Aroma Compounds of Carrier Oils

: Carrier oils are used with essential oils to dilute and enhance skin penetration. They are composed of fatty acids, triglycerides, monoterpenes, and sesquiterpenes and are added to reduce potency and odor. Carrier oils have pharmaceutical applications and reduce cytotoxicity. Solvent extraction is a common practice in the production of industrial-scale carrier oils, but harmful to the environment, so new eco-friendly methods are being researched. This review documents the available characteristics of various carrier oils and identiﬁes knowledge gaps for future studies.


Introduction
Carrier oils, which include vegetable or seed oils, are used in conjunction with essential oils to dilute, reduce potency, and enhance penetrating the skin by breaking down the lipids in the stratum corneum [1].Carrier oils mainly comprise fatty acids, triglycerides, monoterpenes, and sesquiterpenes, allowing for enhanced penetration of the stratum corneum [1].Carrier oils typically have fewer volatile compounds than essential oils and so absorb into the skin before they can evaporate and typically also have a less potent odor [2,3].Carrier oils absorb many biologically active compounds and have promising pharmaceutical applications in the delivery of drugs through the skin.The addition of carrier oils to essential oils has been found to reduce the cytotoxicity of essential oils and even give the oil an antimicrobial effect [4].Generally, small-scale extractions of carrier oils are performed using steam or hydrodistillation, while industrial-scale extractions generally use solvent extraction or mechanical press methods [1].Solvent extraction typically utilizes a Soxhlet apparatus using n-hexane as a solvent, which results in very high oil extraction yields.However, the use of n-hexane is environmentally harmful and typically requires additional refinement steps for the extracted oil which might remove important bioactive compounds and alter the overall chemical composition [5][6][7].The United States Environmental Protection Agency (EPA) has labeled n-hexane as a hazardous air pollutant and so solvent extraction using this solvent has become a less desirable extraction method in recent years despite the high oil extraction yield [7]. Environmental concerns have prompted research into new methods of oil extraction that are safe for the environment, high-yielding, able to maintain the chemical composition of the oil, and are cost-effective.Some of these methods include supercritical fluid extraction, microwaveassisted extraction, ultrasound-assisted extraction, and many other techniques.Seed oils extracted from vegetables are comprised almost entirely of triacyl glycerides making up 95 to 98% of the total chemical composition [8].However, the remaining percentage likewise contributes to the biological activity of the oils and consists of sterols, tocopherols, phenols, and other compounds [8].
Though there exist large amounts of literature on the many different carrier oils, there are many in which important information such as the chemical composition, the volatile organic compounds (VOCs), the industrial production methods, extraction methods, and the origin are not documented or are hidden in obscure pieces of literature.The goal of this review is to consolidate the large amounts of information on carrier oils.This review seeks to document the characteristics of the following carrier oils: almond, apricot, argan, avocado, babassu, baobab, black cumin seed, Brazil nut, camelina, castor, chia seed, coconut, cranberry seed, grapeseed, hazelnut, hemp seed, jojoba, kukui nut, macadamia nut, marula, moringa, neem seed, olive, palm kernel, passionfruit seed, pomegranate seed, evening primrose, pumpkinseed, raspberry seed, rosehip, sacha inchi, safflower, sea buckthorn, sesame seed, shea butter, soybean, sunflower, and tamanu.This will identify the gaps in knowledge that exist in the literature regarding the listed carrier oils to guide future studies (Figure 1).The evaluation of different extraction methods included in this review will expectantly provide guidance for future research into new extraction methods.
Though there exist large amounts of literature on the many different carrier oils, there are many in which important information such as the chemical composition, the volatile organic compounds (VOCs), the industrial production methods, extraction methods, and the origin are not documented or are hidden in obscure pieces of literature.The goal of this review is to consolidate the large amounts of information on carrier oils.This review seeks to document the characteristics of the following carrier oils: almond, apricot, argan, avocado, babassu, baobab, black cumin seed, Brazil nut, camelina, castor, chia seed, coconut, cranberry seed, grapeseed, hazelnut, hemp seed, jojoba, kukui nut, macadamia nut, marula, moringa, neem seed, olive, palm kernel, passionfruit seed, pomegranate seed, evening primrose, pumpkinseed, raspberry seed, rosehip, sacha inchi, safflower, sea buckthorn, sesame seed, shea butter, soybean, sunflower, and tamanu.This will identify the gaps in knowledge that exist in the literature regarding the listed carrier oils to guide future studies (Figure 1).The evaluation of different extraction methods included in this review will expectantly provide guidance for future research into new extraction methods.

