The microbial production of a biopolymer, xanthan gum, was used to demonstrate that transport resistances inherent in immobilized cell systems can be exploited to concentrate macromolecular products within the porous support matrix. Mathematical models were developed to predict the diffusional limitations responsible for concentrating the xanthan internally in the immobilization matrix. However, total xanthan accumulation is very sensitive to oxygen transport limitations. Therefore, immobilized cell fermentations were conducted with the external oxygen concentration controlled at different levels. In agreement with model simulations, xanthan accumulated to the greatest extent in the absence of oxygen transport limitations, reaching a final internal concentration of 55 kg/m 3 ‐pore volume. In comparable free‐cell fermentations, the maximum achievable xanthan concentration was only 35 kg/m 3 .
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Robinson et al. (1988) studied this question.