Diacetyl

• Common flavor descriptors: Buttered Popcorn, Buttermilk, Yogurt

• Possible causes: Diacetyl is produced by all yeast during fermentation, but is usually reabsorbed by the yeast cells. Non-reabsorption or over production is caused by too low temperatures during fermentation, mutated yeast, or racking too soon. It can also be formed by bacterial contamination.

• Avoid by: increasing temperature at end of fermentation (diacetyl rest), allowing fermentation to completely finish, allowing beer to condition in the fermenter before racking, maintaining solid yeast health, practicing good sanitation.

• Note: low levels acceptable or even desirable in some styles

DMS (Dimethyl Sulfide)

• Common flavor descriptors: Canned Corn, Cabbage, Tomato Sauce/Ketchup

• Possible causes: DMS comes from a sulfur-based organic compound (S-methyl methionine, or SMM) produced when grain germinates during the malting process. SMM changes to DMS during the boil, which can concentrate in the wort if the boil is not vigorous or long enough, or not properly vented. It can also be formed by bacterial contamination.

• Avoid by: Vigorous and adequate length boil, adequate evaporation, quickly chilling wort post-boil to halt production of DMS

Butyric Acid

• Common flavor descriptors: Baby Vomit, Rancid Butter, Sour Milk

• Possible Causes: Bacterial contamination, especially during packaging. Can occur during kettle souring, especially if pH is above 4.2 at inoculation and wort is exposed to oxygen.

• Avoid by: thorough sanitization, pre-acidifying and CO2 purging kettle sours

Papery Oxidation

• Common flavor descriptors: Wet Cardboard/Paper, Sherry, Stale

• Possible causes: exposure to oxygen post-fermentation, especially prevalent at packaging

• Avoid by: Careful attention to airlocks, avoiding suckback during cold crashing, closed transfers into CO2 purged kegs, careful bottling/canning.

• Note: some levels expected/desired in aged beers

Acetaldehyde

• Common flavor descriptors: Green Apple, Squash, Latex Paint

• Possible causes: Acetaldehyde is present in all beers at some level as it is produced by yeast during fermentation as a precursor to ethanol. Can also be caused by oxidation in packaged beer where excess oxygen catalyzes the transformation of ethanol back to acetaldehyde.

• Avoid by: Pitch adequate quantity of healthy yeast into well-oxygenated wort, increase temperature near the end of fermentation, avoid oxygenation post-fermentation

• Note: low levels are part of the accepted flavor profile in some styles (American lager)

Ethyl Acetate:

• Common flavor descriptors: solvent, paint-thinner, nail-polish remover, hot alcohol

• Possible causes: Ethyl acetate is an ester produced by the “esterification” of alcohol during fermentation. In small quantities, ethyl acetate produces a pear-like, fruity or rose flavor and aroma, however in higher connotations it leads to the solvent-like off flavors and aromas. Ester production increases with yeast stress, so higher fermentation temperatures and low pitch rate often lead to excess ethyl acetate.

• Avoid by: pitching an adequate quantity of healthy yeast, controlling fermentation temperature appropriate for strain and original gravity.