Citrus Nutritional and Nutraceutical Importance

#### **Chapter 9**

## Citrus Fruits: Nutritive Value and Value-Added Products

*Abu Saeid and Maruf Ahmed*

#### **Abstract**

Citrus fruits are essential sources of food and energy and play a critical role in supplementing healthy diets. Citrus fruits contain mostly carbohydrates such as sucrose, glucose, and fructose and are good dietary fiber sources, which help prevent gastrointestinal disease and promote high circulating cholesterol. Besides, citrus fruits are also significant sources of vitamin C and various bioactive compounds. It is suggested that these components are of vital importance in improving human health due to their antioxidant properties and being converted to vitamin A. However, citrus fruit is still being used for different purposes like juice, jam, jelly, squash, pies, cake, candies, marmalades, etc. Most citrus waste materials are currently used as animal feed. Innovations are occurring in the conversion of citrus by-products into valuable commodities with the development of innovative technologies. This chapter has put up primary and secondary research findings of citrus fruits, especially lemon and pomelo, their chemical properties, composition, and their use in health and cosmetic needs.

**Keywords:** citrus, lemon, pomelo, nutritional properties, value-added products

#### **1. Introduction**

Citrus is an evergreen shrub that belongs to the Rutaceae family from South Asia, China, India and the Malay Archipelago, which is native to the subtropical and tropical regions of Asian regions [1]. The genus of citrus includes sweet orange (*C. sinensis*: 61.1 % of world citrus production), tangerine (*C. reticulata*: 19.9 %), limon and lime *(C. limon* and *C. aurantifolia*: 12.1 %) and grapefruit (*C. paradisi*: 5%). Minor types of citrus, which constitute much of the remaining 2.0%, include sour orange (*C. quarantium*), shaddocks (*C. grandis*), citrus (*C. medica*), which seem to be promising sources for many beneficial human nuts [2]. Citrus fruit is divided into two sections like peel and flesh (**Figure 1**). Peel is made from epicarp or flavedo (colored peripheral surface) and mesocarp or albedo (white soft middle layer). The peel (60–65%), internal tissues (30–35%), and seeds (0–10%) comprise citrus fruits [3]. Citrus fruits provide carbohydrates, such as sucrose, glucose, and fructose mostly. Fresh citrus fruits are also an immeasurable source of dietary fiber associated with gastrointestinal disease prevention and lowered circulating cholesterol. Citrus fruits also have a distinct aroma and delicious taste along with low protein and fat content.

Citrus fruits also provide the most potent source of vitamins C and B (thiamines, pyridoxines, niacins, riboflavin, pantothenic acids, and folate). The fruit also leads to the use of phytochemicals, such as carotenoids, flavonoids, and limonoids [1].

**Figure 1.** *Structure of the citrus fruit [2].*

Citrus phytochemicals contain antibacterial, antiviral, antifungal, anti-carcinogenic, anti-thrombotic, or anti-inflammatory agents [4]. Several studies have proposed citrus fruit evaluation as a healthy and delicious diet [5]. Prior research suggested that citrus and citrus products are rich sources of vitamins, minerals, and dietary fibers [6]. However, the bioactive and non-nutrient compounds in citrus are appreciated to reduce the risk of various chronic diseases [7].

Citrus fruits are eaten as fresh goods and juice throughout the world. Peel is discarded as waste containing many secondary components with significant antioxidant activity related to other fruit portions [8]. In recent years, flavonoids such as polymethoxy flavones (PMFs), which are present in citrus fruits, have been attracted growing attention by their antioxidants [9] and anti-cancer properties [10]. Various bioactive compounds in citrus peel extract and powder may reduce overall cholesterol, triglycerides, LDL, and glucose levels [11]. Citrus by-products produce a range of value-added products, including essential oils, pectin, enzymes, single-cell collagen, natural antioxidants, ethanol, organic acids, and prebiotics. Orange, lemon, mandarin, and grapefruit contained essential oils show antifungal activity upon the fungi *A. niger*, *A. flavus, P. chrysogenum,* and *P. verrucosum.* The essential oil may be regarded as acceptable for the food industry as alternatives to chemicals [12]. Pectin extracted from *Citrus* peel is used in various industrial food processes as gelling agents, including jam, jellies, and as thickener, texturizer, emulsifier, and stabilizer in dairy products. Pectin is also used to jellify properties in the pharmaceutical, dental, and cosmetic industries [13]. Therefore, this chapter highlighted the nutritional values of major essential nutrients such as Vitamin C, carotenoids and vitamin A, Folate, Dietary fiber, flavonoids, and limonoids, as well as value-added products such as food ingredients, pectin, essential oil, enzymes, a natural antioxidant, and packaging film retrieved from citrus especially lemon and pomelo fruit.

#### **2. Characteristics of citrus fruits (lemon and pomelo) and their chemical compositions**

#### **2.1 Lemon**

Lemon (*Citrus limon L.* from Rutaceae*)* is one of the most common globally and ranks third among the Citrus species globally by 4,200,000 metric tons after orange and mandarin [14]. Lemon fruits typically consist of three parts: pulp, skins (albedo and flavedo), and seeds. It offers an extensive supply of natural compound

*Citrus Fruits: Nutritive Value and Value-Added Products DOI: http://dx.doi.org/10.5772/intechopen.95881*

products such as citric acid, ascorbic acid, minerals, flavonoids and essential oils [15]. Lemon bioactive compounds like flavonoids, vitamins, minerals, dietary fiber (**Table 1**), and essential oils are used in the food, cosmetic, and pharmaceutical industries. Most by-products of the lemon juice industry can provide functional foods with nutritional substances such as non-digestible carbohydrates, dietary fiber and bioactive (flavonoids and ascorbic acid). Lemon fruits can function against photo-oxidamage because carotenoids exist. Lemon fruit, rich in flavonoids, has a significant role in the healthy diet, particularly in preventing diseases such as obesity, diabetes, lowering blood lipids, cardiovascular disease, and some forms of cancer [18]. The citrus fruits used for direct consumption or converted into juices, jam, jelly, molasses, lemoncello beverage and more in addition to the lemon skin are added value products such as pectins, essential oil and functional ingredients [12, 18].

#### **2.2 Pomelo**

Pomelo is one of the most commonly grown and eaten citrus fruits and orange, mandarin, lemon, and grapefruit [19]. Pomelo (**Table 1**) is a promising source of carbohydrates, proteins, fiber, vitamins and minerals originating in warm tropical climates in south-eastern Asia [20]. The presence of bioactive (carotenoids, lycopene,

#### **Table 1.**

*Chemical composition of Citrus fruits as [15–17].*

polyphenols, flavonoids, limonoids, fiber and vitamin C) contributes to their protection against oxidative stress, hyperglycemia, and high blood pressure. Due to its essential health promotion properties, pomelo segments in food products are growing in importance in producing functional foods [21]. Pomelo is eaten fresh or made into juice [19], or pomelo fortified noodles help the diabetic population [21]. On the other hand, researchers have investigated alternative ways of restoring pomelo peels to the advantage of value-added products such as pectin, essential oils, polysaccharides, phytochemicals [19]. Production of juice and consumption of fresh fruit create large quantities of agricultural waste. The main components of wet Pomelo Peel waste, like other citrus fruits, include water, cellulose and hemicellulose, soluble sugars, lipids (mainly D-limonene), and bioactive compounds (i.e., polyphenols, mostly flavonoids).

#### **3. Nutritional values of citrus fruits**

Citrus has many natural plant compounds such as vitamin C, carotenoids (some can convert to vitamin A), folic acid, flavonoids, and fiber. **Table 2** shows the amount of vitamin and mineral consumption in lemon and pomelo fruits.

#### **3.1 Vitamin C (ascorbic acid)**

Citrus is a valuable source of vitamin C. By consuming a moderate amount of citrus fruits each day, an individual can achieve 100 percent Vitamin C level. Vitamin C is an essential water-soluble vitamin essential for the body's defense [22]. It is transmitted through muscle fibers, carnitine biosynthesis, neurotransmitters, collagen, and bones because these particles connect the fibers. The immune system

#### **Table 2.**

*The number of nutrients and the percent of the recommended daily allowance or adequate intake met from the consumption of 100 g of selected citrus fruit [22].*

*Citrus Fruits: Nutritive Value and Value-Added Products DOI: http://dx.doi.org/10.5772/intechopen.95881*

can be effectively stimulated by consuming vitamin C, which boosts white blood cells [23]. When Vitamin C is taken for pregnancy, it can decrease pre-eclampsia risk [24]. Some studies indicate that vitamin C supplementation can reduce the severity of colds symptoms or duration [23]. Anti-oxidants such as Vitamin C could reduce the risk of artery stiffening and cardiovascular diseases [25]. Above 200 mg of vitamin C daily is a healthy intake, and citrus fruits are a huge source of this vitamin. Lemon provides 37 mg of ascorbic acid per 100 g of fruit [16]. Pomelos have 52.3 mg of ascorbic acid in 100 g of the flesh [26].

