Study On The Microwave Vacuum Drying Of Agrocybe Cylindracea

Agrocybe cylindracea, scientifically named as Agrocybe aegerila, Glossy ganoderma, Liu matsutake etc., belongs to the Homobasidiomycetes. Agrocybe cylindracea is recommended for its high medicinal and hygienic value as it is rich in polysaccharide, amino acids, proteins and other nutritional ingredients. Due to the seasonal and regional characteristics of planting and sale, drying treatment of agrocybe cylindracea fresh product is required for long-term storage and transportation. Dehydration is an effective way to enhance storage life and facilitate commodity circulation to gain economic interest. In this paper, the technology of microwave vacuum drying (MVD) was applied in the treatment of the agrocybe. The MVD characteristics of agrocybe under different conditions were studied and drying kinetic models were established. Meanwhile, the influence of parameters during microwave-vacuum drying on the quality of agrocybe cylindracea was analyzed and the optimal values were established. Finally, we introduced the tempering technique into MVD and studied the influence of different factors on vaccum-tempering process. We optimized the technique and compared the effect of MVD with that of tempering microwave vacuum drying (T-MVD) on the main quality changes of agrocybe. The main research points and results are as follows.1. We investigated the MVD drying characteristics of agrocybe cylindracea and found three phases emerged throughout the MVD drying process. These phases include the accelerating phase, constant-speed phase and decelerating phase. Drying time could be reduced with increasing microwave power, increasing vacuum degree, and decreasing loading capacity. The dehydration rate of MVD varied depending on the microwave power, vacuum degree and loading capacity. Microwave power and loading capacity had greater impact than vacuum degree on the drying characteristics. Higher vacuum degree caused higher consumption of energy and easy breakdown of the agrocybe. For the consideration of both sensory quality and energy consumption, we selected appropriate microwave power, middle-high vacuum degree and medium load capacity for MVD.2. Kinetic models of MVD were established with the consideration of drying time, drying parameters and moisture ratio (MR) to determine the relationship between time and moisture ratio in different drying parameters. The model was shown to have accurately described and predicted the moisture changes during MVD process, according to the correlation test. This suggests the model has good practical applications in the online monitoring and the forecast of drying process for all kinds of fruits and vegetables.3. We studied the influences of MVD parameters on the main quality changes of agrocybe cylindracea, including the contents of polysaccharide, vitamin C, and free amino acids. Within a certain range, the content of polysaccharide increased gradually as the microwave power increased, and then decreased. This trend also occured with the growing loading capacity. Beyond this range, the agrocybe became locally coked with a significant decrease in polysaccharide content. High vacuum degree is beneficial for keeping polysaccharide contents. For vitamin C, increasing microwave power or loading capacity leads to increased content at first and then decreased, and the influence diminished when the microwave power and loading capacity increased beyond the range. High vacuum degree is favorable for retaining vitamin C as well. For free amino acids, the amount increased to the maximum and then decreased with increasing microwave power, vacuum degree and loading capacity. Microwave power exerts greater impact than vacuum degree and loading capacity on the main qualities of agrocybe cylindracea under the same parameters. As a result, to keep the maximal level of nutritional content and guarantee sensory quality, MVD drying process requires medium microwave power, a higher vacuum degree and a proper loading capacity.4. Box-Behnken composite center design was applied to evaluate the influences of MVD parameters (microwave power, vacuum degree and the loading capacity) on the main qualities of agrocybe cylindracea. The indexes for quality evaluation include dehydration time, polysaccharide contents, unit energy consumption and sensory quality. Based on the experimental data, a fitting regression model of evaluation indexes was established to evaluate the effects of independent and combined action of parameters on each index. Parameters were analyzed with response surface methodology. The appropriate values of technological parameters were determined with Evaluation Function Method. The parameters were optimized as:microwave power 2.93kW, vacuum degree 85.00kPa and the loading capacity 193g. It was indicated that the quadratic fitting regression model could be used to accurately describe and predict the change in evaluation indexes and the effectiveness of response surface methodology.5. We introduced tempering techniques into MVD process and explored the drying characteristics with different influencing factors (tempering time, initial tempering moisture content, and tempering surroundings humidity). The results indicated the T-MVD process comprised of four phases:accelerating phase, speed-down, constant-speed and decelerating phase. Drying time was positively related with initial tempering moisture content and surrounding humidity while inversely related with tempering time. The dehydration rate of T-MVD varied inversely with tempering time, initial tempering moisture content and surroundings humidity. Compared with MVD, T-MVD reduced the drying time and speeded up the dehydration rate.6. We studied the effects of T-MVD parameters on the main qualities of agrocybe cylindracea. The parameters were optimized with the Lg(34) orthogonal experiment. Taking polysaccharide contents, unit energy consumption and sensory quality as evaluation indexes, the fitting regression model of each index was established. The optimal parameters were determined to be:agrocybe cylindracea with 60% initial tempering moisture was dried in an environment of 30% surroundings humidity for 60min. The regression model could be used to accurately describe and predict the variations of indexes for T-MVD by comparing the test results with simulation data.7. MVD was compared with T-MVD for their influences on the main quality indexes of agrocybe, including the content of polysaccharide and vitamin C, the amount of free amino acids, the sensory quality, and the energy consumption. T-MVD showed a significant increase in the content of polysaccharides, vitamin C, and the amount of free amino acids than MVD. Furthermore, the application of T-MVD reduced the drying time as well as energy consumption. Taking into consideration the color, shape, size, flavor and other qualities of drying agrocybe cylindracea, MVD is superior than AD (Air Drying) however inferior than T-MVD.