Menthol

Menthol is the molecule that gives peppermint oil its cooling sensation. The peppermint (Mentha × piperita) oil standard requires a menthol content between 32 and 49% [1]. The cooling effect is not actual cooling. Menthol stimulates cold-sensing nerve endings. In 18 participants who inhaled it, nasal mucosa temperature did not change [2]. When applied to the skin, it produced a cooling sensation but did not change the skin's threshold for feeling cold or warm [3]. Peppermint aroma is not menthol alone. Menthofuran contributes significantly to the aroma of the oil [4]. Menthone and menthyl acetate are the other major components of the oil [1]. Menthol is used in a broad product range from confectionery to toothpaste and cosmetics [5]. Food manufacturers purchase peppermint oil as a flavouring, and cosmetic manufacturers purchase it as a fragrance material [6].

Where is it in peppermint, and how much is present?

The oil is located in the glandular trichomes of the leaf. Essential oil accumulates in small glands on the peppermint leaf surface [7]. Dried leaves yielded 2.6% oil in Kırşehir and 3.2% in Konya. Fresh leaves yielded 0.8 and 2.9% oil [8], [9]. In the oil of dried peppermint, menthol and cineole (1,8-cineole, the eucalyptus-scented component) were higher than in fresh plants [10]. Drying temperature drives off the oil. Slow drying in warm air caused a 2.3% oil loss. Fast drying in hot air caused a 22% oil loss. With hot drying, the menthol share decreased from 47.6 to 43.3% compared to fresh leaves [11].

Maturity converts menthone to menthol. The plant synthesises menthol sequentially: first limonene, then pulegone and menthone, and finally menthol. In mature leaf oil, menthol was measured at 47%, menthone at 24%, and limonene at 1%. In young leaves, limonene reached approximately 3% [12]. Mature leaves convert menthone into menthol and menthyl acetate [13]. Young plants have high menthone and isomenthone contents. As the plant matures, menthol, neomenthol, and menthyl acetate increase [14], [15]. In Norway, peppermint was harvested at early, full, and late bloom over three years. The best balance occurred at full bloom: 43 to 54% menthol and 12 to 30% menthone [16]. Menthofuran and pulegone increase with flowering [14]. Menthofuran is produced in flower buds [4]. In water-stressed greenhouse plants, menthone decreased and menthofuran increased [15].

Variety and climate can reverse the ratio. Five peppermint varieties were cultivated in Isparta and Salihli, Turkey, for two years. In warm climates, peppermint varieties produced menthone-dominant oil instead of menthol. Menthone reached 57.7% in one variety [17]. In Konya, dried leaf oil contained 50.8% menthone and 34.6% menthol [9]. In Kırşehir, the order was reversed: 46.0% menthol and 19.0% menthone [8]. In India, a menthofuran-rich peppermint variety yielded 32.9 to 42.3% menthofuran, 22.3 to 37.7% pulegone, and only 9.8 to 21.0% menthol [18]. The ratio is determined by analysis, not plant name.

Cornmint is a different plant. Cornmint (Mentha arvensis) is much richer in menthol. In the US, two varieties yielded 61.7 and 71.9% menthol [19]. This oil is the commercial source of crystal menthol [20]. The two oils are separate substances in the standard [1].

Menthol, menthone, menthofuran: what is the difference?

Values are taken from PubChem. XLogP shows whether a substance prefers oil or water. The higher the value, the more it prefers oil. With an XLogP of 3.0, menthol is on the oil side, poorly soluble in water and volatile [21]. It is crystalline at room temperature and melts between 42.4 and 43.1 °C [22].

Cooling comes from a single menthol isomer. Menthol has several isomeric forms with identical atoms and different configurations. Only one form provides cooling. Peppermint leaves convert menthone into l-menthol [13]. Forty participants were exposed to l-menthol, d-isomenthol, and d-neomenthol. All three smell like mint, but only l-menthol increased the nasal sensation of airflow [23]. The mouth behaves similarly. Eleven trained and 15 consumer panellists evaluated menthol solutions at 0.2 and 0.4 g/L. They found l-menthol cooler, more nose-clearing and more bitter than d-menthol. At 0.1 g/L, the difference was indistinguishable [24].

Menthofuran and cineole indicate the mint species. Menthofuran contributes significantly to peppermint oil aroma and is found at 5 to 12% in Indian peppermint varieties [4]. The standard peppermint oil specification requires 1 to 8% menthofuran and 3 to 8% cineole. Menthofuran is not specified for cornmint oil and was not detected in it, and cineole is below 2% [1], [25].

Carvone belongs to a different mint. Spearmint (Mentha spicata) contains carvone rather than menthol. Carvone was measured at 51.7% in one oil sample [26]. The word "mint" covers both plants. The analytical certificate separates them.

What is it used for?

