Page 55 - Advances in bioenergy (2016)

significant progress has been done on all these issues. On the other hand, consolidated
bioconversion or, in other words, direct conversion of cellulosic materials into advanced
biofuels is, up to now, partially successful. Cell-surface display engineering enabled the
4
expression of cellulolytic activities and minicellulosomes on S. cerevisiae cell surfaces. The
present review is aimed at summarizing both the hydrolysis technologies and bioconversion
processes employed for cellulosic ethanol production.

Figure 2.1 Cellulose structure is formed by β-(1,4)-linked D-glucose units, where adjacent D-
glucoses are flipped making cellobiose the fundamental repeating unit. The inter- and

intramolecular hydrogen bonds (shown as dots) and van der Waals interactions form
recalcitrance microfiber structures.

HYDROLYSIS TECHNOLOGY

Hydrolysis is one of the main technologies which are in use for converting cellulosic
feedstocks into ethanol. In hydrolysis, the polysaccharides (cellulose and hemicellulose)
present in a feedstock are broken down to free sugar molecules (glucose, mannose, galactose,
xylose, and arabinose). These free sugar molecules are then fermented to produce ethanol. As

lignin cannot be used for ethanol production, it is removed during the conversion process, and
is generally utilized to meet electricity or heat requirement of an ethanol mill.

In order to ensure an efficient enzymatic hydrolysis of cellulose, pretreatment (also called
first-stage hydrolysis) is needed to break down the shield formed by lignin and hemicellulose,
resulting in the disruption of the crystalline structure and the reduction of the degree of
polymerization of cellulose. The mixture obtained at the pretreatment stage is separated into
liquid (hemicelluloses hydrolysate consisted by xylose, mannose, arabinose, galactose, and
small amount of hydrolyzed cellulose in the form of glucose) and solid (lignin + unhydrolyzed
cellulose). Solids are sent for another round of enzymatic hydrolysis (also called second-stage
hydrolysis).

Pretreatment Methods (First-Stage Hydrolysis)

5
Physical pretreatment, such as hammer and ball milling, involve the breakdown of biomass
feedstock into smaller particles to increase the surface area of cellulose and have been shown