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Understanding and Avoiding Off flavors: Acetylaldhyde

By George Bach
(appeared in the July and September 1999 issues of the Home Imbrewment)


Off flavor of freshly cut green apples, and / or acetic-cider / rotten apple flavor. This is a chemical that occurs normally along the fermentation process, when acetylaldehyde is processed into ethanol. But in a young, or highly alcoholic beer this conversion may not have been completed. Thus giving off the smell of fresh green apples. One problem (especially in the alcoholic beers) is the usage of a weak yeast, in ales or stouts this can be prevented by using either the Wyeast # 1056 (American), or #1084 (Irish).

Another possible problem is the ethanol becomes oxidized back to acetylaldhyde and acetic acid, which gives off the "cider" / rotten apple smell and taste. This can be prevented by allowing beer to ferment and age for a proper amount of time. Lastly, bacterial infection which, gives off the rotten apple taste, can be eliminated by using more sterile methods. Next month we'll discuss Alcoholic / Solventlike off flavors.

Astringent - A drying, puckering taste felt throughout the mouth (just like that in red wine). Can be tannin, vinegary, or tart in sensation. May also feel metallic or powdery coating of mouth.

Sources: Boiling of grain husks, fruit skins, excessive hopping, or wort trub. These characteristics are easier noticed in lighter, drier beers than in the stronger sweeter types. Tannins are the key source to eliminating this problem, and their extraction from the grain results from over crushing, overly alkaline mashes, excessive sparge water. Boiling of specialty grains releases tannins, which is why you need to withdraw them before the start of a boil.

Likewise, boiling fruits release tannins. If you are using fruit in a brew, the best way to avoid astringency is to sanitize by heating up water prior to boiling (for 15 - 30 minutes). Once sanitized add fruit to fermenter several days after fermentation has occurred to allow your flavoring to be more pronounced.

Over-hopping causes excess hop oils and resins to make a brew astringent. The poor separation of krausen and the fermenting wort also are factors. All the tannins from hops and grain husks are collected in the trub and are great sources that create astringency. If you currently are not using a blow-off tube, you may want to reconsider since this is a great way to force out tannins.

Proper crushing of grains (i.e., lightly cracked but not torn, shredded, or crushed), as well as the temperature and alkalinity of your mash and sparge waters. Specialty  grains are okay heated up but never boiled, then removing the grain bag before boil. Avoid boiling fruits in the wort, if you must do so limit the amount the time that skins are boiled. Avoid over hopping (which for some of our club members will be a real challenge). Lastly, use those blow-off tubes!!! Happy brewing,
and make sure you save a couple for me, remember the best beer out there is home brewed.

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