Off-Flavors Caused By Yeast

SATURDAY 04/15/2017

Have you ever opened a beer and been like, “Huh, that doesn’t seem right….”? While we always think about all the positive and delicious things about beer, there are some tell-tale signs of contamination or poor brewing methods. By studying and becoming more familiar with these flavors; breweries, brewers, and drinkers alike can help make beer be the best it can be.

There are many off flavors caused by a multitude of things, but today let’s concentrate on off flavors as they relate to yeast.

  • 4-ethyl phenol: brett, barnyard, horse blanket aroma
    • Produced by Brettanomyces yeast. While this flavor can sometimes be positive, too much inoculation can be a bad thing.
  • 4-vinyl guaiacol: clove aroma
    • Formed during the fermentation of ferulic acid formed during malt kilning. Also a partially positive, partially negative aroma.
  • acetaldehyde: green apple, green leaves aroma
    • A common symptom of beer that has not fermented long enough. Formed in the metabolic process as yeast rids itself of excess CO2, most acetaldehyde is later taken up by the yeast and converted into ethanol.
  • autolysed: muddy, soy sauce, umami aroma and flavor
    • Result of the death or disruption of yeast cells.
  • butyric acid: rancid butter aroma
    • Develops in spent grain, often a marker for Lactobacillus.
  • caprylic, caproic, capric acids: goaty, sweaty socks aroma
    • Due to contamination from common outside bacteria or yeast that produces organic acids and aromas reminiscent of animal sweats.
  • diacetyl: buttery or butterscotch aroma and flavor
    • Leaks out of yeast cells during amino acid synthesis. Large amount can be indicative of stressed or mutated yeast from heat, poor yeast storage, or acid levels in the water used for the wort. In very high amounts, it can be a sign of bacterial contamination in dirty or infected draft lines.
  • ethyl acetate: estery/solvent aroma, can even induce eye watering sensation in large quantities
    • Formed during fatty acid synthesis, later leaking out of the yeast cells. Very high amounts can be a result of Acetobacter contamination, the bacteria that produces vinegar.
  • fusel: harsh on the palate/wine-like aroma
    • An alcohol produced by elevated fermentation temperatures.
  • isoamyl acetate: banana/circus peanut aroma
    • Formed during fatty acid synthesis, later leaking out of the yeast cells. Common and very desirable in Bavarian weizens, but undesirable in other styles.
  • sulfur: rotten eggs, sewer gas, burned match aroma
    • Metabolic byproducts of yeast, most commonly a symptom of young, under-fermented beer, especially in some lager strains. Large amounts of hydrogen sulfide may be indicative of Zymomonas bacterial infection.

While malts and adjuncts can add tons of flavor to beer, yeast is the king of flavor and aroma, and can be attributed to a large variety of positive flavors as well (the naming and flavor/aroma profile becomes much more complicated at this point.)

Next time you second guess your beer see if you notice any of these flavors. Refine your palate and dedicate yourself to good beer.


P.S. If you’ve read this article please mention it to an employee next time you’re in and we’ll be happy to take $1 off your first beer in Detroit, or a $1 off your first six pack in Ferndale from now until the end of April.