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Old 08-28-2010, 03:19 PM   #11
rayg
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Originally Posted by ajdelange View Post
Yeast do scrub oxygen, of course, but it is not enough to "neutralize" the oxygen to achieve the reduction of diacetyl and acetaldehde. Hydrogen (and in particular, the electrons it carries) must be supplied to diacetyl and acetaldehyde to reduce them.
This is a uneccessarily confusing. You make it sound as if someone has
to introduce hydrogen gas into the wort. The hydrogen is supplied by
the enzyme (for example, butanediol dehydrogenase) in an acid-base
reaction, many times the hydrogen is supplied by a histidine residue in
the enzyme. Overall the chemistry can be described by redox processes
but the individual reactions are almost always aqueous acid-base
chemistry.

Yeast while alive and in a liquid medium is a chemical reaction
factory, there are probably many thousands of reactions going on
continuously, oxygen is used for some of them and free oxygen in
the wort is absorbed through the cell membrane and used for
various purposes in the yeast cell. Inorganic reactions take place
outside the cell with oxygen, and these undesirable byproducts can then
be absorbed by the yeast and transformed by the cell metabolism
to other things.

Ray


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Old 08-28-2010, 03:40 PM   #12
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I usually secondary after two weeks for pipline purposes and end up with enough yeast on the bottom of the secondary after another two weeks to pitch another beer. I used to drop a carbonation tab or two into the secondary to help with the head space but have decided it isn't worth the trouble.



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Old 08-28-2010, 07:05 PM   #13
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This is a uneccessarily confusing. You make it sound as if someone has
to introduce hydrogen gas into the wort. The hydrogen is supplied by
the enzyme
As yeast's main source if reducing power is NADH I'd guess (but I do not know - I don't know that much biochem in general) that NADH is the reducing agent for conversion of diacetyl to Acetoin:

2(CH3)(CHO)(CHO)(CH3) + H+ NADH --> 2(CH3)(CHOH)(CHO)(CH3) + NAD+

and for the conversion of acetoin to 2,3 butane diol:

2(CH3)(CHOH)(CHO)(CH3) + H+ NADH --> 2(CH3)(CHOH)(CHOH)(CH3) + NAD+


Seems to me that if the H were supplied by the enzyme, it would become oxidized and unable to catalyze further reduction.

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Originally Posted by rayg View Post
(for example, butanediol dehydrogenase)
The main enzyme involved is diacetyl reductase. While acetoin reductase is mentioned it is thought by some to be the same as diacetyl reductase. Sort of makes sense. First you do the carbonyl on one side of the molecule and then the one on the other.

But other reductases, in particular acetaldehyde reductase (alcohol dehydrogenase) apparently reduce it too.


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Originally Posted by rayg View Post
in an acid-base
reaction, many times the hydrogen is supplied by a histidine residue in
the enzyme.
Electrons are sometimes passed through the enzyme (oxidative phosphorylation) but I have never heard of the enzyme supplyng the electrons. Or the H+. Those usually come from acids produced by the yeast. But as I say, I'm pretty weak in biochem.

Quote:
Originally Posted by rayg View Post
Overall the chemistry can be described by redox processes
but the individual reactions are almost always aqueous acid-base
chemistry.
One of the "advantages" of the Lewis model is that as all bonding involves the exchange of electron pairs and an electron pair donor is an Lewis base and an electron pair acceptor a Lewis acid all chemical reactions are acid/base reactions. Sort of the unifying field theory of chemistry. If you think my post confused people, "clarifying" by taking them to this level is bound to send them from their terminals screaming.


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Originally Posted by rayg View Post
Inorganic reactions take place
outside the cell with oxygen, and these undesirable byproducts can then
be absorbed by the yeast and transformed by the cell metabolism
to other things.
This is exactly what happens with diacetyl. It is formed after packaging as acetolactate is oxidized to diacetyl non enzymatically (but it's still organic). This diacetyl is then be taken up by and reduced in the cell.
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Old 08-28-2010, 07:28 PM   #14
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So it seems to me then, that the cake is not important in the stages immediately following primary fermentation, because there is so much active yeast still in suspension that the reaction surface area is many times greater than what's accumulated at the bottom. But further down the line when the beer clears, then sediment activity is all there is. Not to say the sediment isn't *at all* active, but just that it's more of an issue during lagering, for example. Does that jive?

BTW, lagers are primarily what I want to brew, so this is all great info.

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Old 08-28-2010, 07:47 PM   #15
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Yes.

