A Few Experiments on Hop Stands

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Brewers on this and other boards describe hop stands with or without whirlpooling and lasting from a few minutes to well over an hour. Most toss the hops in at knockout and let room temperatures gradually cool the kettle. Others chill to 80 or 70 and then add the hops. It all seems to work, but I went looking for relevant, controlled experiments and found a few. Other members may know of more.

In one, hop stands of 80 minutes gave the finished beer 14% more hop flavour and 9% more hop aroma than 50 minute stands. The hops were added at knockout. Chilling was done after the stand, and how much the wort cooled during the stand is unclear. See Van Havig, Maximizing Hop Aroma and Flavor through Process Variables, MBAA Technical Quarterly 47 (2), 2009.

Havig appears to have used whole flowers. Pelleting crushes lupulin sacs and reduces hops to a coarse powder with a large surface area. Quite possibly substituting pellets would speed the extraction of aroma and flavour, but I didn't find relevant experiments.

As for temperature, one experiment compared 30-minute whirlpools at 170 F (77 C) and only 120 F (49 C). Tasting panels found only slight differences. “Drew” seems to have used pellets.

If stands extract flavour and aroma over a wide range of temperatures, then brewers can do whatever is convenient, except that bittering becomes a consideration at the higher temperatures.

Experiments have found that iso-alpha acids accumulate half as fast at 90 or 92 as at 100, when isomerization and subsequent degradation are both taken into account. See in particular Malowicki, M.G., Hop Bitter Acid Isomerization and Degradation Kinetics in a Model Wort-Boiling System, M.S. Thesis, Oregon State University, 2004.

I found little solid evidence concerning what happens below 90. Some brewers claim isomerisation stops below a threshold around 80, mostly based on brewing AIPAs, in which small additions to bittering could easily be lost.

“Alchemy Overlord” tried hop stands at five temperatures from 100 down to 61. There was bittering down to 61, but with only one data point at each temperature, the experiment is not a basis to predict rates of iso-α acid accumulation.

In using IBU calculators I extrapolate from Malowicki's model, estimating that at 80 degrees iso-α acids accumulate about half as fast as at 90 and one-fourth as fast as at 100. For a 40-minute stand at 80 degrees I add 10 minutes to every hop addition. Knockout hops go into the calculator as 10-minute additions, while boil hops 15 minutes before knockout are entered as 25-minutes, etc. At 70 I would assume a rate one-eighth as fast as at boiling.

It seems to work, but a better test would be to add all the hops post-boil. I might try it first at 90 and then at 80 and have a vial of isomerised extract handy in case the bittering falls too low.
Malowicki gives the activation energy for isomerisation as 98 kJ / mol so the isomerisation rate ratio will be given by

E^ (Ea/(R.T1)- Ea/(R. T1-10))

For 100 vs 90 oC this is

E^ (98000 / 8.3 x 373 - 98000 / 8.3 x 363)

which is about 1 / 2.5.

Repeating the calculation for 80 degrees gives a ratio of about 1 / 6.

This was all covered a while ago in the thread on IBU calculation
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