**1. Introduction**

Longan (*Dimocarpus longan* L.) is an attractive subtropical fruit of the evergreen tree of the Sapindaceae family. The fruit is widely cultivated in many countries, especially China, Thailand, Vietnam, and Australia. The fruit has high nutritional value and is best when eaten fresh. However, longan is non-climacteric; the fruit is harvested at optimum maturity and does not continue to ripen once harvested. The fruit matures in high temperature and

**Citation:** Khan, M.R.; Huang, C.; Ullah, R.; Ullah, H.; Qazi, I.M.; Nawaz, T.; Adnan, M.; Khan, A.; Su, H.; Ren, L. Effects of Various Polymeric Films on the Pericarp Microstructure and Storability of Longan (cv. Shixia) Fruit Treated with Propyl Disulfide Essential Oil from the Neem (*Azadirachta indica*) Plant. *Polymers* **2022**, *14*, 536. https:// doi.org/10.3390/polym14030536

Academic Editor: Hiroshi Ito

Received: 23 December 2021 Accepted: 24 January 2022 Published: 28 January 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

humidity, meaning it deteriorates rapidly once harvested due to pericarp browning and microbial rot. Further, due to the unique pericarp structure of longan fruit, the dehydration and microbial invasion expedite the senescence and browning, consequently shortening its postharvest life [1]. Several studies have been conducted on treatments such as chlorine dioxide [2]; adenosine triphosphate (ATP) [3]; hydrogen peroxide [4]; chitosan [5]; SO2, ClO2, or their combination [6]; thymol coatings and thymol fumigation [7,8]; and many other studies in order to preserve the quality and extend the shelf life. However, due to consumer awareness of the health concerns regarding the residues of the synthetic compounds and the resistance of the microbes to the existing preservatives, there is a need to develop other preservatives that are safe to humans and the environment. Therefore, the continuous use of synthetic compounds needs to be eliminated to ensure the availability of safe and fresh fruits for longer periods of time.

Recently, plant-based extracts have attracted much more interest from researchers due to their biologically active components with antioxidant and antimicrobial properties. Various essential oils have been extracted from plants and have been utilized in the food industry. The neem plant (*Azadirachta indica*) is a rich source of about 300 primary and secondary metabolites, which possess antifungal, antibacterial, and antioxidant properties [9–11]. In another study, more than 140 biologically active compounds have been isolated from different parts [9], which have anti-inflammatory, antihyperglycemic, antiulcer, antimalarial, antifungal, antibacterial, antioxidant, antimutagenic, and anticarcinogenic properties [10,12,13]. So far, most studies have been conducted on neem extracts in the pharmaceutical industry or traditional medicines. Few studies have reported on controlling plant diseases. Propyl disulfide compound is the major volatile compound in neem plant seeds, and our research group previously assessed the antifungal activity levels of propyl disulfide, which effectively inhibited the mycelial growth of fungi, which causes anthracnose [14] and stem end rot [15] in mango fruit, obtaining very promising results. Zakawa et al. [16] studied the effects of neem leaf extract on the fungi causing anthracnose in wild mango. In 2011, Suleiman [17] reported on the effects of neem leaf extract against tomato anthracnose.

Neem extracts have also been used to preserve a wide range of other food products. For example, Serrone et al. [11] preserved the quality of fresh retail meat using neem oil and reported its efficacy against a wide range of bacterial populations. In another study, neem cake oil was used to preserve the quality of fresh retail meat [18]. The antioxidant activity of neem oil was reported in regard to beef lipid oxidation reactions, which extended the shelf life of raw beef patties to 11 days at 4 ◦C [19].

Enzymatic browning, microbial decay, and water loss are the major concerns in longan fruit; therefore, in this study, we focused on the antioxidant and antimicrobial activity levels of propyl disulfide from neem. Different packaging films were tested to find the best storage conditions. A very simple, cost-effective, and practical method was proposed for PD fumigation. The in-depth antioxidant mechanism of propyl disulfide was assessed regarding the enzymatic browning reaction in longan fruit, which includes phenolic substrates, enzymes, and browning. Pearson's correlation coefficient analysis was carried out to find the relationships between these parameters. The effect of PD on the unique pericarp structure of longan fruit and its three components were analyzed via scanning electron microscopy and are reported in detail in this paper.