Almond Oil
Almonds trees, or Prunus dulcis D.A. Webb (syn.Prunus amygdalus Batsch) of the Rosaceae family, are believed to have originated in Central Asia [9].Almond trees are grown primarily in Mediterranean climates such as Morocco and Spain but are also grown in France, Argentina, and the USA [10,11].As of 2011, California was responsible for 84% of the world's almond production producing 2.02 billion pounds of almonds [12].Almond oil has historically been used in ancient medicine.Chinese, Ayurvedic, and Greco-Persian schools of medicine utilized almond oil throughout history for its uses in health and beauty [13].Almond oil is produced by first dehulling almonds and drying them out.Once the almonds are dried out, the shell must be separated from the almond seed.Then the almond oil is extracted from the almond.Industrial methods of almond oil extraction sometimes involve using solvents and high temperatures to produce a high oil yield [14].However, these methods can diminish the quality of the oil and often require the oil to undergo a refining process which can diminish the health benefits of raw almond oil by

Almond Oil
Almonds trees, or Prunus dulcis D.A. Webb (syn.Prunus amygdalus Batsch) of the Rosaceae family, are believed to have originated in Central Asia [9].Almond trees are grown primarily in Mediterranean climates such as Morocco and Spain but are also grown in France, Argentina, and the USA [10,11].As of 2011, California was responsible for 84% of the world's almond production producing 2.02 billion pounds of almonds [12].Almond oil has historically been used in ancient medicine.Chinese, Ayurvedic, and Greco-Persian schools of medicine utilized almond oil throughout history for its uses in health and beauty [13].Almond oil is produced by first dehulling almonds and drying them out.Once the almonds are dried out, the shell must be separated from the almond seed.Then the almond oil is extracted from the almond.Industrial methods of almond oil extraction sometimes involve using solvents and high temperatures to produce a high oil yield [14].However, these methods can diminish the quality of the oil and often require the oil to undergo a refining process which can diminish the health benefits of raw almond oil by removing or inactivating the vitamins and other natural elements.The supercritical fluid extraction method is a common way to avoid harsh solvents and temperatures which would diminish the almond oil [14].

Apricot Seed Oil
Apricot (Prunus L., Rosaceae) originated from Asia [19].P. armeniaca L. is the most common species of apricot cultivated for fruit production [19].Apricot trees are cultivated and harvested in several Mediterranean and Central Asian countries with Turkey being the largest producer of apricots and Uzbekistan being the second largest producer [20,21].The oil from apricot kernels has been used historically in oriental medicine to treat a variety of skin diseases [22].Prior to oil extraction, the apricot seed coating must be removed, and the seeds must be made into a powder typically by crushing with the use of a blender [23].Apricot seed oil is most commonly extracted through solvent extraction or with the use of a cold press.Still, it can also be extracted by enzyme-assisted extraction or supercritical fluid extraction [5,21].Cold-press extraction was shown to have a lower yield than solvent extraction methods, but the fatty acid content, triacyl glyceride content, phenol content, and color of the oil showed no significant differences between the different methods [24].However, tocopherol content, antioxidant activity, and oxidative stability had significant differences between the two methods [24].

Argan Oil
Argan oil is produced in southwestern Morocco as it is the only area where Argania spinosa Skeels, more commonly known as argan, is endemic and naturally grows [31,32].The argan tree also grows in the Tindouf region of Algeria, though the tree is not endemic to Algeria [33].The production of argan oil used to be limited both in quality and yield due to the persistence of traditional extraction methods historically used in southwestern Morocco.However, these extraction methods have been improved using a cold-press extraction method [34].Modern methods of argan oil extraction begin first with the picking and drying of the fruit from argan trees.Dehulling, collecting the kernels, and possibly roasting depending on the intended use of the oil, before finally being cold pressed to extract the argan oil [34].The industrial extraction of argan oil is typically performed by solvent extraction using a lipophilic solvent such as hexane [32].The fatty acid composition of argan oil was oleic acid (45.0-50.3%),linoleic acid (29.0-36.8%),palmitic acid (12.3-13.8%),stearic acid (4.7-5.7%),arachidic acid (0.3-0.4%), gadoleic acid (0.3-0.4%), myristic acid (0.2%), linolenic acid (0.1-0.2%), and palmitoleic acid (0.1%) [34,35].The volatile organic compounds in argan oils were primarily made of alcohols, pyrazines, acids, furans, pyrroles, lactones, phenols, ketones, and aldehydes [36].The squalene content of argan oil ranged from 303-321 mg/100 g while the total sterols were in the range of 272-357 mg/100 g [37].

