Publications by Urs von Stockar

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Bench-scale heat flux calorimetry is applied to monitor complex microbial systems producing secondary metabolites and having a rheol. complex behavior. With such systems, biol. metabolic activity can only be measured accurately if calorimetry is improved by online correction for stirring power variations using torque measurement.First, a successful application to the prodn. of the antibiotic erythromycin by Saccharopolyspora erythraea is presented. During a batch-culture study, it was shown that heat-flux c

The influence of residual ethanol on metab. of food grade Gluconacetobacter xylinum I 2281 was investigated during controlled cultivations on 35 g/l glucose and 5 g/l ethanol. Bacterial growth was strongly reduced in the presence of ethanol, which is unusual for acetic acid bacteria. Biomass accumulated only after complete oxidn. of ethanol to acetate and carbon dioxide. In contrast, bacterial growth initiated without delay on 35 g/l glucose and 5 g/l acetate. It was found that acetyl CoA was activated by t

This paper describes the development of a new method to obtain aqueous-core microcapsules from organic-core capsules. The direct production of microcapsules, using tripropionin as organic material, followed by the hydrolysis of the core by a lipase was inv ...

A novel in situ product removal (ISPR) method that uses microcapsules to ext. inhibitory products from the reaction suspension is introduced into fermn. technol. More specifically, L-phenylalanine (L-phe) was transformed by Saccharomyces cerevisiae to 2-phenylethanol (PEA), which is inhibitory toward the yeast. In order to continuously remove PEA from the vicinity of the cells, the reaction suspension was brought into contact with capsules of 2.2-mm diam. that had a hydrophobic core of di-Bu sebacate and an

The bioconversion of L-phenylalanine to 2-phenylethanol by Saccharomyces cerevisiae in fed-batch expts. has shown that concns. of 2-phenylethanol of >2.9 g/L have a neg. impact on the oxidative capacity of the yeast. Without tight control on ethanol prodn., and hence on the feed rate, ethanol rapidly accumulates in the culture media, resulting in complete inhibition of cell growth before the maximal 2-phenylethanol concn. of 3.8 g/L, obtained in the absence of ethanol prodn., could be achieved. This effect

The effective biotechnol. process development depends on the system for monitoring the activity of the culture during cultivation. The feasibility of monitoring the metabolic flux during the cultivation of Bacillus thuringiensis var. galleriae (Btg) in pilot scale bioreactor was studied. All possible sources of heat generation and transfer in the system were accounted to det. the metabolic heat flux caused by the culture alone. Successful demonstration of the heat measurements has revealed the scale-up char

Measurements of the heat produced by Saccharomyces cerevisiae CBS 426 were used in conjunction with elemental and enthalpic balances to control fed-batch fermns. A proportional control using the ratio of carbon dioxide evolution rate to heat prodn. resulted in high biomass yields and minimal ethanol prodn. For the special case where the carbon source contains no nitrogen, biomass prodn. estd. from heat measurements agreed well with measured values. When the controller gain was set below the max. specific ce

The effect of the presence of metabolism-induced concentration correlations in the calibration samples on the prediction performance of partial least-squares regression (PLSR) models and mid-infrared spectra from Chinese hamster ovary cell cultures was investigated. Samples collected from batch cultures contained highly correlated metabolite concentrations as a result of metabolic relations. Calibrations based on such samples could only be used to predict concentrations in new samples if a similar correlati

Bacterial exopolysaccharides (EPS) have unique rheol. properties due to their high purity and regular structure. Thus, the food industry frequently uses EPS as thickening, gelling or stabilizing agents. The strain Gluconacetobacter xylinus I 2281 is capable of producing with a very high yield a new sol. EPS named gluconacetan, which is composed of rhamnose, glucose, mannose and glucuronic acid. Growth of these bacteria presents many features that represent a real challenge for both monitoring and control. H

Dielec. spectroscopy was applied to two industrial high cell d. culture processes and used to det. online the concn. of CHO cells immobilized on macroporous microcarriers in a stirred bioreactor and in a packed-bed of disk carriers. The cell concn. predicted from the spectroscopic data was in excellent agreement with off-line cell counting data for both processes. Deviations between the two counting methods only occurred in the case of a significant decrease of the cell viability, from 93% to 64%, which ind

In this work, dielec. spectroscopy was used to monitor two CHO perfusion culture expts. (B14 and B16). The capacitance of the cell suspension was recorded every 20 min over an excitation frequency range of 0.2 MHz to 10.0 MHz. A phase plot of the capacitance at a low excitation frequency vs. the value at a higher frequency proved to be an accurate indicator of the major transition points of the culture, i.e., max. cell viability, end of lactate consumption, point of zero viability. For both expts., the capa

Batch cultures of a mouse/mouse hybridoma cell line secreting a dimeric IgA were performed in combination with a membrane process for the continuous removal of ammonia from the culture medium. The latter involves hydrophobic porous membranes with an assocd. pH gradient. The process was efficient in reducing the ammonia concn. by approx. 50% by comparison to control expts. Two series of expts. were performed with an initial glutamine concn. of 4 mM and 10 mM, resp. The growth characteristics, antibody produc

