of living bacteria in medium with 2 mM ferulic acidwas 32.1 × 106cells/mL, while the reference mediumwithout ferulic acid only contained 23.4 × 106cells/mL.This was not due to a buffering effect, as the pH decreasedto 2.8 in all the media (Figure 3B). The concentration offerulic acid did not change very much during the cultiva-tion (Figure 3D). Most cultures exhibited a decline in theconcentration of ferulic acid at the end of the cultivation,but it is not clear that it was significant.Results obtained with vanillin are shown in Figure 4and in Table 1. Figure 4 indicates that vanillin did notseverely inhibit the growth ofG. xylinusuntil the concen-tration exceeded 2.5 mM (Figure 4A-C). An initial vanillinconcentration of 0.5 mM hardly affected glucose con-sumption, pH value or BC yield (Figure 4 and Table 1).With an initial vanillin concentration of 2.5 mM, theglucose consumption rate declined from 3.5 g/(L∙d) to1.7 g/(L∙d) (Table 1A). The concentration of living bacteriaat the end of the cultivation was 4.0 × 106cells/mL, muchlower than in the cultures with reference medium, whichreached 23.4 × 106cells/mL (Figure 4C). The volumetricyield of BC declined as the concentration of vanillin in-creased (Table 1B). When the initial concentration of van-illin was 5 mM or higher, the glucose consumption ratewas <0.4 g/(L∙d) (Table 1A), the concentration of livingbacteria in the cultures was very low (Figure 4C), andthe yield of BC was <0.4 g/L (Table 1B).The vanillin content in cultures with initial concentra-tions of 0.5 or 2.5 mM seemed to decrease during the cul-tivation (Figure 4D). Control experiments (with vanillin inthe medium but without bacterial inoculation) showed noproduct formation making it plausible that conversionproducts detected in bacterial cultures would be the resultof a biotransformation. Only very small concentrations of
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