Species name is the basic unit of classification of plants. It describes one kind of plant within a genus. The chemical makeup of the EO extracted from three wild species of Moroccan thyme (T. satureioides Coss., T. willdenowii Boiss., and T. zygis L.) was studied and compared. Carvacrol (44.95%) and p-cymene (23.36%) were found to be the most prevalent components in T. zygis L. EO. Thymol, carvacrol, and p-cymene were discovered to be the main volatile components in T. willdenowii Boiss. EO. Carvacrol (27.57%) and borneol (21.56%) make up the majority of T. satureioides Coss. EO. On the other hand, the two main ingredients in T. zygis L. were carvacrol (72.33%) and thymol (10.70%). Thymol (65.01%) and carvacrol (21.74%) predominated in T. willdenowii Boiss. Carvacrol (47.35%), borneol (25.04%), β- and ɣ-terpineol were all present in considerable concentrations in T. satureioides Coss (Moukhles et al., 2022). Additionally, Limam et al. (2020) examined the leaf EO diversity of 13 Eucalyptus species, finding that the majority of these species were abundant in the key component 1,8-cineole (82.64% in E. microcarpa) (Limam et al., 2020). A combination of chromatographic and spectroscopic methods has been used to analyze the fruit oil of five species of kumquat (Fortunella japonica, F. margarita, F. crassifolia, F. obovata, and F. hindsii) cultivated in the same pedoclimatic circumstances. Limonene (84.2%–96.3%) had a substantial dominance over the compositions of the five fruit oils. Myrcene (1.3%–12.9%) and germacrene D (0.3%–2.4%) were additional substances that were detected in significant amounts (Sutour et al., 2016). EO distillation of Salvia dugesii and S. gesneriiflora yielded 0.08% and 0.07%, and their chemical compositions revealed the presence of 27 and 25 compounds as main components, respectively. Caryophyllene is highlighted as the most abundant component in essential oils from both studied plants. Bornyl acetate, valeranone, monocyclic geranyl-α-terpinene, and hedycaryol (Lo et al., 2005) showed relative concentrations over 5% in S. gesneriiflora, which is a native species in Mexico. Bicyclic compounds, including spathulenol, caryophyllene oxide, and γ-muurolene, are predominant in S. dugesii (Calderón-Oropeza et al., 2021).

Ecotypes are plants which belong to the same species but possess different phenotypical features because of environmental factors such as elevation, climate, and predation. Ecotypes can be seen in wide geographical distributions and may eventually lead to speciation (Mayr, 1947). Kaškonienė et al. (2016) examined the impact of ecotypes on the yield and composition of EOs in seven samples of the Lithuanian plant Chamerion angustifolium (L.) Holub, which was gathered in six different locations; anethole and caryophyllenes (α- and β-) were discovered in all samples. Only three samples had a high concentration of caryophyllenes (α- and β-), making up 54.0–78.5% of the EOs. Two of the samples, collected in Panara, had an anethole concentration of more than 51%, while the sample (from Aleksotas) stood out for having a rather high level of cis-arbusculone (8.5%) (Kaškonienė et al., 2016). Four ecotypes of Citrus deliciosa var. tangarina (Fina Clementine, Nour Clementine, Spinosa Clementine, and Thornless Clementine) were examined in another study for the EOs of their peel and leaf. Three oil samples under evaluation contained limonene as their primary element, and the Nour Clementine leaves contained sabinene as their primary constituent. Generally, total hydrocarbons outnumbered total oxygenated compounds in percentage, and total monoterpenes outnumbered total sesquiterpenes (El-hawary et al., 2013). Citrus sinensis Osbeck (forma Salustiana, forma Valencia, and forma Washington navel) were grown in Kenya. The volatile components of their peel EOs were isolated by cold pressing and were identified by GC and GC-MS. Salustiana, Valencia, and Washington navel peel oils have totals of 56, 73, and 72 components, respectively. The detected elements made up, respectively, 98.7%, 97.8%, and 97.4% of the total volatiles in each oil. The volatile percentages of the Salustiana (96.9%), Valencia (94.5%), and Washington navel (92.7%) oils were predominantly dominated by monoterpene hydrocarbons. The main constituents in each oil were limonene, α-pinene, sabinene, and α-terpinene. Valencia and Washington navel oils had 0.1% of the total volatiles as sesquiterpene hydrocarbons, with (E,E)-farnesene as the predominant component. Salustiana, Valencia, and Washington navel volatiles contained 1.7%, 3.4%, and 4.5% of the total oxygenated compounds, respectively. The key ingredients were linalool, decanal, (Z)-carvone, (Z)-carveol, (E)-carveol, nootkatone, and sabina ketone. The presence of β-phellandrene and α-terpinene in Salustiana, α-phellandrene (Z)-nerolidol, and aromadendrene in Valencia, and p-cymene, α-sinensal, and dodecanoic acid in Washington navel oils can distinguish the three sweet Citrus kinds (Njoroge et al., 2005).

