Simple diffusion is the primary mechanism for glucose uptake during the production phase of the Aspergillus niger citric acid process.

Models for the known glucose transporters in Aspergillus niger and for simple diffusion of glucose through the hyphal membrane were prepared. The results from the models were compared with fermentation data from published studies on citric acid. It was found that the purely physical and uncontrolled process of diffusion could explain the specific rate of glucose uptake observed during the production phase in several different types of fermentation.