Study on the Microwave Vacuum Drying Characteristics and Drying Technology Optimization of White Fungus
White fungus which parasitizes on withered wood, is an advanced edible fungus, and has special nutrition, cosmetology as well as medicinal value. Therefore, it is in good graces of masses of consumers. Owing to high moisture content and great difficulty of storage and preservation, dehydration becomes an important method of processing and storage. Microwave vacuum drying (MVD) technology was applied to white fungus drying in this paper. The MVD characteristic of white fungus was studied and kinetic models of MVD were established correspondingly. Meanwhile, effects of drying parameters on main quality of white fungus were studied, the operating conditions were optimized, and the consumption with efficiency and main quality of dried white fungus by MVD were compared with airflow drying (AD) to confirm the feasibility of MVD technology.1. Drying characteristics of MVD for white fungus were systematically studied. The MVD process for white fungus was divided into three stages that were speed-up, constant-speed and speed-down. The more microwave power, loadage and microwave intensity were, the quicker dehydration rate was and the shorter drying time was. Vacuum degree had great influence on the dehydration rate of constant-speed and speed-down period. It could obviously increase dehydration rate and shorten drying time by increasing vacuum degree. However, the higher vacuum degree was not always good. The center of dried white fungus presented the coking phenomenon which affected the quality of product. Therefore, it was very important to choose suitable vacuum degree to dry white fungus. The lower initial moisture content was, the fewer dielectric loss constant was and microwave power absorbed decreased. Therefore, the drying time of constant-speed stage and speed-down stage was shortened.2. According to the rule of moisture variation, MVD kinetic model between moisture ratio and drying time for white fungus was established, and the drying course was consistent with Page model. It was proved that predicted data and experimental data were nearly accordant and the model could be used for predicting the change of moisture and dehydrating rate of white fungus during MVD. 3. Effects of MVD parameters on main quality of white fungus were studied. Influence law of shrinkage, rehydration ratio and polysaccharide contents of white fungus affected by microwave intensity and vacuum degree was got. Shrinkage of dried white fungus decreased when microwave intensity 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 microwave intensity. 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. It was comprehensively considered that microwave intensity was neither too high nor too low and -90kPa was the suitable vacuum degree for drying.4. The optimization of MVD technology for white fungus was studied. A two factors quadratic regression general rotary unitized design was adopted to optimize MVD technology for white fungus and the effects of microwave intensity and vacuum degree on drying time, rehydration ratio, tremella polysaccharide contents and unit energy consumption were analyzed. Based on the experimental data, the quadratic regression model of four indexes were established, the effects of independent and combined action of microwave intensity and vacuum degree on consumption and quality were analyzed, then variables were analyzed with response surface methodology, and evaluation function method was applied to optimize drying technology. It was indicated that microwave intensity and vacuum degree had significant influences on each index. Meanwhile, regressive equation of each index was significant, which was proved by statistical test of variance analysis. The optimal MVD technology for white fungus was determined as follows: microwave intensity was 10 W/g and vacuum degree was -90kPa.5. Comparison of consumption and quality of white fungus between AD and MVD was studied. It was indicated that MVD could shorten about 98% of drying time and reduce about 45% of unit energy consumption than AD when white fungus of the same weight was dried. The shrinkage of airflow drying sample was less than that of MVD, AD sample presented better rehydration capacity when rehydrating for 15 minutes, but balance water-holding capacity was poor, and the final rehydration capacity of MVD sample was better than that of AD. Rehydration capacity by rehydrating intermittently was better than that by rehydrating continuously in certain time. Retention rate of tremella polysaccharide by MVD was more than that of AD obviously. The appearance and color of MVD sample were near to that of AD, while hardness and brittleness of MVD sample was superior to that of AD, so its general score was higher than that of AD. AD sample showed superposed microstructure, slight collapse and shrinkage, and distortion happened to white fungus after airflow drying. Sample structure became dense due to shrinking after MVD, but porosity structure was still retained to prepare for adequately rehydrating. Based on the above results, MVD could greatly improve the quality of dried white fungus, enhance production efficiency and reduce unit energy consumption. Hence, MVD was one of new drying techniques which had a bright prospect of deployment.