2.3. Preparation and propagation of yeast cells
The dried yeast powder (S. cerevisiae) was aseptically inoculated into ten 150-ml Erlenmeyer flasks each containing 50 ml sterilized yeast malt extract (YM) broth. The flasks were incubated at 30 °C for 48 h and 100 rpm in an incubator shaker (Innova 4000, New Brinswick Scientific, USA). Ten percent of the inoculum was aseptically transferred to each of 10 sterilized 250-ml Erlenmeyer flasks containing 100 ml sterilized PYGAL medium composed of (%; w/v) peptone 0.3, yeast extract 0.3, KH2PO4 0.1, MgSO4·7H2O 0.1 and galactose 2.0. The pH of the medium was adjusted to 5.6 and the flasks were incubated at 35 °C for 24 h at 100 rpm in an incubator shaker. Fifty milliliters of the inoculum obtained from each flask after 24 h incubation was aseptically transferred to a 1-l flask containing 500 ml sterilized PYGAL medium. The 10 1-l flasks were incubated at 35 °C for 24 h at 100 rpm in an incubator shaker. PYGAL medium was prepared fresh each time, just prior to the fermentation experiments. Cells were concentrated by centrifugation of culture in sterilized centrifuge tubes at 10,000g at 4 °C for 10 min in a refrigerated centrifuge (Sorvall superspeed FCC B, Sorvall Inc., USA). The supernatant was discarded from the centrifuge tubes and the tubes containing pellets were votexed on a vortex shaker. The contents from all the tubes were aseptically transferred to two sterilized 50-ml centrifuge tubes. Depending upon the cell count in suspension, the process was repeated for further concentration of the cells. Since the objective of this study was to achieve high ethanol productivity, the cells were concentrated to a level of 3 × 109 cells/ml for inoculation purpose. Previous studies reported that S. cerevisiae grown on galactose helped in simultaneous fermentation of glucose and galactose in high cell density fermentation ( Ernandes et al., 1992). In a previous study we found a 30% increase in ethanol production from kinnow (Citrus reticulata) waste using S. cerevisiae cells grown on galactose compared with the cells grown on glucose in a SSF process (data not shown).