Contact: Rodney B. Croteau, 509-335-1790; 335-0586
Cool, Soothing, Lucrative Mint: 30 Years of Research
If you drive through Central Washington's mint-growing country
in mid-summer, you're likely to be overwhelmed by the scent of mint
rising like an exhalation-at once delightful and inescapable-from
the surrounding fields. In fact, your senses might deceive you into
believing that not much has changed in the last 30 years or so. But
during that time Rod Croteau, professor at the Institute for
Biological Chemistry at Washington State University, has been doing
research that has helped make Washington mint plants produce more
and better peppermint.
Peppermint plants produce menthol, which is a terpene, as are all
the other compounds Croteau researches. Terpenes are chemicals put
together in specific ways from units containing five carbon atoms.
Menthol is a small terpene, with just 10 carbons-small enough to
evaporate, which is why we can smell it, says Croteau. Rubber is a
terpene, a huge one with hundreds of thousands of carbons. Taxol,
used in the treatment of breast cancer, is a terpene, as are some
of the resin compounds made by trees to protect themselves from
bark beetles.
"Nature uses only so many tools," says Croteau, so what he and his
fellow researchers have learned through the work with mint has been
applicable to work on these and other terpenes.
Peppermint flavoring is made from oil produced by the peppermint
plant, oil that is a mixture of at least 25 different components.
The primary ingredient is always menthol, however. "It's what gives
you that cooling sensation," says Croteau. The oil mix produced by
a given plant varies with location and growing conditions. Yakima
oil is easy to tell from Flathead Valley oil or oil from other
countries. American peppermint oil is a high-end, quality product,
says Croteau.
The taste of a given peppermint product depends on the mix of oils
used to flavor it, a mix that usually results from a blending of
oils from plants of different regions.
Croteau began his work with peppermint in 1976, when he received
the start of what has become continuous funding from the Washington
Mint Commission. The Commission originally was looking for ways to
improve the State's production of mint. The costs were high, there
was competition from less expensive foreign oil, and synthetic oils
could be made from petrochemicals. Croteau chose to concentrate on
increasing yields and improving the composition of the oils.
At the start, the research involved classic biochemistry and the
manipulation of agronomic practices. "We had little idea of
practical molecular techniques then, for they didn't exist," says
Croteau. Nor was it possible to use breeding programs or mutants to
improve production. The mint plant is a sterile hybrid created
hundreds of years ago in nature.
The biochemical research was aimed at determining the pathway the
plant uses to make menthol. The experiments were tedious and time
consuming-it took the laboratory a decade to elucidate the nine
steps in the main pathway the plant uses to produce menthol.
Understanding that pathway was important in that it helped the
researchers understand the effects of environmental and agronomic
conditions on oil production and indicated agronomic manipulations
that might alter yield or composition.
The lab studied those manipulations primarily in the greenhouse,
where conditions could be standardized, and determined that both
water use and irrigation method were important. Most irrigation of
mint had been done by furrow irrigation but is now done mostly by
overhead systems, a more economical method that unfortunately
reduces yield. The plant's oil glands, which are where peppermint
oil is made, are on the leaf surfaces, and overhead sprinkling
disturbs them. In addition, the lab found that moderate, well-timed
water stress increased the yield of oil with good composition.
Croteau's group also looked at harvest timing, traditionally based
on word of mouth, and found that harvesting when 10 percent of the
plants are in flower maximizes the yield of peppermint oils with
good composition.
In the early 1990s, the lab developed a method for isolating oil
glands from the leaves, providing a highly enriched source of
material not only for biochemical studies but also for what would
follow, studying the genes responsible for making the proteins
active in the pathway that produces menthol.
The lab's approach to genetic manipulation was different than most,
however. To assuage the public's fear of moving genes from one
organism to another, the lab used only mint genes to improve
mint.
Once the genes active in the oil glands were isolated, they were
compared to already characterized genes. Several genes were further
studied and manipulated, and two were chosen to move into mint
plants. One improves yield and was overexpressed so that the plant
would make more than a normal amount of the protein it coded for.
The other reduced undesirable oil components and was knocked out so
that the plant would make less than normal amounts of these
components. The two plants that resulted showed roughly a 50
percent higher yield and 50 percent fewer undesirable oil
components, respectively.
Both of these genes have been incorporated into a plant that is
the focus of much of the lab's current work. This plant will be
used as parental stock, and a variety of other genes will be moved
into individual parental stock plants. The goals for this work are
to further increase the yield, up to double that of the parent
plant, and to improve the quality of the peppermint oil produced.
Differences that, again, you won't see or smell, but that will make
Washington mint growers even more competitive.