| Green tea, which is also called unfermented tea, is produced with tea shoots by heating or steaming to de-enzyme, rolling, drying and other processes. Green tea has rich nutrition and good health care effect. Dring is a key technology in the initial processing of green tea. In this study, hot-air drying, microwave drying, vacuum drying and microwave vacuum drying were applied to green tea drying. The drying characteristics of the four drying methods were studied, drying dynamic models were also established. Meanwhile, effects on qualities characteristics of the four drying methods of green tea were studied to get the optimal technological condition. Therefore an important recommendation for controllable and continuous processing of green tea was offered.1. The rules of moisture change were known, by researching drying characteristics of four drying technologies, including hot-air drying, microwave drying, vacuum drying and microwave vacuum drying. (1) The processes of hot-air drying, microwave drying, vacuum drying and microwave vacuum drying for green tea were all divided into three stages that were accelerated phase, constant-speed phase and decelerated phase. Among the four drying technologies, three including hot-air drying, vacuum drying and microwave vacuum drying, had shorter accelerated phase and longer decelerated phase, except microwave drying course, which included obvious accelerated phase, constant-speed phase and decelerated phase. (2) During the process of hot-air drying, the required time decreased with an increase of temperature; during the process of microwave drying, the required time decreased with an increase of microwave power; during the process of vacuum drying, the required time decreased with an increase of temperature; during the process of microwave vacuum drying, the required time decreased with an increase of microwave power. (3) Among the four drying technologies, the required time of vacuum drying was the longest, hot-air drying was longer, microwave drying and microwave vacuum drying was the shortest.2. According to the drying characteristics of hot-air drying, microwave drying, vacuum drying and microwave vacuum drying, drying dynamic models for green tea were established. For hot-air drying, the drying dynamic model agreed with Page equation MR = e-rt N,of which r = e0.6085 - 0.0077T + 0.0002T 2, N = 0.7644 - 0.0060T + 0.0001T2; for microwave drying, the drying dynamic model agreed with Page equation MR = e-rt Nby two sections, of former section, r = e-1.0061 + 0.4551P + 0.3583P 2, N = 0.1801 + 0.4279P - 0.4610P2, of latter scetion, r = e-4.8506 + 6.2814P - 1.4483P 2 , N = 1.7557 - 0.0464P - 0.5548P2; for vacuum drying, the drying dynamic model agreed with Page equation MR = e-rt N, of which r = e0.2836 - 0.0142T + 0.0003T 2, N = 0.3314 + 0.0041T; for microwave vacuum drying, the drying dynamic model agreed with Page equation MR = e-rt N, of which r = e-0.6303 + 0.3997P - 0.0436P 2, N = 0.2483 + 0.2455P - 0.0469P2. It was proved that predicted date and experimental date were nearly accordant and these models could be uesd for describing the change of moisture in green tea during the processes of hot-air drying, microwave drying, vacuum drying and microwave vacuum drying.3. Effects of drying methods on tea polyphenol, amino acid, caffeine, water extract and sensory quality were studied, and the optimum drying parameters were determined as follows: the optimum temperature of hot-air drying was 80℃, the optimum microwave power of microwave drying was 539w, the optimum temperature of vacuum drying was 50℃,the optimum microwave power of microwave vacuum drying was 2kw.4. Effects of four drying methods, including hot-air drying, microwave drying, vacuum drying and microwave vacuum drying, on the contents of total tea polyphenols, total amino acids, caffeine, water extracts, chlorophyll, vitamin c, free amino acids, catechin, the color and sensory evalution of green tea were studied. It was showed as follows: (1) Among four drying technologies, the contents of total tea polyphenols of vacuum drying was the highest, which tasted most bitter, and the second was hot-air drying and microwave drying, microwave vacuum drying was the lowest, which tasted good. (2) The content of total free amino acids of microwave vacuum drying was the highest, and the second was vacuum drying and hot-air drying, microwave drying was the lowest. (3) The color of microwave vacuum drying was the best, and the second was hot-air drying and microwave drying, vacuum drying was the worst. (4) The contants of chlorophyll and vitamin c of microwave vacuum drying was the highest; then was hot-air drying. Thus, microwave vacuum drying could maintain the thermosensitive elements, such as chlorophyll, vitamin c. (5) The sensory evalution of microwave vacuum drying got the highest score, and the second was hot-air drying and microwave drying, vacuum drying got the lowest score.5. The conclusion can be obtained from synthetical analysis: microwave vacuum drying technology was beneficial to produce green tea with high qualities.
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