1. Introduction
The pomegranate (
Punica granatum L.) is a typical cultivation of Asian and Mediterranean countries and of Middle East and some USA areas [
1,
2]. Pomegranate fruit is composed of about 80% of water, 15% of carbohydrates, mainly sugars (like fructose, sucrose and glucose), and the remaining part is represented by fibers, vitamins (like vitamin C) and natural bioactive compounds (NBCs), namely polyphenols [
3].
Although the pomegranate is present in our territories since ancient times, the data of the Italian “Istituto Nazionale di Statistica” (ISTAT) show that until 10 years ago this kind of crop was uncommon in Italy [
4]. Initially, pomegranate was considered a simple ornamental plant, while now it has registered a growing trend in its consumption and this is due to the healthy properties emerged from numerous scientific studies that have highlighted the antioxidant and anti-radical activities of this “superfood”. Thanks to the high content of NBCs, the consumption of pomegranate appears to exert significative beneficial effects on human health, allowing to strengthen the immune system and to counteract the free radicals formation, responsible for many pathophysiological processes [
5].
According to Aviram et al [
6], it is estimated that in 1 kg of pomegranate, about 40% is represented by juice, while the components discarded amount up to 60%. Of the latter, the 50% is represented by mesocarp, endocarp and exocarp, while the 10% is represented by the seeds of the arils. These matrices, considered wastes from the agro-industrial sector, if properly managed, represent a precious resource, as they are a source of bioactive polyphenolic molecules, such as flavonoids, anthocyanins, tannins and in particular ellagitannins (like punicalagina) [1,7-9]. These compounds have been identified using various analytical techniques of liquid chromatography-mass spectrometry (LC-MS), like the liquid chromatography-tandem mass spectrometry (LC-MS/MS) and the liquid chromatography-high-resolution-mass spectrometry (LC-HR-MS) [
10,
11]. In recent years, the use of LC-HR-MS instrumentation is increased rapidly for the qualitative identification of polyphenolic compounds present in plant matrices, such as the cultivation object of this work, which is particularly rich in ellagitannins.
The chemical composition of pomegranate differs, based on the variety and cultivation conditions. The peel, comprising 50% of the fruit's weight, is rich in phenolic compounds, such as phenolic acids, flavonoids and tannins. Flavonoids, notably anthocyanins in arils, contribute to the red colour and offer antimicrobial and antioxidant properties. Pomegranate components, including tannins, punicalagin, punicalin, strictinin A and granatin B, inhibit nitric oxide production and suppress inflammatory cytokine expression, due to ellagic acid (EA) action [
12].
This review focuses on the biological activities of the bioactive polyphenolic compounds present in pomegranate and on the possible beneficial effects of these compounds in internal medicine and dentistry.
3. Bioactive Polyphenolic Compounds of Pomegranate and Their Biological Activities
Pomegranate is rich in bioactive polyphenolic compounds, which exert numerous biological activities, demonstrated by several i
n vitro and
in vivo studies (
Figure 1).
Among the polyphenolic classes of pomegranate, there are Ellagitannins. They belong to the subclass of hydrolysable tannins and represent one of the most diverse groups of polyphenols derived from plant matrix, as they comprise over 1,000 identified NBCs [
13]. Tannins, in general, exert a function of defense for the plant against attack by pathogens and herbivores. In fact, these compounds, including ellagitannins, are found in the vacuoles and in the cytoplasm of plant cells and play an important protective role in natural growth conditions [
14]. Furthermore, these compounds, with strong astringent properties, are capable of complexing proteins and polysaccharides [
15], they have a molecular weight between 300 and 20,000 Da and are found in the form of monomers (C-glycosidic ellagitannins with an open chain glucose portion), oligomers and complex tannins [
15,
16] but all these molecules have in common the presence in the chemical structure of at least one HHDP unit esterified in a polyol, generally glucose or quinic acid [
17]. Several antimicrobial and antiviral properties are recognized in this subclass [18-21] that exert also beneficial effects in the prevention of chronic non-communicable diseases [13,18-22].
