/treatment) for ‘Nova’ mandarins and ‘Unique’ tangors, respectively, and (C, F) juice ethanol values for ‘Nova’ mandarins and Unique’ tangors, respectively (five replicates of three fruit each). Columns with different letters are significantly different for each storage period according to Duncan’s test or the Games Howell test (P < 0.05). Dashed line represents the critical level according to Hagenmaier (2002) at which the mandarin flavor may be affected (internal CO₂ and juice ethanol >14% and >1500 μL·L⁻¹, respectively, and O₂ <4%).

Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 146, 1; 10.21273/JASHS04972-20

• Download Figure
• Download figure as PowerPoint slide

‘Unique’ tangors showed the same trend as ‘Nova’ mandarins: fruit coated with shellac showed the lowest O₂ along with highest CO₂ and ethanol levels, followed by the carnauba microemulsion and nanoemulsion (Fig. 3D–F), especially after 7 d at 20 °C. Uncoated fruit had the lowest ethanol and CO₂ levels, along with the highest O₂ level. There were no differences between the nanoemulsion and control for ethanol levels or between the nanoemulsion and the microemulsion for O₂ or ethanol after 14 d of cold storage and after the simulated marketing conditions for ethanol. Lower ethanol levels in the nanoemulsion coating could indicate better flavor, similar to that of uncoated fruit. Therefore, both carnauba coatings in this study were shown to be suitable for mandarins and tangors. Similar results were reported by Navarro et al. (2007) for ‘Valencia’ oranges and ‘Marisol’ tangerines. These authors reported that internal CO₂ and ethanol were highest for fruit coated with shellac.

Shellac coatings can reduce gas exchange, thus modifying the internal atmosphere and creating an anaerobic/fermentative environment (Baldwin et al., 1995a; Hagenmaier, 2000; Hagenmaier and Baker, 1994). Usually resin coatings have low oxygen permeability properties, and shellac and wood resin are generally less permeable than waxes such as polyethylene, candelilla (Hagenmaier, 2002), and carnauba (Assis et al., 2008; Lin and Zhao, 2007). Therefore, lipid-based coatings, such as carnauba wax, present a more effective moisture barrier and are relatively permeable to gases (Assis et al., 2008; Lin and Zhao, 2007), resulting in less off-flavor.

Low oxygen atmospheres have been shown to result in an increased production of ethanol, methanol, and acetaldehyde in citrus fruit (Baldwin et al., 1995b; Davis, 1970; Shaw et al., 1990). To summarize this study, carnauba wax emulsions exhibited intermediate modifications of the fruit internal atmosphere of uncoated controls and shellac, presented a better moisture barrier than shellac, and resulted in less ethanol, with the nanoemulsion ethanol levels not different from those of uncoated fruit (Fig. 3C and F).

Aroma volatile analysis.

The PCA plots of aroma volatiles of ‘Nova’ mandarins and ‘Unique’ tangors are shown in Fig. 4A and B, respectively. The PCA for ‘Nova’ mandarins (Fig. 4A) explained 59.5% of the data variation (33.2% and 26.3% for components 1 and 2, respectively) and 62.4% of the variation (mostly in component 1) for ‘Unique’ tangors (Fig. 4B), offering more evidence of the altered internal atmosphere due to coatings. For ‘Nova’ mandarins, shellac coatings were associated with ethanol, methanol, and acetaldehyde, indicating fermentation and other volatiles. The carnauba wax emulsions were clustered with control and 0-d samples or further down and to the right on component 1, especially one nanoemulsion outlier, that was associated with common citrus terpenes, esters, and an aldehyde (limonene, myrcene, α-pinene, γ-terpinene, ethyl hexanoate, and decanal among others) (Fig. 4A). For ‘Unique’ tangors, there were not many volatile differences after 14 d at 10 °C for any of the treatments, which were similar to those of 0-d juice, because the metabolism at this temperature is relatively slow (Fig. 4B). However, after cold storage followed by the simulated marketing period at 20 °C, there were more differences between treatments. The aroma volatiles in the juice from 0-d fruit, controls, and the carnauba nanoemulsion treatment were associated with fewer volatiles (on the left side of component 1) but associated with methanol and α-terpineol. The juice from fruit coated with the microemulsion and that from fruit coated with shellac were associated with more aroma volatiles on the right side of component 1, including acetaldehyde, ethylacetate, ethanol, and acetone (Fig. 4), which are associated with anaerobic respiration. One control outlier was also associated with acetaldehyde as well as valencene. Although the other volatiles shown in Fig. 4 are mostly desirable and comprise the citrus aroma, the coatings may have trapped them and/or affected their synthesis, resulting in an altered flavor profile (as evidenced for 0-d and control uncoated fruit juice) and possible off-flavor.

View Full Size

Effects of shellac and carnauba coatings on citrus fruit aroma volatiles. Principle component analysis of 26 aroma volatile compounds measured in the juice of 15 citrus fruit (five composite replicates of three fruit each) for untreated fruit [control (C)] or fruit coated with three different coatings and stored at 20 °C [day 0 (O)]. (A) ‘Nova’ mandarins stored for 7 d at 20 °C [C, shellac (S), nanoemulsion (N), and microemulsion (M)] and (B) ‘Unique’ tangors stored for 14 d at 10 °C (red/orange C, S, N, and M) or for 14 d at 10 °C plus 7 d at 20 °C (green/blue C, S, N, and M).

Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 146, 1; 10.21273/JASHS04972-20

• Download Figure
• Download figure as PowerPoint slide

Fruit flavor evaluation.

During the second experiment with ‘Unique’ tangors, the juice used to measure the aroma volatiles was also evaluated by a sensory panel for flavor (Fig. 5A–C). After 14 d of cold storage, control fruit juice was ranked as having the best mandarin flavor (lowest value), although it was not different from that of the carnauba microemulsion; however, the carnauba nanoemulsion and shellac-coated fruit were ranked as having the worst mandarin flavor (highest value), although it was not different from that of the carnauba microemulsion (Fig. 5A). For off-flavor at 14 d, there were no differences between treatments (Fig. 5C). This reflects the volatile data that showed few differences between treatments after cold storage (Fig. 4). After 14 d of cold storage followed by 7 d at 20 °C, control fruit juice and the carnauba nanoemulsion-coated fruit juice were ranked highest for mandarin flavor, and the shellac-coated and microemulsion-coated fruit juices were ranked the lowest (Fig. 5B). This also reflects the increased volatile changes after the simulated marketing period at 20 °C (Fig. 4). Similarly, regarding off-flavor, the control and carnauba nanoemulsion-treated fruit juices were rated lowest for off-flavor, and the shellac-treated and microemulsion-treated fruit juices were rated highest (Fig. 5C). This is consistent with the ethanol and other volatile levels, especially for shellac. The CO₂, O₂, and ethanol levels for the carnauba microemulsion were at or near levels that would predict flavor problems, but it is surprising that there were no flavor differences compared to shellac-coated fruit, which induced a much more extreme modification of the atmosphere than did the carnauba microemulsion, resulting in higher ethanol. Off-flavor descriptors for the shellac-coated and microemulsion-coated fruit were “rancid, “fermented,” “chemical,” “rotten fruit,” and “soapy,” which might be explained by higher amounts of ethyl acetate, ethyl hexanoate, ethyl-3-hydroxy hexanoate, and some aldehydes (Plotto et al., 2008).

