Effect of nitrogen fertilization on peppermint oil quality and content Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/q237hv47s

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  • 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 professional graders. 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 contents. 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 50-60% menthol. 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 both cases. 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.
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