Hop Alpha Acid Testing (at Home)

[Smashin]

From Brewing Science and Practice

8.2.3 Analysis of the hop resins
Procedures for the estimation of the total resins, total soft resins, and hard resins, by difference, (see definitions above) in hops and hop products are given in Analytica-EBC but since it was found that the A-acids are the most important brewing principles few brewers bother to measure the total and soft resin contents. The _-acids were originally estimated gravimetrically as their lead salts but since the precipitate is soluble in excess of the lead acetate reagent, trials had to be made so that only the correct amount of reagent was used making it a lengthy process.

View attachment 23432

It was found that if the conductivity of a methanolic solution of the A-acids was measured during titration with methanolic lead acetate solution it did not increase until there was an excess of the reagent. Thus if regular aliquots (0.20 ml) of the methanolic lead acetate reagent (2 or 4%) are added and the conductivity measured after each addition, a graph can be drawn where the intersection of the two straight portions provides the endpoint (Fig. 8.5). The absolute value of the conductivity is not needed and the lead acetate reagent can be standardized by a similar titration against 0.100 N sulphuric acid. The shape of the graph is different in the presence of other solvents, so, in the approved method, pyridine (1 ml) is added to the titration. The conductivity may be plotted against the volume of the reagent on Cartesian coordinate paper (Fig. 8.5) or the resistance may be plotted directly on to reciprocal ruled paper. Since the reaction of lead acetate is not specific for A-acids, the result is expressed as the Lead Conductance Value (LCV). However, with fresh hops the LCV is very similar to the A-acid content but, as hops age on storage, oxidation products are formed which may react with lead acetate.

Hope that helps

MHB

Cheers MHB, thats all in line with what ive read so far, for bittering calculations LCV seams to be a very useful measure. Especially for home brewers. Personally i dont consider the other hop oil components when designing a beer so it seam very useful for young and old hops. Picked up 4L of methanol today, its denatured metho, should be fine for this purpose.

Heres to yellow salt!!!

Very useful that the actual conductance isn't so important as is the turn point interpereted on the chart.

 

[MHB]

Mate are you sure its Methanol, you need Methanol, not Ethanol with bad flavourings (aka -Denatured Alcohol, Mentholated Spirits, Metho).

If for no other reason than that chlorophyll is very soluble in Eth but only sparingly so in Meth and I have no idea how soluble either Lead Acetate or more importantly the precipitate that should form would be in Ethanol as apposed to Methanol.
God knows what the denaturing agents would do to your results, they dont even tell you which of the dozen some approved agents they have used.

Chlorophyll caused me a pile of grief during my final year chemistry project; I had to design and then execute a testing procedure to determine the levels of Capsaicine in Chillies; the red and yellow ones were fine, green ones gave wacky results, until I changed solvents (dam Magnesium).

This really is a pretty basic test, you will however have to use the right reagents and processes, if you arent sure of your methodology or the chemicals you need, drop into the Chemistry department of your local tech college and find your self a beer-head chemist, they can be amazingly useful people to know.

Equipment wise you need (just at a glance) a decent burette, good scales, a couple of beakers, filter funnel and papers, a magnetic stir plate and bar, and a conductivity meter.
The last you can get from a hydro shop or some pool shops (go for one that reads in millisiemens rather than TDS)

I would strongly recommend that you write down your procedure, elucidate each step carefully and then get someone with relevant experience to go through it with you.

The first thing to look at would be your sample size, just say you were testing 10g samples of a hop, if you wash the sample (say) 3 times in 100mL portions of Methanol to extract the AA, you would want 500mL beakers to hold the sample, that should leave room for the titration to be added and for stirring

The accuracy of your scales will be important to, if you weighed the 10g sample on scales that are good to 1g as apposed to 0.01g your results are 1/100 the as accurate from the get go. You would need a 1 Kg sample, 5 litre beakers and the burette from hell to get the same accuracy.

Dont even want to start talking about representative samples but you will have to think about it.

Like I said, get someone with relevant experience to go through it with you.
I thought about doing something similar, decided it was too much of a PITA but then I dont grow hops, that might make it worth while.

MHB

 

[Smashin]

Mate are you sure its Methanol, you need Methanol, not Ethanol with bad flavourings (aka -Denatured Alcohol, Mentholated Spirits, Metho).

If for no other reason than that chlorophyll is very soluble in Eth but only sparingly so in Meth and I have no idea how soluble either Lead Acetate or more importantly the precipitate that should form would be in Ethanol as apposed to Methanol.
God knows what the denaturing agents would do to your results, they don't even tell you which of the dozen some approved agents they have used.

Chlorophyll caused me a pile of grief during my final year chemistry project; I had to design and then execute a testing procedure to determine the levels of Capsaicine in Chillies; the red and yellow ones were fine, green ones gave wacky results, until I changed solvents (dam Magnesium).

This really is a pretty basic test, you will however have to use the right reagents and processes, if you aren't sure of your methodology or the chemicals you need, drop into the Chemistry department of your local tech college and find your self a beer-head chemist, they can be amazingly useful people to know.

Equipment wise you need (just at a glance) a decent burette, good scales, a couple of beakers, filter funnel and papers, a magnetic stir plate and bar, and a conductivity meter.
The last you can get from a hydro shop or some pool shops (go for one that reads in millisiemens rather than TDS)

I would strongly recommend that you write down your procedure, elucidate each step carefully and then get someone with relevant experience to go through it with you.

The first thing to look at would be your sample size, just say you were testing 10g samples of a hop, if you wash the sample (say) 3 times in 100mL portions of Methanol to extract the AA, you would want 500mL beakers to hold the sample, that should leave room for the titration to be added and for stirring

The accuracy of your scales will be important to, if you weighed the 10g sample on scales that are good to 1g as apposed to 0.01g your results are 1/100 the as accurate from the get go. You would need a 1 Kg sample, 5 litre beakers and the burette from hell to get the same accuracy.

Don't even want to start talking about representative samples but you will have to think about it.

Like I said, get someone with relevant experience to go through it with you.
I thought about doing something similar, decided it was too much of a PITA but then I don't grow hops, that might make it worth while.

MHB

yes, yes good point on the metho being essentially ethanol. I'll have to order some methanol in and a 50ml burette.

I did some error calcs based on measurement accuracies, with a set of scales with +/-1g my compound error is 6.85%, with +/-0.1g scales this error falls to 1.85%, given in both cases a 0.2ml division on the burette. Which is goodenough for the home brewer (note new scales on the shopping list). Its fairly easy to standardise the testing procedure for repeatabilty. Anyway progress is being made.

 

[Smashin]

Thought I'd post a bit of an update. For those interested the project is nearing critical mass. I now have all equipment with the exception of a magnetic stirrer, which should be built by the weekend. So once again this weekend should be the commencement of some hop testing (at home).


pictured. Technical grade methanol (20L), Lead Acetate (50yrs worth), 50ml burette (0.1ml graduations) and pipette sucker thingy modified for burette, Scales (0.01g and 1g increments), Conductivity meter, thermometer.

First up is some testing of mixing times to determine typical leaching durations and some testing of known a% hops. A bit of typical homebrew style ingenuity may be required to mount everything to keep hands free and away from the nasty stuff but functionality is what is required before beauty.

Heres to a can do attitude, will post more later. :blink:

 

[tcraig20]

I've used HPLC (and GCMS) quite a bit but have never come across the pressure description.

From what Ive seen High Pressure Liquid Chromatography is an older title that has largely fallen out of use. It was more common back in the 70s and early 80s, at least from what Ive seen of the literature.

It did, however, enjoy a brief revival in the title of my honours thesis