Annick Lejeune

Stochastic models to study the impact of mixing on a fed-batch culture of Saccharomyces cerevisiae

The mechanisms of interaction between microorganisms and their environment in a stirred bioreactor can be modeled by a stochastic approach. The procedure comprises two submodels: a classical stochastic model for the microbial cell circulation and a Markov chain model for the concentration gradient calculus. The advantage lies in the fact that the core of each submodel, i.e., the transition matrix (which contains the probabilities to shift from a perfectly mixed compartment to another in the bioreactor representation), is identical for the two cases. That means that both the particle circulation and fluid mixing process can be analyzed by use of the same modeling basis. This assumption has been validated by performing inert tracer (NaCl) and stained yeast cells dispersion experiments that have shown good agreement with simulation results. The stochastic model has been used to define a characteristic concentration profile experienced by the microorganisms during a fermentation test performed in a scale‐down reactor. The concentration profiles obtained in this way can explain the scale‐down effect in the case of a Saccharomyces cerevisiae fed‐batch process. The simulation results are analyzed in order to give some explanations about the effect of the substrate fluctuation dynamics on S. cerevisiae.

Influence of bioreactor hydraulic characteristics on a Saccharomyces cerevisiae fed-batch culture: hydrodynamic modelling and scale-down investigations

Yeast is a widely used microorganism at the industrial level because of its biomass and metabolite production capabilities. However, due to its sensitivity to the glucose effect, problems occur during scale-up to the industrial scale. Hydrodynamic conditions are not ideal in large-scale bioreactors, and glucose concentration gradients can arise when these bioreactors are operating in fed-batch mode. We have studied the effects of such gradients in a scale-down reactor, which consists of a mixed part linked to a non-mixed part by a recirculation pump, in order to mimic the hydrodynamic conditions encountered at the large scale. During the fermentation tests in the scale-down reactor, there was a drop in both biomass yield (ratio between the biomass produced and the glucose added) and trehalose production and an increase in both fermentation time (time between inoculation and beginning of stationary phase) and ethanol production. We have developed a stochastic model which explains these effects as the result of an induction process determined mainly by the hydrodynamic conditions. The concentration profiles experienced by the microorganisms during the scale-down tests were expressed and linked to the biomass yields of the scale-down tests.

Characterization of Lactobacillus strains isolated from Algerian children faeces for their probiotic properties

Lactic acid bacteria termed probiotics have preventive as well as curative effects on several types of diarrhoea of different aetiologies. The main objective of this study was to screen lactobacilli strains having probiotic traits, isolated from Algerian healthy children faeces on the purpose of using them further in children diarrheal illnesses. One hundred and twenty (120) lactic acid bacteria isolates were selected from faecal samples of healthy Algerian children aged between five and ten years. Gram positive rods and catalase negative bacteria (52 isolates) were screened,

Screening of Candida boidinii from Chemlal spent olive characterized by higher alkaline-cold adapted lipase production

A total of 24 lipolytic yeasts were isolated from the spent olive derived from olive fruits of the Algerian variety Chemlal . One strain, G5, had the highest lipolytic activity (20 mm) on the tributyrin agar plate. The morphological, biochemical characterization and 18S rDNA gene analysis of the selected strain, confirms that it is Candida boidinii KF156789. The production of lipase and biomass were carried out in liquid and solid (spent olive) media. In submerged fermentation, it seemed that the production of enzyme reached its maximum 7.3 U/ml, whereas; the growth cells reached its maximum at 1.9 × 10 8 cell/ml. That can be explained by the assimilation of free fatty acids by this strain after degradation of olive oil by the enzyme. The production of lipase and biomass, in solid state fermentation, gave the maximum yield for cell growth (1.3 × 10 9 cell/ml), while lipolytic activity reached 4.8 U/g. The highest activity of the studied enzyme was at pH 7.0 and 37°C. The enzyme maintained more than 90% of its activity at pH 8.0-9.0 and 70% at temperature range of 4-40°C; it was concluded that the lipase from C. boidinii KF156789 has the potential to be an alkaline cold-adapted enzyme. Keywords: Candida boidinii KF156789, higher lipase, alkaline cold-adapted lipase, chemlal spent olive, solid state fermentation. African Journal of Biotechnology , Vol 13(11), 1287-1294

Electrostatic and Water Content Effects on Yarrowia lipolytica Lipase Immobilization by Adsorption.

Aims: In the present investigation, an attempt has been made to explain lipase immobilization by adsorption on three minerals matrixes, i.e. Celite 545, Silica gel (60G) and Avicel (PH 101). Study Design: immobilization by absorption on minerals matrixes, water content by volumetric karl Fischer titration and surface potentials using a particle charge detector Mutek PCD 03 were used. Place and Duration of Study: Walloon Centre of Industrial Biology (CWBI) Unit of BioIndustries, University of Liege, Gembloux Agro-Bio Tech, Passage des Deportes 2, B5030 Gembloux, Belgium between Jun 2012 and jun 2013. Methodology: A methodical order was developed whereby the influences of water Original Research Article British Microbiology Research Journal, 4(6): 640-653, 2014 641 content, surface potentials and pH, on immobilization by adsorption were explored. Adsorbed YLL was used to understand an interesterification reaction between rapeseed oil and milk fat in comparison with a commercial silica-granulated Thermomyces lanuginosus lipase (Lipozyme TL IM). Results: Maximum immobilization yield was obtained with Celite (70%) and the lowest with silica gel (29%). Total water content of free and immobilized lipase was determined by volumetric Karl Fischer titration. The water content of Silica gel was higher than the one of other supports. Water content of silica gel could prevent the enzyme fixation. These results could be explained by the adsorption being governed mainly by electrostatic interactions between the enzyme and matrix. This hypothesis was further reinforced by measurements of electrical potential. They showed a lowest negative potential of Silica gel after enzyme adsorption in comparison to Celite. Conclusion: From these results celite was designated as an efficient matrix to immobilize Yarrowia lipolytica lipase (YLL) by adsorption. This performed system was used to realize an interesterification reaction between rapeseed oil and milk fat in comparison with a commercial silica-granulated Thermomyces lanuginosus lipase (Lipozyme TL IM).