Isopulegol

The terpenes (−)-isopulegol (≥99%), menthol (≥99%), p-cymene (≥99%), eucalyptol (≥99%), (R)-(+)-pulegone (97%), γ-terpinene (97%), α-terpinene (≥95%), linalool (≥97%), (S)-(+)-carvone (≥96%), citronellal (≥95%), (−)-terpinene-4-ol (≥95%), citral (≥95%), menthone (≥90%), farnesene (mixtures of isomers, ≥90%), α-phellandrene (≥90%), β-myrcene (≥90%), and TPTZ (2,4,6-Tri(2-pyridyl)-s-triazine)) were obtained from Sigma-Aldrich (St. Louis, MO, USA). Acetic acid (ACS), FeCl₃ × 6H₂O, FeSO₄ × 7H₂O, phosphoric acid (ACS), hydrochloric acid (ACS), chloroform (ACS), ethyl acetate (ACS), 2-butanone (ACS), and methanol (analytic purity grade) were obtained from Polish Reagents (Gliwice, Poland). The standards trolox ((±)-6-hydroxy-2,5,7,8-tetramethylchromane-2-carboxylic acid, 97%) and gallic acid (>98%) were purchased from Sigma-Aldrich (St. Louis, MO, USA).

Bornyl acetate (99%), camphene (95%), camphor (96%), carvarol (98%), (−)-carveol (95%), (+)-carvone (96%), β-caryophyllene (98.5%), caryophyllene oxide (95%), β-citronellal (95%), citral (95%), 1,8-cineole (99%), p-cymene (99%), decyl chloroformate (97%), dodecanoic acid (98%), β-farnesene (90%), geranyl acetate (97%), geraniol (98%), isopulegol (98%), linalool (97%), limonene (97%), limonene oxide (97%), methyl linolenate (99%), myrcene (90%), myristic acid (99%), palmitic acid (99%), α-phellandrene (85%), α-pinene (98%), pivalic acid (99%), sabinene (75%), α-terpineol (90%), γ-terpinene (97%), 4-terpineol (95%), terpinolene (90%), and tymol (99%) were obtained from Sigma-Aldrich (St. Louis, MO, USA). Achillea millefolium L. flowers, Citrus aurantium L. fruits, Leptospermum petersonii F. M. Bailey leaves, Ruta graveolens L. leaves, and Thymus vulgaris L. leaves were collected from a local store in Chonju, Korea. Sample specimens were authenticated by Jeongmoon Kim at Chonbuk National University, Korea. Essential oils of the five plants were obtained by steam distillation extraction, and finally dried over Na₂SO₄ to extract the pure essential oils (Table 1).

Eight high-purity synthetic odorants (four minty: isopulegol, L-carvone, methyl salicylate, eucalyptol; four citrusy: citral, (R)-(+)-limonene, citronellyl acetate, nonanal; Fig. 1a) were obtained from Sigma Aldrich (St Louis, MO) and delivered directly to subjects' noses using a custom-built olfactometer. This system is capable of mixing odourized air, diverted via separate channels through the gaseous headspace of 20-ml amber bottles containing 1 ml of undiluted liquid odorant, with odourless air diverted through an empty amber bottle. To equalize perceived odour intensity as best as possible across the stimulus set, before all testing sessions we adjusted the ratio of odourized to odourless air individually for each odorant while maintaining a constant total flow rate of 3.2 l min⁻¹ (see Supplementary Table 1 for the list of flow ratios (odourized to odourless air) used for each odorant). For all testing sessions odours were delivered using the olfactometer, with the exception of pairwise odour similarity rating sessions (see below), during which subjects sniffed as prompted from labelled amber bottles containing odorants diluted with diethyl phthalate at low concentrations (6–20%).