For the next part of our guide to off-flavors we'll take a look at acetaldehyde, with persistance and consistency you can avoid this undesireable quality. 

Acetaldehyde smells and tastes like green apples.  Sometimes it’s described as “oxidized apples” or “acetic cider”. 

How does it form in beer?

Acetaldehyde is the immediate precursor to ethanol in fermentation. Like diacetyl, acetaldehyde is found in large quantities during early fermentation as the yeast produces it en masse early in their metabolic cycle.  If there is a high amount of dissolved oxygen present in the young beer, then the oxygen could react with ethanol and oxidize it back into acetaldehyde.

Acetaldehyde is also formed during too long sitting on the yeast. When yeast health is poor, cells can die and burst open (autolysis) which releases a lot of acetaldehyde into the beer. This is why it is important to stick to a strict yeast dumping regimen during aging, and avoid the heavy buildup of yeast in the cone of the fermentor.

How do you solve the problem?

Just like diacetyl, kraeusening is the best way to remove excess acetaldehyde. Brewers need to make sure that they aren’t removing the beer before fermentation has finished. Raising the fermentation temperature a few degrees (diacetyl rest) will help resolve acetaldehyde issues. The other cause of acetaldehyde is too much dissolved oxygen in the beer.  Brewers need to make sure that oxygen inclusion is minimal during the brewing process and any cellar activities (i.e. dry hopping).