The on-line direct measurement of cell mass concentration by using optical density principles promises to dramatically improve the knowledge of the metabolic processes underway within a bioreactor. This measurement is most effective on spherical cells such as E. Coli. The measurement technology is packaged in a sterilizable stainless steel probe which is inserted directly into the bioreactor itself via a flange or quick-disconnect mounting (Fig. 1).
By comparing the mass over time, cell growth rate can be determined. This measurement can be used in conjunction with metabolic models which employ such physiological parameters as oxygen uptake rate (OUR), carbon dioxide evolution rate (CER) and respiratory quotient (RQ) along with direct measurements such as dissolved oxygen concentration, pH, temperature, and offgas analysis to more precisely control nutrient addition, aeration rate and agitation. Harvest time can be directly determined as can shifts in metabolic pathways possibly indicating the production of an undesirable by-product.
Cell mass concentrations of up to 100 grams per liter are directly measured using the optical density probe. In this probe, light of a specific wavelength, created by laser diode or passing normal light through a sapphire crystal, enters a sample chamber containing a representative sample of the bioreactor broth and then passes to optical detection electronics. The density is determined by measuring the amount of light absorbed, compensating for backscatter. Commercial versions such as those manufactured by Cerex, Wedgewood, and Monitec are packaged as stainless steel probes that can be mounted directly into bioreactors ten liters or greater, and offer features such as sample debubblers to eliminate interference from entrained air.
Another technique used to determine cell density is spectrophotometric titration which is a laboratory procedure which employs the same basic principles as the probes discussed above. This requires a sample to be withdrawn from the broth during reaction and therefore exposes the batch to contamination.
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