#### **3.2 Carotenoids and Vitamin A**

There are many types of carotenoids, including terpenes responsible for pigments commonly found in plants, and there are about 600 carotenoids in foods and 50 in human bodies [27, 28]. The highest carotenoid levels, such as lutein, zeaxanthin, lycopene, and vitamin A, are found in fruits and vegetables, including orange and carotene. Benefits of carotenoids in foods include improving immune function, promoting bone formation, promoting eye health, and maintaining visual quality [22]. There is a large amount of data supporting that carotenoids reduce the risk of cancer, macular degeneration, cataracts, skin damage to the sun, and cardiovascular diseases [29]. Higher consumption of β-carotene is linked to a lower breast cancer risk [30]. Beta carotene, lycopene, or lutein may decrease the rate of UV-induced lipid peroxidation in human skin fibroblast cells [30]. Lutein is inversely related to colorectal cancer in both men and women [31]. The levels of lutein, zeaxanthin, β-cryptoxanthin, and β-carotene in the lemon and pomelo, were around 2.95, 0.81, 0.81 and 10.3 (μg/g, db), respectively [32]. The content of carotenoids in pummelos' peel was 0.012-0.015 mg/gdb [33].

#### **3.3 Folate (folic acid)**

Folic acid, which is a water-soluble vitamin, and its derivatives are collectively called folate or folacin. The most notable folate compounds in Citrus are the reduced 5-methyl tetrahydrofolate (monoglutamate) and polyglutamate compounds [34]. Folate plays a vital role in DNA, which is involved in homocysteine regulation and protein production primarily through the methylation transfer reactions [22]. Because there is a high DNA production during pregnancy, a folate deficiency is significantly linked to birth defects such as neural tube defects [35]. Lack of folic acid caused higher levels of homocysteine, raising heart disease and atherosclerosis [22]. Previous studies show that citrus fruits' daily consumption can help improve folate levels, which will subsequently decrease blood homocysteine (tHcy), thus reducing cardiovascular disorder and neural tube defects [36]. Citrus is a parallel source of dietary folate that can help to cover up to 10% to 20% of the recommended daily allowance of adults, children, and infants with a consumption of 100 g of citrus fruits. The consumption of citrus fruit is an easy way to obtain vitamin C and dietary folate, which is vital for absorption in the body. Lemon, a citrus fruits representative, has eleven to sixteen micrograms of Folate in 100 grams [22]. According to El-Otmani and Ait-Oubahou [37] Citrus limon contained 11mg of folic acid per 100 g of citrus.

#### **3.4 Dietary fibre**

The fiber is found in vegetables and fruits cannot be digested and absorb in the small intestine. There are two kinds of dietary fiber; soluble and insoluble fiber. Insoluble fibers are highly fermentable and connected with carbohydrate and lipid metabolism, while soluble fibers contribute to fecal bulk and reduce transit time [34]. Although pectin, cellulose, and hemicellulose comprise the most abundant dietary fiber on the plants, they also contain only trace amounts of lignin. Pectin is citrus' primary fiber, which occurs primarily in citrus peels and rinds. Consumption of citrus fruit can contribute significant quantities of pectin in a diet. Dietary incorporation of pectin appears to affect several metabolic and digestive processes; principal interest affects glucose absorption and cholesterol level [38, 39]. There is a significant benefit in consuming citrus fruit because of its pectin content. Dietary incorporation of pectin appears to have many implications for metabolic, digestive, and health affairs. One way fiber can reduce colon cancer is by diluting and trapping the harmful chemicals in the colon from bile-absorption and bile-excretion [34]. Scientific studies have proven that fiber can help promote laxation and satiety, the uptake and reabsorption of glucose, fat, cholesterol, and bile acids, thereby lessening heart disease risk and possibly enhancing healthy intestinal microbial fermentation [40, 41]. Citrus fruits significantly reduce cholesterol levels depending on the esterification degree of fiber consumption, viscosity, and molecular mass [22]. A fiber-rich diet has a low risk of deadly chronic diseases such as diabetes, heart disease, weight, and cancer and lowers cholesterol levels and blood sugar [42]. Several epidemiological studies reported that citrus peel support reducing plasma liver cholesterol, total serum cholesterol, serum triglyceride levels, and total liver lipids [43].

#### **3.5 Flavonoids and limonoids**

Citrus pulp, peel are rich sources of flavonoids. Toh et al. [44] found that pomelo peeled (356.95 mg/QE) had higher total flavonoid content than pomelo pulp (13.06 mg/QE). Makni et al. [15] found the amount of quercetin in lemon flesh (56.16 mg Eq Quercetin/g dry weight) was higher than in peel (27.50 mg Eq Quercetin/g dry weight). Citrus fruits are also rich in flavonoids such as hesperidin, hesperetin, naringin, naringenin, diosmin, quercetin, rutin, nobiletin, tangeretin, and others [45]. Citrus flavonoids have both antioxidant and anti-inflammatory properties, and because of that, it can increase the antioxidant capacity and effect reducing cholesterol and triglycerides levels and provide more excellent bone health [22]. Preclinical studies and clinical trials demonstrated that flavonoids' effects in the forms of hesperidin and its aglycone hesperetin prevent various types of diseases, including neurological, psychiatric, and cardiovascular disorders [46]. Over the years, naringin and hesperidin are gaining attention for their great antioxidant capacity, contributing sweet flavor to foods and beverages [47]. Some known naturally occurring flavonoids have potency in defending against certain types of RNA and DNA viruses [48].

Limonoids are also known as flavonoids, which are compounds found in citrus fruits. In citrus fruits, there are two groups of limonoids: aglycones and their corresponding glucosides. Bitter taste in citrus results from limonoids present. Limoninin's most important constituents are glycosides called limonin and nomilin [22]. In animal and human cell lines, limonoids slow down the development of aggressive cancers like the pancreas, colon, stomach, and breasts. On the other hand, limonoids are also reported to reduce skin cancer in animal models. Limonoids are known for their medicinal or health beneficial effects like anti-cancer, anti-microbial and antimalarial activities [49]. Limonoids have antibacterial and antiviral effects. Some limonoids are known to stimulate the in vivo production of the detoxifying enzyme glutathione S-transferase in the liver and inhibit the formation of chemically induced tumor cells in the oral cavity, forestomach, small intestine, colon, lung, and skin of

*Citrus Fruits: Nutritive Value and Value-Added Products DOI: http://dx.doi.org/10.5772/intechopen.95881*

#### **Figure 2.**

*Proposed model for signaling pathways leading to growth inhibition by obacunone in estrogen-responsive breast cancer (MCF-7) cells [53].*

animals [50, 51]. Limonoids found in citrus fruits decreased the spread of cancer cells in animal studies [52]. Studies showed that lemon-lime oil in the form of obacunone appeared to prevent breast cancer by inhibiting aromatase enzyme and antiinflammatory pathways [53] (**Figure 2**). Several studies have revealed that limonin and nomilin are found in fruits, pulp, and seeds. Pummelo juice contained 18 ppm of limonin and 29 ppm of total limonoid glucosides [54]. Wattanasiritham et al. [55] reported limonin content of 18 ppm in the juice of pomelo cultivars. Limonin levels in extracted juice from seven pummelo cultivars from Florida ranged from 10.07 to 29.62 ppm. As shown by analysis, mature lemon seeds contain 1300 μg/g of different limonoid glucosides on a fresh weight basis [56]. The mature seeds contain much higher amounts of glucosides than commercialized juice, therefore. Fong et al. [57] documented that the commercial lemon juice had only 82 μg/g of glucosides.

#### **4. Value-added products**

Citrus fruits are known for being highly fragrant, with a tart taste and higher vitamin C content. The world has a wider variety of citrus fruit because of the continued type changes, such as sour oranges, oranges, pummelos, lemons, and limes, among others [22]. Nowadays, citrus pulp/pomace, seed, and peel are used for various commercially valuable products such as food ingredients, pectin, essential oils, enzyme production, a natural antioxidant, and packaging film formation.

#### **4.1 Food ingredient or food products**

The nutritional supplement of pomelo fruit segments has been added to products to be developed like noodles prepared with 30% new segments and 5% dry. These noodles can satisfy those with regular diabetes and the general public [21]. A high dietary fiber food was created by reducing the dietary fiber-rich pomelo peel to a powder that contained nearly 50% of dietary fiber. Lemon fruit is usually eaten fresh, but it is also processed to make juices, jams, jellies, molasses, candies and much more [18]. Lemon juice has been used as a coagulant during the manufacture of wara cheese [58]. Another innovation implemented in the beef burger is to use lemons for "enhancing the cooking properties of the burger" [59]. Lario et al. [60] had reported that the high-fiber lemon powder extract from lemon peel debris by-products is an ideal additive for food products (as meat, dairy, and bakery products).

#### **4.2 Pectin**

Pectin is an agent of gelling, emulsifying, stabilizing, texturizing, which appears as a white to light brown powder broadly accepted as a functional ingredient [61]. Fruit peels are a highly desirable pectin source because they cover up to 20% of the fruit's total Pectin [62]. Pomelo is a highly valued source of natural Pectin. About 20.75% pectin is derived from lemon peel for jams. In the study, the high extraction (36.71%) of Pectin from lemon peel has something to contribute to this industry [63]. Moneim et al. [64] recommended utilizing 20.75% of the lemon peels' total weight in making pumpkin jam. The researchers were added 16.740% of Pectin from pomelo peel to the pressed carrots before storage [65]. Studies by Methacanon et al. [66] have shown that pectin yield was 23.19% for pomelo peel. On the other hand, Roy et al. [65] were found that pomelo peels are a good source of Pectin, and then carrot jam made by extracting Pectin from pomelo peel.

