Comparison of drying rate
The drying curves of moisture content vs. time were shown in Figure 2 for different drying methods to reach a final moisture content of about 4%–5%. It was observed that drying time was quite different from four drying methods to reach the final moisture content. The required drying time was about 100 min for hot air drying, 70 min for vacuum drying, 10 min for microwave drying, and 5 min for microwave vacuum drying, respectively. 5 min drying time to reach the desired final moisture content indicated the high efficiency of the microwave vacuum drying method, which was 20 times faster than hot air drying. The boiling point of water was decreased at the vacuum drying conditions, and the water migration rate was increased, resulting in a fast drying rate. The quality of tea was highly related to the drying time. The
long drying time caused serious quality deterioration in color, nutritional composition and organizational structure. At the vacuum drying conditions it was very difficult to transfer heat from outside to materials at the low pressure environment using conventional drying methods; while the microwave drying using dielectric theory, the green tea was heated inside with electromagnetic energy, resulting in a fast heat transfer rate even at the vacuum conditions. Microwave vacuum
drying combined two major advantages of vacuum and microwave heating, and greatly reduced the drying time.
In addition, it can be seen that the different drying rates were resulted from microwave drying and microwave
vacuum drying. In the early drying period, the drying rates were similar to microwave drying and microwave vacuum drying. But in the later drying period, microwave vacuum drying had a significant higher drying rate than microwave drying. It indicated that water was easily reduced with both microwave heating and vacuum sucking, resulting in short drying time and less damaged cell structure with improved green tea quality.