Alcohol Production

2.1. Alcohol Production

In general, the requirements for a glaring cost-effective fermentation of organic feedstocks for

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Reasonably lower transportation costs of raw materials. Low costs involved for conversion of polymers viz., wood, cellulose, hemicellulose, starch

etc. into their corresponding mono- and disaccharides. Application of ‘mixed cultures’ so as to catabolize specially altogether different types of

substrates and to convert them into the desired metabolite. Usage of highly characteristic thermophilic strains to minimize and save costs for cooling,

to bring about excessive conversion rates, and also to lower drastically contamination risks. High energy consumption and demand for aeration, anaerobic methods are invariably pre-

ferred. Amenable to a feasible continuous process. Low recovery and concentration costs.

It is however, pertinent to state at this juncture that ‘ethanol’ for utilization as a chemical feedstock was solely generated via fermentation in the early era of industrial microbiology. Interestingly, with the passage of time for many years it has been duly produced via chemical means instead, primarily through

the catalytic hydration of ethylene (CH 2 = CH 2 ). Again in the recent past adequate and legitimate atten- tion has turned towards the copious production of ethanol urgently required both for fuel and chemical utilities by the phenomenon of fermentation exclusively.

Examples : In 1986, three major countries in the world produced the following quantum of

‘industrial ethanol’ :

USA

: 2.5 × 10 9 L

Germany : 1 × 10 8 L

Brazil

: 1.1 × 10 10 L

Agriculturally rich and intesive areas in Brazil, United States, and South Africa are under vigorous and intensive investigations with respect to the viable production of ethanol from various carbohydrates e.g., starch and sucrose. The cardinal objective of such research oriented activities is to maximise

exploitation of ‘ethanol’ as a substitute for automobile fuel (i.e., mixed usually with ‘gasoline’). Of course in certain countries the ‘ethanol’ obtained from fermentative procedures is used exclusively to produce petrochemicals and ethylene.

Efficiency of energy conversion by fermentation to ‘ethanol’ varies appreciably depending solely upon the type of ‘raw material source’ used.

Examples : Under optimal parameters the efficiency of energy yield* is as stated under :

Carbohydrate Source Efficiency of Energy Yield (%)

Sugar beet

Sugat Cane

* Ratio of energy demand to energy produced.

PHARMACEUTICAL BIOTECHNOLOGY

Biosynthesis of Ethanol : Bacteria and yeast possessing enormous potential have been em- ployed for commercial production of ethanol, such as : the most abundantly used —

Organism (Bacteria) : Zymomonas mobilis, and Yeast : Kluyveromyces fragilis ; Saccharomyces cerevisiae It has been amply demonstrated that under aerobic fermentative parameters and also in the

presence of reasonable high glucose concentrations, the yeast Saccharomyces cerevisiae no doubt grows very well but fails to yield any ethanol at all. Nevertheless, under strict anaerobic conditions the following steps occur sequentially as shown in Fig. 6.1 below :

Pyruvate Decarboxylase

PATHWAY

[Mg ; Thiamine pyrophosphate] 2+

Alcohol Dehydrogenase

[NADH ] 2

ETHANOL

Fig. 6.1. Biosynthesis of Ethanol.

Explanations : The various steps involved in the biosynthesis of ethanol may be explained as under :

Step 1 : Growth of said yeast cells slows down. Step 2 : Pyruvate from ‘glycolytic pathway’ gets split under the influence of enzyme ‘pyruvate

decarboxylase’ into corresponding acetaldehyde and carbon dioxide. Step 3 : Ultimately ethanol is produced from acetaldehyde by reduction with alcohol

dehydrogenase (NADH 2 ).

Ethanol Tolerant Mutants : It has been duly established that by using pure sugar solutions

one may accomplish ethanol concentration upto a maximum level of 10% only ; whereas, the skilful application of specifically designed ethanol-tolerant mutants one may even produce 12-13% ethanol, which are currently being scaled-up to full fledged production. Importantly, in recent years the widely acclaimed bacterium Zymomonas mobilis has, in fact, proved to be of tremendous potential advantages, namely :

(i) Osmotic tolerance to higher sugar concentrations (upto 400 g.L –1 ). (ii) Relatively enhanced ethanol tolerance (upto 130 g.L –1 ). (iii) Increased specific growth rate in comparison to yeast (growth rate being, µ of 0.27 com- pared to 0.13 for yeast i.e., almost twice in laboratory culture studies).

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(iv) Anaerobic carbohydrate metabolism is invariably performed via the Entner-Doudoroff path- way, wherein only one mole of ATP is produced per mole of glucose consumed, thereby minimising the quantum of glucose remarkably which gets converted to biomass rather than ethanol ultimately.