Higher substrate
amount may also impede the fermentation process due to formation
of higher concentration of ethanol and byproducts that are
unfavorable to the cells. Thus, a balance between suitable substrate
and yeast loading under constant stirring fulfills the extent of
ethanol production, where the carbon dioxide formed during the
reaction is continuously vented. The ethanol production rate,
therefore varies between 1.1 g and 0.70 g h1 kg1 dry substance,
showing relevance for a lignocellulosic material, where no pretreatment
steps were conducted. Fermentation of the lignocellulosic
hydrolysates by microorganisms would mean conversion of
both hexose sugars of glucose, mannose and galactose as well as the
pentose sugars of xylose and arabinose. However, yeast cells are not
capable to make use of the pentose sugars and thus a significant
amount of sugars remains unutilized