View Full Size

Nanoemulsions and microemulsions of carnauba wax compared with a commercial shellac microemulsion for effects on citrus fruit juice flavor. Fruit juice was ranked for the tangerine flavor of ‘Unique’ tangors after 14 d at 10 °C (A) and after an additional 7 d at 20 °C (B). Columns with different letters are significantly different according to the critical absolute rank sum differences table (value on figure bars, shown in order of ranking) at P < 0.05 (data shown in order of ranking) (Newell and MacFarlane, 1987) for the four samples as determined by 24 panelists. Color shading depicts the percentage of panelists’ selections. (C) Juice rated for off-flavor using a scale of 1 to 10, where 1 = none and 10 = extreme. Numbers (mean rating, n = 24) followed with the same letter are not statistically different according to Fisher’s least significant difference test (P < 0.05).

Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 146, 1; 10.21273/JASHS04972-20

• Download Figure
• Download figure as PowerPoint slide

Gloss analyses.

Fruit visual shine boosts sales; therefore, coatings that impart shine are sought after by the coating industry, especially for citrus. During preliminary tests with paper sheets, gloss units on coated paper sheets increased when hot air was used to dry the coating surface and were generally higher when coatings were dried at 50 °C compared to 20 °C (Fig. 6A), which is significant for the two carnauba emulsions. This indicates that the application of heated air helps coatings to dry and tends to impart more visual shine to the fruit. In fact, heated air-drying tunnels are often used for citrus by the industry (Hall, 2012). Shellac, as expected, had the highest gloss, which was significantly different from that of carnauba wax nanoemulsions and microemulsions and the uncoated paper (Fig. 6A).

View Full Size

Nanoemulsions and microemulsions of carnauba wax that impart shine measured as gloss units by a gloss meter. (A) Gloss units for uncoated (control) and coated black paper (five test papers with five measurements each). Differences were determined by the t test (*P < 0.05, **P < 0.1). (B) Gloss units for uncoated (control) and coated ‘Nova’ mandarins (C) and for ‘Unique’ tangors (10 fruit/treatment with two measurements/fruit). Coatings: shellac (shellac microemulsion), nano (carnauba nanoemulsion), and micro (carnauba microemulsion). For each storage period, columns with different letters are significantly different according to Duncan’s test or the Games Howell test at P < 0.05.

Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 146, 1; 10.21273/JASHS04972-20

• Download Figure
• Download figure as PowerPoint slide

For gloss on ‘Nova’ mandarin fruit (Fig. 6B), the shellac coating showed higher gloss units than the uncoated control initially; the gloss units of the carnauba wax coatings were not different from those of the shellac coatings or control. However, after 7 d of storage, all coatings showed more gloss units than the control, and there were no differences among coating treatments. For ‘Unique’ tangors (Fig. 6C), initially, the shellac showed the highest gloss units, which were significantly different from those of the other treatments, which were not different from each other. After 14 d of storage at 10 °C, shellac and the carnauba wax nanoemulsions showed the highest gloss units, with no difference between them, and there was no difference between the gloss units of the carnauba microemulsion and control. However, after 7 d at 20 °C, the nanoemulsion showed the highest gloss units; the gloss units of the microemulsion were not different from those of the nanoemulsion or shellac. Citrus fruit loss of gloss units was observed for all treatments over time in storage; however, this loss was greater for shellac-coated fruit.

Similar results were reported by Navarro et al. (2007) for ‘Valencia’ sweet oranges and ‘Marisol’ tangerines treated with different coatings; the fruit gloss was highest with the shellac coating. In another study, after storage, the shellac gloss/shine decreased but was still shinier than that of the uncoated fruit. For that study, the shellac coating provided fruit with more shine and exhibited better weight loss control when compared with commercial cellulose-based coatings (Baldwin et al., 1995b).

Sensory visual shine analyses.

For ‘Nova’ mandarins, shellac-coated fruit were ranked highest for visual shine by panelists after 7 d at 20 °C, followed by carnauba nanoemulsion, carnauba microemulsion, and the uncoated control (Fig. 7A). For ‘Unique’ tangors, the carnauba nanoemulsion had the highest visual shine score, although it was not different from that of the microemulsion; however, its shine score was higher than that of the shellac, and all shine scores were higher than that of the uncoated fruit (Fig. 7B) after 14 d at 10 °C and 7 d at 20 °C. There was no difference in the visual shine with the carnauba microemulsion and shellac. During the sensory pairing test (tetrad) with ‘Nova’ mandarins, the panelists recognized shellac-coated and uncoated fruit but could not differentiate between carnauba wax microemulsions and nanoemulsions based on the appearance of coated fruit (Fig. 7C).

