How do you like them apples! Green apples, cidery, crushed ants (?), are all flavours I’ve heard to describe acetaldehyde.
Acetaldehyde is a common organic compound found in loads of ripe fruit, veggies and even coffee. It’s also a common fault found in beer. It is the penultimate compound in the catabolism of glucose into ethanol. This, of course, is the foundation of yeast fermentation. So it is produced in abundance early during fermentation by brewers yeast.
A normal, happy, healthy fermentation will result in the full conversion of acetaldehyde to ethanol, and so should never be seen in a good beer. It can arise, however, due to a few different mechanisms.
Firstly, if there is copious levels of dissolved oxygen in the young beer, acetaldehyde can be oxidised back to ethanol, it is a reversible reaction. Secondly, if you remove the yeast too early during fermentation, there is insufficient activity to drive the reaction to completion. Thirdly, if you get oxygen ingress during packaging you can produce acetaldehyde. Finally, if the yeast is in poor health or the young beer is left on the yeast cake for too long, yeast can autolyse and release a lot of acetaldehyde.
Fun fact, the first thing your body does with ethanol is convert it back to acetaldehyde in an effort to clear the body of alcohol. Acetaldehyde is pretty toxic for us old hominins so it is further metabolised to acetate and then other things. Heat Flush, a reaction to alcoholic beverages that results in a red, hot face, is due to a build up in acetaldehyde. Individuals who suffer from this condition have a mutation in the enzyme that converts acetaldehyde to acetate.
Green apples, tastes good, but not good in beer. Now you know.