Antimicrobial Activity of Pomegranate Peel and Its Applications on Food Preservation

Pomegranate (Punica granatum L.) fruit is being cultivated since the civilization is known, and its production and consumption have been increased since the last century due to the scientific confirmation of its health benefits. Pomegranate fruits, fruit juice, its seeds, and peels are known to have higher contents of bioactive compounds, viz., phenolic acids, flavonoids, and hydrolysable tannins. -e peels of pomegranate fruits are the major by-products produced during food processing of pomegranate enriched in antioxidants and broad-spectrum antimicrobial agents and can prevent food deterioration even. -is health potential of pomegranate is known to vary significantly upon the varieties, growing conditions, cultivation practices, stages of the development, and the extraction methods. Herein, the biochemical composition of the pomegranate peel extract (PPE), its efficacy in food preservation, and antimicrobial activities are discussed to provide a comprehensive guide for farmers, food processing, and storage sectors and academia.


Introduction
Pomegranate (Punica granatum L.) plants are among the first cultivated plants by humanity; however, its consumption had been limited most commonly as a result of the hassle of extracting the juicy arils [1]. Due to the increasing number of scientific studies about its health benefits, production and consumption of pomegranate fruits have been increasing since the beginning of the 21st century. Pomegranate fruits are consumed as both fresh and processed mainly in the forms of juice, oil, wine, and jams. Both the fruits and its peel are known to have high levels of numerous phytochemicals, including phenolic acids, flavonoids, and tannins.
is diverse characteristic of phytochemicals is thought to be responsible for its high antioxidant potential and health benefits [2]. During processing, a considerable amount of by-products are developed from peels and is known to have high contents of hydrolysable tannins (HTs) [3]. Recently, by-products of pomegranates, especially pomegranate peel extract (PPE), have been increasing attention due to its scientifically confirmed therapeutic properties such as antioxidant, antimicrobial, anticancer, antiulcer, and anti-inflammatory activities [4,5]. Numerous scientific studies have suggested that PPE exhibits excellent antimicrobial activity against several foodborne pathogens and improves the postharvest storability of food products [6,7]. is paper will describe and discuss the recent advancements about the biochemical composition, antimicrobial potential, and food preservation characteristics of PPEs.
e antioxidant activity of the PPEs is attributed to the phenolic acids, flavonoids, and tannins. Among these, ellagitannins are noted to be most responsible for the antioxidant activity of the pomegranate peels [9]. It was also stated that both concentrations of phytochemicals and antioxidant activity are highly dependent upon the solvents used for the peel extraction. Previous studies also suggested that the methanolic extracts of the pomegranate peels exhibit higher antioxidant activity as compared with other extractions methods [27]. e concentrations of phenolic acids, flavonoids, and tannins in PPEs mainly depend on the extraction method. For example, Orak et al. [28] reported the highest concentrations of tannins identified from methanol extracts, as compared with water and ethanol extracts. Acetone extracts of pomegranate peels were also noted to have higher antioxidant activity than the water and ethanol extracts [19].

The Extract Process of Pomegranate Peel
One of the most widely used methods for pomegranate peel extraction is the methanol extraction method reported by Dahham et al. [29]. In this method, first of all, the fine peel powders are obtained by an electric blender and oven-dried at 40°C for 24 h. en, the powders are sieved through a 24mesh, and 10 g of powder sample is extracted with 250 ml of 80% methanol at room temperature (∼25°C) for 24 h. e final extract is then filtered and used. If the aqueous extract is needed for use, the powdered sample (10 g) is extracted with 100 ml of distilled water and used. e drying temperature and duration was reported as 50°C for 48 h in some other studies [11].
Water, ethanol, and acetone are the other solvents reported to be used instead of methanol for the extraction of the pomegranate peel [19,28]. Most of the published literature about the different extraction methods/solvents recommended that the methanol extraction method is better than others in terms of the abundance of phenolic compounds and increase antioxidant activity [28,[30][31][32]. e extraction method is reported to have a strong influence on the biochemical composition of PPEs. e conventional techniques generally need high amounts of solvents and results with low extraction yields. Moreover, the use of high temperature is reported to result in degradation in the extracts [33]. Recently, high-pressure extraction is observed and noted as a practical methodology, which results with no detrimental effects on bioactive compounds. Additionally, to be a green technology, the high-pressure extraction is faster and has a high yield [34].
Another important extraction method is the enzymatic extraction. It was reported to assist in the extraction of bioactive compounds. e mechanism behind the enzymatic extraction involves some cell wall degrading enzymes (i.e., pectinases). ese enzymes break down the cell wall and improve the extraction of the bioactive compounds [35,36].