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Old 08-28-2010, 10:52 PM   #16
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The hydrogen is supplied by
the enzyme
Just thought of something - did you mean the co-enzyme ( NADH)? It is the nicotinamide ring in NADH that provides the 2 electrons and H+ and the nicotinamide is quite similar to the histidine that you mention. But NADH isn't part of the enzyme - it get's tucked into the notch in the enzyme (assuming that NADH is the reducing agens). And it delivers a proton and 2 electrons i.e. a hydrogen atom and an electron. I don't see how you can call that an acid/base reaction (except in the sense that all reaction are under the Lewis approach). It looks like a classical biochem redox to me. Maybe the nomenclature has changed in the years since I've looked at this.
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Old 08-29-2010, 04:27 PM   #17
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Quote:
Originally Posted by ajdelange View Post
As yeast's main source if reducing power is NADH I'd guess (but I do not know - I don't know that much biochem in general) that NADH is the reducing agent for conversion of diacetyl to Acetoin:
I have confirmed that it is indeed NADH which reduces diacetyl catalyzed by diacetyl reductase (EC 1.1.1.5).

So the equations are indeed

Quote:
Originally Posted by ajdelange View Post
2(CH3)(CHO)(CHO)(CH3) + H+ + NADH --> 2(CH3)(CHOH)(CHO)(CH3) + NAD+

and for the conversion of acetoin to 2,3 butane diol:

2(CH3)(CHOH)(CHO)(CH3) + H+ + NADH --> 2(CH3)(CHOH)(CHOH)(CH3) + NAD+
A proton and an electron (i.e. a hydrogen atom) are passed to each of the 2 diacetyls in the first reaction which shows 2 protons and 2 electrons because we get 2 electrons and 1 proton from the NADH and need an extra from the solvent (beer). The 2 electrons and 1 proton (a hydride ion if you prefer) come from the pyridine ring in the NADH: NADH --> 2e- + H+ + NAD+ = H- + NAD+. The hydrogen ion from the solvent is needed to pair with the extra electron. This is classical bio redox (including the fact that the redox potential depends on pH).

So we know the most of the hydrogen comes from the NADH (with the rest from the solvent)but the remaining question is "Where does the NADH come from". The answer is NAD+. It is constantly being reduced by yeast in the energy generation phase of glycolysis. In fact one of the reasons alcohol is produced in fermentation is because it is a byproduct of the process which transforms NADH into the NAD+ required in that phase. IOW as long as the yeast are not totally dormant i.e. there is something for them to munch on they will produce NADH by recycling NAD+ and thus remain able to reduce diacetyl. There should be no question in anyone's mind that it is NADH which reduces acetaldehyde to ethanol: CH3CHO + NADH + H+ --> CH3CH2(OH) + NAD+. So again, as long as the basic metabolism is running, if slowly, acetaldehyde should be reduced too.
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Old 08-30-2010, 04:20 PM   #18
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Quote:
Originally Posted by ajdelange View Post
As yeast's main source if reducing power is NADH I'd guess (but I do not know - I don't know that much biochem in general) that NADH is the reducing agent for conversion of diacetyl to Acetoin:

2(CH3)(CHO)(CHO)(CH3) + H+ NADH --> 2(CH3)(CHOH)(CHO)(CH3) + NAD+

and for the conversion of acetoin to 2,3 butane diol:

2(CH3)(CHOH)(CHO)(CH3) + H+ NADH --> 2(CH3)(CHOH)(CHOH)(CH3) + NAD+


Seems to me that if the H were supplied by the enzyme, it would become oxidized and unable to catalyze further reduction.
The reaction you wrote above is an acid/base reaction. The ultimate
source of the H+ (the acid) is some redox reaction far earlier in
the sequence, but ultimately it is used in an aqueous acid base reaction.
For a mechanism involving histidine as an acid/base, see the diagram
labeled "Serine Protease Mechanism" on this page:
http://employees.csbsju.edu/hjakubowski/classes/ch331/catalysis/olcatenzmech.html

Your molecular structure and stoichiometry above is incorrect. You've
written:
2(CH3)(CHO)(CHO)(CH3) + H+ NADH --> 2(CH3)(CHO)(CHO)(CH3) + H+ NADH --> 2(CH3)(CHOH)(CHO)(CH3) + NAD+ + NAD+

But the diketone on the left should be

(CH3)(CO)(CO)(CH3) + 2H+ + 2NADH - - > (CH3)(CHOH)(CHOH)(CH3)

Each carbonyl requires 2 moles of H, one formally H+ and one
formally H- (minus), the NADH supplies the H-, the H+ is
supplied by the enzyme usually through some acidic source such
as a protonated histidine or a protonated serine or protonated
tryptophan. A reverse reaction would be the conversion of
ethanol to ethanal:
CH3CH2OH + NAD+ ---> CH3CH=O + NADH + H+
Here the NAD+ picks up an H- and the H+ is picked
up by some part of the enzyme (a histidine or serine residue
or other etc).

But all of this is overkill for most of the members of this
list. All the op needs to know is that the yeast are chemical
factories that absorb things from solution and convert them
to something else, one thing in particular being O2.

