Keywords

12.1 Introduction

Essential oils (EOs), also known as essences, volatile oils, etheric oils, or aetheroleum, are natural products formed by several volatile compounds (Sangwan et al. 2001; Baser and Demirci 2007). In nature, they play an important role in the protection of the plants as antibacterials, antivirals, antifungals, insecticides. According to the International Standard Organization on Essential Oils (ISO 9235: 2013) and the European Pharmacopoeia (Council of Europe 2004) an essential oil is defined as the product obtained from plant raw material by hydrodistillation, steam distillation or dry distillation or by a suitable mechanical process (for Citrus fruits). The term ‘oil’ denotes the lipophilic and viscous nature of these substances, while the term ‘essential’ signifies their preciousness and typical fragrance of plants. They are used all over the world and their use is constantly increasing because of the strong demand for pure natural ingredients in many fields: cosmetics, flavors, fragrances, agriculture, food and health industries (with aromatherapy and phytomedicine). Essential oils of Citrus are the most popular natural essential oils and account for the largest proportion of commercial natural flavors and fragrances: in 2012, production of orange oil was around 55,000 tons and production of lemon oil was around 9,500 tons. Depending on the plant source, Citrus essential oils are extracted from pericarp, flower, fruit juice, crushed fruits, leaf and twigs with sometimes little green fruits.

12.2 An Historical Overview

The origin of the essential oil industry began in ancient times in the Orient, especially in Egypt, Persia and India, where the process of distillation was first employed (Guenther 1950). From the scarce and extremely vague data obtained from Herodotus (484–435 B.C.), Pliny (23–79) and his contemporary Dioscorides, appear that essential oil of which the preparation was established was turpentine and camphor oil. The Muslim civilization strongly promoted the development of the spice trade and the distillation techniques, afterwards. During the Arab domination, between the end of the first and the beginning of the second millennium, the botanist Al-Beithar reported in his ‘Dictionary of the Simple Remedies’ (1200) the first technical description of essential oil extraction from citron fruits. The art of distillation was diffused in Europe by the catalan physician Arnald de Villanova (1235–1311). It was the Swiss medical reformer Bombastus Paracelsus von Hohenheim (1493–1541) that named the effective component of a drug ‘Quinta essentia’, so opening the way for research in the preparation of essential oils after his time. A noticeable progress in the knowledge of the nature of essential oil was made in the 16th century when the Neapolitan scientist Giovanni Battista Della Porta (1537–1615), in his ‘De Destillatione libri IX’ distinguish the nature of essential oils, describes their preparation, the ways of separating the volatile oils from water and the apparatus for this purpose. In this period, the industrial exploitation started in Sicily with the extraction of essential oils from orange, lemon and bergamot fruits. In the 17th and 18th centuries, chiefly the pharmacists improved methods of distillation and made valuable investigations into the nature of essential oils. Modern investigation starts with the 19 century when, thanks to French chemists disciple of Lavoisier (J.B. Dumas, M. Barthelot and others) a systematic study of essential oil allowed to understand their chemical structure. The work of O. Wallace (1847–1931), a German chemist, is considered a milestone in investigation and thoroughly comprehension of essential oils composition.

12.3 Chemical Composition

As mentioned, an essential oil is a natural matrix produced by steam distillation or hydrodistillation; the essential oils of Citrus are the unique obtained by a mechanical procedure (Rubiolo et al. 2010; Tranchida et al. 2012; Palazzolo et al. 2013). In other words, the Citrus essential oils are necessarily (forced) by-products of the Citrus fruits juices production, since the first step of any industrial procedure of the juice production, known as cold pressing process (‘sfumatrice’, ‘pelatrice’, in line F.MC. Food Machinery Corporation) is the removal of the essential oil to avoid their mixing with juice (Arce and Soto 2008). This is because in citrus fruits the essential oils are contained in glands localized in the epicarp or flavedo, more precisely in the region immediately below the epidermis of the fruit. These glands are between 0.4 and 0.6 mm in diameter and have no walls, but are enclosed instead by the remains of decayed cell matter, with no excretory outlets.

Citrus essential oils are complex mixtures of more than 200 compounds (Ruberto 2002, Tranchida et al. 2012; Palazzolo et al. 2013), whose content depends on several factors: genotype and chemotype of the plant, ripeness of fruits, vegetative stage of plants, agro-climatic conditions, extractive and analytical processes (Fanciullino et al. 2005; Hosni et al. 2010; 2013; Luro et al. 2012). Rootstocks may also affect volatiles contents and concentrations (Verzera et al. 2003; Benjamin et al. 2013).

