Takashi Tsurusaki

Accelerated drying of welsh onion by far infrared radiation under vacuum conditions

Abstract Far infrared radiation has excellent radiation characteristics and high energy conversion rates can be achieved using ceramic coated radiators. Using such a radiator, the dehydrating synergy, generated by far infrared radiation heating under vacuum condition, the drying of welsh onion was studied. The radiation intensity levels influenced dramatically the drying rate and the product qualities: the a ∗ for rehydrated onions, and the L ∗ and ΔE ∗ ab for dehydrated onions. A rising-rate period, a constant-rate period, and a falling-rate period were ascribed to the drying behaviour. The radiation also had significant effects on chlorophyll content. The long time in drying and the high temperature may have contributed to a decrease in rehydration properties.

FAR INFRARED–VACUUM AND –CONVECTION DRYING OF WELSH ONION

The drying of welsh onion by a far–infrared (FIR) heater under reduced pressure and convective conditions were studied to understand the drying characteristics. The effects of radiant power input and vacuum levels on the drying phenomenon were investigated. In addition, drying rate as a function of moisture content was studied vis–a–vis drying qualities such as rehydration potential, shrinkage, and color change. When onion samples were dried from about 10.1 to 0.05 w/w dry basis moisture content, the three characteristic periods, namely, rising, constant, and falling rate periods, were obtained. The drying rate increased with increasing radiant power input and was higher in a reduced–pressure environment than under a convective condition. With 0.17 W/cm2 radiant power input, the drying rate was 50% higher in the vacuum operation than under the convective condition. There was a higher shrinkage with a concomitant higher rehydration potential and superior quality of onion at lower radiant power input with the FIR–vacuum system than with the FIR–convection system.

Kinematic Model for a Far Infrared Vacuum Dryer

Abstract Energy analyses in drying of welsh onion from one source of far infrared radiation at an absolute pressure of 2.6 and 5 kPa were carried out to determine the energy input at which the highest level of efficiency would be attained. The experimental results were used not only to formulate an advanced mathematical model for the prediction of moisture content of onion as a function of drying time but also to study the dryer efficiencies. Less than half of energy input was utilized for evaporating water from the onion. Approximately 73–99% of operating efficiency, which was dependent on the dryer configuration, was converted into radiant energy. The highest radiant efficiency was obtained at 40 W initial power supply, which was equivalent to the source temperature of about 100°C. An 80 W initial electrical power supply at a 10 cm distance of heater from the onion surface showed the highest drying efficiencies. At 80 W power, efficiency decreased with increasing distance from the heater source but at the expense of quality. For optimum drying efficiency without compromising quality, distance of heater from onion surface was proposed to be 10 cm.

Far Infrared Radiation Drying of Welsh Onion under Reduced Pressure and Convection

A comparative study of the effects of initial power supply on the drying of the leafy part of welsh onion by a far infrared heater under reduced pressure and convection were studied to understand the drying phenomenon. The drying rate as a function of moisture content was studied vis-a-vis drying qualities such as rehydration potential, shrinkage, and color change. The three characteristic preheating, constant, and falling rate periods were obtained in both drying techniques. Drying rate increased with increasing radiative energy input and was higher under the reduced pressure environment than in the convective condition as was shrinkage and rehydration ratio. For optimum drying speed and product quality vis-a-vis energy savings in the drying of welsh onion, it was recommended that FIR- vacuum operation at 70-watt initial power supply be used.

Modeling of a Far Infrared Vacuum Dryer on Welsh Onion

Abstract A study of the energy efficiencies in the drying of welsh onion from one source of far infrared radiation and at an absolute pressure of 5kPa was carried out to determine the energy input at which the highest level of efficiency would be attained. The experimental results were used not only to formulate a simple mathematical model for the prediction of moisture content of onion as a function of drying time but also to study the dryer efficiencies. At 70-watt energy input, the highest drying efficiencies were attained.