Citrus sinensis (L.) Osbeck is a very common cultivar belonging to the Rutaceae family. It is largely diffused in several areas of the world characterized by mild to warm climate conditions. Its abundant worldwide production (up to 107 Tons. per year) and consumption both as the edible part of the fruit and as several types of derivative products imply the production of a huge amount of waste, such as the fruit pomace. Several ways of recycling this material have been developed in recent years: employment as fertilizer, fodder ingredient, and even cloth material. However, the chemical added value of Citrus sinensis peel has been underestimated despite the diversified and significant content of useful chemicals, such as polyphenols, polymethoxylated phenols, glycosylated flavonoids, volatile and non-volatile terpenoids, pectins, enzymes, etc. This work aims to highlight the outstanding chemical potential of Citrus sinensis peel.
- biological activity
- Citrus sinensis
- essential oil
- orange peels
CS is an evergreen tree, 3 to 9 mts. high with sparingly barbed branches, alternate leaves with toothed blades differently shaped, oval or elliptical, connected to the stem by winged-petioles. Axillary flowers are present singly or in whorls of 6 and possess 5 white petals and up to about 25 yellow colored stamens. The pericarp of CS has a spherical or oval shape of 6–10 cm diameter with the color changing from green to yellow-orange during the ripening; the endocarp containing juice sac glands is enclosed within a wrinkled epicarp or exocarp or flavedo containing a great number of essential oil glands protected by a waxy epidermis. Below the flavedo is the albedo, also called mesocarp, a white filamentary tissue composed of tubular-like cells.
The principal industrial application of CS is the production of frozen concentrated juice. The procedure of juice extraction eventually accompanied by the extraction of the essential oil, implies the generation of a major “by-product” constituted by a pomace, mainly containing peels, accounting for up to around 60% w/w of the original fruit mass processed . This huge amount of biomass does pose serious environmental concerns because of its high level of total organic carbon (TOC) and biological oxygen demand (BOD) that make disposal procedures rather complex and demanding from both the legal and industrial points of view. This is because there is an increasing trend to modify the way of approaching this problem by reconsidering the post-production orange pomace more like a by-product rather than a waste. In the last years, many producers have subjected this material to processings involving partial acidic fermentation, drying, and packaging to biologically and chemically stabilize the biomass before its application as animal feed in zootechnics, soil conditioners in agriculture, or the manufacturing of compost and biogas .
Beyond the standard workup of the
In the following sections, the chemical structures and the biological effects of these compounds will be discussed.
2. The chemistry of
2.1 Essential oils
The essential oil (EO) is mainly obtained from the CS peel as a major by-product of the juice production process by a cold-pressing method that can provide the intact blend of compounds without losing the lighter, more volatile, components of the complex mixture that can be lost in the standard EO extraction procedure that is the hydrodistillation. The last one is mainly used in small scale applications, for example in research laboratories.
The chemical composition of CSP EO [13, 14, 15] is reported in Table 1. As it can be seen, the major component is D-Limonene, accompanied by several minor components belonging to the classes of monoterpene alkenes, oxygenated monoterpenes including alcohol aldehydes and esters, sesquiterpenes as well as linear alkanes and aldehydes. This rather complex blend accounts for the numerous deal of biological activities reported for the CSP EO [14, 15, 16], which include anthelmintic, anti-aflatoxigenic , antibacterial [18, 19, 20], anticarcinogenic, antifungal , antioxidant , anti-tumor , anxiolytic , food preservative ,
hepatocarcinogenesis suppressant, insecticidal and larvicidal , pain relief and relaxant . It can be argued that the main effects are due to the presence of the major component Limonene that showed several bioactivities when tested as pure compound . However, it is possible that synergistic effects due to the combination of Limonene with other minor components may be speculated and should have to be demonstrated.
