Far Infraredvacuum and Convection Drying of Welsh Onion

The drying of welsh onion by a farinfrared (FIR) heater under reduced pressure and convective conditions werestudied to understand the drying characteristics. The effects of radiant power input and vacuum levels on the dryingphenomenon were investigated. In addition, drying rate as a function of moisture content was studied visvis dryingqualities such as rehydration potential, shrinkage, and color change. When onion samples were dried from about 10.1 to0.05 w/w dry basis moisture content, the three characteristic periods, namely, rising, constant, and falling rate periods, wereobtained. The drying rate increased with increasing radiant power input and was higher in a reducedpressure environmentthan under a convective condition. With 0.17 W/cm2 radiant power input, the drying rate was 50% higher in the vacuumoperation than under the convective condition. There was a higher shrinkage with a concomitant higher rehydration potentialand superior quality of onion at lower radiant power input with the FIRvacuum system than with the FIRconvection system.