1. Introduction
Exporting fresh citrus fruit to distant markets carries the challenge of maintaining quality for extended transport periods. Fruit appearance on the market is one of the main quality factors that directly affect the price(1). The use of coatings, mainly waxes, with different concentration of solids aims to compensate for some of the natural waxes that are lost in the cleaning process of the fruit, avoid evaporation and transpiration loss that result in weight loss, extend fruit preservation while maintaining its firmness and freshness, and improve presentation, giving it a special shine(1)(2)(3)(4)(5).
The coating quality is defined by the wax content and the percentage of solids. An important factor when increasing the latter is the need for an adequate emulsion-resin ratio so as not to affect surface tension. The percentage of solids defines the coverage degree and is very important, since a poor coating (reduced level of solids) may not provide the desired final brightness and promote dehydration during storage, leading to premature aging, firmness loss and possible appearance of dehydration symptoms, spots and remarked and blackened oil glands. On the other hand, excessive coating (high content of solids) prevents gaseous exchange through the peel, reducing oxygen levels and increasing carbon dioxide levels inside, resulting in a fermentation process, product of anaerobic breathing(6)(7)(8)(9)(10)(11)(12). This process modifies the concentration of different volatile compounds such as ethanol and acetaldehyde, which are associated with the appearance of unwanted flavors and odors known as off-flavors(2)(13)(14)(15)(16)(17)(18).
Most citrus fruits are sensitive to low temperatures, developing different symptoms of skin damage, known as chilling injury (ci). ci symptoms do not affect the internal quality of the product but do impair its commercial quality(19). Therefore, quarantine temperatures mandatory for export, which are close to 1°C, result in ci symptoms in sensitive varieties that reduce commercial life. The application of coatings may affect the appearance of physiological disorders. Waxing (regardless of the level of solids, 12 vs 18%) minimizes the intensity of chilling injuries, reducing their severity in different species and varieties of citrus fruits(20). In line with this, several authors determined that coatings based on shellac (E-904), carnauba (E-903), or oxidized polyethylene (E-914) prevent ci in different grapefruit varieties(21)(22). Waxing has also been found to decrease the incidence and severity of post-harvest spotting in Satsuma Okitsu mandarin(23).
Uruguayan citrus companies currently apply coatings based on oxidized polyethylene wax and shellac, with 18% solids, in most citrus fruits for export. However, variable environmental conditions of humidity and temperature during the packing of the fruits and the high-quality standards in the different destination markets led to the current use of coatings that do not meet companies’ requirements in terms of brightness and drying efficiency.
The objective of this study was to evaluate the effect of different commercial waxes (with modifications in their composition) on external and internal quality, weight loss, and production of acetaldehyde and ethanol in different citrus varieties: Nules Clementine, Nova, Navelina, New Hall and Salustiana, during postharvest storage. The working hypothesis focuses on the fact that changes in the composition of commercial coatings can affect the quality of citrus depending on the cultivar, and that new coatings should maintain the fruit quality and register a similar or lower weight loss than the wax currently used by Uruguayan citrus companies (briue), with no negative effect on the production of compounds associated with off-flavors.
4. Discussion
The environmental conditions during the citrus fruit packing in Uruguay (high relative humidity and medium-low temperatures), as well as the diversity of markets at different distances and with varied requirements (quarantine treatments, external and internal quality requirements), lead to the selection of suitable coatings. The evaluation of new coatings as alternatives to the briue control, widely used by all Uruguayan citrus companies, showed interesting results and supported the decision to change at a commercial level.
In the experimental evaluation, citaue and citsun showed higher brightness levels and good general appearance. In the commercial evaluation, brif6, citak and tec coatings stood out. The increased concentration of shellac in brif6 compared to briue, as well as the presence of carnauba in the composition of citak and tec are relevant factors that contribute to superior visual quality and a more intense brightness. Carnauba is the compound that generates the highest shine, followed by shellac and finally oxidized polyethylene(21)(22). Differences between coatings with the same ingredients could be due to the concentration of each particular compound and the other chemical components that accompany the formulation, such as surfactants and emulsifiers(1)(22). The ac36 (1:2) coating gave the fruit a natural appearance in terms of brightness. Diluting this coating to 1:5 in the experimental phase, negatively influenced the general appearance of the fruits, presenting a lower brightness. According to Hassan and others(5), a very thin layer of wax, or with a very low percentage of solids, may not be effective in preventing senescence and fruit quality loss. As the storage period advanced, some authors reported decreases in fruit brightness with coatings based on oxidized polyethylene(7), or oxidized polyethylene and shellac(15). During the commercial evaluation, the same trend was observed in the three most promising coatings in terms of brightness, without detecting large differences with the rest of the treatments after shelf life.