View Full Size

Nanoemulsions and microemulsions of carnauba wax that impart visual shine to citrus fruit. Sensory shine ranking for ‘Nova’ mandarins stored for 7 d at 20 °C (A) and ‘Unique’ tangors stored for 10 d at 10 °C and then for 7 d at 20 °C (B) at end of the storage period. Columns with different letters were significantly different according to the critical absolute rank sum differences table at P < 0.05 (value on figure bars, shown in order of ranking) (Newell and MacFarlane, 1987). Color shading depicts the percentage of panelists’ selections. (C) Sensory tetrad test for ‘Nova’ mandarins at the end of the storage period. Columns with different letters were significantly different according to the approximation equation for tetrad (Z-test) at P ≤ 0.05. (D) Sensory firmness perception for ‘Unique’ tangerine using a 10-point scale (0 soft to 10 hard) at the end of the storage period. Columns with different letters are different according to the Kruskal-Wallis multiple comparisons test (P < 0.05).

Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 146, 1; 10.21273/JASHS04972-20

• Download Figure
• Download figure as PowerPoint slide

The results for ranking based on visual shine are in agreement with those of Navarro et al. (2007), who reported that shellac-coated fruit ranked third for appearance (out of five treatments) and was considered too shiny. At the end of the cold storage (14 d at 10 °C) and simulated marketing conditions (7 d at 20 °C), uncoated tangors were more firm (hard, dried out). Those coated with the microemulsion and the nanoemulsion were the softest. Those coated with shellac were not different from those coated with either of the carnauba emulsions (Fig. 5D).

In conclusion, carnauba wax emulsion coatings allowed more gas exchange, resulting in fewer fruit internal atmosphere modifications, more sustainable shine (as measured by gloss units), and reduced water loss more effectively than a commercial shellac microemulsion on citrus fruit. Carnauba coatings did not affect the fruit sugar and acid levels, whereas shellac effects were minimal. The shellac coating induced the lowest O₂ and highest CO₂ and ethanol levels in fruit, which indicated anaerobic respiration and could predict undesirable flavor changes, as confirmed by a sensory panel. The panel found that the uncoated controls and the nanoemulsion-coated fruit had the most tangerine flavor and least off-flavor. An aroma volatile analysis showed that the shellac and microemulsion resulted in increased volatile levels, indicating alterations of the normal volatile profile. When the two carnauba coatings were compared, the microemulsion showed slightly better water loss control and the nanoemulsion generally exhibited higher gloss/shine, fewer modifications of the fruit internal atmosphere, fewer alterations of the aroma volatile profile, and lower ethanol levels. This resulted in better flavor quality for nanoemulsion-coated fruit juice, which was not different from the uncoated control fruit juice, which represented unaltered flavor. The effect on flavor is important because the use of coatings often detrimentally affects flavor, which affects repeat sales (Baldwin et al., 2014). For the carnauba emulsions, the materials used to make both emulsions are very similar, but the equipment is different. The nanoemulsion needs a closed reactor and a high-pressure homogenizer, which is a financial investment for coating companies, but the cost of the material is the same. The preparation time is ≈3 h for the nanoemulsion but ≈30 m for the microemulsion. The use of edible coatings is an environmentally friendly technology that can reduce postharvest losses and maintain fruit quality when used appropriately, and the carnauba nanoemulsion is as good as or better than the carnauba microemulsion for shine and flavor. Future work should evaluate decay control for carnauba and shellac microemulsions compared with the carnauba nanoemulsion for citrus fruit with and without antimicrobial essential oils.