3.2. Effect of inoculum size in hemicellulosic hydrolyzate supplemented with molasses “B”
The effect of inoculum size on ethanol production from 50% hemicellulosic hydrolyzates supplemented with molasses B (8%) by S. cerevisiae ITV-01 is shown in Table 2. When the initial yeast concentration increased gradually from 0.2 to 10 gL−1, ethanol yield slightly decreased, while the maximum ethanol concentration exhibited remained unchanged, although ethanol productivity (Qp) increased significantly. These results showed a faster conversion of sugars present in the medium (glucose and fructose) to ethanol corresponding to a higher cell density, although the xylose present converted to xylitol reached maximum concentration of 6.11 gL−1 using 10% w/w inoculum (data not shown). Complete glucose and fructose consumption was achieved for all the different inocula sizes evaluated. Even though S. cerevisiae cannot utilize xylose, it possesses a gene for xylose reductase (XR) activity [29] and [30], albeit with very low activity. A gene similar to xylitol dehydrogenase (XDH) of S. stipitis was also found in S. cerevisiae [30] but xylitol was not converted to xylulose, which explains the accumulation of xylitol in the medium. Additionally, high- and low-affinity transport systems for glucose have been found in S. cerevisiae [31] and [32], and apparently both are used in xylose uptake [32] and [33]. However, the affinities of these systems for xylose are considerably lower than those for glucose, indicating that glucose is the primary substrate [32]. Additionally, xylose was consumed in part by the end of fermentation and converted into xylitol.