Ethanol and concentrated alcoholic drinks by fermentation- used since long for consumption.
Microbial Fermentation in early days. But now, chemical synthesis
In 1906 when the Industrial Alcohol Act was passed, the production of industrial alcohol, ethanol, became commercially feasible on a large scale.
There are various uses of alcohol such as,
1. Ethanol is used as an organic solvent in many different products as paints, perfumes, markers inks, varnishes, soaps, and other products like explosives.
2. The most popular use of ethanol is as motor fuel and fuel additive (gasohol)- renewable source for environmentally friendly fuels.
4. Ethanol is used as a potent food preservative in homes.
5. Ethanol is used in making antiseptic soap and cosmetics as it is effective against bacteria, fungi, and many viruses, but is not much effective against bacterial spores.
6. Ethanol is routinely consumed in different forms around the world like beer, wine, gin, whisky, etc. Beer is the most popular ethanol beverage in the world.
7. Ethanol plays an important role in making drugs and pharmaceuticals.
8. Ethanol is used in the preparation of essences and flavorings
9. Ethanol is used as the fluid in thermometers.
10. Ethanol is used in preserving biological specimens.
Microbial Fermentation
- Ethanol fermentation is a biological process in which sugars are utilized to produce ethanol and carbon dioxide as metabolic waste products.
- This process is carried out by yeast cells that converts glucose to CO2 and alcohol.
- C6H12O6 (glucose) → 2 C2H5OH (ethanol) + 2 CO2 (carbon dioxide)
- Before fermentation, one glucose molecule is converted into two pyruvate molecules by glycolysis.
- Under aerobic condition, some species of yeast (e.g., Kluyveromyces spp.) will oxidize pyruvate completely to carbon dioxide and water which is termed as cellular respiration.
- These species of yeast will produce ethanol only under anaerobic condition.
- So ethanol fermentation is classified as anaerobic process.
- However, many
yeasts such as Saccharomyces cerevisiae (baker's yeast), Schizosaccharomyces pombe (fission
yeast) will produce ethanol even under aerobic conditions
- Microorganisms
Many yeasts and bacteria such as Zymomonas mobilis are suitable for ethanol production. But, all yeasts are not suitable for brewing. Choice of yeast for the alcohol production depends upon the composition of the medium, particularly the type of carbohydrate used in the medium.
.
|
Raw materials |
Fermentation Strain |
|
Starch and sugar
|
Saccharomyces cerevisiae |
|
Lactose of whey |
Candida pseudotropicalis
|
|
Sulfur waste liquor
|
Candida utilis |
Selected strain should
ü grow rapidly.
ü tolerate high concentrations of sugar.
ü be able to produce abundant amounts of alcohol
ü be resistant to the produced high concentrations of alcohol.
Ethanol Fermentation
Three stages
- Media Preparation
- Fermentation
- Product
harvesting/Distillation
Ethanol biosynthesis
1. Medium
The media for the commercial production include any of these Carbon sources- blackstrap
Molasses/beet molasses, corn, potatoes, grains, wood wastes, sulfite waste
liquor, whey.
Molasses (Blackstrap molasses) is commonly used.
Molasses must be diluted with water or water plus stillage to a final sugar
concentration between 10 -18%. Concentrations greater than 20% are not employed
as they could inhibit the growth of yeast.
Molasses wort can be directly used or after the addition of
ammonium salts (ammonium phosphate/sulphate). The pH of the medium is set
between 4.8 to5 by adding sulfuric acid or lactic acid. An initial lactic acid
fermentation by lactic acid bacteria is preferred since it inhibits butyric
acid bacterial contaminants and a pH value of <5 also inhibits LAB.
Other microbial contaminants are inhibited by low pH, high sugar
concentration, anaerobic fermentation conditions and high amount of alcohol
production by yeast.
So molasses medium require only pasteurization and not
sterilization
If beet molasses is used, a small amount of black strap molasses
is added to meet biotin requirements.
In case of starchy media (corn, rye and barley), initial starch
hydrolysis is necessary. This can be done by squashing with barley malt, by
mixing with dilute acids or by utilizing amylolytic enzymes of fungi like
Aspergillus and Rhizopus.
