Chemical Content and Pharmacology of Sweet Orange ( Citrus sinensis ) Fruit Peel: A Review

. The sweet orange ( Citrus sinensis ) fruit peel is one of the often overlooked part of the sweet orange plant. However, recent research indicates that sweet orange peel possesses various bioactive properties and intriguing health benefits. The purpose of this article is to provide a comprehensive overview of the chemical compounds, pharmacological activities, and potential development of sweet orange peel, serving as an introduction to further research. Sweet orange peel contains a variety of chemical compounds, including essential oils, flavonoids, carotenoids, steroids, terpenoids, alkane groups, and ethyl esters. These chemical compositions confer antioxidant properties to sweet orange peel, which can protect the body from oxidative damage caused by free radicals. In vivo and in vitro studies have demonstrated that sweet orange peel extracts exhibit strong antioxidant activity and may aid in preventing degenerative diseases such as cancer. Additionally, sweet orange peel shows potential antimicrobial activity. Certain compounds in sweet orange peel have proven effective against various types of bacteria and pathogenic fungi.


Introduction
Sweet orange is considered one of the most beloved and popular fruits worldwide.Its sweet taste when ripe is a unique attraction for consumption by people of all ages, from children to adults.This has led to a large market demand, automatically increasing the commodity value for this plant.The global production of oranges is estimated at 115 million tons each year [1].Another reason why sweet oranges are a preferred choice for consumption is the presence of active compounds such as phenolics, flavonoids, carotenoids, vitamin C, and polysaccharides, which play crucial roles in human nutrition [2][3][4].Additionally, it contains folate and thiamine (vitamin B), contributing to nutritional needs [5].The high level of sweet orange production poses another problem, namely that the consumed part of the fruit is only the flesh, leaving the fruit peel as a significant waste.If not properly managed, this large amount of wast 1 e can contribute to environmental pollution.However, this overlooked fruit peel still contains active compounds comparable to the fruit flesh.Several studies have been reported on the peel of sweet oranges, covering chemical components such as essential oils, flavonoids, carotenoids, steroids, terpenoids, alkane groups, and ethyl esters.Additionally, pharmacological activities including antioxidant, anticancer, antimicrobial, and anti-inflammatory properties have been investigated.These Corresponding author: sutrisno.kimia@um.ac.id aspects will be the focus of discussion in this article, aiming to provide a comprehensive overview of the chemical compounds, pharmacological activities, and potential development of sweet orange peel.This serves as an introduction to further research, with the goal of transforming what has traditionally been considered waste into a product with commercial value.

Method
The method employed in this study is based on a literature review, utilizing book searches, journals, and scientific works through databases such as Google Scholar and Online Library.The search was conducted using keywords such as Citrus sinensis, chemical content, and pharmacological activities.This approach allowed for a comprehensive exploration of existing knowledge and research related to the chemical composition and pharmacological activities associated with Citrus sinensis.

Taxonomy and Morphology
The sweet orange is a plant belonging to the Rutaceae family.Common characteristics of this plant include reaching a tree height of 3-10 meters, having short thorns almost all over the plant, leaf stalks measuring about 0.5-3.5 cm in length with varying leaf shapes (elliptical, elongated with blunt pointed tips, and egg-shaped).The fruit is round in shape, green when young, and turns yellow or orange when mature or ready for harvesting, with the edible flesh being orangish-yellow in color [6]  The orange fruit generally consists of two main parts: the peel and the fruit flesh.The outermost part of the orange fruit is called the peel.The peel is divided into two parts: albedo and flavedo.The inner layer of the peel has a spongy tissue and is called albedo, while the outermost layer of the peel contains oil sacs and chromoplasts and is called flavedo.The edible part of the fruit is the fruit flesh or endocarp.Within the endocarp, there are fruit juice sacs with thin layers or walls called carpels or locules [8].

Chemical Constituents
The chemical content of sweet orange peels has been reported.Sweet orange peels contain various chemical compounds, with secondary metabolites taking the focus in this discussion.The compounds present in sweet orange peels consist of essential oils, flavonoid compounds, steroids, terpenoids, alkanes, and ethyl ester.
It should be noted that other identifications of essential oils from sweet orange peel have indicated the presence of the monoterpenes group, namely, α-thujene (1.6), α-pinene (1.

