What is Acetaldehyde in beer? Acetylaldehyde (as it is sometimes misspelled), is an organic compound found frequently in nature. It is often perceived as green apples (and sometimes dry apple cider) in both aroma and flavor.
In normal fermentation, acetaldehyde is a precursor to ethanol. It is noticed mostly in young beers where the yeast were not able to reabsorb or finish the conversion of glucose to pyruvic acid to acetaldehyde and finally to ethanol.
Glucose > Pyruvic Acid > Acetaldehyde > Ethanol
In other words, the beer was removed from the yeast too quickly. Give your homebrew plenty of time on the yeast and then condition as long as practical to prevent these flavors from occurring.
Not conditioning your beer long enough is an obvious reason for the compound's presence, and the cure being to condition for longer periods of time giving the yeast time to fully convert the acetaldehyde in beer to ethanol and eliminate the off flavors.
Many people preach it, but patience is definitely important in lagering to give yeast time to condition your beer and remove many of the off flavors you will find in young green beers.
You will taste or smell the apple-like acetaldehyde notes more frequently when using corn or cane sugar in your beer recipes. It normally dissipates with further conditioning.
The Oxidized Apple Cider Version
The oxidized or acetic-cider version is produced when ethanol oxidizes, or sometimes from bacterial contamination.
When your beer becomes oxidized, the reaction which forms ethanol can be reversed and the intermediary compounds will become evident. This is how the acetic-cider off flavor often occurs.
It is produced when ethanol is oxidized back to acetaldehyde, and then the acetaldehyde is converted to acetic acid.
Preventing Cider Off Flavors
To prevent this type of off flavors, keep oxygen exposure to a minimum. When transferring your beer, flush the receiving container with CO2, keep the hose below the surface of the beer, keep a lid on top while transferring, make sure all connections are air-tight, etc.
These are things you should be doing anyway, but if you are having a problem with acetaldehyde flavors or aromas in your homebrew, oxidation is one of the things to look at.
Green apple-like aromas and flavors are typically not appropriate for any style, although Salvator has some apple-like nuances and Budweiser is probably the most famous brand with these flavors.
If you want these flavors and aromas in your beer for some reason, just pull the beer off the yeast early and filter with a very fine filter to remove the yeast.
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