Solid-State-Fermentation of starter cultures in fluidized bed. Part 2: Mathematical Modelling

: Bahr, D.; Menner, M.

Abstract
A dynamic model was evaluated that gives a detailed description of internal diffusion and growth of baker's yeast in pellets in addition to external mass transfer resistance. The model predicts the temporal and local profiles of substrates and product in the support, which cannot be measured. The model clearly shows that substrates (glucose and oxygen) do not reach the core of the sphere of pellets, whereas in the outer shell in a film of 100 mm the substrates rise to their maximum. The results demonstrate that significant substrate limitation (glucose, oxygen) will occur in the centre of the pellet, wheras in the aerobic shell high glucose concentration gives rise to catabolic repression. This results in a significant reduction of the growth rate and in a rise of the ethanol production rate. It is clearly shown that internal diffusion is the most sensitive parameter for the limitation of the growth rate by solid state fermentation of yeast in fluidized beds.