A “chemotype” is plants which belong to the same species that have the same morphological characteristics (relating to form and structure) but produce different quantities of chemical components in their EOs (Clarke, 2008). Rosmarinus officinalis EO chemotypes from eight sites in north Tunisia were studied. According to analyses, the principal constituents of the oils include 1,8-cineole (44.5–57.88%), (+)-camphor (9.27–18.99%), α-pinene (9.18–3.66%), borneol (13.21%), and camphene (3.15–4.69%). Three chemotypes were found, with the first one predominating: 1,8-cineole/camphor/α-pinene, 1,8-cineole/camphor, and camphor/1,8-cineole (Abada et al., 2020). The content and composition of EOs from Juniperus oxycedrus L. sensu lato collected at 20 populations (interpopulations) across Bulgaria were examined. The leaf EOs yield ranged from 0.03% to 0.20% (interpopulation) and from 0.04% to 0.18% (intrapopulation), but the galbuli EOs ranged from 0.83% to 2.6% (interpopulation) and from 1.2% to 1.9% (intrapopulation), respectively. Galbuli EO’s major chemical class ranged from 53.4% to 69.5% (intrapopulation) and was monoterpenes, whereas leaf EO’s dominant chemical class ranged from 46.2% to 67.4% (intrapopulation) in sesquiterpenes. To categorise the populations into eight chemotypes, the major constituents of leaf EOs (α-pinene, limonene, α-curcumene, γ-cadinene, δ-cadinene, manoyl oxide, germacrene D, β-caryophyllene, α-caryophyllene, caryophyllene oxide) were considered. Semerdjieva et al. (2020) established 27 chemical groups for their interpopulation study. Due to its antibacterial properties, the triketone chemotype of manuka, Leptospermum scoparium, has significant commercial value. On the East Cape of New Zealand, oils from 36 different plants all had high triketone concentrations (>20% total triketones), with no seasonal change. The high triketone chemotype was localised to the East Cape, according to analyses of oils from 261 individual manuka plants gathered from 87 sites across New Zealand, while oils with triketone levels as high as 20% were discovered in the Marlborough Sounds region of the South Island. Ten further chemotypes were listed: high geranyl acetate, sesquiterpene-rich with high c-ylangene, α-copaene, and elevated triketones, sesquiterpene-rich with no distinguishing components, sesquiterpene-rich with high trans-methyl cinnamate, high linalol, and sesquiterpene-rich with elevated elemene and selinene. One of the most noticeable chemotypes, with a high concentration of geranyl acetate in the oil, contained aroma components at rather high levels (Douglas et al., 2004). Variations in EO chemotype chemical compositions impact their biological activity (Abada et al., 2020; Melo et al., 2018; Melo et al., 2021; Peixoto et al., 2015; Salem et al., 2015; Blanco et al., 2013). An investigation of European Origanum vulgare focused on the qualitative and quantitative composition of essential oil compounds isolated from 502 individual O. vulgare plants from 17 countries. The content of EO compounds of O. vulgare ranged between 0.03% and 4.6%. The monoterpenes were primarily made up of sabinene, myrcene, p-cymene, 1,8-cineole, β-ocimene, ɣ-terpinene, sabinene hydrate, linalool, α-terpineol, carvacrol methyl ether, linalyl acetate, thymol, and carvacrol. Among the sesquiterpenes, β-caryophyllene, germacrene D, germacrene D-4-ol, spathulenol, caryophyllene oxide, and oplopanone were often present in higher amounts. According to the proportions of cymyl-compounds, sabinyl-compounds, and acyclic linalool/linalyl acetate, three different main monoterpene chemotypes were defined (Lukas et al., 2015).

The yields of Origanum majorana essential oils from Kathmandu and Bhaktapur were found to be 0.5% v/w (50 compounds—91.2%) and 0.8% v/w (41 compounds—98.8%), respectively (Paudel et al., 2022). For O. majorana oil Kathmandu, terpinen-4-ol (32.1%), linalool (13.8%) and γ-terpinene (9.5%) were the most common compounds, followed by linalyl acetate (5.9%), α-terpinene (5%), cis-sabinene hydrate (4.4%), α-terpineol (3.7%), terpinolene (2.5%), bornyl acetate (2.4%), β-caryophyllene (2.4%), cis-p-menth-2-en-1-ol (1.8%) and p-cymene (1.8%) in smaller amounts. Similarly, in O. majorana essential oils from Bhaktapur, terpinen-4-ol (33.35%), linalool (15.37%), p-cymene (6.90%), and linalyl acetate (6.67%) were the most common compounds, followed by cis-sabinene hydrate (3.48%), 1,4-hydroxy cineole (3.35%), bornyl acetate (2.83%), α-terpineol (2.63%), and caryophyllene oxide (2.54%) in smaller amounts. Terpinen-4-ol is the major oxygenated monoterpene found in the EO of the two samples, in which dextrorotatory (+) enantiomer is the most predominant chiral compound. In this study, the essential oil of O. majorana essential oils from Bhaktapur has nearly the racemic mixture of terpinen-4-ol, (+) 52.6% and (−) 47.4%, whereas terpinen-4-ol, (+) 58.31%, is dominant over terpinen-4-ol, (−) 47.69% for O. majorana EOs from Kathmandu. In both EO samples, (−) β-caryophyllene, (−) bornyl acetate, and (−) linalyl acetate were detected as enantiomerically pure, with 100% in levorotatory form. Camphene is the enantiomerically most dominant component that exists in levorotatory form with slight variations, including (−) 96.8% to (−) 97.87% in both EO samples, which is followed by (+) sabinene and (+) cis-sabinene hydrate, with the variation in the enantiomeric distribution in this study. The antioxidant activity for O. majorana EOs from Bhaktapur was found to be relatively moderate, with an IC₅₀ value of 225.61 ± 0.05 μg/mL and an EC₅₀ value of 372.72 ± 0.84 μg/mL, as compared to the standard used. Furthermore, with an MIC value of 78.1 μg/mL, O. majorana EOs from Bhaktapur demonstrated effective antifungal activity against Aspergillus niger and Candida albicans. Moreover, both samples displayed considerable antimicrobial activity (Paudel et al., 2022).

