α-Terpineol

Aroma and sensory profile

Alpha-terpineol is lilac grown on a pine tree. Fragrance references type it terpenic with medium strength and describe it neat as pine, terpene, lilac, citrus, woody, and floral; at 1 percent in alcohol it is pine-like, woody, and resinous with a slight cooling lemon and lime nuance and a floral dry-out, and its flavor is citrus, woody, lemon, lime, and soapy [1].

The molecule, p-menth-1-en-8-ol, has one stereocenter and is chiral, and the enantiomers differ. (+)-alpha-terpineol is typed floral and described neat as lilac and floral [2]. (−)-alpha-terpineol is typed terpenic and described as lilac, floral, and terpenic [3]. The (+) form is the cleaner flower; the (−) form carries more of the pine. Fenaroli's handbook places the (+) form in cypress-family oils, cardamom, star anise, marjoram, clary sage, and neroli, the (−) form in winter savory, lavandin, cajeput, lime, lemon, cinnamon leaf, and pine distillates, and the racemate in cajeput, Eucalyptus globulus, and petitgrain [4].

Effects and research

No human trial of alpha-terpineol was found. The Research Institute for Fragrance Materials reviewed it in 2008 and concluded that sensitization, phototoxicity, and genotoxicity do not present a concern under intended fragrance use [5]. The pharmacology is rodent, and it has recently acquired a toxicology that matters.

The benefit-side data are mice. Alpha-terpineol reduced nociceptive behavior in formalin, writhing, glutamate, and capsaicin tests and showed central analgesic activity in the hot plate test [6]. Oral doses of 6.25 to 50 milligrams per kilogram reduced stool output by 24 to 55 percent and diarrheal episodes by 10 to 66 percent through muscarinic inhibition [7]. In rats on a high-fat diet, both enantiomers at 25 to 100 milligrams per kilogram lowered tumor necrosis factor alpha and interleukin-1 beta, with the (−) form reducing TNF-alpha 3.4-fold against 1.5-fold for the (+) form [8].

The toxicology is rats and it is recent. In a 90-day dietary study, the no-observed-adverse-effect level was 146 milligrams per kilogram per day in males, with excessive sperm fragmentation and reduced motility at the high dose, and 500 in females [9]. In a reproductive study, gavage doses of 150 to 300 milligrams per kilogram per day produced azoospermia, reduced testosterone, testicular damage, and lowered thyroxine in both sexes [10]. These doses are far above any flavor, fragrance, or inhalation exposure, but they are the reason alpha-terpineol should not be treated as inert when taken orally in quantity.

Natural sources

Alpha-terpineol is everywhere. HSDB records it as occurring naturally in more than 150 plant species and in more than 150 derivatives from leaves, herbs, and flowers, and as an emission from plants and molds [4]. Tea tree oil carries it at 1.5 to 8 percent under the international standard, and it rises in stored oil as the terpinenes oxidize [12]. Enantiomer-specific records place the (+) form in cardamom, cinnamon, lovage root, marjoram, and Australian tea tree oil at 2 percent, and the (−) form in bay laurel, cinnamon, mint, cajuput, pine oil, petitgrain, and tea tree at 0.7 percent [2][3]. Its largest source is not a plant but pine turpentine, from which it is made by hydration of the pinenes, and it is the main constituent of pine oil [3]. In cannabis it is a minor terpenoid, present in most chemotypes at low levels.

Boiling point and vaporization

PubChem compiles alpha-terpineol's boiling point as 217 °C from the Joint FAO/WHO Expert Committee on Food Additives and 218 to 221 °C from Sax's handbook [4]; fragrance references list 214 to 218 °C at 760 mmHg and 136 to 137 °C at 50 mmHg [1]. This profile uses 217 °C (423 °F), the JECFA value. That places it with linalool and isopulegol in the heart-note band: it persists on skin and fabric for hours. It is a viscous liquid or low-melting solid near room temperature and a combustible rather than flammable one [4]. As a tertiary alcohol with one ring double bond it is far more oxidation-stable than the terpinenes it is made beside. No thermal degradation study of alpha-terpineol was found.

Safety and regulation

Alpha-terpineol is listed by the US Food and Drug Administration as a permitted synthetic flavoring substance at 21 CFR 172.515, as "α-Terpineol; p-menth-1-en-8-ol" [11]. The Research Institute for Fragrance Materials found sensitization, phototoxicity, and genotoxicity not to be a concern in 2008 [5]. No cancer classification applies.

Its skin record is that of an irritant, not an allergen. Under Europe's classification inventory, 99 to 100 percent of notifications flag it as a skin irritant and 82 to 100 percent as a serious eye irritant, with no sensitization code above threshold [4]. It is not among the fragrance allergens that require labelling in Europe. The concern that has emerged is systemic and oral: the 90-day and reproductive studies above put the no-effect level at 146 milligrams per kilogram per day and found azoospermia and thyroid suppression at two to three times that [9][10].

Industries and uses

Fragrance: alpha-terpineol is a foundation lilac and pine material in floral, fougère, and household accords and one of the highest-volume fragrance alcohols [1][2][3]. Culinary: it holds FDA flavoring status and contributes citrus, lime, and soapy-floral nuances; it is a natural constituent of cardamom, marjoram, and citrus flavor [1][11]. Aromatherapy: tea tree, cajeput, and pine oils are its carriers; the irritation classification and the oral toxicology are the relevant cautions [4][10][12]. Cleaning: pine oil disinfectants are built on it, and its irritant classification is the reason those products carry eye and skin warnings [4].