Mathematical Modeling Of Microwave-vacuum Drying And Its Application In Food Processing

The objective of this work was to develop, simulate and apply the microwave-vacuum drying in food processing.A lab-scale microwave-vacuum dryer was developed and manufactured in which the material to be dried could rotate in the cavity. The microwave power output and vacuum pressure of the dryer could be adjusted in wide ranges. The kinetics of microwave-vacuum drying of thin layer sliced food was studied by introducing a theoretical mathematical model using carrot as test material. The model is based on the energy conservation of sensible heat, latent heat and source heat of microwave power. The model was tested with data produced in a lab-scale microwave-vacuum dryer. The theoretical and experimental drying curves showed that the theoretical model was in agreement with experimental data, and drying rate was a constant until the dry-basis moisture content was about 2. As 1(<2, the experimental drying curves showed a little deviation from the theoretical drying curves. While <1, the experimental drying curves showed a sharp deviation from the theoretical drying curves. To predict the changing of moisture content with time by the theoretical model in the period of , a correction factor, , was introduced and obtained using non-linear regression analysis. The investigation involved a wide range of microwave power and vacuum pressure levels. Both the theoretical model and experimental data also showed that the drying rate was linear to the microwave power output, and inversely proportional to the first order of latent heat of evaporation for water at the vacuum pressure of .The temperature changes during microwave-vacuum drying of sliced food were also investigated using carrot as test material. Sliced samples were dried to 7-10% moisture content (wet-basis) at a wide range of microwave power and vacuum pressure levels. The experiments showed that for sample thickness less than 8 mm, the core temperature of the sample was the same as its surface temperature, with uniform temperature distribution within the sample. However for sample thickness more than 8 mm, temperature gradient developed along the thickness of the sample. The experiments also showed that, with the decrease of moisture content (dry-basis), for samples with thickness 8 mm, the drying process of sliced carrots experienced three distinct periods: a warming-up period () without removal of moisture when the product temperature increased linearly with drying time until it reached the corresponding saturation (boiling) temperature of water in the food at the vacuum pressure; a constant temperature period () in which most of moisture evaporated and flowed out of the sample efficiently with little resistance; and a heating-up period () in which the drying rate decreased and sample temperature increased rapidly. The theoretical mathematical models for predicting sliced sample temperature were developed based on energy conservation principle for the warming-up and constant temperature periods during microwave-vacuum drying of samples with thickness 8 mm, while in the heating-up period, an empirical model was developed by regression using the experimental data. Combination of microwave-vacuum drying and air drying was examined as a potential mean for drying garlic slices. The sample was dried by microwave-vacuum until the moisture content reached 10% (wet basis), and then by conventional hot-air drying at the temperature of 45oC to final moisture content less than 5% (wet basis). Pungency, colour, texture and rehydration ratio of garlic slices dried by this method were evaluated and compared with those dried by freeze drying and conventional hot-air drying. The comparison showed that the quality of garlic slices dried by the current method was close to that of freeze dried garlic slices and much better than that of conventional hot-air dried ones. Microwave-vacuum drying, and combined microwave-vacuum with air or vacuum drying were also examined as potential methods for drying green and high-carotenoids vegetables of Chinese chiv...