A cultivar is a plant that is propagated through human intervention to develop a desirable characteristic and to ensure it retains the characteristics of its parent plant. Cultivars are created from stem cutting, grafting, tissue cultures, or cross-pollination (Merckx et al., 2013). Verma et al. (2013) investigated the effect of cultivars on the production and makeup of Pelargonium graveolens—the rose-scented geranium. In India’s Western Himalayan region, three cultivars—Bourbon, CIM-Pawan, and Kelkar—were grown. It was discovered that Kelkar’s chemical makeup differed from those of the other two cultivars (Verma et al., 2013). A similar investigation was conducted on the EOs of 17 cultivars of fresh Indian Zingiber officinale (Ravi Kiran et al., 2013). The Assam fiberless cultivar stood out among the other cultivars investigated for having the highest EO production (4.17%) and the highest monoterpene hydrocarbon content (38.65%) compared to sesquiterpene hydrocarbon (25.38%). Assam Tinsukia had the greatest citral content (23.66%), and Meghalaya Mahima had the highest zingiberene content (29.89%) of all these cultivars.

A plant “variety” is found growing and reproducing naturally in the plant kingdom. It varies from its standard species in some way because of natural evolution. Plant varieties are most often found in groupings that have cross-pollinated as adaptations to differences in habitat. Plants grown from the seeds of a species variety are often exact copies of the parent plant (Clarke, 2008). The EO concentration of Daucus carota variety “Cubic” roots was higher than that of D. carota variety “Nanco F1” roots, and the same was true for the leaves of both kinds (Habegger and Schnitzler, 2000). The composition of D. carota leaf EOs varied greatly between types (Kainulainen et al., 1998). Carum carvi EO content is reduced from the “Rekord,” “Prochan,” and “Kepron” varieties to the “Konczewicki” kind (Sedlakova et al., 2001). Additionally, research found that several C. carvi types had 1–9% (m/m) of the EOs that give C. carvi its distinctive flavor and aroma. Compared to the varieties “Dulce” and “Selma,” the sweet Foeniculum vulgare variety “Zefa Fino” recorded the highest levels of EO content in the leaves and bulbs (Semiz et al., 2012). F. vulgare varieties “Sweet Fennel” and “Shumen” had the lowest and greatest EO contents, respectively (Bowes and Zheljazkov, 2004). Variation significantly changed the ratio of the following compounds in Petroselinum crispum’s EOs: β-phellandrene, 1,3,8-p-menthatriene, α-p-dimethylstyrene, myristicin, α-myrcene, and apiole (Petropoulos et al., 2004). The leaves of diverse D. carota kinds (“Flakker 2,” “Nantura,” “Parano,” “Napoli”, “Panther,” “Splendid,” “Nantes 3 Express”) have considerably diverse oil compositions (Kainulainen et al., 2002). The oil output and content of the Coriandrum sativum varieties “Jantar” and “Alekseevski” were different (Zheljazkov et al., 2008).

For medicinal plants, there are different relationships between climatic conditions and EO content (Moghaddam et al., 2023). C. sativum oil yields are highest in the coolest, wettest seasons (Olle et al., 2010). In addition, the ratios of the following chemicals varied significantly depending on the climate, which had a substantial impact on the EO composition of P. crispum: 1,3,8-p-menthatriene, β-phellandrene, α-,p-dimethylstyrene, β-myrcene, myristicin, and apiole (Petropoulos et al., 2004).

Higher temperatures often promote the creation of EOs, while particularly hot days could result in an excessive physical loss of oil. Leaching caused by persistent rain can cause the loss of EOs in leaves and roots (WC, 2002). Menthone, menthol, and small amounts of menthofuran have been found in M. piperita L. leaves grown under long-day settings; menthofuran is a prominent component of the EOs in plants cultivated under short-day conditions. Additionally, the reduction pathway is activated with the conversion of menthone to menthol when immature leaves have a long photoperiod. A significant increase (approximately ten-fold) in aromatic compounds, including benzyl alcohol, was found at night in studies on the day–night variations in the relative concentrations of volatiles from flowers of Nicotiana sylvestris and other species, but no such increase was seen in other EO constituents (such as linalool or caryophyllene) (Loughrin et al., 1990). It is also crucial to consider the type of radiation that plants receive. Johnson et al. (1999) discovered that with Ocimum basiclicum, more UV-B radiation enhances levels of both phenylpropanoids and terpenoids in the leaves in comparison to herbs grown under glass and receiving no UV-B radiation. Karousou et al. (1998) raised two chemotypes of M. spicata that were exposed to elevated UV-B radiation, which is comparable to the l5% ozone depletion over Patras, Greece, in regards to aromatic plants. The therapy increased the production of EOs in one chemotype, while the other showed a comparable but unimportant trend.

