Control of Fed-Batch Yeast Cultivation Using a Capacitance Sensor

Abstract

Abstract Saccharomyces cerevisiae biomass is still one of the most important biotechnological products in the world, mainly in the form of baker's yeast. Costs of substrates have an important position in the overall economics of baker's yeast production, which is usually run in fed-batch bioreactors. In this context, the maximization of biomass yields, aiming at the highest achievable volumetric productivity is a driving force for improving the control of the process. Classical model-based control strategies for this process, however, face difficulties due to the inherent variability of this system: microorganisms have a complex growth dynamics, lumped in very simplified growth models; raw materials are variable and may be not traceable; the system response depends on non-controlled previous process stages, such as strain selection and inoculum preparation. Hence, the possibility of using feedback information from in situ sensors for re-tuning control parameters is an important issue to ensure sub-optimal performances, at least. A capacitance sensor is a device that can monitor cell concentration on-line. In fed-batch cultivations, the feed rate and correlated inputs can be controlled by systems coupled with softsensors that infer the state of the system from on-line measurements of primary variables. Several estimation techniques have been proposed in the literature, and among them the gas balance technique is widely used. The specific respiration rate (qO2), the specific carbon dioxide production rate (qCO2), and the respiratory quotient (RQ) are the main variables determined from the gas balance. Values of RQ for complete oxidation of some carbon sources to carbon dioxide and water are found in the literature. For the baker's yeast production process growing in glucose, a value of RQ close to unity indicates the preponderance of the aerobic route. The capacitance sensor, after calibration, may provide information about cellular growth and viability: the capacitance of the medium is linearly proportional to viable cell concentration. In this work, the signal of capacitance sensor and the RQ value are coupled to a fuzzy algorithm in order to control the glucose feed rate during baker's yeast aerobic cultivation.