Higher Alcohols

Yeast create other alcohols aside from ethanol; these are fundamental component of brandy aroma. Rodrigo Blazquez, 2004

Higher alcohols (also called Fusel oil) are alcohols that have more than 2 carbons (Ethanol has two carbons CH3-CH2-OH) and thus have higher molecular weight and higher boiling point.

Higher alcohols are present in wines and are formed in small amounts by yeast metabolism during alcoholic fermentation process. Higher alcohols are produced either from sugars and from amino acids (Erlich mechanism). Sugars and amino acids by a sequence of reactions are transformed into higher alcohols.
A higher alcohol can be related with its amino acid from which it is formed, some examples: Leucine with 3-methylbutanol, Isoleucine with 2-methylbutanol, Valine with 2-methylpropanol, Threonine with propanol and Phenylalanine with 2-phenylethanol. 
The amount produced during fermentation is dependent of the genus, specie and strain of yeast, the specific nutrient status (nitrogen and amino acids) and composition of the must and temperature, aeration and pH during fermentation. 

Higher alcohols can have an aromatic effect in wines and some higher alcohols can be considered positive and others can be considered negative to the aromatic wine profile. However, due to the concentration that are found in wines and its high threshold, higher alcohols does not have many sensory effects in wine.
Higher alcohols have a major importance in wine distillate (grape spirits or brandy), due to the fact that in distillates higher alcohols are found in grater concentration.

The total concentration range in wine is between 100-500 mg/L. Guymon and Heitz, 1952 (cited in Boulton et al 1996) reported the following range of concentration for white wines (162 – 266 mg/L) and for red wines (140 – 417 mg/L) produced in California.
Isoamyl alcohol is the major Higher alcohol found in wines (more than 50%) and its concentration has been reported in the range of 90 to 292 mg/L (Usseglio-Tomasset 1975, cited in Boulton et al 1996).

List of compounds:
The major four higher alcohols found in wines in order of amounts produced are:

3-methylbutanol (isoamyl alcohol):        (CH3)2-CH-CH2-CH2-OH
2-methylbutanol (active amyl alcohol):    CH3-CH2-CH(CH3)-CH2-OH
2-methylpropanol (isobutyl alcohol):     (CH3)2-CH-CH2-OH
1-propanol (n-propyl alcohol):        CH3-CH2-CH2-OH

others that can be found, not limited to below, are:
isopropanol:                 (CH3)2-CH-OH
n-butanol:                CH3-CH2-CH2-CH2-OH            
n-amyl alcohol:            CH3-CH2-CH2-CH2-CH2-OH
n-hexanol:                CH3-CH2-CH2-CH2-CH2-CH2-OH
2-phenylthanol:            (C6H5)-CH2-CH2-OH

Determination of Higher alcohols is performed with Gas Chromatographic analysis.

Boulton, R.B., V.L. Singleton, L.F. Bisson, and R.E. Kunkee.  1996. Principle and Practices of Winemaking. Chapman & Hall, New York. Pages 150-166.
Guymon, J. F., and J. E. Heitz. 1952. The fusel oil content of California wines. Food Technology. 6: 359-362. 
Margalit, Yair. 1997. Concepts in Wine Chemistry. The wine appreciation guild. San Francisco. CA. Pages 23-24.
Useeglio-Tomasset, L. 1975. Volatiles of wine dependant on yeast metabolism. Proc. 4th Intl.Oenol. Symp. Valencia, Spain. p. 346-370.
Zoecklein, B., K. Fugelsang, B. Gump, and F. Nury. 1995. Wine Analysis and Production. Chapman & Hall, New York. Pages: 101-104 and 422.

Rodrigo Blazquez, 2004