Charles Ghommidh

Dielectric monitoring of growth and sporulation of Bacillus thuringiensis

On-line permittivity and optical density measurements have been used to monitor biomass concentration and sporulation status during growth of a spore-forming bacterium, Bacillus thuringiensis, in fed-batch culture. The correlation between permittivity, optical density and other observations showed three distinct phases of growth: growth itself, transition and sporulation. The permittivity variations during the transition and sporulation phases could be related to the sporulation development: the evolution pattern of the ratio of optical density to permittivity was representative of the culture state, and during the sporulation phase, a permittivity index could be build to measure the extend of spore liberation.

Continuous alcoholic fermentation of sucrose using flocculating yeast. The limits of invertase activity

SummaryAt high flow rates, the continuous alcoholic fermentation of sucrose in a laboratory fermenter with internal cell recycle, using a strongly flocculating yeast can be limited by the substrate hydrolysis. This system is sensitive to glucose catabolic repression and to mineral deficiency. The release of invertase activity in the medium is negligible. From theoretical and experimental considerations, the hydrolysis rate is imposed by diffusionnal limitations in the biomass particles. Nevertheless, ethanol productivities as high as 68 g/L.h can be reached, without biomass retention problems. A better understanding of the basic phenomena involved in floc formation and evolution is required to control reactor performances.

Assessing yeast viability from cell size measurements

During microbial cell cultures, environmental conditions affect cell physiology and subsequently process efficiency. Physiological changes result in changing cell morphology, such as cell size variations. The aim of this work was to study cell size evolution of a Saccharomyces cerevisiae population exposed to various stresses during alcoholic batch fermentations, and to evaluate the potential use of cell size measurements to infer cell viability. During a reference culture, without perturbation, viability as assessed by propidium iodide staining (PI) remained 100% and mean cell diameter was found to be above 5microm. A rapid temperature shift from 33 to 43 degrees C at 50gl(-1) of ethanol resulted in an immediate arrest of growth and triggered a progressive loss of viability from 100% to 0% and a decrease of mean cell diameter from 5.2 to 3.7microm. Cell size distribution curves obtained with a cell counter showed an increasing subpopulation of significantly smaller cells. At single-cell level, combined microscopy size measurements and PI staining showed that this subpopulation was exclusively composed of dead cells. Similar results were obtained after acetic acid or furfural additions. Accordingly, a multivariate data analysis was achieved to estimate the ratio of dead cells from cell size distributions obtained using the cell counter.

Influence of Yeast Flocculation on the Rate of Jerusalem Artichoke Extract Fermentation

Variations in residual sugar composition have been observed during Jerusalem artichoke extract fermentations by using Saccharomyces diastaticus NCYC 625, a flocculating yeast strain. In batch cultures, these differences were due to the inulin polymer size distribution of the extracts: measurements of enzymatic activities on different polymerized substrates have shown that the hydrolysis and fermentation yield decreased when the fructose/glucose ratio of the extract increased. Inulin hydrolysis appeared to be the limiting factor of the fermentation rate. A comparison of continuous and batch cultures with the same extract showed that fermentability differences were related to the structure and size of the yeast flocs. This led to an hydrolysis selectivity of the inulin polymers according to their size: the chemostat culture in which the floc average size was larger gave longer chained residual sugars.

Influence of medium composition on surface properties and aggregation of a Saccharomyces cerevisiae strain

Abstract In an attempt to understand the factors governing the aggregation of yeast in alcoholic fermentation, the influence of the medium composition was investigated. The surface physicochemical properties (hydrophobicity and electrical properties) and the surface chemical composition of the cells, measured by X-ray photoelectron spectroscopy, were evaluated concomitantly. Phosphate depletion in the culture medium led to a lower phosphate concentration at the cell surface; depletion of other nutrients resulted in an increase of phosphate surface concentration due to a lower amount of biomass. The electrical surface properties of the cells depended strongly on the phosphate surface concentration and influenced the tendency of the cells to aggregate. The aggregation of this yeast can be considered in most cases to be caused by the interactions between solvated macromolecules at the surface, the aggregates behaving like weak gels.

