Yield of alcohol

In the chapter on fermentation it was explained that the theoretical yield of pure, 100 percent alcohol from 10 kg of cane sugar is 6.25 litres. This is equivalent to 6.58 litres of 96 percent alcohol or 15.63 litres of 40 percent alcohol. While it is possible to approach such a yield you will find in practice that you only reach 70-80% of this value due to various losses along the way. One place where you can expect losses to occur is in the fermentation process — for example, you may not have left the brew long enough for all the sugar to have been completely used up. Or the yeast may have lost some of its activity. And then there are all those unwanted side reactions which produce the congeners such as methanol, fusel oils, etc., instead of ethanol. However, the major place where losses occur is in the last stages of beer-stripping where time and energy consumption require that the stripping cease long before the last drop of alcohol has been extracted. As a result, the practical yield of 96 percent alcohol is likely to be no better than about 5 litres which is a yield of 73% of the theoretical value. This is equivalent to 11V2 litres of vodka or gin, which is not too bad.

In commercial practice such a low yield would not be tolerated, but for us it should be quite acceptable, particularly on economic grounds. Higher yields, which are certainly possible, offer an interesting challenge to the dedicated amateur.

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