Antimicrobial Activity of Pomegranate Peel
Pomegranate has a broad spectrum of antimicrobial effects, which has an apparent inhibitory effect against Gramnegative, Gram-positive bacteria (Table 1), fungi, and mould (Table 2). However, different extracts from different parts of pomegranate have various antimicrobial activities. e study of many scholars showed that the antimicrobial activity of PPE was more potent than other parts, and the antimicrobial activity of PPE was related to the total flavonoids and tannins content. PPE is well known for its antimicrobial activity against bacterial and fungal pathogens [29,37,38]. However, the number of studies which investigated the effects of PPE against plant pathogenic bacteria and fungi is limited [11,32,39,40]. PPE was noted to have antibacterial effects on different foodborne pathogens including Escherichia coli, Fusarium sambucinum, Penicillium italicum, and Bacillus subtilis [11,31,38,41]. In one of these comprehensive studies, the antimicrobial activity of extracts derived from different parts of pomegranates (peel, seeds, juice, and whole fruits) was tested on seven bacteria (Bacillus coagulans, Bacillus cereus, B. subtilis, Staphylococcus aureus, E. coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa). e maximum inhibitory of all bacteria was noted from the peel extracts of pomegranates [29]. Pomegranate peel was also tested as an incorporation agent into bio-based films and was found to improve the antibacterial activity of materials. In one of these studies, Ali et al. [42] reported that PPE inhibited the growth of S. aureus (Gram-positive) and Salmonella (Gram-negative). As discussed in Section 3, the extraction method can significantly influence the antimicrobial activity of pomegranate peels. In a recent study, antimicrobial effects of high pressure (300 and 600 MPa) of the pomegranate peels were tested by [13]. In this study, the maximum antioxidant activity and phenolic content were observed at the extraction of 300 MPa; thus, the higher antimicrobial activity against pathogenic bacteria was noted from 300 MPa. e ethyl acetate extract of pomegranate peel was previously reported to include high concentrations of tannins which are very active against Staphylococcus aureus strains [43]. Water extract of pomegranate peel was also reported to control Gram-positive significantly and Gramnegative bacteria including E. coli, B. subtilis, Enterobacter aerogenes, Serratia marcescens, Brucella spp., Saccharomyces cerevisiae, and Rhodotorula glutinis [44].
In a different study, the effects of PPEs for controlling the growth and development of Fusarium sambucinum were tested in in vitro as curative and preventive. e researchers noted that the PPE (20 mg/ml) exhibited complete inhibition of spore germination and 75.5%  Figure 1: Chemical structures of reported compounds from pomegranate peels.
Journal of Food Quality inhibition on mycelia growth of fungi [11]. In a recent study, El Khetabi et al. [45] studied the in vitro and in vivo effects of aqueous PPE on the brown rot (caused by Monilinia laxa and M. fructigena). ey reported an inhibition varying from 76.65% to 90% on the control of mycelia growth. Intense fungicidal activity of PPE was then reported against Botrytis cinerea, Penicillium digitatum, and Penicillium expansum by Nicosia et al. [46]. 20 h of incubation with PPE resulted with almost complete inhibition of all fungal spores at lemons and grapefruits. A recent study investigated the antifungal activity of 21 different pomegranate genotypes [47]. Researchers noted that all of the tested genotypes have varying antifungal activities, against Fusarium oxysporum f. sp. lycopersici, where some of them completely inhibit the fungus, and some others had deficient inhibitory activity. e two different extracts of pomegranate peel (water and methanol) were incorporated with chitosan and locust bean gum for controlling P. digitatum at orange fruits. Results suggested that the addition of 0.361 g water PPE into both locust bean gum and chitosan coatings significantly reduce the P. digitatum by 28% and 49%, respectively [48]. Phenolic profile and antimicrobial activity of "Gabsi" PPEs were studied by Kharchoufi et al. [32]. Similar to the previous studies of the researchers, the water and methanol solvents were tested separately. Researchers noted different results than their other studies and recommended that the methanol extraction is more effective against the P. digitatum and Saccharomyces cerevisiae.
ere are still significant obstacles for the industrial use of PPEs to control postharvest pathogens. is limitation is due to the unclear mode of action of PPE, high costs, lack of curative effect, significant variation among varieties, climatic conditions, cultivation practices, and extraction methods and limited range of activity against different fungal pathogens [9,49].

