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17 Utilization of Microbial Potential for Bioethanol Production from Lignocellulosic Waste

17.5.2

Fermentation Process

Post–pretreatment and hydrolysis of LCBs, basic monomeric sugars are deliv-

ered because of depolymerization of the individual components cellulose and

hemicellulose that are then fermented by important microorganisms. The general

procedure is alluded to as fermentation. Ethanol maturation should be possible

either by solid state fermentation or submerged fermentation. Water is a significant

fluid in submerged fermentation that is utilized to make fermentation. Dampening

of LCB is done in a solid-state fermentation. The most widely recognized commer-

cial process of ethanol formation is submerged fermentation. A novel innovation

concentrated to increment xylose utilization by Pichia stipitis gave an increment in

bioethanol yield by 20–51%. Fermentation is carried out in different modes namely

batch, fed–batch, and continuous fermentation. It depends on kinetic characteristics

of microorganisms causing fermentation and different types of feedstocks. Batch

culture is primarily carried out in contained culture environment by inoculating

fermenting microorganisms into a primary fermentation media having calculated

amounts of nutrients and is set to ferment till the total depletion of the nutrients.

Batch culture is fermentation at its simplest where no supplementation is given

after inoculation apart from acid or alkali for maintenance of pH. Microorganisms

have been found to worn in high substrate concentration initially in batch mode of

fermentation. It also yields higher amount of bioethanol as a product.

Fed-batch systems are broadly utilized to make ethanol for commercial purpose.

In fed-batch mode, the microorganism is on low substrate feeding but it yields higher

amount of bioethanol. Fed-batch cultures are known to obtain higher yield with

respect to batch cultures for the generation of microbial metabolites. Primary bene-

fit of fed-batch over batch mode of fermentation is its potential to obtain maximum

viable cell concentration, allow product accumulation, and prolong culture lifetime.

The rate of feed flow limits the yield portions of fed-batch modes. Critical process

variables like pH, temperature, and dissolved oxygen at definite levels maintain the

optimum operation of fed-batch systems. Different kinds of bioreactors are suit-

able for continuous fermentation like plug flow reactors or stirred tank reactors. It

gives a higher product yield, highest productivities being obtained at lower dilutions.

The feed contains the nutrients for microbial growth, culture medium, etc. which is

pumped into a vessel continuously which has agitation system installed to ensure

equal distribution of nutrients.

17.5.3

Product Recovery of Bioethanol Post Fermentation

Distillation and distillation combined with adsorption are the two techniques by

which bioethanol is recovered from the fermentation broth [42]. The first step for

ethanol discovery is distillation of the fermentation broth to separate ethanol from

the broth water to obtain a concentration of 95%, and the residual liquid which

contains residual lignin, hemicellulose, unreacted cellulose and ash, organisms,