Drying of Pyrethrum Flowers

Abstract

Pyrethrum is the fourth largest export commodity, for Kenya and one which today earns the country £5 ,000,000 annually. It is grown at high. altitudes in Nyanza, Central and Rift Valley Provinces, picked and dried on the farm, and transported to Nakuru for extraction and export. Prior to World WarI, Dalmatia and Japan were respectively the leading producers of pyrethrum flowers However,over the last thirty years, Kenyahas outstripped the rest of the world in production and today she produces two-thirds of the global crop. Over the years, pyrethrum growers in Kenya have developed a variety of natural and forced convective driers which have presented a number of operational problems: excessive use of heat on flowers, contamination of flowers, fires, improper distribution of air, and drier maintenance. For a start, a survey of different types of driers used in more developed countraes is done in an attempt to identify those which have potential for use in drying pyrethrum. It is concluded that only the deep-bed, spouted-bed, fluidised-bed and the solar drier are worthy of further investigations . Literature survey on drying and thennal degradation of pyrethrins is done in order to gain deeper insight into past research activities in these two fie Ids • This survey has formed the basis for formulating theoretical models of drying and pyrethrins loss and subsequent results compared with experimental data. Experimental work was carried out on the two related aspects of drying, namely, moisture removal and thermal degradation of pyrethrins Prior to conducting major experiments on moisture renoval, it was found necessary to carry out preliminary work on the establishment of methods of determination of moisture content of pyrethrum flowers in general, and equilibrium moisture content in particular. Results obtained from moisture removal experiments show that the rate of drying is heavily dependent on air temperature and to a lesser extent on initial flower moisture content, air mass velocity and the type of clone being dried. Three distinct but equivalent methods f drying pyrethrins control samples were established. Experimental work on pyrethrum Clones 4331, M/22/246, Ma/65/99 and 1708 showed that pyrethrins loss increases with air temperature and drying time. Furthernore, it was shown that when subjected to the same drying conditions, all clones tested exhibit the same relative loss of pyrethrins. However,the absolute loss in terms of quantities of pyrethrins lost is especially high for high yield clones such as Ma/65/99. It was also shown that the rate of thermal degradation of Pyrethrin is the same as that for Pyrethrin II, for all clones tested. Analysis of test data on pyrethrins loss showed that it was a first-order reaction, implying that the mechanism of this loss is diffusion and evaporation rather than by some complex chemical reaction. Laws governing the loss of pyrethrins were developed and a drying air temperature of 800Cis recommended on grounds of 10\'1 loss of pyrethrins and reasonable drying time. On moisture removal, a model which treats the pyrethrum flower as a sphere through which moisture diffuses radially outwards followed by surface evaporation is presented. Good agreement is obtained between the resulting theoretical equations and experimental data. Design and development work on large-scale pyrethrum driers is be the natural follow-up of the present work, bearing in mind the following specifications: low pyrethrins losses; drying tine of 6 hours; initial drier cost of Sh. 8,000/=; drier life-time of 10 years; drier capacity of 1,000 kg of wet flowers; no electricity and a drying cost of 10 cents per kilogram of dried flowers