Burst agitation rate promotes sustained semicontinuous cultivation of filamentous fungi in stirred tank reactors

Edible filamentous fungi L. edodes (shiitake mushrooms) were cultivated in submerged fermentation in stirred tank bioreactors (STR) both in batch and semicontinuous cultivation in a corn steep liquor (CSL) medium. The adjustment of a combination of constant impeller agitation speed, a short duration of a high-speed agitation (burst), and the frequency of bursts improved biomass (cell dry weight (CDW) titre from 1.75 to 4.95 g/L in a 96-h batch cultivation. These bioreactor process conditions were applied to a semicontinuous culture strategy to produce similar biomass density at a dilution rate of 0.02 h^-1 for up to 10 days without washout over the duration of the fermentation. An increase in the dilution rate above 0.02 h^-1 resulted in washout of L. edodes over time. Using a richer growth medium through the addition of malt extract, peptone, and molasses allowed L. edodes to grow to 4.7 g/L at a dilution rate of 0.025 h^-1 without washout. The maximum biomass productivity (396 mg CDW/h) of the semicontinuous cultivation (D = 0.02 h^-1) was 1.9-fold higher than the batch cultivation 206 mg CDW/hour. Use of the richer growth medium at D = 0.025 h^-1 improved biomass productivity further to 470 mg/h. Glucans, known bioactives, were present in the fungal biomass at a maximum of 14% of the cell dry weight (CDW) with b-glucans predominating over a-glucans. KEY POINTS: • Short, high-speed impeller bursts homogenise mycelia preventing bioreactor clogging. • Semicontinuous fermentation at D of 0.02 h-1 generated 5.1 g/L of fungal biomass. • Cultivation of fungal biomass in STR resulted in biomass with 14% of total glucans.