The amount of EOs varied depending on the time of day and night. C. sativum EO concentration variations during the day were investigated by Ramezani et al. (2009). EO yields (w/w-%) at various times were in the range of 6 (0.432%), 12 (0.436%), 18 (0.404%), and 24 (0.319%) treatments because EO concentration at midday was higher than at other times.

The time of year that the aromatic plant is gathered is typically of great significance. Karamanos and Sotiropoulou (2013) examined Greek O. vulgare oil harvested in two successive seasons to determine the effect of harvesting season on EO output and composition. From approximately 1.5% to 2.0% in the dry warm season to approximately 5.5% in the following wet cold season, the concentration of EOs per plant increased dramatically. Inflorescences had a higher concentration of EOs than leaves. Carvacrol, the primary component of O. vulgare EOs, was more abundant in both leaves and inflorescences during the drier and warmer seasons (70.75–84.88%), while other compounds tended to accumulate at higher levels during the wetter and colder season with only slightly higher levels of carvacrol (56.46–75.12%) (Karamanos and Sotiropoulou, 2013). Another crucial factor in harvesting is the time of day. In Kiyombe (Rwanda), a study on the rose-scented P. graveolens investigated how the time of day affected the yield of EOs. Before EOs were extracted by steam distillation, the P. graveolens plants were manually gathered at 10:00 a.m., 12:00 noon, 2:00 p.m., and 4:00 p.m. They were then allowed to dry for 17 h in the shade. The maximum EO concentration (0.22%) was found in the P. graveolens plants that were collected in the early afternoon (2:00 p.m.) (Malatova et al., 2011).

Medicinal plants have different relationships between latitude and EO content (Moghaddam et al., 2023). C. sativum grown in Lithuania had more EO concentration than that grown in southern Russia (Olle et al., 2010). Johreniopsis seseloides from Teheran and Kurdistan were collected by Fouladi et al. (2006). Monoterpenes (24.05%) and sesquiterpenes (43.19%) were present in the EOs of J. seseloides from Teheran, while those from Kurdistan province had a high monoterpene content (69.67%) and a low sesquiterpene content (26.69%). Between the two growing zones under study, the EO content of C. sativum saw a substantial alteration (Msaada et al., 2007).

Planting and sowing dates were reported to influence the composition of EOs (Delibaltova, 2020). The planting date of P. crispum significantly changed the content of β-phellandrene, 1,3,8-pmenthatriene, α-,p dimethylstyrene, myristicin, β-myrcene, and apiole (Petropoulos et al., 2004). Belgium, and Rafieiolhossaini et al., 2009 investigated the impact of planting date and seedling age on Matricaria chamomilla according to agro-morphological parameters, EO content, and composition. Four distinct planting dates (15 April, 1 May, 15 May, and 30 May) and five different seedling ages (30, 45, 60, 75, and 90 days) were used in the treatments, which were reproduced three times in a randomized full block design. All the agro-morphological parameters were significantly influenced by the planting date. Early planting dates yielded the highest values for them, whereas later planting dates saw values decline. Planting date had a considerable impact and negative relationship with days after sowing (DAS) and days after transplanting (DAT) to flowering. The chemicals that were significantly influenced by planting date for seedlings that were 30 days old were (E)-farnesene, α-bisabolol oxide A, α-bisabolone oxide, and spathulenol; the highest levels were obtained for the latest planting date. The only agro-morphological traits that were significantly impacted by seedling age at transplanting were plant height and spread. DAS to flowering was significantly enhanced by seedling age, although the effect varied with age. The only compound whose first planting date was significantly impacted by seedling age at transplanting was spathulenol, with 90-day-old seedlings having the maximum yield (Mohammad et al., 2010). Reduced watering frequency caused water stress, which raised P. crispum and F. vulgare’s EO percentages (Mohamed and Abdu, 2004). With watering, F. vulgare’s output of EOs also increased (Mohamed and Abdu, 2004). C. sativum seeds’ EO content increased due to irrigation, although the percentage of linalool in the EOs decreased (Arganosa et al., 1998). P. anisum oil production was decreased by a water shortage during stem elongation and umbel development (Zehtab-Salmasi et al., 2001).