Avocado Oil
Avocado fruit, or Persea americana Mill. of the Lauraceae family, originated from Southern Mexico where the trees have been cultivated since ancient times up to 6000 years ago [38].Modern-day avocado oil production takes place mainly in New Zealand, Mexico, Chile, South Africa, and the United States [39].Avocado oil has been popularized due to its use in skin care and human nutrition [40].Avocado oil is unique in that it is absorbed by the skin far easier than almond, corn, olive, and soybean oils, making it an ideal oil for skin care [40].Avocado oil has been used historically as far back as the sixteenth century when it was used in the treatment of scars and rashes [40].The bioactive compounds of virgin avocado oil, notably α-tocopherol and β-sitosterol, assist in reducing cholesterol and hypertension [41].The most common way that avocado oil is produced is through solvent extraction [42].Avocado oil production starts with the drying of the fruit to remove any water.The dried avocado fruit is then crushed and ground into a powder [43].The powder is exposed to hexane in an extractor, and the oil is separated from the powder [42].

Baobab Oil
Adansonia digitata L., commonly known as Baobab, is found throughout tropical regions of Africa, islands in the Indian and Atlantic Oceans, and even in some parts of southern Arabia [52,53].The crude oil extraction of baobab oil starts with the picking of the fruit and the removal of its shell [54].The pulp is separated from the seeds of the fruit and the seeds are pressed using a mechanical method such as a cold press [54].Mechanical and solvent methods of extraction of baobab oil were compared and it was found that mechanical methods, more specifically the cold-pressing extraction method, better maintained the physicochemical properties of the oil and preserved the high concentrations of phenolic compounds [55].It was also suggested that the physicochemical properties of the oil were less impacted by the solvent extraction method when a mixture of acetone and hexane was used rather than either solvent alone [55].Baobab has high percentages of palmitic and oleic acids which makes it ideal for use in cosmetic products [56].The oil is mainly made of oleic acid (23.3-36.1%),linoleic acid (29.0-30.6%),palmitic acid (21.9-22.9%),and stearic acid (3.7-5.9%), and a total sterol content of 4630 mg/kg [55,57,58].

Brazil Nut Oil
Brazil nuts (Bertholletia excelsa Humb.& Bonpl.)originate from the northern and eastern regions of the Amazon rainforest in Brazil and have expanded to several other South American countries including Bolivia, Columbia, Peru, and Venezuela [67].The Brazil nuts or seeds are gathered from Brazil nut trees and then dehydrated and prepared for oil extraction [68].The drying process is very important as Brazil nuts have a high moisture content which can lead to fungal growth [69].The Brazil nuts which are broken or too small to be sold commercially are used in the extraction of Brazil nut oil [70].Brazil nuts can be extracted by using n-hexane and ethanol as solvents in a Soxhlet apparatus or through the use of hot or cold pressing to better retain the bioactive compounds of the oil [68,71].Hydraulic press extraction, solid-liquid (solvent) extraction by petroleum ether and hexane as the solvent, and supercritical fluid CO 2 extraction were compared, and it was found that supercritical fluid CO 2 extraction resulted in the highest average percentage of rich unsaturated fatty acids [71].Industrial-scale extraction of Brazil nut oil typically involves mechanical methods such as hot pressing or cold pressing using a hydraulic press [68,71].The major fatty acids of Brazil nut oil were linoleic acid (42.8%), oleic acid (29.1%), palmitic acid (13.5%), and stearic acid (11.8%) [72,73].The total phenolic and total flavonoids of the oil were 169.2 and 107.8 mg/100 g, respectively [72,73].β-Sitosterol accounts for 76% of the sterol composition and β-tocopherol 88-93% of the tocopherol content of Brazil nut oils [74].The volatiles of Brazil nut oil were a mixture of carbonyls, alcohols, hydrocarbons, and terpenes [75].

