Mathematical modelling and experimental investigation of drying of Piper Nigrum L.

The aim of this study was to identify the optimum drying conditions of Piper Nigrum L. berries that would yield quality black pepper. Experimental trials were carried out to find out the drying characteristics of Piper Nigrum L. berries. The berries were dried under natural convection and forced convection. The effects of drying temperature and air flow on drying rate, drying duration, retention of chemical constituents, and the changes to the internal structure of the individual drupes were studied. In natural convection trials, the drying rate was found to have increased at higher drying temperatures. This reduced the drying duration, the moisture retained and hastened the changes in the internal structure of individual drupe. In forced convection trials, the increase in drying rate was obvious only within the first 24 hours of drying at 45°C and 55°C. The increase was significant throughout trials in 35°C. At different drying temperatures studied, the drying duration, the chemical constituents, and the changes to the internal structure of individual drupes were minimally affected by air flow. The Fickian diffusion model for a spherical solid was used to describe the drying characteristics of the berries. This model was solved via the analytic solution and the finite difference method. The moisture distribution within the individual drupe for different drying conditions was simulated. The results obtained from the solutions were validated with the results obtained from experiments conducted in this work using the bulk moisture loss method. The relationship of drying temperature and air flow with the effective diffusivity of these chemical constituents was identified. The validation of the results obtained from the solutions was satisfactory. The effective diffusivity of piperine was much lower than the effective diffusivity of moisture in all trials. The effective diffusivity of moisture was higher when berries were dried at higher drying temperatures while the effective diffusivity of piperine in berries was about the same at all drying temperatures studied except at 30°C.An increase in the speed of air flow neither increased nor decreased the effective diffusivity of all the chemical constituents at 35°C, 45°C and 55°C. In this work, it is concluded that the air flow is a less influential driving force compared to drying temperature on the drying of the berries. The optimum drying condition to produce black pepper is a combination of drying temperature from 45°C to 55°C and air flow from 0.5ms^-1 to 1.0ms^-1.