Preparation, purification, and antioxidant activities of low-molecular-weight polysaccharides from Dendrobium stem

Dendrobium is famous for its edibility and medicinal uses, with polysaccharides as active ingredients. Optimal fermentation conditions were obtained through single factor experiments combined with response surface methodology. The results showed that the extraction of CDP (Crude Dendrobium polysaccharides) was up to 5.84% with the inoculum 1.0%, sucrose dosage 0.9%, fermentation broth pH 5.0 of fermentation condition. A homogeneous polysaccharides component, designated as DOP-1, was successfully isolated and purified from Dendrobium officinale stem. The structural characterization of DOP-1 was conducted utilizing ultraviolet (UV) spectroscopy, Fourier transform infrared spectroscopy (FT-IR), high-performance liquid chromatography (HPLC), nuclear magnetic resonance spectroscopy (NMR), and scanning electron microscopy (SEM). Structure analysis revealed DOP-1 is mainly made up of mannose and glucose, with a molecular weight of 5526 Da. The structure of DOP-1 is composed of alternating or segmented D-mannose and D-glucose connected by β - (1→4), and carries monosaccharide side chains at the C2 positions of the residues. DOP-1 showed antioxidant capacity by scavenging DPPH, ABTS and •OH radicals in vitro antioxidant experiments, with IC 50 values of 5.43, 8.62 and 5.88 mg/mL respectively. During in vitro digestion test, no significant differences ( P > 0.05) were observed in the contents of total sugars and reducing sugars of DOP-1 before and after digestion, whereas its antioxidant activity dynamically increased initially and then remained stable. After digestion, the scavenging rates of DOP-1 against DPPH, ABTS, and •OH radicals were 73.18%, 79.55%, and 78.41%, respectively. The results showed that DOP-1 exhibited good gastrointestinal tolerance and potential for prebiotic development.