| White fungus is one of main edible fungus in China, and it is also one of important characteristic agricultural products exported to earn. At present, drying method of white fungus usually adopts traditional airflow drying, which consumes high drying consumption and presents low drying efficiency and poor product quality. Microwave vacuum drying (MVD) technology is one kind of high-efficiency, energy-conservation and environmental protection drying technology. The application of MVD will start a new treating approach to white fungus drying. In this study, MVD equipment was designed to solve the problem of drying uniformity, the MVD kinetics process of white fungus was studied, MVD quality of white fungus was explored, effects of different drying methods on the quality of white fungus was studied, combined airflow and MVD technology of white fungus was optimized, testing method of Bongkrekic acid (BA) in degenerative white fungus was established and removal effect of BA by microwave treatment was investigated.1. MVD equipment was developed. MVD system was designed to solve drying uniformity and long-drying-time consumption with load tray of solid bottom. The type with size of drying chamber and resonator was determined as follows: rectangular resonator with the volume of 90×90×100cm3, which had 242 resonant frequencies. The total microwave power was calculated base on the productivity was 4kW. It could guarantee rotary drying uniformity by using diagonal crossover of upper and underside walled surface and uniform arrangement for designing microwave fed entrance. Using carrot as testing material, it was found that load tray of solid bottom could enhance drying rate and shorten drying time than load tray of ethmoidal bottom by studying effects of load tray on drying uniformity.2. MVD kinetics of white fungus was studied. The experiment results showed that the MVD process of white fungus was divided into three stages that were speed-up , constant-speed and speed-down. Vacuum condition could obviously enhance drying rate, however, which was limited. If vacuum degree was too low, the quality of dried white fungus would be influenced. Therefore, appropriate vacuum degree should be chosen to drying white fungus. Unit mass microwave power and initial moisture content had great influence on drying rate of white fungus. The higher unit mass microwave power, the less initial moisture content was, and then the quicker the drying rate was, the shorter the drying time was. Quadratic polynomial could express drying kinetics curves of every drying condition by using Matlab software for polynomial curves fitting. It was feasible to reduce experiment times and predict drying results by applying BP neural network simulation technique. Predictive value of neural network and actual value had good fitting results, and prediction precision was good.3. MVD quality of white fungus was studied. Shrinkage of dried white fungus decreased when unit mass microwave power and vacuum degree increased, rehydration ratio increased when microwave intensity and vacuum degree increased, and vacuum degree had less important effect on the shrinkage and rehydration ratio of white fungus than unit mass microwave power. Polysaccharide contents of white fungus increased when microwave intensity decreased and vacuum degree increased. The lower vacuum degree was, the easier the coking phenomenon occurred in the center of dried white fungus. In order to avoid the loss of polysaccharide, high vacuum degree should be chosen as possible. Comprehensively considered, unit mass microwave power was neither too high nor too low, using 10W/g, and vacuum degree used -90kPa.4. Effects of different drying methods on the quality of white fungus were studied. Five drying methods, viz., airflow drying(AD), vacuum drying, microwave drying(MD), MVD and freezing drying were applied to drying fresh white fungus, effects on shrinkage, rehydration ratio, sensory quality and organization structure of dried white fungus were analyzed, and the drying time, drying rate and consumption between airflow drying and MVD were compared. The comparison results showed that MVD was the most advantageous method and worthy of popularization, for it consumed lower energy and generated a better quality in spite of deficient gloss.5. Optimization of combined AD and MVD technology for white fungus was studied. Choosing shrinkage ratio, rehydration ratio, sensory quality of dried white fungus and unit energy consumption as evaluation indexes, effects of airflow temperature, conversed moisture content and unit mass microwave power on the quality of dried white fungus and drying energy consumption were studied. By orthogonal optimization, the optimal combined AD and MVD technology of white fungus was determined as follows: airflow temperature was 70℃, conversed moisture content was 30%, and unit mass microwave power was 5 W/g. The better technology of combined AD and MVD for white fungus was compared with single AD and single MVD. It showed that combined AD and MVD could greatly improve the quality of dried white fungus and obviously reduce the energy consumption than single AD.6. The effect of MVD on main safety index of white fungus was studied. Testing method of BA content in degenerative white fungus, that was uv spectrophotometry, was established by experiment. There was no significant difference in testing results between uv spectrophotometry and high pressure liquid chromatography. Precision and accuracy of uv spectrophotometry were both good, and this method was suitable to popularize due to its low equipment cost and convenient operation. Effects of AD, MD and MVD on BA content in degenerative white fungus were compared. Results showed that MD and MVD both had removal effect compared with AD, and removal rate was respectively 23.8% and 9.5%, which signified detoxification effect of MD was more obvious.
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