User:Dstrevey

Off Flavor Compounds
4-ethylphenol – Generally referred to horsey, barnyard, earthy, and band aid medicinal like aromas and flavors

4-ethylguaiacol – Generally thought of as producing spicy, clove like, smoky flavors

Isovaleric acid – Common descriptions include sweaty socks, rancid cheese, mousy

Biosynthesis


4-ethylphenol and 4-ethylguaiacol - The biosynthesis of 4-ethylphenol is attributed to the yeasts ability to convert volatile phenols to 4-ethylphenol through enzymatic reactions. The volatile phenol responsible for the catalyst of this reaction is mainly cinnamic acid.  Cinnamic acid is very prevalent in the wine as it come is in grape juice and the skin of the grape.  In beer production cinnamic acid is mainly provided from malted barley, however a small proportion may come from hops.  Cinnamic acid is then esterified with anthocyanin esters also provided from malted barley and hops.  The hydroxycinnamic acid then interacts with the enzyme cinnamate decarboxylase produced by the [[Brettanomyces species.   This enzyme decarboxylates the hydroxycinnamic acid producing a hydroxystyrene, and then reduced to the volatile phenol form by a vinyl phenol reductase (VPR).  The unique portion of this reaction is the enzyme Cinnamate decarboxylase.  The weak hydroxycinnamic acids are inhibitory to growth of many microorganisms, but Brettanomyces spp. have overcome this problem by generating an enzyme to convert the weak acids to volatile phenols.

Isovaleric Acid - Isovaleric Acid – The biosynthesis of isovaleric Acid by Brettanomyces bruxellensis is not well understood. What is known is that isovaleric acid is produced via the degredation of L-Leucine. The L-Leucine first undergoes a reaction with branch chained amino acid aminotransferase and 2-ketoglutarate. This forms one glutarate and 2-keto-isocaproic acid. Next a 2-ketoacid oxyreductase and coenzyme A produce one CO2 two H and Isovaleryl-CoA. This is acted upon by phosphate acyltransferase which introduces a phosphate and releases the CoA to produce Isovaleryl-P. The final step a fatty acid kinase and ADP act on the Isovaleryl-P producing Isovaleric Acid and an ATP molecule.

Relevance To Beer
The importance of these molecules is both negative and positive to a brewer. If a brewer has an infection of Brettanomyces bruxellensis in their beer then the production of these flavors will be seen as a flaw in the finished product. If a brewer intentionally inoculates a beer with Brettanomyces bruxellensis then, promoting the formation of these compounds in varying degrees will not only be sought after, but accepted for that beer style. Wine makers generally consider the presence of Brettanomyces bruxellensis as an infection to the wine. Beer makers on the other hand have developed beer styles and techniques to promote the growth of this yeast to produce unique flavors in very unique beers.