Extraction and purification of Dendrobium Officinale polysaccharides

The extraction and purification of DOP were conducted according to Wang et al. with slight modifications (15). Briefly, dry powder of the D. officinale herbal (250 g) was suspended in 95% ethanol (250 ml) in a beaker with constant stirring for 24 h. The suspension was subjected to two more cycles of 24-h soaking and subsequent centrifugation at 10,000 g and 25°C for 20 min. Then, the residue was extracted with water (1:20 w/v) stirred at 70°C for 4 h followed by centrifugation at 10,000 g and 25°C for 20 min. The obtained supernatants were concentrated (to 1/4 of the original volume) using a rotary evaporator, and then ethanol precipitated (1:3 ratio, v/v) at room temperature to accumulate the crude polysaccharide. Subsequently, thermostable α-amylase (3,000 units/ml) was added to 1% polysaccharide solution, stirred at 70°C for 2 h, and then cooled to room temperature. The small molecular weight contaminants produced by the hydrolysis were removed by dialysis against deionized water (with 3,500 Da cut-off) for 72 h. The solution was further freeze-dried to obtain a dry sample of purified polysaccharide (DOP).