Volatile Constituents of Some Myrtaceous Edible and Medicinal Fruits from the Brazilian Amazon

1. Introduction

The Amazon region is the last stronghold of potentially valuable plants awaiting domestication and economic exploitation. The use of exotic fruits introduced worldwide, such as apples and oranges, has been improved for centuries in a continuous process, whose initial memory has already been lost to time. Humanity began to domesticate plants around ten thousand years ago, while the history of the domestication of Amazonian fruits has only begun to be written [1].

Several fruit species native to the Amazon have been commercialized and consumed for medicinal and nutritional purposes. They are known for having a pleasant flavor and juicy pulp, representing significant economic potential; however, they still require domestication and genetic improvement studies. These studies must be resumed urgently due to the increase in deforestation, which represents a high risk of species extinction, in addition to the fact that some still need to be adequately studied and scientifically classified [2,3,4,5,6,7].

A significant variety of fruits are consumed in Brazil, representing one of the primary sources of vitamins, minerals, fiber, aromas, and antioxidants in the diet of the native population. In this context, considered one of the great centers of global biodiversity, Brazil has many tropical fruits with different and pleasant flavors [2]. Due to its territorial extension, geographical position, climate, and soil, Brazil produces fruits in tropical, subtropical, and temperate areas, being the third largest fruit producer in the world, after China and India, with 42 million tons per year and more than 2.2 million hectares planted across the country. More than 30% of fresh fruits produced in Brazil are exported to different parts of the world [3,4].

In this scenario, the region of the Middle Amazon River, with occurrence in the State of Pará and made up of the municipalities of Óbidos, Juruti, Oriximiná, Terra Santa, and Santarém, has made available several species of native fruits, some produced on a large scale and sold for different purposes, in addition to others fruits unnoticed by consumers, resulting from limited stocks and logistics (IBGE, 2019) [8]. In addition to nutritional value, other attributes existing in native fruits and those cultivated in this region of the Middle Amazon River are their outstanding aromas, composed of a significant diversity of different volatile constituents which, despite representing a low percentage of the total mass of the fruit (around 0.05% to 1.0%), contribute to the taste, flavor, and acceptability of these fruits. Furthermore, scientific knowledge of the chemical constituents responsible for the characteristic aromas of tropical fruits is justified by the importance they can play in the quality of their products. The attractive tropical fruit flavor has stimulated growing consumer interest around the world. In this context, the Amazon stands out for its outstanding natural diversity of fruits, with characteristic flavors that require the identification of their volatile constituents, also representing a promising area for research into the typical aromas of this region [7].

In fruits, the biosynthetic routes for forming volatile constituents involve enzymatic reactions, producing volatile components terpenes, sulfur compounds, derivatives of fatty acids, derivatives of amino acids, and those originating from fermentation. The enzymatic generation of volatile constituents derived from fatty acids is one of the main routes that leads to the formation of the characteristic flavor of fruits. As the reactions occur, the aroma of the fruit can change, and aldehydes and ketones, for example, can be converted into the corresponding alcohols, presenting more prominent aromas [9].

The present work aimed to characterize the chemical composition of the volatile concentrates of the Myrtaceae edible fruits Eugenia stipitata (Araçá-boi), Eugenia uniflora (Ginja), Myrciaria dubia (Camu-Camu), Psidium guajava (Goiaba), Psidium guineense (Araçá), sold in fairs and markets in the cities of Santarém, Juruti, Oriximiná, and Terra Santa, in the Lower Amazon River region, Brazil (see Figure 1).

2. Materials and Methods

2.1. Plant Material

The fruits Eugenia stipitata (Araçá-boi), E. uniflora (Ginja), Myrciaria dubia (Camu-camu), Psidium guajava (Goiaba), and P. guineense (Araçá) (Figure 2, Figure 4, Figure 6, Figure 8 and Figure 10), which provided their pulps for this work, were obtained in fairs and markets in the cities of Santarém, Juruti, Oriximiná, and Terra Santa, occurring in the Lower Amazon River region, Brazil. The selection of fruits was made considering those with integral characteristics, a natural shape without deformations, and the absence of possible microbiological contamination. The fruits were washed in running water, measured, and weighed, and their pulp (edible part) was processed to remove seeds and skins. Then, the fruit pulps were frozen for subsequent analysis.