#### **4.3 Essential oil**

Essential oils (Eos) are volatile, complex, natural mixture of aromatic oils obtained from plants [67]. Citrus essential oil is commonly known to produce a good fragrance and has been officially approved for healthy public consumption. All over the world, EOs are used in cosmetics, perfumery, toiletries, flavoring, beverage, pharmaceuticals and other personal hygiene products [68–70]. Lemon oil is often used on the skin because of its antimicrobial and antifungal properties. Pomelo peel (PP) has approximately 299 recognized volatile compounds. A significant number of these volatile compounds is considered as terpenoids (189 volatiles, 63.2%). Various kinds of chemicals released are monoterpenoids, monocyclic monoterpenoids, bicyclic monoterpenoids, diterpenoids, acyclic sesquiterpenoids, monocyclic sesquiterpenoids, bicyclic sesquiterpenoids, and tricyclic sesquiterpenoids. Another major volatile present in PP EO is nonterpenoid alcohols (4.7%), nonterpenoid aldehydes (6.0%), nonterpenoid hydrocarbons (5.7%), and esters (8.7%). The unknown volatiles covered 11.7% (35 volatiles) of total volatile compounds. The structures of widely known terpenoids in Polypropylene EO has shown in **Figure 3a,b**. The most critical monoterpenoids (1 to 16) and sesquiterpenoids (17 to 29) are present in PP EO [19]. According to studies conducted, lemon essential oils retain aroma in foods because of their natural preservative and flavoring properties [71]. The effect of lemon essential oils on the cheesemaking process dramatically reduces microorganisms' population, especially those of the Enterobacteriaceae family [58]. In the food and pharmaceutical industries, citrus

*Citrus Fruits: Nutritive Value and Value-Added Products DOI: http://dx.doi.org/10.5772/intechopen.95881*

#### **Figure 3.**

*(a) Major monoterpenoids in pomelo peel essential oils (1-16); (b) Major sesquiterpenoids in pomelo peel essential oils (17-29).*

EOs can be employed to inhibit mold and fungal growth. Lemon EO (Citrus limon) was used as a possible fungicide to manage the pathogenic fungi attacking grapevines, namely Eutypa sp., Botryospaeria dothidea, and Fomitiporia mediterranea. The antifungal activity was observed for EO against all the three fungi with the highest action against strain Eutypa sp. (82% inhibition) and the lowest tolerance (33.1%) towards F. Mediterranean [72]. These essential oils repress the growth of mold and yeast. The raspberries coated with alginate and lemon EO (0.2 percent) or orange EO (0.1 percent) halted bacteria, yeast, and mold and also reduced the quality deterioration right after harvest [73]. The lemon EOs mixed in the chitosan films can be used to control L. Multicellular pathogens in refrigerated foods Researchers Rahmawati et al. [74] observed that an edible coating applied with lemon essential oil only delayed aging of tofu and fresh strawberry.

#### **4.4 Enzymes**

The most basic usage of citrus peels is to produce the pectinolytic enzyme for beneficial purposes. Larios et al. [75] studied endo-polygalacturonase production by Aspergillus sp. CH-Y-1043 using untreated lemon peel and citrus pectin as carbon sources. Lemon peel being employed as a substrate in submerged cultures to obtain high pectinase titers by A. flavipes FP-500 and A. terreus FP-370 [76].

A. niger produces approximately 2,181.23 U/L pectinases from lemon, peel pomace in a solid-state reactor [77]. Seyis and Aksoz [78] have shown that lemon pomace and peel are suitable substrates for heterotrophic xylanases enzyme production using fungus Trichoderma harzianum. Aspergillus niger LFP-1 was studied in solidstate fermentation (SSF) using pomelo (Citrus grandis) peels as a substrate [79]. Maller, et al. [80] determined that lemon peels are extremely capable of triggering the production of Polygalacturonase in the aspergillus niveus. Pectin lyase yield increased through fungal strain Aspergillus oryzae process derived from lemons peel and used in solid-state fermentations [81]. Studies said that Polemo pericarp powder utilized as a substrate for Aspergillus oryzae JMU316 has Naringinase enzyme [82]. Pectinase enzyme produced from pomelo peels by By Aspergillus niger through Solid State Fermentation [83]. Lemon peels could be a good source of naringin, which could be used as a carbon source in submerged fermentation for naringinase production using Aspergillus niger [84]. Naringinase is essential for the production of sweetener precursors, preparation of prunin, aroma enhancement in winemaking, biotransformation of antibiotics, and rhamnose manufacturing [82].

#### **4.5 Natural antioxidant**

Antioxidants are chemical substances that can reduce or prevent the damage caused by free radicals in the body, thus reducing the risks of cardiovascular disease and cancer [85]. Results of studies showed that lemon peel contained almost 75.9% of antioxidant content. The unique ability of Paneer was derived from compounds found in the Peel of orange, lemon and pomegranate [85]. Peel taken from Tambun White pomelo type contains higher levels of antioxidants and is also a rich source of natural antioxidants [44]. Lemon peel and flesh had the highest antioxidant capacity, and they had a significant impact on the prevention of cardiovascular diseases and other diseases [86].

#### **4.6 Packaging film formation**

More sustainable, biodegradable plastic has gained popularity among environmental scientists. The researcher created plastic films from citrus peels. By applying the peels of citrus, biodegradable packaging material could be made [87]. Wu et al. [88] prepared fruit peel as the edible packaging film with high content biopolymer to form film for packaging. The film was designed to incorporate tea polyphenols, which causes interacting molecules to become more closely crowded. Soy protein with essential oil of lemon peel was used to create a degradable film and create cheese curdorants for preservation [89]. Dias et al. [90] reported that the use of citrus essential oil and its aroma significantly improved consumers' health and significantly increased the acceptance of biscuits' packaging. Das et al. [91] demonstrated that chicken feather keratin combined with pomelo peel pectin to form biodegradable composite film and wrapping of fried fish fillets resulted in less weight loss, hardness value, and reduction in the surface microbial count.

#### **5. Conclusion**

Citrus has positive effects on human health, and it could be an essential raw material to the biotechnological industry. Citrus is a mighty source of vitamins, minerals, and dietary fibers. Bioactive and non-nutrient compounds in citrus are valuable for controlling chronic diseases such as diabetes, cholesterol, obesity, cardiovascular disease, and some forms of cancer. Besides citrus, vitamin C also

#### *Citrus Fruits: Nutritive Value and Value-Added Products DOI: http://dx.doi.org/10.5772/intechopen.95881*

has other benefits, including fighting diseases such as cardiovascular disease, boosting white blood cells, immune function, and symptoms or duration of colds. Peel, flesh/pomace, and seed from the citrus fruit are employed in making different novel foods like noodles, extract pectin, enzyme extracts, and essential oil. Therefore this information might be necessary for the readers because it gives facts about the popular citrus fruit. Also, choosing the best citrus for an edible ingredient can be beneficial for citrus processors.

### **Author details**

Abu Saeid1 and Maruf Ahmed<sup>2</sup> \*

1 Department of Food Engineering, NPI University of Bangladesh, Manikganj, Bangladesh

2 Department of Food Processing and Preservation, Hajee Mohammad Danesh Science and Technology University, Dinajpur, Bangladesh

\*Address all correspondence to<EMAIL_ADDRESS><EMAIL_ADDRESS>
© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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#### **Chapter 10**

## Citrus Essential Oils: A Suite of Insecticidal Compounds

*Bulbuli Khanikor, Kamal Adhikari and Bikash Rabha*

#### **Abstract**

Citrus essential oils (CEOs) and their constituent compounds are being reported to have multifarious activities. In this chapter an attempt is made to discuss the insecticidal activities, as well as CEO profile of different vegetative part of *Citrus* species and biocidal potentiality of their constituent compounds against diverse insect pests. It is observed that in most of the CEO constituent profile, limonene is the major constituent compound. Other important constituents present in different percentages in different CEOs are β-citronellal, linalool, pinene, β- caryophyllene, β-myrcene, terpinene, citral etc. These plant EO constituents are reported to have insecticidal effects against diverse insect species. Taking the four peel EOs of *Citrus limon, Citrus paradisi, Citrus medica, Citrus maxima* commonly grown in North Eastern part of India, study on their insecticidal effects against *Dolichoderus affinis* (Hymenoptera: Formicidae) was made and result is presented showing higher fumigant toxicity of *C. medica* and *C. limon* oil against the ant sp. With the increasing awareness for using safe insecticidal products among consumers, the citrus EOs with their attracting terpene compounds having good insecticidal potency bear all attributes to be used as commercial green pesticides in coming days both in indoor and outdoor management of insect pests.

