The obtained concentration of ethanol versus the concentration of furfuryl alcohol is shown in Fig. 6. The amount of produced ethanol increased from 10 g/L with pure glucose to 24 g/ L at the dose of 1.5 mL/L furfuryl alcohol. At higher concentrations a decrease in ethanol is observed with almost no ethanol being produced at the dose of 15 mL/L. This indicates that with the presence of small doses of furfuryl alcohol, ethanol production can increase.

To provide information about the sharp increase between measurement one and two, another experiment was conducted with the same method as the previous, except the dose of furfuryl alcohol were (0.0, 0.5, 1.0, 1.5, 2.0 and 2.5) mL/L. Samples of this experiment were analyzed by HPLC and the results are shown in table 3.

As shown in table 3, the fermentation will peak with a dose of 1.5 mL/L and at levels above that the amount of ethanol produced will decrease versus the concentration of furfuryl alcohol. It was further shown that there was plenty of leftover glucose for the yeast to ferment. Therefore in none of the cases the loss in fermentation rate was due to lack of glucose.

As shown in Table 3 there is a strong correspondence between the amount of ethanol produced and the dose of furfuryl alcohol. Furfuryl alcohol is a microbial growth inhibitor, but when added in small concentrations it will force the yeast cells to increase their metabolism to survive. This effect should be present for the norm of microbial inhibitors. Despite of the fact that furfuryl alcohol is toxic to the yeast, the stress that it causes can lead

to approximately 80% increase in ethanol production given that sufficient amounts of glucose are available. For SSF the amount of glucose needed for this effect to be visible is only present in the start of the SSF which is also where the positive feedback effect of the yeasts fermentation rate is mainly present.

To describe the effect of lacking inhibitory stress on the yeast the maximum fermentation rate with 2 g/L yeast of both 80 g/L DM and 170 g/L DM in the SSF experiments with rape straw are compared and shown in Fig. 7.

The rate of fermentation for the furfuryl alcohol experiment is calculated for the period of 5 to 19 hours, which is estimated to be the highest rate of fermentation for the experiment. As shown in Fig. 7, it seems that even though microbial inhibitors are generally restricting the fermentation process, a medium completely without inhibitors (such as the control from Fig. 8) will have a decreased fermentation rate compared to medium with a small dose of microbial inhibitors.