Modelling diffusion-reaction phenomena in yeast flocs of

Limitations in the diffusion of substrates into the ¯ocs 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 pro®les and metabolic ¯uxes of glucose and oxygen inside the ¯ocs would bring valuable information about the conditions under which a fermentation should run.Direct measurement of such pro®les is rather dif®cult but their simulation has been performed and is presented in this work.Calculations were made for different possible sizes of the yeast ¯ocs, considering also the presence or absence of a polymeric additive.Only a small percentage of the cells in the ¯ocs 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 ¯oc natural shape.