Fertilisers are utilized to maintain soil fertility and enhance plant growth and development. Less N applied to D. carota produced more EOs (Schaller and Schnitzler, 2000). C. sativum produced the most oil when treated with ammonium nitrate, but A. graveolens responded better to urea. Lower nutrient concentrations (1.2, 2.4, and 3.6 mS/cm) led to an increase in EO content in A. graveolens leaf (Udagawa, 1994). In curl leaf P. crispum, the content of EOs declined with increasing N rate (3.2, 16.4, 32.4, and 48.6 g m⁻²) but was unaffected by N treatment in the roots and leaves of both turnip-rooted and plain leaf P. crispum (Petropoulos et al., 2009). In two successive crops, the growth, foliar EO composition, and yield of A. graveolens L. “Ducat” were examined in relation to N fertilization, cultivation season, and harvest stage. For the autumn–winter crops, seeds were seeded in October, and for the spring crop, in January. In a randomised design, nitrogen (NH₄NO₃) was supplied at four different amounts (50, 150, 300, and 450 ppm). After sowing, the plants were harvested after 158 days (for the autumn–winter crop) and 83 days (for the spring crop). Weighing the plant leaf allowed the separation and analysis of the EOs. The maximum fresh weight of leaf per plant was reached at 300 ppm of N in the spring, but it gradually grew with increasing N up to 450 ppm in the autumn and winter. In the autumn and winter, N treatment had no effect on the amount of EOs present in the leaf; however, in the spring, the concentration of EOs was much higher at 300 ppm N than at other N levels. The primary ingredients in foliar EOs were phellandrene, A. graveolens ether, phellandrene, phellandrene, thujene, myrcene, and cymene. Phellandrene was the main component in both crops. In the autumn–winter crop, the concentrations of phellandrene, β-phellandrene, and A. graveolens ether were also higher at 300 ppm N, but the other constituents were unaffected by higher N. In spring, the concentration of all the EO constituents identified (aside from π-cymene) was highest at 300 ppm N. Consequently, for the autumn–winter crop, 450 ppm N was ideal for biomass and EO yield, whereas 300 ppm N was advised in the spring (Tsamaidi et al., 2010). The effects of four phosphorus levels (0, 20, 40, and 60 kg ha⁻¹) on the production and content of EOs in Cuminum cyminum were examined by Tuncturk and Tuncturk (2006). Their findings indicated that the treatment of 40 kg ha⁻¹ P₂O₅ produced the maximum oil content and EO yield. Using solutions of NiSO₄ in varying concentrations, P. crispum plant soil was enriched with 0, 25, or 100 mg of Ni per kilogram of soil. The findings of this experiment demonstrated that P. crispum oil yield was significantly increased by modest amounts of Ni fertilization, notably 50 mg kg⁻¹ in clay soil (Atta-Aly, 1999). Organic fertilizers (2.5 kg m⁻²) considerably increased oil yield in F. vulgare (Mohamed and Abdu, 2004).

With an increase in salt, the oil content of sweet F. vulgare seeds gradually reduced (Ashraf and Akhtar, 2004). Based on dry matter weight, C. sativum EO yield was 0.06% in the control and did not vary at low concentrations (25 mM), but it rose considerably with increasing sodium chloride (NaCl) concentrations to reach 0.12% and 0.21% at 50 and 75 mM NaCl, respectively (Neffati and Marzouk, 2009). When F. vulgare plants were irrigated with saline water (3,355 ppm), the oil output per plant decreased and the anethole content of the oil decreased to 23.2% (Abd El-Wahab, 2006). With 25 and 50 mM NaCl, respectively, the C. sativum EO output significantly increased to 18% and 43%, and it significantly decreased with high salinity (Neffati and Marzouk, 2008).

C. carvi and F. vulgare plants were sprayed with the growth inhibitor (B-9) in concentrations ranging from 500 to 4,000 ppm, which increased the EO content in the seeds. By spraying the plants with 4,000 ppm B-9, maximum oil output was attained (Abou-Zied, 1974).

One of the most popular preservation techniques is drying, which can increase the shelf life of plants by lowering their moisture content. This means that, depending on their nature and environmental factors, drying procedures may cause the degradation of heat-sensitive plant metabolites and a drop in quality. On Citrus sp. peel, the effects of seven drying techniques—sun, shade, oven, hoover oven, microwave, and freeze drying—were examined. Results revealed that the freeze-dried sample had the greatest EO extraction yield (6.90%) (Farahmandfar et al., 2020). The EOs of M. piperita L. leaf were quantitatively and qualitatively analyzed using various drying techniques. The leaves were dried in thin layers in a hot-air dryer at 50, 60, and 70 °C as well as in a microwave oven at 200, 400, and 800 W. Hydrodistillation was used to extract EOs from the fresh and dried samples, which were then analysed using GC/MS. The hot air-dried leaf at 50 °C and the microwave-dried leaf at 800 W yielded the highest (22.24 g/kg dry matter) and lowest (1.33 g/kg dry matter) oil yields, respectively. In general, the amount of EOs reduced as drying temperature rose. According to the GC/MS analysis of the EOs, the chemical components primarily belonged to the class of oxygenated monoterpenes (72.34%–86.41%). Microwave drying at 200 and 400 W power levels considerably (p < 0.01) reduced the chemical compounds group. The principal components of the M. piperita L. leaf EOs, including menthol, menthone, menthofuran, 1,8-cineole, and menthyl acetate, varied significantly (p < 0.05 and/or p < 0.01) depending on the drying techniques evaluated. The primary component of the oil, menthol, was found to have minimum (35.01%) and maximum (47.50%) concentrations in hot air-dried leaves at 50 °C and microwave-dried leaves at 400 W, respectively. Under microwave drying, the percentage of menthone, the EO’s second ingredient, considerably decreased (p < 0.01) (Beigi et al., 2018). The quantity and chemical makeup of the EOs from Laurus nobilis L. leaves were examined after being subject to six different drying techniques. Fresh and dried samples were hydrodistilled in Clevenger equipment to separate the EOs, which were then examined using GC-MS. The EO content was greatly increased by infrared drying at 45 °C and air drying at room temperature. It was discovered that there were 47 components, the majority of which were oxygenated monoterpenes. Except for air drying at room temperature, the drying technique has no discernible impact on this class of chemicals. With different drying techniques, the primary constituents—1,8-cineole, methyl eugenol, terpinen-4-ol, linalool, and eugenol—showed notable changes. When these compounds were dried by air at room temperature, their concentrations dramatically rose. These outcomes made it possible to conclude that this drying technique generated the greatest outcomes in terms of EOs and bioactive component levels (Sellami et al., 2011).

