Effects of different pretreatment methods on the hot-air drying characteristics and quality of Dendrobium officinale stems

Abstract: Dendrobium officinale has been one of the most precious medicinal plants in China. The functions of antioxidant, antitumor, and immune can be attributed to the biologically active components in the plant, including polysaccharides, polyphenols, and dendrobine. The quality of Dendrobium officinale is prone to deteriorate during storage, due mainly to the stems rich in the water content. Therefore, the dried Dendrobium officinale stems can often inhibit the reproduction of spoilage microorganisms and chemical degradation. The present study aims to optimize the drying process for the high quality of dried Dendrobium officinale stems. A systematic investigation was also implemented to clarify the effect of directly cutting, blanching, and freeze-thaw cycles on the drying characteristics and quality of dried Dendrobium officinale stems. The water mobility and distribution were characterized using 1H Low-Field Nuclear Magnetic Resonance (LF-NMR) and Magnetic Resonance Imaging (MRI) techniques. The results indicated that the pretreatments played an important role in the drying process of Dendrobium officinale stems. Compared with the directly shearing, the blanching and freeze-thaw cycle treatments were beneficial to enhance the effective water diffusion coefficient for the less consumption of energy, and thereby speeding up the drying process. Especially, the freeze-thaw cycle treatment was more effective to improve the drying rate, where the drying time and energy consumption were reduced by 44.4% and 42.76% (P<0.05), respectively. The LF-NMR results showed that there were still three states of water in the stems of Dendrobium officinale. Free water accounted for the largest proportion, followed by the non-flowing water, and the bound water was the least. The MRI images indicated that the moisture on the surface of Dendrobium officinale stems was first removed in the drying process, and then the internal moisture migrated to the surface for evaporation. Scanning Electron Microscopy (SEM) images showed that the intercellular space of dehydrated Dendrobium officinale slices increased, and the cell wall was damaged after blanching and freeze-thaw pretreatments, leading to the changes in water fluidity and distribution for the accelerated migration and removal of water during drying. In the drying characteristics, the free water was removed first in the drying process. Both blanching and freeze-thaw cycle pretreatments were used to reduce the removal time of free water. The freeze-thaw cycle pretreatment took the shortest time (120 min) to remove the free water. It showed that the freeze-thaw cycle greatly contributed to weakening the binding of tissue structure to water for the better fluidity of water. The MRI found that the brightness area of Dendrobium officinale slices decreased with the extension of drying time, whereas, the red gradually decreased, indicating the decrease in water during drying. The freeze-thaw cycle pretreatment was led to the higher retention of polysaccharides and polyphenols in Dendrobium officinale stems, which increased by 10.66% and 12.32%, respectively, whereas, the blanching pretreatment was led to the lower retention of polysaccharides and polyphenols in Dendrobium officinale stems, which was reduced by 11.73% and 14.73%, respectively, compared with the directly shearing during drying. The three pretreatments had no significant effect on the content of dendrobine, which were 1.03, 1.01, and 1.02 mg/g, respectively (P>0.05). In a word, the freeze-thaw cycle pretreatment performed better to speed up the drying process for the high quality of dried Dendrobium officinale stems. The finding can provide the experimental reference to optimize the drying process of Dendrobium officinale stems, further reducing the risk of quality deterioration for the standardization of primary processing of Dendrobium officinale.