*3.1. Decay Rate and Weight Loss*

Decay rate is a distinctly important indicator that affects the preservation effect of horticultural produces. As shown in Figure 2A, CH-HFE and 1.5% chitosan coatings delayed the appearance of fungal infection in comparison to uncoated group, which started to decay from 60 days of storage. Many of the uncoated orange fruits (10.4%) were rotten at the termination of storage, while fruits treated by CH-HFE and 1.5% chitosan coating exhibited significantly lower decay rate than the control group at the level of *p* < 0.05 (5.2% and 6.8%, respectively). As the data in Figure 2B show, for the whole time of experiment, the percentage of weight loss in control group was higher than in both coating treatments, with high weight loss (9.12%) of uncoated orange fruits at the end of storage. However,

orange fruits treated with CH-HFE coating displayed the lowest weight loss at 5.16%, which was much lower than that of 1.5% chitosan and control fruits throughout the 120-day storage period.

**Figure 2.** Changes in decay rate (**A**) and weight loss (**B**) of navel orange fruits stored at 5 ◦C for 120 days. Bars indicate standard error of three replicates.

#### *3.2. Fruit Quality*

TSS, TA, total sugar, and VC were the key determinants of citrus fruit quality. Figure 3 showed that the content of TSS, TA, total sugar, and VC varied within coated and uncoated groups as storage progressed. As illustrated in Figure 3A,C, the content of TSS and total sugar increased continuously during the early stage of storage, and decreased slightly in the subsequent storage period. The content of TSS and total sugar in the control group reached the highest level at 30 days, while the content of TSS and total sugar in coating groups reached their peaks at 60 days, and were delayed for 30 days more than the control group. During the late 60 days of storage stage, the content of TSS and total sugar in coated fruits was significantly higher (*p* < 0.05) than the control group. The TA content of all samples fell greatly after 120 days of storage (Figure 3B), and the value was significantly higher (*p* < 0.05) in CH-HFE-coated treated fruit, compared to 1.5% chitosan coating and the control group. During the storage, the control group showed a greater TA loss. The low level of TA in the control group, relative to coating treatments, suggested that the coating delayed ripening by providing a semi-permeable film around the fruits. The CH-HFE and 1.5% chitosan-coated fruits showed a slight increase in VC content during the first 30 days, reached their peak concentration (56.87 mg/100 g and 54.04 mg/100 g), followed by a decline in the subsequent storage period, while the control group presented a gradual decrease in VC content during the whole storage (Figure 3D). Under the storage conditions, the VC content in CH-HFE coated fruits was significantly higher (*p* < 0.05) than 1.5% chitosan-coated and control group fruits.

#### *3.3. Respiration Rate and MDA Content*

As illustrated in Figure 4A, the respiration rate decreased gradually, and reached its valley value at 60 days after storage, and recovered in the subsequent storage period. The valley value in coating-treated fruits was significantly lower (*p* < 0.05) than that in control group. During storage, the respiration rate of CH-HFE-coated fruits was significantly lower (*p* < 0.05) than those with 1.5% chitosan coating and the control group.

MDA is the final product of lipid peroxidation, and its content has been used as one of the direct indexes of cell oxidative damage [27]. As shown in Figure 4B, MDA content increased during storage, and a significant difference was shown between the coating treatments and the control group. At the end of the storage (120 days), the MDA content of CH-HFE, 1.5% chitosan, and control groups reached their maximum (2.47, 2.85, and 3.25 mmol g−1, respectively). The MDA content of control fruits was significantly higher than that of CH-HFE and 1.5% chitosan coatings, 23.7% and 12.3%, respectively.

**Figure 3.** Changes in total soluble solid (TSS) (**A**), titratable acid (TA) (**B**), total sugar (**C**), and vitamin C (VC) content (**D**) of navel orange fruits stored at 5 ◦C for 120 days. Each value represents the mean ± SE of three replicates, each consisting of 30 randomly selected fruits.

**Figure 4.** Changes in respiration rate (**A**) and malondialdehyde (MDA) content (**B**) of navel orange fruits stored at 5 ◦C for 120 days. Bars indicate standard error of three replicates, each consisting of 30 randomly selected fruits.

