COMBINED CONVECTIVE-MICROWAVE DRYING OF AGAR GELS: INFLUENCE OF MICROWAVE POWER ON DRYING KINETICS

The use of microwave energy in the drying of deformable material such as gel considerably reduces drying time and enables the control of retraction in the sample. A further advantage is that no hot spots are produced, allowing a dry product of superior quality to be obtained.The aim of this work has been to determine the kinetics of the convective-microwave drying process of agar gel plates. For this purpose, we developed a pilot closed loop, computer-controlled apparatus of convective-microwave drying, that enables the drying air conditions to be changed and the microwave power to be supplied over a wide value range. The equipment also records the sample surface temperature by means of an infrared thermometer. The drying curves obtained for plane geometry present four different drying phases: an initial phase where a rapid increase in the drying rate and in the surface temperature can be observed, as well as a constant rate phase that ends in the so-called convective critical moisture content, a first falling rate phase that concludes in the microwave critical moisture point, and finally a second falling rate phase. Combined convective-microwave drying enables a considerable reduction in drying time compared to convective drying, the time required being inversely proportional to the microwave power supplied. The empirical equation that best represents the kinetics is of the Page type. The absorbed volumetric power in terms of the moisture content was experimentally estimated, with the experimental data fitting an empirical equation.