Submerged culture conditions for the production of mycelial biomass and exopolysaccharides by the edible Basidiomycete Grifola frondosa

Optimization of submerged culture conditions for mycelial biomass and exopolysaccharide (EPS) production by Grifola frondosa was studied. The optimal temperature and initial pH for both mycelial growth and EPS production in shake flask cultures were 25 °C and 5.5, respectively. Glucose (30 g/l) was the most suitable carbon source for both mycelial biomass and EPS production. Yeast extract and polypeptone were favorable nitrogen sources for both mycelial biomass and EPS production. To find the optimal operational parameters in a 5 l stirred-tank fermenter, the self-directing optimization technique was used and the results were as follows: culture pH, 5.06; aeration rate, 1.16 vvm; agitation rate, 166 rpm. In the course of mycelial submerged culture, the mycelial morphology was significantly altered by culture pH, aeration rate, and hydrodynamic behavior, which subsequently affected the yield of EPS production. While compact pellets were formed at low aeration conditions, freely suspended mycelial growth was observed at high aeration conditions. The most desirable morphology for EPS production was a feather-like mycelial clump. Under optimal culture conditions, maximum biomass concentration and EPS production in a stirred-tank fermenter were 16.8 and 5.3 g/l, respectively, which were significantly greater than results prior to optimization (13 and 4 g/l, respectively). For a comparative study, the fungus was further cultured in a 5 l airlift fermenter, but mycelial biomass yields (10 g/l) and EPS yields (4.53 g/l) were lower than those in the stirred-tank fermenter. Eventually, three groups of polysaccharides of diverse molecular mass (470–1100 kDa) were produced either from the mycelial extract or from the culture filtrate precipitate.