#### *3.4. Activities of Protective Enzymes*

SOD, POD, CHI, and GLU were the main protective enzymes to alleviate lipid peroxidation and delay fruit senescence. Figure 5 showed the activities of SOD, POD, CHI, and GLU varied within coated and uncoated groups, as storage progressed. As illustrated in Figure 5A,B, the activities of SOD and POD gradually increased to reach their peak, and then dropped quickly for the following day. The activities of SOD and POD in the control group reached peak values at 30 days and 60 days, while peak values of SOD and POD activities in coating groups were significantly higher (*p* < 0.05) than the control group, and delayed for 30 days. Moreover, SOD and POD activities in the CH-HFE coated fruits was significantly higher (*p* < 0.05) than those with 1.5% chitosan coating and the control group, during the later stage of storage. The CHI activity decreased slightly during the first 30 days, and then increased notably at the end of storage period (Figure 5C). The coated fruits had a relatively high CHI activity, and the average activity of CHI of the CH-HFE and 1.5% chitosan-coated treatment increased to approximately 1.30 and 1.19 times higher than that in the control group. The GLU activity of all samples fell greatly after 120 days of storage (Figure 5D), and the value was significantly higher (*p* < 0.05) in CH-HFE-coated treated fruit compared to 1.5% chitosan coating and the control group.

**Figure 5.** Changes in the activities of SOD (**A**), POD (**B**), CHI (**C**), and GLU (**D**) of navel orange fruits stored at 5 ◦C for 120 days. Each value represents the mean ± SE of three replicates selected randomly from 30 fruits each treatment.

#### **4. Discussion**

As a medicinal plant and food, hairy fig (*Ficus hirta* Vahl.) fruits have strong antifungal, antioxidant, and antiseptic effects [28,29]. Our previous studies have been demonstrated that HFE was able to significantly inhibit both *P. italicum* and *P. digitatum* in vitro [21,30]. In the present work, we found that HFE had an abundance of antioxidants and more effective DPPH, OH, ABTS radical scavenging activities, and Fe3+ reducing power than ascorbic acid in different solvent extracts (data not shown). A similar result has been reported where the roots of *Ficus hirta* Vahl. showed prominent anti-inflammatory activity [31]. A recent in vitro test has shown that HFE had a broad antifungal

spectrum and high efficacy in controlling fungal growth of fungal pathogens, such as *P. italicum*, *P. digitatum*, and *Geotrichum citri-aurantii* and *Alternaria citri* in citrus, *Botryosphaeria dothidea*, *Phomopsis* sp., and *Botrytis cinerea* in kiwifruits, *Alternaria alternata* in pears, and *Phomopsis vexans* in eggplants [21]. HFE has been shown to be a natural and safe fungistat when applying to control blue mold in citrus, and incorporated into alginate-based edible coatings to enhance Nanfeng mandarin preservation [28]. According to these results, there is no doubt that HFE can be safely used as a natural preservative for horticultural products.

Decay rate and weight loss are the main distinctly important indicators that affect the preservation effect of fresh fruit and vegetables, mainly due to postharvest diseases by pathogen infection, and the loss of water caused by transpiration [32,33]. Edible coatings provide a semi-permeable film around fruit, reducing disease incidence and depressing the rate of dehydration and respiration [28,34]. Coating "Newhall" navel oranges with CH-HFE is clearly effective in conferring a beneficial barrier to pathogen infection and water loss; thus, the decreased rate of fruit decay and weight loss in CH-HFE-coated fruit was evaluated during cold storage in this study. Our results are supported by Shah et al. [35], where fruit rotting intensity and weight loss of Kinnow mandarin can be reduced by coating with carboxymethyl cellulose (CMC) or guar gum-based coatings enriched with silver nanoparticles. Apart from citrus fruit, chitosan-based coatings containing natural antifungal agents, such as plant extracts or essential oils, have been effective at controlling the rate of fruit decay and weight loss from other horticultural products, including strawberry [36–39], guava [40,41], grape [42–44], tomato, and cucumber [45–47].

In general, TSS and TA, known as a respiration substrate, are continually consumed, and affect firsthand the taste flavor of navel oranges. Meanwhile, VC is a nutritional component of citrus fruits, and is also an important antioxidant which scavenges active oxygen. Due to the post-maturing action of citrus fruit, the fruit quality of harvested "Newhall" navel oranges, after a certain period of storage time, was improved, and the content of nutritional components were also increased. As the film acts as a gas barrier, the respiration process slows down the consumption of nutritional components, thus, coating treatment has a positive effect on the increase or maintenance of VC and other components [33]. In the present study, we found that the changes of CH-HFE and 1.5% chitosan coatings on nutritional components, such as TSS, TA, and total sugar of "Newhall" navel oranges are probably due to the slowing down of respiration, hence effectively delaying fruit ripening and senescence. It is shown by us, that CH-HFE and 1.5% chitosan coatings result in a beneficial semi-permeable film surrounding the fruit, modifying the internal atmosphere by elevating CO2 and/or reducing O2, and delaying the degradation rate of nutritional components in navel orange fruits during the mid–later storage period. Our results are in accordance with those of Chen et al. [23] and Shah et al. [35], where a slow decline in TSS, TA, and VC was recorded in citrus fruits treated with CMC composite coatings. However, other studies have demonstrated that the levels of total soluble solids and acidity, and flavor preferences in "Navel" oranges and "Star Ruby" grapefruits treated with CMC/chitosan bilayer coating, were the same as in the uncoated fruits [48].