2.2. Obtaining and Analyzing Volatile Concentrates

The fruit pulps were subjected to micro distillation-extraction in a Likens & Nickerson-type apparatus (30 g in total, 2h, duplicate) to obtain their volatile concentrates, using n-pentane (99% HPLC grade, 3 mL) as the solvent [10].

The volatile concentrates were submitted to GC and GC-MS analysis. It was performed on a GCMS-QP2010 Ultra system (Shimadzu Corporation, Tokyo, Japan), equipped with an AOC-20i auto-injector and the GCMS-Solution software containing the Adams (2007) and Mondello (2011) libraries [11,12]. A Rxi-5ms (30 m × 0.25 mm; 0.25 μm film thickness) silica capillary column (Restek Corporation, Bellefonte, PA, USA) was used. The conditions of analysis were as follows. Injector temperature: 250 °C; Oven temperature programming: 60–240 °C (3 °C min^-1); Helium as the carrier gas, adjusted to a linear velocity of 36.5 cm s^-1 (1.0 mL min^-1); split mode injection (split ratio 1:20) of 1.0-2.0 µL of the n-pentane solution; electron ionization at 70 eV; ionization source and transfer line temperatures of 200 and 250 °C, respectively. The mass spectra were obtained by automatically scanning every 0.3 s, with mass fragments in the 35–400 m/z. The retention index was calculated for all volatile components using a homologous series of C8-C40 n-alkanes (Sigma-Aldrich, Milwaukee, WI, USA) according to the linear equation of van den Dool and Kratz (1963) [13]. Individual components were identified by comparing their retention indices and mass spectra (molecular mass and fragmentation pattern) with those existing in the GCMS-Solution system libraries [11,12]. The quantitative data regarding the volatile constituents were obtained using a GC2010 Series gas chromatograph, operated under conditions similar to the GC-MS system. The relative amounts of individual components were calculated by peak-area normalization using a flame ionization detector (GC-FID). Chromatographic analyses were performed in duplicate.

2.3. Multivariate Statistical Analysis

Principal Component Analysis (PCA) was applied to verify the interrelationship of the samples of volatile concentrates analyzed with the classes of identified compounds, monoterpene hydrocarbons (MH), oxygenated monoterpenes (OM), sesquiterpene hydrocarbons (SH), oxygenated sesquiterpenes (OS), benzenoids/phenylpropanoids (BP), and fatty acids derivatives (FA). The data matrix was standardized for multivariate analysis by subtracting the mean and dividing it by the standard deviation. Hierarchical Cluster Analysis (HCA), considering the Euclidean distance and complete linkage, was used to verify the similarity of the samples based on the distribution of the constituents selected in the PCA analysis (Software Minitab, free version 390, Minitab Inc., State College, PA, USA) [14].

3. Results and Discussion

3.1. Eugenia stipitata McVaugh - Myrtaceae

Botanical description: It is an ornamental leafy tree or shrub known as Araçá-boi, with 3.0-15.0 m tall, densely branched habit, without apical dominance; stem with brown to reddish-brown; bark flaking; young branches covered with short, velvety, brown hairs that are lost with age. Leaves opposite, simple, without stipule; petiole short, 3 mm long; blade ovate to somewhat broadly elliptic, 8-19 cm long, 3.5-9.5 wide; apex acuminate; base rounded and often subcordate; margins entire; leaves dull, dark green above, with 6-10 pairs of impressed lateral veins; pale green, shortly pilose, with scattered hairs below. Inflorescence racemose pedicles long; bracteoles linear, 1-2 mm long; calyx lobes rounded, broader than long, overlapping in bud; petals 5, white, obovate, 7-10 mm long, 4 mm wide, ciliate; stamens about 70, 6mm long; ovary 4 locular, each locule with 5-8 ovules; style 5-8 mm long. Fruit an oblate or spherical berry, 2-10 x 2-12 cm, weighing 50-750 g, light green at first, turning pale or orange-yellow when ripe, soft, with a thin, velvety skin enclosing a juicy, thick pulp that accounts for as much as 60% of the fresh fruit. There are approximately 12 seeds in each fruit (see Figure 2) [15]. They are fruiting from November to May in all Amazon regions. The pleasant-tasting Araçá-boi fruit is rich in vitamins A, B1, and C, and it is used in soft drinks, juices, ice creams, and sweets.

