Heat flux measurements for the fast monitoring of dynamic responses to glucose additions by yeasts that were subjected to different feeding regimes in continuous culture.

Heat measurements have been successfully as an analytical tool for the study of the dynamics of energy metabolism of Saccharomyces cerevisiae and Candida utilis grown in continuous culture under fluctuating substrate supply. A low average dilution rate (D = 0.05 h-1) was maintained either by adding the medium as continuously (dropwise) as possible or (blockwise) by adding the medium at high speed during a short period (D = 0.5 h-1 for 40 s) and not at all during the following period (D = 0.00 h-1 for 360 s). The resulting biological activity was monitored on-line with conventional (O2 and CO2) off-gas analyses, DOT measurements, and heat flux measurements. In C. utilis cultures, the biomass-specific maximum oxygen consumption rate (qO2,max), the biomass yield (Ys,x), and the dynamic responses to a glucose pulse and to a change in feeding regime were not significantly affected by different preceding feeding regimes. In contrast, S. cerevisiae grown in continuous culture with blockwise feed showed a 50% increase in qO2,max and a 25% drop in Ys,x compared to the culture grown with dropwise feed. The dynamic response to a glucose pulse (0.6 g L-1) was slower for the continuous (dropwise) than for the blockwise grown S. cerevisiae. With a second testing method for the dynamic response of the yeasts, the feeding regime was changed. The blockwise fed S. cerevisiae proved to be better "trained" to cope with sudden changes in glucose supply and, therefore, was more "shockproof" toward a change in feeding regime. This clearly points to major differences in the intracellular metabolic flux control between the yeasts. These findings are of relevance for industrial baker's yeast production, where reactor mixing times of one to several minutes are not uncommon. The observed, heat production, together with the dissolved oxygen concentration, appeared to give the fastest response to actual changes in the culture. It is suggested that heat measurements can be a very useful tool to monitor and control the growth of S. cerevisiae in laboratory and industrial fermenter operations.