Practice question

Industrial fermentation aims to maximize volumetric productivity of biomass, recombinant proteins or primary metabolites in minimal time with reproducible quality. Log phase cells are physiologically most uniform population with high ribosome content, active glycolysis and TCA cycle, abundant ATP and NADPH and low stringent response. Enzyme systems for substrate uptake like PTS and product synthesis operate at peak specific activity providing fastest conversion per cell per hour. Transferring mid-exponential inoculum into production vessel shortens nonproductive lag phase improving batch turnaround and consistency. Lag cells are still adapting and synthesize transporters, stationary cells undergo sigmaS mediated stringent response shifting to maintenance and secondary metabolism which while useful for antibiotic production not ideal for growth-associated products where synthesis coupled to growth, death phase suffers lysis releasing proteases that degrade product. Therefore optimal harvest and inoculum selection for probiotics, starter cultures and recombinant protein processes coincides with mid to late exponential phase where viability and catalytic productivity peak together giving maximum yield and process efficiency.