People say "Green Apple" as an off flavor when describing beer. What is that?

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What is "Green Apple" to you?

  • Like a Granny Smith Apple

  • Like any under-ripe apple (You'd have to be around fresh growing apples to know)

  • Jolly Rancher Green Apple

  • Other: Explain for us please


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cannman

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Looking to get some clarification on the matter! For me, and everyone else. Please vote.
 
It's acetaldehyde which smells/tastes different depending on the matrix and the concentration. It can vary from a very pleasant fruity experience to a nasty aldehydic one. Some words borrowed from a website: pungent, fresh, fruity, ethereal, aldehydic, musty, green. I suppose my personal descriptions would be, in order of strength of stimulus, pleasantly apple-like (no particular variety) to rotting apple to really raw aldehyde (almost like formaldehyde).
 
Acetaldehyde is a flaw in beer because it has multiple causes and generally points to process or sanitation issues. It is an intermediate stage of yeast converting sugar to alcohol, so it's presence in young beer tells you the yeast isn't finished.

Yeast: Maltose/Glucose > Pyruvic Acid > Acetaldehyde > Ethanol.

This happens swiftly and is nearly complete at the end of primary fermentation if the yeast is healthy and the wort had sufficient nutrients. In a healthy fermentation, it only takes hours to a day or so for yeast to complete the conversion to alcohol.

Post fermentation, acetaldehyde is be caused by the breakdown of ethanol. The breakdown can be from oxidation; though if that's the mechanism, you'll notice a lot of other artifacts. More often than not, the breakdown is biological as something is consuming the ethanol. The usual suspect is Acetobacter, but there are other organisms that can feed on alcohol, too. However, the process to control them is identical to controlling acetobacter, so it's moot to worry about all of them.
 
I'm also in the acetaldehyde crowd. As a new homebrewer, one of the things I've noticed when bottle conditioning at room temperatures below 70F, more time and patience is needed before cracking a seal and drinking. Learning from experience, other people, and taste-testing my own initial brews along the way reinforced my perception that this "green" flavor can be attributed to unfinished bottle conditioning. If your sanitary habits are flawless, you can write the "green" flavor off as an immature beer especially if your taste-testing shows it diminishes as your beer ages.
 
My focus was to zero in on what "green apple" perception is like, not the causes. A Jolly Rancher is sweet and candy like, where a granny Smith apple is a quite different aroma. BOTH are encompassed by the term green apple and BOTH are very different.

So when you read the term "green apple" in a brew book, which is it?
 
When you read the term 'green apple' in a brewing book it means acetaldehyde is present. As I noted in my earlier post, the way it presents depends on the concentration and the matrix. Same is true of Granny Smith and Jolly Rancher. Both contain acetaldehyde, doubtless at different concentrations, but also accompanied by a different matrix of other aroma compounds. That is why those two cultivars smell different.
 
Acetaldehyde created as a pre-cursor to ethanol tastes different than acetaldehyde created from the oxidation of ethanol. Not all acetaldehyde will taste the same. Not all people will perceive it the same.
 
OK. I've got a bottle of acetaldehyde in my fridge. According to Wikipedia it was probably produced by the Wacker process (oxidation of ethylene using a copper/paladium catalyst). Now will this taste and smell the same as acetaldehyde created by oxidation of ethanol? And will each of those taste/smell different than the acetaldehyde you find in beer which is produced by decarboxylation of pyruvate?
 
I read somewhere where a lack of zinc in the brewing process can contribute to the production of acetaldehyde. Any truth to that?

I recently started using Wyeast Yeast Nutrient which contains nutrients not found in other nutrient brands, one of which is zinc. I use distilled water, so I figured it would be beneficial to add some items back in for the yeast.
 
I've only tasted it once in a lager that I was getting too fussy with and it tasted like a sharp, underripe apple. Like it was almost astringent.
 
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