RUNAWAY HEATING OF ONIONS DURING MICROWAVE DRYING

摘要

Runaway heating is one of the major problems encountered during the process ofrnmicrowave drying of onions as well as other fruits and vegetables. As a result ofrnrunaway heating, drying of the products is not uniform. The aim of this study was torndetermine the relationship between the power level and dry weight of the onions whenrnhot spots are formed. In this study, two onion varieties, namely, Spirit and Niz, werernused. The dry matter content of Sprit and Niz were 12.0 ± 1.8 and 17.6 ± 1%,rnrespectively. Eighteen experiments were carried out for each variety using differentrnpower levels (1.2, 2.4, 3.6, 4.8, 6.0, 7.2 kW) and product loads (1.0, 1.5, 2.0 kg).rnDrying process was performed until a brown or black spot appears. The results showedrnthat the dark spot appeared at a constant specific power (power/weight) regardless of therninput power level. At higher input power, the dark spots appeared at a higher productrnweight and this critical weight depends upon the initial onion load. If the initial onionrnload is lower, the critical weight is also lower. The critical water content is somewhatrnsimilar regardless of the initial load but depends on the power level. The resultsrnobtained allowed us to determine the weight (or water content) of onions at which thernpower should be decreased during drying. A drying procedure to overcome thernrunaway heating and to achieve uniform drying of onions was defined.