Dewatering and drying characteristics of Coffee Pulp

Abstract

The major constraint in the utilisation of coffee pulp is its high moisture content. For easy storage, the moisture content of the pulp needs to be lowered to about 15% wet basis. A two stage process comprising mechanical Dewatering followed by drying is effective in removing water from the coffee pulp. However, to design appropriate equipment for dewatering and drying coffee pulp, the dewatering and drying characteristics of the pulp need to be known. For this purpose, studies on dewatering and drying of coffee pulp were undertaken in order to identify and relate the parameters important to these two stages of processing. The dewatering process was investigated by subj ecting coffee pulp samples to consolidated drained (CD) triaxial compression tests. Four levels of constant cell pressures and continuously increasing axial pressure were used to vary the dewatering conditions. Studies into the drying of coffee pulp were undertaken in a ventilated laboratory oven. Drying temperatures were between 75°C to l500C because coffee pulp dries fastest within this range, besides the ease to achieve constant temperature and relative humidity in the oven. Drying experiments were conducted at four levels of initial moisture content obtained by mechanical removal of 0%, 5%, 10% and 20% of the fluid from four separate coffee pulp samples. The results indicate that the behaviour of coffee pulp collected at different times of the picking season is generally very diverse even when subjected to similar dewatering conditions. For the increasing pressure conditions the relationship between expressed fluid and applied pressure was exponential. No relationship was found between expressed fluid and rate of applying pressure because the rat.t er varied in unpredictable manner. However when under constant pressure an exponential relationship was identified between expressed fluid and pressing duration. Empirical equations for these relationships were developed. A linear relationship was found to exist between expressed fluid and reduction in sample volume. However, the reduction in sample volume is bigger than the corresponding expressed fluid. The drying studies culminated with the development of an empirical equation for predicting the moisture content of coffee pulp during drying. However, for this equation to be applicable in the field, further work is required to establish the equilibrium moisture content at different conditions of temperature and relative humidity. The empirical equations developed for both dewatering and drying of coffee pulp seem to be dependent on other unknown factors inherent to the samples besides those covered by this study. These equations can be refined by studies into the physical and biological properties of coffee pulp.