To have a complete picture of their use in the biomedical field, scientific studies have also taken into account the bioavailability of these molecules, characterized by a complex structure that does not permit their absorption from the gut microflora [
23,
24]. Their effect can be attributed to the fact that these compounds undergo a hydrolysis process in the digestive system, which transforms ellagitannins into smaller and less complex compounds, such as EA and urolithin, whose main effect on human health is attributable to the antioxidant and antiradical capacity. These molecules are able to counteract the action of free radicals and reactive oxygen species (ROS) [
25].
The main bioactive compound present in pomegranate wastes is punicalagin [
26,
27], which, like the various polyphenolic compounds present in plant matrices, shows to have numerous biological and functional properties: antioxidant [
20,
28], antiviral [
28] anti-inflammatory [
29,
30], antidiabetic [
31], anticancer [32-34], cardio-protective [35-37] and antimicrobial [38-43].
The punicalagin is part of the subclass of ellagitannin and is the most representative compound within the pomegranate waste, in fact it represents about 70% of the total ellagitannins in the peel of the fruit. Punicalagin (C48H28O30) is a water-soluble polyphenolic compound, with a high degree of hydroxylation and high molecular weight, equal to 1084.7 Da. This compound is naturally found in the forms of two α and β reversible anomers [10,44-46].
Another of the most representative compounds in pomegranate fruit is EA (C
14H
6O
8), which is a dimeric derivative of gallic acid (C
7H
6O
5) [
47]. EA is a thermostable molecule with a melting point of 350.3 C and a molecular weight of 302.19 g/mol, which is chemically identified as 2,3,7,8-tetrahydroxy-chromium [5,4,3-cde]chromene-5,10-Dione [
21]. EA is a naturally occurring bioactive polyphenolic compound that is a secondary metabolite in many plant matrices.
This compound, identified as a dilatton of hexahydroxydiphenic acid (HHDP), showed beneficial effects in
in vitro and
in vivo models and it is characterized by a high free radical scavenging activity, as reported by Fischer et al. [
27].
In addition to the antiradical ability, this compound has attracted particular attention from the scientific world in consideration of its antioxidant, anti-inflammatory, antimutagenic, antiproliferative, cardioprotective, hepatoprotective, nephroprotective and neuroprotective properties.
Both compounds most present in pomegranate wastes, such as punicalamine and EA, show to have an inhibitory activity of α-glucosidase [
48], while another compound present in pomegranate wastes, in lower amounts, is gallic acid (GA).
Individual samples of pomegranate extracts can give different responses in tests for the evaluation of functional and biological activities and properties. This is due to the fact that these properties are related to the content of the polyphenolic compounds in the extracts, which can vary depending on the cultivars, as well as on the extraction procedures [
40,
49].
Furthermore, some scientific studies have compared the antioxidant activity
in vitro on cell lines of single analytical standard of punicalagin and EA and of
Punica granatum L. extracts. The authors demonstrated a superior bioactivity of the latter [
20] and this evidence confirms, as highlighted in tests relative to other vegetal matrices, that the phytocomplex has a greater effectiveness, compared to the purified standard.
4. Sustainable Application of a Circular Economy Model for Pomegranate Wastes Recovery
In recent years, scientific evidences have led to an increased demand for sustainable products with remarkable healthy properties, highlighting the need to recover polyphenolic compounds. The latter origin from waste matrices in order to promote the sustainability goals, through the application of circular economy principles based on "zero waste” concept. Waste materials coming from pomegranate cultivation and mainly consist of plant tissues, such as exocarp, mesocarp, endocarp and seeds found in the arils.
Literature data showed that pomegranate wastes present biological activities ; among these, there are antioxidant, anti-inflammatory, antiproliferative and antimicrobial ones. For this purpose, the recovery models of pomegranate peel through the integration of extraction and purification phases, during the transformation processes, have been developed. These processes aim to obtain concentrated fractions rich in hydrolysable tannins as active principles for various purposes. In fact, these compounds can be used primarily as food additives, secondarily as functional foods ingredients, useful in the formulation and in the prototipation of products, bound to different markets [50-53].
Moreover, as for pomegranate, as well as for other plant matrices, it is possible to use dried and micronized wastes, in order to avoid the extraction process [
54].
