Production of exopolysaccharides by submerged mycelial culture of a mushroom Tremella fuciformis

The optimization of submerged culture conditions for mycelial growth and exopolysaccharide (EPS) production in an edible mushroom Tremella fuciformis was studied in shake flasks and bioreactors. The temperature of 28 °C and pH 8 in the beginning of fermentation in agitated flasks was the most efficient condition to obtain maximum mycelial biomass and EPS. The optimal medium constituents were as follows (g L−1): glucose 20, tryptone 2, KH2PO4 0.46, K2HPO4 1 and MgSO4·7H2O 0.5. The fungus was cultivated under various agitation and aeration conditions in a 5 L stirred-tank bioreactor. The maximum cell mass and EPS production were obtained at a relatively high agitation speed of 200 rpm and at an aeration rate of 2 vvm. The flow behavior of the fermentation broth was Newtonian and the maximum apparent viscosity (35 cP) was observed at a highly aerated condition (2 vvm). The EPS productivity in an airlift reactor was higher than that in the stirred-tank reactor. The morphological study revealed that the fungus grows in mainly three different yeast-like forms: ovoid, elongated, and double yeast forms. The high population of the elongated yeast has a very close relationship to high EPS production. The EPS were protein-bound polysaccharides consisted of mainly mannose, xylose, and fucose. The molecular weights of EPS were determined to be (1.3–1.5) × 106.