Evaluation of moisture diffusivity in copra at different drying condition

Abstract

Copra is one of the major traditional products processed from coconuts and is used primarily as a source of coconut oil. It is the kernel of coconut after reducing the moisture content from about 50% (dry basis) to about 6% (dry basis) by drying. Traditional drying processes are vastly used in manufacturing of copra and that has created many quality problems leading to hygienic and health issues which can be minimized by using controlled drying techniques. Controlled drying is also a primary requirement in producing edible copra and premium products like virgin coconut oil. Accurate prediction of moisture diffusivity of porous materials like food under given conditions is important in analysing the drying process. In this study drying behaviour of copra was examined and two methods were suggested to predict the moisture diffusivity of copra. In the first method, the moisture diffusivity of copra was determined for the first and second falling rate periods. A critical moisture content of 30% (dry basis) was identified as the probable limit between the first and second falling rate periods. A computational fluid dynamic model was used to fine-tune the system parameters with experimental data and the effective moisture diffusivity values at 55 ºC for first and second falling rate periods were found to be 1.10 × 10-8 and 1.99 × 10-9 m2s-1 respectively. In the second method, moisture diffusivity of copra was found as a function of drying temperature and dry basis moisture content. Drying experiments were performed for seven different temperatures in the range of 45 – 75 ºC to obtain drying curves of copra. The moisture diffusivity was found to be an exponential function of moisture content where the model parameters were linearly varied with temperature. Further the volume shrinkage of copra was linearly correlated with moisture content. A three-dimensional numerical model was developed to predict the spatial distribution of moisture inside the copra using computational fluid dynamics (CFD) with OpenFOAM software. Results of the spatial moisture distribution were graphically presented. The results of simulation were in agreement with the experimental observations and the optimum temperature for drying of copra was found to be about 60 ºC for 20 hours of drying time.