Biochemical Changes and Food Preservation of Pomegranate Peel
PPE is a valuable by-product for the food preservation industry. As discussed previously, PPE is a rich source of bioactive compounds, including tannins consisting of ellagic acid and gallic acid [42,50,51]. Most of the bioactive compounds, which are abundant in the PPE, were previously tested as natural additives for improving the preservation quality of food [52]. PPE was yet tested alone or in combination with edible films and coatings for food preservation. Renewable, bio-based, environment friendly active packaging systems, which are usually composed of biopolymers such as proteins, lipids, and polysaccharides [53], have been extensively used in food packaging since the  F. oxysporum f. sp. lycopersici Water extract (80%), methanol extract (40%), and ethanol extract (20%) of pomegranate peel Due to the high levels of total phenol and punicalagin contents. [47] P. digitatum Incorporation of 0.361 g water PPE into locust bean gum and chitosan coatings Due to the high contents of phenolic compounds [48] beginning of the 21st century [54]. Legume seeds as a good source of plant proteins (25-28%) have a good potential as a bio-based film [55]. Moreover, incorporation of natural compounds (with high contents of antioxidant activity, phenolic compounds, and essential oils) into bio-based films is known to improve the activity of edible coatings and enhance the storage duration of food products [56]. Among these, fruit peels constitute an essential part, which is a rich source of bioactive compounds. PPEs have also been widely used in the formulation of bio-based edible coatings/films [57]. e PPE was also noted to reduce the water vapour permeability of chitosan-based film material and its antimicrobial activity [58]. Another recent study showed that the incorporation of different concentrations of PPE with mung bean protein films provides a biofunctional edible film for packaging of food products [59]. e incorporation of PPE was reported to give flexibility to the films, increase the thickness and water vapour permeability, and decreases the moisture content. It was determined that PPE retains its semicrystalline structure in bio-based edible films and improves the efficacy of the material [42]. In a similar study, incorporation of PPE in chitosan was tested for food packaging [60]. e addition of PPE to active polyvinyl alcohol (PVA) composite film also resulted in high antioxidant activity and antibacterial ability [61]. e incorporation of PPE (1% w/v) into chitosan (1% w/ v) and alginate (2% w/v) coatings was then reported to improve the postharvest storability of "Allahabad safeda" guava fruits. Results suggested that the edible coatings reduced the respiration rate, protected ascorbic acid content, total phenolics, total flavonoids content, and antioxidant activity, and maintained the overall fruit quality [62]. e phenolics in foods capture free radicals, produced during oxidative stress, and prevent the deterioration of the foods [63]. Several studies with other edible coatings confirm that the edible coatings reduce the respiration rates and help to maintain the postharvest quality of fresh fruits [64,65]. Reduction in respiration rate results with a decrease in the enzyme activity and thereby resulting in a reduction of ascorbic acid oxidation [66] and improves the postharvest storability of foods. Higher respiration rate results in the breakdown of total phenols, and this accelerates the ageing process. us, the reduction of the respiration rate by the edible coatings enriched with PPE results with a high amount of phenolic compounds [31] and improved storability of the foods.
Rather than the direct influence on the pathogens, PPEs also induce plant resistant to pathogens. In one of these studies, PPE was reported to cause a transcriptomic response at orange fruits. It was noted that PPE upregulates 273 significant genes and downregulates eight genes. Changes in the gene expressions were noted to enrich antibiotic biosynthesis and induce defence pathways [67].

Prospective
Pomegranate peel is the waste produced in the process of pomegranate food processing, accounting for 20-30% of the total weight of pomegranate. e high antioxidant activity, inhibition of lipid peroxidation, and broad-spectrum antimicrobial efficiency of pomegranate peel play an intrinsic quality foundation for its development as a food preservative. To make pomegranate peel more widely used in food preservation, the following aspects need to be further studied and explored: (1) Study on active antimicrobial components of pomegranate peel. Pomegranate peel composition is complicated. Although the chemical composition of pomegranate peel was well studied, the number of specific studies about the mode of action is relatively few. us, the direct causes of antimicrobial activities of pomegranate peel are still unclear, and there are other antimicrobial substances in pomegranate peel, which still need further research. (2) Study on the antibacterial mechanism of pomegranate peel. Pomegranate peel has an excellent bacteriostatic effect, but the mechanism of its bacteriostatic effect has not been studied intensely. Furthermore, research on the mechanism of bacteriostatic impact will be helpful to promote the application of pomegranate peel in the field of food preservation. (3) Study on the compatibility of pomegranate peel and other chemical food preservatives. Food preservatives from different sources have a synergistic preservative effect. Studying the optimization of the formula of pomegranate peel and other chemical preservatives can enhance the preservative effect and reduce the amount of chemical preservative.

Data Availability
e data used to support the findings of this study are included within this article.

Conflicts of Interest
e authors declare that they have no conflicts of interest.