Camelina Oil
Camelina has the highest species diversity in Turkey and the Caucasus regions which hints at those areas being its place of origin [76].However, that is not the only hotspot for Camelina species diversity [77].In Russia and Ukraine, the high camelina species diversity suggests that it is likely to have been the origin of camelina [77].Camelina (Camelina sativa L.) was first cultivated during the bronze age in which northern Europeans used the plant primarily for oil, which was used in medicine, as nutrition, and as a fuel source.Camelina plants grow quickly, can self-pollinate, and are over 40% oil in composition, making them an ideal oil crop [78].The traditional process of camelina oil extraction begins with the harvesting of the dried plants.The seeds are collected from the dried plants, cleaned, and then milled into a powder.Water is added to the powder to make a paste and then heated again.This paste is then used in the extraction of camelina oil [79].
Mechanical extraction, solvent extraction, and enzymatic extraction are some of the most common methods of camelina oil extraction [80].Mechanical extraction was traditionally used in the extraction of camelina oil by use of a hydraulic or screw press.This method, however, does not result in as high a yield as other methods of extraction such as solvent extraction, but the refinement process is much simpler as only filtration and degumming are required to refine camelina oil for human consumption.Solvent extraction results in higher oil extraction yields than mechanical or enzymatic extraction methods but is more harmful to the environment and requires refinement to remove any solvent in the oil.Enzymatic extraction is a method that results in high oil extraction yields and does not require the use of harsh solvents or refinement processes.However, this method takes much longer to extract than both mechanical and solvent methods of extraction.Solvent extraction is the preferred method of camelina oil extraction on an industrial scale due to the high oil yields and to avoid economic losses due to waste in mechanical extraction [79].Using a Soxhlet apparatus and hexane as the solvent was found to result in camelina oil yields of 30.5% by weight, making it a profitable method of extraction on an industrial scale [80].The major fatty acids of camelina oil were α-linolenic acid (34.7-37.1%),oleic acid (14.8-19.1%),linoleic acid (15.3-19.3%),gondoic acid (11.7-16.7%),palmitic acid (4.8-6.4%), and stearic acid (1.6-2.9%)[81,82].A total of 168 volatile and semi-volatile compounds were identified in camelina seeds between acids, alcohols, esters, phenolics, aldehydes, ketones, alkanes, alkenes, aromatics, ethers, and terpenes [79].

Castor Oil
Castor oil (Ricinus communis L.) contributes to only around 0.15% of the world's vegetable oil production but is, more importantly, used in the production of chemicals due to the oil being the only commercially available source of a hydroxylated fatty acid, ricinoleic acid, which comprises that majority of castor oil (75.7-94.6%)[83][84][85].The exact origins of castor are unknown, though four regions are believed to be the possible origin of castor.These regions are East Africa, Northwest and Southwest Asia and the Arabian Peninsula, India, and China [86].In modern times, castor is now grown all over the world in tropical, sub-tropical, and even temperate climates [86][87][88].India is currently the largest producer of castor oil and castor seeds with an annual production between 250,000 and 350,000 tons of castor oil [83].Castor seed consists of anywhere between 30 and 50% oil by mass.The first step of castor oil production, right after harvesting, begins with the drying of the seeds so that their hulls can be cracked open, and the seeds isolated from the hulls.After the seeds are isolated, they are cleaned so that any other foreign matter is removed.A steam-jacketed method is used to heat the seeds and dry them before they are finally used in the extraction of castor oil [83].The extraction of castor oil is performed by a variety of different methods including hydrate press extraction, continuous screw-press extraction, solvent extraction, or even a combination of mechanical hot-press extraction and solvent extraction to result in a high oil extraction yield [88].Crude castor oil must be subjected to a refinement process to extend the storage life of the oil by removing impurities that could contribute to the oil's deterioration [83].Colloidal matter, phospholipids, and excess free fatty acids are just some of the impurities removed by castor oil's refinement process [83].

Chia Seed Oil
Salvia hispanica L., belonging to the Lamiaceae family and more commonly known as chia, originates from Mexico and Guatemala but can be grown in nearly every continent [89][90][91].Chia has been used for human nutrition for about 5500 years and was used as a medicine by the Aztec and Mayan peoples [92].Chia seeds contain around 40% oil which contains high concentrations of essential fatty acids, ω-3 α-linolenic acid, and ω-6 linoleic acid [93].The ingestion of chia seeds, along with chia seed oil, has been shown to result in health benefits for cardiovascular diseases, diabetes, and metabolic syndrome [92].Chia also has high concentrations of phytosterols for an oil seed crop, which has cancer and cardio-protective effects [92].The European Parliament declared chia a Novel Food in 2019 and found that the ingestion of chia seeds had resulted in no identifiable health hazards.A common process of chia seed preparation for oil extraction begins with the removal of any foreign matter or seeds that are not mature enough for extraction by using a sieve.The seeds are further cleaned and then ground down to a fine powder before finally being used in the extraction of chia seed oil.The extraction of chia seed oil was found to yield more than 30% more oil by solvent extraction than by pressing [94].It was also found that the fatty acid content had no significant differences between the two methods of extraction [94].Traditional methods of chia seed oil extraction typically involve the use of an expeller for mechanical methods or solvent extraction using n-hexane [95].However, the current method of industrial-scale chia seed oil extraction is performed through cold pressing.The main fatty acids of chia seed were linolenic acid (60.5%), linoleic acid (22%), oleic acid (7.4%), palmitic acid (7.3%), and stearic acid (2.8%) [96].

