A model based simulation of brewing yeast propagation

A comprehensive kinetic model of yeast propagation in breweries is introduced. It represents the basis for a control strategy aimed at the provision of optimal inoculum at the starting time of subsequent industrial beer fermentations. In the metabolic modelling approach presented, respiratory metabolism on sugars and ethanol and fermentative metabolism respectively, are included. Occurring limitations, due to specific nutritional data of the growth medium, were taken into account for sugar, nitrogen, ethanol and oxygen. Correspondingly, inhibitions of the metabolism by ethanol and high sugar concentrations were formulated. The model especially represents the Crabtree-effect. For model validation, literature data were used and selected experiments within the relevant range of manipulated variables (temperature, dissolved oxygen) were conducted. Model based simulations matched these data with a deviation of 5.6% and with standard deviations of 5.6% after an adaptation of three variable parameters. A relationship between the temperature and the variable parameters could be extracted, which allowed predictive simulations of the yeast propagation process using non-isothermal trajectories. It was shown, that with a precise adjustment of trajectories of both, temperature and dissolved oxygen, the crop time of the inoculum could be varied within a period of 7 to 50 h to maintain high fermentation activity for the subsequent anaerobic fermentation.