As mentioned in the Chapter 6, S. cerevisiae converts hexoses into pyruvate through glycolysis, which is decarboxylated to obtain acetaldehyde that is finally reduced to ethanol generating two moles of adenosine triphosphate (ATP) for each mol of consumed hexose under anaerobic conditions. In addition, this micro­organism also has the ability to convert hexoses into CO2 by aerobic respiration favoring the production of yeast cells. Therefore, aeration is an important factor for both cell growth and ethanol production. Although these yeasts have the abil­ity to grow under anaerobic conditions, small amounts of oxygen are needed for synthesis of such substances as fatty acids and sterols. In the case of continu­ous cultures, cell concentration, cell yield from glucose, and yeast viability are enhanced by increasing air supply while decreasing ethanol concentration under both microaerobic and aerobic conditions. Inhibition of cell growth by etha­nol decreases at microaerobic conditions related to fully anaerobic cultivation. Specific ethanol productivity is stimulated with the increase of oxygen percentage in the feed (see, for example, the work of Alfenore et al., 2004). In the case of fed-batch cultures, Alfenore et al. (2004) show that higher ethanol concentrations (147 g/L) can be obtained in cultures without oxygen limitation (0.2 vvm) in only 45 h in comparison to microaerobic conditions. In addition, a 23% increase in the viable cell mass was achieved. Similar studies for fed-batch cultures were performed for assessing the synergistic effect of temperature and ethanol content on the behavior of S. cerevisiae cultures (Aldiguier et al., 2004). Best results were found at 30°C and 33°C for around 120 g/L of ethanol produced in 30 h. Slight benefits for growth at 30°C and for ethanol production at 33°C were observed. These data suggest the possibility of designing two-stage, high-cell-density biore­actors. One proposed method to reduce the inhibition by ethanol of yeast cultures is the preadaptation to a medium with an initial content of ethanol. In this way, yeasts could be acclimatized to acquire ethanol tolerance by serially transferring them into a medium with ethanol. Vriesekoop and Pamment (2005) point out that this approach has not provided important success because which preadaptation to ethanol causes a decrease in the rate of yeast death, it does not prevent it. These authors showed that the addition of acetaldehyde to preadapted cultures of S. cer — evisiae eliminates the long lag-phase of yeasts caused by the sudden exposure to initial ethanol concentrations as high as 50 g/L.