Citrus essential oils are characterized by the predominance of terpenoidic compounds (in particular monoterpene hydrocarbons); the oxygenated components (mono and sesquiterpene) are in most of the species/cultivars at very lower levels. The Fig. 12.1 shows selected molecular formulas of the main volatile components of Citrus essential oils. In all citrus varieties, limonene is the main component with a percentage ranging between 60 and 95% of the total oil.

Fig. 12.1
figure 1

Selection of the most representative volatile components of Citrus essential oils

Full size image

Unfortunately, the large amount of terpene hydrocarbons in Citrus essential oils represent a problem since these compounds scarcely contribute to the Citrus oil aroma and are rather unstable, being subjected to oxidation with formation of undesirable off-flavors. The removal of these components, with a procedure called ‘deterpenation’, produces oils with enhanced Citrus flavor, higher stability and increased solubility in water and alcohols, being of easier use in food and other employments. The deterpenation process can be carried out with a classic fractionated distillation or adopting new procedures, such as membrane separation, supercritical fluid extraction (SFE), extraction with ionic liquids (Ruberto 2002; Arce and Soto 2008).

A specific chemical feature characterizes the Citrus essential oil with respect to other oils. As consequence of the mechanical extraction, Citrus oils contain a variable amount of not volatile lipid components, which stratify together with the essential oils over the water layer used in the production process. These not volatile components are present in the external layer of the peel (flavedo): their concentration ranges from 1 to 15%, depending on the Citrus species, variety or cultivar and their composition is quite variable containing carotenoids, sterols and waxes, mixed with different groups of oxygen heterocyclic metabolites, such as polymethoxyflavones (PMF), coumarins and psoralens.

12.4 Main Citrus Essential Oils

Sweet orange (Citrus sinensis L. Osb) is the main and most cultivated fruit around the world. The increasing production of orange juice entails the recovering of thousands of tons of essential oils (around 75,000 tons/year, in 2013). In sweet orange essential oils are contained in fruit pericarp. All varieties/cultivars of sweet orange contain more than 90% of limonene. The large production of orange juice causes very low prices for the essential oil (it rarely reaches only 2 $/L) and the need to find new and alternative uses for it, as well as for limonene (Thomas and Bessière 1989; Ciriminna et al. 2014, 2018; Lubbe and Verpoorte 2011; Vieiria et al. 2018). The oxygenated components (the terpenoidic derivative linalool, neral, geranial and sinensal), together with some esters (neryl and geranyl acetate), and some not terpenoidic components (such as octanal and decanal aldehydes), strongly affect the sweet orange fragrance. The blood orange cvs Tarocco, Moro and Sanguinello, typical Sicilian products, have a higher content of oxygenated compounds than that of blond orange, cvs Washington navel, and Valencia. The composition of sour orange (C. aurantium L.) essential oil is very similar to that of sweet orange, with a slight difference due to a higher content of terpenoid esters that makes this oil particularly appreciated, mainly in the cosmetic sector.

Mandarin (C. reticulata Blanco) and clementine (C. clementina Hort. ex Tan.) represent the second group of Citrus fruits, after sweet orange. In mandarin, essential oils are synthesized from flowers, fruit pericarp, leaves and twigs with sometimes little green fruits. With respect to sweet orange oil, the oil of mandarin is characterized by a lower content of limonene (65–75% of total), followed by γ-terpinene (15–22%); in any case, the total amount of monoterpene hydrocarbons is similar to that observed for sweet orange. The oxygenated component is present at very low extent: the terpenes linalool, α-terpineol and sinensal, and the non terpene octanal and decanal, are the major compounds. However, the characteristic component of mandarin oil is the presence of an unusual nitrogen derivative, methyl-N-methyl anthranilate, which, notwithstanding its low amount, confers the typical mandarin fragrance. Clementine is a natural hybrid of sweet orange and mandarin and its essential oil is very similar to that of orange oil with over 95% of limonene, and very low amounts of the oxygenated components linalool, octanal and decanal.