Organic acids oxidation by Candida utilis: application to industrial waste water treatment

Abstract A process including an anaerobic acidogenic reactor followed by a yeast reactor in series is proposed to treat highly concentrated effluents of the food industry. Candida utilis was selected and operated in batch or in a continuous stirred tank reactor with acetic acid or propionic acid or butyric acid or a mixture of these acids as the main carbon source while the pH was maintained at 3.5 to minimize bacterial contamination. The yeast growth kinetics was inhibited by the substrate when its concentration was higher than a critical value. A significant uncoupling between growth and substrate utilization was observed in the batch reactor and the transient state of the perfectly mixed reactor could not be predicted from batch data. If the continuous reactor was conveniently operated, the 97% TOC abatement obtained was independent of inlet concentration and space time provided that the space time was higher than the washout value. Such large loading rates as 30 kg TOC m −3 day could then be treated. Sludge production was of the same order of magnitude as in a conventional activated sludge reactor. The solids were easily flocculated and settled after neutralising the suspension. The same overall abatement was obtained with a pilot reactor treating on line the effluent of an acidogenic reactor fed with beet molasses. The organic acids were completely oxidised and the TOC abatement depended on the conversion of the initial compounds in acids. However, the liquid–solids separation required then a flocculation step with chitosan at low concentration.

Diffusivity measurement in a flocculating yeast layer

The diffusion of small molecules (sugars and alcohols) through a dense layer of flocculating yeast has been studied and compared to the theorical value calculated from parameters such as porosity and yeast density. Lactose, ethanol and methanol were chosen as model molecules. In comparison with water, the diffusion rate of lactose is divided by three. Alcohols which cross the cell envelope diffuse relatively faster than lactose which was assumed not to penetrate the cells. An increase of the lactose diffusion rate is observed when the yeast are heat killed.

Biomass characterization by dielectric monitoring of viability and oxygen uptake rate measurements in a novel membrane bioreactor

The application of permittivity and oxygen uptake rate (OUR) as biological process control parameters in a wastewater treatment system was evaluated. Experiments were carried out in a novel airlift oxidation ditch membrane bioreactor under different organic loading rates (OLR). Permittivity as representative of activated sludge viability was measured by a capacitive on-line sensor. OUR was also measured as a representative for respirometric activity. Results showed that the biomass concentration increases with OLR and all biomass related measurements and simulators such as MLSS, permittivity, OUR, ASM1 and ASM3 almost follow the same increasing trends. The viability of biomass decreased when the OLR was reduced from 5 to 4 kg COD m(-3)d(-1). During decreasing of OLR, biomass related parameters generally decreased but not in a similar manner. Also, protein concentration in the system during OLR decreasing changed inversely with the activated sludge viability.

On-line determination of flocculating Saccharomyces cerevisiae concentration and growth rate using a capacitance probe.

During continuous alcoholic fermentation, Saccharomyces cerevisiae 38A floc concentration was monitored using an on-line impedance probe. Since the sensor response is linear and does not depend significantly on yeast particle size, an automatic technique of determining the yeast growth rate has been developed and validated against the conventional mass balance method.

Simultaneous production of sugars and ethanol from inulin rich-extracts in a chemostat

SummaryIncomplete fermentation of inulin-containing extracts by Saccharomyces diastaticus allows the simultaneous production of ethanol and syrups with increased fructose content. The yeast strain used ferments sucrose and inulin small polymers but does not easily ferment inulin large polymers. After batch fermentation a production of 62.5 g/L ethanol and 75 g/L of sugars containing up to 94 % fructose can be obtained. A continuous fermentation was performed in a chemostat permitting the adjustment of both productions according to the dilution rate with a maximal ethanol productivity of 3.9 g/L.h.