EO content drops with longer storage times (Sedlakova et al., 2001). Storage losses of EOs from dried C. sativum fruit kept in piles or cotton bags over a 2-year period were just 3–5% (Olle et al., 2010).

Different techniques were used for kumquat peel extraction, including hydrodistillation, solvent, and supercritical CO₂ (SC-CO₂) extractions at densities of 281 g/dm3 (80 bar, 40 °C) and 875 g/dm3 (250 bar, 40 °C). The hydrodistillate of Citrus japonica was a colorless oil that smelled like fruit. The pentane extract was an intensely scented scarlet oil. Red oils from the supercritical extracts precipitated a white substance. Eight monoterpene hydrocarbons, six terpene alcohols, one terpene aldehyde, one terpene ketone, four terpene esters, and four sesquiterpene hydrocarbons were all quantified—24 substances in total. The quantity of the EO’s main constituent, was limonene. In comparison to EOs derived through steam distillation and solvent extraction, those obtained through SC-CO₂ extraction had higher levels of linalyl acetate, neryl acetate, and geranyl acetate (Sicari and Poiana, 2017). Cold pressing (CP) and SC-CO₂ extraction techniques were used to obtain the peel extract from Turkish-grown clementines (Citrus clementina Hort. Ex. Tan.). At total of 69 components were found, accounting for 99.8% of the total volatiles in both samples. Compared to SC-CO₂ extraction, CP extraction produced less oxygenated compounds (3.7%); most of them were carbonyls (2.09%–2.10%), followed by alcohols (1.32%–1.60%) and esters (0.12%–0.40%). Myrcene (4.64%–3.77%) was second to limonene (88.12%–89.28%) as the primary constituent. Linalool (1.02%–1.24%) and decanal (0.71%–0.72%) were the oxygenated substances that were more abundant (İsmail Kirbaşlar et al., 2012).

Moisture has been suggested as a potential cause of EO degradation (Guenther and Appendix, 1948). For instance, it has been demonstrated that compound spectra are distorted during water distillation processes at approximately 100 °C. It is known that citral can react with acids to form methylacetophenone, p-cymene-8-ol, p-cymene, α-p-dimethylstyrene, and cresols in aqueous solutions (Schieberle et al., 1988). In contrast, Rao et al. (2011) found that even at a water source concentration of 20% (v/v), various oils held in the presence of water did not significantly alter. This concentration is greater than what, under ideal conditions, might be released in EOs during hydrodistillation. Kaul PN. et al. (1997) suggested drying EOs after distillation by adding water-binding materials because their study revealed some notable alterations, such as reduced contents of linalool and geraniol as well as an increase in citronellol and geranyl formate in P. graveolens oil that had been kept at room temperature for 1 year and which contained an undefined amount of water. However, the observed chemical alterations could have also been brought on by the container having 50% air space.

The monoterpene hydrocarbon sabinene is a key component of Juniperus sabina L. oil. When oil samples from different distillation durations were analysed, it seemed that sabinene levels continuously fell while the proportion of an oxygenated molecule, terpinen-4-ol, gradually rose. In the first decade of this century, it was already documented how diluted mineral acids may cause sabinene to convert into terpinen-4-ol (Wallach, 1907). Cooper et al. (1973) and Norin and Smedman (1971) validated the rearrangement of sabinene and reported that, in addition to terpinen-4-ol, other reaction products included α-terpinene, ɣ-terpinene, and terpinolene. Given that the levels of these chemicals increased concurrently with the reduction in sabinene, it was presumed that some conversion had taken place because of the acidity of the distillation water. Therefore, it appeared worthwhile to investigate how the pH of the distillation water affected the separation of the oil. For this, plant material soaked in buffer solutions with a pH range of 2.2–8 was used in distillations. Over the tested pH range, the amount of sabinene in the oil varied significantly. The percentage of sabinene in the distillate was roughly 7.5% at low pH levels. However, when the solution was made alkaline, this percentage rapidly rose to approximately 40% at pH 8 (Wallach, 1908).