Figure 2. Eugenia stipitata fruits – trivial name Araçá-boi.

Figure 2. Eugenia stipitata fruits – trivial name Araçá-boi.

Synonimy:Eugenia stipitata subsp. stipitata McVaugh, E. stipitata subsp. sororia McVaugh [5].

Geographic distribution: It is a fruit tree native to the Peruvian Amazon. It is found in the wild in many areas of the region, and its multiplication occurred in the Ucaiali River basin in Peru. In the state of Amazonas, Brazil, it is cultivated on a domestic scale by the caboclo and indigenous populations of the Solimões River [5].

Monoterpenes hydrocarbons (28.5%), oxygenated monoterpenes (25.5%), and oxygenated sesquiterpenes (20.9%) predominated in the volatile concentrate of E. stipitata, followed by sesquiterpene hydrocarbons (13.1%) and fatty acid and derivatives (10.8%). The main constituents were α-pinene (17.4%), citronellyl butanoate (15.6%), pogostol (13.5%), α-terpineol (9.6%), β-pinene (6.8%), δ-elemene (4.1%), hexyl hexanoate (3.5%), epi-α-muurolol (3.2%), and γ-muurolene (2.6%) comprising 76.3% of its volatile concentrate (see Figure 3).

The volatile composition of the fruits and leaves of E. stipitata have been previously reported: a fruit sample collected in Manaus, Brazil, showed germacrene D, β-pinene, and α-pinene as main constituents [16]; a fruit sample collected in Caquetá, Colombia exhibited ethyl octanoate, ethyl dodecanoate, ethyl decanoate, 1-hexanol, 2-methyl-butanoic acid, hexanoic acid, and octanoic acid, in decreasing order [17]; a leaf sample collected in Azores, Portugal, showed (E)-caryophyllene, caryophyllene oxide, and α-pinene as primary compounds [18]; and in a leaf sample collected in the Araripe region, Pernambuco, Brazil, β-eudesmol, γ-eudesmol, elemol, and caryophyllene oxide predominated as the main constituents [19].

Figure 3. Ion-chromatogram of the Eugenia stipitata fruit volatile concentrate.

Figure 3. Ion-chromatogram of the Eugenia stipitata fruit volatile concentrate.

3.2. Eugenia uniflora L. - Myrtaceae

Botanical description: It is a shrub 1.5 to 8.0 m, branched from the base, known as Ginja and Pitanga. Leaves simple, opposite, chartaceous, ovate, 1.5-5.0 m long and 1.0-3.5 m wide, dark green and shiny, shortly petiolate, rounded base, and short obtuse-acuminate apex. Flowers solitary or in groups of 2 to 3, axillary, filiform pedicels 2-3 cm long; corolla 4 white petals, slightly fragrant, numerous stamens. Fruit, an oblate berry 2-3 cm in diameter with 7-10 longitudinal buds, persistent calyx, smooth, shiny skin, red when ripe; orange pulp, juicy, sweet flavor, little astringent, 1-2 greenish-white seeds [5] (Figure 4). Its fruiting has been observed throughout the year. With a pleasant flavor, the Ginja, or Pitanga, fruit is consumed fresh, in salads and in the preparation of jellies and ice cream.

Figure 4. Eugenia uniflora fruits – trivial names Ginja and Pitanga.

Figure 4. Eugenia uniflora fruits – trivial names Ginja and Pitanga.

Synonimy: Eugenia brasiliana L. (Aubl.), E. costata Cambess, E. indica Nicheli, E. michelii Lam., E. microphylla Barb. Rodr., Myrtus brasiliana L., M. willdenowii Spreng., Plinia rubra L., Stenocalyx affinis O. Berg, S. Michelii (Lam.) O. Berg, S. uniflorus (L.) Kausel, Syzygium michelii (Lam.) Duthie, among others [20].