Food: peppermint aroma and cooling. In food, menthol does more than cool: it makes sweetness stand out and keeps the aroma in the mouth longer. Too much of it masks the taste with bitterness and cooling. Both effects have been measured in tasting studies. In a sensory panel, menthol reduced the recognition threshold (the lowest amount at which sweetness is noticed) for corn syrup sweetness from 5.98 to 5.02 g/L. It enhanced and prolonged sweetness perception. Menthol at 4 to 30 mg/L increased sweetness, whereas 60 mg/L suppressed it with bitterness and cooling [27]. In twenty subjects, when menthol was added to a water-based flavouring, the amount of limonene released in the mouth rose to 225% of the control and citral (a lemon-scented flavour material) to 125%. Menthol did not make the aroma more volatile. It held it longer in the mouth [28]. Both studies were conducted with pure menthol. In peppermint oil, identical menthol amounts occur alongside menthone and menthofuran.

Oral care and cosmetics: cooling fragrance material. In creams and oral care products, peppermint oil gives a fresh scent and a lasting cooling feel. It has also been tested in the lab against dental plaque. Peppermint oil containing 42% menthol and 26% menthone reduced cariogenic (cavity-causing) bacterial biofilms (the layer bacteria build on the tooth surface) on teeth and inhibited new biofilm formation [29]. In cosmetics, peppermint oil has been used up to 3% in rinse-off products (such as shampoo and shower gel) and 0.2% in leave-on products (such as creams and lotions) [6]. Menthol applied to the skin produced cooling for up to 70 minutes in 12 of 18 subjects [3].

Room fragrance and aromatherapy: inhalation for alertness and nausea. Peppermint scent is used in room fragrance and aromatherapy for alertness and against nausea. In one trial it helped memory and attention, and it also reduced nausea after surgery. In a trial, 144 volunteers performed attention and memory tests in peppermint, ylang-ylang (a tropical flower oil), or odourless environments. Peppermint aroma improved memory performance and increased alertness [30]. For postoperative (after surgery) nausea, two systematic reviews (reviews pooling many studies) show differing conclusions. A review pooling 19 studies reported that inhaling peppermint oil reduced postoperative nausea by 0.6 points within 2 to 6 hours. Chemotherapy-induced nausea decreased by 2.2 points at 48 hours [31]. An older review of 4 studies and 115 patients found no difference between peppermint and placebo (an inactive comparison product) at five minutes, rating evidence quality (the reliability of the findings) as low [32].

30% or 50%: which form?

Essential oils can also be supplied diluted in an edible carrier oil. The certificate must state whether the percentage applies to pure oil or the diluted product.

Why peppermint oil instead of an isolated molecule? For food manufacturers, the reason is aromatic character. Menthofuran contributes significantly to the aroma of the oil [4]. Menthol also prolongs the retention of accompanying flavourings in the mouth: limonene and citral release increased with menthol [28]. For cosmetic manufacturers, peppermint oil is a fragrance material labelled under its own name [34], enriched by these secondary components.

The choice depends on four questions.

• Cooling or peppermint character? For cooling, the line to read is menthol. Peppermint character is established by menthofuran and cineole [4]. Four lines on the certificate decide: menthol, menthone, menthofuran, and cineole.
• Oil or aqueous product? Menthol has low water solubility [21]. Aqueous products require an emulsion or a carrier. Peppermint oil nanoemulsions were prepared and monitored for 90 days [35].
• How much is required? For a target of 0.1% menthol in 1 tonne of product (a level favoured in a sports gel study [36]), 1 kg of menthol is required. If the oil contains 43% menthol [16], 2.3 kg of peppermint oil is needed. These figures are illustrative.
• Price. Menthol content in published peppermint oils ranges from 0.8 to 53.3%. Authors attribute the low values to carvone-dominant chemotypes and analytical quality [1]. The comparison is cost per kilogram of menthol, not per kilogram of oil. If the buyer pays for peppermint character, the comparison includes the menthone and menthofuran lines.

How to read the certificate of analysis?

Method. Peppermint oil is analysed by gas chromatography (GC), which separates volatile components and determines relative proportions. Percentages usually represent relative peak areas rather than absolute quantitative amounts [8]. The standard defines lower and upper limits for twelve components. None of the 26 published peppermint oil profiles complied with all twelve ranges. Authors note that the standard reflects selected commercial oils [1]. The lines are read together, not one by one.

Lines that indicate the mint species. If cineole is below 3% and menthofuran is absent, the oil may be cornmint or partly dementholised cornmint oil [25], [1]. The standard uses three lines to distinguish the two species: isomenthone, cineole, and 3-octanol. Isomenthone is 2 to 8% in peppermint oil and 6 to 14% in cornmint oil [1].

How to detect adulteration. Fifty commercial peppermint oils were screened by NMR, a molecular fingerprint method, and 42% were adulterated. Adulterants included synthetic compounds, dementholised cornmint oil, and low-cost oils [37]. Infrared spectroscopy distinguishes spearmint oil and added menthol [38]. Origin tests show whether the menthol came from the plant or from synthesis [39].