**Keywords:** essential oils, limonene, *Dolichoderus*, *Citrus medica*

#### **1. Introduction**

The genus Citrus has tremendous industrial value all over the globe not only for its nutritive juicy high valued fruits but also for the essential oils present in its different vegetative parts. Thus, both the Citrus fruits and citrus essential oils bear potential to generate livelihood & to boost the country's economy. Citrus essential oils (CEOs) with diverse biologically active compounds of terpene groups with pleasant aroma have already achieved significant positions in flavor, food, cosmetic industries. At the same time, because of their antimicrobial activities as well as anticancer, antioxidant, anti-inflammatory, metabolic disorder alleviating activities etc. these oils and their compounds have been getting importance in pharmaceutical and medical sectors for the last few decades [1]. A good number of studies also reported insecticidal potential of citrus EOs extracted from different citrus sp. and their constituents at different times, a few of which are commercialized to be used by the consumers against insect pests. There are 33 recorded species of citrus worldwide (ThePlantList.org) with many recorded and unexplored varieties present in different parts of the world. The essential oil profile of different citrus species varies although some of the constituent compounds are common but present in different amounts in the total bulk oil. Even the oil profile of different vegetative parts of a single citrus species are not identical. Understanding of essential oil profile of diverse citrus species grown in wild, semi wild and cultivated state across the globe at different seasons is the much-needed task as the quality of the oil, oil yield percentage, consistency of the constituents even varies with seasonal changes, geographical location, harvesting time of the plant parts, soil type etc. however from the existing GC–MS profile of different *Citrus* sp. reported at different times and from different places, it is apparent that two -three dominant compounds are mostly present in most of the Citrus species. The literature revealed that the Citrus EO comprises more than 200 compounds of which 85–99% are volatile and 1–15% nonvolatile compounds. The volatile compounds comprise mostly monoterpenes (predominant limonene), some sesquiterpenes and their oxygenated derivatives [2].

Pest control sector is dominated by synthetic pesticidal products for many decades. At recent times with increasing concern to ecofriendly product, plant essential oils are getting renewed interest as they are not only effective but also comparatively safe and environment friendly in comparison to synthetic counterparts. Essential oils are part of natural plant defense system and many of them are proved effective and some are exploited for integrated management practices of pest and pathogens. As some citrus species are naturally resistant to certain group of pests and or pathogens, it is assumed that certain bioactive compounds may present in the essential oil part of those citrus species. It is already established that citrus essential oils of different citrus species are effective against wide range of pest and pathogens. It is also important to have an insight about the interaction of citrus constituents against its own insect pest and pathogen complex to be used as insecticidal, repellent and bactericidal etc. A few papers highlighted beneficial effects of using citrus essential oil against its own pest and pathogen complex. The added advantage of considering CEO as insecticidal and insect repellent is that the plant is edible therefore safe for residual contamination or toxicity to consumer. At the same time the pleasant aroma offers consumer acceptance.

#### **2. Citrus EO against insect sp**

CEO and extracts have been tried against a wide range of insect pests for assessing their insecticidal as well as repellent properties. In some parts of the world citrus plants have been traditionally used to ward off a insect pests. Some recent reports especially of the last two decades of the insecticidal and repellent effects of different citrus sp. are presented below. Most of the works were carried out on dipteran, lepidopteran, hemipteran and coleopteran insect pests.

Topical toxicity of the essential oil of *Citrus hystrix* with LD50 of 26.748 μL/g and antifeedant properties leading to severe growth inhibition has been reported against tobacco armyworm *Spodoptera litura* [3]. The fumigant toxicity and repellent effect of the n-hexane extract of the plant leaf was documented against stored grain pest *Lasioderma serricorne* [4]. Fumigant toxicity of peel oils of lime, orange, mandarin, tangerine, grapefruit and lemon were reported against three store grain pest species *Callosobruchus maculatus, Sitophilus zeamais and Dermestes maculatus* [5].

The peel essential oil of the plant is reported to possess repellent effect against *Callosobruchus maculatus* [6], *Aedes aegypti* and *Anopheles minimus* [7]. Similarly, the insecticidal and repellent activity of *Citrus reticulata*, *Citrus limon* and *Citrus aurantium* peel oils was demonstrated against *Callosobruchus maculatus* [8]. Insecticidal activity of *Citrus limon* and *Citrus sinensis* against vine mealybug**,** *Planococcus ficus* [9]. The larvicidal and adulticidal effects of *Citrus limon* and

*Citrus Essential Oils: A Suite of Insecticidal Compounds DOI: http://dx.doi.org/10.5772/intechopen.95887*

*Citrus sinensis* are mentioned against *Attagenus fasciatus* and *Lasioderma serricorne* [10]. The seed and peel extracts of *Citrus limon* L. was reported to have the highest larvicidal toxicity (LC50 values of 395.59 ppm for seed; 468.69 ppm for peel) after 24 hours over EOs of *Citrus grandis, Citrus sinensis, Citrus paradise, Citrus reticulate* [11]. Essential oil of *Citrus reticulata* and *Citrus sinensis* was reported effective against the fourth instar larvae and adults of *Tribolium castaneum* with higher potency of *Citrus reticulata* [12].

The seed EOs of *Citrus reticulate* var. kinnow, *Citrus reticulate* var. freutrall, *Citrus sinensis* and *Citrus jambhiri* was tested against *Tribolium castaneum* with promising efficacy in terms of LC50 for *Citrus jambhiri* followed by Citrus reticulate and *Citrus sinensis* [13]. Similarly Oboh et al. [14] recorded insecticidal efficacy of *C. sinensis* peel essential oil against Callosobruchus mamulatus, *Tribolium confusum*, *Sitophilus oryzae* with LC50 value of 21.8, 38.9, 60 μl/l.

Comparative evaluation of toxicity of EOs of *C. limon, C. aurantifolia, C.sinensis* in filter paper impregnation method showed highest toxicity of *C. limon* (95% mortality) followed by *C. aurantifolia* (92.5%) and *C. sinensis* (82.5%) against carpenter ant *Camponotus nearcticus* [15]. But, Guerra et al. [16] comparatively lower topical toxicity (15% mortality) of *C. limon* EO against *Camponotus pennsylvanicus* among the eleven different EOs tested. Essential oils and or extracts of *C. maxima* or *C.grandis* have been reported effective against different mosquito species. In our earlier studies we recorded differential biocidal activities of essential oil extracted from peel and leaf part of *Citrus grandis* grown in Assam against different developmental stages of *Aedes aegypti* and *Culex quinquefasciatus* [17, 18]. EO extracted from leaves was found more effective against egg stage while oil from peel was recorded more effective against larval and adult stages of *A. aegypti* [18]. The leaf and peel oil of the plant was recorded highly effective against egg and larval stage with LC50 value of below 50 ppm but did not found much effective against adult stage of *Culex quinquefasciatus* although having repellent properties with good protection time [19]. In a recent study we observed synergistic larvicidal response of *Citrus grandis* leaf oil with *Allium sativum* bulb oil against *C. quinquefasciatus* [19]. Manorenjitha et al. [20] tested hexane, ethyl acetate, methanol, water and essential oil extract of *C. grandis* peel extract for evaluating oviposition deterrent and repellent properties on *Aedes aegypti*. They observed promising oviposition deterrent activity of ethyl acetate fraction (10 ppm concentration) in breeding plates kept within mosquito cage and effective repellency (94.7%) of 20% essential oil fraction of the peel while offering animal bait in modified tunnel test. A study for toxicity assessment on worker termites *Odontotermes feae*, essential oils of *Citrus grandis* with LC50 value of 273.36 ppm was found to show maximum toxicity out of *Citrus paradisi, Cassia fistula*, *Citrus grandis* EOs [21].

The peel essential oils of *Citrus aurantifolia* has been reported as insecticidal, repellent, and larvicidal against *Aedes aegypti* [22]. In our previous study, we observed the ovicidal, larvicidal and adulticidal effects of leaf and peel essential oil of *Citrus aurantifolia* against *Aedes aegypti* [23].

Promising fumigant toxicity of the peel EO of *Citrus aurantium* and *Citrus sinesis* from the north east Brazil with LC50 value of 5.80 μL/L of air and 3.80 μL/L of air respectively and oviposition deterrent activity at 3.5 and 7.0 μL/L of air against the whitefly *Bemisia tabaci* [24]. The same oil was reported effective against the larval and adult stages of tomato leafminer *Tuta asoluta* (Lepidoptera: Gelechiidae) [25]. The insecticidal activity of *Citrus aurantium* EO against adult housefly *Musca domestica* was reported next to the activity of EO of *Citrus sinensis* [26]. *Citrus aurantium* leaf EO was found as effective fumigant against sawtoothed grain beetle *Oryzaephilus surinamensis*, cigarette beetle *Lasioderma serricorne* and rice weevil *Sitophilus oryzae* with LC50 value of 64.94, 202.49 and 364.25 μL/L of air

respectively [27]. Similarly, Bnina et al. [28] noted fumigant toxicity of peel, leaves and flowers essential oil of *Citrus aurantium* against four stored grain pests namely *Tribolium castaneum, Liposcelis bostrychophila, Sitophilus granarium, Cryptolestes ferrugineus* with LC50 value of 64.78%, 23.11%, 101.50% and 20.62% respectively. They also noted repellent property of the oil against these pests. Yazdgerdian et al. [29] tested contact and residual toxicity of eleven essential oils including *Citrus aurantium*, *Citrus sinensis*, *Citrus limon* against wooly beech aphid*, Phyllaphis fagi* (Hemiptera: Aphididae) and rice weevil, *Sitophilus oryzae* (Coleoptera: Curculionidae) and recorded highest residual toxicity (40%) of *C. aurantium* among the citrus sp. tested against the targeted species. Similarly, without affecting the seed viability of stored cowpea and consumer acceptability, peel EOs of *Citrus nobilis* and *Citrus medica* was reported to show significant reduction of egg laying, egg hatching, and adult emergence percentage of pulse beetle *Callosobruchus maculatus*. Both the oils showed dose dependent repellency with higher effect for EO of *C. nobilis* [30]. Like that of EOs, the nonpolar petroleum ether extract of ripe fresh fruit of *C. aurantium* was reported effective against adults of olive fruit fly *Bactrocera oleae* (Diptera:Tephritidae) in petri dish residual exposure test [31]. The same solvent extract also reported to have good toxicity against medfly *Ceratitis capitata* adults (LC50 value of 70.6 and 147.1 lg/cm2 for male and female respectively at 96 h in Petri dish residual bioassay) [32].