Terpenoids
Sweet oranges have a distinctive aroma attributed to the presence of triterpenoid compounds, specifically limonoids (4.1), which are a group of terpenoids found in their peel [16].

Alkanes and Ethyl ester
The groups of alkane and ethyl ester were also identified from sweet orange peel was extracted using hot methanol, namely tetracosane (5.1) and ethyl pentacosanoate (5.2) [17].

Pharmacological Activities
Pharmacological investigations into the sweet orange peel and the chemistry of its main compound groups, namely essential oils, flavonoids, and other compound categories, have been extensively conducted by researchers.The research findings have shown that these 6 E3S Web of Conferences 481, 06002 (2024) ICSChem 2023 https://doi.org/10.1051/e3sconf/202448106002compounds possess beneficial pharmacological properties that align with their traditional medicinal uses long recognized by communities.

Antioxidant
The presence of flavonoid hydroxylation and flavonoid polymethoxylation in sweet orange peels indicates high antioxidant activity in reducing DPPH radical agents [16].Decanal, linalool, valencene, and octanal isolated from sweet orange peels exhibited good inhibitory values against DPPH with an IC50 value of 66.10 mg/mL [11].Similar findings were revealed in other research studies, where sweet orange peels extracted using acetone-water solvent were able to inhibit DPPH radicals and showed the ability to reduce ferric ions (Fe 3+ ) to ferrous ions (Fe 2+ ) [3].The antioxidant activities of methanol and ethanol extracts from sweet orange peels showed significant free radical scavenging activities, as determined by ABTS with values of 55.8% and 60.7% respectively, and DPPH scavenging activities based on their hydrogen donating abilities at 70% and 80%, where ascorbic acid was used as a control [21,22].The ethyl acetate fraction exhibited the highest antioxidant activity due to the presence of flavonoid C-glycosylation, flavonoid O-glycosylation, flavonoid polymethoxylation, flavanone O-glycosylation, and phenolic acid esters [23,24].
In the antifungal study of sweet orange peel, it has also been stated that there is good inhibitory activity, as reported in [29,30].Isolated essential oil functions as a good antifungal against Aspergillus niger, which is a filamentous fungus causing post-harvest damage to oranges.It has also been reported that ethyl acetate extract from sweet orange peel can inhibit the growth of the fungus Candida albicans [28].Hexane extract from sweet orange peel showed activity against Penicillium digitatum and Cladosporium cucumerinum [31].Another study indicated that a combination of oils (1:1) from C. maxima Burm and C. sinensis obtained by hydrodistillation resulted in 100% inhibition of mycelium growth of Aspergillus fumigatus, A. terreus, Alternaria alternata at 750 ppm [32].Polymethoxy flavones obtained from C. sinensis peel extract (flavon-7-O-[6-acyl]-glucoside, tetramethyl-O-scutellarein, nobiletin, natsudaidai, tangeretin, heptamethoxyflavone) showed inhibitory activity against the growth of Aspergillus niger [33].In practical applications, the addition of orange peel powder to wheat flour can extend cakes self life due to its high antioxidant activity [34].

Antiinflamation and antiosteoporotic
Reportedly, essential oil from sweet orange peel has shown anti-inflammatory activity in egg albumin-induced rat edema [35].In vivo analysis of ethanol extract of C. sinensis peel (5 mg/kg) in ovariectomized rats showed an increase in trabecular bone mineral content, compacted tibial bone mineral, and increased levels of phosphorus and calcium, thereby preventing and reducing bone resorption.[36].

Conclusion
With the increasing public interest in a healthy lifestyle and the rise in consumption of functional foods, harnessing the potential of sweet orange fruit peel presents a relevant solution.The compounds present in sweet orange fruit peel, such as essential oil, flavonoid groups, steroids, and terpenoids, have demonstrated good pharmacological activities as antioxidants, anticancer agents, and antimicrobials, as well as anti-inflammatory and antiosteoporosis agents.As a result, sweet orange fruit peel can be utilized as a component of herbal medicine and is believed to have fewer harmful effects compared to synthetic drugs.Both the powder and extract of sweet orange fruit peel can be developed as food additives to extend shelf life due to their antimicrobial activity.Their antioxidant capabilities can be applied as a sunscreen.The potential as an anticancer agent could offer an affordable herbal treatment alternative, given that current chemotherapy treatments for cancer are very expensive and have adverse effects on the body.