Effects of different drying methods on the quality characteristics and rehydration performance of Morchella esculenta

Morchella esculenta was prone to decay and deterioration after harvesting and they were mainly circulated in the market as dried products. The choice of different drying methods was related to the quality characteristics of dried and rehydration products of Morchella esculenta, mainly including sensory and nutritional characteristics. The color, aroma, taste, type, and qualities of Morchella esculenta were mainly evaluated through five dimensions. In this study, Morchella sextelata (Mel-6) was used to explore the effect of different drying methods and rehydration treatments on its quality changes. Three strategies including natural drying (ND), constant temperature drying (CTD), and vacuum freeze-drying (VFD) were used to study their effects on the microstructure, taste, nutritional qualities, and rehydration performance of Morchella esculenta. The differences in texture, color, and flavor before and after rehydration, and their drying characteristics were analyzed. The results showed that the amino acids, crude polysaccharide, and sweetness of Morchella esculenta under VFD treatment were 944 mg/100 g、6.26 g/100 g、19.73 respectively, significantly higher (P<0.05) than the other two drying methods. The brightness of the stipe and cap of Morchella esculenta under VFD treatment was significantly improved (P<0.05) with less color difference, the value of △E was 5.83. Moreover, the volatile profiles analysis of Morchella esculenta before and after drying and rehydration treatment suggested a clear difference with distinguishable flavor components, according to the principal component analysis (PCA) of electronic nose data. The VFD-treated Morchella esculenta maintained the optimal rehydration ratio due to its porous microstructure with uniform pores distribution. The low-field nuclear magnetic resonance(LF-NMR) combined with magnetic resonance imaging (MRI) showed that the immobilized water of Morchella esculenta during the rehydration process was increased continuously with faster rehydration speed within 20 minutes and the best rehydration uniformity under VFD treatment. The color difference (△E) of the stipe and cap after VFD rehydration were 12.28 and 11.47 respectively, they changed less significantly (P<0.05) compared to the other two drying methods. The brittleness of the stipe and cap of Morchella esculenta after VFD rehydration were 35.87N/mm and 10.42N/mm, they remained similar with CTD. The distribution of sensory and nutritional variables in samples with different drying methods was intuitively demonstrated by the results of cluster and correlation analysis, The high-quality characteristics were maintained with vacuum freeze-drying technology during the drying and rehydration process of Morchella esculenta, further demonstrating the feasibility of vacuum freeze-drying in the drying of Morchella esculenta. These results would provide solid guidelines for the optimal processing method of dried Morchella esculenta products.