Lemon (C. limon L. Burm. f) and lime (C. aurantifolia L.) represent the third group of Citrus fruit in order of importance. In lemon, essential oils are contained in flower, fruit pericarp, leaf and twigs with sometimes little green fruits, while in lime in pericarp, fruit juice or crushed fruits. The monoterpene hydrocarbon portion of these two species is very similar, being characterized by about 60% of limonene and 10–12% of the 2 monoterpene hydrocarbons, β-pinene and γ-terpinene. The difference between the two species is due to the oxygenated portion: in lemon, two terpenoidic aldehydes, neral and geranial (usually defined together as citral), confer the typical lemon fragrance (other components concurring to the total aroma are the esters neryl and geranyl acetate), while in lime, the oxygenated portion is due to 1,8-cineole, terpinen-4-ol and α-terpineol. It is to underline that the lime oil is the only Citrus essential oil obtained by distillation of whole fruit.

Grapefruit (C. paradisi Macf.) is notified as a natural hybrid between sweet orange and pummelo (C. maxima (Burm.) Merr). In grapefruit, essential oils are obtained from the fruit pericarp. As well as in sweet orange, grapefruit oil is characterized by over 90% of limonene. Octanal, decanal and linalool are recorded as the major components of the aromatic portion. A specific component, the oxygenated sesquiterpene nootkatone, confers a typical aroma to grapefruit essential oils.

Bergamot (C. bergamia Risso) is produced in Calabria-Italy (95% of worldwide cultivation) and, to a small extent, in Ivory Coast, Guinea and Brazil. Not edible owing to its high bitterness, the fruit is cultivated mainly for the production of essential oils. The essential oil of bergamot is used mainly in cosmetic and perfumery industries (Forlot and Pevet 2012), where it has a great commercial value, but also in the food and confectionery industries as a flavoring for liqueurs, teas, toffees, candies, ice creams, and soft drinks. It has a peculiar composition: the content of the monoterpene hydrocarbon fraction rarely reaches 60% (limonene, γ-terpinene and β-pinene are the main components), whereas the oxygenated fraction is highly represented, unlike the other Citrus oils, being linalyl acetate about the 30% and linalool about 10% of the total oil composition. These two compounds define the flavor notes of the bergamot oil; for this reason, international buyers evaluates the quality of a bergamot oil according to the amount of oxygenated compounds and, in particular, of linalool and linalyl acetate.

Yuzu (C. junos Sieb. ex Tanaka) is well-known in far Eastern countries because of the pleasant aroma from the outer rind. Recently, yuzu essential oil has gained a great interest due to its unique properties industrially used in sweet production, beverages, cosmetics and perfumery, and also in aromatherapy (Sawamura 2005). Limonene is the most predominant compound of yuzu oil (63.1–68.1%).

Petitgrain oil is produced by the distillation of leaves and twigs of all Citrus species (orange, mandarin, lemon), even though the most appreciated one is that coming from sour orange, which is particularly rich in linalyl acetate (ca. 50%) and linalool (ca. 30%). Neroli oil is obtained by distillation of flowers of C. aurantium. Also in this case, the oxygenated terpenes linalool, linalyl acetate, nerolidol and geranyl acetate are the main components. Because of these particular features and the very low oil yield, both these oils are very expensive. They are used mainly in the preparation of exclusive perfumes.

12.5 Uses

The current use of citrus essential oil sweeps in a wide range of fields:

Pharmaceutical and Therapeutic. A vast number of studies demonstrate the pharmaceutical and therapeutic potential of essential oils and their individual constituents (Burt, 2004; Edris 2007; Bakkali et al. 2008). Their role and mode of action have been studied with regard to the prevention and treatment of cancer, cardiovascular diseases including atherosclerosis and thrombosis, as well as their bioactivity as antibacterial, antiviral, antioxidants, anti-inflammatory, analgesic and antidiabetic agent. The phenolic component present in essential oils has been recognized as the bioactive constituent with antimicrobial activity. The mechanism of action is not fully understood: essential oils components act involving several targets in the bacterial cells, rendering the microbial cell membrane permeable and leading to loss of homeostasis, leakage of cell contents and death. Essential oils and their individual components showed cancer suppressive activity when tested on a number of human cancer cell lines including glioma, colon cancer, gastric cancer, human liver tumor, pulmonary tumors, breast cancer, leukemia and others (Bhalla et al. 2013). Recent studies performed with the Citrus essential oils established their potential anticancer effectiveness and assessed their efficiency in reducing local tumor volume or tumor cell proliferation by apoptotic and/or necrotic effects (Visalli et al. 2014). Syrian C. limon essential oil showed a cytotoxic effect on the human colorectal carcinoma cell line LIM1863 when studied in vitro (Jomaa et al. 2012). Celia et al. (2013) and Navarra et al. (2015) verified that bergamot essential oils exhibited anticancer activity in different in vitro assays against human neuroblastoma cells. More recently, a Chinese group showed the inhibitory effect on the proliferation of human lung cancer and prostate cell lines of the essential oil from a Navel orange peel (Yang et al. 2017). A review dealing with several therapeutic effects of limonene reported how this compound alone or in combination with other natural products exerted anticancer effects against human gastric and colon cancer cells (Vieria et al. 2018).

