*3.6. TSS, TA, and pH*

In all treatments, TSS decreased with increased storage time (Table 2). No significant differences were found between the treated and control fruit samples, with few exceptions. Similarly, packaging films did not affect the TA and pH values, as no significant differences were found in the treated and control fruit samples, with few exceptions. However, storage time had an effect, as TA and pH values increased in all treatments with increasing storage time (Table 2).

**Figure 8.** Polyphenol oxidase (**A**) and peroxidase (**B**) activity levels in longan fruit samples in different packaging films fumigated with propyl disulfide and control, stored at 12 ± 1 ◦C. Vertical bars represent means ± standard deviations (*n* = 3). Different small and capital letters show significant differences between the treatments within the same day and between the days in the same treatment, respectively, via LSD test at *p* ≤ 0.05.




**Table 2.** *Cont.*

Different small letters in each row and capital letters in each column show the significant differences between the treatments within the same day and between the days in the same treatment, respectively, via LSD test at *p* ≤ 0.05. SDs are presented in parentheses.

#### *3.7. Scanning Electron Microscopy and Pericarp Microstructure*

Longan peel consists of three major parts, namely the exocarp, mesocarp, and endocarp. In this experiment, SEM analysis of the longan fruit samples treated with propyl disulfide was conducted to observe the structural changes in these parts and to compare them with controls. The SEM micrographs of the various parts of the treated and control longan peel samples are presented in Figure 9. The exocarp is the outermost part of the longan peel. The propyl disulfide had a positive effect; as shown in Figure 9, the exocarps of the treated fruit samples were very smooth, complete, tight, and connected. This honeycomb-like structure was composed of cup-shaped cells that were uniform and more clear, while the exocarp of the control fruit was not complete, had free space, a loose structure, a non-uniform honeycomb appearance, less cup-shaped cells, and more mycelial pathogens. The mesocarp is the second layer of the longan peel and comprises about 70% of the longan pericarp. The mesocarp layer of the propyl-disulfide-treated fruit was more uniform and continuous than the control, where rough structures and many cracks and openings were observed. The rills were deeper and broader in control samples than treated samples. The endocarp is a single layer of cells. The endocarp layer of the propyl-disulfide-treated fruit showed a regular surface omamentation compared to the control fruit, where some irregular surface omamentation was observed.

**Figure 9.** Scanning electron micrographs of treated and control longan pericarp. Exocarp samples at 1500× magnification, mesocarp samples at 400× magnification, and endocarp samples at 1500× magnification.

#### **4. Discussion**

Water loss is one of the main reasons for pericarp browning in longan fruit, whereby shrinkage occurs when substrates, enzymes, and other cell constituents come into contact and initiate browning. This can be seen from the strong positive correlation of water loss with BI (r = 85) and L\* values (r = 87), as shown in Table 3. Weight loss was significantly higher in control fruit samples than in all the other treatments. This is one of the benefits of polymeric films, which prevent water loss from fruit samples. On the other hand, PD also might play a role in water loss prevention, probably maintaining the pericarp's integrity and reducing water loss. Additionally, it can be stated that PD worked as a barrier against water loss, as previously found in thymol-treated longan fruit samples [8] and fresh-cut beans treated with tea tree essential oil and peppermint essential oil [27].

PD effectively inhibited the growth of microbes on the longan fruit surfaces. Although DI percentages increased in all treatments at the end of storage, the DI percentage of the control longan fruit samples was comparatively higher than all other treatments (Figure 2). A possible mechanism of the decay prevention of PD could be attributed to the sulfur compound, as it is well known for its antimicrobial activity levels. Ramos et al. [28] reported that the antiadhesive mechanism of neem extract could be the hydrophobicity of the cell surfaces' and the formation of biofilm, which could affect the microbial colonization. Koul [29] stated that either the characteristic odor of sulfur compounds or some physiological mechanism of interaction make the PD an effective grain protectant against insect pests. Kumar and Kudachikar [30] reported that the antifungal mechanisms of natural plant extracts against pathogens could be attributed to the disruption of membrane integrity and cellular component leakage. As microbial growth symptoms appear on the longan fruit's surfaces, in the current research work we did not study the antimicrobial efficiency of PD on specific microorganisms, but generally evaluated the decay of longan fruit. Hence, no decay and less symptoms were found in PD-treated fruit samples than control fruit. The term decay incidence was used to describe this process, and the data are presented in Figure 3. However, in our previous study on mango fruit, we found that PD was very efficient in inhibiting the growth of major fungi in mango fruit samples. We found that PD effectively inhibited the mycelial growth of *Colletotrichum gloeosporioides* and *Colletotrichum acutatum*, causing anthracnose, as well as *Lasiodiplodia theobromae* and *Neofusicoccum parvum,* causing stem end rots in mango [14,15]. Future research should be directed toward the specific microorganisms found in longan fruit samples.

