Industrial production of ethyl alcohol by fermentation.

1. Industrial production of ethyl alcohol by fermentation

              The raw material used for the large scale production of ethyl alcohol by the process of fermentation Molasses and starch.

1.1. FROM MOLASSES 

In our country molasses is the chief raw material for the production of ethyl alcohol.
Molasses contains about 30% sucrose and about 32% invert sugar.
As strong sugar concentration is not suitable for fermentation by yeast, it is diluted with water to a 10% sugar solution called " mash". It is then transferred to a large steel tank called fermenter and made slightly acidic (PH 4.0 to 5.0) by adding sulphuric acid.
Acid prevents the growth of unwanted bacteria and promotes vigorous growth of yeast cells beneficial for alcoholic fermentation.
Required amount of ammonium sulphate and ammonium phosphate are added as nutrient for yeast. At 21°C  yeast culture is added to a liquor.
Within a few hours evolution of carbon dioxide gas can be observed and after 24 hours the solution appears to boil due to excessive frothing.
The enzyme invertase present in yeast cell hydrolyses sucrose to glucose and fructose.
Another enzyme zymase then acts upon glucose and fructose converting them to ethyl alcohol and releasing carbon dioxide.

       The fermenters are closed vessels for the collection of CO2 as by-product. They are water cooled and the temperature is kept below 37°C.
Fermentation is completed within 2 to 3 days. The fermented liquor  called " wort" contains 6-8% alcohol by volume which is concentrated by fractional distillation. Is the alcohol percentage goes above 14, the growth of yeast is inhibited.

1.2. FROM STARCHY MATERIALS 

Common sources of starch are wheat, Barley, potato, rice, maize etc.
Grains are broken to uniform size in a mill (potatoes cut into thin slices) and made into a slurry with water and cooked for 2-3 minutes at about 65°C. The slurry, now called mash, is subjected to super heated steam at 180°C for one minute when starch grains become exposed. The enzyme diastase in the malt hydrolyses starch to sugar maltose.

      The liquor is further cooled to 15°C, Pumped to fermenter and yeast is added. The enzyme maltase present in yeast hydrolyses maltose to glucose. Glucose is converted to alcohol by enzyme zymase.

    The fermented liquor called 'beer' (general term applied to such fermented liquor) contains 8-10% alcohol.
As mentioned earlier,the dilute alcohol solution is concentrated by fractional distillation.

1.3. DISTILLATION OF DILUTE ALCOHOL         

The Dilute alcohol obtained either from molasses or starch containing 6-8% alcohol is concentrated by fractional distillation to give 95.6% alcohol (by volume) known as rectified spirit.
The fractional distillation is carried out in a special still called 'coffey's still' which consists of analyser, rectifier, condenser and heat exchangers.
The dilute alcohol enters at the top of the analyser from the bottom of which steam is injected.
The steam  volatilises alcohol and as the steam-alcohol vapour rises up the tower, the vapour becomes gradually richer in alcohol towards the top of the tower.
The alcohol rich vapour then enters at the bottom of the rectifying column and further separation takes place as less volatile water vapour and fusel oil condense towards the bottom of the column and rectified spirit is collected at the top of the rectifying column.
By this process three fractions are collected separately.
FIRST FRACTION : it contains mainly acetaldehyde (bp. 21°C).
SECOND FRACTION : this fraction is rectified spirit which is a mixture of 95.6% ethyl alcohol and water by volume (bp. 78.15°C, azeotropic mixture).
THIRD FRACTION : it is fusel oil which contains higher alcohols like n-propyl,n-butyl, n- amyl and iso amyl alcohol (bp. 125°-140°C).
Residue : the residue contains glycerol and succinic acid.

1.4. PREPARATION OF ABSOLUTE ALCOHOL 

By fractional distillation of rectified spirit absolute alcohol (100% ethyl alcohol) cannot be prepared as 95.6% alcohol and 4.4% water forms constant boiling mixture at 78.15°C.
However, remaining water can be removed from rectified spirit by the following processess -
(a) distillation of rectified spirit over quicklime furnishes, 99.7 % ethyl alcohol. The remaining trace of water can be removed by distillation over metallic calcium. The method is used in a small scale for laboratory work.
(b) however, commercial production of ethyl alcohol is done by distilling rectified spirit with proper quantity of benzene when azeotropic distillation occurs.
First fraction comes out at 64.8°C consisting of benzene (74.1%), ethyl alcohol (18.5%) and water (7.4%).
After removal of all water, the second fraction comes out at 68. 21°C containing benzene (67.6%) and ethyl alcohol (32.4%).
When benzene is completely removed, absolute alcohol distilles at 78.1°C as the third fraction.

1.5. OTHER ALCOHOLIC PRODUCTS 

Ethyl alcohol is subjected to heavy excise duty in most of the countries of the world which makes it too costly for industrial use.
To avoid this difficulty, ethyl alcohol is mixed with certain chemicals to make it unfit for drinking purpose and sold duty free or with lower duty.
(a) denatured spirit : ethyl alcohol is mixed with methyl alcohol, pyridine, raw rubber juice to make it unfit for human consumption.
(b) methylated spirit : ethyl alcohol is mixed with methyl alcohol only and made unfit for drinking purpose.
Other major yeast fermentation process in which ethyl alcohol is an important end product is used in the industrial production of beverage alcohol (beer, whisky, rum, wine etc).
A brief outline is given in the table 1. :

Table.1. Brief outline of the method of preparation of some alcoholic beverages.

Name	Type Of Yeast	Method Of Preparation	Important condition	Percent Of Alcohol
Beer	Saccharomyces Cerevisiae	Barley malt and starch are mixed with warm water.after enzymic starch conversion,wort is filtered.it is boiled with hops (bitter cones of a climbing plant)and fermented with yeast.	Temp.8-12°C, PH= 4-5.5,fermentation time : 4-9 Days.	3-8
Scotch Whisky	-do-	cooked grain mash is saccharified with peated malt and then fermented with yeast. The distilled liquor is aged in oak casks for at least 3 years. it is then blended with grain whisky.	Temp.26°C,PH= 4-5, Fermentation time: 72 hours.	50
Rum	-Do-	To blackstrap molasses (12-14% fermentable sugar) required amount of ammonium sulphate and ammonium phosphate are added as nutrient. After fermentation with yeast the liquor is distilled.	Temp.21°C (initial)to 35°C (final), PH= 4-4.7,Fermentation time: 3-7 days.	40
Wine	S.ellipsoideus	grapes (sugar concentration upto 22%) are allowed to ferment with special strain of yeast or yeast naturally present on the outer skin of grapes. after fermentation the liquor is stored for ageing to improve the aroma and flavour derived from the original fruit.	Temp. below 29.4°C, PH=natural acidity of grape juice, time:7-11 days.	8-14