Fruit weight loss increased during storage, in general, it was greater in mandarin than in orange. This may be due to anatomical differences(5), such as fruit size; since smaller fruits have greater interaction with the outside, or to the peel thickness, which is greater in oranges (data not shown). These authors also describe that the weight loss of fruits is due to the respiratory process, the loss of humidity and some oxidation processes, therefore, the conservation temperature and the high relative humidity were key factors in the prevention of weight loss. Differences between treatments in the experimental evaluation were not observed, nevertheless, in the commercial evaluation, treatments with carnauba (citak and tec) showed a smaller weight loss (5.7% in oranges and 7.8% in mandarins) compared to the rest (6.4% in oranges, 9.2% in mandarins). Carnauba is the ingredient/compound that most limits gaseous interchange, therefore, it reduces the weight loss caused mainly by dehydration. In this aspect, it is followed by shellac and, finally, by oxidized polyethylene(22). Therefore, the coating composition is a factor that impacted directly on the weight loss.
Firmness loss was more intense in oranges than in mandarins, especially in Navelina and New Hall. This was directly associated with weight loss, which was greater in these varieties. In general, no differences were observed in the commercial evaluation between coatings, except for ac36 (1:2) which further reduced firmness in New Hall and Clementine but was the firmest in Nova and Salustiana.
The evaluated coatings did not impact the color of the fruits since the pci remained unchanged during refrigerated storage for most treatments and varieties. Variations in the color of fruits coated with oxidized polyethylene waxes have been observed, but not with shellac coatings, although storage temperature was higher: 2-3°C and 9-10°C(21). A decrease in the pci was observed in Nova, only in those with carnauba-free coatings, after shelf life. In Salustiana, this decrease was observed with the citaue, citsun and ac36 (1:2) coatings.
In general, coatings did not affect the internal quality, with some exceptions regarding the SS. Variations depended mainly on the evaluated varieties since coatings did not show a constant between them. The reduction of acidity content caused by the use of acids as an energy source, as well as the increase in soluble solids due to the hydrolytic activity of starch, are metabolic processes that depend directly on temperature(5), therefore, surely, conservation at 1°C influenced these results. Other authors did not observe differences in the SS content and acidity in fruit stored at 5°C(10) and 7°C either(12). However, there are also records of acidity reductions and SS increases after prolonged storage at 5°C, although these were accentuated after periods of shelf life at 20-25°C(5)(9)(15)(16)(17).
Regarding the general condition of the calyx, a senescence increase was observed over time, which was aggravated after the rise in temperature, during shelf life. Degreening was a key factor that favored the blackening and senescence of the calyx, detecting differences in the varieties with or without ethylene treatment. This application generates a color change and accelerates senescence, producing premature fruit aging, an increase in perspiration with weight loss, desiccation of calyxes and even an increase in rotten fruits(2)(26)(27). Within the degreened varieties, in Clementine, briue limited senescence compared to the rest of the treatments (47 vs 30% of green calyxes respectively, after shelf life), while in New Hall senescence was so severe that no treatment stood out (100% black and/or fallen calyxes after shelf life). Among the varieties without degreening, in Salustiana, the most occlusive coatings were highlighted (brif6, citak and tec) together with citaue, limiting senescence compared to the rest of the evaluated coatings (93 vs 82% of green calyxes respectively, after shelf life). In Nova mandarin, despite having an advanced state of maturity, the senescence of the calyxes was minimal, due to a varietal characteristic, with no differences between treatments (97% of green calyxes after shelf life).
The application of wax coatings is described as a postharvest handling tool that can minimize the appearance or aggravation of the symptoms of chilling injury(2)(20)(22). Results obtained demonstrate that the coatings brif6, citak, ac36 (1:2) and tec could prevent or reduce the intensity of the symptoms of ci in New Hall and Salustiana oranges, compared to briue, citaue and citsun, which had a higher incidence of ci. All these species, under these storage conditions(2)(19), are susceptible to this disorder, determining differential behavior of the coatings in interaction with the variety under study.
Coatings reduce gas exchange between fruits and the environment, especially as the contents of carnauba and shellac are higher, and the content of oxidized polyethylene is lower(22)(28). The lower this exchange, the lower the water loss (less dehydration and weight loss), but also the production of acetaldehyde and ethanol could be stimulated, as temperature increases and, with it, respiration. Fermentative processes can occur if the oxygen demand exceeds its capacity to enter the fruit, which would lead to the development of off-flavors and a higher acetaldehyde and ethanol concentration(26). Detecting these off-flavors in citrus fruits is associated with ethanol levels above 800-5000 mg.L-1(7), 900 mg.L-1(11), 1500 mg.L-1(8)(9) or 2000 mg.L-1(29).
