A study of the influence of the Parchment Hull in the thin-layer drying of Parchment coffee

Abstract

This research was conducted at the University of Nairobi with the broad objective being to determine the influence of the parchment hull on the thinlayer drying of parchment coffee. Parchment coffee consists of a soft tissue bean enclosed in a woody husk (the parchment hull). Drying of parchment coffee is very important in maintaining the intrinsic high cup quality. The convectional drying process is sun drying, a process that consumes large ground space, is labour intensive, takes long and is vulnerable to bad weather. However, research has shown that the parchment coffee can be partly dried using mechanical dryers without significant loss of quality. In the development of mechanical dryers, the role of the thin-layer equation for accurate prediction of the drying process is pivotal, yet none existed for parchment coffee. A laboratory dryer with air conditioning system was designed and fabricated for thin-layer drying studies. The dryer was tested and found to provide constant drying air conditions for temperature ranges between 31 and 66°C Data were collected for five thin-layer drying conditions for the parchment and the green coffee beans respectively. The drying conditions were composed of five dry bulb temperatures viz. 31°C, 38°C, 46°C, 50°C and 66°C and one dew point temperature viz. 14°C. A mathematical model based on the Fick's second law of diffusion was developed to describe thin-layer drying of parchment coffee. Further two mathematical models were selected from literature. The models selected were the diffusion model and the Page equation. The proposed model and those selected from literature were evaluated to fit the data. The results of the drying experiments indicated that the time taken to achieve any given moisture ratio was shorter for the green coffee beans than for the parchment coffee beans, hence the parchment hull retards the drying rate of parchment coffee beans. The Page equation was found most suitable for describing thin-layer drying of parchment coffee beans for the drying air temperature range of 31 to 66°C, relative humidity of 3 to 31% and air velocity of 0.6 m/s. UoN·