Pulegone and menthofuran lines relate to regulations. In Oregon, 375 peppermint oils were analysed across three years. Pulegone varied with harvest date, showing the lowest levels in late-season first-year crops. In commercial oils, pulegone varied by geographic origin [40]. In commercial peppermint teas and essential oils, pulegone ranged from 0.02 to 2.6% and menthofuran from 0.02 to 6.6%. All fell within pharmacopoeial limits [41]. These two lines are read together with the food limits.

Specification and result are distinct. A specification is the manufacturer's commitment. The measured value is the lot result. The technical data sheet (TDS) is a general document, whereas the certificate of analysis (CoA) is specific to the lot.

What defines quality?

Maturity markers. Menthone and isomenthone indicate immature plants. Menthol, neomenthol, and menthyl acetate indicate maturity [14]. If menthone exceeds menthol, the crop was harvested early [14] or grown in high temperatures [17].

Carvone line indicates spearmint. The presence of carvone in peppermint oil indicates spearmint blending or a carvone-dominant chemotype [1]. Infrared spectroscopy detects this admixture [38].

Limonene line indicates young plants or adulteration. The standard expects limonene between 1 to 3% [1]. Levels near the upper limit may come from young plants [12]. Levels well above the range raise suspicion of adulteration. Limonene exceeded all standard ranges in one published commercial oil [1].

Solvent residue and raw material testing. Steam distillation and supercritical CO₂ extraction use no organic solvents. A solvent residue specification applies only to solvent-extracted products. Raw botanical materials are also tested for pesticides and heavy metals.

How does it behave in formulations?

Higher menthol does not mean higher cooling. Topical gels with 0.5, 4.6, and 10.0% menthol were applied to thighs in ten subjects. All three gels reduced skin temperature for at least one hour, independent of concentration. The 4.6% gel provided the strongest perceived cooling, outperforming the 10.0% gel [42]. The upper limit has also been measured in a food. In 40 endurance athletes, a 0.1% menthol gel provided cooling and achieved equal acceptability to placebo. A 0.5% gel was perceived as cooler but scored lower overall [36]. An oral 0.3% menthol solution administered once per minute for ten minutes showed decreasing sensory irritation over time while cooling persisted [43]. In eleven trained panellists, perceived bitterness increased with dose [44]. These studies used pure menthol.

Heat and storage stability. Products containing menthol lose aroma during storage because menthol is highly volatile [45]. In spray drying, beta-cyclodextrin retained menthol most effectively among three tested cyclodextrins [46]. Starch-based microcapsules preserved menthol for 300 days and improved thermal stability [47]. When creams containing free menthol were stored at 37 °C for 28 days, cooling intensity dropped. Encapsulated menthol maintained cooling performance [48]. In peppermint oil, menthofuran declined during room temperature storage, whereas cineole remained stable [25]. Store in tightly closed, full containers in a cool, dark place.

Skin: penetration enhancement. In an ex vivo skin model, a gel containing 2% menthol doubled the transdermal penetration of an analgesic drug [49]. The cosmetic safety panel warns that menthol can also carry through the skin an ingredient that is meant to stay on it. The same panel limits pulegone in peppermint oil to 1% [6]. Topically applied menthol increased transepidermal water loss [3]. These studies used pure menthol.

Cosmetic labelling: menthol and peppermint oil listed. Menthol was added to the EU list of declarable fragrance allergens in 2023. Peppermint oil (Mentha piperita oil) and spearmint oil were also added. Limonene, which comes with the oil, was already listed. Carvone and beta-caryophyllene were added in 2023. Menthone, menthofuran, pulegone, and menthyl acetate are not listed. Allergens exceeding 0.001% in leave-on products or 0.01% in rinse-off products must be listed on the label. Products placed on the market after 31 July 2026 must comply. Existing stocks may be sold until 31 July 2028 [34].

Pulegone and menthofuran limits in food. The EU flavouring regulation prohibits adding pulegone and menthofuran to food as such and set maximum limits for mint-containing foods. Pulegone cannot exceed 250 mg/kg in mint confectionery, 350 mg/kg in chewing gum, 20 mg/kg in mint non-alcoholic beverages, and 100 mg/kg in alcoholic beverages. Menthofuran is limited to 500 mg/kg in confectionery, 1,000 mg/kg in chewing gum, and 200 mg/kg in alcoholic beverages. Limits apply to ready-to-consume foods [50]. For example, the sweetness-enhancing menthol dose of 4 to 30 mg/L [27] equals 10 to 70 mg/L of peppermint oil containing 43% menthol. Even if the oil contains the maximum standard limit of 3% pulegone [1], the beverage will contain 0.3 to 2.1 mg/kg pulegone, well below the 20 mg/kg limit. Adding 1 g/L (0.1%) of the same oil yields 30 mg/kg, exceeding the limit. In chewing gum, 0.2% oil with 8% menthofuran yields 160 mg/kg, below the 1,000 mg/kg limit. These figures are illustrative and must be calculated using lot-specific CoA values.

It passes through the body quickly. Peppermint oil is rapidly absorbed after oral intake. Menthol is metabolised in the liver and excreted via bile and urine [51].

Safety documentation. Follow the safety data sheet (SDS) when handling extracts. Transport and storage classifications are specified in the SDS.