Hot Oxidation of Wort

[houstonbg]

I originally posted this in my beer journal thread (which you can view here), but I didn't pose it as a question and the thread itself isn't getting much visibility, so I'm posting it here for some input.

For some quick context, I did some research into what happens when wort oxidizes and what happens when ethanol oxidizes and reverts back to acetylaldehyde.

From the brief research I did, I put together the following breakdown of what happens:

The way I understand it is that wort contains dissolved oxygen. During a boil, much of this oxygen is released. If the yeast is pitched before the wort is able to cool to sub-80*F temps, the wort continues to release the aforementioned oxygen which binds with lipids and melanoidins faster than the yeast is able to utilize it to reproduce.

As a result, it quickly jumps into 2nd gear, breaking down glucose to ethanol. However, so long as the wort is burnin' up above 80*F, it is releasing oxygen as the yeast does its thing, and ultimately reverting any glucose it breaks down into ethanol right back into acetylaldehyde.

Later, the oxygen that binded to the lipids and melanoidins gets broken down into compounds that lead to off-flavors such as diacetyl and fusel alcohols.

My question is whether or not my understanding is valid, and if it isn't, I welcome links, comments, et cetera that can help me better inform myself.

Thanks!

 

[houstonbg]

Quick bump.

I don't know if bumping is kosher here, so I apologize if this is against the rules!

 

[ajdelange]

Akin to and sort of dual to acid/base chemistry which is concerned with the transfer of electrons (as opposed to protons in acid/base). The problem with hot side aeration is that oxygen in the air steals electrons from materials in the wort putting them into an oxidized state. These become reservoirs of oxidation potential (sort of like pH - in fact symbolized by rH) that hang around in the beer to steal electrons from flavor components in the finished beer. For example and alcohol R-CH2OH can lose two electrons to an oxidizing agent to become an aldehyde:
R-CH2OH + NAD+ --> R-CHO + NADH + H+

NAD+ is an oxidizing agent used as an example here - this reaction is, reversed, the one by which yeast reduce acetaldehyde to ethanol. So yes, one could suffer reversion of EtoH to aceataldehyde in his beer but this is not the reason we try to avoid hot side aeration so much as to avoid other aldehydes such as trans-2- nonenal (the one that always seems to get mentioned in the context of staling).

Conversely, if hot wort is denied access to oxygen it keeps its electrons in the form of 'reductones' which are the opposite of oxidizers. They supply electrons so that if some oxidizing agent (and, as you might expect, oxygen is a powerful oxidizing agent) makes its way into the finished beer, the reductone will supply it with electrons rather than some flavor component that you'd rather didn't.

These redox reactions take place after fermentation is complete. They are slow and the main reason we are concerned with them is that they allow the beer to stale faster than it would if it can be kept in reduced state.

Diacetyl is formed by oxidation of alpha aceto lactate. It happens fast. Live yeast cells can reduce diacetyl to less flavor active compounds and that is desirable but we often want to accelerate the formation of diacetyl in order to give yeast the opportunity to remove it before they are themselves removed from the beer.

Fusel alcohols are alcohols and thus represent highly reduced chemical compounds.

 

[houstonbg]

ajdelange, thank you very, very much for your thorough response.

The chemistry behind the brewing process is fascinating to me, so I'm trying to study up on it as best as I can. I'm brushing up on my chemistry and am actively jumping into things like the University of Oklahoma's online class, Chemistry of Beer.

That said, do you have any book or site recommendations that dive deep into the chemistry of things? It's OK if they're advanced, as I tend to learn better when things are over my head because it forces me to do a lot of research, thus I pick up a lot of things when all is said and done.

 

[ajdelange]

ajdelange, thank you very, very much for your thorough response.

It was actually pretty sketchy.

The chemistry behind the brewing process is fascinating to me, so I'm trying to study up on it as best as I can. I'm brushing up on my chemistry and am actively jumping into things like the University of Oklahoma's online class, Chemistry of Beer.

That said, do you have any book or site recommendations that dive deep into the chemistry of things?

I understand that course was pretty thorough and should go into this aspect of brewing science. O/W I'd start with a good biochemistry text. It should discuss biochemical redox thoroughly in an early chapter. Then you could move on to one of the brewing texts. I'm not making specific recommendations here because I am away from my library for the summer and we don't want to rely on my memory.

 

[houstonbg]

O/W I'd start with a good biochemistry text. It should discuss biochemical redox thoroughly in an early chapter.

Any objections to this one: Lehninger Principles of Biochemistry? A search of "oxidation-reduction reactions" within the book (courtesy of Google Books) shows that it's first mentioned on page 23 (of over 1,000).

Also only $2 used!

 

[ajdelange]

That's the one I started with with no background in organic and only 2 courses in inorganic from engine school. I found it quite readable nevertheless.

 

[houstonbg]

That's the one I started with with no background in organic and only 2 courses in inorganic from engine school. I found it quite readable nevertheless.

Snaggin' it here shortly. Thanks, man. Very much appreciated!

 

[DSorenson]

AJ saves the day again.

I love this guy.

 

[houstonbg]

AJ saves the day again.

I love this guy.

Quality guy, that's for damn sure. Funny thing is I created this thread hoping he'd chime in since I saw the knowledge bombs he was dropping in other threads on Brew Science.

And boom!

 

[DSorenson]

He has been patient with me, that's for sure.

Cheers, AJ!

 

[marcopolo]

THANKS for the info, guys. I took the OSU online beer chem course this year, and I really enjoyed it. I just bought the referenced biochemistry book (used) on amazon. I also have The Chemistry of Beer by Roger Barth - seems approachable for a non-science type person like myself. Cheers !