Modeling the mechanisms of glucose transport through the cell membrane of Aspergillus niger in submerged citric acid fermentation processes

Data from batch fermentations of citric acid producing Aspergillus niger cultures in shake flasks, loop and stirred tank bioreactors, were used to construct diffusion models for the transport of glucose. It was found that the mediated diffusion model does not reflect the relationship between the observed uptake rate and glucose concentration, nor for the lack of sensitivity to citrate. This is due in part of the low value of K-m in relation to the actual substrate concentration, which means that the carriers are saturated until the end of the process. The membrane barriers must be strongly inhibited under the standard production conditions. Instead, the simple diffusion model fits all the observed data and it explains the relationship between the specific uptake rate and the concentration of glucose, which should not exist under carrier-saturated conditions. This may account for the overproduction or organic acids under the specific process conditions. The simple nature of this mechanism also explains the similarity of the uptake relationships from different sources, despite the use of different growing conditions. (C) 2004 Elsevier B.V. All rights reserved.