Geographic distribution: Originally from Brazil, this fruit is spread throughout South America, the Caribbean islands, Central America and South Florida.

The primary compound classes of E. uniflora volatile concentrate were sesquiterpene hydrocarbons (41.3%), oxygenated sesquiterpenes (39.8%), and monoterpene hydrocarbons (17.5%), while its main constituents were curzerene (30.5%), germacrone (15.4%), atractylone (13.1%), (E)-β-ocimene (11.1%), (Z)-β-ocimene (4.6%), and trans-β-elemenone (4.1%) comprising 78.8% of the volatile concentrate (see Figure 5).

The volatile composition of the fruits and leaves of E. uniflora have been previously reported: a fruit sample collected in Pinar del Rio, Cuba exhibited curzerene, bergaptene, myrcene, (E)-β-ocimene, and limonene as primary constituents [21]; a fruit sample collected in Pernambuco, Brazil, showed (E)-β-ocimene, (Z)-β-Ocimene, and β-pinene [22]; in a fruit sample collected in Pelotas, Rio Grande do Sul, Brazil, predominated hexadecanoic acid, (E)-β-ocimene, α-selinene, and germacrene B [23]; in the fruit and leaves samples collected in Ibadan, Nigeria, the major compounds were curzerene, selina-1,3,7(11)-trien-8-one, selina-1,3,7(11)-trien-8-one epoxide, atractylone, furanodiene, and germacrone [24]. The essential oil of leaves and thin branches of E. uniflora, cultivated in the city of Belém, Brazil, was investigated and the main components were germacrone, curzerene, and germacrene B (15.6%) [25]; in the oil of leaves collected in Goiânia, Santo Antonio de Goiás, Nova Veneza e Anápolis, Goías, Brazil, the main constituents were germacrene A, B, and C, atractylone, curzerene, selina-1,3,7(11)-trien-8-one, and selina-1,3,7(11)-trien-8-one epoxide [26].

Figure 5. Ion-chromatogram of the Eugenia uniflora fruit volatile concentrate.

Figure 5. Ion-chromatogram of the Eugenia uniflora fruit volatile concentrate.

3.3. Myrciaria dubia (Kunth) McVaugh - Myrtaceae

Botanical description: It is a small shrub measuring 1-3 m, reaching up to 8 m, known as Camu-Camu. Leaves simple, opposite, elliptical or broadly ovate, 6-10 cm long and 1.5-3.0 m wide, obtuse or rounded base, long-acuminate apex, delicate lateral veins. Axillary inflorescences, formed by subsessile flowers arranged in decussate pairs, white, fragrant. The fruit is a spherical berry measuring 2.0-2.5 cm in diameter, with a thin, smooth, shiny skin, red to blackish-purple in color, slightly pinkish juicy pulp, with 2 seeds [5] (Figure 6). They are fruiting from November to March in all Amazon regions. The fruit has an acidic flavor due to its high vitamin C content. The Camu-Camu fruit has an acidic flavor due to its vitamin C content and is used as a soft drink, ice cream, liqueur, jellies, and sweets.

Figure 6. Myrciaria dubia fruits – common name Camu-Camu.

Figure 6. Myrciaria dubia fruits – common name Camu-Camu.

Synonimy: Psidium dubium Kunth, Eugenia grandiglandulosa Kiaersk, Marlierea macedoi D. Legrand, Myrciaria divaricata (Benth.) O. Berg, M. lanceolata O. Berg, M. obscura O. Berg, M. paraensis O. Berg, M. phillyraeoides O. Berg, M. riedeliana O. Berg, M. spruceana O. Berg, Myrtus phillyraeoides (O. Berg) Willd., Psidium dubium Kunth, among others [27].

Geographic distribution: This species is distributed northwest of the Brazilian Amazon, Peru, and Venezuela in semi-flooded areas.

