Farnesene

Aroma and sensory profile

Farnesene smells like the skin of a green apple. Fragrance references type alpha-farnesene woody and describe it neat as citrus, herbal, lavender, bergamot, myrrh, neroli, and green, or at 1 percent as woody and green-vegetative with a hint of floral, with a flavor at 10 to 20 parts per million of fresh green vegetative with celery and hay nuances and fatty, tropical-fruity afternotes [2]. (E)-beta-farnesene, its isomer, is described as woody, citrus, herbal, and sweet [1]. Both are quiet, green, and slightly waxy; alpha-farnesene is the fresher and more apple-like, beta-farnesene the warmer and more woody-herbal.

The molecule is an acyclic sesquiterpene with four double bonds. Alpha-farnesene, 3,7,11-trimethyldodeca-1,3,6,10-tetraene, and beta-farnesene, with the double bond moved to an exocyclic methylene, have no stereocenter, so there are no enantiomers to compare; what they have are geometric isomers, and the natural forms are (E,E)-alpha-farnesene and (E)-beta-farnesene. The two are the commonest sesquiterpene hydrocarbons in the plant world after caryophyllene, and they are made by many plants precisely because insects can smell them.

Effects and research

No human trial of farnesene was found. Its research is chemical ecology, plant physiology, and one unexpected mouse result.

(E)-beta-farnesene is the principal component of the alarm pheromone of many aphid species, and many plants release it as a volatile [4]. Predators listen: the hoverfly Eupeodes corollae detects it through specific odorant receptors, its larvae using aphid-derived farnesene to find prey at short range and adults using plant-derived signals from farther away [5]. Transgenic Arabidopsis emitting it constitutively did not repel green peach aphids or reduce their performance, which suggests habituation [4], and in field colonies of pea aphids, natural pheromone levels did not increase predator visits, though exaggerated amounts did [6]. Chamomile makes it in response to aphid feeding and it repels them [7].

Alpha-farnesene's physiology is about fruit. In stored apples and pears it autoxidizes to conjugated trienes, hydroperoxides, and trienols that induce superficial scald, the brown skin injury of cold storage; farnesyl hydroperoxide applied to apple skin reproduced the symptoms [8], and in d'Anjou pears conjugated trienols were the major oxidation products accumulating in the peel [9]. Apple varieties that make more of it are more susceptible to fungal infection, and silencing the synthase reduced disease establishment [10].

The mouse result concerns vaping. Farnesene, used as a green-apple flavorant in e-cigarette liquids, produced reward-related behavior in male and female mice at 0.1 to 10 milligrams per kilogram by acting on high-sensitivity nicotinic receptors in the ventral tegmental area, with about 30 percent of nicotine's potency [11].

Natural sources

Farnesene is a fruit-skin, hop, and rhizome terpene. Fragrance databases log (E)-beta-farnesene at 18.2 percent in galangal oil, 14.2 percent in ginger root extract, 10.5 percent in juniper berry oil, 9.6 percent in Vietnamese Artemisia annua oil, and 1 to 2 percent in lavandin, calamus, cedrela, yuzu peel, and pine cone oils [1], and alpha-farnesene in apple fruit, skin, and pericarp, at 5 percent in an Artemisia flower oil and 3 percent in a Brazilian flower oil, with traces in basil, bay leaf, bergamot, blood orange, and cajuput [2]. South African hop varieties carried beta-farnesene at 2.1 to 7.6 percent of their oil [12], and German chamomile oil is rich in it [7]. Apple skin is the classic alpha-farnesene source and the reason stored apples smell as they do [8]. In cannabis farnesene is a minor sesquiterpene, reported in a number of chemotypes at low levels.

Boiling point and vaporization

PubChem compiles farnesene's boiling point as 260 to 262 °C at 760 mmHg, imported from fragrance industry data [3], and this profile uses 262 °C (504 °F), replacing the 122 °C the site previously listed, a vacuum-distillation value. Fragrance references give (E)-beta-farnesene a lower figure, 206 °C at 760 mmHg [1], which is not carried in PubChem and is not adopted here; the difference, if real, would place the beta isomer with the heavier monoterpene alcohols and the alpha isomer with caryophyllene. Both are volatile enough to work as airborne insect signals at ambient temperature [4][5]. Their instability is oxidative and severe: four double bonds make farnesene one of the fastest-autoxidizing terpenes, which is the chemistry behind apple scald [8][9]. No thermal degradation study of farnesene was found.

Safety and regulation

Farnesene is not listed by name in the FDA flavoring tables at 21 CFR 172.515; its flavor use rests on FEMA GRAS numbers 3839 for alpha-farnesene and 4971 for (E)-beta-farnesene [1][2], and no Research Institute for Fragrance Materials assessment was found. No cancer classification applies.

Its hazard record is the hydrocarbon pattern. Under Europe's classification inventory, 97.5 percent of notifications flag farnesene as an aspiration hazard, and for beta-farnesene 26 percent add skin irritation and 24 percent possible skin sensitization [3]. No patch-test series naming farnesene was found. Two points deserve more weight than the classification gives them: the autoxidation chemistry means aged farnesene is a hydroperoxide source of the kind that sensitizes for other terpenes [8], and the mouse reward study means farnesene in a vaping liquid is not an inert flavor [11]. Neat material should not be swallowed or drained.

Industries and uses

Fragrance: farnesene supplies a green, woody, apple-skin freshness in fruity, green, and hop accords and is used as a natural constituent of those oils and as an isolate [1][2]. Culinary: both isomers hold FEMA GRAS status and contribute green apple, celery, and hay nuances; alpha-farnesene is the note of apple skin and beta-farnesene part of hop and ginger flavor [1][2][12]. Aromatherapy: chamomile, ginger, and juniper berry oils are its carriers; the oxidation and aspiration points above are the relevant cautions [3][7].