RQ determination
To facilitate a reliable determination of respiratory quotients, experimental conditions involved high cell densities and relatively low gas flow in order to maximize sensitivity in the quantification of O2 consumption. Yeast inocula were incubated in 25 mL YPD broth at 25 °C and 250 rpm orbital shaking for 48 h. Cells were then washed twice with sterile deionized water and the OD600 determined. Five hundred OD600 units were suspended in 25 mL of a 2X chemically defined medium previously described by Herwig et al. (2001) with two modifications: glucose was omitted, and pH was adjusted to 3.5. The suspension was then transferred to 250 mL shake flasks, and incubated in the resulting resting conditions for 24 h at 25 °C. After that time, 25 mL of a 400 g/L glucose solution was added to resting cultures in order to reach a final concentration of 200 g/L glucose, similar to the sugar content of a standard grape must. Cells were then incubated for 24 h at 25 °C and 250 rpm orbital shaking. Immediately after that, cultures were transferred to sterile 100 mL flasks and supplemented with 200 μL of antifoam 204 (Sigma Aldrich) for the determination of the respiratory quotient (RQ) in fully aerobic conditions. To this end, cultures were sparged with air at a flow rate of 0.6 L/h (0.2 vvm) and magnetically stirred at 250 rpm. Fermentation off-gas was led through silica gel filters and the concentration of O2 and CO2 determined every 10 s in a GA4 gas analyzer (DASGIP AG, Jülich, Germany) for 90 min. Concentration of both gases was used to calculate RQ values as the ratio between released CO2 and consumed O2 for each time point.