Ehrlich Pathway Explained

Ehrlich Pathway: Fusel Alcohol Creation
Ehrlich Pathway: Fusel Alcohol Creation

Fusel alcohols or what are also called higher alcohol add aromatic complexity to hard cider and other fermented beverages. Yes, too much of them can lead to undesirable or overwhelming flavors. But, like all things in life, moderation is the spice of life. While most Yeast Assimilable Nitrogen comes from amino acids, so to do fusel alcohols. Felix Ehrlich, a German biochemist, noted similarities between several amino acids and fusel alcohols in 1904 and this led to a series of discoveries by Ehrlich and others into the workings of what is known as the Ehrlich Pathway. This pathway can convert amino acids into fusel acids but in most anaerobic environments with Saccharomyces cerevisiae, it will result in mostly fusel alcohol generation(1). It is speculated that the Ehrlich pathway may also be regulatory in nature. Meaning that under certain environmental or nutritional conditions, it’s output will signal the yeast to turn on or off various genes in order to generate the needed nutrients or prepare for stressful situations. It may also provide an energy efficient means for NADH regeneration. More research is needed but the one thing we know for certain is that this pathway creates fusel alcohols and these add to the aromatic complexity of ciders. It’s a great example of why amino acids are an important component of hard cider. While most of the work on the Ehrlich Pathway has been done on Saccharomyces cerevisiae yeast, it has also been found in some non-Saccharomyces yeasts.

If you are like me, you might be asking yourself what exactly is a pathway. Is it some type of organ inside a yeast cell that eats amino acids and secretes fusel alcohols? Pathways are a series of chemical reactions within a living organism. For the Ehrlich Pathway, it is the processing of amino acids into alpha-keto acids using enzymes. This first step is a transamination reaction. Next, it is the conversion of these alpha-keto acids into fusel aldehyde. Again, enzymes are used but this is a decarboxylation reaction. That means CO2 gas is formed, like during fermentation or MLF. The last reaction is either the conversion of these fusel aldehydes into fusel acids or fusel alcohols. The reaction is oxidative if it converts it into fusel acids and reductive if it converts the aldehydes into fusel alcohol. Oxidation and reduction are the reactions involving the release or absorption of electrons. Oxidative reactions release electrons while reductive reactions gain electrons. While the Ehrlich Pathway is interesting and important in making great hard cider. It’s not the only pathway in yeast. In fact, there is an entire website and database dedicated to the Yeast Pathways for Saccharomyces cerevisiae yeasts. Don’t believe me, check out YeastPathways at the genome database, which is part of the work to sequence the DNA genome for Saccharomyces cerevisiae. See, hard cider making is part art and part science.

(1) L. Hazelwood and associates, The Ehrlich Pathway for Fusel Alcohol Production: a Century of Research on Saccharomyces cerevisiae Metabolism, APPLIED AND ENVIRONMENTAL MICROBIOLOGY, Vol 74, No.8, Apr. 2008

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