|Abstract or Summary
- Two field experiments were established during the spring of
1970 in an effort to characterize the effects of nitrogen fertilizer and
plant maturity on the yield, composition and quality of peppermint
(Mentha piperitta var. Mitcham) oil. These experiments were located
in central Oregon, near Madras, and in the Willamette Valley, near
Corvallis, thus representing the two major peppermint producing
regions in the state of Oregon. The objectives of these experiments
were (1) To evaluate the effects of various nitrogen treatments on oil
yield when harvested at different stages of plant maturity and (2) To
determine how the various terpene compounds, which characterize oil
quality were affected by nitrogen fertilizer treatments and crop
maturity. The experiments were maintained over a period of two
years, 1970 and 1971. Both the Corvallis and Madras experiments were randomized
block designs having five replications. The nitrogen fertilizer treatments
applied to the plots varied in both rate and time of application.
Plant maturity treatments consisted of sampling plots at various
degrees of inflorescence. These samples were evaluated for oil
yield and composition.
Oil yield determinations were made using steam distillation and
water separation techniques similar to commercial mint oil distilleries.
Measurement of terpene hydrocarbons, menthyl acetate, menthone,
menthol and menthofuran was accomplished using liquid gas
chromatography. The samples of oil were also evaluated for odor by
Generally, higher oil yields were obtained with heavier nitrogen
rates. For Willamette Valley peppermint, rates of 200 to 250 pounds
of nitrogen per acre appeared optimum. Slightly higher rates of 250
to 300 pounds of nitrogen per acre seem optimum for high oil yields
in the central Oregon production areas.
The level of terpene hydrocarbons, the light boiling fractions of
mint oil constituents, generally decreased with crop maturation.
Nitrogen treatments had little effect on terpene hydrocarbon contents.
Acceptable levels of terpene hydrocarbons (9.5% to 13.5%) were
obtained for samples taken at and beyond 50 percent bloom regardless
of nitrogen treatments. Menthone contents of 20% to 30% are considered normal for
Oregon peppermint oils. Samples taken during 50% bloom appeared to
best satisfy this criterion. Heavier rates of nitrogen fertilization (200
to 300 pounds of nitrogen per acre) resulted in increased menthone
Menthol contents decreased with heavier nitrogen fertilization
and increased with crop maturity. Results indicate that 200-250
pounds of nitrogen per acre is optimum for peppermint that is harvested
at 50 percent bloom and beyond; producing oil having approximately
Generally, menthyl acetate levels increased with crop maturity.
The 5.0 to 7.5% menthyl acetate content considered desirable for
Oregon peppermint oil may be achieved by harvests timed at 50%
bloom in the Willamette Valley and just prior to 50% bloom in central
Oregon with 250 to 300 pounds of nitrogen per acre being applied in
Menthofuran contents increased with crop maturity and
inflorescence. Plots harvested at 50 percent and full bloom at
Corvallis showed decreases in menthofuran contents with increasing
nitrogen rates. Menthofuran contents of less than 3% are desired for
Oregon peppermint oil. Results indicate that harvests should be made
prior to 50% bloom in order to avoid excessive menthofuran contents.
Heavy nitrogen rates (200 lb N/A or greater) may also aid in
maintaining low menthofuran contents.