Friday, November 16, 2007

Tannins and icky beer

Tannins are astringent, bitter tasting compounds. Chemically, tannins refer to a broad class of polyphenols. They are often differentiated from other polyphenols in that, tannins precipitate proteins. Ann Hagerman at Miami of Ohio University has a great pdf online that explains tannins much better than I could.

They are not desired in beer. However, they are found in the hulls of the grains whence comes the goodness that eventually becomes beer. During the mashing process, the starches get enzymatically broken down to maltose. However, if the mashing or sparging water is too hot, detrimental amount of tannins can be leached from the grains. It is well known that for most things, the solubility in water increases as the temperature of the water increases. So, this makes sense.

However, the pH of the mash can also affect the solubility of tannins. If the pH is too high (the "normal" pH of a mash is around 5.2) the solubility of tannins increases. This has to do with the presence of the phenolic functional group.

Shown is catechin, a common (and fairly simple) tannin found in a brew kettle near you [edit: The structure shown is missing one OH group. I'll fix it later].

As any sophomore organic chemistry student will tell you, the OH groups bonded to the aromatic rings (the group that puts the "phenol" in "polyphenol") are willing to lose their protons to a base. The pKa of phenol is about 10. I don't know what it is for catechin, but I'm sure it is in that vicinity.

As a result, if the pH of the mash get too high, more and more of the tannins will lose the proton. The result is charged, ionic compound which has a higher water solubility than the neutral species. As a result, the result of this result results in more tannins dissolved in the water.

So, what affects the pH of the water in the first place? You'll just have to wait and find out.

7 comments:

milkshake said...

another way the tannins can produce off-flavors is the complexes with Ca and Mg. It turns out tea tastes much better from DI water or soft rain water - in high lime water the tea gets quickly cloudy, brownish flakes tart falling out and the taste is horrid. But I don't know if this makes any difference with beer.

Can you add a tiny bit of some soluble protein to the wort, to sequest tannins - like a pich of gelatine or a dried fat-free milk?

Ψ*Ψ said...

milkshake, i think you might have just explained why i'm a coffee drinker...we have crazy-hard water here since the whole area is built on limestone

Chemgeek said...

Gelatin is often used as a clarifying agent. I recently used it in a secondary with exceptional results. I think there is more to it that just sequestering tannins, but I bet it has a lot to do with it. I have not opened any of these bottles yet, but I suspect there will be little chill haze.

Chemgeek said...

Oh, and I meant to mention, the level of Ca and Mg affects the amount of tannins present in beer, but it is a bit more complicated. I hope to write a post about that as well someday when I learn more about it.

Joy said...

'As any sophomore organic chemistry student will tell you, the OH groups bonded to the aromatic rings (the group that puts the "phenol" in "polyphenol") are willing to lose their protons to a base.'

I believe part of that statement is wrong, but then, you didn't know me as a sophomore organic chemistry student. ;-)

Chemgeek said...

OK:

"As any sophomore organic chemistry student should tell you..."

That's probably more accurate.

Beer Aficionado said...

The problem with using gelatin as a clarifying agent when bottling beer is that it attracts and settles the yeast in the suspension of the beer. This is not a problem when kegging because the carbonation needs of kegs are different than those of bottles. When most of the suspended yeast is left in the fermenter after bottling, your bottles will take an eternity to condition the proper level of carbonation; if they do at all. The same goes for using polyclar.

I just stick to Irish moss in the last 15 minutes of boiling and patience for clarifying my beers. The Irish moss will sequester the proteins during boiling and leave nothing for the tannins to bind with and cause chill haze.