HerbalEGram: Volume 7, Number 8, August 2010

Controversy Over Recent Mint Research

Recently published research on peppermint (Mentha x piperita), Japanese cornmint (Mentha canadensis), and spearmint (Mentha spicata) suggests possible new production locations for these economically important crops.^1,2,3 Some experts in the herb industry, however, doubt the validity of the research methods and conclusions.

Peppermint, spearmint, and Japanese cornmint, and their essential oils, are valuable products used in the flavoring, food, fragrance, and pharmaceutical industries. Peppermint and Japanese cornmint are the only species whose oil contains usable quantities of menthol, with Japanese cornmint having a higher concentration. Because menthol crystallization from peppermint oil is an expensive and difficult process, cornmint is the most economically feasible option for menthol crystallization production. But Japanese cornmint has never been successfully grown commercially in the United States, despite its essential oil being a major import item and that some reports state that products imported from China and India are of an inconsistent supply and quality.³ 

In the January­ and February 2010 issues of Agronomy Journal, researchers discuss their attempts to successfully grow peppermint, spearmint, and Japanese cornmint in two Northern Mississippi locations. They state that this was the first attempt at investigating commercial peppermint and spearmint cultivation in the Southeastern United States.

Current thinking generally considers commercial production of peppermint to be unsuccessful in locations below the 40^th parallel of latitude (the Prime Meridian).¹ Long days and shorter nights, common in the Northwestern United States during the summer, enable the peppermint plant to form flowers and encourage the synthesis and accumulation of monoterpenes, which are secondary metabolites that largely comprise the plant’s essential oil, aroma, and some of its medicinal activities. Some reports state that at least 15 hours of daylight are needed to produce the highest oil yields.⁴ Because the summer sun rarely shines so long in locations below the 40^th parallel, large-scale cultivation of peppermint has taken place only in the Northwestern United States for all of its modern history. Additionally, spearmint production has always been considered ideal in the Northwestern and Midwestern United States,³ and Japanese cornmint is grown commercially only in India, China, Vietnam, Brazil, and some parts of Eastern Europe and Asia.²

In the cropping seasons of 2007 and 2008, researchers studied the effects that growth stage, fertilization, and harvest time or cut had on the yields, oil content, and composition of these three mints. They planted the mints in raised beds on plots about 6 meters long in a field at the North Mississippi Research and Extension Center in Verona, MS, and also planted and studied spearmint in a field at the Delta Research and Extension Center in Stoneville, MS. They used drip irrigation to supply moisture and the plants were harvested by hand at bud formation and at flowering and then dried. Their essential oils were extracted using steam distillation, then weighed and chemically analyzed using gas chromatography-mass spectrometry methods.

The researchers reported that the peppermint grown in Mississippi did flower and actually produced similar or higher average oil yields at bud formation and flowering than the US average peppermint essential oil yields in 2008. Additionally, the Scotch and Native spearmints had similar essential oil concentrations and higher productivity yields as those grown in other US locations and regions around the world, they wrote. The menthol concentrations of Japanese cornmint grown in Mississippi were similar to or more than reports of menthol concentrations in Japanese cornmint grown in India and Bulgaria, as well as trials in Indiana and Michigan. The researchers concluded that Mississippi and locations in the region with similar conditions might be a suitable area for commercial production of peppermint, spearmint, and Japanese cornmint.

“Essential oil composition of peppermint, spearmints, and Japanese cornmint grown in Mississippi met the industry standards for the respective oil quality,” said lead researcher Valtcho Jeliazkov, PhD (e-mail, April 28, 2010). “I would say the results were expected because mint has been previously grown south of the 41 parallel, in Europe, Asia, especially in India, and in Africa. By reading these studies I was confident that peppermint would perform well in Mississippi, at least in the northern part of the state where I am located. Although Japanese cornmint is considered a subtropical plant, there was no question in my mind about the suitability of Japanese cornmint for Mississippi.”

Dr. Jeliazkov added that mints grown in Mississippi could be marketed at the international market. “Because the mint essential oil industry is looking for expansion, Mississippi seems to be a good place for the establishment of peppermint, spearmint, and Japanese mint as cash crops,” he said. “Mississippi has fertile soil and inexpensive abundant water in some regions, which are two important factors for successful mint production.

According to Dr. Jeliazkov, some parties, including representatives from essential oil broker companies in the Northwest and individual growers in Mississippi, are already expressing their interest in Mississippi as a commercial production site. “I think prospective growers in Mississippi would need some state support to make that happen; essential oil extraction from mints requires steam distillation equipment, an infrastructure that is available in the Northwest and Midwest, but not in Mississippi. And we need some funding from state and federal funding agencies to address other aspects of mint production, drying and extraction for Mississippi environmental conditions. Hence, we have been submitting project proposals in that respect.”

Not everybody in the mint industry is excited about the new research reports, however. Rocky Lundy, the executive director of the Mint Industry Research Council who was initially involved with the research projects, thinks that the methods and findings are “really off-base” (oral communication, May 20, 2010). According to Lundy, the conditions under which the mints were cultivated, including the small plot sizes, raised beds, and drip irrigation, did not create a “real-world” situation and thus make the experiments less valid.

“Anything is going to look fantastic under these conditions,” said Lundy, noting that mint is a perennial that has to be vegetatively propagated and grown like hay or alfalfa using large sprinkler systems. “I think [Dr. Jeliazkov] overstated what the potential is.” The researchers stated in their Agronomy Journal articles that raised beds are common in the southern United States as they help drain surface moisture after heavy rains.²

Lundy said that he approached Dr. Jeliazkov about 3 or 4 years ago with the idea that Japanese cornmint might grow well in Mississippi and throughout Dr. Jeliazkov’s experiments, he visited several times to check on the progress. During the second harvesting year, Lundy said he found the mint growing much too high and thick, with only a few mint leaves growing on the top of the plants. According to Lundy, the mints in Mississippi were not going dormant, perhaps due to climate or the relatively equal length of daylight in the South, and thus were crowding themselves out. When this happens, the sunlight can only reach the topmost mint leaves, which is where the essential oils are produced.

Lundy said that it might still be possible for these mints to be grown in Mississippi if researchers can overcome the “mint not going dormant situation,” but that may result in an increase in production costs, which could make it economically unattractive. “If mint was to successfully be grown in Mississippi, it probably would have been done so a long time ago.”

Two mint industry sources who wished to remain anonymous due to the controversial nature of the research said they did not find any issues with the research methods or the conclusions (oral communication, May 20, 2010). While recognizing that they have not done research in this specific area nor did they visit the research plots, the sources said that the yield data and other details are not outside theoretical boundaries for mint plants.

Additionally, it is possible to obtain valuable information from cultivation on small plots, and drip irrigation is sometimes used when researching cultivation and doesn’t necessarily obscure the results, they said. The industry sources noted, however, that the researchers need to expand cultivation to larger field plots in order to get information on the commercial viability of mint production in Mississippi.