Energy balance calculations were performed for different physiological states during batch growth of Saccharomyces cerevisiae with glucose as carbon and energy source. For the different physiological states, energy recoveries close to one were obtained, which permitted a continuous control that the constantly changing growth process was quantified accurately. During the respiro-fermantative phase of growth, during which glucose served as the carbon and energy source, a low-heat-yield value (DeltaQ(x)) of -8.6 kJ/g dry biomass formed was obtained. This low-heat-yield value was due to the mainly fermentative metabolism during the middle of this phase of growth. After a transition phase, the ethanol produced during the respiro-fermentative growth was respired. During this respiratory phase, the heat yield values increased markedly, resulting in a lowest value of -42.7 kJ/g. The low-heat-yield values of the respiro-fermentative growth is not a reflection of the most efficient metabolism of S. cerevisiae. On the contrary, during the middle of this phase, 74% of the energy input was dissipated as ethanol, 6% was dissipated as heat, and the energy conserved as biomass was just 13%, while during the early respiratory phase, 69% of the energy input was dissipated as heat, and 22% of the energy input was conserved as biomass. By mathematical modeling and direct monitoring on-line of the rate of heat production, continuous calculations of (1) glucose consumption, and (3) biomass production were performed, and were shown to correlate closely with measured values for the continuously changing growth process.