Change of pyrethrin content during drying of pyrethrum flowers
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Pyrethrum is one of the major cash crops in Kenya. To extract the insecticidal constituents found in the pyrethrum flower, the flower has to be dried. High temperature drying may result in pyrethrin loss . .This work looks at pyrethrin loss trends as a function of temperature with the aim of generating information that may be used to reduce the loss. The flowers were dried from an initial moisture content of 80 to 84% (wet basis) at temperatures of 70 to 175°C. Drying was done in a thermostatically controlled forced air oven while the pyrethrin content for various lengths of drying was measured using photo-spectrometric method. The rate of moisture removal and change in pyrethrin content was determined. It was observed that like any other crop pyrethrum loses moisture exponentially at a constant drying temperature. However I a unique pattern of pyrethrin loss with time during drying was observed. There exists a period during which there is no loss of pyrethrin. The results show that a Newtonian equation can be used to depict the moisture loss. Two expressions for predicting pyrethrin loss were developed and both show good agreement with experimental Data. An equation for predicting when the no-loss duration end after the beginning of drying for the temperature range of 70 to 175°C is also given. The equation which depicts the no-loss duration and the expression for moisture removal can be used to set time-temperature schedules which will ensure minimum loss of pyrethrin during drying.