Search

Systematic errors due to the neglect of water and/or ethanol partition between liq. and gaseous phases are discussed for bioreactors equipped with or without a condenser. Both water vapor and ethanol vapor are present in the off-gas leaving the condenser. Presence of residual water vapor largely influences the gas measurements by diln. As a consequence, the oxygen consumption rate can be overestimated by a factor of 3 if calcns. are not cor. for water vapor content or if no addnl. device is implemented.after the condenser to completely dry the off-gases. The mass balance and partition equations predict that the condenser has only a small effect on redn. of the ethanol vapor content of the off-gas. The reason is the high ethanol concn. of the condensate droplets on the condenser wall in contact with the off-gases. Model predictions as well as exptl. results show that ethanol evapn. represents a large fraction of the ethanol prodn. rate and influences greatly the elemental recoveries. For a reactor working at 30 DegC without condensation of the vapors and for a volumetric aeration rate of 0.63vvm, stripping of ethanol resulted in a gaseous diln. rate of 0.016 h-1 for ethanol. The diln. rate by stripping was reduced to 0.014 h-1 when a condenser at 12 DegC was implemented. The fraction of ethanol that is stripped is mainly dependent on the ratio D/vvm (liq. to gaseous flow rates), and the effect is only slightly influenced by low condenser temp. The evapn. of ethanol may account for more than 20% of the ethanol formation rate. Therefore, the condenser does not succeed to reflux all ethanol to the reactor broth. In terms of a unit operation, ethanol vapor can be efficiently reduced by absorption instead of condensation. To demonstrate the feasibility, a simple modification of the reactor was tested for continuous cultures: the feed port was changed from the top-plate to the top of the condenser, which was used as an absorption column. Ethanol stripping was reduced by a factor of 4 as compared to the condensation setup (at 12 DegC): it accounted for 2% of the ethanol prodn. rate as compared to 8.2% at D = 0.19 h-1 and 0.63vvm. [on SciFinder (R)]

A review with 29 refs. of lab.-scale investigations carried out on the usefulness of biol. heat-release measurements, as a means for monitoring and controlling the metabolic state of microbial cultures. Such studies are carried out in high-quality bench-scale calorimeters, but measuring heat generation rates by establishing energy balances ought to be applicable to large-scale bioreactors without resorting to sophisticated instrumentation. The signal received can either be interpreted by more qual. correlation with the evolution of the culture, or it may be quant. exploited -together with other online measurements - in order to assess the rates at which various types of metabs. proceed in the culture. The work described shows how this can be used to keep a culture in a desired metabolic state during fed-batch and transient continuous cultures of the yeast, S. cerevisae, and how a bacterial fed-batch culture can be controlled in order to optimize biosynthesis of an antibiotic. [on SciFinder (R)]

The influence of substrate compn. on the yield, nature, and compn. of exopolysaccharides (EPS) produced by the food-grade strain G. xylinus I-2281 was investigated during controlled cultivations on mixed substrates contg. acetate and either glucose, sucrose, or fructose. Enzymic activity anal. and acid hydrolysis revealed that 2 EPS, gluconacetan and levan, were produced by G. xylinus. In contrast to other acetic acid strains, no exocellulose formation has been measured. Considerable differences in metabolite yields have been obsd. with regard to the carbohydrate source. It was shown that glucose was inadequate for EPS prodn., since most of this substrate (0.84 C-mol/C-mol) was oxidized into gluconic acid, 2-ketogluconic acid, and 5-ketogluconic acid. In contrast, sucrose and fructose supported a 0.35 C-mol/C-mol gluconacetan yield. In addn., growing G. xylinus on sucrose produced a 0.07 C-mol/C-mol levan yield. The compn. of EPS remained unchanged during the course of the fermns. Levan sucrase activity was found to be mainly membrane assocd. In addn. to levan prodn., an anal. of levan sucrase's activity also explained the formation of glucose oxides during fermn. on sucrose through the release of glucose. The biosynthetic pathway of gluconacetan synthesis has also been explored. Although the activity of key enzymes showed large differences to be a function of the C source, the ratio of their activities remained similar from one C source to another and corresponded to the ratio of precursor needs as deduced from the gluconacetan compn. [on SciFinder (R)]

The dynamic behavior of a continuous culture of Saccharomyces cerevisiae subjected to a sudden increase in the diln. rate has been successfully modelled for anaerobic growth on glucose, and for aerobic growth on acetate, on ethanol, and on glucose. The catabolism responded by an immediate jump whereas biosynthesis did not. Thus catabolism was in excess to anabolism. The model considers the decoupling between biosynthesis and catabolism, both types of reactions being modelled by first-order kinetic expressions evolving towards maximal values. Yield parameters and maximal reaction rates were identified in steady state continuous cultures or during batch expts. Only the time const. of biosynthesis regeneration, Tx, and the time const. of catabolic capacity regeneration, tx, had to be identified during transient expts. In most expts. tx was around 3 h, and tcat varied between 2 and 2.5 h for the different metabs. investigated. [on SciFinder (R)]