Application of porous teflon tubing method to automatic fed-batch culture of microorganisms II. Automatic constant-value control of fed substrate (ethanol) concentration in semibatch culture of yeast

An automatic feedback control system incorporating a porous Teflon tubing sensor was developed and a strain of yeast was cultivated semibatchwise in mineral salt medium by feeding pure ethanol as the sole carbon source. In the control system, The ethanol concentration was continuously measured by the porous Teflon tubing sensor combined with a flame ionization detector, and its output signals were furnished to an automatic feed controller which controlled an ethanol feed pump so that deviations from the set level of ethanol concentration might be corrected. The controller was constructed on the basis of proportional-differential negative feedback control of which the proportional sensitivity and differentiation constants were estimated from the dynamic mass balance of ethanol. Precise measurement of temperature and compensation of the detector output signals for temperature fluctuations of culture broth were necessary to achieve good control. Cultivation experiments were carried out with three levels of concentrations: 10², 10³, and 10⁴ ppm. The relative deviations of the concentrations were less than ±0.5% for the 10³- and 10⁴- ppm levels but a little offset arose for the 10²-ppm levels. The growth of cells was at first exponential and then almost linear when the dissolved oxygen concentration dropped considerably.