Study of the oxygen transfer in a disposable flexible bioreactor with surface aeration in vibrated medium.

The oxygen mass transfer is a critical design parameter for most bioreactors. It can be described and analyzed by means of the volumetric mass transfer coefficient K(L)a. This coefficient is affected by many factors such as geometrical and operational characteristics of the vessels, type, media composition, rheology and microorganism's morphology and concentration. In this study, we aim to develop and characterize a new culture system based on the surface aeration of a flexible, single-used bioreactor fixed on a vibrating table. In this context, the K(L)a was evaluated using a large domain of operating variables such as vibration frequency of the table, overpressure inside the pouch and viscosity of the liquid. A novel method for K(L)a determination based on the equilibrium state between oxygen uptake rate and oxygen transfer rate of the system at given conditions was also developed using resting cells of baker's fresh yeast with a measured oxygen uptake rate of 21 mg g(-1) h(-1) (at 30 degrees C). The effect of the vibration frequency on the oxygen transfer performance was studied for frequencies ranging from 15 to 30 Hz, and a maximal K(L)a of 80 h(-1) was recorded at 30 Hz. A rheological study of the medium added with carboxymethylcellulose at different concentrations and the effect of the liquid viscosity on K(L)a were determined. Finally, the mixing time of the system was also measured using the pH method.

An on-line technique for monitoring propionic acid fermentation.

An on-line technique, based on measuring the increase in pressure due to CO(2) release in a closed air-tight reactor, was used to evaluate the fermentation of lactate by propionibacteria. The method was applied to batch cultures of Propionibacterium shermanii grown in yeast extract/sodium lactate medium containing lactate as a carbon source under micro-aerophilic conditions. Gas pressure evolution was compared both with substrate consumption and metabolites production and with acidification and growth. Linear relationships were found between gas pressure variation, lactate consumption and propionate and acetate production. The technique also enabled the evaluation of total CO(2) produced, by taking account of pressure, oxygen and pH measurements. These results tend to show that this simple and rapid method could be useful to monitor propionic acid bacteria growth.

Microbial production of citric acid

O ácido cítrico é o ácido mais produzido em termos de tonagem e é extensivamente utilizado pelas indústrias alimentícia e farmacêutica. É produzido principalmente por fermentaçäo submersa utilizando o fungo Aspergillus niger e leveduras do gênero Candida sp. à partir de diferentes forntes de carbono, como a glicose e meios à base de amido. No entanto, outras técnicas de fermentaçäo, e.g. fermentaçäo no estado sólido e em superfície, e fontes alternativas de carbono tem sido intensamente estudadas mostrando grande perspectivas para o processo. O presente trabalho apresenta um resumo dos últimos avanços sobre a produçäo do ácido cítrico, descrevendo de maneira sucinta os trabalhos mais recentes, descrevendo microorganismos, técnicas de produçäo e substratos empregados, etc