Mathematical modelization of a packed-bed reactor performance with immobilized yeast for ethanol fermentation

Immobilization of cells for application in the food industry

Immobilization of cells offers advantages to the food process industries, including enhanced fermentation productivity and cell stability and reduced downstream processing costs due to facilitated cell recovery and recycle. This article summarizes the varied immobilization methodologies, including adsorption, entrapment, covalent binding, and microencapsulation. Examples of interest to the food industry are provided, together with a review of the physiological effects of immobilization. Topics in process engineering include immobilized cell bioreactor configurations and the scale-up potential of the various immobilization techniques.

Determination of glucose and ethanol effective diffusion coefficients in Ca-alginate gel

Glucose and ethanol diffusion coefficients in 2% Ca-alginate gel were measured using the experimental technique based on solute diffusion into or out of gel beads in a well-stirred solution. The aim of the study was to make the measurements under typical conditions found in alcoholic fermentations, such as the concentrations of glucose (100 g l-1) and ethanol (50 g l-1), the simultaneous counter-diffusion of glucose and ethanol, and the presence of cells in the gel beads at a level of 10(9) cells g-1 of beads. Previously, an evaluation of the error associated with the methodology used indicated how the experimental procedure would minimize the error. The individual measurement of glucose and ethanol coefficients in 2% Ca-alginate with no cells gave values of 5.1 and 9.6 x 10(-6) cm2 s-1, respectively, which are lower than those in water. When the effect of counter-diffusion was investigated, both coefficients decreased: glucose by 14% and ethanol by 28%. When cells were incorporated into the beads, only the ethanol coefficient decreased significantly, while the glucose coefficient apparently increased its value to 6.9 10(-6) cm2 s-1.

An effectiveness factor you can see. Experimental visualization of the effectiveness factor concept in a biological system

Effectiveness factor values corresponding to carrageenan immobilized yeast beads packed in a continuous tubular fermenter are obtained from the mathematical modelization of the reactor performing ethanol fermentation. Simultaneously, microscopical direct observation of transversal sections corresponding to two different points in the fermenter is in good agreement with the calculated values: An effectiveness factor near to unity gives a uniform cell distribution in beads, as an effectiveness factor near to 0.75 corresponds to a non-uniform cell growth in beads. This observation gives a visual evidence of the validity of the approach used to treat diffusional limitations in biocatalytic particles.