Drying properties of parchment coffee beans

Experiments were conducted to determine the drying properties of parchment coffee. Air temperature of 140, 160 and 180 deg. F. and air flow rates of 10, 15 and 20 m3/min/m2 were tested using the forced convection dryer. The drying rate and drying time in the forced convection drying were compared to that of the natural convection and sun drying. Increasing both airflow rate and air temperature while decreasing the relative humidity, increased the drying rate. Drying rate was maximum, 46.95%/hr at air temperature of 180 deg. F. (13-15% RH) and at air flow rate of 20 m3/min/m2. The least drying rate, 19.52% hr was noted at lower air flow rate of 10 m3/min/m2 and air temperature of 140 deg. F. (36-38% RH). Drying time was shorter as air temperature and air flow rate increase. It took 2 to 4 hours to dry the parchment coffee from 97.51% to 13% moisture content dry basis at various air temperature and air flow rate. Shorter drying time (1.8 hours) was noted at an air temperature of 180 deg. F. and air flow rate of 20 m3/min/m2. Longest drying time, 4.33 hours, was observed at lowest air temperature (140 deg. F.) and air flow rate (10 m3/min/m2). Mass diffusivity ranged between 0.21 x 10 ** (4) and 5.31 x 10 ** (4) cm2/min. It increased as air temperature and air flow rate were increased. Six equilibrium moisture content models were tested, namely: Henderson, Smith, Chung-Pfost, Oswin, BET and Halsey. The data fitted well with the Henderson equation. Results revealed that it took two hours to dry the coffee from 97.51% to 13% moisture content using forced convection while 3 hours in natural convection and 30 hours using sun drying. The drying rates were 42.68%, 28.26%, and 2.81%/hr in the forced, natural convection and sun drying respectively. Based on the results, forced convection is advisable to use in drying parchment coffee to attain the shortest drying time