|Comp.||Comp. name||%||Compound.||Comp. name||%|
Polyphenols extracted from the CS peel belongs to the general structural categories of flavanones (Figure 1a), flavones (Figure 1b), flavonols (Figure 1b), with and without sugar moieties attached to one or more of the hydroxyl groups . It is worthy of particular mention the rarely occurring class of C-glycolflavones (Figure 1b, compounds
These compounds are produced
The composition of the peel extracts described in the literature may slightly vary depending on the cultivar and the region of harvesting but some general points are common, that is the presence of the high amount of bioactive polymethoxyflavonoids [29, 30](PMF) some of which are rather ubiquitous, e.g. Nobiletin
The biological role of these secondary metabolites in the plant is still matter of debate. It has been proposed their involvement in the mechanism of defense of the fruits exposed to the attack of phytopathogens, such as
Further, the composition of the PMF blend can be employed for the chemiotaxonomic characterization of the
However, it needs to be stressed that in many cases the reported compounds were recognized by mass spectrometry and electronic spectroscopy. It is not always a matter of simplicity to discern the exact structure of a given PMF and to discriminate between different regioisomers, normally quite similar in terms of mass and electronic spectra, if an isolation procedure is not conducted and followed by a complete bi-dimensional NMR characterization. Significant differences in the extract composition do arise also in consequence of the extraction method; non-polar solvents such as Methanol, Chloroform Ethyl acetate let to obtain PMFs-rich extracts while, on the other hand, hydroalcoholic and aqueous extracts do contain a low concentration of PMFs and a higher concentration of un-methylated polyphenols as well as glycosylated compounds.
The biological activities disclosed for the flavonoids extracted from CSP are variegated. They include antioxidant [9, 34, 35, 36, 37, 38, 39], anti-inflammatory [40, 41], antimicrobial [39, 42, 43, 44], antimalarial , antitrypanosomal , cardio-protective , anti-osteoporosis , anti-ulcer , vascular protective , anti-diabetes [51, 52], hepatoprotective [53, 54], neurotrophic , anti-adipogenesis and anti-obesity [56, 57, 58], anti-hypertensive , cataract prevention , sun protection , metabolic syndrome control . Further, it has been demonstrated  that while both flavonoid set
The most abundant PMF occurring in CSP, Nobiletin
PMFs can be considered as especially promising lead compounds for cancer therapy as asignificant cytotoxic activity has been demonstrated toward a number of cancer cells [70, 71] with several mechanisms of action [72, 73]; the cytotypes investigated include MCF-7 [73, 74, 75, 76], Hs578T triple-negative breast cancer [73, 77]; colon cancer cells CaCo-2 , LoVo , HTC-116 [79, 80] and HT-29 [79, 81]; lung cancer cells A549 [80, 82], H460 [82, 83], H1299 [82, 83]; gastric cancer cell lines AGS, BGC-823, and SGC-7901 ; leukemia cells HL-60 . However, data regarding a possible antitumor activity
Given the development of pharmacological applications of CSP extract components, further investigations are needed to better understand the bioavailability, safety, and efficacy of these compounds in humans. Most of the data reported concern
In general, it should be emphasized as the body of evidence concerning the actual efficacy of sweet orange-derived compounds in human health is still far to be exhaustive. For example, while this work is under typewriting, a severe acute respiratory syndrome pandemic due to a COVID-19 virus is in act and a big deal of research has been being directed toward antiviral remedies and therapies. Research on nutraceuticals is not an exception and in particular some authors have shown by computational and molecular docking methods how Hesperidin
The pharmacological potential of pure Hesperidin
Several hydroxylated carboxylic acids belonging to several structural sub-classes are present foremostly in the extract obtained with mixed hydro-organic solvents, such as MeOH/water and EtOH / water [37, 38, 51, 78]; these include the aliphatic Ascorbic, Citric, Kojic, Lactic, and L-Malic acids; the aromatic 4-Hydroxybenzoic, Protocatechulic, and Gallic acids. Further, the cinnamyl compounds (Figure 3) Cinnamic (
These organic acids are mainly found in free form but in some cases, they are esterified with a variety of alcoholic compounds, such as Ethanol in Ethyl gallate
It was shown  that the antioxidant properties of a CSP extract better correlated with the total phenols content (TPC) of the sample rather than with its total flavonoid content (TFC), as it can be expected from the known relevant antioxidant character of hydroxycynamic derivatives.
