In the case of sugarcane the sugar can be pressed from the cane and used directly in fermentation to produce ethanol. Starch on the other hand cannot be used by yeasts in fermentation and so has to be converted to glucose before it can be used. This is the main difference between the processes used in the USA and Brazil.

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Figure 6.3 outlines the process that is used to obtain glucose from maize. The process was developed initially to produce starch from maize where the starch was used in food formulation, for the production of high fructose maize syrup, a low- calorie sweetener and a wide range of starch products. The harvested maize is soaked in water to loosen the kernels which are passed through a mill which removes the germ or embryo. The wash water is known as maize steep liquor and is one of the components of the medium used to grow penicillin. The separated germ is used for germ meal as a high protein supplement or pressed to extract maize oil (Mazola) used widely in cooking. The kernels, which now contain mainly starch, are ground, washed and centrifuged to remove fibre and gluten which are used in the food indus­try. What remains is starch which can be used in food and other products. Some of the starch is converted into glucose so that it can be processed into a mixture of glu­cose and fructose known as high fructose maize sugar (HFCS) for use as a low-calorie sweetener. The conversion of starch into glucose is carried out by starch-degrading enzymes, the amylases. Starch is synthesized in the chloroplast where glucose molecules are linked together with a-D-1,4 glycosidic linkages to form long chains (Fig. 6.4). At some stages, a branching by enzyme adds a side chain with a a-D-1, 6 linkage which gives starch a more rigid structure. The starch molecules can be bro­ken down either by hydrolysis with acid or by enzymatic breakdown. The enzymatic

process is normally used as it produces fewer by-products. The starch forms a stiff paste with water, which is first liquefied by heating to 60-80°C and the enzyme a-amylase added. The enzyme breaks the long glucose chains in the starch into shorter sections, known as long chain dextrins. In some cases, a high temperature a-amylase is used and the starch heated to 100°C. After a short period of time, the liquefied starch is cooled to 50-60°C and another enzyme, amyloglucosidase, added. This enzyme converts the dextrins into glucose.

Once glucose has been produced, it can be used as a substrate for ethanol fermen­tation by yeast. Figure 6.5 shows a typical process for the production of ethanol from glucose. The glucose from the starch is mixed with salts, nitrogen and phosphorus

and the mixture sterilized by heating to 120°C for 2-5 min in a continuous sterilizer. Once sterilized, the medium is run into a large bioreactor (200,000 l and above), yeast added from a seed bioreactor (1-10% volume of the main reactor) and the culture incubated at 30°C for a few days. Carbon dioxide produced during the fermentation can be adsorbed and used to make solid carbon dioxide, ‘dry ice’. Once fermentation has finished the yeast cells are removed by centrifugation and the medium, sometimes known as ‘beer’, is warmed by passing through a heat exchanger and then distilled. Distillation is needed to concentrate the ethanol and is the major energy-consuming stage. The fermentation yields about 10% ethanol and it needs to be more than 95% to be used as E95 or 100% if used as a blend. Heating a 10% ethanol solution will yield a vapour containing more ethanol than water and the remaining solution will contain more water so that a limited amount of enriched ethanol can be obtained. But with a series of distillations a concentration of 95.6% ethanol can be obtained. At 95.6% the liquid and vapour have the same concentration so no further concen­tration is possible. The mixture is known as an azeotrope. However, by using a distil­lation column separated by plates a series of separate distillations can be produced and this will give the azeotrope in one distillation. To produce anhydrous ethanol a second distillation is required where benzene is added and this on distillation gives pure ethanol and the benzene can be recovered and used again.