**5.2 Fermentation**

Microorganisms are employed to metabolize the liberated single sugars from enzymatic hydrolysis to convert them to bioethanol. There are two approaches:


SSF is currently considered the optimal method to convert lignocellulose to bioethanol. The process is reported with high conversion yield [21]. However, there are still some small backwards of this method. The optimal temperature for enzymatic hydrolysis is 45–50°C, while fermentation is at its highest efficiency at 28–35°C. Moreover, some intermediate products also resist the growth of microorganisms [25, 38].

Different microorganisms can be employed to enhance the fermentation. **Table 3** presents popular microorganisms, which can metabolize sugars and excrete ethanol.

From **Table 3**, it is easy to understand why *Saccharomyces cerevisiae* is a favorable choice of yeast to ferment sugar solution to bioethanol. Thanks to its tolerance to high ethanol concentration and material's inhibitors, *Saccharomyces cerevisiae* is known not only a traditional but also the most popular yeast in bioethanol production.

In fermentation process, an additional nutrient is necessarily added to provide organic nitrogen source for the growth of microorganisms. Peptone, corn steep liquor (CSL), urea, and even the distillation residue of bioethanol production process have been employed and investigated [32].

Production of lignocellulosic ethanol is still cost-inefficient. In attempts to improve bioethanol fermentation yield, more than one microorganism strain can


#### **Table 3.**

*Some popular microorganisms for bioethanol production [39].*

be loaded to the fermentation mixture as simultaneous saccharification and cofermentation (SSCF) method [39]. Hereby, both hexose and pentose can be utilized to produce bioethanol.
