**3.3 Ultrasonic pretreatment**

*Elements of Bioeconomy*

**3. Pretreatment**

**3.1 Mechanical treatment**

**3.2 Steam explosion**

consisted of p-coumaryl alcohol derivatives [31].

and sinapyl alcohol units (**Figure 4**). It is categorized as softwood lignin when the coniferyl alcohol derivatives predominant, hardwood lignin where both coniferyl and synapyl alcohol derivatives exist together and grass lignin where it chiefly

Lignin is a recalcitrant component of the lignocellulosic biomass. Resistance to chemical and enzymatic attack increases with increase in lignin content [32]. Lignin the natural cement, acts as a ceiling for microbial/enzymatic attack. Hence, it is one of the major hurdles in using lingo-cellulosic materials in fermentation. Pretreatment is one of the most important steps in the process of converting renewable lignocellulosic biomass into useful products. The main target of any pretreatment is to alter or remove structural and compositional resistant to hydrolysis which further enhance digestibility of biomass [33]. It exposes cellulose and hemicellulose chains by breaking the crystalline matrix (**Figure 5**). To remove the obstacles for enzymatic scarification of lignocellulosic material following pretreatment used.

Major mechanical treatment includes chiping, grinding and milling to reduce the particle size which is responsible to increase surface area and increased surface area responsible for better interaction between substrate and enzyme [21, 35]. Physical treatment includes un-catalyzed steam explosion, hot water pretreatment and high energy radiations. By the process size reduces to 10–30 mm after chipping

Mason [36] first time introduced steam explosion in which biomass is pretreated at 180–240°C under 1–3.5 MPa pressure for 1–10 min with hot steam, followed by

the biomass and finally after milling or grinding 0.2–2 mm size is attained.

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**Figure 5.**

*Effect of pretreatment on lignocellulosic biomass [34].*

Scanning electron microscopy images reveal that ultrasonic treatment have the capacity to modify structure of lignocellulosic biomass [42]. Ultrasonic waves work by creating pressure difference within a solution [43]. The pressure wave travels through the liquid medium creating alternate regions of high (compression) and low (rarefaction) pressure (**Figure 6**).

### **3.4 Acid pretreatment**

In this method lignocellulosic material is dipped in an acidic solution (typically H2SO4), and subjected to optimum temperature. Dilute sulfuric acid had been used at commercial scale for pretreatment of various biomasses such as Switch grass [44] Corn Stover [45] and Poplar [46]. By acid catalyzed hydrolysis (**Figure 7**) most of the hemicelluloses are almost removed from the micro fibrils of the biomass but delignification is achieved to a lesser extent. Dilute acids are highly effective in removing hemicelluloses as dissolved sugars as a result of which glucose yield from cellulose increase to almost 100%. The optimal conditions to attain maximum sugar yield depends on the target to be achieved [47].

#### **3.5 Alkaline pretreatment**

It is responsible for the saponification of inter molecule delignification of the hemicelluloses. The biomass is exposed for the enzymatic hydrolysis of cellulose and hemicelluloses. As compared to other methods of pretreatment, alkali pretreatment is carried out for longer duration at low temperature and pressure [39]. It is supposed to act by saponification of inter-molecular ester bonds which are found to present between hemicelluloses and other components [48] (**Figure 8**). It is mainly responsible for

**Figure 6.**

*A pressure wave traveling through a solution [36].*

**Figure 7.** *Cellulose hydrolysis in acidic media [47].*

**Figure 8.** *Ether bond cleavage in alkaline solution [48].*

delignification of lignocellulosic biomass. But it also removes some acetyl and uronic acid substitutions on hemicelluloses, which expose the biomass for enzymatic hydrolysis of cellulose and hemicelluloses [49]. A major limitation of alkaline pretreatments is formation of some salts which are either irrecoverable or incorporated as salts into the biomass [50]. Reactor costs for alkali pretreatment are lower than those for acid pretreatments [51]. For a given quantity of biomass, lowest operating cost is for lime pretreatment [39]. However the use of more pricey salts at higher concentrations is the major drawback that poses environmental threats and may also hinder the recycling process [52].
