The ancient Greeks have a legend about mint. Mint, or Minthe, was once a beautiful nymph from the underworld river of Cocytus. She was said to be of nobler form and more beautiful than Persephone, queen of the underworld herself. Hades, the god of the underworld and husband to Persephone, became infatuated by the young river maid after she made an attempt to seduce him. The wife of Hades was enraged by the nymph, and intervened by trampling the girl under her heel into nothing more than dust. Sorrowful for the loss of the young girl, Hades brought her back to life with his power as a fragrant mint plant.
The association between mint and the underworld came about from ancient burial traditions that used mint to cover up the smell of the dead. The aromatic leaves have also been used historically to mask the odors of households and alleyways due to inadequate sanitation. On top of that, mint was greatly admired for its ability to freshen the breath and clear body odors in a time when bathing wasn’t widely practiced. The mint plant was so highly regarded for its power to cleanse that mint was commonly used as a form of currency in Egypt during Biblical times.
Modern Mint Production
Mint plants are incredibly fast-growing herbaceous perennials, and are actually known to be rather invasive plants, which makes commercial cultivation relatively easy. Commercial production of mint began in England during the 1750s. Mint was quickly transplanted to New York after the revolution, and was readily grown in the United States. The cooling ability of mint leaves were especially important for Southern cuisine because of the high heat and humidity of the Southern states. Four varieties of mint are commonly cultivated today: peppermint, native spearmint, Scotch spearmint, and cornmint. A more recent variety, apple mint, has been introduced into commercial cultivation in Europe for its unique hint of apple flavor. The major producer of fresh mint is the United States, with a total output of 75% of the global supply, with Indiana, Wisconsin, Washington, and Oregon being important mint producing states. While mints grow rapidly in the presence of cool pools of water, production output is easily affected by droughts. Seasonal high heat also reduces the mint oil output of cultivated crops.
Mint Flavor Chemistry
The major chemical constituents of mint oils are menthol and its oxidized relative, menthone, with minor components that impart unique flavors to the different varieties of mint oils. German chemist and physician Hieronymus David Gaubius isolated menthol from mint leaves in 1771 to identify the compound responsible for the cooling effect of mints. Only the enantiomer (-)-menthol is capable of triggering the cooling sensation commonly associated with mint flavor, and mint essential oils are highly prized in the chemical and food industry for their (-)-menthol content. Menthol can uniquely trigger the TRPM8 receptors in skin to induce the cooling experience when applied to the body or taken orally, in a similar mode of action as capsaicin, the compound responsible for the hotness of chili. TRPM8 is an ion channel that allows the passage of sodium and calcium ions, which induces action potentials that lead to cold sensations through low temperatures and application of menthol.
Mint is used mostly in food and nutraceutical applications where there is a desire to impart a sense of cleanliness. For example, mint is widely used in gum, breath fresheners, mouthwash, antacids, and toothpaste. Of course, mint is also incorporated in foods to add that distinct minty fresh flavor as a secondary sensation, especially in chocolates, ice creams, confections, and beverages. Mint is the third most popular flavoring ingredient in the world, behind vanilla and citrus flavors, and continues to be one of the fastest growing flavor segment in the market driven by consumer demand for clean, fresh flavors. Other applications for mint are in cooling balms, essential oils, perfumes, pest control, and antimicrobial agents.
The demand for mint, and especially its active component menthol, currently outstrips the supply from natural crop sources. Global demand for menthol has reached 20,000 metric tons, with only 13 metric tons sourced from plants. Three companies were responsible for discovering and developing processes for the modern production of synthetic menthol. Haarmann & Reimer GmbH, the company that first commercialized synthetic vanilla, first developed and patented a process to synthesize menthol from the petrochemical m-cresol in 1976. In the Symrise process, renamed after a 2003 merger of Haarmann & Reimer with Dragoco, m-cresol (3-methylphenol) is first reacted with 1-propene through a Friedel-Crafts alkylation step using an anhydrous acid catalyst. The thymol product is catalytically hydrogenated to form a mixture of menthol isomers. The mixture is fractionally distilled to collect racemic (-) and (+)-menthol, and the remaining isomeric residues are catalytically epimerized to produce more racemic menthol. A synthetically powerful final step is conducted by esterifying the racemic menthols with benzoic acid, and slowly recrystallizing the mixture with a seed of optically pure (-)-menthol benzoate. The (-)-menthol benzoate crystals are collected and hydrolyzed to give pure (-)-menthol up to an excellent 90% yield. The mother liqueur rich in (+)-menthol benzoate and minor iso-menthol benzoates is hydrolyzed and recycled back into the process. The benefit of the process is that while the undesired (+)-menthol and iso-menthols are formed, they can be continuously reprocessed into (-)-menthol.