Monoterpenes hydrocarbons (79.6%) and oxygenated monoterpenes (11.5%) predominated in the volatile concentrate of E. stipitata, followed by sesquiterpenes hydrocarbons (5.2%). The main constituents were α-pinene (55.8%), (E)-β-ocimene (13.1%), α-terpineol (10.0%), (E)-caryophyllene (4.2%), limonene (3.7%), terpinolene (2.9%), and β-pinene (2.6%) comprising 92.3% of the volatile concentrate (see Figure 7).

Franco and Shibamoto (2000) [16] also identified α-pinene, limonene, and β-caryophyllene as the major constituents of the volatile concentrate of Camu-Camu fruit collected in Manaus, Brazil. Furthermore, Quijano and Pino (2007 [28] highlighted limonene, α-terpineol, and α-pinene as significant components of a volatile concentrate extracted from fruits sampled in Caquetá, Colombia. The characterization of the aroma of Camu-Camu was recently reported, and limonene, (E)-caryophyllene, a-pinene, and isoamyl acetate were the compounds that most contributed to the fruity, herbal, citrus, and woody notes of the M. dubia fruit also collected in Caqueta, Colombia [29]. The essential oil from M. dubia leaves sampled in Belém, Brazil, exhibited α-pinene, (E)-caryophyllene, and caryophyllene oxide as its primary constituents [30].

Figure 7. Ion-chromatogram of the Myrciaria dubia fruit volatile concentrate.

Figure 7. Ion-chromatogram of the Myrciaria dubia fruit volatile concentrate.

3.4. Psidium guajava L. - Myrtaceae

Botanical description: It is a small tree, 10-12 m; stem irregular, tortuous, very branched, light green, quadrangular branches; thin, smooth, greenish-brown bark, exfoliating frequently. Leaves simple, opposite, short-petiolate; limbus sub-coriaceous, elliptical, 5-15 cm long, 4-6 cm wide, apex obtuse, acute or sub-acuminate, base obtuse-rounded; conspicuous, straight and parallel lateral ribs. Flowers axillary, solitary, tubular-swollen hypanthium, thick greenish-white sepals; 4-5 white petals, rounded and very deciduous; stamens numerous white; inferior ovary. Fruit is a rounded, ovoid, or pyriform berry of varying size, greenish or yellow skin, with numerous seeds, fleshy and edible [5] (Figure 8). Fruiting in two periods, from April to June/July and from November to January/February, the Goiaba is much appreciated in its natural state, with its sweet, aromatic pulp. Its primary use is in sweets, jams, jellies, juices, and ice creams.

Figure 8. Psidium guajava fruits – common name Goiaba.

Figure 8. Psidium guajava fruits – common name Goiaba.

Synonimy: Guajava pumila (Vahl) Kuntze, G. pyrifera (L.) Kuntze, Myrtus guajava (L.) Kuntze, Psidium angustifolium Lam., P. aromaticum Blanco, P. fragrans Macfad., P. guajava L. var. guajava, P. guayava Radii, P. pyriferum L., Syzigium ellipticum K. Schum. & Lauterb., among others [31].

Geographic distribution: It is a fruit of pre-Columbian culture, originating from Mexico to Brazil, currently cultivated in almost all New and Old-World tropical countries.

Oxygenated sesquiterpenes (44.0%) and fatty acid derivatives (41.6%) predominated in the volatile concentrate of P. guajava, followed by sesquiterpenes hydrocarbons (7.4%). The main constituents were (2E)-hexenal (21.7%), hexenal (15.4%), caryophylla-4(12),8(13)-dien-5-β-ol (10.5%), caryophyllene oxide (9.2%), pogostol (8.3%), muurola-4,10(14)-dien-1-β-ol (4.8%), (E)-caryophyllene (4.1%), and (Z)-β-ocimene (2.6%) comprising 76.6% of the volatile concentrate (see Figure 9).

