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Hops
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The hopping rate (weight) and IBU saturation have a drastic effect on IBU's. Aaron Justice reported a higher utilization when using Polaris hops (17.6% alpha acids) versus Tettnang hops (1.9% alpha acids). Two beers were brewed using enough of each of the hop varieties to target a calculated 40 IBU. However, the beer brewed with Polaris hops had 42.6 IBU and the beer brewed with Tettnang hops had only 28 IBU. It was hypothesized that the higher surface area of more hop matter reduces IBU's by binding to iso-alpha acids and possibly other compounds that register on the standardized IBU test. Justice also reported that more IBU pickup from whirlpooling was possible in beers without hops added in the boil, indicating that IBU saturation can limit IBU's, with a maximum IBU being around 100 <ref name="justice_2018" />.
The pH of the wort can also have a small effect on the isomerization of alpha acids to iso-alpha acids, although this variable is less significant than other variables such as alpha acid content, time (to a certain extent), temperature, and original gravity. Aaron Justice reported a slight rise in utilization of hops ("utilization" as being the conversion of alpha acids to iso-alpha acids) when the boil pH was raised from 5.05 to 5.35 <ref name="justice_2018" />. Bastgen et al. (2019) found that at a boil pH of 5.6, the percentage of iso-alpha acids increased by 32% by extending the boil from 60 minutes to 120 minutes. However, there was no increase at all in iso-alpha acids when the boil pH was 7, but a pH of 7 is not typical in the brewing process <ref name="Bastgen_2019" />. Time is thought to play a large role in isomerization, however, Justice reported that the majority of the IBU from iso-alpha acids in 60 minute additions and in whirlpool additions occurs within the first 10 minutes, with only a 12-30% increase after another 50 minutes of boiling/whirlpooling (higher gravity beers had more isomerization during the final 50 minutes while lower gravity beers had less isomerization during the final 50 minutes of boiling) <ref name="justice_2018" />.
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