| Agaricus bisporus is one of the most extensively consumed and widely cultivated in the world. According to statistics, the annual output of the Agaricus bisporus amounts to 2,184 thousand ton. It is considered as a valuable heath food not just because of its abundance of nutrients and delicious taste, but also its plentiful active substance. However, Agaricus bisporus are frequently subjected to browning, umbrella opening and even decay during preservation and transportation. Currently, freeze drying (FD) is considered as one of the most effective drying method to dehydrated high quality dried products. It can dehydrate high value dried products with good quality including color, texture, nutrition and flavor retention. Nevertheless, freeze drying is a very high energy consuming operation with disadvantages of long production cycle and high production cost. Therefore, the study on high efficency and energy saving drying process has important theoretical significance and practial value for deep processing of Agaricus bisporus.In this research, Agaricus bisporus were used as raw materials, and freeze drying and three different combinations of drying methods:freeze drying combined with hot air drying (FD+AD), freeze drying combined with vacuum drying (FD+VD) and freeze drying combined with microwave vacuum drying (FD+MVD) were used to dry Agaricus bisporus slices. A comprehensive analysis of dried products was performed on their colour, texture, nutrient retention, microstructure and energy consumption. The results showed that, under conditions of 38% moisture content changing point, most of the parameters including L values, a values, average density and hardness of FD+VD and FD+MVD samples had no remarkable changes compared with FD samples. Moreover, FD+VD and FD+MVD products were better than FD+AD products in nutrient retention except that content of vitamin C was comparatively lower during FD+MVD process. The energy consumption of all these three combination drying methods could reduce by 34.51%,36.36% and 35.27%, respectively in comparison with freeze-drying, but only FD+MVD could reduce the total drying time by 35.63% in comparison with freeze drying. In addition, the microstructure of FD+MVD products was better than the other two combination drying methods, and had no significant difference with FD ones. Therefore, we ensured that FD+MVD was a high efficiency, energy-saving drying method, which can be implied in high quality Agaricus bisporus dehydration.Drying parameters including drying temperatures (20,30, and 40 ℃), chamber pressures (70,100, and 130 Pa) and material layer thicknesses (single, double, and triple) during FD process, and microwave power densities (20,40, and 60 W/g) and material layer thicknesses (single, double and triple) during MVD period of FD+MVD process, were investigated for their drying characteristics. The proper well-known mathematical models were used to describe drying kinetics and rehydration characteristics of their productions, and the effective diffusivities (Deff) in the FD and FD+MVD processes were also calculated. The results showed that freeze drying temperatures, material layer thickness, chamber pressures and microwave power densities had significant effects on the drying rate of FD and FD+MVD processes. The drying rate increased with increasing freeze drying temperature and microwave power density, and with decreasing material layer thickness and chamber pressure. Based on nonlinear regression analysis, the Page model and logarithmic model provided the best fit for FD (in both FD and FD+MVD processes) and MVD (in FD+MVD process) curves, respectively. The mathematical drying models were obtained by using multiple linear regression analysis as follows:For FD process: MRFD=exp(-kFDt1.388), kFD= 0.012T-0.177LFD-0.005V+0.844, R2= 0.970For FD+MVD process:Both of the two mathematic models can accurately predict the relationships between moisture ratio and time. Moreover, the Deff values of the MVD period in FD+MVD process were about ten times greater than those in FD process, which indicated that the MVD of combined drying process has much better mass transfer efficiency than FD process. In addition, the Peleg model gave a better fit for rehydration conditions applied in both FD and FD+MVD products. The We values of FD and FD+MVD products were almost similar during the same condition of rehydration process, which indicated that the rehydration capacities of the two dehydrated products were comparable.The non-volatile components of Agaricus bisporus slices, including soluble sugar and polyols, free amino acids,5’-nucleotides, organic acids were determined and compared during FD and FD+MVD processes. The EUC values of mushrooms dehydrated at different drying stages were also evaluated. The results showed that most of non-volatile components in the Agaricus bisporus, including mannitol, L-glutamic acid, L-histidine, L-glutamine, L-phenylalanine,5’-AMP,5’GMP and organic acids components significantly (p<0.05) increased during the sublimation period of FD and FD+MVD processes, and then reduced during desorption period of FD process and MVD in FD+MVD process. Similar trend could also be seen in EUC values. In comparison with FD products, FD+MVD products possessed lower contents of soluble sugars and polyols but had significantly higher content of total free amino acids (especially the taste-active amino acids, which make a great contribution to the EUC values), which was close to fresh ones. Besides, the contents of 5’-nucleotides and organic acids in both FD and FD+MVD products had no significant difference. In addition, the EUC values of FD and FD+MVD products did not differ remarkably from fresh ones, which indicated that both drying processes could effectively preserve the MSG-like components for Agaricus bisporus.The conditions of SPME extraction for were optimized based on the content of total volatile components and 1-octen-3-ol in Agaricus bisporus. GC-MS and electronic nose were used to study and compare changes in volatile components of Agaricus bisporus during FD and FD+MVD processes. The results showed that under the optimum conditions, the volatile components were extracted at 60℃ for 40 min.32 volatile components were determined by GC-MS during different stages of FD and FD+MVD processes. The C8 components (including 1-octen-3-ol,3-octanol,2-octen-1-ol and 3-octanone, etc), benzyl alcohol, benzaldehyde and esters were the main flavours of the fresh Agaricus bisporus. The content of C8 compounds, esters, alcohols and aldehydes decreased during drying process, and more alkanes and heterocyclic compounds were formed in the later drying period. Compared to FD process, the thermal sensitivity volitle compounds could be destroyed by FD+MVD process more easily. Nevertheless, the contents of new volitle compounds in FD+MVD products such as alkanes and heterocyclic compounds, which might be formed from Maillard reaction, were significantly lower than FD ones. According to the clustering analysis and principal component analysis, we presumed that the turning point of volatile components in Agaricus bisporus might be occurred between the sublimation period and latter drying period during FD and FD+MVD processes, and more importantly, the constitute of volatile compounds in FD+MVD products were more similar to fresh ones compared to FD products. From the results of electronic nose analysis, flavour constitutes of samples determined from different stages of FD and FD+MVD periods were significantly different. The trend of radar map variation obtained by electronic nose was in accordance with changes in volatile components of Agaricus bisporus determined by GC-MS during FD and FD+MVD processes. |
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