268

17 Utilization of Microbial Potential for Bioethanol Production from Lignocellulosic Waste

Bacterial/fungi

/enzyme

Biological

delignification

Hydrolysis

Fermentation

Biofuel

Agricultural refuse

Streptomyces, P.

chrysosporium,

LiP

Bacillus sp.,

cellulases,

xylanases,

etc.

S. cerevisiae,

C. shehatae

Bacterial/fungi

/enzyme

Bacterial

and/fungi

Figure 17.2

Overview of bioethanol production from lignocellulosic biomass.

17.2

Processing of Lignocellulosic Biomass to Ethanol

Pretreatment of LCB is the primary step in bioethanol production protocol. It

is considered to be the basic step which largely affects edibility of cellulose and

that unequivocally affects downstream expenses including detoxification, catalyst

stacking, squander treatment requests, and different factors (Figure 17.2). Pretreat-

ment establishes for over 40% of the all-out handling cost. The cellulose in LCB

is secured by lignin and hemicelluloses. Subsequently, it decreases the available

surface area accessible for enzyme-mediated saccharification. Pretreatment is

necessary to change the LCB to naturally visible and minute size. Appropriate

pretreatment might expand the convergence of fermentable sugars post-enzymatic

saccharification in this way increasing the general procedure efficiency. A perfect

pretreatment process is essential to enhance the hydrolysis of LCB [5]. The various

pretreatment methods of LCB include mechanical pretreatment, physico-chemical

pretreatment, chemical pretreatment, and biological pretreatment. Mechanical

pretreatment reduces cellulose crystallinity, is easy to handle, reduces degree of

polymerization, and increases surface area. But unfortunately, it has no high energy

input nor removes any lignin. Summary of commonly used pretreatment methods

are listed in Table 17.1. Physico-chemical pretreatment increases pore volume,

improves enzyme accessibility, removes hemicellulose, and reduces particle size.

But, it contributes to less lignin removal, it has high energy demands, and it

decomposes sugars.

Chemical pretreatment has high reaction rates, it removes hemicellulose more

efficiently, increases surface area, and creates an alteration in lignin structure.

Little lignin removal, requirement of neutralization process, inhibitors formation,

and requirement of disposal of neutralization salts are some of the drawbacks of

chemical pretreatment. To fill up the voids of all these pretreatments of LCB, a new

method of biological pretreatment has come up. It can degrade lignin successfully,

has less formation of inhibitors, has low energy and capital cost demands, no

chemical reagents are required, and it can reduce the polymerization degree of

cellulose and hemicellulose in mild environmental conditions. It just has a few

disadvantages like slow rate of delignification, longer residence timings, and loss of

carbohydrates as they are metabolized by the microbes [3]. Plenty of LCB namely