Coconut Oil
The Indo-Malayan tropical regions are believed to be the origin place of the coconut palm (Cocos nucifera L.).However, the coconut palm has spread to most tropical regions of the world largely thanks to European and Polynesian exploration and cultivation [97].Indonesia is the largest producer of coconuts followed by the Philippines and India [98].Coconut oil is one of the oldest oils with its use being traced as far back as 4000 years ago [99].Lauric acid comprises about half (49.6-51.5%) of the total fatty acids present in coconut oil [100,101] and is a precursor to monolaurin, which has many health benefits along with antiviral, antibacterial, and antiprotozoal monoglyceride properties [102].The process of coconut oil extraction through low-pressure methods starts with the procurement of coconut meat, which is grated and dried to an 11% moisture content.The dried coconut gratings are placed in a low-pressure press to extract the oil, which is collected, and later filtered, and centrifuged [103].Coconut oil can also be extracted using a cold press in which the coconut meat is grated, but rather than drying the grated coconut, water is added to the gratings.The gratings are then placed in mesh bags where the coconut milk is squeezed out, collected, and then left to sit for a day.After a day has passed, the coconut milk gets separated into three distinct layers.The middle layer is collected, filtered, and left to sit for 72 h so that any water can evaporate.Coconut oil can also be extracted in a variety of other ways, including chilling, freezing, and thawing methods.These methods start with pressing coconut kernel and collecting the residue.Pressure is applied to the residue, and the emulsion is extracted and then centrifuged [103].Centrifugation separates the residue into distinct layers in which the coconut cream layer is collected.Enzymes are added to the coconut cream, and it is frozen, thawed, and then centrifuged.Hot-press and cold-press extraction have been traditionally used to extract coconut oil.However, both methods result in low oil yields, and the hot press method will alter the chemical composition of the oil, compromising the important bioactive compounds [103].It was found that coconut oil contains much lower sterol content than most other vegetable oils, including soybean oil and corn oil [100].Ethyl acetate, acetic acid, hexanal, 2-heptanone, limonene, nonanal, octanoic acid, ethyl octanoate, δ-octalactone, δ-decalactone, and dodecanoic acid were identified as the volatile compounds in virgin coconut oil [104].

Cranberry Seed Oil
Cranberries are believed to have originated in North America but are also grown and cultivated in Europe [105].The European cranberry (Vaccinium oxycoccos L.) and the American cranberry (Vaccinium macrocarpon Aiton) are rich sources of bioactive compounds which include anthocyanins, proanthocyanins, and quercetin [106].These bioactive compounds have several health benefits due to the antioxidant activity of the cranberry fruit, especially a cardioprotective effect.These compounds are important as they decrease the risk of cardiovascular disease by reducing low-density lipoprotein cholesterol oxidation, inhibiting platelet aggregation, and reducing blood pressure [107].Cranberry seed oil is extracted from cranberry seeds, which are typically procured as a by-product from the fruit juice processing [108].The cold-pressing extraction technique, along with ultrasonographic extraction, are common methods of cranberry oil extraction as the chemical composition and bioactive compounds are not significantly altered or diminished [105].It was found that ultrasound assisted extraction, when compared with microwave assisted extraction, heat reflux extraction, and cold-press extraction, produced the highest cranberry oil yield and the highest tocopherol concentrations, revealing that ultrasound assisted extraction produced the highest quality oil which was most rich in phytonutrients [108].Traditional methods of cranberry oil extraction include cold-press extraction and heatreflux extraction.However, these methods are being improved upon and replaced by microwave assisted extraction and ultrasound assisted extraction due to these methods resulting in an increase in oil yield and a higher tocopherol concentration [108].Linoleic acid (37.4%), linolenic acid (34.9%), oleic acid (20.2%), and palmitic acid (5.3%) were the main fatty acids in cranberry seed oil [105].The volatile organic compounds from cranberry seed oil were 1,2-dimethyl cyclopropane, 1,3,5,7-cyclooctatetraene, 1-penten-3-one, (E,E)-2,4-heptadienal, (E)-2-butenal, (Z)-2-heptenal, 2-hexenal, (E)-2-penten-1-ol, 2-pentenal, (E)-2-pentene, (Z)-4-heptenal, acetic acid, ethyl acetate, 2-ethylfuran, furfural, hexanal, hexane, (S)-methyl-oxirane, pentanal, and trimethylene oxide [109].

