Author : Satip Ratanapaskon

Summary : Parchment coffee beans (Robusta variety) were used in the experiments to evaluate the effect of process variables in intermittent drying on drying time and heat energy consumption. Three levels of air flow rate (10, 15, and 20 m3/min/m2), three levels of number of passes (2,3 and 4 passes) and three levels of tempering periods (4, 8 and 12 hours) were the variables considered in the experiments. Results showed that air flow rate had significant effect on drying time and heat energy consumption. High air flow rates resulted in more rapid drying and higher heat energy consumption. Also, number of passes had a highly significant effect on drying time and heat energy consumption. Both were significantly reduced as the number of passes were increased. Moreover, statistical analyses showed that the interaction of air flow rate and number of passes had significantlyy affected the drying time. An air flow rate of 15 m (superscript 3)/min/m (superscript 2) with 3- and 4-pass drying were not significantly different from an air flow rate of 15 m (superscript 3)/min/m (superscript 2) with 2-pass drying in their effect on drying time. An air flow rate of 15 (superscript 3)/min/m (superscript 2) with 4-pass drying had no significant difference from an air flow rate of 20 m (superscript 3)/min/m (superscript 2) with 3-pass drying on drying time. Tempering period also had highly significant effect on drying time and heat energy consumption. Both were reduced with longer tempering period. However, 8-hour tempering period was not significantly different from 12-hour tempering period. A good combination of process variable in intermittent drying is an air flow rate 15 (superscript 3)/min/m (superscript 2) at drying air temperature of 60 degree C, a 3- to 4-pass drying and an 8-hour tempering period. Intermittent drying was found to be an effective method of drying parchment coffee beans based on the comparison of intermittent and continuous drying systems of the drying time at different air flow rates with fixed drying.