Variation Rules Of Flavor Components And Formation Mechanism Of Volatile Sulfur-Containing Compounds In Lentinula Edodes During Vacuum Freeze-Drying

Lentinula edodes(L.edodes),being the edible fungus with the second-largest yield worldwide,not only tastes delicious but also has great edible and medicinal values.However,fresh L.edodes has a high moisture content,which makes it difficult to preserve L.edodes after harvest.L.edodes can be preserved for a long term after drying,but the quality of dried products can be directly influenced by the changes in nutrition and flavor during the drying process.Volatile sulfur-containing compounds(VSCs)are the most characteristic flavor components in dried L.edodes and are also key components in the aroma of L.edodes.Accordingly,it is very essential for quality control of products to study the change rules and mechanism of flavor components in L.edodes during the drying process.In this work,headspace solid-phase microextraction(HS-SPME)method of vacuum freeze-dried L.edodes was optimized,and then the variation rules of flavor components and the formation mechanism of VSCs in L.edodes during vacuum freeze-drying(VFD)were investigated.These results will provide theoretical support for the optimization of the VFD process of L.edodes and guide the development of flavor products based on L.edodes.The contents and results of this work are as follows:1.The effects of different SPME fibers,sample pretreatments,extractive temperatures,equilibrium times,and sorption times on the determination of volatile compounds in vacuum freeze-dried L.edodes were investigated by gas chromatography-mass spectrometry(GCMS).SPME fiber of 50/30 μm DVB/CAR/PDMS,sample pretreatment of adding 2 m L deionized water to 0.5 g of L.edodes powder,the extractive temperature of 50 °C,equilibration time of 30 min and sorption time of 50 min were determined as the optimal HSSPME conditions for the analysis of flavor components in VFD L.edodes.2.The variation rules of volatile compounds in fresh L.edodes during vacuum freezedrying which covers the stages of pre-freezing,primary drying,and secondary drying were investigated by electronic nose(E-nose)and GC-MS.Results showed that there were different characteristic flavor components in L.edodes at different VFD stages.Alcohols,aldehydes,and VSCs were the main flavor constituents of fresh L.edodes,pre-frozen L.edodes,and secondary dried L.edodes.Aldehydes,ketones,and VSCs were the main aroma substances in L.edodes after primary drying.Esters were no longer the key odorants of L.edodes after secondary drying.Compared with fresh L.edodes,the odors of L.edodes changed obviously at the stage of secondary drying,and these changes were affected by the temperature of secondary drying: the total content and quantity of volatile compounds significantly decreased(P < 0.05)after the secondary drying at 20 °C;the secondary drying process between 30～50 °C facilitated the formation of alcohols in L.edodes;the secondary drying process at 50 °C had no effect on increasing ketones in dried L.edodes,while was good for the formation of VSCs.3.The variation rules of taste components in fresh L.edodes at different stages of VFD process were investigated by electronic tongue(E-tongue),automatic amino acid analyzer,and high performance liquid chromatography.Results showed that the contents of soluble sugars/mannitol and 5’-nucleotides in L.edodes significantly increased after pre-freezing(P< 0.05);after primary drying,the contents of soluble sugars/mannitol and free amino acids(FAAs)in L.edodes reached the maximum of the whole VFD process,which were 185.20mg/g and 44.76 mg/g,respectively;during secondary drying,the content of organic acids increased and was not affected by the temperature,while the contents of soluble sugars/mannitol and FAAs showed different decreasing trends with the changes in temperatures.The umami intensity of L.edodes greatly increased after the secondary drying of 30 °C,40 °C,and 50 °C,whose equivalent umami concentration(EUC)values were about twice as much as the fresh sample.4.The changes in the activities of two key enzymes of L.edodes related to VSCs formation including the γ-glutamyltranspeptidase(γ-GGT)and the S-alkyl-L-cysteine sulfoxidelyase(C-Slyase)were monitored during vacuum freeze-drying.Meanwhile,the changes in VSCs and Maillard reaction substrates of raw L.edodes and enzyme-inhibitory L.edodes were compared during vacuum freeze-drying.Thus,the formation mechanism of VSCs in L.edodes during vacuum freeze-drying was investigated.According to the results,the formation of VSCs in L.edodes under the VFD process was mainly related to enzymatic reaction and the secondary drying stage of 50 °C was beneficial to the formation of VSCs in L.edodes,especially the formation of dimethyl disulfide.In addition,the content of 1,2,4-trithiolane,2,3,5-trithiahexane,and 1,2,4,6-tetrathiepane in dried products increased by a large margin compared with fresh L.edodes.The correlation analysis indicated that the γ-GGT and C-Slyase in L.edodes were closely related to the synthesis of various VSCs in the VFD process,which further proved the outstanding contribution of enzymatic reaction to the formation of VSCs in L.edodes during vacuum freeze-drying.