Hazelnut Oil
Hazelnut (Corylus avellana L.) naturally grows in Europe, Africa, the Caucasus Mountains, and eastern Asia [117].Hazelnuts are the third most produced nut in the world by mass, with over 863,000 tons of hazelnuts produced every year.Over half of the world's hazelnut production comes from Turkey, with the rest coming from a variety of countries, including Italy, Azerbaijan, the United States, and China [118].After hazelnuts are harvested, the skin is peeled away, and the nut is crushed and ground into a powder or paste, which is heated and dried before being used in the extraction of hazelnut oil [119].Traditional methods of extraction, those being mostly mechanical or solvent extraction, resulting in low yields, are less environmentally friendly, are time-consuming, and often require a refining process which removes important bioactive compounds [120].Modern methods of extraction include supercritical CO 2 extraction, enzymatic extraction, microwave-assisted extraction, and ultrasonic-assisted extraction, and have greatly improved the production of hazelnut oil by increasing the oil yield, reducing extraction times, reduced alteration in chemical composition, and reducing the risk of harming the environment [120].The industrial chemical refinement of hazelnut oil was found to not cause any significant changes to the fatty acid composition.The sterol content, however, was greatly diminished.The tocopherol content was found to be reduced as well, but not significantly [121].Hazelnut seeds themselves are high in the amino acids arginine, leucine, isoleucine, phenylalanine, and valine [122].Hazelnut oil is a rich source of oleic acid (72.8-83.5%)[123].

Kukui Nut Oil
The kukui nut tree (Aleurites moluccanus Willd., Euphorbiaceae) is native to many islands in the Pacific Ocean due to Polynesian exploration and cultivation of the tree.The kukui nut was very important to native Hawaiian people as it was believed to have important medicinal properties.They also used the nut and its oil as a fuel source for light and as food [136].The kukui nuts are ground into a powder, even with the shell still attached, and then dried prior to oil extraction [137].Linoleic acid (61.6%) is the main fatty acid in kukui oil, followed by oleic acid (17.0%), linolenic acid (5.2%), stearic acid (4.7%), and palmitic acid (2.1%) [137].No information was found about the volatile composition of the oil.

Macadamia Oil
Macadamia (Macadamia spp., Proteaceae) nuts originate from the New South Wales and Queensland states of Australia [138].In modern times, Hawaii has been a major producer of macadamia nuts along with Australia [139].Macadamia nut oil, when refined by traditional methods, turns rancid quickly and has important storage requirements [140].

Marula Oil
Marula (Sclerocarya birrea Hochst., Anacardiaceae) trees are native to southern Africa but have been introduced to several other countries including Australia, India, and Israel [143,144].The only sample preparation necessary for marula oil extraction is the grinding of the seeds into a powder [145].Cold-pressed extraction is the preferred method of marula oil extraction as the oxidative stability of the oil is maintained [146].Aqueous extraction is also a common method of extraction that does not utilize harsh solvents, pressures, or temperatures which could diminish the quality of the oil.Solvent extraction using hexane as the solvent is typically used to extract marula oil on an industrial scale due to the high oil extraction yields which leaves behind roughly only 0.5% of the oil in the cake or meal [146].Oleic acid is the major fatty acid in marula oil, accounting for 70-78% [144,145,147].The total tocopherols and sterols were 13.7 mg/100 g and 286.6 mg/100 g, respectively [135,136,138].The main volatiles reported from the fruit pulp were β-caryophyllene (91.3%) and α-humulene (8.3%), while the intact fruit had heptadecene (16.1%), benzyl 4-methylpentanoate (8.8%), (Z)-3-decen-1-ol (8.4%), benzyl butyrate (6.7%), (Z)-13-octadecenal (6.2%), and cyclopentadecane (5.7%) [148].

Moringa Oil
Moringa (Moringa oleifera Lam.) trees are cultivated all over the world but originated in Africa and Asia [146,149].Moringa trees are most easily grown in dry tropical areas such as the northwestern regions of India, which is why cultivation of the tree is so prevalent and economically valuable there.On an industrial scale, solvent extraction is the traditional method of extraction of moringa oil as many of the important lipids are recovered.The solvent typically used in this extraction is either hexane or petroleum ether.Mechanical methods, including cold-pressing, are standard as well but are typically not used on an industrial-scale as the yield is much lower than through solvent extraction [149].The fatty acid composition of moringa seed oil was oleic acid (73.4%), palmitic acid (6.1%), behenic acid (5.7%), and stearic acid (5.4%) [150].