Mahattanatawee and co-workers (2005) [32] identified hexanal and (E)-caryophyllene as the major constituents of the volatile concentrate of Goiaba fruit sampled in Florida, USA. Also, Chen, Sheu, and Wu (2006) [33] highlighted (E)-caryophyllene, globulol, α-pinene, 1,8-cineole, hexanal, and ethyl hexanoate as significant components in the volatile concentrate of Goiaba fruit collected in Linnei, Taiwan. The odor-active compounds of a Goiaba specimen sampled in Alquizar, Cuba, showed (E)-caryophyllene, hexanal, and 1-hexanol as principal constituents [34]. In Brazil, the behavior of Goiaba fruit volatile compounds at the maturation stages was: in immature fruits predominated the aldehydes (E)-2-hexenal and (Z)-3-hexenal, and in mature fruits were the esters (Z)-3-hexenyl acetate and (E)-3-hexenyl acetate and the sesquiterpenes (E)-caryophyllene, α-humulene, and β-bisabolene [35]. The major constituents of the essential oil of leaves and fruits from a specimen of Goiaba sampled in Cairo, Egypt, were (E)-caryophyllene and limonene for the fruit and (E)-caryophyllene and selin-7(11)-en-4α-ol for the leaves [36].

Figure 9. Ion-chromatogram of the Psidium guajava fruit volatile concentrate.

Figure 9. Ion-chromatogram of the Psidium guajava fruit volatile concentrate.

3.5. Psidium guineense Sw. - Myrtaceae

Botanical description: It is a species of variable size, from 0.7 to 6.0 m. Leaves elliptical or obovate, 8-15 cm long and 4-7 cm wide; apex and base obtuse or rounded, lower surface more hairy, lateral veins 8-10 pairs. The inflorescences are isolated flowers or small axillary dichasia, up to 3 flowers; white corolla with free shell-shaped petals facing downwards, stamens about 200. The fruit is a yellowish-white globose berry, about 4 cm in diameter, with numerous 2-3 mm seeds, hard test, creamy-white pulp, and quite acidic [6] (Figure 10). It flowers from June to December and fruits from October to March. The fruits are naturally consumed in soft drinks, ice cream, sweets, and liqueur.

Figure 10. Psidium guineense fruits – Araçá.

Figure 10. Psidium guineense fruits – Araçá.

Synonimy: Campomanesia multiflora (Cambess.) O. Berg, C. tomentosa Kunth, Eugenia hauthalii (Kuntze) K. Schum., Guajava albida (Cambess.) Kuntze, G. guineensis (Sw.) Kuntze, G. multiflora (Cambess.) Kuntze, Mosiera guinensis (Sw.) Bisse, Myrtus guineensis (Sw.) Kuntze, Psidium albidum Cambess., P. araca Raddi, P. guyanense Pers., Psidium multiflorum Cambess., among others [37].

Geographic distribution: The region where Araçá occurs ranges from Mexico and the West Indies, passing through Brazil and reaching Argentina. The species has an African name due to a mistake by Swartz, who assumed it was introduced to the Antilles from Africa. Araçá is cultivated or spontaneously throughout the Amazon region in open areas, fields, and pastures.

In the volatile concentrate of P. guineense predominated monoterpene hydrocarbons (36.4%), fatty acid derivatives (29.8%), oxygenated sesquiterpenes (18.9%), and sesquiterpene hydrocarbons (12.1%), followed by minor content of benzenoids/phenylpropanoids (1.1%) and oxygenated monoterpenes (0.4%). The primary constituents of Araçá were limonene (25.2%), ethyl butanoate (12.1%), epi-β-bisabolol (9.8%), α-pinene (9.2%), and ethyl hexanoate (5.9%), comprising 62.2% of its volatile concentrate (see Figure 11).

The volatiles ethyl butyrate, ethyl hexanoate, (E)-caryophyllene, and selin-11-en-4-α-ol were previously identified in the Araçá fruits occurring in Colombia [38]. Furthermore, the main constituents of the fruits and leaves of an Araçá specimen collected in Hidrolândia, Goiás, Brazil, were also reported, such as (2Z,6E)-farnesol, α-copaene, δ-cadinene, γ-himachalene, and cubenol in the fruits, and (2Z,6E)-farnesol, α-copaene, muurola-4,10(14)-dien-1-β-ol, and epi-α-cadinol in leaves [39]. Volatile compounds isolated from Araçá leaves were also reported: β-bisabolene and α-pinene as the main constituents of a specimen sampled in Tempe, Arizona, USA [40]; spathulenol at high content in leaf samples collected in Dourados, Mato Grosso do Sul, Brazil [41]; and limonene, α-pinene, β-bisabolol, epi-α-bisabolol, epi-β-bisabolol, β-bisabolene, α-copaene, and (E)-caryophyllene from specimens collected in the Amazon region, Brazil [42,43]. A review of essential oils from the leaves of Psidium species, emphasizing the description of monoterpenes and sesquiterpenes from P. guineense, was recently reported [44].