Moravvej et al. [33] tested fumigant toxicity of EOs from four citrus species namely *C. paradisi, C. limonium, C. sinensis* and *C. aurantium* among which *C. paradisi* was the most effective with LC50 value of 125 μl/L and *C.sinensis* is the least effective with LC50 value of 269 μl/L against *Callosobruchus maculatus.* However, fumigant toxicity of *C.sinensis* was reported against *Solenopsis invicta* (Hymenoptera: Formicidae) with 100% mortality at 3 mg/tube after 24 hrs [34]**.** Ethanolic extract of the same plant was found effective against *Anopheles stephensi, Aedes aegypti* and *Culex quinquefasciatus* with larval and adulticidal LC50 value of below 500 ppm along with more than 50% repellency at 150 ppm concentration till 180 min [35].

Ezeonu et al. [36] reported the insecticidal properties of the volatile peel extracts of *Citrus sinensis* and *Citrus aurantifolia* against mosquito, cockroach and housefly and recorded higher insecticidal potency of the peel extract of *Citrus sinensis* with maximum fumigant effect (85% at 60 min) against cockroach.

Zewde and Jembere [37] evaluated the solvent extract and essential oil of *Citrus sinensis* against *Zabrotes subfasciatus* (Coleoptera: Bruchidae) for their repellent, fumigant and protectant properties and recorded no progeny emergence on application of oil at low dose (30 mg of EO), prominent percent mortality at high dose (750 mg of essential oil killed 67% of Z. subfaciatus after 96 hours). Fumigant toxicity of *C. sinensis* EO was also reported effective against second instar larvae of *Musca domestica* (Diptera: Muscidae) with LC50 values of 71.2 μl/L as well as percent inhibition of pupae with 46.4% at 40 μl/L exposure concentration [38].

Orange oil extract was also recorded effective against the subterranean termite *Coptotermes formosanus* (Isoptera: Rhinotermitidae). Application of the oil extract at 5 ppm concentration resulted in 96% and 68% mortality respectively in closed container. At the same time termites did not show tunneling behavior on the 0.2 & 0.4% oil treated soil [39].

Majeed et al. [40] reported the insecticidal activity of the acetone, ethanol and aqueous extracts of seeds, leaves and fruit peels and leaves of *Citrus aurantium* and *Citrus sinensis* and two other plants against mealy bug *Drosicha mangiferae* (Hemiptera; Pseudococcidae) of which ethanol extracts of *C. sinensis* seeds and *C. aurantium* leaves were suggested as considerably toxic against the said insect.

*C. medica* peel essential oil was reported as more effective in filter paper fumigation method against stored grain pest *Tribolium castaneum* (Coleoptera) with LC50 value of 29.5 mg/L air than the leaf EO [41].

Abdel-Kawy et al. [42] showed *Citrus trifoliata* essential oil loaded nanocubosome significantly enhanced insecticidal property of the essential oil against the second instar larvae of *Spodoptera littoralis*.

While working as biocidal and repellents, plant products including EOs and constituent terpene compounds are reported to act on cholinergic system [43], voltage – gated sodium channel of the nerve membrane, glutamate-gated chloride channel [44], GABA-system [45], Octopaminergic system [46], mitochondrial system [47], endocrine system disrupting the endocrinological balance and respiratory system of insect body. However, not much studies yet conducted on detailed study on the mode of action of EOs and their constituent compounds.

#### **3. GC-MS profile of citrus EO**

With the development of GC–MS technique, profiling of essential oil became easier. The composition of different citrus species from different parts of the world have been reported utilizing this technique. Most of the profiling results although detected average 20–50 numbers of compounds, a few compounds mostly occupy the major share of the bulk oil. The dominating compounds in most citrus species is limonene. In some species like *C. hystrix* the major compound is β-citronellal. The other common constituent compounds observe to be present in many GC–MS results of citrus species are linalool, pinene, β- caryophyllene, β-myrcene, terpinene, citral etc. Some of the reported constituent profiles of citrus species are mentioned below.

#### **3.1** *Citrus hystrix*

From the leaf essential oil of *C. hystrix* in Malaysia, 29 compounds were reported, out of which beta-citronellal was the major compound (66.85%) [3]. The other compounds present in more than 1% total compositions were β-citronellol (6.59%), linalool (3.90%), 5,9-dimethyl-1-decanol (4.96%), methyl citronellate (1.90%), geranyl acetate (1.80%), citronellol (1.76%), 3-undecanol (1.04%). The same compound (beta-citronellal) with 86.43% amount was reported in another study from Indonesia along with 11.48% citronellol and 1.65% β-linalool [4].

#### **3.2** *Citrus maxima* **(synonym** *Citrus grandis***)**

In the leaf EO of *C. maxima*, 42 constituent compounds were reported to be present with citronellol as the major compound (28.26%) of the essential oil. The compounds comprised more than 1% of the oil compositions were β- caryophyllene (16.89%), −spathulenol (9.32%), α-caryophyllene (2.48%), γ-cardinol (3.16%), α-cardinol (2.51%), 2n-hexylcyclopentanone (2.22%), caryophyllene oxide (1.03%). Again, 34 compounds were identified from the peel essential oil of the same plant with limonene comprising bulk of the oil constituents (89.04%) [48]. The major compounds identified in more than 1% of the total composition were β-pinene (2.25%), β-myrcene (2.06%), β- copaene (1.76%). Compounds comprising more than 0.3% amount but below 1% were linalool, β-phellandrene, α-pinene, terpinene-4-ol. Earlier 35 compounds with limonene as the major constituent compound (93.2%) was reported from the fruit peel oil [49]. Myrcene comprised 2.9%

and other six compounds namely α-pinene, octyl acetate, germacrene -D, linalool, decanal, geranial comprised above 0.2% but below 1% of the total composition.

#### **3.3** *Citrus aurantium*

Phytochemical profiling of essential oil of *C. aurantium* showed the presence of 25 compounds with limonene occupying 87.52% followed by linalool (3.365%) and β-myrcene (1.628%) as dominant compounds [25]. From Tunisian *Citrus aurantium*, limonene percentage in leaves, flowers and peel EOs were 6.52, 5.03, 73.6% respectively, linalool occupied 37.24, 41.82, 4.8% respectively, linaly acetate occupied 7.87, 13.75, 1.6% respectively and neral share was almost of similar (3.40, 4.80, 3.26% respectively) percentage. β-pinene composition for leaves and flowers were 9.68 and 9.21% respectively and α-thujene composition were 10.65 and 6.15% respectively but both β-pinene and α-thujene present below 0.5% in peel EO of the plant [28].

#### **3.4** *Citrus aurantifolia*

In our recent studies 31 compounds from the leaf oil and 26 compounds from the peel essential oil was recorded from GC–MS analysis of EO of *Citrus aurantifolia* in India. Citral and limonene were noted as the major constituent compound of leaf oil and limonene and palatinol-1C as the major constituent of peel essential oil of the plant [19, 23]. The EO of the plant from Italy reported to comprise limonene (53.8%), γ-terpinene (16.5%), β-pinene (12.6%), β-Bisabolene (1.33%), Geranyl acetate (1.06%), Neryl acetate (1.12%), Geranial (1.84%), sabinene (1.74%), and α-Pinene (1.97%) [50]. Phytochemical analysis of leaf and peel EOs of the plant from Brazil [51] showed limonene as the dominant compound in both leaf (32.7%) and peel (77.5%) part. Other prominent compounds in the leaf EO were linalool (20.1%), citronellal (14.5%), citronellol (14.2%), trans-β-Ocimene (2.7%), geranial (2.6%), neral (2.1%), trans-β-Caryophyllene (2%), myrcene (1.4%) etc. The other major compounds of leaf EO were myrcene (4.4%), linalool (3.5%), citronellal (3.2%), citronellol (2%), β-Bisabolene (1.5%) etc.