Neem Seed Oil
Neem (Azadirachta indica A. Juss.) trees are indigenous to the Karnataka forests of southern India and the island forests of Myanmar.However, neem trees grow in nearly 80 countries around the world [151].Neem oil is most commonly extracted by mechanical methods which result in high yields but is typically not pure and may require a refinement process to remove undesirable metals and compounds.Supercritical fluid extraction results in an oil high in purity but very expensive to produce [152].Solvent extraction using a Soxhlet apparatus is another common method of extraction that results in high oil yields and includes very few undesirable metals and compounds [153].The average fatty acid composition of neem oil is oleic acid (25-54%), palmitic acid (16-33%), stearic acid (9-24%), and linoleic acid (6-16%) [154].γ-Tocopherol is the main tocopherol in neem seed oil accounting for 68.9% [155].The main volatile organic compounds from the hydrodistilled oil were 3,5-diethyl-1,2,4-trithiolane (57.8%), 1-hexadecanol (5.7%), and (E)-15-heptadecenal (5.1%) [156].

Olive Oil
Domestication of the olive tree (Olea europaea L.) is believed to have begun in the Near-East, though in modern times, over 90% of the world's olive production takes place in Mediterranean countries [157,158].The origin of the olive tree has been debated, but it is believed to have originated in the Mediterranean basin or the Middle East [157].Olive trees are indigenous to Italy, Greece, Palestine, Syria, and many other surrounding countries [159].The processing and extraction of olive oil has been traditionally carried out via mechanical methods which included crushing, malaxation, and centrifugation [160].A reason that mechanical methods are still commonly used even in modern times is the lack of need for further refinement, as olive oil is safe for consumption directly after mechanical extraction [161].Olive oil extractions on an industrial scale have, in recent years, been adopting microwave, pulsed electric field, and ultrasound assisted olive oil extraction to increase the yield and quality of the extracted oil [160].The sterol content of extra virgin olive oil was 319 mg/100 g [37].The main fatty acids in virgin olive oil were oleic acid (78.7%), palmitic acid (8.7%), and linoleic acid (8.3%) [162].(E)-2-hexenal (23.2-27.7%),(E,E)-α-farnesene (10.5-13.2%),and acetic acid (6.4-8.5%) were the major volatile organic compounds [163].

Palm Kernel Oil
It is believed that the oil palm, or Elaeis guineensis Jacq., originated in Africa, though some have proposed the origin to be in South America [164].The most common methods of palm kernel oil extraction, at least in one of the largest palm kernel oil producers in the world, Malaysia, include mechanical methods, which utilize a screw press, direct solvent extraction, and a combination of both methods in which mechanical pressing is followed by solvent extraction [165].With the dangers of solvent extraction becoming more evident, newer methods, such as the supercritical CO 2 extraction method, have become a more common alternative extraction method due to the use of a solvent that is environmentally friendly [165].Solvent extraction is typically used to extract palm kernel oil on an industrial level as it requires very expensive equipment but results in very high oil yields [7].The use of hexane as the solvent has made this method less desirable in recent years as hexane has been labeled a hazardous air pollutant by the United States Environmental Protection Agency, or EPA [7].The use of supercritical fluids as solvents resolves this issue and greatly reduces harm to the environment [165].Myristic acid (72.3%) and palmitic acid (25.8%) are the major fatty acids in palm kernel oil [166].

Pomegranate Seed Oil
Pomegranate (Punica granatum L.) trees are believed to have originated in Iran or the Near-East.In modern times, pomegranate trees have been distributed and cultivated all over the world and are especially prevalent throughout Eurasia [172].The pomegranate juice industry provides the left-over seeds to be used in pomegranate oil extraction.These seeds are first cleaned of any other parts of the pomegranate until only the seeds remain.The seeds are then dried and powdered before being used in the extraction of pomegranate oil [173].Traditionally, extraction methods of pomegranate seed oil include Soxhlet extraction, distillation, cold pressing, hot pressing, and maceration.However, some of these conventional methods degrade the bioactive compounds and result in an oil of lesser quality [173].Multiple methods of pomegranate seed oil extraction were compared, and it was found that solvent extraction utilizing a Soxhlet apparatus produced the highest yield when compared to extraction by normal stirring, microwave irradiation, ultrasonic irradiation, and supercritical fluid [174].Stirring had the lowest pomegranate oil yield.It was found that the differences in fatty acid composition were negligible between the compared methods [174].Production of pomegranate oil on an industrial scale is greatly limited by the waste necessary to extract the oil, and it results in large amounts of waste which has a risk of becoming an environmental hazard [173].Pomegranate seed oil is a rich source of punicic acid (73.7-80.7%)[175,176].Ethyl valerate (8.9-16.1%),2-methylbutyl formate (10.1-17.5%),2-methylbutyl acetate (3.3-7.8%),hexanal (1.9-7.9%),2-methyl-1-butanal (1.1-7.2%),pentanal (2.4-5.6%),3-methyl-1-butanal (0.2-5.4%), butyraldehyde (1.4-9.5%), and 2-butanone (0.2-4.8%) were reported as the major volatile organic compounds [177].

