In order to overcome the preferential oxidation of glucose overlactose as well as the potential C-source catabolite repression inP. taetrolens cells during batch mode, mixed carbon source strategieswith lactose/glucose/glycerol feedings in combination with apH-shift cultivation strategy at 6.5 were carried out (Fig. 3). Ascan be seen in Fig. 3A, both glycerol and glucose feedings supported higher cell densities (1.7–1.8 g/L) in comparison tothose values achieved with lactose feeding (1.5 g/L) at the onsetof the stationary growth phase. These results suggested that fractionsof non-lactose C-sources (glucose and glycerol) were usedfor cell biomass, promoting higher cell density conditions thanthose achieved in a whey/lactose fed-batch co-fermentation.As Fig. 3B shows, DOT became limiting at early growth phase(<1 h). Dissolved oxygen availability was therefore controlledabove 10% throughout the co-fermentation systems to improvethe microbial fitness of P. taetrolens cells as well as to avoid criticallong-term oxygen-limiting conditions (Alonso et al., 2012a). Underwhey/lactose co-fermentation, the transition of DOT to saturationvalues occurred earlier (after 24-h) than with glucose or glycerolas co-substrates (after 36-h) (Fig. 3B), suggesting that lactose waspreferentially preferred as bioconversion substrate when afed-batch co-fermentation system was applied.
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