Fermentation productivity

The simplest term for productivity is the volumetric productivity, called q, and it is the amount of product formed per time per volume.

If one wants to compare the production rate of two different microorganisms, one would have to consider that the growth rate of the two organisms could be different. The specific productivity, called r, is the amount of product formed per time per g DW. It can be calculated as follows:

The specific productivity is both dependent on the yield (how much product is produced in relation to biomass) and how fast the cells are growing (how much biomass is generated). It is related to the volumetric productivity with the concentration of biomass.

The volumetric production is dependent of the biomass concentration. In batch fermentation the biomass concentration is growing, and so the productivity is not constant. If one want to make an estimate over the overall volumetric productivity it is calculated as the maximum product concentration, divided by the time to reach that concentration.

Often, there is a tradeoff in productivity and yield. Better yield makes the process cheaper (less substrate is needed to form same amount of product), but higher productivity means faster production and thereby smaller production facilities. As a rule of thumb, yield is most important for low-value products (e.g., bulk chemicals) and productivity is most important for high-value products (e.g., pharmaceuticals)

Abbreviations

x: concentration of biomass produced

DW: dry weight, a measurement of biomass

Y: yield factor

S_R: initial substrate concentration

s: residual substrate concentration

p: concentration of product

q_p: specific rate of product formation (mg product g^-1 biomass h^-1)

Y_xp: yield of product in terms of biomass (g product g^-1 biomass)