Zheljazkov et al. (2013a) postulated that the distillation time (DT) can be adjusted for the best P. anisum EOs output and composition. They examined how nine steam DTs—5, 15, 30, 60, 120, 180, 240, 360, and 480 minutes—affected the yield and content of P. anisum seed EOs. To forecast the production of EOs, the concentration of certain constituents, and their yield as a function of DT, researchers created regression models. At 360-min DT, the highest EO output (2.0 g/100 g seed, 2%) was attained. The primary P. anisum oil element trans-anethole’s concentration ranged from 93.5% to 96.2% (as a percentage of the overall oil) and was typically highest at 15- to 60-min DT and lowest at 240- to 480-min DT. However, the yield of trans-anethole rose with increasing DT to reach its maximum at 360-min DT (derived from the EO’s output and the concentration of trans-anethole in the oil). Other oil ingredient concentrations also varied significantly with respect to the DT, with some of them being higher at the shorter DT than at the longer. However, lengthier DT (either 360 or 480 min) produced the maximum yields of these components. P. anisum EOs with variable composition can be produced using DT, which is advantageous for the EO sector (Zheljazkov et al., 2013a). The EO yield and oil compositions of M. piperita L., Cymbopogon citratus, and Cymbopogon martinii oils were modeled through experiments as a function of the steam DT. At a DT of 20 min, the highest EO yields of M. piperita L., C. citratus, and C. martinii were attained; subsequent increases in DT had no further effect on oil yields. In tests with C. citratus and C. martinii, DTs of 240 min resulted in 25–40% lower oil yields than yields at 20–160 min. This study showed that DT can be utilized as a method for extracting EOs from M. piperita L., C. citratus, and C. martinii that have a specific desired composition. Second, Zheljazkov et al. (2013a) discovered that the ideal duration of the DT for maximising the yields of M. piperita L., C. citratus, and C. martinii EOs was significantly shorter than that often employed by researchers and processors. Reduced DT could help producers and processors save money on energy and other resources (Cannon et al., 2013). When Lavandula angustifolia EOs were extracted from dried flowers, the impact of DT on the production and content of the oil was studied. In this experiment, the following DTs were tested: 1.5, 3.75, 3.75, 7.5, 15.5, 30.1, 60.2, 90.2, 120.2, 150.2, 180.2, and 240 min. At 60 min DT, EO output (0.5%–6.8%) peaked. The levels of fenchol (range 1.7–2.9%) and cineole (range 6.4–35%) were highest at the 1.5 min DT and declined with longer DTs. At 7.5–15 min DT the concentration of camphor (range 6.6–9.2%) peaked, while at 30 min DT the concentration of linalool acetate (range 1%–38%) peaked. The findings imply that the output of L. angustifolia EOs might not increase after 60 min DT. The asymptotic nonlinear regression model did a very good job of simulating the change in EO yield as well as the concentrations of cineole, fenchol, and linalool acetate as DT changes. DT can be used to alter the chemical composition of L. angustifolia oil and produce oils from the same L. angustifolia flowers with different chemical compositions. When citing or comparing reports on the production and composition of L. angustifolia EOs, DT must be considered (Zheljazkov et al., 2013b).

In simulated experimental settings like pyrolysis-GC or thermal heating in the presence of oxygen or an inert environment, the thermolability of several EOs and their constituent aroma compounds, primarily monoterpenoids, was studied by Jakab et al. (2018). Citrus aurantiifolia oil underwent pyrolysis, increasing the amount of total monocyclic monoterpenes (limonene, ɣ-terpinene, p-cymene, β-phellandrene, and p-cymenene) from 55.74% to 66.21%. However, total bridge bicyclic monoterpenes, such as α- and β-pinene, sabinene, and α-thujene, showed a drop in abundance (22.98–12.41%). The pyrolysate of Bergamot orange oil showed the same trend (monocyclic 44.26–48.57% and bicyclic 9.48–6.50%). Additionally, linalyl acetate-dominated total linear alkyl esters (30.60–15.31%) were found to significantly decrease in level. This was due to acetic acid being eliminated, causing their breakdown (about 60%) into linear monoterpene hydrocarbons (1.13%–18.60%) such as β-myrcene and β-ocimene. Elettaria cardamomum oil’s main component α-terpinyl acetate followed a comparable process to breakdown by 48% into monocyclic monoterpenes, primarily limonene.

Additionally, some monoterpenes’ thermos-sensitivity has been examined in their pure form (McGraw et al., 1999). For instance, when heated separately at 90–130 °C with air present, α- and β-pinene demonstrated degradation rates of 23–37% and 22%, respectively. Verbenone, myrtenol, pinocamphone, α-pinene oxide, β-pinene, α-pinene oxide, and α-campholene aldehyde were the degradation products obtained from α-pinene, while myrtenol was the primary product of β-pinene. In a closed system that simulated the conditions of a wood dryer, the thermo-oxidative degradation of limonene, α-terpinene, camphene, and 3-carene was investigated at 120 °C in the presence of oxygen (1.00:1.24). The observed degradation was 50% (5, 24 h), 100% (8, 4 h), 38% (9, 72 h), and 36% (10, 72 h), respectively. Dehydrogenation, elimination, hydrolysis, C–C double bond cleavage, epoxidation, allylic oxidation, and thermal rearrangement all produced a variety of oxidised compounds; the former degradation pathways have been discussed in detail (Stolle et al., 2009; Vandewiele et al., 2011; Mahanta et al., 2021; Turek and Stintzing, 2013) (Table 1).