Aromatherapy is a complementary and alternative therapy that has gained a lot of attention in the last 15-20 years. It uses essential oils as the main therapeutic agents. Essential oils are administered through inhalation, massage or application on the skin surface, providing a feeling of well-being to the body and showing a curative potential on mind and spirit (Maeda et al. 2012). Compounds from essential oils enter the body (via the olfactory mucosa or the bloodstream by lung absorption) and may directly influence the brain’s limbic region, affecting a person’s emotional responses, heart rate, blood pressure and breathing. Recent clinical trials revealed the positive effect of lemon oil inhalation on nausea and vomiting of pregnancy, of citrus aurantium oil on anxiety, and bergamot oil on mood states, parasympathetic nervous system activity and salivary cortisol levels (Yavari Kia et al. 2014; Namazi et al. 2014; Watanabe et al. 2015).

Cosmetic. The pleasant odor and the distinctive taste make the essential oils one of the most important components in flavoring and perfume industries (Burt 2004; Sawamura 2011). Due to its intense fragrance and freshness and the ability to fix the aromatic bouquet of aromas, the essential oil of bergamot is used as one of the main basic constituents for the manufacture of perfumes.

Agricultural. The environmental problems caused by the massive application of pesticides (high toxicity, non-biodegradable properties, residual effects in soils, water resources and crops, selective resistance) in agriculture have been the matter of concern for the public opinion in the last years. In addition, the regulatory measures for pesticides use have become stricter. Current control focuses more on the use of alternative contact pesticides and other innovative greener phytosanitary methods. Natural products are excellent alternatives to synthetic pesticides. The properties that make them suitable for use in insect management include multiple modes-of-action and sites-of-action in the insect nervous system and elsewhere; these may account for the wide range of pesticidal actions (viz., contact, knockdown, fumigant toxicity) and sublethal behavioural actions (viz., deterrence, repellence). Owing to their volatility, the oils and their constituents are environmentally non-persistent. Toxicological tests indicate that most essential oil chemicals are relatively non-toxic to mammals and fish, and meet the criteria for ‘reduced risk’ pesticides. The insecticide activity of orange oils and the repellent capacity of lemon oils has been proved (Koul et al. 2008; Raina et al. 2007; Jaenson et al. 2006).

Food industries. Control of food spoilage and pathogenic bacteria is mainly achieved by chemical control, but the use of synthetic chemicals is often associated to undesirable aspects, such as carcinogenicity, acute toxicity, teratogenicity and slow degradation periods. Moreover, the emergence of bacterial antibiotic resistance in the food chain is a further concern. Demand of consumers for food without synthetic and harmful chemicals is therefore increasing. Consequently, interest in natural, non-synthesized food additives as potential alternatives to conventional antimicrobials to extend shelf life, combat food pathogens, improve the quality of stored food products and protect the environment, has heightened. Among natural products, EOs are gaining interest as potential food additives and are widely accepted by consumers because of their relatively high volatility, ephemeral and biodegradable nature (Burt 2004; Holley and Patel 2005; Hyldgard et al. 2012; Rivera Calo et al. 2015). Among the great variety of essential oils, citrus fruit EOs and their major components have received attention in the food industry since they have been recognized as safe (GRAS) by the Food and Drug Administration (2005) and many foods tolerate their presence (Fisher and Phillips 2008). The antimycotoxigenic activity of Citrus EOs in food system has been proved by Phillips et al. (2012) that reported a strong inhibition of mycelial growth for the phytopathogenic fungi Penicillum chrysogenum, Aspergillus niger and Alternaria alternata on grain, following the application of vapour of citrus EO. Essential oils from C. reticulata, C. maxima, C. sinensis and C. aurantifolia displayed broad fungitoxic spectrum and anti-aflatoxigenic activity against different food contaminating moulds (Razzaghi-Abyaneh et al. 2009; Singh et al. 2010a; 2010b; Velazquez-Nunez et al. 2013; Jing et al. 2014; Trabelsi et al. 2016). Moreover, there would be no chance of alteration in the organoleptic properties of food commodities when citrus EOs or their components are used as preservatives because the monoterpenes present in these oils are widely used as natural ingredients in many food products, soaps, soft drinks, cosmetics and perfumes for their lemon-like flavour and odor (Shukla et al. 2009).

