ABSTRACT
ObjectiveThe hygroscopic properties of Mume Flos decoction pieces were studied from the perspectives of macroscopic［water activity（Aw）］ and microscopic（water molecular mobility）， which provided a theoretical basis for the determination of the safe storage moisture content. MethodAdsorption isotherm of Mume Flos decoction pieces was obtained by static weighing method， and seven common hygroscopic models were fitted and estimated. The best model was selected according to the principle that determination coefficient（R2） was closer to 1， residual sum of squares（RSS） was closer to 0 and Akaike information criterion（AIC） was smaller. According to the optimal model， the absolute and relative safe moisture contents of Mume Flos decoction pieces at 25， 35， 45 ℃ was calculated. Low-field nuclear magnetic resonance（LF-NMR） was used to measure the water molecular mobility in the hygroscopic process of Mume Flos decoction pieces. ResultThe best model to describe the adsorption isotherm of Mume Flos decoction pieces was the Peleg model. According to the model expression， the absolute safe moisture contents of Mume Flos decoction pieces at 25， 35， 45 ℃ were 9.59%， 7.96% and 7.68%， and the relative safe moisture contents were 13.05%， 11.99%， 11.77%， respectively. Mume Flos decoction pieces all contained two water states during the process of hygroscopic absorption at different temperatures， namely bound water T21 and free water T22. During the process of hygroscopic absorption， bound water had the largest increase in peak area. The sum of peak areas of the bound water and free water had a good linear relationship with the moisture contents， and the R2 were 0.959 9， 0.911 8 and 0.974 7 at 25， 35， 45 ℃， respectively. When Aw<0.57， T21 did not change， and the water molecular mobility remained unchanged. When Aw>0.57， T21 showed an increasing trend， and the water molecular mobility increased. The moisture contents of Mume Flos decoction pieces were 8.44%， 6.81% and 6.25% when the water molecular mobility increased at 25， 35， 45 ℃， respectively. ConclusionCombined with the theory of water activity and water molecular mobility， 6.25% is recommended as the safe storage moisture content of Mume Flos decoction pieces， this study can provide reference for determining the safe storage moisture content of other decoction pieces.
ABSTRACT
Objective To study the moisture transfer laws of Chaenomeles sinensis in different drying processes. Methods Using the non-destructive and non-invasive technique of low field-nuclear magnetic resonance (LF-NMR), the transverse relaxation time (T2) inversion spectrum of C. sinensis slice was monitored under different drying methods (hot air drying, drying after evaporation, segmental drying and drying in the shade) to analyze the changes of moisture migration. Results There were three different types water that were detected in C. sinensis (free water > bound water > immobilized water). The internal water distribution and water content changed during drying process. The moisture changes were similar in hot air drying, drying after steaming, and drying in shade, the total water gradually decreased, and the combining degree between moisture and non-water components enhanced. Steaming promoted the water loss rate of C. sinensis slice, the water loss rate was higher in drying after steaming than in hot air drying, and the difference was significant (P < 0.05). During the intermittent drying, the conversion of different states of water would occur in order to return to a relatively stable equilibrium. During the low temperature drying process, immobilized water content decreased and free water content increased. The low-temperature drying has less damage to the tissue, which is more conducive to the conversion of immobilized water into free water, and thus the water dissipated faster. During the early of drying, high temperature caused tissue structure damage, the bonding force between water and non-aqueous tissue would be strengthened because of the tissue shrinkage. Conclusion The three different types water content and peak area in T2 was positively correlated. The LF-NMR technique would provide useful guides for the investigation of water distribution and variation of C. sinensis, which will provide a theoretical basis for C. sinensis processing.