Ethanol fermentation
- 2. INTRODUCTION • Alcohol fermentation, chemically Ethanol fermentation is a biological process in which sugars such as glucose, fructose and sucrose are converted into cellular energy and thereby produces ethanol and carbon dioxide as metabolic waste products. • Yeast perform this conversion in the absence of oxygen, alcoholic fermentation is considered an anaerobic process. • In the developing countries, microbial fermentation processes are preferred for the production of alcohol. • This is mainly because of the cheap raw materials available. • With increasing oil process, many countries now realise the potential of alcohol production by fermentation.
- 3. PRODUCTION OF ETHANOL BY FERMENTATION • Many countries have started production of ethanol by fermentation process. • The organisms and the raw materials used, along with the production and recovery processes for alcohol are briefly described below… Microorganisms: • Certain yeasts and bacteria are employed for alcohol fermentation. • The type of the organism chosen mostly depends on the nature of the substrate used. • Among the yeasts, Saccharomyces cerevisiae is the most commonly used, while among the bacteria, Zymomonas mobilis is the most frequently employed for alcohol production.
- 4. Raw materials: • There are a large number of raw materials that can serve as substrates for alcohol fermentation. • They may be broadly categorized as sugary materials (e.g. molasses, whey, glucose, sucrose), starchy materials (e.g. wheat, rice, maize, potato) and cellulosic materials (wood, agricultural wastes). Microorganisms used in alcohol fermentations Microorganism Source of carbohydrate Yeasts Saccharomyces cereviciae Starch, sugar Kluyveromyces fragilis Starch, sugar Bacteria Zymomonas mobilis Starch Candida pseudotropicalis Lactose, whey
- 5. Pretreatment of raw materials: • Most of the raw materials of alcohol fermentation require some degree of pretreatment. • The actual process depends on the chemical composition of the raw material. • In general, the sugary raw materials require mild or no pretreatment while the cellulosic materials need extensive pretreatment. • This is because the cellulosic substances have to be subjected to acidic or enzyme hydrolysis to release monosaccharide units that are needed for alcohol production.
- 8. PRODUCTION PROCESS OF ETHANOL • Ethanol production can be carried out in three stages- preparation of nutrient solution and inoculum, fermentation proper and recovery. Preparation of nutrient solution (media) : • The most commonly used raw materials are molasses, whey, grains, potatoes and wood wastes. • When molasses is used for fermentation, it is diluted with water so that the sugar concentration is in the range of 10- 18%. A concentration higher that this is detrimental to the yeast. • When starchy materials are used , they have to be first hydrolysed by pretreatment for use as nutrients.
- 9. • This may be done by barley malt, dilute acids or fungal amylases (e.g. Aspergillus sp, Rhizopus sp.) Preparation of inoculum: • After selection of the desired organism (yeast or bacteria) and its isolation in pure form, the inoculum is prepared under aseptic conditions. • For this purpose, the organisms are first cultured in flasks under aerobic conditions to increase the size of the inoculum which can be used for inoculation. Fermentation proper: • Originally batch fermentation was adopted. Now, continuous fermentation is used. • It has been possible to increase alcohol production by 10-12 fold by continuous fermentation compared to batch fermentation.
- 10. • Industrial production of alcohol is carried out in huge fermenters up to a size of 1,25,000 gallons. • The ideal pH is around 4.0-4.5. the initial temperature is kept between 21-26˚C. • Ethanol gets evaporated at temperature above 27˚C. • Aeration is initially required for good growth of the organisms. • Later, anaerobic conditions are created by withdrawal of oxygen coupled with production of carbon dioxide. • It takes about 2-3 days for the fermentation to be completed. • As the fermentation is complete, the fermentation broth contains ethanol in the range of 6-9% by volume. This represents about 90-95% conversion of substrate to ethanol.
- 11. Recovery of ethanol: • The cell mass is separated by centrifugation or sedimentation. • Ethanol from fermentation broth can be recovered by successive distillations. For a concentration above 95%, special techniques of distillation have to be adopted. • For a preparation of absolute (100%) alcohol, an azeotropic mixture of benzene, water and alcohol is first prepared. This mixture is then distilled by gradually increasing the temperature. • By this technique, it is possible to first remove benzene- ethanol-water mixture, and then ethanol-benzene mixture. Thus, absolute alcohol is left out.
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