Modelling diffusion-reaction phenomena in yeast flocs of Saccharomyces cerevisiae

Abstract

Limitations in the diffusion of substrates into the flocs will condition cell metabolic behaviour, having obvious consequences on growth and product formation. Polymeric additives have been used aiming the reduction of those limitations. The knowledge of the concentration profiles and metabolic fluxes of glucose and oxygen inside the flocs would bring valuable information about the␣conditions under which a fermentation should run. Direct measurement of such profiles is rather difficult but their simulation has been performed and is presented in this work.

Calculations were made for different possible sizes of the yeast flocs, considering also the presence or absence of a polymeric additive. Only a small percentage of the cells in the flocs metabolise glucose oxidatively due to severe oxygen limitations. The presence of the polymeric additive increases the ratio of cells operating under respiratory metabolism over those under fermentative metabolism: from 0.4% to 5.7% without additive to 1.2% to 8.5% with additive, depending on the bulk glucose concentration. Also, based on this data, it is possible to justify the yeast floc natural shape.