Sea Buckthorn Oil
Sea buckthorn trees (Hippophae rhamnoides L.) grow in the temperate zones of Europe and Asia where they are indigenous [208].Sea buckthorn oil can come from either seeds or fruit pulp, in which the seeds have a slightly higher yield than the fruit pulp [209].The chemical composition of sea buckthorn oil changes significantly depending on what part of the fruit was used in extraction [209].Cold-press extraction, solvent extraction using hexane, and supercritical CO 2 extraction methods of sea buckthorn seed oil were compared, and it was found that supercritical CO 2 extraction resulted in the oil with the highest sterol percentage [210].The fatty acid composition of sea buckthorn oil was palmitic acid (49.4%), palmitoleic acid (28.0%), oleic acid (11.7%), linoleic acid (4.1%), cis-vaccenic acid (4.5%), and α-linolenic acid (4.5%) [211,212].The total sterols, carotenoids, and tocopherols were 748.1, 22.2, and 421.7 mg/100 g, respectively [211,212].

Soybean Oil
It is believed that the origin of soybean (Glycine max (L.) Merr.) is in either the northeastern or southern regions of China.Its domestication began in East Asia somewhere between 6000 and 9000 years ago [222].Mechanical methods of extraction are the most common method of soybean oil extraction even though it results in a lower yield [223].This method allows for the later use of the left-over soybean cake as it has not been contaminated with solvents that are harsh for the environment.The solvent extraction method has a very high extraction efficiency of 98% but is a less environmentally friendly extraction method.The industrial level extraction of soybean oil usually utilizes combinations of mechanical and chemical methods to result in very high oil yields [223].The fatty acid composition of soybean oil was linoleic acid (53.2%), oleic acid (23.1%), palmitic acid (11.4%), linolenic acid (7.5%), and stearic acid (4.1%) [223].The total sterol and tocopherol contents were 397 and 94.9 mg/100 g, respectively [223].

Tamanu Oil
Tamanu (Calophyllum inophyllum L.) is native to tropical parts of Asia and is distributed not only across Asia but also Polynesia, Micronesia, and Melanesia [227,228].Two high oil yield extraction methods, supercritical CO 2 and Soxhlet extraction, were compared and it was found that supercritical CO 2 resulted in a higher percentage of fatty acids than the Soxhlet extraction method [229].The oil extracted through supercritical CO 2 extraction was found to have more antibacterial properties than the oil extracted through Soxhlet extraction [229].Tamanu oils contained oleic acid (37.7-38.9%),linoleic acid (31.8-33.8%),palmitic acid (12.9-13.5%),and stearic acid (12.4-14.0%)as the major fatty acids [229].

Volatile Composition
Table 1 summarizes the volatile composition of carrier oils.Interestingly, no information was found on the VOCs of babassu, baobab, chia, hazelnut, jojoba, kukui nut, palm kernel, sacha inchi, sea buckthorn berry, shea butter, sunflower, and tamanu.

Conclusions
Aromatherapy is an increasingly popular complementary and alternative treatment, and many essential oils are used in conjunction with carrier oils.However, many volatile components of carrier oils may have offensive organoleptic properties (e.g., dimethyl sulfide, dimethyl disulfide, butyric acid) or unacceptable toxicities (e.g., benzene).Furthermore, there are several components reported that are likely contaminants (e.g., 2,4-di-tertbutylphenol, diethyl phthalate, dibutyl phthalate) rather than actual carrier oil components.It is also useful to know if the contaminants that are present in the carrier oils to be used have undesirable side effects (e.g., phthalate esters have shown anti-estrogenic activities and developmental and reproductive toxicity).Future studies should be carried out to verify the volatile components of carrier oils, determine variability in the volatile components based on origin, and assess possible contamination or adulteration.Interestingly, no information was available about the volatiles of babassu, baobab, chia, hazelnut, jojoba, kukui nut, palm kernel, sacha inchi, sea buckthorn berry, shea butter, sunflower, and tamanu.Additional research is needed in order to determine the volatile components of these oils as well as any forthcoming carrier oils.This review has presented a compilation of volatile components of carrier oils and includes evaluation of extraction methods.The information serves to identify gaps in information regarding carrier oils and should provide guidance for future research into new carrier oils and new extraction methodology.

Figure 1 .
Figure 1.A summary of the information used for carrier oil authentication.

Figure 1 .
Figure 1.A summary of the information used for carrier oil authentication.

Table 1 .
Volatile composition of carrier oils.