Figure 11. Ion-chromatogram of the Psidium guineense fruit volatile concentrate.

Figure 11. Ion-chromatogram of the Psidium guineense fruit volatile concentrate.

3.6. Fruit Scent: Chemistry and Ecological Function

Like other plant parts, the fruits are also composed of secondary metabolites. These related compounds act ecologically, attracting frugivorous and seed-dispersing little animals and repelling other so-called fruit antagonists. It has been said that secondary metabolites in fruits act mainly as defensive agents for the plant. The discussion about the defense of fruits by secondary metabolites has been attributed to molecules with higher molecular weight and non-volatile character, and, on the other hand, less attention has been paid to volatile organic compounds and lighter, odorous hydrophobic constituents. The volatile organic compounds not only play a role in the defense of fruits but are also responsible for the aroma and attractiveness to human consumers [45].

Fruit aroma is a significant contributor to fruit quality. In the wild, the aroma of volatile organic compounds released from fruits influences herbivore behavior. It attracts animal dispersers, such as fruit bats, that recognize ripe and non-ripe fruits based on the emitted volatiles. Also, volatile organic compounds from fruits have biological activities against bacteria and fungi. For example, volatiles extracted from citrus species exhibit significant antifungal and antibacterial activities against pathogenic strains [46].

Fruits are generally classified into berries, melons, citrus fruits, drupes (fruits with stones), pomes (apple and pear types), and tropical fruits, as in the present case of Myrtaceous species. Most fruits release a wide range of volatile organic compounds, which determine the profile of their aromas and which, in general, are fatty acid derivatives (esters, ketones, aldehydes, lactones, and alcohols), terpenoids (mono- and sesquiterpenes, and benzenoids, phenylpropanoids (aromatic compounds). Each species of fruit has a characteristic aroma based on the mixture of its volatile organic compounds [9].

Many factors regulate the aroma of fruit emission, while the genotype of the fruit influences the flavor. The final flavor fruit profile is affected by environmental conditions (climate, sunlight, soil, fruit ripening, harvesting time, and post-harvesting processes. For example, environmental stresses (high temperature and drought) influence the metabolism of fruit and the aromatic compound content [47]. The volatile organic compound profiles of fruits change according to the maturation stage. Terpenoids dominate the aroma profile in some fruits during ripening, such as apples, apricots, and peaches, while in grapes, some phenylpropanoids increase with maturation. Furthermore, fatty acid and amino acid-related compounds increase during the maturation of apples and apricots. Therefore, maturation is vital for the emission of volatile organic compounds in fruits and affects commercial production [46].

As seen, fruit aromas serve as a signal to their pollinators or eaters. However, most horticultural varieties and cultivars have been selected according to human preference. Identifying volatile organic compounds relevant to human sensory preference is essential to meet consumer demand for fruits. Furthermore, biotechnological modification of the aromatic characteristics of fruits or the engineering of synthesis pathways in microbial cell factories could increase the production of their aromatic metabolites for commercial exploitation [48].

3.7. Multivariate Statistical Analysis

The variability of Myrtaceae fruit volatile constituents was evaluated using multivariate statistical analyses (PCA, principal component analysis; and HCA, hierarchical cluster analysis) based on their classes of compounds. The percentage values of monoterpene hydrocarbons (MH), oxygenated monoterpenes (OM), sesquiterpene hydrocarbons (SH), oxygenated sesquiterpenes (OS), fatty acid derivatives (FA), and benzenoids/phenylpropanoids (B/P) were obtained based on the GC-MS analyses of their volatile constituents. The data of compound classes from Table 1, Table 2, Table 3, Table 4 and Table 5 were used as variables (see Table 6).