I know you are trying to help by explaining, but if you are
going to be very technical, you have to at least be correct,
otherwise you are just creating more confusion. Your comment
about Lewis structures involving nothing but acid/base
chemistry is also not correct, but I'm not going to get into
that here because I don't think too many here would care.

Ray
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Old 08-30-2010, 08:37 PM   #19
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I wish I could follow you guys...it sounds interesting...Ill prolly eventually pick up my chem book and go over it and a biochem book sooner or later...

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Old 08-30-2010, 09:39 PM   #20
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Originally Posted by rayg View Post
Your molecular structure and stoichiometry above is incorrect...
But the diketone on the left should be

(CH3)(CO)(CO)(CH3) + 2H+ + 2NADH - - > (CH3)(CHOH)(CHOH)(CH3)
Yes, that was a stupid mistake on my part.

Quote:
Originally Posted by rayg View Post
Each carbonyl requires 2 moles of H, one formally H+ and one
formally H- (minus),
Yes though you've shown it all in one step and it usually goes diacetyl -> acetoin -> butane dione.

Quote:
Originally Posted by rayg View Post
the NADH supplies the H-,
So at least we agree that part of the hydrogen comes from the coenzyme - that's progress.

Quote:
Originally Posted by rayg View Post
the H+ is
supplied by the enzyme usually through some acidic source such
as a protonated histidine or a protonated serine or protonated
tryptophan.
Why would it take it from some protonated residue on the enzyme if it were readily available from the solvent. Indeed with respect to...

Quote:
Originally Posted by rayg View Post
A reverse reaction would be the conversion of
ethanol to ethanal:
CH3CH2OH + NAD+ ---> CH3CH=O + NADH + H+
Here the NAD+ picks up an H- and the H+ is picked
up by some part of the enzyme (a histidine or serine residue
or other etc).
...Stryer, in Biochemistry p 320 describes exactly this reaction with the phrase "one hydrogen atom of the substrate is directly transferred to NAD+, whereas the other appears in the solvent". In the reverse reaction, especially in fermentation where the pH is low, why wouldn't it come from the solvent by way of the multiple acids produced by the yeast in order to establish the low pH levels at which they compete best? I'll agree that any acid residue with a pK < pH of the solvent would give up it's protons but why would the acetaldehyde (in the reverse reaction) prefer those? Are you perhaps so used to thinking in terms of physiologic pH that you've forgotten that fermenting beer pH is a couple of units lower?

In the reaction Quinone + 2H+ + 2e- <--> Hydroquinone no enzyme is required but the protons have to come from somewhere and somewhere is the solvent. That's why the redox potential for this reaction depends on pH as do the redox potentials computed (in, for example Mathhew and van Holde (Biochemistry - p526) for the acetaldehyde/ethanol couple. Stryer bears a copyright of 1988 and Matthew and van Holde 1996. Has something new come to light? Is there a different way of looking at things today? The literature abounds with references to this as a redox reaction - in fact it (the ethanol/acetaldehyde reaction) is often cited as an example of a biochemical redox, as I expect you must know. If I want to compute the change in Gibbs energy for this reaction I can do it using the published redox potential. If that's no longer valid, how would I do it?

Quote:
Originally Posted by rayg View Post
But all of this is overkill for most of the members of this
list. All the op needs to know is that the yeast are chemical
factories that absorb things from solution and convert them
to something else, one thing in particular being O2.
Agreed, but this is the science topic.

Quote:
Originally Posted by rayg View Post
I know you are trying to help by explaining, but if you are
going to be very technical, you have to at least be correct
I'll stand by what I've posted (with the exception of the errors I've acknowledged).



Quote:
Originally Posted by rayg View Post
otherwise you are just creating more confusion.
I thought my original explanation was fine as it is. Untill convinced otherwise I have to believe that these are redox reactions - I've just seen that stated i n too many places over the years. Telling readers that they are acid base, given that so many sources call them redox, seems to be more likely to confuse whether that model is valid or not. The only way I can, at this point see that as working would be the Lewis definitions. I almost thought I had it figured out because the nitrogen in the pryridine ring in the nicatinamide part of NADH is clearly a Lewis base but I'm not knowledgeable enough to take it any further than that.

Quote:
Originally Posted by rayg View Post
Your comment
about Lewis structures involving nothing but acid/base
chemistry is also not correct, but I'm not going to get into
that here because I don't think too many here would care.
That comment, and I'm amazed I was able to find the reference, comes from the description of the Lewis definitions in "Concise Inorganic Chemistry" by J.D. Lee. He says on p 265 "Almost all reactions become acid-base reactions under this system". I forgot the "almost". Are we quibbling over the "almost" or is this statement just flat out wrong? Again, it's an older book - 1991 copyright.

Do I need to buy a whole new set of textbooks?


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