Dehydration of fruits and vegetables in biomass-fired dryer

A study investigates the technical performance of a biomass-fired dryer designed and fabricated by the UP School of Technology for the dehydration of fruits and vegetables. This included the determination of the effects of loading density and thickness. At a fuel (rice hull) feeding rate of 1.0-1.5 kg/hr, the desired temperature level of 60-80 deg C was obtained, but in the front half of the dryer alone. Charcoal was used to initiate combustion. Regular fuel tending was necessary. Food materials to be dried were loaded in the front half of the dryer alone. Drying was performed until the equilibrium moisture content (M Ce) level was reached. Shredded squash (initial MC (w.b.)=90.33 percent) was loaded at densities of 1.92, 3.20 and 4.48 kgs/sq.m dryer area. Results showed that: 1.) the drying rate was most rapid at the upper part of the drying chamber, 2.) uniform drying and the shortest drying time (approx. 5 hrs) was obtained at the lowest loading rate, and 3.) the drying curves clearly exhibited the three drying stages, namely: unsteady state, constant-rate and falling-rate. Differences in drying rates were most explicit during the constant-rate period; the change in drying rate was not, however, proportionate to the change in the loading density. The deposition of shredded squash on the trays, e.g., density of packing, apparently affected the drying behavior. MCe was approximately 2.8 percent. Tests with sliced saba banana (initial MC (w.b.) + 66.9 percent; thickness (mm) of 2.4, 6.0 and 9.0) showed that 1.) drying was uniform in all trays, 2.) increasing the thickness decreased the drying rate, and 3.) the drying curves had brief or negligible unsteady-state stages. Apparently, the drying mechanism was different for the drying of shredded squash; the presence of space or voids between slices enables a through-flow mechanism which allows for drying to occur at the cross-sectional surfaces as all as the edges/sides. MCe was 3.0 percent. The dehydration of glazed pineapple proceeded similarly. However, no constant rate period was observed, apparently due to the presence of sugar and its behavior upon heating. Definitely, design modifications are necessary to further improve the performance of the biomass-fired dryer