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A Review on Fusel Alcohol Formation by Yeast - Based on a lecture given at the 32nd European Brewery Convention Congress in Hamburg, 10-14 May 2009
Hazelwood, L. A., Daran, J.-M., van Maris, A. J. A., Pronk, J. T., Dickinson, J. R.

Fusel alcohols and the esters derived therefrom are important flavour and aroma constituents in beers. Consistent batch-to-batch maintenance of the desired concentrations of these compounds is essential. The formation of fusel alcohols is one of the longest-studied biochemical processes: investigations having been begun by Ehrlich in 1907. The aims have been (i) to define the steps of the biochemical pathway (the Ehrlich pathway), (ii) to identify the enzymes involved and the genes which encode them, and (iii) to understand the biochemical and genetic regulation associated with changes in yeast's growth and environmental conditions. The methods included the use of amino acids specifically labelled with 13C followed by 13C NMR spectroscopy to identify the metabolic sequences, specific mutants suspected or known to encode particular enzymes/isoenzymes, overexpression of structural genes and transcriptome profiling. Leucine, isoleucine, valine, phenylalanine, tyrosine, tryptophan and methionine that are present in wort serve as the starting materials for the formation of isoamyl alcohol, activeamyl alcohol, isobutanol, 2-phenylethanol, tyrosol, tryptophol and methionol (respectively). The steps of the Ehrlich pathway are transamination in which the amino acid is converted into an -keto acid, then decarboxylation in which the -keto acid is converted to an aldehyde. The aldehyde is then reduced resulting in formation of the appropriate fusel alcohol. In aerobic conditions (not found in beer production), the aldehyde could be oxidized to the corresponding fusel acid. Four transaminases, 5 TPP-dependent decarboxylases, 16 alcohol dehydrogenases, 6 aldehyde dehydrogenases and 2 broad-spectrum reductases have roles in the pathway depending mainly upon the amino acid, growth phase of the yeast and other cultivation conditions. Transcriptional regulation of the structural genes explains most, but not all of the regulation observed. Posttranslational modification(s) of enzymes remain to be discovered. Timely use of the Ehrlich Pathway likely offers both metabolic and developmental advantages to a yeast.

Descriptors: Amino acid, fusel alcohol, genetics, physiology

BrewingScience - Monatsschrift fr Brauwissenschaft, 62 (September/October 2009), pp. 147-154