In 2001, the Japanese flavor house Takasago International Corporation developed a more sophisticated route to asymmetric (-)-menthol that did not require the meticulous recrystallization step, using a state-of-the-art catalyst discovered by Nobel laureate Ryoji Noyori. At the time, Dr. Noyori was director at Takasago, and together with his team, used the relatively abundant myrcene as the starting material. Myrcene is a pleasant-smelling terpene produced from the pyrolysis of β-pinene, a product obtained from pine-derived turpentine. The compound is reacted with a lithium amide to form an addition product, which is then catalytically isomerized with a chiral ruthenium catalyst to form an optically active enamine. The intermediate is hydrolyzed under acidic conditions to give a single enantiomeric isomer of the aldehyde, citronellal, which is then cyclized using a Lewis-acid catalyst such as zinc bromide. The resulting olefin, isopulegol, is further hydrogenated under a nickel metal catalyst to give enantomerically pure (-)-menthol (94% e.e.) in the desired stereochemical configuration without any further purification steps. Since (+)-menthol or iso-menthols are not formed here, there is no need for the lengthy recrystallization step that’s used in the Symrise process.
While both companies would go on to lead in synthetic menthol production, global demand for the product continued to rise well into the 21st century. European-based BASF, the largest chemical manufacturer in the world, currently has established methods for the production of a series of isomeric terpenes using cheap petrochemical butene as feedstock. Geraniol and nerol, which are structural isomers of one another, are the primary products of these processes. But by diverting the citral intermediates geranial and neral, the company has found a way to create value-added (-)-menthol using a similar process described previously for Takasago. The citral products are first asymmetrically hydrogenated using a proprietary chiral ruthenium catalyst to give enantiomeric (+)-(R)-citronellal. Much like the Takasago process, the intermediate is catalytically cyclized to form (-)-isopulegol, then hydrogenated to form (-)-menthol with a reported purity yield of 99.7%. In 2012, BASF announced the construction of a continuous process plant in Germany to capitalize on this discovery.
Other advances in mint flavor technology have come in the form of developments in synthetic analogues and mimics to (-)-menthol. Wilkinson Sword, a shaving and personal product company, initiated a long-term research program in the 1970s to find compounds that could illicit a more intense and longer cooling sensation than menthol. The need for longer-lasting freshness and intensity in confectionery and chewing gum applications in the past 30 years has led to the discovery of a chemical family known as Wilkinson Sword carboxamides, which boast stronger and longer cooling effects without impacting flavor. Other synthetic derivatives found and developed by other flavor house companies include compounds with the (-)-menthol structure attached to polymeric appendages, long-chain carboxamides, ketals, and carboxylic acid esters, many of which are now labeled by the FDA as GRAS (Generally Recognized As Safe). Such research programs have opened up the range of ingredient possibilities, which has allowed flavor companies to develop products that work synergistically with lower contents of natural mint flavors and synthetic menthol to boost cooling sensations while reducing costs.
A Minty Fresh Outlook
Mint continues to be an important flavor for the food, personal care, and pharmaceutical industries. As consumer demand for mint continues to exceed current production capacity, new methods of production will be needed to meet that demand. And while mint flavor has benefited greatly from the technological advances made by chemistry, the upcoming use of synthetic biology in flavor production has yet to still impact mint flavor synthesis. The future of mint looks fresh, and there will be many opportunities for entrepreneurs and technologists to make their mark on a globally significant flavor.
P.S. For a more thorough, in-depth look into the fascinating chemical technology of advanced mint flavors, read this article by Dr. John Leffingwell.