Inoculum preparation
Inoculum is yeast culture.
Optimum pH 4.8-5.
Optimum temperature 28-30 o C.
High degree of aeration and agitation
required for rapid growth of yeast to obtain large amount of biomass.
Inoculum built up in 10 ml test tube
followed by 200 ml flask, 1l glass container, 5l glass container and finally
small tanks/yeast vessels
Inoculum added to the fermenter by pumping
or under gravity; this process is called pitching.
Fermentation process
8-10% yeast culture used as inoculum
Carbon source- 10-18% sugar (molasses wort)
Nitrogen source- organic/inorganic
nitrogen compounds like ammonium sulphate- limited amount of nitrogen since
growth is promoted by excess nitrogen.
Optimum pH 4.8-5.
High pH- more chances of contamination
Low pH- low yield of ethanol
Optimum temperature 28-30 o C
Heat gets evolved during fermentation and it
raises the temperature, which is brought down by cooling coils. Intermittent
agitation ensures uniform cooling. Otherwise, evaporation loss of ethyl alcohol
is caused. At high temperatures, there are more chances of bacterial
contamination also.
Fermentation continues for 2-3 days, but
exact time period is based on the substrate utilized and temperature.
Aeration not applied during fermentation
and the medium quickly turns anaerobic because of
·
Withdrawal
of O2 from the medium by yeast
·
Rapid
evolution of CO2 which starts shortly after pitching
Growth starts within a few hours after
addition of inoculum- very rapid till 24h
Alcohol production from 30-72 h.
At the end, fermentation broth contains 6
-9 percent alcohol by volume.
Continuous fermentation processes used
nowadays commercially.
90 -98% theoretical conversion of substrate sugar to alcohol.
Fermented broth is allowed to settle for
few hours and then subjected to fractional/successive distillation to obtain
rectified spirit (Ethanol of 95% concentration) and fusel oil fractions.
Ageing, dilution and blending results in matured rectified spirit such as rum, gin, whisky and brandy.
Alcohol can be marketed as rectified spirit, denatured spirit or special denatured spirit.
Alcohol is also used for the manufacture of ether, acetaldehyde, acetic acid, butanol etc.
Fusel oil is high boiling fraction recovered during distillation of fermentation broth and forms about 0.5% of crude distillates. It is mainly a mixture of n-amyl and iso amyl alcohols. It can be used as lacquer solvent, in perfumes and liquors. It is converted into acetates (n-propyl acetate, amyl acetate and iso propyl acetate) by esterification and used in paint and lacquer industry.
(lacquer-liquid which is painted on wood or metal ito protect it and to make it shiny)
Other byproducts include solid CO2 or dry ice (soft drink industry), trace amounts of succinic acid, large amounts of glycerol, some acrolein from microbes degrading glycerol. Spent wash and bottom sludge has high potassium salt content and can be used as potassium fertilizer.
To obtain 100% or absolute alcohol, special distillation technique is required. For that 5% water is removed by forming an azeotropic mixture of benzene, water & ethanol, which then is distilled at increasing temperature. This procedure removes first the azeotropic benzene- ethanol- water mixture, and then benzene-ethanol mixture, so that absolute ethanol remains.
(azeotrope- a constant boiling point mixture is a mixture of two or more liquids whose proportions cannot be altered or changed by simple distillation.)
Glycerol
Addition of sodium bisulphite during fermentation modifies the alcohol fermentation pathway. Sodium bisulphite binds with acetaldehyde thus removing it. Dihydroxy acetone phosphate remains which is reduced to form glycerol phosphate which is further dephosporylated to glycerol. Osmotolerant yeasts such as Saccharomyces rouxii, Torulopsis magnoliae and Pichia farinosa is used in glycerol fermentation.
References
Principles of Fermentation Technology: (2nd edition, by Peter F. Stanbury, Allan Whitaker and
Stephen J. Hall, Butterworth-Heinemann, An imprint of Elsevier Science.)
Industrial Microbiology: (By Casida L. E.New Age international (P) ltd publications)
A Text Book of Industrial Microbiology: (2nd edition By Wulf Crueger & Anneliese Crueger)
No comments:
Post a Comment