The Effect Of Combined Drying On Water Holding Capacity Of Rehydrated Shiitake Mushroom And Process Optimization

Combined drying is the technology that combines two or more drying methods,which could shorten the drying time and save energy.Shiitake mushroom(Lentinula edodes)is rich in nutrients with high nutritional values.Compared with other fruits and vegetables,shiitake mushroom is unique in its porous structure.Generally,dried shiitake mushrooms need to be rehydrated before consumption.The porous structure is not conducive for keeping the water holding capacity(WHC)of rehydrated shiitake mushrooms,which could lead to the poor sensory experience.In this study,shiitake mushrooms were subjected to hot air drying,vacuum freeze drying and vacuum freeze-hot air combined drying.Nuclear Magnetic Resonance(NMR)and microscopic imaging technology(TEM)were utilized to investigate the structural changes under different drying conditions in order to explore the relationship between the structural changes and WHC.Then the process was further optimized in order to obtain the high-quality shiitake mushrooms with better WHC and flavor.The results provide a theoretical basis for optimizing the drying process of shiitake mushrooms.The main conclusion of the thesis is listed as follows:(1)Effects of hot air drying with different temperature on WHC of rehydrated shiitake mushrooms was analyzed from the perspectives of cell wall,cell membrane and protein.The results have shown that the cell membrane was not the main factor for WHC variation of rehydrated mushrooms as the integrity of cell membrane for all shiitake mushrooms was lost by dehydration.Protein denaturation played the most important role as the protein denaturation was largely correlated with the stiffness of the cell wall,thereby affecting the porous structure.The state of cell wall fibrous material had a relatively weak effect on WHC changes,which was mainly related to the WHC changes at high drying temperature(>80 ?).(2)The rehydration property and WHC of rehydrated hot air drying and vacuum freeze-dried samples were compared from perspective of pores and fiber matrix.The free water content of all rehydrated freeze-dried products was comparable with the ones of 40 ?-50 ? dried and rehydrated samples.The results of the pore size distribution of dried products,indicated the significant collapse of the internal structure of dried shiitake mushrooms.The structure collapse could be considered as a combined result of the loss of cell membrane integrity,mechanical destruction of cell wall by ice crystals,and protein denaturation.The hydration characteristics of the cell wall fibrous material have shown that the hydration capacity of the fiber matrix was significantly increased after freeze-drying,which was mainly caused by the uneven surface induced by the ice crystals formation.(3)Vacuum freezing-hot air combined drying was carried out for processing shiitake mushroom.The moisture transition point for combined drying was set as 2 g/g db.The structure of the dried product was relatively fixed at this moisture content and change of structural rigidity in the later stage of the combined drying could be limited.It was beneficial to the maintenance of the porous structure of shiitake mushrooms.Compared with the FD and FD-HAD(40 ?)samples,the color of the FD-HAD(80 ?)sample was significantly changed.FD-HAD(80 ?)also enhanced the volatile substances of the shiitake mushrooms.In addition,FD-HAD(80 °C)had a great influence on the internal structure of shiitake mushrooms.However,since the structure of shiitake mushrooms remained stable at low moisture content,the structural changes induced by the high drying temperature can be reflected by the structural changes during and after rehydration.The post-high temperature drying was also beneficial to the improvement of the WHC of rehydrated freeze-dried shiitake mushrooms.