Stability of Pyrethrins in Drying Methods of Pyrethrum Flowers

Abstract

Pyrethrins are extracted from pyrethrum flowers and are classified into two groups; Pyrethrins I and Pyrethrins II. They are used as a broad spectrum natural insecticide active ingredient in agriculture and public health. Pyrethrins are degradable on exposure to air, moisture and high temperatures. The aim of this research was to investigate the optimum drying temperature, time, light intensity and moisture content of drying pyrethrum flowers for maximum yield of pyrethrins on extraction. Mature pyrethrum flowers from experimental farm, College of Agriculture and Veterinary Sciences, Kabete campus, University of Nairobi were harvested into brown paper bags, divided into sixteen portions and taken to the laboratory. The first six portions of the flowers were dried at varying temperatures of 30, 40, 50, 60, 70 and 80 ºC respectively to total dryness in an oven. The seventh to twelfth portions of flowers were dried in the oven at 30, 40, 50, 60, 70 and 80 ºC for a maximum period of 18 hrs. The thirteenth portion of the flowers were dried in darkness to a constant weight at temperatures below 50oC. The fourteenth portion of the flowers were dried in sunlight to a constant weight at temperatures below 50oC. The fifteenth and sixteenth portions were dried in darkness and in direct sunlight respectively for two weeks. The determination of moisture content of the flowers dried in the oven was carried out at given temperatures at one hour intervals. Hexane was used for solvent extraction after the flowers were ground into fine particles. Analysis of the extracts was carried out using Ultra High Performance Liquid Chromatography and Titrimetric method after refining the extracts. Maximum moisture loss of about 10% was achieved by the flowers at varying times and temperatures. The time recorded for drying at temperatures of 70 and 80 ºC was 18 hours. A percentage yield of 0.84 was obtained when the flowers were dried to constant weight at 30 ºC while drying for 18 hours yielded 0.75. Extractable Pyrethrins II were found to reduce by 8.6% when the drying temperature was raised from 50 to 60 ºC, by 11.3% from 60 to 70 ºC and by 8.5% when temperature was raised from 70 to 80 ºC for flowers dried for 18hrs. Extractable Pyrethrins I were found to reduce by 2.5% when the temperature was raised from 50 to 60 ºC, by 5.2% from 60 to 70 ºC and by 5.5% from 70 to 80 ºC when flowers were dried to constant weight. The percentage reduction of Pyrethrins II when the temperature was raised from 50 to 60 ºC during drying was 8.2%, 12.5% from 60 to 70 ºC and 12.2% from 70 to 80 ºC. Percentage reduction of Pyrethrins I when the drying temperature was raised from 50 to 60 ºC was 6.0%, 5.1% from 60 to 70 ºC and 9.5% from 70 to 80 ºC. Percentage of pyrethrins obtained from flowers dried at 50 ºC for 18 hours was 1.37% and 1.44% when drying was done at the same temperature to constant weight. Total pyrethrins obtained from the flowers dried in darkness was 1.38% and 1.02% when drying was done in direct sunlight. Moisture level for the flowers dried to a constant weight was below 10%. There was a variation in the ratio of Pyrethrins I : II over the temperature range of 30-80oC. The optimum conditions for drying pyrethrum flowers were found to be at a temperature of 50 ºC, a duration of 21 hours in darkness to moisture level of less than 10%.