Pericarp browning of longan is another major concern that limits its postharvest life. The combined effect of PD and polymeric films delayed the pericarp browning. Pericarp browning is a complex phenomenon that may include many interconnected factors that can be represented in terms of color values, phenol contents, activity levels of enzymes, and so on. The effect of PD on browning inhibition was obvious in maintaining the color values of longan pericarp samples (Table 1). Compared to the control treatment, fruit samples treated with PD and packaged in various polymeric films had high L\* values, lowest redness (a\*), and high yellowness (b\*). The presence of high TPC and TFC contents in longan pericarps treated with PD further confirmed the antioxidant efficiency of neem plant extracts. Interestingly, PD also affects the enzymes involved in the browning reaction. Longan fruit samples treated with PD and packaged in polymeric films showed lower PPO and POD activity levels than those in the control treatment. Regarding the overall phenomenon of the browning reaction and the antioxidant properties of neem, the high TPC and TFC contents (Figure 7), reduced activity levels of PPO and POD (Figure 8), high color values (Table 1), and lower BI levels (Figure 4) indicate the high antioxidant effect of PD. This can be seen from the correlation coefficient values in Table 3, whereby BI is highly correlated with PPO (r = 92), POD (r = 83), phenols (r = −85), flavonoids (r = −88), L\* (r = −99), a\* (r = 92), and b\* (r = −93) (Table 3). This good fit of the correlation coefficients in these parameters and compounds involved in the enzymatic browning reaction and their dependency on each other shows the antioxidant efficiency of PD against the enzymatic browning in longan fruit samples. The apparent color changes in the longan pericarp further confirm the antioxidant activity of PD, as can be seen in Figure 6, where the longan fruit pericarp turned brown on day 9. Changes in PD-treated fruit and packaged in the polymeric films were very slow, and the fruit samples were still acceptable on day 18 in OPP-20 and OPP-40 packages. This trend of high phenols, reduced enzymes activity levels, and consequently low BI levels was also seen in our previous study, when longan fruit cv. Daw was treated with plant essential oil (thymol) [7,8]. Valero et al. [31] fumigated table grapes with thymol, eugenol, and menthol, and reported that these essential oils maintained the phenol contents and high color values better than the control. Similar to our study on the propyl disulfide compound, other phytochemicals, such as diallyl disulfide and diallyl trisulfide obtained from garlic essential oil, showed strong antioxidant activity against nicotinamide–adenine dinucleotide phosphate (NADPH) oxidase enzymes [32].

The antioxidant activity of neem extract was also studied regarding the preservation and shelf life extension of other food products. Ouerfelli et al. [19] preserved the quality and extended the shelf life of raw beef patties to 11 days storage at 4 ◦C. These authors reported that neem extract prevented the loss in color, reduced the metmyoglobin formation, scavenged the DPPH free radicals, and possessed high antibacterial potential against beef patties. Serrone and Nicoletti [18] preserved fresh retail meat using neem cake oil and reported its efficacy against a wide range of bacteria. Serrone et al. [11] reported that neem oil effectively preserved the quality of fresh retail meat.

The peel firmness of the longan was higher in the treated fruit than in control. As shown in Figure 6, the peel and aril breakdown in the control on day 9 meant they were not suitable for further storage and were discarded, while the complete aril and peel in the treated fruit showed that PD treatment and storage in various polymeric films, particularly OPP-20 and OPP-40, prevented softness and aril breakdown.


**Table 3.** Pearson's correlation coefficient values for different quality parameters in longan fruit samples stored at 12 ± 1 ◦C.

Longan fruit has a unique pericarp structure, and besides the effect of PD on the physicochemical characteristics, the impact of PD was also evaluated on the pericarp structure. The scanning electron microscopy analysis indicated that PD maintained a uniform smooth surface. The SEM analysis confirmed that the PD maintained the cell integrity and compactness, prevented water loss, and minimized the chance of pathogen growth. On the other hand, the pericarp of the control treatment was cracked and damaged, showing an irregular surface and free space (Figure 9). This kind of loose structure was beneficial for the pathogens' invasion, as shown in Figure 3 (decay incidence), and enhanced the water loss, as shown in Figure 2.

Another reason for the good quality of the longan fruit being maintained could be the application of PD in the vapor phase due to the slow release time, which might maintain the quality and suppress the microbes for a longer time. Another benefit of the vapor phase application is that low quantities of PD are required, without altering the sensory properties of the products. Hence, the shelf life of the longan fruit treated with PD was extended up to 18 days compared to 9 days in control.