#### **3.5** *Citrus sinensis*

Phytochemical analysis of peel essential oil from three varieties of *C. sinensis* from Kenya showed presence of 56 components in Salaustiana variety, 73 in Valencia and 72 in Wshington varirty. Limonene occupied more than 90% in all the essential oil (Salustiana 94.6%, Valencia 92.5% and Washington 90.5%); alpha terpinene occupied 1.5% in Valencia and Washington and 1.7% in Salustiana [52]. Limonene with 90% share and β-myrcene, γ-terpine, linalool with 1.88%, 1.21% and 0.88% share out of 32 identified compounds in peel EO of *C. sinensis* has been reported from China [53]. GC–MS analysis of the plant EO from Argentina showed limonene (92.47%), linalool (1.43%), and β-myrcene (0.88%) as the major constituent compounds along with terpineol (0.28%) in lesser amount [26]. Almost similar constituents were identified from EO of *C. sinensis* with D-limonene (65.28–80.18%), Linalool (0.32–2.20%) and β- pinene (1.71 5.58%) as major part in another study [34].

#### **3.6** *Citrus nobilis*

Phytochemical study on constituents from the peel of *C. nobilis* from China showed D- limonene (12,601 μg/g) as the major constituent compound followed by β-myrcene (1600 μg/g), β-pinene (82.64 μg/g), p-mentha-1,8-dien-3-one (41.33 μg/g), α-pinene (25.41 μg/g), geranial (21.32 μg/g), sabinene (21.18 μg/g), E-β-ocimene (14.97 μg/g), linalool (10.38 μg/g), α-terpineol (6.36 μg/g). Other compounds are present in lower amounts (below 5 μg/g) [54]. In another study from Sri Lanka, 37 compounds were reported from peel part of which D-limonene (45%) was the major one followed by cyclopentane-2-methyl-1-methylene-3- (1-methylethenyl) (3.94%), p-mentha-4,8-diene (3.73%), α-terpinolene (3.03%), methyl-2-(methylamino) benzoate (2.4%), α-fernesene (1.1%). Other compounds were present in below 1% [30].

#### **3.7** *Citrus limon*

Phytochemical analysis of citrus leaf EO from Iran showed presence of 27 compounds of which the major compound was linalool (30.62%). The other compounds present in significant amount were geraniol (15.91%), α-terpineol (14.52%), linalyl acetate (13.76%), geranyl acetate (6.75%), Β-pinene (4.51%), neryl acetate (4.24%), p-Cymene (1.86%), and limonene (1.13%) [55]**.** Chemical composition of EO of *C. limon* grown in Iraq [56] showed presence of 24 compounds with limonene as principal compound with 29.52% share. Other major compounds were β-Pinene (23.89%), α-Pinene (2.25%), Myrcene (1.31%), (Z)-β-Ocimene (2.09%), Linalool (1.41%), (R)-Citronellal (15.10%), α-Citronellal (3.57%), (+)-α-Terpineol (1.57%), Neral (Z-Citral) (1.19%), Geranial (E-Citral) (1.73%), Thymol (9.79%), Citronellyl acetate (1.87%), Caryophyllene (1.36%), Phytol (1.36%). The analysis of the essential oil of *Citrus limon* from North-East India reported presence of 43 constituent compounds of which limonene (55.40%), neral (10.39%) trans-verbenol (6.43%) and decanal (3.25%) were the major constituent compounds [57].

#### **3.8 Citrus paradisi**

Phytochemical analysis result of *C. paradisi* peel EO from Turkey demonstrated presence of 25 constituent compounds of which limonene occupied the highest percentage (88.6%) of the oil. The other major compounds were α-terpinene (1%), and β-pinene (1.2%) [58].

From Nigeria, fifteen phytochemical constituents of the plant oil were reported. Among the compound limonene (94.2%) occupied the major share [59].

#### **3.9** *Citrus medica*

A total of 19 constituent compounds were identified from leaf essential oil of *Citrus medica* from Bangladesh of which erucylamide (28.43%), limonene (18.36%), citral (12.95%), Mehp (8.96%), 2,6-octadien-1ol,3,7-dimethyl-acetate, (Z) (5.23%) were the major compounds. From peel essential oil 43 compounds were reported out of which isolimonene (39.37%), citral (23.12%), limonene (21.78%) were the major constituents. Three other compounds namely β-myrcene, neryl acetate and neryl alcohol were reported to present at around 2% each in total composition and remaining compounds were present in traces amount [60]. In a study carried out by Li et al. [61], all total 28 compounds were reported to present in the fruit essential oil of *Citrus medica* of which limonene (45.36%), γ-terpinene (21.23%), dodecanoic acid (7.52%) were documented as major constituent compounds. Compounds like β-bisabolene, tetradecanoic acid, α-terpineol, terpinene-4-ol, hexadecenoic acid, α-bergamotene, α-pinene, β-pinene comprised between 5–1% range in total composition. In another study fruit peel EO of the plant was reported to comprise limonene (38.7%), γ- terpinene (28%) and ο-cymene (15.2%) as major compounds [41].

#### **4. Citrus EO compounds against insect sp**

Essential oil composition of different citrus sp. across the globe although may vary but some of the compounds are observed as common in most of the oil profile. The most dominating and commonly present compound is limonene. Other common compounds are citronellal, citronellol, linalool, pinene, myrcene, ocimene, terpinene, caryophyllene etc. The bioactivity of EOs is often related to the activity of major compounds present in the crude oil and some of the studies have already established this fact. Individual assessment for insecticidal property of these common constituent compounds have been performed by different researchers and some of them were found active against insect pest. Limonene and other *Citrus limon*oids are reported as insect repellents, feeding deterrents, growth disruptors, and reproduction inhibitors against a wide range of pest complexes. Insecticidal activity of limonene was reported effective against *Tuta asoluta* (Lepidoptera: Gelechiidae) [25]. Yoon et al. [62] revealed repellent property of different citrus oil and its major compound limonene against different species of cockroaches like *Blatella germanica, Periplaneta americana* and *Periplaneta fuliginosa.* However, Karr and Coats [63] did not get significant insecticidal activity of d-limonene against *Blattella germanica, Musca domestica, Sitophilus oryzae* and *Diabrotica virgifera virgifera.* In contrast they reported enhanced growth of nymph of *Blatella germanica* after oral administration of d-limonene.

In another study against cat flea species *Ctenocphalides felis* (Siphonaptera: Pulicidae), d-limonene (LD 50 against larvae, adults 226, 160 μg/cm2 respectively) and d-limonene with piperonyl butoxide (PB) (LD 50 against larvae, adults 157, 49 μg/cm2 respectively) were reported effective against all the life stages except the pupal stage of the flea species [64]. Fumigant toxicity of d-limonene, α- terpineol etc. also reported against honey bee *Apis mellifera* and tracheal mite parasite species *Acarapis woodi* [65].

Fouad and da Camara [66] extracted the essential oil from *Citrus aurantiifolia* and *Citrus reticulate* and analyzed the phytochemical constituents using GC–MS and found limonene as the major constituent compound, 38.9% of the *C. aurantiifolia* oil and 80.2% of the *C. reticulata* oil. They analyzed the enantiomers of limonene against the said insect. They found that *Citrus reticulate* was more toxic than *Citrus aurantifolia* towards the said insects. (R)-limonene was shown to have greater toxicity against *S. zeamais* than the (S)-limonene as found in the ingestion bio assay. Repellent bioassay showed (S)-limonene more susceptible to *S. zeamais* than (R)-limonene.

After identifying limonene as major compound in the EOs of *Citrus aurantiifolia* (38.9%) and *Citrus reticulate* (80.2%) Fouad and da Camara [66] tested enantiomers of limonene against *Sitophilus zeamais* and recorded greater toxicity of (R)-limonene than the (S)-limonene in the ingestion bio assay. But in the repellency test they found more susceptibility of *S. zeamais* towards (S)-limonene than (R)-limonene.

In a recent study, Sowler et al. [67] comparatively evaluated the effect of laboratory grade limonene and a commercial limonene-based insecticide against *Haematobia irritans irritans* in terms of deterrence, mortality, and reproduction. They showed that the egg viability was decreased in both the treatment, however, commercial limonene that caused loss of viability at 5.8% concentration was ovicidal in case of laboratory grade limonene. However, in terms of knockdown effect commercial limonene was better. Interestingly, at a concentration of less than 0.1%, both the commercial and laboratory grade limonene were acted as attractant.

#### *Citrus Essential Oils: A Suite of Insecticidal Compounds DOI: http://dx.doi.org/10.5772/intechopen.95887*

Giatropoulos et al. [68] tested essential oil of *Citrus sinensis, Citrus limon,* and *Citrus paradise* and their constituents and recorded γ- terpinene as the most toxic compound against *Aedes albopictus* larvae. They also reported that the constituent compound tested for repellency were better mosquito repellent than the parent essential oil. Similarly, Luo et al. [41] analyzed composition of leaf and peel essential oil of *C. medica* and tested both crude oil and major compounds viz. limonene, terpinene, o-cymene, β-caryophyllene against *Tribolium castaneum* and recorded γ-terpinene as the most effective insecticidal compound having LC50 value of 4.1 mg/l air and β- caryophyllene as the effective repellent compound. Limonene was reported to have almost similar fumigant toxicity like that of crude EO of *C. aurantium* against *Tribolium castaneum, Sitophilus granarium* and *Cryptolestes ferrugineus* [28]. In our earlier studies we found higher toxicity of citral, that is the major compound of the essential oil of *Citrus aurantifolia* than the crude oil as mosquito larvicidal, ovicidal and adulticidal against *Aedes aegypti* [23]. Plata-Reuda et al. [69] reported the insecticidal activity of citral and geranyl acetate against peanut beetle *Ulomoides dermestoides*. These compounds affected the survivorship, locomotor activity and reduced the respiration rate of the said species.