Optimum growth conditions for the fermn. of nonconcd. whey permeate by K. fragilis NRRL 665 were defined. Use of 3.75 g yeast ext./L, a growth temp. of 38 Deg and a pH of 4.0 allowed a max. productivity of 5.23 g EtOH [64-17-5]/L-h in continuous culture with a yield 91% of theor. Complete batch fermn. of permeate with 100 g lactose [63-42-3]/L was possible with a max. specific growth rate of 0.276/h without any change in EtOH yield. Fermn. of concd. permeate resulted, however, in a general decrease of speci

The calcn. of material balances during transitions implies the evaluation of cumulated quantities consumed and produced. This article presents the techniques to achieve this goal by evaluating the quantities involved in the reactions at each observation and by calcg. the material balances (carbon, nitrogen, degrees of redn.) at each observation. Balance profiles are assessed by statistical tests and an expt. involving different metabolic pathways is analyzed. [on SciFinder (R)]

Yeast strains were examd. for their ability to produce glycerol, and Kluyveromyces marxianus NRRLY-665 was chosen for further optimization due to its comparatively high growth rate (0.15/h) and glycerol yield (9.5% by wt. on lactose) at 37 Deg and pH 7. Optimal conditions for glycerol formation were 30-37 Deg and pH 6-7. Increasing whey permeate concn. above the equiv. of 4.5% lactose decreased growth rate and cell prodn. and increased HOAc prodn. When SO32- was added throughout the fermn. to complex AcH be

Oxygen is a key substrate in animal cell metab. and its consumption is thus a parameter of great interest for bioprocess monitoring and control. A system for measuring it based on an oxygen balance on the liq. phase was developed. The use of a gas-permeable membrane offered the possibility to provide the required quantity of oxygen into the culture, while avoiding problems of foaming or shear stress generally linked to sparging. This aeration system allowed moreover to keep a known and const. kLa value thro

The applicability of neural networks to a typical biochem. engineering problem was investigated. The data treatment required for a novel technique for measuring the mean bubble size and the sp. interfacial area in aerobic bioreactors was chosen as an appropriate example in case. The new measuring technique is based on either light or ultrasound transmission measurements through the dispersion. The function relating the results of such measurements to bubble size cannot be explicitly modelled. It was therefo

In order to extend the shelf life of refrigerated dough, a new method based on thermo-gelling immobilization is proposed. This slows fermn. at low temps. (

A balanced supplementation method was applied to develop a serum and protein- free medium supporting hybridoma cell batch culture. The aim was to improve systematically the initial formulation of the medium to prevent limitations due to unbalanced concns. of vitamins and amino acids. In a first step, supplementation of the basal formulation with 13 amino acids, led to an increase of the specific IgA prodn. rate from 0.60 to 1.07 pg cell-1 h-1. The specific growth rate remained unchanged, but the supplementa

The isolation and sepn. of the mol. variants of monoclonal IgA from cell culture supernatants is possible using several filtration and ion-exchange chromatog. steps, followed by size-exclusion chromatog. for the actual sepn. of the mol. variants. The latter step is esp. time consuming and laborious. This report presents possible improvements of the procedure. Use of the displacement rather than the elution mode may render the ion-exchange step more productive (higher product concns. and space-time yield). F

Urs von Stockar

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Use of reaction calorimetry to monitor and control microbial cultures producing industrially relevant secondary metabolites

Influence of residual ethanol concentration on the growth of Gluconacetobacter xylinus I 2281

Enzymatic hydrolysis of organic-core microcapsules to produce aqueous-core microcapsules

xxvel type of in situ extraction: Use of solvent containing microcapsules for the bioconversion of 2-phenylethanol from L-phenylalanine by Saccharomyces cerevisiae

Enhancement of 2-phenylethanol productivity by Saccharomyces cerevisiae in two-phase fed-batch fermentations using solvent immobilization

Heat flux as an on-line indicator of metabolic activity in pilot scale bioreactor during the production of Bacillus thuringiensis var. galleriae-based biopesticides

Calorimetric control of fed-batch fermentations

The influence of correlated calibration samples on the prediction performance of multivariate models based on mid-infrared spectra of animal cell cultures

Adaptive control of a G. xylinus fed-batch fermentation using in situ mid-IR spectroscopy

On-line determination of animal cell concentration in two industrial high-density culture processes by dielectric spectroscopy

On-line biomass monitoring of CHO perfusion culture with scanning dielectric spectroscopy

Batch cultures of a hybridoma cell line performed with in situ ammonia removal

Metabolic, physiological and kinetic aspects of the alcoholic fermentation of whey permeate by Kluyveromyces fragilis NRRL 665 and Kluyveromyces lactis NCYC 571

Energetic investigation of Saccharomyces cerevisiae during transitions. Part 1.Mass balances

Glycerol production by yeast fermentation of whey permeate

A new method for on-line measurement of the volumetric oxygen uptake rate in membrane aerated animal cell cultures

Characterizing bubbles in bioreactors using light or ultrasound probes: data analysis by classical means and by neural networks

Immobilization as a tool to control fermentation in yeast-leavened refrigerated dough

The role of vitamins and amino acids on hybridoma growth and monoclonal antibody production

Development of a downstream process for the isolation and separation of monoclonal immunoglobulin A monomers, dimers and polymers from cell culture supernatant