What is ethanol fermentation ;Brewing beer and Production Biofuel

  FIGURE 1 Industrial-scale fermentations to produce biofuel and other 
products are typically carried out in tanks that hold thousands of liters of 
medium.          

            The carbohydrates, largely polysaccharides, must first be degraded to disaccharides and monosaccharides. In a process called malting, the barley seeds are allowed to germinate until they form the hydrolytic enzymes required to break down their polysaccharides, at which point germination is stopped by controlled heating. The product is malt, which contains enzymes that catalyze the hydrolysis of the Beta linkages of cellulose and other cell wall polysaccharides of the barley husks, and enzymes such as alpha-amylase and maltase.         

            The brewer next prepares the wort, the nutrient medium required for fermentation by yeast cells. The malt is mixed with water and then mashed or crushed. This allows the enzymes formed in the malting process to act on the cereal polysaccharides to form maltose, glucose, and other simple sugars soluble in the aqueous medium. The remaining cell matter is then separated, and the liquid wort is boiled with hops to give flavor. The wort is cooled and then aerated.

          Now the yeast cells are added. In the aerobic wort, the yeast grows and reproduces very rapidly, using 
energy obtained from available sugars. No ethanol forms during this stage because the yeast, amply supplied with oxygen, oxidizes the pyruvate formed by glycolysis to CO2and H2O via the citric acid cycle. When all the dissolved oxygen in the vat of wort has been consumed, the yeast cells switch to anaerobic metabolism, and from this point, they ferment the sugars into ethanol and CO2. The fermentation process is partly controlled by the concentration of the ethanol formed, the pH, and the amount of remaining sugar. After fermentation has been stopped, the cells are removed, and the “raw” beer is ready for final processing.

              In the final steps of brewing, the amount of foam proteolytic enzymes that arise in the malting process(or head) on the beer, which results from dissolved proteins, is adjusted. Normally this is controlled by proteolytic enzymes that arise in the malting process.  

              If these enzymes act on the proteins too long, the beer will have very little head and will be flat; if they do not act long enough, the beer will not be clear when it is cold. Sometimes proteolytic enzymes from other 
sources are added to control the head.

              Much of the technology developed for the large-scale production of alcoholic beverages is now finding 
application to a wholly different problem: the production of ethanol as a renewable fuel. With the continuing depletion of the known stores of fossil fuels and the rising cost of fuel for internal combustion engines, there is increased interest in using ethanol as a fuel substitute or extender. The principal advantage of ethanol as a fuel is that it can be produced from relatively inexpensive and renewable resources rich in sucrose, starch, or cellulose—starch from corn or wheat, sucrose from beets or cane, and cellulose from straw, forest industry waste, or municipal solid waste. The raw material (feedstock) is first converted chemically to monosaccharides, then fed to a hardy yeast strain in an industrial-scale fermenter. The fermentation can yield ethanol for fuel and side products such as proteins that can be used as animal feed.