Analysis of the C-source uptake rates revealed that the specific
lactose uptake rate was 1.66 g/g h in a whey/lactose
co-fermentation, 21- and 33-fold times higher than the uptake
rates achieved for glucose and glycerol in their respective
co-fermentation systems (Fig. 3F). Interestingly, all the substrate
consumption profiles followed a straight trend upon the onset of
the lactobionic acid production phases (Fig. 3E). Whereas glycerol
was barely consumed at a rate of 0.09 g/L h in a whey/glycerol
co-fermentation (leaving 15.6 g/L after 52 h), glucose was completely
exhausted in a whey/glucose co-fermentation, resulting in
the formation of 12.5 g/L of gluconic acid after 14-h post-feeding.
However, such glucose-limited conditions led to the progressive
co-utilization of the gluconic acid at a rate of 0.15 g/L h when no
more glucose was co-supplied (Fig. 3F). Overall, the combination
of a non-lactose C-source like glycerol resulted in higher cell densities
but lower lactose bioconversion abilities. Likewise, the
co-supply of glucose led to the preferential channelling of glucose
into the direct oxidative pathway to produce gluconic acid which
was further over-time consumed upon C-source exhaustion.