The biochemical reaction which converts sugar to ethanol is depicted below:
glucose ethanol carbon dioxide
This equation tells us that one molecule of sugar (glucose) in the presence of yeast produces two molecules of ethyl alcohol and two molecules of carbon dioxide. The yeast itself, which is a living organism, is not consumed in the reaction but merely acts as a catalyst. The yeast cells die, however, and in the absence of oxygen will not replenish themselves, so eventually the yeast becomes inactive.
The atomic weights of carbon, hydrogen and oxygen are 12, 1 and 16 respectively, and when these weights are applied to each of the atoms in the above equation we find that 180 parts of glucose will lead to the production of 92 parts of ethyl alcohol and 88 parts of carbon dioxide. As a close approximation, therefore, a given weight of sugar will produce about one-half its weight of alcohol, i.e. 1 kg of sugar should give about 500 grams of alcohol. Because the specific gravity of ethyl alcohol is 0.8 the 500 grams represent 625 ml of absolute alcohol or 1V2 litres of 40 per cent alcohol, the normal strength of vodka and other spirits.
It should be understood that the above figures represent the ideal situation, the theoretical yield. Such yields are approached very closely in commercial practice and in well-equipped laboratories, but in the hands of amateurs the yield is unlikely to reach more than about 70 to 80 per cent of theory. There are two main reasons for this, one being the occurrence of side reactions which convert the sugar into a whole range of unwanted organic compounds such as methanol, acetic acid, fusel oils, etc. The second and major reason is a failure to recover all the alcohol from the fermentation broth during beer stripping. Losses such as these would not be tolerated in a commercial operation but are acceptable for the amateur. After all, even with a recovery as low as 70% of theory a kilogram of sugar valued at a dollar or so would produce over a litre of gin or vodka.
The conversion of sugar to alcohol by means of yeast is an anaerobic reaction; that is to say it occurs in the absence of air. If air is present the yeast, instead of producing alcohol, will multiply and grow. Wine-makers habitually buy a small quantity of an expensive, specialty yeast and let it grow in the presence of a little air and nutrients until they have the quantity they require. Then they cut off the air supply and the yeast starts making alcohol instead. In our situation such refinements are unnecessary because we use massive quantities of cheap baker's yeast which generate high yields of alcohol and large quantities of carbon dioxide. The CO2 is quite effective in excluding air without the use of air-locks.
Under such crude conditions the yeast and sugar will produce a wide range of organic compounds in addition to ethanol, a situation which would be unacceptable if we were making wine or beer and had to drink these unpleasant and even harmful substances. However, the presence of such impurities is of small concern to us because they will all be removed during distillation.
The production of extraneous compounds will be aggravated by sloppy practices so, although it is not as necessary to be as careful as it would be during wine-making, reasonably hygienic conditions should be maintained at all times. Otherwise one is simply wasting sugar.
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