The evolution of the acetaldehyde and ethanol concentration registered similar behaviors in all the evaluated varieties, with a progressive increase during storage that was accentuated after shelf life. This coincides with what various researchers have observed(6)(10)(11)(16)(24). Once again, storage temperature was influential, being one of the main factors that stimulate the synthesis of both compounds(26).
In the experimental phase, the concentration of acetaldehyde and ethanol, in general, were higher in Clementine than in Navelina. This may be due to anatomical differences in the peel of both species, which provide different levels of permeability and gas exchange as well as due to genetic differences in the natural production of both compounds. In this sense, differences were observed when comparing W. Murcott mandarin and Star Ruby grapefruit, concluding that the grapefruit flavedo is thinner and with fewer oil glands (impermeable to gases), and the albedo is less dense than that of W. Murcott, so the diffusion of gases would be greater, which can reduce the formation of volatile compounds derived from anaerobic respiration(14). After shelf life, no great differences were observed between treatments with respect to acetaldehyde, while briuej was the coating that showed a higher concentration of ethanol in Clementine (4619 vs 2201 mg.L-1 average), and Navelina (5267 vs 1824 mg.L-1 on average).
In the experiments under commercial conditions, the accumulation of both volatiles was higher in New Hall, reaching the detectable threshold of ethanol (2060 mg.L-1 on average) and widely exceeding it in the case of the control (briue, 3196 mg.L-1). In mandarins, coatings that reduced weight loss (brif6, citak and tec), being more occlusive, favored the synthesis of this volatile (1467 mg.L-1 in Clementine and 1724 mg. L-1 in Nova), while citaue, citsun and ac36 (1:2) stood out with lower concentration (652 mg.L-1 on average in Clementine and 843 mg.L-1 on average in Nova). In Salustiana, tec favored, once again, ethanol formation (1874 vs 1490 mg.L-1 on average in the other coatings). These differences are associated with a different composition of the waxes. Not only the main coating components are important, but also their concentration(22). Depending on the destination of the fruits, the composition of coatings should vary, choosing for short trips carnauba or shellac components (higher brightness) and for longer transports, oxidized polyethylene, assuming the loss of brightness(22).
5. Conclusions
Differences in the behavior of the evaluated coatings were observed which were related to the citrus variety. Coatings did not negatively affect the external (visual) quality, the firmness or the internal quality of the fruit compared to Brillaqua ue 18% (briue-control), so all of them could be used as an alternative, depending on each company's requirements. The Citrosol ak and Teycer glk coatings with carnauba and shellac were the ones with the lowest weight loss after storage and shelf life.
Brightness and general appearance of the fruit were superior in the Brillaqua f6, Citrosol ak and Teycer glk treatments at the end of the drying oven. After shelf life, no great differences were observed, highlighting Concentrol ac-36 (1:2) and Citrosol aue.
In the most susceptible varieties to CI (New Hall and Salustiana), most of the coatings offered better protection than Brillaqua ue 18%, with the exception of Citrosol (aue and Sunseal) in Salustiana.
The continuous control of storage temperature is very important regarding the levels of ethanol and acetaldehyde. The concentration of these volatiles depended on the type of coating, its application form and the application of the degreening treatment. In the commercial phase, in general, no coating exceeded the detection threshold of 2000 mg.L-1, except for briue in New Hall. The most occlusive treatments provide greater brightness and less weight loss, presenting higher concentrations of these volatiles. In these cases, it is important to achieve strict temperature control during periods of shelf life, especially in mandarins.
Finally, it is concluded that Brillaqua f6, Citrosol ak and Teycer glk coatings were the most promising for postharvest application on citrus fruits and subsequent storage at 1°C. These presented a higher level of brightness and a superior general appearance of the fruit, less weight loss and levels of acetaldehyde and ethanol that did not exceed the proposed thresholds. It is important to bear in mind that its use on overripe fruit or with a more severe degreening process can cause the appearance of off-flavors in some varieties if the storage temperature increases.
The Concentrol ac-36 (1:2) and Citrosol aue coatings are not ruled out, as they had acceptable levels of brightness, especially after conservation, as well as acceptable general visual appearance and levels of acetaldehyde and ethanol.
Fernández, G.
