**4.1 Pretreatment**

Recently, the conversion of valuable food resources into alcohol fuel is facing very negative criticism worldwide, and work is underway to switch the raw material for bio-ethanol to non-edible biomass. However, the production of bio-alcohol *Alcohol Fuels: Current Status and Future Direction DOI: http://dx.doi.org/10.5772/intechopen.89788*

#### **Figure 8.**

*Schematics of bio-butanol production process [4].*

from non-edible cellulosic biomass requires solving the problem of breaking the hard biomass structure before converting into alcohol fuels. Pretreatment step is important. The pretreatment process is costly since it involves several process steps and costs for enzymes. It is very important to develop the low-energy/energysaving process scheme and the suitable enzyme to overcome such technical/cost barriers.

The first challenge in the conversion of biomass to alcohol fuels starts with the difficulty in breaking down the recalcitrant structure of biomass cell walls and further breaking down the cellulose to 5–6 carbon sugars that can be fermented by microorganisms [8]. Size reduction and uniformization in density/size are the first preparation step. Pretreatment by steam, hot water, or slight carbonization is a common procedure.

Various ways of pretreatment are used in biomass conversion to alcohols as illustrated in **Table 5**. Recent types include steam explosion auto-hydrolysis, wet oxidation, organosolv, and rapid steam hydrolysis (RASH) [16]. Organosolv is a pulping technique that uses an organic solvent to solubilize lignin and hemicellulose. The principal purpose of most pretreatment is to increase the susceptibility of cellulose and lignocellulose parts of biomass at the next process in which acid and enzymatic hydrolysis occur. Cellulose enzyme systems react very slowly with un-pretreated biomass, whereas the rates of enzymatic hydrolysis enhance dramatically when the lignin barrier around the plant cell is partially disrupted [16].

### **4.2 Saccharification (hydrolysis)**

Saccharification is basically a step of breaking down the cellulose/hemicellulose through hydrolysis to make sugars such as glucose and xylose. The overall hydrolysis is based on the synergistic action of three distinct cellulase enzymes depending on the concentration ratio and the adsorption ratio of the component enzymes (endo-beta-gluconases, exo-beta-gluconases, and beta-glucosidases) [16].

Two main procedures exist in hydrolysis: acid hydrolysis and enzymatic hydrolysis. Most commonly employed procedure is the enzymatic one because it has a


#### **Table 5.**

*Pretreatment technologies currently available for alcohol fuels [12].*

better environmental and economic performance. Acid hydrolysis operates under severe conditions of high temperature and low pH, which results in corrosive conditions and requires a special construction material [17].
