S91: The Substratostat, a novel automated and continuous fed-batch approach to industrial fermentations

Though batch fermentations remain the standard in industrial fermentations, they involve large fluctuations in substrate and product concentrations. High initial substrate concentrations may be desirable for economic process efficiency, but can lead to increased by-product formation and decreased product yields. In yeast fermentations, high sugar concentrations cause a hyperosmotic stress response, increasing the risk of fermentation failures and causing higher acetic acid concentrations.

A FT-NIR spectroscopy-based system is presented that provides accurate real-time quantification of key-fermentation parameters including glucose, fructose, and ethanol throughout fermentations. Integrating this in-line sugar analysis with process control enables innovative fermentation strategies that include carrying out automated and continuous fed-batch fermentations at constant substrate concentrations (substratostat). For this, medium is added to the yeast starter until reaching a certain substrate concentration, which is then held constant. The substratostat thus allows very high sugar-containing media to be used without eliciting the yeast hyperosmotic stress response, thereby increasing specific product yields and concentrations.

To demonstrate the potential of the substratostat, the traditional batch fermentation of a medium initially containing 340 g/l of sugar was compared with the substratostatic fed-batch approach, where a pump fed the medium at suitable rates to maintain sugar levels at 50 g/l. The substratostat showed constant fermentation rates and similar fermentation duration, and led to a reduction of final acetic acid levels from 1 to <0.2 g/l compared with the traditional batch fermentation. Calibrated for other substrates, the substratostat can be used in other industrial fermentations, too.