For the homebrewer that grows and harvests his/her own hops; how best can one approximate the alpha acids in the hops they have?
Is it worth it or should brewers stick to using home grown hops for aroma and flavor?
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I was going to ask this very question on here last week but I googled and found that you basically have two options:
I'm not a professional chemist so I can't recommend any quantitative tests, but you could make a controlled subjective comparison against a package of purchased hops with known alpha acid content. The goal is not an exact number but a relative bitterness.
Micro batches of a quart to 1/2 gallon size using either sugar water or a basic light malt extract, use your standard tap/filtered water to start and top off evaporation losses with distilled water. Quart(or liter) is the smallest I would go, even with this you need a mass balance with at least 0.002oz or 0.1g resolution for the dry hops to get better than +-10% measurement accuracy.
Use a light wort and lower IBU because it should make small changes in bitterness more obvious, require less total hops, and avoid some taster burnout/tolerance-drift.
If starting with no baseline I would make three known standard batches of approximately 10, 20 and 40 ibu. As an example: 0.04 ounces of 6%aa hops in one quart of 1.040 wort boiled 60 minutes should be 21.5ibu, so then use .02, .04 and .08 ounces per quart to represent 3% 6% and 12%aa. Then make a batch with 0.04 ounces of the unknown hops. I see cultivated hops sold from 2% to 18% alpha acid but I expect random crosses or non-professional cultivation to clip the upper extreme, so you could start by assuming something between 4% and 8% alpha acid and adjust from there. (A bit like a binary search.)
Then taste test. After the initial test round you can blend the standards to get intermediate values and narrow the scope. If the unknown hops seem to be above "12%" standard then dilute the unknown sample by half,(and note that the original standards then become 6, 12 and 24%) or if below the "3%" standard brew up a new batch of the unknown hop at double strength.(and note that the original standards then represent 1.5, 3 and 6%) By diluting or doubling the strength of the unknown batch, rather than the comparison samples, you keep the total IBUs in a moderate range for best detection.
The other option for hops under 3% is to ignore the alpha acid and use them for aroma only.
The following is an addition to the post. An alternate, possibly more simple method.
Make a single known 40 IBU sample. An unhopped wort.(also boiled) And a batch with the unknown hops, calculated to an assumed 40 IBU. What I mean by assumed is to just assume some an alpha acid percentage for the initial brewing calculation, and write it down. (There are many brewing calculators available for IBU so I won't explain that here.) Then compare and dilute the more bitter sample until the two samples match, recording the total dilution factor.
If the unknown sample is less bitter than the known sample: You need to dilute the known sample, and the alpha content of the unknown sample is less than your assumed percent. The fraction of known-hopped volume to total diluted volume is the multiplier for the assumed alpha in the unknown hops. example: 3 parts known-hopped wort diluted with 1 part unhopped wort is 3/4. If you had originally assumed 7% in your brewing calculation for the unknown sample then your actual alpha content is
7*(3/4)=7*0.75=5.25%(same as 1 part hopped plus .333 part unhopped,
7*(1/1.333)=7/1.333 or 7*0.75=5.25%)
If the unknown sample is more bitter than the known sample: You need to dilute the unknown sample, and alpha of the unknown is greater than the your assumed percent. The fraction of total unknown-hopped plus unhopped divided by unknown-hopped is the alpha acid multiplier. Example: 3 parts unknown-hopped with 1 part unhopped wort is 4/3. If you originally assumed 7% in your brewing calculation for the unknown sample then your actual alpha content is
7*(4/3)=9.33% (this works even more easy with 1 part hopped and eg 0.333 part unhopped.
Neither of these methods will tell you much about beta acids, which are more important in dry hopping and lagering. You will not get essential oil content either, which is closely related to aroma, and while it can be measured quantitatively there are many qualitative elements and most home brewers don't bother to use the oil content in any calculations.