The temperature changes during microwave-vacuum drying of slicedcarrots were investigated. Sliced samples were dried to 7-10% moisture content(wet-basis) at a wide range of microwave power and vacuum pressure levels. Theexperiments showed that for sample thickness less than 8 mm, the core temperatureof the sample was the same as its surface temperature, with uniform temperaturedistribution within the sample. However for sample thickness more than 8 mm,temperature gradient developed along the thickness of the sample. The experimentsalso showed that, with the decrease of moisture content Xs (dry-basis), for sampleswith thickness ≤ 8 mm, the drying process of sliced carrots experienced threedistinct periods: a warming-up period (Xs = 7.68) without removal of moisture whenthe product temperature increased linearly with drying time until it reached thecorresponding saturation (boiling) temperature of water in the food at the vacuumpressure; a constant temperature period (2 ≤Xs 7.68 ) in which most of moistureevaporated and flowed out of the sample efficiently with little resistance; and aheating-up period (Xs < 2 ) in which the drying rate decreased and sample
展开▼
