Maximizing the amount of product formed during cultivation
processes of a given size means simultaneously maximizing, all
time, the product of biomass x and specific product formation rate
. Consequently, high cell density cultures with well performing
cells, i.e. those with a high specific formation rate with respect to
the desired product, are requested.
During most aerobic fermentations the mass transfer requirements
with respect to oxygen, supplied by dispersing air in the
culture in form of bubbles, is an important constraint to high cell
density operation. As long as the biomass concentration is low,
mass transfer does not make severe problems. At some critical
cell concentration, however, oxygen can no longer be supplied
fast enough to meet the oxygen demand. Thereafter oxygen limits
productformation and cell growth.Many examples of practical limitations
on oxygen transfer rates are discussed in the biochemical
engineering literature