Using a PH meter to determine home grown hops acid level?

[Hermit]

This idea came to me tonight when talking to someone. Boil down a specific hop weight in a specific volume of distilled water and measure the PH. We are talking acids after all. It only needs to get a number close enough so that home brewers growing hops can be close enough to use them in brewing. Now that AJ has us all out buying ph meters.....

Seriously, I have NO idea if this is valid so I just thought I'd throw it out there.

 

[ajdelange]

pH actually, while it is intimately related to the amount of acid in a solution, is not, in itself, a measure of the amount of acid. To measure the actual amount of acid we need to know a lot about all the acids and bases in the solution in particular the dissociation constants or every proton in every acid. If we have all that information we can add controlled amounts of base to a solution and observe how much the pH changes with each addition. From this it would be, theoretically, possible to estimate the amount of the various bittering acids. An example of where we do this in brewing is where we measure the alkalinity of water by adding controlled amounts of acid up to the point where pH 4.3 is reached. In this case we are able to assume that there is only one acid system (carbonic) present and we know the 2 pKs of carbonic acid very well. The situation with hops is much more complex.

Another comparison would be where the titratable acidity of a grain is measured. Here the number of acids is as multiple as it would be with hops but we are only concerned about the total. We could certainly get the total acidity (or alkalinity) of a hops solution but we would have no way of knowing what part of that came from the bittering acids and how much from other acids.

A much simpler explanation is that if it were possible to estimate the amount of alpha and beta acids in a hops sample by something as simple as a pH measurement or even a titration someone would be doing it but this is not how hops bitterness is measured. The current methods include UV absorption or HPLC of extracts of the plant/beer.

 

[Hermit]

I started thinking of the "adding base solutions" after I posted this but in terms of a taste test. As I type this another question actually pops up. Is it even the acid component that is directly responsible for the bitter flavor or is it just the carrier? I know acidic isn't one of the taste senses, but I don't consider things line lemons bitter.

 

[ajdelange]

As I type this another question actually pops up. Is it even the acid component that is directly responsible for the bitter flavor or is it just the carrier?

An acid consists of a proton(s) (hydrogen ions) bound to an anion. What makes something taste sour is the hydrogen ion but the cation may have a flavor of its own. Lactic acid certainly tastes sour as does citric acid and phosphoric acid but the 'sourness' is very different for the three with lactic and citric having pronounced distinct flavors and phosphoric little. It is the anions that are responsible for the other flavors.

The same principle applies with hop acids. They aren't acidic enough to release enough protons that the solution tastes sour but the anions taste very bitter.

I know acidic isn't one of the taste senses, but I don't consider things line lemons bitter.

Actually it is: the sour taste is caused by acid i.e. protons (though there is more to it than just that). It is alkaline (basic) things that tend to taste bitter but I don't think the mechanism is as clearly understood as it is for acids. I'm a bit out of my depth here.

 

[josephus23]

If you're growing a lot of hops at home the best way to test the AA% is to buy a UV/Vis spectrometer. The measurement isn't that difficult.

 

[ajdelange]

It's pretty simple to measure the bittering level in a beer but the procedure for estimating alpha and beta acids from hops is appreciably more involved, prone to error and, in general, a PITA. Besides which a UV/Vis does not come cheap and you will also need a good shaker, spectrophotographic grade toluene and spectrophotographic grade methanol. You can do it without a shaker but I don't think you'll want to do it more than once (i.e. you have to shake by hand and it's for a pretty long time).

 

[Hermit]

It's pretty simple to measure the bittering level in a beer but the procedure for estimating alpha and beta acids from hops is appreciably more involved, prone to error and, in general, a PITA. Besides which a UV/Vis does not come cheap and you will also need a good shaker, spectrophotographic grade toluene and spectrophotographic grade methanol. You can do it without a shaker but I don't think you'll want to do it more than once (i.e. you have to shake by hand and it's for a pretty long time).

For home grown hops the ratio of the acids may not be as relevant unless you want to age the beer. I have no idea if it is plant dependent or not? Will the same plant produce vastly different ratios year to year?

Is the equipment reasonable for a home brew club if you don't care about the ratio that much?

 

[josephus23]

I got a few Spec 20 spectrometers a while back when a college was throwing them away. I recently sold a few on Ebay, which went for about $150 each. There are a few issues with this process:
1. the instrument is heavy, so it costs about $40 to ship it.
2. you need a good balance to weigh out the hops
3. you need solvents to extract the hop oils (hexanes....) not all that available to most people
4. you need glassware, such as a separatory flask to extract the hops into these solvent.

It's probably easier to send the hops to a place such as HopUnion to get them analyzed.

 

[ajdelange]

The equipment is the same if you are just doing beer or hops. Shaking is required in both cases. And the hops procedure is not a pain because of the ratio. It is a pain because of the extraction and because the extract has to be diluted by trial and error until the absorption falls within certain limits. Once you get all that done you just make readings at two wavelenghts, stick them in a couple of formulas for, respectively, the alpha and beta levels.

Reasonable for a club? Depends on the club

DR 5000

is pretty dear but is a good unit because it comes pre-programmed for a lot of the water tests and brewer wants to do and also contains the software for the ASBC tristimulus color test. More practical than shelling out for one of these would be to try to obtain one of their older models (DR-4000UV) used. There are, of course, other manufacturers (I believe the Hach units are actually made by Beckman).

For the shaker you want something like the Burell 75 wrist action shaker. They have a newer model that looks pretty expensive so I'd look for an older unit. You will need different clamps for hops (which are extracted in volumetric flasks) and beer (which is extracted in a 50 mL centrifuge tube).