Effects Of Different Pretreatment On Hot Air Drying Process And Quality Of Dendrobium

Dendrobium officinale which belongs to Dendrobium orchidaceae is a rare Chinese herbal medicine.Since Dendrobium officinale stems is rich in water content,which is prone to sprouting and mildew resulting in the decline of their quality during storage.Therefore,it is important to adopt appropriate method to dry Dendrobium officinale stems after harvesting.The characteristics and quality of Dendrobium officinale stems in drying process were studied by hot-air drying.The effects of different pretreatment(shearing,freezing-thawing,blanching)on the drying process and quality of dried Dendrobium officinale stems were investigated.The changes in water mobility and water distribution were characterized using low-field nuclear magnetic resonance(LF-NMR)and magnetic resonance imaging(MRI)techniques.The changes of the quality of dried Dendrobium officinale stems were analyzed by rehydration,shrinkage,activities of components,antioxidant and polysaccharide dissolution in Dendrobium officinale stems.The main results are as follows:1)In the hot air drying process,with the increase in temperature,the drying rate of Dendrobium officinale stems accelerated,the drying time was shortened,and the effective water diffusion coefficient increased.At different drying temperatures,Logarithmic model can basically reflect the drying process of Dendrobium officinale stems.With the increase in temperature,the contents of polysaccharides and polyphenols in Dendrobium officinale stems increased and reached the maximum value at90 ℃ and then reduced.The content of dendrobine in Dendrobium officinale stems is not affected by temperature.The optimum temperature for drying Dendrobium officinale stems was 90 ℃.2)Compared with shearing,after blanching and freezing-thawing pretreatment,the effective diffusion coefficient of water increased,the activation energy of drying decreased,and the drying time was shortened.Especially the freezing-thawing treatment,which shortened the drying time by 44.4%,reduced the energy consumption by 42.76%.The Weibull model could describe the drying process of Dendrobium officinale stems.Low field NMR analysis showed that there were three states of water in Dendrobium officinale stems,the free water content was the most,the nonflowing water content was the second,and the bound water content was the least.In the drying characteristics,the free water was removed first in the drying process.Both blanching and freeze-thaw cycle pretreatments reduced the removal time of free water.The freeze-thaw cycle pretreatment took the shortest time(120 min)to remove the free water,showing that the freeze-thaw cycle was more conductive to weakening the binding of tissue structure to water for the better fluidity of water.Scanning electron microscopy images showed that the intercellular space of dehydrated Dendrobium officinale slices increased,and the cell wall was damaged after blanching and freeze-thaw pretreatments,which led to the changes of water fluidity and distribution,and accelerated the migration and removal of water during drying.3)Compared with directly shearing,the rehydration of Dendrobium officinale stems after blanching and freezing-thawing pretreatment was improved,and the shrinkage of Dendrobium officinale stems after freezing-thawing pretreatment was the lowest.Compared to the directly shearing,freeze-thaw cycle pretreatment led to higher retention of polysaccharides and polyphenols in Dendrobium officinale stems,while blanching pretreatment led to lower retention of polysaccharides and polyphenols in Dendrobium officinale stems in the drying process.The three pretreatment methods had no significant effect on the content of dendrobine in Dendrobium officinale stems.In the same antioxidant system,the antioxidant activities of the crude extracts from Dendrobium officinale stems after different pretreatment methods were significantly different.The scavenging activities to DPPH,ABTS and hydroxyl radical of crude extract from Dendrobium officinale stems after freezing-thawing pretreatment was the best.Compared with directly sheering and blanching,the polysaccharides were more easily dissolved in Dendrobium officinale stems after freezing-thawing pretreatment.In conclusion,freezing-thawing pretreatment was more effective in shortening the drying time and maintaining the quality of Dendrobium officinale stems.The results could provide reference for further optimizing the drying process of Dendrobium officinale,reducing the risk of quality deterioration,and promoting the standardization of postnatal processing of Dendrobium officinale.