Coated "Newhall" oranges generally had lower respiration rate than the uncoated fruits (Figure 5A), likely due to the modification of internal atmosphere by elevating CO2 and/or reducing O2 [28]. Similar observations have been reported by Arnon et al. [48], Chien et al. [49], Contreras-Oliva et al. [50], and Xu et al. [51] in citrus fruits coated with chitosan-based edible coatings. In our research, the incorporation of HFE into 1.5% chitosan coating formulation resulted in a significantly lower respiration rate than 1.5% chitosan coating alone, and uncoated fruits, during the whole storage. Similarly, the respiration rate of citrus fruits coated with CMC edible coatings containing clove oil, *Impatiens balsamina* L. extract, and silver nanoparticles, has been shown to decrease, compared to uncoated fruits [23,35,52].

In this present study, the activities of protective enzymes, such as SOD, POD, CHI, and GLU, are enhanced by CH-HFE coating, which suggests that CH-HFE coating induces natural resistance in "Newhall" orange fruit by activating the fruit's antioxidant and defense-related system to control the

level of reactive oxygen species (ROS) and delay fruit senescence (Figure 5A–D). Chen et al. [21,28] and Zeng et al. [18] have also found that *Ficus hirta* Vahl. extracts have a powerful antifungal activity against *Penicillium* spp. and abundant antioxidants, as well as inhibitory efficacy for HeLa cells. Chen and co-authors also maintain that an alginate-based edible coating containing HFE can significantly increase the activities of antioxidant and defense-related enzymes and enhance the preservation effect of Nanfeng mandarin [28]. An advantage of the activities of protective enzymes has been reported, previously, for navel oranges treated with a 1.5% CMC coating enriched with *Impatiens balsamina* L. extract, strawberry treated with a 1.0% chitosan coating containing lemon essential oil, guava treated with 2.0% sodium alginate and 1.0% chitosan coatings enriched with pomegranate peel extract, and cherry tomato treated with a 1.0% chitosan coating incorporating grapefruit seed extract [37,40,46,52]. Thus, our results may imply that these different protective enzymes in the antioxidant and defense-related system may be collectively induced by CH-HFE coating in navel orange fruits. The induced activities of protective enzymes may be a crucial part of the mechanism of CH-HFE coating in enhancing disease resistance and prolonging storage-life for navel orange preservation.

### **5. Conclusions**

Edible coatings containing natural antifungal agents, such as plant extracts and essential oils, are being used widely to enhance the preservation effect of horticultural products by prolonging their storage life. In this study, our work indicates that the application of plant extracts, such as hairy fig fruit extract (HFE) with chitosan, could enhance the postharvest quality of navel orange. A combination of HFE into chitosan-based coatings not only resulted in a lower percentage of rotting fruits and weight loss, but also in maintenance of good fruit quality of navel orange, by depressing the respiration rate and, thereby, delaying fruit senescence. Moreover, protective enzymes, including SOD, POD, CHI, and GLU in CH-HFE coating treatment, had obviously higher than those activities of the control group or 1.5% chitosan coating alone. According to the results from this study, CH-HFE coating treatment showed the best preservation effect for harvested "Newhall" navel orange. The probable preservation mechanism of CH-HFE coating of navel oranges is mainly through reducing the percentage of fruit decay, weight loss, and MDA content, depressing the respiration rate, and maintaining a high level of fruit quality and protective enzymes. In short, our work indicates that CH-HFE coating has become a natural and safe alternative treatment for enhancing postharvest quality and prolonging the storage life of navel orange.

**Author Contributions:** Conceptualization, J.C. and W.C.; Methodology, C.C. and X.P.; Data Analysis, C.C. and Z.N.; Writing—Original Draft Preparation, C.C.; Writing—Review and Editing, W.C.; Project Administration, J.C.; Funding Acquisition, J.C., C.C. and W.C.

**Funding:** This research was funded by National Natural Science Foundation of China (No. 31760598), and Natural Science Foundation and Advantage Innovation Team Project of Jiangxi Province (No. 20171BAB214031 and No. 20181BCB24005).

**Conflicts of Interest:** The authors declare no conflict of interest.

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