3.1.5. Changes in Flesh Color (h◦)

Data shown in Figures 1 and 3B and Table 3 presented the changes in flesh hue angle according to chitosan, HW treatment, and the cold storage period. Chitosan-treated fruits had the highest significant values of flesh hue angle, followed by the combined treatment and HW, whereas the control treatment showed a lower flesh hue angle content than the other treatments.

3.1.6. Changes in Vitamin C Content, Peroxidase (POD) Activity, Catalase (CAT) Activity, and Membrane Stability Index Percentage (MSI%)

Peroxidase and catalase activities increased during the storage period in all treatments. However, after one week of the storage period, POD and CAT activities were lower in all treatments, including the control (Figures 3C and 4A–C, and Table 3). At the end of the storage period, all the treatments showed higher POD and CAT activities than the control. The chitosan and combined treatment presented the highest POD and CAT activities. Vitamin C content was lower than initial in all treatments and decreased during storage. At the end of the storage period (28 days), all treatments obtained a higher vitamin C content than the control. However, the chitosan and combined treatments showed higher vitamin C content than the other treatments. In all treatments, the membrane stability index (MSI) recorded lower values than initially and declined throughout the storage period (Figure 4C and Table 3). All treatments applied at the end of the storage period maintained a higher MSI than the control. The chitosan treatment and the combination treatment had a greater MSI in both seasons than the other treatments, including the control.

**Figure 4.** Effect of hot water treatment and chitosan coating on peroxidase (POD) activity (**A**), catalase (CAT) activity (**B**), and membrane stability index (MSI) percentage (**C**) of mango fruits during storage at 13 ± 0.5 ◦C and 85%–90% relative humidity (RH) for 28 days during 2020 and 2021 seasons. Values are means ± SE from three replicates. Statistical analysis was performed using LSD test.

### **4. Discussion**

As a climacteric fruit, mango fruit exhibits relatively high levels of bioactivity, including high respiration rates and postharvest ethylene production, thus reducing its shelf life. Due to consumer concerns about the use of synthetic chemicals, various natural coatings are currently being investigated throughout the shelf life for their effectiveness in slowing ripening and maintaining fruit quality [37–39]. Our investigation demonstrated that chitosan coating, alone or in combination with HW treatment, could efficiently delay ripening, increase postharvest quality, and control the decay of mango fruits. These results revealed that chitosan coating could be an alternate and effective technique for prolonging the postharvest life of mango fruits. In the current study, chitosan coating, alone or combined with HW treatment, significantly decreased the decay percentage after 3 weeks of cold storage compared to the control and the other treatments (Tables 1 and 2). The fruit coating with a concentration of 0.2% chitosan significantly inhibited the decay incidence of mango fruits caused by disease and maintained the fruit quality [5]. Chitosan postharvest application is known to influence the host–pathogen as antibacterial and antifungal activity [22,40]. It can destroy the plasma membrane of the spore of several pathogens, inhibit mycelial growth, and induce damage to the fungal cytoplasm [41]. Moreover, chitosan may also induce a defense mechanism in host tissues [42]. Several studies have shown that the use of HW is beneficial for tropical fruits [43–46]. The main cause of fruit weight loss is water loss induced by respiration and transpiration processes [47]. Chitosan has been reported to decrease the respiration rate in mango fruit [47–49]. The lower weight loss observed in chitosan-treated fruits may be related to the higher vapor barrier of chitosan. On the contrary, one study reported that 'Ataulfo' mango coated with chitosan film had greater weight loss [50]. Chitosan coating is reported to minimize transpiration losses by forming a semipermeable layer on the fruit's surface and acting as a selective permeability to water vapor [51]. In addition, it is reported to decrease the transpiration rate and retard senescence by modifying the internal atmosphere of the fruits [52]. The application of HW treatment has been reported to reduce or increase the weight loss of fruits. In this study, the HW treatment decreased fruit weight loss. Consistent with our result, Fawaz [5,53] mentioned that the weight loss of 'Alphones' mango fruits was reduced by a 45 ◦C HW treatment, suggesting that a mild heat treatment would dissolve the cuticle wax and decrease water loss [54]. The ripening of mango fruit is marked by a softening of the texture and a change in the color of the surface. The results of our study showed that the chitosan coating alone or in combination with the HW treatment effectively slowed the ripening of mango fruit, as evidenced by the retention of firmness and the delayed color change. In addition, TA and vitamin C reduction, TSS increase, and the weight loss of mango fruit were significantly suppressed by the chitosan coating. Thus, the application of the chitosan coating alone or in combination with the HW treatment effectively maintained the postharvest quality of the mango fruits according to the result obtained on the mango fruits treated by chitosan-based coating after harvest [49,55]. In the previous studies, chitosan maintained the firmness of the mango fruit [47,56,57]. Amin et al. [58] reported that mango fruit firmness was found to decrease linearly with storage time. The rate of firmness loss, however, was consistently decreased with the addition of a up to 2% chitosan–Aloe vera coating. The results of the HW treatment are consistent with [9,43]. The slowing of the softening might be attributed to the prevention of the formation of cell wall hydrolysis enzymes, which maintain membrane stability and reduce firmness [59]. Consistent with our result, chitosan has been shown to reduce fruit TSS during storage in mango [47,56,60]. Chitosan-treated fruits had the highest significant values of flesh hue angle, followed by the combined treatment and the HW treatment. These findings are consistent with those of Zhu et al. and Djioua et al. [25,47] on mango. They reported that the application of a 2% chitosan coating and a HW treatment at 50 ◦C for 30 min were effective treatments to maintain firmness and delay color change during fruit storage. Chitosan formed a semipermeable layer over the fruit peel and altered the atmosphere by elevating CO2 and lowering O2, which inhibited ethylene production and delayed ripening [61]. As the ripening process was delayed, it consequently reduced

