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Old 08-16-2011, 12:21 AM   #1
passedpawn
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Default IBU Assay: where to get chemicals?

I need smallish amounts of iso-octane (trimethylpentane) and HCl for measuring bitterness units.

Where to buy inexpensive chemicals online? Thanks.

( I can't help but wonder if I could use a different common solvent than the iso-octane, like something in my garage).

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Old 08-16-2011, 12:26 AM   #2
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Great question -- I'll stay tuned for results.

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Old 08-16-2011, 12:31 AM   #3
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Originally Posted by passedpawn View Post
I need smallish amounts of iso-octane (trimethylpentane) and HCl for measuring bitterness units.

Where to buy inexpensive chemicals online? Thanks.
Which procedure are you using? Absorbance measurement? You're best off using spectral-grade isooctane, and that's not cheap. You can order from places like Sigma Aldrich or Alfa Aesar


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( I can't help but wonder if I could use a different common solvent than the iso-octane, like something in my garage).
I know you're mostly kidding, but no. You'd have to recalibrate the scale to the new solvent, which would have a different solubility for the iso-alphas that you want to measure.
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Old 08-16-2011, 12:35 AM   #4
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There's another procedure that I've seen around that is far more bootstrap, but certainly good enough for homebrewing use.

Take 1oz of unknown hops, and boil for a specified length of time in a quart of water. Cool, strain, and successively dilute the solution by half until you can't taste bitterness. That puts you at ~5 IBUs, and you can work backwards to figure out the AA% of the hops.

You'd primarily use it for homegrown hops, but I imagine you could try it for a beer too.

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Old 08-16-2011, 12:52 AM   #5
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Thanks for all the info dwarven. I'm planning on measuring absorbance @ 275nm per this ASBC method.

Argh, $50 for the iso-octane. Better than what I found previously, but still more than I wanted to spend on a whim.

I read that the anti-knock additives for fuel are mostly iso-octane, which was my point about finding something in my garage. Since absorbance is a ratio, I thought any minor differences between said garage chemical and the "spectral-quality" stuff would cancel out. Thoughts?

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Old 08-16-2011, 01:29 AM   #6
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A small impurity (as %) could really screw up your assay if it has a high extinction coefficient in the wavelength range of interest, in your case 275nm.

I couldn't follow the link to your method, but it might be feasible to zero/blank out the background from your garage solvent, but if this is a high proportion of your total signal that makes it tough. Is this solvent just for measuring or is it also needed for extraction?

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Old 08-16-2011, 02:55 AM   #7
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Sorry, ran out of the house in a hurry (pick up kid from soccer practice).

Link is here: http://www.realbeer.com/hops/ASBC.html

More good info here: http://beersensoryscience.wordpress....the-ibu-assay/

... and here... http://www.compacitypro.nl/Portals/3/F_BEER_T_EN.pdf

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Old 08-16-2011, 03:09 AM   #8
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A small impurity (as %) could really screw up your assay if it has a high extinction coefficient in the wavelength range of interest, in your case 275nm.
That's exactly the problem. (that, and upsetting the Abs --> IBU equation that I mentioned above). For example, alcohols absorb very strongly in the UV range, and a mix of alcohols can get pretty wild. Iso-octane is used because its absorbance is known, and a small amount of additives can play havoc with your measurement. I think another additive in those anti-knock formulas is methanol, which will certainly screw up UV absorbance.


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Is this solvent just for measuring or is it also needed for extraction?
It's used for extraction, but I imagine that's just for simplicity's sake, so that the whole procedure is done in one solvent.
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Old 08-16-2011, 03:15 AM   #9
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Quote:
Originally Posted by JimTheHick View Post
A small impurity (as %) could really screw up your assay if it has a high extinction coefficient in the wavelength range of interest, in your case 275nm.

I couldn't follow the link to your method, but it might be feasible to zero/blank out the background from your garage solvent, but if this is a high proportion of your total signal that makes it tough. Is this solvent just for measuring or is it also needed for extraction?
My understanding (from the docs I linked to above) is that the isomerized hop oils are made "more hydrophobic" by lowering the pH of the beer (with HCl). When the non-polar solvent is added, I guess they are then they are extracted from the beer and dissolve into the solvent. The organic phase of this mixture is then measured for UV absorbance.

I'm not a chemist, but if I can get the chemicals I think I can take a stab at it. I'd then like to test some other solvents and acids to see if there is a simpler way to do this with household chemicals (i.e, mineral spirits and vinegar). Just for fun.
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Old 08-16-2011, 04:48 AM   #10
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( I can't help but wonder if I could use a different common solvent than the iso-octane, like something in my garage).
There is indeed something you can use in your garage - gasoline. If you read the MOA carefully you will note that it says that triple distilled gasoline will do. I will happily pay the freight for spectrographic grade 2,2,4 trimethyl pentane before I will distill any gasoline in my lab and would advise you to do the same.

You are correct in your assumption that the HCl is there to lower the pH thus enhancing extraction into the gasoline phase.

Note that you will also need some octanol (or hexanol). This is only used to keep the beer from foaming when it is picked up in the pipet (the tip is dipped into octanol and then the bit remaining in it shaken off before dunking it into the beer). Accordingly, the blank also has a wee bit of octanol in it.

I suppose you could take plain old gasoline, add a bit of octanol and measure the absorbance against distilled water. If it's not too great you could use that as a reagent blank and get readings but I wouldn't be too comfortable doing that.

Also note that you sometimes need a centrifuge as the protein (I assume) in the beer can form a sort of slush which you need to get out of the way to pipet out the solvent phase.

WRT other solvents - when you ask for spectrographic grade you pay a high price for that. Spectrographic grade methanol (used as a dilutant when determining hops alpha and beta acids) is expensive as is the toluene which is used for the extraction.
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