dry hopping
process / process-dry-hopping
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Adding hops to beer after fermentation or during cold fermentation, rather than to boiling wort. It does not involve isomerization of α-acids.
On this page (7 sections)
Reading the bitterness
Dry hopping adds hops to beer after fermentation or during cold maturation, rather than at the wort-boiling stage. Because there is no boiling, α-acids do not undergo isomerization into iso-α-acids, the main source of bitterness. The process is often described as transferring hop aroma alone after fermentation, but studies show that the chemical composition of beer bitterness changes as well.
In dry-hopped beer, IBU (International Bitterness Units), a common bitterness indicator, can rise. Yet in a study of 15 commercial beers before and after dry hopping, total iso-α-acid concentration instead fell in more than half. In an experiment dry-hopping an unhopped beer at 0–16 grams per liter for 0–72 hours, the IBU increase was attributed to extraction of humulinones (oxidized α-acids) and, in some cases, polyphenols. A review by another group also supports this divergence in directions and estimates humulinone bitterness at about 66% (±13%) of iso-α-acid bitterness.
Dry hopping was once considered not to contribute to bitterness because α-acid isomerization does not occur. However, a study examining hop freshness concluded that humulinones were a principal source of bitterness in dry-hopped beer. They increased during dry hopping but began to decline as oxidation progressed further after hop storage condition passed a certain threshold. An IBU value therefore cannot be read directly as a measure of iso-α-acid content.
Beer that is dry hopped after fermentation, because the wort is not boiled, tends to let hop-derived constituents transfer more directly, giving a strong aroma; bitterness felt in the beer overall, however, is described as weaker than in kettle-hopped beer. Two reasons are given for why conversion into iso-α-acids progresses poorly: alpha acids have low solubility in water, so much of it remains undissolved, and the temperature during dry hopping is not high enough for thermal isomerization to occur.
In dynamic (agitated) dry hopping used in US commercial brewing, the target dosage is described as 500-800 g/hL; exceeding this saturates the environment with hop oils, and extraction efficiency per unit dosage is reported to drop noticeably instead. In static dry hopping, there is also a report that around 24 hours at a process temperature of about 13.3-15.0°C is enough for an average of 75% of humulinones to transfer.
As a concrete numerical example, in a primary study that dry hopped Cascade hops at a dosage equivalent to 386.54 g/hL for 3 days at 16°C, iso-α-acid concentration is reported to have fallen from 51 mg/L to 32 mg/L, while both humulinones and alpha acids increased from below the detection limit to 13 mg/L. In another experiment where Cascade hops were dry hopped at 1.43 g/L, polyphenol concentration increased significantly over the control from about 2 hours after the start of dry hopping, but did not increase uniformly over time: it dropped below the control concentration once around 24 hours, then increased again over the final 3 days.
Sources for this overview
Impact of Dry Hopping on Beer Flavor Stability
Checked for this overview: Checked the description of adding hop aroma after fermentation and the finding that more than aroma changes. / Basis for the behavior in which polyphenols increase significantly at 2 hours with 1.43 g/L dry hopping, drop once at 24 hours, then increase again over the final 3 days
Toward Understanding the Bitterness of Dry-Hopped Beer
Checked for this overview: Checked addition rates, contact times, rising IBU, and falling iso-α-acids in 15 commercial beers. / Checked the description of humulinones as potential contributors to dry-hopped beer bitterness.
Effects of Dry-Hopping on Beer Chemistry and Sensory Properties - A Review
Checked for this overview: Checked the convergence of sources on the opposing trends and the estimate of 66±13% relative bitterness for humulinones. / Basis for the US standard dosage of 500-800 g/hL and the drop in extraction efficiency above it / Basis for the statement that an average of 75% of humulinones transfer at 24 hours and 13.3-15.0°C / Basis for the numerical example of iso-α-acids 51→32 mg/L and humulinones/alpha acids 0→13 mg/L at 386.54 g/hL, 3 days, 16°C
Impact of Hop Freshness on Dry Hopped Beer Quality
Checked for this overview: Checked the earlier view that dry hopping was considered not to contribute bitterness and the conclusion about humulinone increase and decline. / Basis for the contrast that aroma is stronger and bitterness weaker compared with kettle hopping / Basis for the reasons (solubility and temperature) why isomerization yield is low
Follow the connections
dry hopping →increases→ bitterness units (BU / IBU)
humulinones →is extracted into beer by→ dry hopping
dry hopping →differs from→ kettle hopping →contributes to→ bitterness
Structured claims — open a section to read each claim's confidence, evidence and limits
Mechanism and receptors (1)
Scope: Discussion of beer dry hopped with 3 hop varieties (Celeia, Aurora, Styrian Wolf) at 5 HSI levels
Note: The authors adopt the mechanistic explanation from another source; this is not direct verification original to this study. It is not an experiment that quantitatively separates the respective contributions of solubility and temperature.
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process-dry-hopping —has_currently_proposed_mechanisms→ "Two reasons have been proposed for the low isomerization yield into iso-α-acids: the low solubility of alpha acids, and that the temperature does not become high enough to bring about thermal isomerization"How processing and cooking change it (6)
Scope: Pilot-scale dry hopping of a hop-free ale (0-16 g/L, 0-72 h), and a survey of 15 Pacific Northwest commercial beer brands sampled before and after dry hopping
Note: The 2 halves move in opposite directions: the conventional IBU proxy rises while the compound class it is supposed to stand in for falls. Central counter-evidence against treating IBU as a reading of iso-α-acids.
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process-dry-hopping —increases→ concept-bitterness-unitsScope: Dry-hopped beer in a pilot-scale study, a survey of 15 commercial brands, and a hop-freshness trial across 3 varieties
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compound-humulinones —is_extracted_into_beer_by→ process-dry-hoppingScope: The process definition used in a dry-hopping time study
Note: This is not treated as 『香りだけ』 (in translation: "aroma only"). Existing data show that humulinones, polyphenols, IBU, and iso-alpha acids can move as well.
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process-dry-hopping —is_used_as→ "A process that adds hop aroma after fermentation"Scope: A general literature description of the process contrasting dry hopping with kettle hopping (not limited to specific hop varieties or dosages)
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process-dry-hopping —differs_from→ process-boil-hoppingScope: Pale ale dry hopped with Cascade hops at 8 oz/bbl (1.43 g/L) for 0-96 hours (freshly made Young beer and Aged beer after 30 days of storage at room temperature)
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compound-polyphenols —is_extracted_into_beer_by→ process-dry-hoppingScope: A review's description of dynamic (agitated) dry-hopping practice in US commercial brewing
Note: This describes US practice only; the authors add that high doses such as 2200 g/hL also exist, so it is not an absolute upper limit.
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process-dry-hopping —has_typical_dosage_range_of→ "The standard dosage for dynamic dry hopping in the US is described as 500-800 g/hL; exceeding this noticeably lowers extraction efficiency per unit dosage due to saturation of the environment with hop oils"