Study On The Quality And Characterization Of Moisture Transfer From Tricholoma Matsutake During Drying Process

This study was performed to compare the characteristics of mushroom(Tricholoma matsutake)by three drying methods,hot-air drying(HAD),vacuum freeze drying(VFD)and microwave vacuum drying(MVD)in the aspect of color,nutrients,animo acids,volatile compounds,polyphenol content and antioxidant capacities during the drying process.The moisture transformation and distribution of three groups(Tricholoma matsutake,pileus and stipe)were measured by low filed nuclear magnetic resonance(LF-NMR)and magnetic resonance imaging(MRI)technology during the whole drying processes to explor the key factors of improving the quality.Meanwhile,moisture transformation profiles in Tricholoma matsutake by LF-NMR and MRI technology during the rehydrating process was measured to analyze the relationship between rehydration and dehydration.All the findings provided great insights in preserving the quality of Tricholoma matsutake during the freeze-thaw treatment,hot-air dried process.The main conclusions of this paper were as follows:(1)The VFD group presented the lowest browning degree.The content of protein,total sugar and fat of Tricholoma matsutake were increased during the drying process.MVD group had the highest amino acid contents,HAD group had more volatile compounds,and the polyphenol content was significantly increased.There were differences in polyphenol content,composition,and antioxidant activities among different drying methods.HAD group got the highest content of polyphenol,VFD group got the lowest content of polyphenol.The major compositions of polyphenol in Tricholoma matsutake were gallic acid,protocatechuic acid,vanillin,cinanmic acid and p-hydroxybmzoic acid.Compared with the antioxidant capacities,the DPPH-.radical scavenging capacity:VFD group>HAD group>MVD group.No significant differences were found in ABTS+ radical scavenging capacities by polyphenols among the groups.The reducing power:HAD group>VFD group>MVD group.OARC values:MVD group>HAD group>VFD group.(2)There were three categories of moisture including free,immobilized,and bound water presented in Tricholoma matsutake.Free and immobilized water were active while bound water was stable during hot-air drying process.At the beginning of drying processes,the initial drying air temperature played an important role in controlling the speed of moisture removal.Later,internal moisture diffusion was the key regulatory parameter,under conditions in which most of the free moisture was lost,immobilized moisture could also become more mobile and undergo considerable loss.MRI results showed that moisture distribution was non-uniform during the hot-air drying process.In the early stage of hot-air drying,the signal intensity attenuated quickly;in the intermediate drying stage,moisture dehydration became difficult due to shrinkage of the surface.Intracellular moisture exhibited a lower migration rate than surface moisture.Therefore,accelerating the transformation of immobilized moisture to free moisture was one of the key factors in shortening the drying time.(3)During the rehydration process,the moisture penetrated into pileus,spreaded from periphery of the pileus to the center of the pileus and gradually transferred to stipe.Dried Tircholama matsutake had no ability to hold free water,and immobiliazed water was the main existence form during this period.Bound water was stable that not influenced by environmental factors.Hence,the key to improve rehydaration capacity is to promote the penetration of immobilized water.(4)The freeze-thaw treatment resulted in the moisture transfer of Tricholoma matsutake.After five cycles of freezing and thawing,the water loss in stipe was more serious than that of pileus,and there was no free water in some areas of stipe.With more cycles of the freeze-thaw,the content of free and immobilizede water was decreased,while,bound water content was increased and then decreased;the thawing loss rate increased,L values(lightness),a*(redness)and b*(yellowness)values decreased,hardness decreased first and then increased.The correlation analysis showed that the times of freeze-thaw had a significant correlation(p<0.01)with T22,T23,a*(redness),b*(yellowness)and thawing loss rates.The T22 and T22 had a significant correlation(p<0.05)with thawing loss rates,a*(redness)and b*(yellowness)values.Freeze-thaw treatment caused the L values decreased first and then increased;hardness increased first and then decreased of hot-air dried Tricholoma matsutake.Among them,the quality of hot-air dried Tricholoma matsutake was worst after 4th freeze-thaw treatment.