Essential oils exert also potent and broad-spectrum antimicrobial activity in vitro, and to a smaller degree in foods, against common food-borne pathogens (Oussalah et al. 2007; Callaway et al. 2011; Muthaiyan et al. 2012). Limonene, the major chemical component of citrus EOs, and orange terpenes, alone or in combination, showed lethal effects against 11 different strains of Salmonella on a disc diffusion assay (O’Bryan et al. 2008), against Campylobacter spp and Cinnamonum coli (Nannapaneni et al. 2009), against Escherichia coli and Salmonella onto beef at the chilling stage of processing (Pittman et al. 2011). The antimicrobial activity, however, seems to be strictly oil-dependent and it is very hard to know which constituents or mixtures of them are responsible for the bacteriostatic or bactericide effect (Mandalari et al. 2007; Espina et al. 2011); in general, Gram-positive organisms seem to be much more susceptible to EOs than Gram-negative organisms (Rivera Calo et al. 2015). The substitution of synthetic additives with EOs with antimicrobial effect is still premature due to high cost, food matrix composition and possible sensory changes of food characteristics as a function of the EOs dose. The application of citrus EOs might be recommended to reduce the use of chemical additives, to maximize the use of existing resources and to minimize adverse effect of by-products in the environment.

Recent research has focused on the development of edible/biodegradable packaging for food product as substitute of conventional plastic materials. In this contest, the incorporation through emulsification of citrus essential oils into edible films positively impact the most relevant properties of edible films and coatings, namely microstructural, physical, antioxidant and antimicrobial (Sanchez-Gonzales et al. 2010; Tongnuanchan et al. 2012; Atarés and Chiralt, 2016).

Nanoformulations can solve problems related to EO application on large scale as volatility, hydrophobicity and tendency to oxidize (Campolo et al. 2017). In order to enhance the antimicrobial activity of essential oils in food, protect the essential oil from oxidation or evaporation and minimize the impact on the quality attributes of the final product, a nanoencapsulation delivery system has been positively tested (Donsì et al. 2011). Nanometric delivery system, due to the subcellular size, may increase the passive cellular absorption mechanisms, reducing mass transfer resistances and increasing antimicrobial activity (Donsì et al. 2011). Several examples of application of nanoformulated citrus essential oils have been reported as biocides for pest control (Campolo et al. 2017) and food preservatives (Ribeiro-Santos et al. 2017) and additive for develop new active food packaging materials (Vilela Dias et al. 2013).

Other uses

Citrus essential oils have been also evaluated in the field of conservation of cultural properties and for their effects in the control of biodeterioration of documentary heritage. C. sinensis oils in the vapor phase showed significant inhibitory activity against fungal and bacterial strains isolated from different documentary supports and indoor environments of repositories, without negative environmental and human impacts (Borrego et al. 2012).

12.6 Future Perspective and Strategies

Consumption of citrus essential oil is constantly enhancing year after year because of the strong demand for pure natural ingredients in many fields. Thus, increasing the production of citrus essential oil has become a crucial objective for several breeders. The target, however, presents severe difficulties. Conventional breeding methods effective in creating useful variability and improve essential oil yield and uniformity are hampered in Citrus because of several factors: apomixis, diffuse pollen and ovule sterility, sexual incompatibilities, long juvenile period, are the most relevant. Moreover, the poor knowledge of the metabolic pathways by which essential oils are biosynthesized, makes the challenge even more complicated. The ever-increasing demand for citrus oils together with their high cost and, for some of them, their scarcity, encourages the flavour industry to consider biotechnology as an appropriate tool for improvement of citrus oil yield and quality. The application of biotechnological approaches to citrus essential oil improvement start from a quite low baseline, however, a range of biotechnological tools, such as somatic hybridization and molecular genetics, can help to circumvent some of the barriers associated with the reproductive biology of citrus. Somatic hybridization via protoplast fusion is an additive process capable to capture the genetic diversity of the gene pools by combining (fusing) the nuclear, chloroplast and mitochondrial genomes of desired parental protoplasts in novel arrangements, therefore creating unobtainable homokaryon or heterokaryon biotypes (Davey et al. 2005; Eeckhaut et al. 2013; Grosser and Gmitter 1990; Johnson and Veilleux 2001). The potential heterozygosity is extremely large depending on cumulative allelic differences between the contributing parents (Grosser and Gmitter 2011). The large extent of genomic arrangement and recombination following ploidy manipulation may have a deep impact on the chemical composition of somatic hybrid fruits, aiming to a presence of distinctive and original traits in phytochemical characters (Gancel et al. 2002, 2003; 2005a; 2005b; Tusa et al. 2007; Abbate et al. 2012; Fatta Del Bosco et al. 2013; 2017; Napoli et al. 2016). Gene expression differences between citrus allotetraploid somatic hybrids and their parents have been analyzed through quantitative RT-PCR assay. It revealed that the genes controlling the biosynthetic pathways of aromatic compounds (of the peel oil) are not inherited in an additive fashion in the allotetraploid hybrids but may be subject to dosage effects, likely over-dominance, co-dominance and other complex interactions in gene expression regulation (Gancel et al. 2003; Bassene et al. 2009a, 2009b).