the color changes by decreasing carotenoid biosynthesis and preserving the chlorophyll content. The hue values decreased with the advancing of the storage period and the values were significant for all treatments, with the change of flesh color from creamy to orange. Previously, the use of 1% chitosan was shown to improve the ascorbic acid content of mango [57,62]. Similar results have been reported in a previous study using the HW treatment [5]. The ascorbic acid content was found to be increased 1.14-fold in HW-treated fruits in contrast with the control fruit. The results showed that the chitosan treatment and the combination treatment had a greater MSI in both seasons than the other treatments, including the control (Figure 3C and Table 3). Ripening is a potentially oxidative process in which the transition from the maturing stage to the ripening stage is accompanied by a dynamic move to an oxidative state [63]. Likewise, excessive ROS generation can lead to the oxidation of the cell membrane lipids and proteins involved in mango ripening, which results in a gradual loss of membrane stability because of the changes in the biophysical and biochemical characteristics of the cell membranes. The expression of genes encoding enzymes involved in the fruit antioxidant system, including POD, PPO, and catalase, increases during ripening [64,65], and endogenous defense against the accumulation of harmful ROS has also been reported [49,66,67]. This might demonstrate that these treatments, especially chitosan and combined treatments, improved the antioxidant network of the fruit [67,68], allowing for the more effective regulation of metabolic free radical levels, hence preserving peel cell membrane integrity and maintaining better flesh firmness. Although many of the previous publications have indicated that both hot water and chitosan could be applied for different fruit protections, the novelty of our study is that, under the synergistic effect of the coating of the chitosan solution with a low concentration (1%) and the hot water treatment, the postharvest decay was suppressed, the quality of the mango fruits was improved, and the shelf life was extended by increasing the temperature of the hot water and shortening the treatment time of the hot water. Moreover, different fruits have different profiles regarding their storage capacity, with different treatment applications.

### **5. Conclusions**

Edible coatings combined with HW treatment were used to induce fruit decay resistance and improve fruit quality parameters. The present study evaluated the effect of a chitosan edible coating combined with a HW treatment on mango fruits during the storage time for 18 days at 13 ± 0.5 ◦C and 85%–90% relative humidity for 28 days. The results revealed that a combination of HW and chitosan treatments dramatically decreased the decay incidence percentage and improved the quality in mango fruits while also elevating the specific activity of POD and CAT defense-related enzymes. The combination of the prestorage HW treatment and chitosan coating maintained higher values of flesh hue angle (h◦), vitamin C content, membrane stability index (MSI) percentage, as well as lower weight loss compared with the untreated mango fruits. Fruits treated with HW ripened the fastest, comparable to the control fruits and the other treatments. However, a combination of the HW and chitosan treatments slowed down the fruits' ripening. As a result, combining HW and the chitosan coating improved the effects of each treatment alone. This application might be a promising technology and a novel strategy for controlling fruit decay, thus maintaining mango fruit quality during the storage period.

**Author Contributions:** Conceptualization, M.F.M.A., S.O.O., I.E.S., H.-E.H.O. and Z.A.A.; Data curation, H.A.K., D.O.E.-A., F.K.M.S., S.O.O., I.E.S., S.A.L., M.A.A. and Z.A.A.; Formal analysis, S.A.L., M.A.A. and Z.A.A.; Funding acquisition, I.E.S. and H.-E.H.O.; Investigation, H.A.K.; Methodology, H.A.K., M.F.M.A., D.O.E.-A., F.K.M.S., S.O.O. and Z.A.A.; Project administration, D.O.E.-A. and F.K.M.S.; Resources, M.F.M.A., S.A.L. and Z.A.A.; Software, A.A.L., F.K.M.S., S.O.O., M.A.A. and Z.A.A.; Supervision, A.A.L., H.-E.H.O. and M.A.A.; Visualization, H.-E.H.O. and M.A.A.; Writing original draft, M.F.M.A. and D.O.E.-A. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors extend their appreciation to Taif University for supporting this work, the Researchers Supporting Project under Project No. (TURSP-2020/116), Taif University, Taif, Saudi Arabia.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Relevant data applicable to this research are within the paper.

**Acknowledgments:** The authors extend their appreciation to Taif University for supporting current work by Taif University Researchers Supporting Project number (TURSP-2020/116), Taif University, Taif, Saudi Arabia.

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