Structure and sucrose hydrolysis activity of Saccharomyces cerevisiae aggregates.
ABSTRACT The sucrose hydrolysis activity of dense spherical yeast flocs, cultivated on a sucrose medium in a continuous reactor with internal settler, is nearly proportional to the particle surface. From computer simulation, in good agreement with experimental determinations, the calculated sucrose penetration depth is in the range 0.2-0.3 mm, a dimension smaller than the usual diameter of strongly flocculating yeast particles. From specific gravity determinations, the flocs can be considered as homogeneous and cannot exhibit a fractal structure, reported in the literature for a number of microbial aggregates. However, the analysis of the sucrose hydrolysis rates reveals that the cell density may be lower in the outer layer of the flocs.
Article: Modeling brewers' yeast flocculation[Show abstract] [Hide abstract]
ABSTRACT: Flocculation of yeast cells occurs during the fermentation of beer. Partway through the fermentation the cells become flocculent and start to form flocs. If the environmental conditions, such as medium composition and fluid velocities in the tank, are optimal, the flocs will grow in size large enough to settle. After settling of the main part of the yeast the green beer is left, containing only a small amount of yeast necessary for rest conversions during the next process step, the lagering. The physical process of flocculation is a dynamic equilibrium of floc formation and floc breakup resulting in a bimodal size distribution containing single cells and flocs. The floc size distribution and the single cell amount were measured under the different conditions that occur during full scale fermentation. Influences on flocculation such as floc strength, specific power input, and total number of yeast cells in suspension were studied. A flocculation model was developed, and the measured data used for validation. Yeast floc formation can be described with the collision theory assuming a constant collision efficiency. The breakup of flocs appears to occur mainly via two mechanisms, the splitting of flocs and the erosion of yeast cells from the floc surface. The splitting rate determines the average floc size and the erosion rate determines the number of single cells. Regarding the size of the flocs with respect to the scale of turbulence, only the viscous subrange needs to be considered. With the model, the floc size distribution and the number of single cells can be predicted at a certain point during the fermentation. For this, the bond strength between the cells, the fractal dimension of the yeast, the specific power input in the tank and the number of yeast cells that are in suspension in the tank have to be known. Copyright 1998 John Wiley & Sons, Inc.Biotechnology and Bioengineering 03/1998; 57(3):330-41. · 4.16 Impact Factor
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ABSTRACT: The aim of this work was to identify the main viable heterotrophic bacteria in a marine fish farm with a recirculating water system and to study their growth dynamics. The experiments were performed with sea bass (Dicentrarchus labrax) in an experimental recirculating water system. The bacteria identified were typical of the marine environment: Pseudomonas, Oceanospirillum, Marinobacter, Paracoccus and Erythrobacter genus from Bergey’s group IV, two genus of Vibrionaceae, one strain of Vibrio and one strain of Aeromonas. These populations were stable when the ingested feed/replacement water ratio was kept constant. Fixed bacteria formed large biofilms, which released about 104 CFU ml−1 into the tank. The biological filter, with its large surface area, was the largest source of bacteria in the farm, but the UV disinfection unit kept the number of free bacteria stable. When properly managed, the majority of bacteria that grew on the biological filter were from Bergey’s group IV; no Vibrio were ever detected on it.Aquacultural Engineering. 01/2000;
- Journal- Institute of Brewing 09/1996; 102:333-342. · 0.88 Impact Factor