After the Dry Hopping mega chapter, we find ourselves stuck in a subset: Dry Hopping and Bitterness. I know I like my Hazebombs to maintain bitterness, as I find it aids drinkability and complexity. But I always thought that came exclusively from hot-side hopping. Think again!
IBUs are determined via a spectrophotometer that measures the iso-alpha-acid content in a beer. Cohumulone, humulone, and adhumulone are the alpha-acids that get isomerized, leaving bitterness behind in a beer. Hops also contain humulinones which are easily extracted into beer due to polarity. These humulinones are 66% as bitter as alpha-acids, so
1 ppm humulinone = .66 IBUs
Humulinones are not present during hot-side hopping, so their time to shine is dry-hopping. .5-2 pounds per barrel of dry-hopping imparts large amounts of humulinones. Though hops can take iso-alpha-acids out of beer as well! During wort cooling or dry hopping, this phenomenon is possible. John Maye found that around 25 IBUs is the threshold. Beers beginning dry-hopping under 25 IBUs tend to get more bitter, whereas beers over 25 IBUs tend to lose bitterness during dry hopping. This is because the iso-alpha-acids were removed and replaced by less bitter humulinones and alpha-acids.
Calculated Sensory Bitterness = ppm iso-alpha-acids + (0.66 x ppm humulinones) + (0.10 x ppm alpha-acids)
The higher the HSI of a hop, the more bittering compounds it possesses.
The larger the dry hop rate, the higher the pH of the beer rises. You can expect a 0.14 pH increase per pound of pellets used in the dry hop. The higher the pH of the beer, the more bitter it will taste.
Hop aroma extracts (on the cold side) increase perceived bitterness.
Magnesium can isomerize alpha-acids on the cold side. Magnesium can be added via brewing salts or are presented by the malts used during the brewday. Hops can also isomerize at room temperature, or during fermentation in small amounts.
Only 8 ppm of alpha acids can thwart a bacterial infection. I have a feeling hops were not only used for storage by those Brits shipping to India.
That's all. Succinct, incredibly useful stuff.
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