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Old 05-09-2011, 11:21 PM   #1
batfishdog37
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Default A-Acid dissociation?

Greetings,

Does anyone have an idea if hop alpha acids dissociate hydrogen ions into the wort during boiling/isomerisation, in a similar way to the way other acids like hydrochloric acid does in a solution? I am searching through many resources like "Principles of Brewing Science", "Technology, Brewing and Malting", some material from Siebel and online sources and for whatever reason cannot find specific descriptions of this.

Ultimately, I am trying to figure out the specific reasons why isomerisation is better at more alkaline pH's. My initial thought is because the alkaline solution would accept the H+ ions more readily and/or if the solution would have a lowered buffering capacity for some reason.

Thanks for reading and for any resource ideas or direct help!

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Old 05-09-2011, 11:25 PM   #2
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Subscribed! I just finished Chem II this semester. Would love to hear more on this.

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Old 05-16-2011, 06:46 PM   #3
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Interestingly enough, hop "acids" are not at first glance actually acids. That is to say, they don't contain any carboxylate groups that would tend to give an organic acid its name.
hop-acids.jpg
There are ionizable groups in the form of hydroxyls, which can also dissociate a proton (H+ ion) at sufficiently high pH. This is most likely why they were originally classified as acids, before their molecular structures were known. However, I estimate that the dissociation would not happen in any appreciable amount below pH 12, so you would never see this effect in wort or beer.

The reason the isomerization works better at higher pH is most likely that it is a base-catalyzed process. Transient deprotonation of one of the hydroxyl groups is the first step in a reasonable reaction mechanism for the isomerization. Of course you still need the heat of boiling to drive this reaction forward in the brewing process.

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Old 05-17-2011, 12:42 AM   #4
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While there are many definitions of "acid", none requires a carboxyl group, organic acid or not. Alpha acids are, in fact, classically acidic by any definition. The pKa of humulone is ~ 4.5 - 5.5 (depending on which source you pay attention to), which shows that it is, in fact, fairly strong as far as weak acids go -- around as strong as acetic acid (pKa=4.76), in fact.

That the proton is on a hydroxyl group makes no difference, because the hydroxyl group is resonance stabilized, which is the classic way hydroxyl groups end in being strongly acidic.

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Old 05-17-2011, 01:56 PM   #5
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Okay, I admit I oversimplified my answer a bit, mostly due to my vastly overestimating the pKa's of the hydroxyls in hop acids. In retrospect I should have noticed the resonance stabilization onto carbonyl oxygens, which makes them essentially vinylogous carboxylates.

So let me try to correct for my mistakes by providing some back-of-the-envelope calculations for what the presence of hop acids in wort does. Based on the pKa range of humulone provided by ni*, and an assumption that most beers are around pH of 4, one would expect anywhere from 5-25% of the hop acids to be deprotonated in a beer.

How does that affect the pH of the beer itself? Hard to say for sure, but I expect not that much. According to Brewing Science & Practice (Briggs, et al, CRC Press, 2004), one can expect a range of isohumulones (isomerized hop acids) between 15 and 80 mg/L, which corresponds to ~0.04 - 0.2 mM. Contrast this concentration to the major buffering inorganic anions (~5 mM phosphate, 1 mM sulfate) and the concentration of dissolved CO2 (~60-100 mM, as CO2, bicarbonate & carbonate), and I would expect that the total buffering capacity of beer is much stronger than that of the hop acids.

I realize I got off topic from the OP's question about how this affects wort. Of course in boiling wort there is no appreciable dissolved CO2, but I still expect that the water chemistry (alkalinity) and grain bill will be what dictates the pH of the wort. Wort pH should start around ~5.4 and go down during the boil as you drive off CO2 from bicarbonate. So at these pH's I would expect to see a fraction (25-75%) of the hop acids deprotonated. If the pH is higher, more is deprotonated. And since this deprotonation is the first step in isomerization, it works better at higher pH.

There are a number of other factors at work in the isomerization of the hop acids, including the solubility of the hop acids, the presence of divalent cations (calcium, magnesium), and the specific gravity of the wort. So while, in isolation, higher pH means faster hop isomerization, it is more unpredictable than that in actual brewing.

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