74 ALDEHYDES AND ALCOHOLS 



about 60°, whereby the starch undergoes hydrolysis with 

 formation of maltose and dextrin : — 



(CeHioOsln + H,O^Ci,H,,0„ + (QHioO.^x 

 Starch Maltose Dextrin 



After about one and a half hours the mixture is rapidly 

 cooled to 15° and mixed with yeast; fermentation at once 

 sets in, accompanied by a considerable evolution of heat ; the 

 mixture is therefore cooled artificially, so that the temperature 

 is maintained steady at about 27-5-30°. 



During this time the maltose is converted first into dex- 

 trose and then into alcohol and carbon dioxide according to 

 the equations — 



CfiHiPa = 2C,H50H + 2CO2 



In order to convert the dextrin, which would otherwise be 

 lost, into a fermentable substance, the temperature towards 

 the end is maintained at about 26-29° in order to give the 

 malt a further opportunity of hydrolysing the dextrin to 

 glucose, and so rendering it capable of being fermented by 

 yeast. When the fermentation is completed after about three 

 days, the mixture contains about 13 per cent of alcohol by 

 volume ; by distilling the mixture through a fractionating 

 column, so much of the water is removed that the distillate 

 contains about 80 to 95 per cent of alcohol.* 



No amount of fractional distillation without dehydrating 

 agents will produce alcohol containing less than 4-43 per cent 

 by weight of water, since such alcohol gives a constant boiling 

 mixture. 



Alcohol containing 0*5 per cent or less of water is, in 

 commerce, known as absolute alcohol, although in a scientific 

 laboratory the term is only correctly applied to alcohol which 

 is quite free from moisture ; such alcohol can only be obtained 

 by careful fractionation from freshly burnt quicklime. f If 



* The residue remaining after distillation contains, in addition to the 

 solid unfermentable materials, a certain amount of other soluble products 

 of fermentation, such as glycerol and succinic acid ; it is used as a cattle 

 food. 



t Occasionally the last traces of moisture are removed by treating the 

 alcohol with sodium wire. 



