Method for extracting and purifying dendrobium officinale polysaccharides

A method for extracting and purifying Dendrobium officinale polysaccharides, comprising following steps:

• (1) taking Dendrobium officinale powder and fully dispersing it in pure water to obtain crude liquid;
• (2) removing insoluble impurities from the crude liquid obtained in the step (1) through a microfiltration membrane to obtain permeate 1 and retentate 1;
• (3) performing ultrafiltration treatment of the permeate 1 collected in the step (2) through a macroporous ultrafiltration membrane with a molecular weight cut-off of 100-800 kDa, and collect permeate 2 and retentate 2;
• (4) adding 0.5-1.5 mol/L of aqueous solution of edible alkali metal inorganic salt to the retentate 2 collected in the step (3), fully stirring and dissolving to obtain polysaccharide crude liquid, performing ultrafiltration treatment through a macroporous ultrafiltration membrane with a molecular weight cut-off of 100-800 kDa, and collecting permeate 3 and retentate 3;
• (5) combining the permeate 2 and permeate 3 obtained in the step (3) and step (4), adding the combined permeate into an electrodialysis device for desalination, and collecting dilute solution and concentrated solution;
• (6) performing ultrafiltration treatment of the dilute solution obtained in the step (5) through an ultrafiltration membrane with a molecular weight cut-off of 3 kDa-8 kDa and collecting retentate 4 and permeate 4;
• (7) carrying out freeze-drying of the retentate 4 obtained in the step (6) to obtain Dendrobium officinale polysaccharides.