The HCA analysis (Figure 12) showed a heterogenous formation of five groups, with a similarity of 55.53% between the species. The first group comprised Eugenia stipitata (Esti, I); the second group was Myrciaria dubia (Mdub, II); the third group was Eugenia uniflora (Euni, III); the fourth group was Psidium guajava (Pgua, IV); and the fifth group was Psidium guineense (Pgui, V), evidencing the statistical differentiation between them.

The analysis of chemical variability was also evaluated by principal component analysis (PCA), which represented 83.9% of the data (Figure 13), in which PC1 explained 54.5% of the data and showed a negative correlation with oxygenated monoterpenes (OM, λ=- 0.389), monoterpene hydrocarbons (MH, λ=-0.564) and positive correlation with oxygenated sesquiterpenes (OS, λ=0.584), sesquiterpene hydrocarbons (SH, λ=0.264) and fatty acid derivatives plus benzenoids/phenylpropanoids (FA-B/P, λ=0.346). PC2 justified 29.4% of the data and showed a positive correlation with monoterpene hydrocarbons (MH, λ=0.002) and fatty acid derivatives plus benzenoids/phenylpropanoids (FA-B/P, λ=0.670) and a negative correlation with oxygenated monoterpenes (OM, λ=-0.060), sesquiterpene hydrocarbons (SH, λ=-0.733) and oxygenated sesquiterpenes (OS, λ=-0.104). Similar to HCA, the PCA analysis confirmed the formation of five distinct groups.

The fruits of Eugenia stipitata (Esti) and Myrciaria dubia (Mdub) were characterized by the presence of monoterpene hydrocarbons (Esti, 28.5%; Mdub, 79.6%) and oxygenated monoterpenes (Esti, 25.5%; Mdub, 11 .5%). The fruit of Eugenia uniflora (Euni) was described by the existence of sesquiterpene hydrocarbons (41.3%). The fruit of Psidium guajava (Pgua) was characterized by the presence of oxygenated sesquiterpenes (43.8%) and fatty acid derivatives (41.6%). The fruit of Psidium guineense (Pgui) was described by the existence of monoterpene hydrocarbons (36.4%) and fatty acid derivatives (29.8%).

Based on the PCA and HCA studies, it was observed that there was no significant statistical grouping between the analyzed samples, whose chemical profiles are characterized by α-pinene (17.4%), citronellyl butanoate (15.6%), pogostol (13.5%), and α-terpineol (9.6%) in Eugenia stipitata; curzerene (30.5%), germacrone (15.4%), atractylone (13.1%), and (E)-β-ocimene (11.1%) in Eugenia uniflora; α-pinene (55.8%), (E)-β-ocimene (13.1%), and α-terpineol (10%) in Myrciaria dubia; (2E)-hexenal (21.7%), hexanal (15.4%), caryophylla-4(12),8(13)-dien-5-β-ol (10.5%), and caryophyllene oxide (9.2%) in Psidium guajava; and limonene (25.2%), ethyl butanoate (12.1%), epi-β-bisabolol (9.8%), and α-pinene (9.2%) in Psidium guineense.

4. Conclusions

The present study contributed to a better knowledge of the chemotaxonomy of Myrtaceae fruit species, as there are few reports in the literature. Thus, considering the main classes of compounds, in Eugenia stipitata, monoterpene hydrocarbons, oxygenated monoterpenes, sesquiterpene hydrocarbons, oxygenated sesquiterpenes, and fatty acid derivatives were very representative; in E. uniflora there was an absence of oxygenated monoterpenes and fatty acid derivatives; in Myrciaria dubia there are only monoterpene hydrocarbons and oxygenated monoterpenes; in Psidium guajava sesquiterpene hydrocarbons, oxygenated sesquiterpenes and fatty acid derivatives predominated; and in P. guineense there was an absence of oxygenated monoterpenes. Therefore, these findings can contribute to a better understanding of the chemical profiles of Myrtaceae fruit species.