Beyond the primary utilization of high-quality fruits in the fresh market, the cultivation of
Punica granatum L. can involve various processes. These include the innovative production of arils through a functional green withering, the production of juices obtained through cold pasteurization, as well as concentrates or innovative based gels, and finally, the production of depleted seed oil and/or flours for feed and food use. An appropriate dried process of fresh peels can lead to the production of powders such as micronized, natural pigmenting principles and standardized phyto-complexes in the hydrolysable tannin content, extending up to the energy production (
Figure 2).
6. Pomegranate and Oral Health
Pomegranate has garnered a significant attention in the field of dentistry for its potential therapeutic properties, particularly in the treatment of dental plaque, gingivitis and periodontitis. The literature data about this fruit have revealed a multitude of benefits, making it a valuable candidate to promote oral health.
Dental plaque, a biofilm of bacteria that forms on tooth surfaces, is a primary contributor to oral health problems. Pomegranate has been studied extensively for its ability to counteract the dental plaque formation. The flavonoids present in pomegranate demonstrated a robust antibacterial action against key contributors to the plaque formation, including
Streptococcus sanguis and
Eikenella corrodens [
136,
137]. Its antibacterial effect is attributed to tannins, compounds that enhance bacteriolysis and impair the bacterial adherence to tooth surfaces. Comparative studies have highlighted the pomegranate's superior inhibition capacity towards specific bacteria, compared to chlorhexidine (CHX), a commonly used oral antiseptic [
138]. Despite limited research about pomegranate's impact on plaque and on salivary proteins, a study conducted on adolescents revealed that pomegranate mouthrinse significantly reduced plaque accumulation and gingivitis, compared to a placebo. This finding aligns with existing literature, suggesting pomegranate extract's potentiality as an adjunct therapy for treating gingivitis with minimal side effects [
139].
Moreover, pomegranate mouthwash has proven to be effective against periodontal pathogens, such as
Aggregatibacter actinomycetemcomitans, Porphyromonas gingivalis and
Prevotella intermedia [
140]. Notably, pomegranate's role in inhibiting quorum sensing in bacteria, a process vital for gene expression related to antibiotic resistance and to biofilm development. This pomegranate function adds an intriguing dimension to its antibacterial properties [
141,
142].
Orthodontic patients, who often face challenges in maintaining oral hygiene, have benefited from a hydro-alcoholic extract of pomegranate, demonstrating a substantial reduction in dental plaque bacteria, compared to CHX [
143]. The ellagitannin punicalagin has been identified as a key contributor to pomegranate's antibacterial activity [
20].
Beyond its antibacterial prowess, pomegranate exhibits a positive synergistic effect with antibiotics against methicillin-resistant
Staphylococcus aureus (MRSA) and methicillin-sensitive
S. aureus, showcasing its potentiality as a complementary agent in the battle against antibiotic-resistant strains [
144].
Gingivitis is characterized by chronic inflammation and early clinical manifestations like gum bleeding [145-147]. This oral ailment leads to increased salivary albumin, cystatin C and amylase. The enhancement of the latter is due to plasma protein leakage into the gingival crevicular fluid, thus offering a non-invasive diagnostic tool.
Clinical studies affirm pomegranate efficacy in reducing gingival bleeding and in diminishing colony-forming units (CFUs) of dental plaque organisms [
148,
149]. Mouthrinse containing pomegranate extract has also demonstrated effective in reducing bacterial protein levels and activities related to cell injury, increasing the antioxidant enzymes activity, thus highlighting its potential role in oral health maintenance [
118,
150].
For periodontitis, studies emphasize pomegranate's anti-inflammatory effects and its potentiality as adjuvant treatment to the conventional periodontal therapy [
151]. Biochemical investigations reveal significant decreases in inflammatory markers (IL-1β and IL-6), indicating pomegranate potentiality in managing periodontal disease [
152].
Pomegranate also enhances the growth of enteric probiotic bacteria, suggesting potential benefits in decreasing the periodontal pathogen load [
153]. Its antibacterial activity extends to
Helicobacter pylori, associated with deep periodontal pockets. While, its anti-viral properties may contribute to treat the periodontitis triggered by viral infections [
154,
155].
