Inhibition by secondary fermentation products may limit the ultimate productivity of new glucose to ethanol fermentation processes. New processes are under development whereby ethanol is selectively removed from the fermenting broth to eliminate ethanol inhibition effects. These processes can concentrate minor secondary products to the point where they become toxic to the yeast. Vacuum fermentation selectively concentrates nonvolatile products in the fermentation broth. Membrane fermentation systems may concentrate large molecules which are sterically blocked from membrane transport. Extractive fermentation systems, employing nonpolar solvents, may concentrate small organic acids. By‐product production rates and inhibition levels in continuous fermentation with Saccharomyces cerevisiae have been determined for acetaldehyde, glycerol, formic, lactic, and acetic acids, 1‐propanol, 2‐methyl‐1‐butanol, and 2,3‐butanediol to assess the potential effects of these by‐products on new fermentation processes. Mechanisms are proposed for the various inhibition effects observed.

Productivity in a CHO perfusion culture reactor was maximized when pCO(2) was maintained in the range of 30-76 mm Hg. Higher levels of pCO(2) (> 150 mm Hg) resulted in CHO cell growth inhibition and dramatic reduction in productivity. We measured the oxygen utilization and CO(2) production rates for CHO cells in perfusion culture at 5.55×10(-17) mol cell(-1) sec(-1) and 5.36×10(-17) mol cell(-1) sec(-1) respectively. A simple method to directly measure the mass transfer coefficients for oxygen and carbon dioxide was also developed. For a 500 L bioreactor using pure oxygen sparge at 0.002 VVM from a microporous frit sparger, the overall apparent transfer rates (k(L)a+k(A)A) for oxygen and carbon dioxide were 0.07264 min(-1) and 0.002962 min(-1) respectively. Thus, while a very low flow rate of pure oxygen microbubbles would be adequate to meet oxygen supply requirements for up to 2.1×10(7) cells/mL, the low CO(2) removal efficiency would limit culture density to only 2.4×10(6) cells/mL. An additional model was developed to predict the effect of bubble size on oxygen and CO(2) transfer rates. If pure oxygen is used in both the headspace and sparge, then the sparging rate can be minimized by the use of bubbles in the size range of 2-3 mm. For bubbles in this size range, the ratio of oxygen supply to carbon dioxide removal rates is matched to the ratio of metabolic oxygen utilization and carbon dioxide generation rates. Using this strategy in the 500 L reactor, we predict that dissolved oxygen and CO(2) levels can be maintained in the range to support maximum productivity (40% DO, 76 mm Hg pCO(2)) for a culture at 10(7) cells/mL, and with a minimum sparge rate of 0.006 vessel volumes per minute.A = volumetric agitated gas-liquid interfacial area at the top of the liquid, 1/mB = cell broth bleeding rate from the vessel, L/minCER = carbon dioxide evolution rate in the bioreactor, mol/min[CO(2)] = dissolved CO(2) concentration in liquid, M[CO(2)](*) = CO(2) concentration in equilibrium with sparger gas, M[CO(2)](**) = CO(2) concentration in equilibrium with headspace gas, MCO(2)(1) = dissolved carbon dioxide molecule in water[C(T)] = total carbonic species concentration in bioreactor medium, M[C(T)](F) = total carbonic species concentration in feed medium, MD = bioreactor diameter, mD(I) = impeller diameter, mD(b) = the initial delivered bubble diameter, mF = fresh medium feeding rate, L/minH(L) = liquid height in the vessel, mk(A) = carbon dioxide transfer coefficient at liquid surface, m/mink (infA) (supO) = oxygen transfer coefficient at liquid surface, m/min.

No abstract

Laminar shear is the primary mechanism of cell damage, limiting flow rate (and hence flux) in crossflow microfiltration of animal cells. Sensitivity to hydrodynamic and interfacial stress is reduced by the addition of 0.1% Pluronic polyol. A critical average wall shear rate of 3000 s(-1) (above which damage occurs) is found for several cell types, including mammalian and insect cells. Hydrodynamic stress also limits the maximum tip speed in a rotary lobe pump to less than 350 cm/s. Turbulent flow in the recirculation loop piping at Reynolds numbers of up to 71,000 does not cause cell damage. Maximum sustainable flux decreases with cell concentration and increases with cell size (in qualitative agreement with the hydrodynamic lift model). A flux of 30 to 75 L/m(2) h (depending on cell size) can be sustained during 20-fold concentration from 2.5 x 10(6) cells/ml, while maintaining high cell viability.