Comparative analysis of drying coffee beans using microwave and conventional oven

Abstract

This article reports a comparative study of experimental results obtained during the drying of Castilla-variety coffee beans from Santander, Colombia. They were performed by two means: thermal and electromagnetic radiation. Twenty experiments were carried out, ten tests in a microwave cavity at 2,450 MHz-1,080W, and ten tests using a conventional electric oven with temperature controlled at 50±2oC. Experiments were made using samples of coffee beans with parchment, without parchment, and of the only-parchment. For each sample, dimensionless moisture ratio and diffusion coefficients were determined, according to the second law of Fick. We found that the diffusion coefficient of the samples dried in a microwave cavity was twenty-two times higher than the diffusion coefficient of samples dried with thermal radiation. Likewise, it was observed that samples in conventional oven showed a uniform temperature, in contrast with those heated by microwave radiation. Such results are useful for designing hybrid systems for drying coffee beans.

The Development of Coffee Bean Drying Process Technology – A Review

“…The disadvantage of microwave fixed-bed drying techniques is that the drying results obtained are not homogeneous [11]. A comparative analysis of the drying of coffee beans was conducted in a continuous flow dryer using an electromagnetic cavity [27] For practical research, equilibrium moisture is being a concern for both continuous and intermittent processes. The intermittent process could reduce energy consumption but extend the drying time.…”

“…The microwave-based drying at 2450 MHz-1080 W allows 17 times faster processing, namely 17h drying time to reach a moisture value below 12% (d.b.) by means of the conventional oven (grains with parchment), meanwhile with microwaves only requires 60 minutes[27]. The temperature of the samples from the electric oven drying stay close to 50 o C (which was the temperature set for the oven), but the samples temperature by means of microwave oven vary from 30 o C to 100 o C, or even reach 120 o C. This fact is the disadvatage of microwave oven.…”

Drying kinetics of organic parchment coffee beans (Coffea arabica L.) using microwave fluidised bed: Semi-theoretical modeling

“…Effective diffusivity (D eff ) was calculated using Fick´s second law, taking into account the following assumptions: negligible volume shrinkage , taking spherical geometry into account, negligible moisture gradients inside the bean, if diffusivity is constant, and if moisture content is distributed uniformly in the material [16,19,25]. For this system, the analytical solution is represented by equation (3).…”

“…Heat generation by microwave occurs in a faster way than in conventional methods [14]; resulting in a higher water vapor migration rate caused by a diffusive phenomenon [15]. However, the use of microwaves can lead to coffee beans overheating, creating "hot spots" with temperatures between 80 and 90 ° C [16], which could lead to degradation of heat-sensitive compounds responsible for the aroma and taste of coffee beverage [17]. To avoid this, it is necessary to use complementary techniques such as a fluidized bed operating at low temperature (25 °C); since air is in contact with the product surface, this causes a cooling effect that counteracts temperature rising by the microwave heating leading to a constant and uniform product temperature [11], [18].…”

“…Dong et al, [20] dried coffee beans at five power levels (0.3, 0.5, 1.0, 1.5, and 2.0 W) using a microwave at vacuum conditions. Muñoz-Neira et al, [16] compared the effect of oven drying at 50 °C and microwave technique in the drying time and effective diffusivity of coffee beans with parchment, without parchment, and parchment only. In these studies, drying time decreased when the power level increased, and microwave technique was also compared with a conventional method.…”