As mentioned above, the liquid fraction resulting from lignocellulosic biomass
pretreatment can be unified with the cellulose hydrolyzate obtained after the
enzymatic treatment of the cellulose contained in the solid fraction coming from
the mentioned pretreatment process. The liquid stream produced contains all the
soluble sugars derived from the biomass, mostly glucose and xylose. This stream
can be fermented by microorganisms able to convert these two sugars into ethanol.
For this, recombinant bacteria can be used. Leksawasdi et al. (2001) employed
an engineered strain of Z. mobilis able to co-ferment hexoses and pentoses to process a solution containing a mixture of glucose and pentose. In addition, they
developed and experimentally tested an accurate mathematical description that
considers substrate limitation by the two sugars, substrate inhibition by both sugars,
and ethanol inhibition. For this, it employs the concepts of threshold ethanol
concentration for which inhibition of growth begins, and maximum ethanol concentration
for which biomass growth becomes zero. Inhibition constants for the
substrates are taken into account for considering glucose and xylose concentrations
inhibiting both cell growth and ethanol biosynthesis as a result of catabolic
repression. Kinetic expressions can be found in the work of Leksawasdi et al.
(2001). This description was used by the authors of this book to consider cofermentation
processes during process synthesis procedures. The equations of the
model are as follows: