Nagvanshi, Shrivastava and Goswami
The objective of this paper is to study the changes in shrinkage and density as a function of moisture ratio during microwave vacuum drying (MVD) under various operating conditions. Fresh grapes (Thompson seedless) procured from local market were used for the present study. Two types of sample were taken for study, first one was untreated and second one was pretreated with solution of sodium hydroxide (0.5%) and ethyl oleate (2%) at 80°C for 30 seconds. Grapes with the initial moisture content of 80% (wb) were dried up to final moisture content of 20 % (wb). The effects of process variables, namely microwave power (100, 110, 120, and 130 W) and pressure (200, 400 and 600mm of Hg) were studied on the physical characteristics viz., shrinkage ratio and apparent density of grapes. Microwave vacuum drying resulted in 70 to 90% reduction in the drying time. Shrinkage ratio and apparent density of grapes (Thompson seedless) were determined during microwave vacuum drying at various moisture contents ranging from 80 to 20% (wb), the shrinkage ratio of grapes however, reduced linearly with the moisture content. Experimental data showed that system pressure had more significant effect on shrinkage and density changes than the power level during microwavevacuum drying. Simple mathematical models were used to correlate the shrinkage ratio and apparent density with moisture content of grapes.
