The Relative Contribution of Ehrlich and Biosynthetic Pathways to the Formation of Fusel Alcohols

Abstract

Ale yeast fermentations were carried out at 20° C with defined media containing a mixture of amino acids totaling 225 mg/L. Radioactive substrates were added individually to the media prior to the fermentation: The resultant beer alcohols were fractionated and their radioactivities were measured. When [14C(U)] glucose was used, 97.7% of the radioactivity acquired by beer alcohols was recovered in the ethanol fraction while the remaining 2.3% was distributed in the four fusel alcohols. When [14C(U)] threonine was added to fermentation media, 30% (on molar basis) of the resultant n-propanol became radioactive. This means that, under the experimental conditions used, the contribution of Ehrlich pathway to the formation of n-propanol is 30%. Similarly, when [14C(U)]-labeled valine, isoleucine, or leucine was used as tracer, the contribution of Ehrlich pathway to the formation of isobutanol, 2-methyl-1-butanol and 3-methyl-1-butanol was found to be 34%, 75%, and 80%, respectively. The results clearly indicate that the relative contributions of the Ehrlich and biosynthetic pathways vary with each fusel alcohol. In another experiment, [14C(U)] glucose was paired with tritiated threonine, valine, isoleucine, and leucine in dual-labeled fermentations. Results obtained also indicate that glucose utilization for the formation of fusel alcohols is relatively small and that the contribution of biosynthetic pathway decreases as the number of carbon atoms in fusel alcohol molecules increases. The application of dual label tracer technique in the present study was discussed. Methods for improving the recovery efficiency of the radioactive materials were suggested.