Study On Quality Control Of Agaricus Bisporus During Microwave Freeze Drying(MFD)

Button mushrooms(Agaricus bisporus)belong to macro-fungi,which are low in calories but higher in minerals,protein and fiber.The beneficial biochemical properties of mushrooms attracted much attention as functional health foods.Agaricus bisporus is susceptible to rapid deterioration because of its high moisture content,and its shelf life is only about 24 h in ambient conditions.Various physiological and morphological changes occur after harvest,which make these mushrooms unacceptable for consumption.Thus it is necessary to remove the moisture of them for prolong shelf life.The convective drying method is the most commonly used to dry mushrooms,nevertheless,due to long drying time and high drying temperature at industrial scale,the problems of darkening in color,shrinkage,loss in flavor and decrease in rehydration ability easily occur during convective drying process of mushrooms.It is well known that freeze drying(FD)is the best method for water removal from heat-sensitive materials to obtain the highest product quality(such as: color,nutrition and rehydration capacity and so on)compared to other drying methods of foods.However,it is well known that FD is an expensive process,and its application for mushroom drying is limited.Using microwave as the heat source of freeze-drying can significantly reduce the time and cost of freeze-drying.This drying method is called as microwave freeze drying(MFD).Consequently,in order to achieve faster drying along with high product quality,microwave freeze drying was applied to mushrooms dehydration processing.In this paper,the drying characteristics and quality characteristics of Agaricus bisporus were studied in the process of microwave freeze drying,in order to replace the traditional freeze-drying technology,and to achieve the efficient and low energy production of Agaricus bisporus.The main conclusions are as follows:1.By studying the drying characteristics and quality of button mushrooms during microwave freeze drying,it could be found that microwave loading level had a more significant influence on drying rate and physical quality index than the system pressure.However,the system pressure had more obvious influence on nutritional content index,energy consumption and sensory evaluation compared to the microwave loading level.the Henderson and Pabis model could well describe the moisture variation in drying process;the effective moisture diffusion coefficient of button mushrooms during the MFD was in the range of 3.423×10-10-5.654×10-10 m2/s,and it was on the 10-10 m2/s order of magnitude and more influenced by microwave loading level.In addition,the products obtained under high microwave loading level and system pressure would be not accepted by consumers.2.By studying the drying strategy of mushrooms(Agaricus bisporus)based on browning behaviors of button mushrooms during microwave freeze drying,it could be found that both whiteness and browning degree can be used to describe the browning kinetics of button mushrooms during MFD process.Both non-enzymatic browning and enzymatic browning took place during MFD button mushrooms,but effect of enzymatic browning was more significant,which should be controlled during MFD.In order to avoid enzymatic browning during MFD button mushrooms,a relatively low microwave power at initial stage of MFD should be adopted.Meanwhile,a low microwave power also should be used at the end stage of MFD so as to reduce nonenzymatic browning.3.By studying the drying strategy of mushrooms(Agaricus bisporus)based on porosity change behavior of button mushrooms during microwave freeze drying,it could be found that lots of open pores could transform to closed pores while the moisture content was below 20 ± 0.05% w.b.,and the closed-pore porosity was arrived at a relatively stable level at moisture content below 0.15 ± 0.03% w.b.Both total porosity and closed-pore porosity had a significant influence(P < 0.01)on texture of MFD mushroom,and open-pore porosity had a significant effect(P < 0.01)on rehydration ratio of MFD mushroom.According to the porosity change behavior of mushrooms,a dynamic microwave loading strategy can be used to reduce drying time and keep product quality during MFD process.4.By studying the drying strategy of mushrooms(Agaricus bisporus)based on non-volatile taste components of button mushrooms during microwave freeze drying,it could be found that microwave power has a significantly affects on the quality of materials.According to the change tendency of non-volatile taste components,a relatively high microwave power can be adopted at initial stage of MFD to obtain high contents of free amino acids and 5'-nucleotides,but if the loading time of high microwave power is too long,may result in more degradation of free amino acids.On the other hand a relatively low microwave power should be used to prevent the degradation of non-volatile taste components during processing.As a result,a step-down microwave power loading program during MFD process can reduce the drying time on the premise of keeping product quality.5.By comparing the comprehensive evaluation of different drying strategies,it could be found that the comprehensive evaluation of the drying strategy which based on porosity change behavior of button mushrooms during microwave freeze drying was the highest,and this drying strategy should be used in the button mushroom MFD process.