All forms of alcohol prepared by fermentation contain a fraction of high boiling-point, which is termed fusel oil, and amounts to about [p086] 0·1 to 0·7 per cent. of the crude spirit obtained by distillation. This material is not an individual substance, but consists of a mixture of very varied compounds, all occurring in small amount relatively to the ethyl alcohol from which they have been separated. The chief constituents of the mixture are the two amyl alcohols, isoamyl alcohol,
(CH3)2·CH·CH2·CH2·OH,
and d-amyl alcohol,
CH3·CH(C2H5)·CH2·OH,
which contains an asymmetric carbon atom and is optically active. In addition to these, much smaller amounts of propyl alcohol and isobutyl alcohol are present, together with traces of fatty acids, aldehydes, and other substances.
The origin of these purely non-nitrogenous compounds was usually sought in the sugar of the liquid fermented, from which they were thought to be formed by the yeast itself or by the agency of bacteria [Emmerling, [1904], [1905]; Pringsheim, [1905], [1907], [1908], [1909]], whilst others traced their formation to the direct reduction of fatty acids. Felix Ehrlich has, however, conclusively shown in a series of masterly researches that the alcohols, and probably also the aldehydes, contained in fusel oil are in reality derived from the amino-acids which are formed by the hydrolysis of the proteins.
The close relationship between the composition of leucine,
(CH3)2·CH·CH2·CH(NH2)·COOH,
and isoamyl alcohol,
(CH3)2·CH·CH2·CH2·OH,