Nootketone and carvacrol, a phytochemical constituent present in essential oil of Citrus [70] acts as insecticidal compound against *Aedes aegypti* [71]. Pajaro-Castro et al. [72] recorded the neurotoxic effects of linalool and β-pinene on *Tribolium castaneum*. They observed that at low concentration both the compounds were attractant towards the insect and at higher concentration the compounds were repellent**.** Individual treatment of limonene and linalool was found to have fumigant toxicity against two ant species namely *Acromyrmex balzani* and *Atta sexdens* with LC50 values of 5.72 μl/L, 5.38 μl/L and 2.40 μl/L, 5.34 μl/L respectively [73]**.** Similarly individual treatment of β-caryophyllene is reported to have good contact toxicity against these two ant sp. but with low fumigant toxicity [74]. Fumigant toxicity of limonene, linalool and β- pinene were also reported effective against fire ant *Solenopsis invicta* (Hymenoptera: Formicidae) [34].

Linalool, α-terpinene was reported to show 100% fumigant toxicity against adult rice weevil *S. oryzae* at 3.9 mg/L [75].

Muller et al. [76] recorded 85.4%, 71.1%, and 29% repellency of the candles prepared with 5% geraniol, 5% linalool and 5% citronella against mosquitoes on human landing bioassay. They observed similar repellency against sand flies too. 78% repellency of *Culex pipiens pallens* was reported after using 30% citronellal [77]. Progeny deterrent, antifeedant, egg hatching inhibition activity was documented after application of 5–10 μl citronellol against *Callosobruchus analis* [78]. Compounds like limonoids act as synergists to enhance the activities of other biological and or synthetic insecticides [79]. We recorded medium larvicidal and adulticidal potential of limonene against *Aedes aegypti*, but found higher toxicity when combined with diallydisulphide and carvone respectively [80]. So, it is not always the individual compound that act as the most active but appropriate combinations of compounds having synergistic effects would be more fruitful as insecticidal against insect pests.

#### **5. Citrus of North East India**

North East India is enriched with Citrus species having documented 23 species and 68 varieties out of the 27 species of Citrus found in India [81, 82]. It is established that some of the citrus species are endemic and some are in endangered status [83]. According to Hore and Barua [84], there are eight citrus species indigenous to this region scattered in the form of semi-wild, wild state and some in cultivated

state. Some of the species are naturally tolerant to viral and bacterial diseases and also for drought, cold and rainfall. For instance, *Citrus limon* is reported to resist scab, canker and gummosis, *C. indica* resistant to greening disease [84]. Due to diverse ecogeographical conditions the Citrus species and varieties of this region may bear specific traits including its aroma and essential oil constituents which needs to be investigated. However citrus crop and citrus essential oil-based industry is not yet flourishing in this part of the country. The information regarding Citrus essential oils extracted from the Citrus species grown in this particular area is relatively scanty. As the Citrus plants are rich in secondary metabolites to naturally defend an array of pathogens and pest complexes, it is expected that some of the key compounds for controlling insect pest may lie within the secondary metabolite compounds especially in the diverse aromatic essential oil part of the plants at least in the resistant citrus species.

Here we have attempted to evaluate insecticidal properties of essential oil extracted from the fruit peel of four citrus species namely *Citrus limon, Citrus maxima*, *Citrus paradisi* and *Citrus medica* grown in North Eastern part of India against one of the household ants *Dolichoderus affinis* (Hymenoptera: Formicidae)*.* Fruits of *Citrus limon, Citrus paradisi* and *Citrus medica* were collected from Udalguri district, Assam (26.7210 ̊ N, 91.9906 ̊ E) and *Citrus maxima* from a daily market at Guwahati, Assam (26.1445 ̊N, 91.736 ̊E), in September, 2020. Essential oils from fresh peels were extracted by hydro-distillation using Clevenger's apparatus. After 6 hours, essential oils were collected, anhydrous sodium sulfate was added to absorb traces of moisture and were stored at 4 ̊C till its use. The worker ants of *Dolichoderus affinis* from naturally existed colony located in the wooden frame of house wall were considered for the assessment. Fumigant toxicity of these oils were assessed following the method described by Hu et al. [34]. Six different concentrations viz. 0.25 μL, 0.5 μL, 1 μL, 2.5 μL, 5 μL and 7.5 μL. of each EO was individually loaded in 1.5 ml centrifuged tubes, evaporation of EOs was allowed by making five small holes of 1–1.2 mm diameter and placed into 500 ml properly cleaned borosilicate conical flask. Twenty worker ants were taken per flask in a replication and the flask was covered by aluminum foil and bound tightly by rubber band to prevent the loss of volatile compounds. For each concentration three replications were made. Equal number of controls set without oil were placed against each treatment. The environmental temperature range was 21–30°C and relative humidity range 56–99 during the experimental period. The Percent mortality [percent mortality = (Total no. of dead ants/Total no. of treated ants) × 100] data was recorded after 12 h and 24 h of treatment. The ants were considered to be dead if touched with a needle but did not show any movement. Based on the results sublethal concentration was determined using probit analysis with the help of SPSS and Minitab software. As shown in the figure, after treatment with *C. limon*, maximum 71.66% mortality was recorded at 12 h and 91.66% was recorded after 24 h at 7.5 μL treatment. The calculated LC50 for the oil was 2.66 μl / 500 air volume. For the EO of *C.paradisi*, maximum 15% mortality was recorded at 12 h and 66.66% mortality was recorded after 24 h of treatment. LC50 for the oil at 24 h was 7.32 μl / 500 air volume. For the EO of *C. maxima*, not more than 10% mortality was recorded even after 24 h at the highest dose applied and LC50 could not be computed for the oil. While for the EO of *C. medica*, maximum 56.6% mortality was recorded at 5 μl concentration at 12 h and maximum 88.33% mortality was recorded after 24 h at 7.5 μl. LC50 for *C. medica* at 24 h was recorded as 2.09 μl/500 air volume (**Figure 1**, **Table 1**). Highest toxic effect was recorded for *C. medica* followed by *C. limon*. Earlier Adusei-Mensah et al. [15] evaluated insecticidal properties of three citrus species viz. *Citrus aurantifolia*, *Citrus sinensis* and *Citrus limon* against *Camponotus nearcticus* (Formicidae) and recorded highest performance from *C. limon* with 95% mortality.

#### *Citrus Essential Oils: A Suite of Insecticidal Compounds DOI: http://dx.doi.org/10.5772/intechopen.95887*

#### **Figure 1.**

*Relation between concentration of EOs and respective percent mortality.*

#### **Table 1.**

*LC50 values of the individual citrus oils against Dolichoderus affinis at 24 h.*

But Guerra et al. [16] recorded only 15% mortality of *Camponotus pennsylvanicus* (Hymenoptera: Formicidae) on topical application of *C. limon*, the efficacy of which was comparatively lower than other eight EOs tested against the ant species. Not much studies on insecticidal activities of citrus EO against ants have been found to be reported. The findings showed the prospect of using *C. medica* and *C. limon* oil for controlling household ants.

#### **6. Conclusion**

With the increasing awareness of consumers for ecofriendly products and at the same time increasing resistance of insect pests against insecticides, the demand for novel, safe and effective products is increasing. As discussed above, the existing literature revealed presence of a good number of terpene compounds in different *Citrus* species which are present in different ratios although in most cases limonene is the predominant constituent. Both the crude oil as well as individual compounds possess good insecticidal and repellent properties against diverse insect pests, both indoor and outdoor. Our study also showed promising potential against *Dolichoderus affinis* while using four Citrus essential oils with higher efficacy of *Citrus medica* and *Citrus limon* essential oils. It is expected that in near future Citrus plant essential oils with their pleasant aroma and array

of chemical compounds shall take leading space in development of insecticidal and repellent products to be used in both indoor and outdoor pest management practices against insect pests.

#### **Author details**

Bulbuli Khanikor\*, Kamal Adhikari and Bikash Rabha Department of Zoology, Gauhati University, Guwahati, Assam, India

\*Address all correspondence to<EMAIL_ADDRESS>
© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Citrus Essential Oils: A Suite of Insecticidal Compounds DOI: http://dx.doi.org/10.5772/intechopen.95887*

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#### **Chapter 11**

## The Orange Peel: An Outstanding Source of Chemical Resources

*Gianfranco Fontana*

#### **Abstract**

*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 10<sup>7</sup> 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.

