We examine the effect of increasing the portion of xylose in the culture medium on ethanol productivity (Fig. 4.1). The total conversion of mixed sugars is set to 0.99 as before. Additional xylose is assumed obtainable by collecting an unconverted sugar from fermentation systems using wild-type yeast which converts glucose only.

First, the change of ethanol productivity with initial glucose concentration in a batch reactor is given in Fig. 4.1 (a). Ethanol productivity may or may not increase with the ratio of xylose to glucose concentration depending on initial glucose concentrations. If, for example, the upper limit of Xxyl, o/xgLC, o is 1.0, xylose addition results in increase (or decrease) of productivity when xgLC,0 is below (or above) about 50 g/L. If the ratio of initial sugar concentration is allowed to vary up to 2.0, such threshold is extended to xgLC, o = 58 g/L. Higher improvement of ethanol productivity is expected for lower initial concentrations of glucose (e. g., 45% up at xgLC, o = 20 g/L, but 5.4% up at xgLC, o = 50 g/L). Optimal operating conditions correspond to segments of curves above other ones. In Fig. 4.1(a), for example, optimal operating conditions imply that xxyL/o/xgLc/0 = 2 when 20 < xGLC/o < 58, and xxyl, o/xglc, o = 0.5 when 58 < xglc, o < 80.

Next, operating curves in a continuous reactor are presented in Fig. 4.1(b). It is shown that, unlike the batch case, it is always recommendable to increase xylose level in the feed to increase the ethanol productivity. The best productivity is obtained when xxyL, in/ xgLC, in = 2.0, which increases the productivity by 56%, 26%, and 12% at xgLC, in = 20, 50, and 80 g/L, respectively.