Coumarins are aromatic compounds biogenetically related to the o-hydroxysubstituted cynamic acids from which originate by the intramolecular condensation between the carboxylic and the o-hydroxy groups. These compounds are most commonly encountered in other species of
The NADPH dependent bioreduction of flavanols is the biogenetic origin of this class of compounds, present as minor constituents in CSP extract possessing significant antioxidant activity ; they are the two enantiomeric forms Catechin
Pectins  are chemically definable as complex mixtures of polyglyconic acids in which a linear polymeric backbone is structured by a series of α (1 → 4) linkages (Figure 6). The main sugar monomer is always Galacturonic acid with the presence of possible heterogeneous domains of other sugars such as Xylogalacturonan and Rhamnogalacturonan-I. A variable amount of the free carboxy functions may be esterified with methyl groups, while the hydroxy groups at C-2 and C-3 positions of the sugar monomers may be acetylated. Even though the primary structure of the main chain is linear, a possible degree of ramification, depending on the pectin source, may also be found. The differences in the pectins composition and structures, depending on their natural source, do confer them different physio-chemical properties, such as water solubility, sol–gel concentrations, etc. On the ground of the degree of methylation of the acid moieties, pectins are classified as “low methoxyl” (LMP, -COOMe/-COOH <50% mol.) or as “high methoxyl” (> 50% mol). A simplified representation of pectin structure is given in Figure 6.
Pectins find many applications in the food and drug industry as a thickening and gelling agents, excipients, and colloidal stabilizers .
As it has been already mentioned, the extraction method does affect the structure and the properties of the final product; the traditional acidic water extraction implies a certain degree of hydrolytic deterioration, so that new extraction technologies have been being investigated to improve the quality of the final pectins, that is microwave-assisted extraction (MAE)  and ultrasounds assisted extraction (USAE) [35, 95].
As it can be easily argued, the CSP cellular system, whose genomic profile has been fully characterized , is the site of a complex network of enzymatic activity. Some of the enzymes of CSP have been characterized and employed in many applications.
The acetylesterase (international enzymatic classification: EC 126.96.36.199) from CSP is known since 1947  and was isolated and characterized . The acetylesterase activity of the partially purified enzyme was used for the removal of the acetyl group at the 3 positions of β-lactamic antibiotics
Recently, partial purification and functional characterization of a Uronic acid oxidase from CSP was accomplished ; this enzyme promotes the oxidation by O2 of Galacturonic acid
2.5.1 Highly lipophilic compounds
The waxy environment of flavedo in CSP does contain several long-chain saturated and unsaturated compounds: alkanes, fatty acids, waxes, higher terpenoids.
Tetracosane, Tetratriacontanoic acid, and Ethyl pentacosanoate were identified in CSP of a Pineapple variety . Further, some carotenoids were identified in the CSP extract obtained with a solvent mixture composed of Ethanol, Ethyl acetate, Petroleum ether 1: 1:1 . This complex blend of carotenoids includes α- and β-Carotene, Phytoene, Phytofluene, (all-E)- and (9Z)-Violoxanthin, (all-E)-Neoxanthin, (13Z)-, (13Z’)- and (all-E)-Lutein, (9Z)-Zeaxanthin, (all-E)-Zeaxanthin; the mono and di-esters of violaxanthin, antheroxanthins, Xanthophyll, β-Citraurin with various fatty acids, including Lauraic, Myristic, Oleic, Palmitic, Stearic. The composition of the blend has been correlated with the maturity stage of the fruit.
Three cyclic peptides have been isolated from the hot water extract of CSP and were structurally characterized by FAB-MS and 2D-NMR techniques . Their amino-acidic sequences, including a mostly lipophlic heptapeptide
The chemical richness of the primary and secondary metabolome of
The chemical composition of the extract from the exocarp of
It is also a matter of fact that several interesting bioactivities were disclosed in the last years for the
Conflict of interest
The author declares no conflict of interest.