During GC-based EO investigations, sesquiterpenoids with a 1,5-diene system tend to rearrange themselves. GC connected to a flame ionisation or mass detector (FID or MS) is regularly used to investigate EOs due to its high sensitivity (nano to femtogram scale) and ability to resolve a complex mixture of EOs. However, the analytes are subjected to high temperatures (typically up to 250–300 °C) in the column and injector during the run, which may result in the degradation of thermosensitive phytochemicals. Numerous investigations have been conducted to demonstrate the heat conversion of the elemene derivatives of their germacrene counterparts. A high level of elemenes was detected using GC-MS while studying EOs from C. carvi roots and plants (Wichtmann and Stahl-‐Biskup, 1987). Germacrene A and B were completely rearranged to create the corresponding α- and β-elemene. The prediction was confirmed by heating oil fractions rich in germacrene A and B at 180 °C and 200 °C for 1 and 2 h, respectively. Many EOs, including black C. carvi, Smyrnium olusatrum, Hyptis alata, Achillea millefolium, black C. cyminum, Stachys sp., Ocotea sp., and Eugenia sp., were discovered to coexist with equivalent elemene derivatives during GC-MS analysis (Faraldos et al., 2007). GC-MS revealed that the major component of a chemotype of the German liverwort Preissia quadrata known as “germacrene C” undergoes full rearrangement to racemic β-elemene (König et al., 1996). At higher intake temperatures (280 °C), the thermal rearrangement was reported to be significant. During preparative GC at 390 °C injector temperature, EOs of the liverwort Scapania undulata converted helminthogermacrene to cis-elemene (Adio et al., 2004). Saccogyna viticulosa EOs from liverwort also included isogermacrene A rearrangement to iso β-elemene (Hackl et al., 2004). At a GC injector temperature of 300 °C, bicycloelemene was discovered to be a completely rearranged product of bicyclogermacrene in the EOs of Ribes nigrum buds (Le Quere and Latrasse, 1990). Bicycloelemene and bicyclogermacrene were found together in the EOs of Chloranthus spicatus and S. viticulosa (Hackl et al., 2004). GC-MS investigation of Dicranolejeunea yoshinagana revealed the production of bicycloelemene from (−)-ent-bicyclogermacrene (pure) (Toyota et al., 1996). The chemical profile of oil from the roots of Pimpinella saxifraga subsp. Alpestris was discovered to be dominated by C-12 hydrocarbons, with geijerene predicted to be the thermally rearranged product of pregeijerene (Tabanca et al., 2006). Furanosesquiterpenoids such isofuranodienone, furanodiene, and furanodiene were also found to undergo a similar rearrangement. The EOs of Eugenia uniflora underwent rearrangement from furanodiene (1.2%) to curzerene (85.1%) as a result of conventional GC-MS analysis (Chang et al., 2011). Similarly, furanodienone and isofuranodienone undergo similar rearrangement to become curzerenone and epicurzerenone, respectively (Baldovini et al., 2001). It has also been reported that the oxygenated sesquiterpene linderalactone, which was isolated from the roots of Lindera strychnifolia, underwent radical rearrangement at 160 °C to produce isolinderalactone (Setzer, 2008). Two oxygenated sesquiterpenoids, hedycaryol and germacra-1-(10,4-diene-6-ol), were found to have thermolability when Thymus praecox ssp. Arcticus EOs were analyzed (Stahl-Biskup and Laakso, 1990). At a high GC working temperature, both compounds underwent structural rearrangement to yield the corresponding rearranged products elemol and shyobunol, respectively. By heating two different fractions rich in 31 and 32 at 180 °C for 1 hour and 200 °C for 2 hours, respectively, the researchers were able to confirm their prediction. Elemol was also discovered in the oil of Cryptomeria japonica as the rearranged form of hedycaryol (Nagahama et al., 1993). Additionally, 4-hydroxygermane-1,5-diene would isomerize to ɣ-cadinol (Nagahama et al., 1993). Three separate species of Curcuma (C. wenyujin, C. phaeocaulis, and C. kwangsiensis) were the subject of a study that looked into the EO composition (Yang et al., 2007). When injected singly in their pure form during a GC-MS run, six of the eleven sesquiterpenoids (furanodiene, furanodienone, germacrone, curdione, neocurdione, and curcumol) were found to be thermolabile and visibly transformed into their respective rearrangement product(s). Notably, the temperature of the injector and the kind of injection (split vs. splitless) both affected the creation of artefacts. No studied conditions, however, could stop the GC-MS rearrangement of furanodiene and furanodienone. EOs from Acorus calamus and Melia dubia Cav underwent GC-MS analysis, and shyobunone was discovered to be an artefact created by the rearrangement of bacchascandon (acoragermacrone) (Nagalakshmi et al., 2001). The expected thermal transformation of neolinderalactone to isolinderalactone in L. strychnifolia Vill root oil was confirmed by heating 37 at 300 °C for 1–2 min (Setzer, 2008). It has also been observed that during the distillation of Saussurea lappa EOs, costunolide and dihydrocostunolide thermally changed into dehydrosaussurea lactone and saussurea lactone, respectively (Setzer, 2008), (Table 1).