**Keywords:** biological activity, *Citrus sinensis*, essential oil, flavonoids, orange peels, polymethoxyphenols

#### **1. Introduction**

*Citrus sinensis* (CS) (L.) Osbeck is a perennial species growing in warm climate areas of the world and largely employed as food in form of fresh fruit, with a global production of ca. 6.7X10<sup>7</sup> tons. per year (TPY) in 2016 [1], or as a processed derivative (ca. 1.85x10<sup>7</sup> TPY) such as juice, marmalade, flavor, fragrance and coloring additive, pectin.

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 [2]. 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 [2].

Beyond the standard workup of the *Citrus sinensis* peel (CSP) waste, new perspectives have been being opened in the context of the high chemical added value of the CSP [3–5] also by the complete knowledge of the rich metabolomics profile of this species. The use of CS peel has been proposed for a variety of purposes that include the production of antioxidant-enriched dietary supplements in veterinary [6], the preparation of human dietary supplements, and nutraceuticals such as citric acid [7] and flavonoids [8, 9]. The extract of CS peel is the source of a huge variety of phytochemicals and has been investigated on several applications including its chemotherapeutic and chemopreventive potential for several relevant human pathologies, such as cancer [10, 11] and obesity [12]. The extraction procedures vary in function of the main components that have to be obtained: from the simple cold pressing of pomace and the extraction with water to obtain highly hydroxylated compounds to the employment of mixtures organic protic solvent/water and finally low polar organic solvents such as Chloroform and Ethyl acetate to obtain polymethoxylated phenols (PMF, see below). New extraction technologies such as ultrasounds and microwaves may help to obtain better extraction yields.

In the following sections, the chemical structures and the biological effects of these compounds will be discussed.

#### **2. The chemistry of** *Citrus sinensis* **peel**

#### **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 [13–15] 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 [14–16], which include anthelmintic, antiaflatoxigenic [17], antibacterial [18–20], anticarcinogenic, antifungal [21], antioxidant [17], anti-tumor [22], anxiolytic [23], food preservative [24], hepatocarcinogenesis suppressant, insecticidal and larvicidal [25], pain relief and relaxant [26]. 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 [27]. 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.

*The Orange Peel: An Outstanding Source of Chemical Resources DOI: http://dx.doi.org/10.5772/intechopen.96298*

#### **Table 1.**

*Composition of* C. sinensis *essential oil obtained from peels.*

#### **2.2 Polyphenols**

#### *2.2.1 Flavanoids*

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 [28]. It is worthy of particular mention the rarely occurring class of C-glycolflavones (**Figure 1b**, compounds **63**–**65**, **85**, **86**).

These compounds are produced *in vivo* from the biogenetic mixed pathway of the Acetate and Shikimate that implies the enantiospecific formation of the basic aromatic bicyclic framework of the flavanone, from which a huge number of flavonoids originate employing selective enzymatic hydroxylations, methylations, and glycosylation steps. As can be seen from the structures shown in **Figure 1**, most of the chemical entities found in the peel extract contain several methoxy fragments that decorate the carbon skeleton. This characteristic makes those molecules to get a rather apolar character that explains their presence in the hydrophobic environment of the waxy peel. On the contrary, compounds containing a major number of hydroxyl groups are less present in the peel and are instead more significantly concentrated in the juice of the pericarp. However, some glycosylated compounds are present in the peel. In these molecules, the aglicone bears a monosaccharide unit (mainly glucose) or a disaccharide, in most of the cases being

Rutinose (**91**) – Rhamnosyl (α1 ! 6) glucose – or Neohesperidose (**92**)- Rhamnosyl (α1 ! 2) glucose (**Figure 2**).

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

#### *The Orange Peel: An Outstanding Source of Chemical Resources DOI: http://dx.doi.org/10.5772/intechopen.96298*

**Figure 1.** *Chemical structures of flavonoids from* C. sinensis *peels.*

common, that is the presence of the high amount of bioactive polymethoxyflavonoids [29, 30](PMF) some of which are rather ubiquitous, e.g. Nobiletin **53**, Sinensetin **51**, 3<sup>0</sup> ,4<sup>0</sup> ,3,5,6,7,8-Heptamethoxyflavone **55**; some other compounds

**Figure 2.**

*Chemical structures of the disaccharides most commonly bound to flavonoids of* C. sinensis *peel.*

containing one to six methoxy groups in place of the hydroxyl groups are present at variable amounts. The presence of one or more residual hydroxy groups in the molecule may result in a higher bioavailability and in other general differences in their mechanism of biological and therapeutic actions [30, 31].

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 *Phytophthora citrophthora* [32].

Further, the composition of the PMF blend can be employed for the chemiotaxonomic characterization of the *Citrus* genus [33].

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; nonpolar 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, 34–39], anti-inflammatory [40, 41], antimicrobial [39, 42–44], antimalarial [45], antitrypanosomal [46], cardioprotective [47], anti-osteoporosis [48], anti-ulcer [49], vascular protective [50], anti-diabetes [51, 52], hepatoprotective [53, 54], neurotrophic [55], antiadipogenesis and anti-obesity [56–58], anti-hypertensive [59], cataract prevention [60], sun protection [61], metabolic syndrome control [62]. Further, it has been demonstrated [63] that while both flavonoid set **40**, **42**, **43** and the PMFs **51**–**53** were able to inhibit the anion transportin polypeptide OATP2B1 in HEK293 cells, only the PMF group displayed this inhibitory activity also for the OATP1B1 and OATP1B3 carriers.

The most abundant PMF occurring in CSP, Nobiletin **53**, was proven to possess sevral bioactivities, such as antioxidant, anti-inflammatory, cancer preventive [64] and also a significant protective effect *in vivo* against the endotoxic shock [65] and ethanol-induced acute gastric lesions [66] in mice. Further, compound **53** demonstrated the capacity to induce autophagy in human keratinocyte HaCaT cells [67], vasodilatator effect in the rat aorta [68] and to protect the intestinal barrier from the demages induced by dextran sulfate sodium [69].

#### *The Orange Peel: An Outstanding Source of Chemical Resources DOI: http://dx.doi.org/10.5772/intechopen.96298*

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 [73–76], Hs578T triple-negative breast cancer [73, 77]; colon cancer cells CaCo-2 [19], LoVo [78], 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 [84]; leukemia cells HL-60 [85]. However, data regarding a possible antitumor activity *in vivo* are still rather uncommon. An interesting example is the case of the significant reduction of the intestinal tumor mass in ApcMin/+ mice treated with a CSP extract containing various PMF [86]. Further, CSP extract and pure Naringin **47** were tested for their efficacy against a YM1 esophageal cancer in an animal model [87].

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 vitro* experimentations or animal model tests. For example, the toxicity of Hesperidin **40** was evaluated [88] in Sprague Dawley rats showing a 50% lethal dose (LD50) of about 5 g/Kg body weight (BW) and a lowest-observed-adverseeffect level (LOAEL) of ca. 1 g/Kg BW.

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 **40**, the most abundant polyphenol obtained from *C. sinensis*, would be able to bind the spyke protein of this virus thus inhibiting its activity [89]. Despite their undoubted interest, these results need to be further investigated with different experimental approaches.

The pharmacological potential of pure Hesperidin **40** was also investigated for several relevant human morbidity, such as cancer, hypertension, and ulcer [90].

#### *2.2.2 Hydroxy-acids*

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 (**93**), p-Cumaric (**94**), Caffeic (**95**), Ferulic (**96**), Sinapinic (**97**) acids, and Artepillin (**98**) were identified in some CSP extracts that showed relevant biological activities, such as antioxidant [34, 37, 38] and antidiabetes [51].

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 **99** [51], 2-Phenylethanol in Phenylethyl ester of Caffeic acid **100** [51] and ()-Quinic acid in Chlorogenic acid **101** [51]. An interesting ester derivative (**102**) in which the anomeric hydroxyl of Glucose is esterified with a O-Caffeylsinapoyl acid unit was found in the methanolic extract of a Greek cultivar of *C. sinensis* [34].

It was shown [38] 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.

**Figure 3.** *Chemical structures of cinnamic acids extracted from* C. sinensis *peels.*

#### *2.2.3 Coumarins*

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 *Citrus* taxa [91], such as *C. aurantium* (bitter orange), *C. limon*, (lemon), *C. limetta* (lime), *C. paradisi* (grapefruit) and only a few molecules of this class were Isolated from extracts of CSP endowed with activity against osteoporosis [48] and antioxidant [92]; these compounds are shown in **Figure 4**. As coumarins are relatively less common in *C. sinensis* cultivars compared to other species of the *Citrus* taxa, their rarity can be considered as a chemotaxonomic marker for *C. sinensis*.

#### *2.2.4 Catechins*

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 [38]; they are the two enantiomeric forms Catechin **113** and Epicatechin **114**, together with Epigallocatechin **115** (**Figure 5**).

*The Orange Peel: An Outstanding Source of Chemical Resources DOI: http://dx.doi.org/10.5772/intechopen.96298*

**Figure 4.**

*Chemical structure of coumarins extracted from* C. sinensis *peels.*

#### **2.3 Pectins**

Pectins [93] 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

**Figure 5.** *Chemical structure of catechins from* C. sinensis *peels.*

**Figure 6.**

*Minimal representation of a Homopolygalacturonic acid domain of the linear primary pectin structure with a 1/3 Mol. /Mol. Esterification degree.*

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**.