Acetaldehyde is a compound that causes off flavors and aromas in beer, often described as tasting and smelling like green apples, cut grass or green leaves, pumpkin, or latex paint, and is sometimes described as giving beer a green character.
 Acetaldehyde in beer
While acetaldehyde is usually considered a fault in beer, some beer styles do use it as part of the flavor profile. The best-known example is Budweiser, but other beers such as EKU-28, Salvator and Ephemere also include acetaldehyde flavors.
 Causes of acetaldehyde
Acetaldehyde is an compound formed by an intermediate step in the conversion of sugar to ethanol by yeast. Under ordinary circumstances, any acetaldehyde formed during fermentation will eventually be taken up and converted by the yeast. The most common cause is removing the beer from the yeast too early, before the yeast has a chance to complete fermentation.
In finished beer, the ethanol reaction can sometimes be reversed by oxidation, resulting in acetaldehyde re-formation.
Adetaldehyde is also a byproduct of the conversion of ethanol to acetic acid (vinegar) by acetic acid bacteria. If this is the cause of your acetaldehyde problems, it will probably be accompanied by a vinegar-like or cidery flavour and aroma.
 Preventing acetaldehyde
 Creating acetaldehyde
To simulate acetaldehyde in beer for tasting or judging testing or calibration, add 3/4 tsp. of white wine vinegar to 12 ounces of beer.