2.1. Strain and cultivation conditions
Three batch fermentations of S. cerevisiae , named BC1, BC2, and BC3, were performed. Cultivations were carried out in a 3 L stainless steel tank bioreactor (Minifors, Inifors HT, Bottmingen, Switzerland) with a working volume of 1.35 L and equipped with temperature and pH control. 5 g, 7 g and 10 g of baker’s yeast S. cerevisiae (fresh baker’s yeast, Oma’s Ur-Hefe) was used in BC1, BC2, and BC3, respectively. The baker’s yeast was suspended into 100 mL Schatzmann medium containing 0.34 g L−1MgSO4·7H2O, 0.42 g L−1CaCl2·2H2O, 4.5 g L−1(NH4)2SO4, 1.9 g L−1(NH4)2HPO4, 0.9 g L−1 KCl. After shaking for 10 minutes they were added into the bioreactor. The medium used for batch cultivations was the same as for the suspended cells, but with 5 g/L, 7 g/L and 9 g/L glucose for BC1, BC2, and BC3 respectively and 1 mL/L trace elements solution (0.015 g/L FeCl3·6H2O, 9 mg/L ZnSO4·7H2O, 10.5 mg/L MnSO4·2H2O, and 2.4 mg/L CuSO4 5H2O) and 1 mL/L vitamin solution (0.06 g/L myoinositol, 0.03 g/L Ca-pantothenate, 6 mg/L thiamine HCl, 1.5 mg/L pyridoxine HCl, and 0.03 mg/L biotin). All three batch runs were operated at the same conditions, that is, a constant temperature at 30 °C and a maintained pH at 5. The aeration and agitation rates were kept constant at 3.5 L/min and 450 rpm, respectively.