In the last two decades, omics approaches (genomics, transcriptomics, proteomics, metabolomics, hormonomics, ionomics or phenomics), has been increasingly employed to gain insight into the biology of yield in plants (Swanson-Wagner et al. 2009; Syrenne et al. 2012; Thao and Tran 2016). Omics technologies have enhanced the knowledge on cellular processes, including gene and protein regulations, and metabolic pathways for economically important traits (Galland et al. 2012; Hong et al. 2016).

Development of high-throughput sequencing technologies, such as Illumina (https://www.illumina.com/techniques/sequencing.html), PacBio (https://www.pacb.com/), Optical Mapping (http://rtlgenomics.com/bionano/), have made easy the whole sequencing and assembly of complex genomes of plants. Nowadays genomes of several crop are sequenced (http://www.genome.jp/kegg/catalog/org_list.html) and available, such as grapevine (Jaillon et al. 2007), apple (Velasco et al. 2010), tomato (Sato et al. 2012), Valencia orange (Xu et al. 2012), asparagus (Harkess et al. 2017). The genome comparison of sequences and re-sequencing of different cultivars are an effective approach to identify genes involved in specific traits, including regulation and production of bio-compound.

Different omic approaches focused on citrus genus. Transcriptome and proteome analyses of late-ripening sweet orange mutant were carried out (Wu et al. 2014; Zhang et al. 2014), highlighting the presence of multiple ripening events in citrus which suggested the key role of abscissic acid (ABA), sucrose and jasmonic acid (JA) in citrus ripening. Recently, Voo and Lange (2014) reported a protocol for the isolation of essential oil gland cells of citrus fruit peel through single cell omics. Katz et al. (2010) identified by proteomic approach 1,500 proteins in citrus fruit juice sac cells, quantifying their amounts at three developmental stages and developing a protein database with a comprehensive sequence database of citrus genes, ESTs and proteins, named iCitrus. Metabolome profiles of different citrus species were associated to sensitivities against greening (HLB) disease. Higher levels of the amino acids, organic acids and galactose were observed in HLB sensitive sweet orange varieties (Cevallos-Cevallos et al. 2012). In addition, differences in the phenylalanine, histidine, limonin and synephrine were observed in asymptomatic and symptomatic fruits (Chin et al. 2014), suggesting as metabolomics can generate biomarkers for important traits in citrus.

Omics were also used to investigate the regulation of oil biosynthesis in seed with the aim to increase their yields in several crops (Hajduch et al. 2011; Gupta et al. 2017). Since, in citrus, the essential oils composition provides valuable information related to organoleptic properties linked to product quality, the comprehensive untargeted analysis of biochemical constituents is the major objective of metabolomic studies. Due to their high added value, careful attention was paid to ensure the oils’ genuineness and authenticity. In this way, Mehl et al. (2015) developed a multiblock data modelling to integrate heterogeneous signals collected from GC-FID, H-NMR, UHPLC-TOF/MS- and UHPLC-TOF/MS + platforms to obtain a complete characterisation of cold pressed lemon oil (CPLO), identifying relevant biomarkers.

12.7 Conclusions

The increasing demand in citrus natural extracts from the manufacturers of foods, cosmetics and pharmaceuticals and the possibility of linking the chemical contents with particular functional properties call for further and strong efforts in developing new studies on essential oils of citrus species and varieties.

In the future, genomic approaches such as genome resequencing, allele mining and genomic selection, will integrate the techniques for genotype obtaining, characterization, and selection, allowing the recovery and build-up of desirable crop phenotypes in novel and targeted citrus hybrid species.