For chronic periodontitis, pomegranate-based compounds have also proven effective. In fact, clinical studies have demonstrated the effectiveness of pomegranate extract in reducing the inflammation in this chronic condition, inhibiting the periodontopathogens. Moreover, it has shown its action against microbial growth, including the growth inhibition of S. aureus and of MRSA strains [148,156-158].
Pomegranate's applications extend even further. It has demonstrated the ability in treating recurrent aphthous stomatitis, promoting wound healing and acting as a medium storage for avulsed teeth [159-161]. Its several applications grant pomegranate to become a promising candidate for various oral health issues.
An optimal therapeutic agent for plaque control must fulfill selected criteria, including specificity for plaque bacteria, substantivity, stability, lack of adverse reactions, toxic safety, ecological safety and user-friendly features [
162]. Pomegranate seems to well align with these criteria [
12].
Research supporting pomegranate's role in oral health is primarily limited to
in vitro studies, although promising
in vivo studies exist. Pomegranate rinsing reduces α-glucosidase activity and increases ceruloplasmin activity in saliva [
12]. Pomegranate extract effectively treats denture stomatitis, associated with candidiasis [
163]. Pomegranate tannins inhibit human salivary α-amylase, a substrate for cariogenic microbes [
164,
165]. Studies on periodontal therapy support the efficacy of a gel containing extracts of
Centella asiatica and
Punica granatum [
152]. Chewing pomegranate seems to enhance antibacterial and antioxidant effects and to boost salivary flow rate. Pomegranate flower extract inhibits the bacterial sucrose-digesting enzyme linked to dental caries and gingivitis [
166]. Antioxidant agents from pomegranate are hypothesized to have preventive effects against oral cavity diseases [
167]. Pomegranate extracts reduce aspartate aminotransferase activities, suggesting benefits in periodontal pathology [
168]. Hydroalcoholic extracts from pomegranate fruit significantly decrease dental plaque CFUs, offering an alternative for reducing plaque bacteria [
143]. Pomegranate contributes to the maintaining of the oral hygiene and reduces microorganisms cultured from dental plaque in one-minute rinses with a mouthwash containing pomegranate extract [
150].
Punicic acid in PSO acts as an anti-inflammatory agent by down-regulating neutrophil activation and LPO [
169,
170].
Tooth decay, initiated by
Streptococcus mutans in the oral cavity, is a prevalent chronic condition, affecting children and young adults. Conventional mouthwashes, like CHX, possess antimicrobial properties but are also associated with drawbacks, such as staining and taste alterations. A recent study explores the antibacterial potentiality of pomegranate peel and guava leaves extracts, comparing them with CHX in a group of children. The results indicate a significant reduction (p < 0.01) in
S. mutans count after using pomegranate and guava leaves extracts. The beneficial effects seem to be directly correlated with the concentration of these extracts. However, the efficacy of CHX outperformed both extracts, emphasizing the need for further research on combined antimicrobial approach [
171]. In fact, CHX is the gold standard for its potent antibacterial and antiplaque properties, but its use is associated with certain side effects. Among these, xerostomia (dry mouth), hypogeusia (reduced sense of taste) and discoloration of the tongue. In particular, long-term use of CHX may lead to the development of calculus on teeth and extrinsic tooth staining. In this contest, it is important to explore alternative oral care solutions with fewer associated side effects [
172,
173]. Among these, herbal products have been investigated, where the pomegranate seems to represent one of the most effective herbal remedies. At this regard, a study aimed to evaluate the ability of pomegranate mouthrinse compared to 0.12% CHX mouthrinse. In detail, this study examined the impact of these solutions on the reduction of bacterial plaque and the gingivitis, among individuals aged 18–25 years. The results demonstrated antiplaque and antigingivitis effects of both mouthrinses. In particular, pomegranate showed a superior antigingivitis power, but it did not match the antiplaque effectiveness of CHX. Although pomegranate mouthrinse represents a convenient and styptic option with acceptable plaque reduction, further clinical trials are warranted to establish its real action and its advantages over CHX, the current gold standard for the plaque control [
174].