The classification is commercial, not chemical
"Aroma hop" and "bittering hop" are marketing categories inherited from the brewing industry of the 1970s and 1980s, when a brewery's hop bill was split into a cheap alpha source for the kettle and an expensive noble variety for late additions. Nothing in the lupulin gland enforces that split. Every hop cone contains alpha acids, beta acids and essential oils; the ratios differ, the chemistry does not.
That said, the categories do track real numbers, and the Hopedia corpus lets us put figures on them. Across 329 varieties, the breakdown of declared purpose is 143 dual-purpose, 140 aroma, 46 bittering. In other words, only about 14% of documented hops are sold purely as alpha machines, while the dual-purpose group is now the largest single category. The old two-bin model is already obsolete in the catalogue — most modern releases are bred to do both jobs.
Alpha acids: where the line actually falls
Alpha acids are the only property that reliably separates the groups, because they are what isomerise into iso-alpha acids during the boil and produce measurable IBU. In the corpus the distribution across all 329 varieties looks like this:
| Percentile | Alpha acid (%) | Practical reading |
|---|---|---|
| min | 1.2 | barely usable for bittering |
| p10 | 4.3 | classic landrace aroma territory |
| p25 | 6.0 | upper limit of "low alpha" |
| median | 8.9 | the typical modern hop |
| p75 | 12.5 | entry point for kettle hops |
| p90 | 15.0 | high-alpha class |
| max | 20.5 | super-alpha |
Two things fall out of that table. First, the median hop at 8.9% alpha is perfectly capable of bittering; nothing about 9% alpha makes a hop "aroma only". Second, the useful working threshold for a dedicated kettle hop sits around the 75th percentile, roughly 12–13% alpha, because that is where the pellet mass needed for a given IBU drops low enough to stop affecting the beer's flavour.
Run the arithmetic. For a 20 L batch at 1.050 OG targeting 35 IBU with a 60-minute addition (utilisation ≈ 27%), you need about 26 g of a 10% alpha hop but only 17 g at 15%. Swap in a 4% aroma hop and you are suddenly boiling 65 g — more than three times the vegetal matter, which means more kettle trub, more wort absorbed by the hop bed, and a distinct chewy, tea-like hop-bitter character that some styles want and most do not.
Where our five reference hops sit
Warrior at 14.5–18% alpha sits above the 90th percentile of the database; Saaz (CZ) at 2–5% sits below the 10th. Those two bracket almost the entire usable range, and the chart below shows just how far apart their acid profiles are.
| — | Saaz (CZ) | Cascade | Citra | Magnum (US) |
|---|---|---|---|---|
| Alpha acid | 2–5% | 4.5–9% | 10–15% | 10–16% |
| Beta acid | 4–8% | 4.8–7.5% | 3–4.5% | 4.5–7% |
| Co-humulone | 23–26% | 30–40% | 20–35% | 21–30% |
| Variety | Purpose | Alpha % | Beta % | Cohumulone (% of alpha) | Total oil (ml/100 g) | Percentile vs. corpus (alpha) |
|---|---|---|---|---|---|---|
| Saaz (CZ) | Aroma | 2–5 | 4–8 | 23–26 | 0.4–1.0 | below p10 |
| Cascade | Dual | 4.5–9 | 4.8–7.5 | 30–40 | 0.7–2.5 | ≈ p25–p50 |
| Citra | Dual | 10–15 | 3–4.5 | 20–35 | 1.5–3.0 | ≈ p60–p90 |
| Magnum (US) | Bittering | 10–16 | 4.5–7 | 21–30 | 1.6–3.0 | ≈ p60–p92 |
| Warrior | Dual | 14.5–18 | 4.3–6 | 22–26 | 1.0–2.5 | above p90 |
Note the awkward fact hiding in that table: Magnum (US) is labelled bittering and Citra is labelled dual, yet their alpha ranges overlap almost completely and Magnum actually carries more total oil at the top of its range. The label reflects how the oil smells, not how much of it there is.
Cohumulone: the quality-of-bitterness argument
Cohumulone is one of the three main alpha-acid analogues. The traditional claim is that high-cohumulone hops give a harsher, coarser bitterness. The effect is real but modest and partly confounded by dose: because cohumulone isomerises slightly more efficiently and its iso form is more soluble, high-cohumulone hops deliver marginally more IBU per gram, which alone shifts perception.
Corpus figures, from 290 varieties with reported data: median 28.5% of alpha, p25 at 24.5%, p75 at 35%, with a full range from 2.3% to 54%. So a hop at 24% cohumulone is not exotic — it is simply first-quartile. Saaz at 23–26% and Warrior at 22–26% both land there, which is exactly why Warrior has a reputation as a "clean" high-alpha hop despite carrying three to four times Saaz's acid load. Cascade at 30–40% sits above the median, and its kettle bitterness is noticeably firmer and more resinous.
Practical rule: if you bitter with more than ~40 IBU, pick something at or below the 25th percentile for cohumulone (≤ 24.5%). Below 30 IBU the difference is largely inaudible.
Oils: the real dividing line for aroma hops
If alpha acids define bittering capacity, essential oils define aroma capacity — and here the numbers are less flattering to tradition. Across 279 varieties the median total oil is 1.5 ml/100 g, with p25 at 1.1 and p90 at 2.6 (max 4.5). Saaz, the archetypal aroma hop, carries only 0.4–1.0 ml — below the 25th percentile. Citra at 1.5–3.0 ml straddles the median to the 90th percentile.
That is the single most counter-intuitive statistic on this page: the classic "aroma" hop has less aroma oil than the average hop in the database, and roughly a third of what a modern American dual-purpose variety delivers. Noble aroma is about composition, not concentration. And because oil, unlike alpha acid, does not survive the boil, low-oil hops must be dosed late and heavily — 5–8 g/L of Saaz at whirlpool is not unusual, versus 2–4 g/L of Citra for a much louder result.
Composition matters more than volume. Corpus medians: myrcene 42.3% of oil, humulene 18.5%, caryophyllene 8.5%, farnesene 0.5%. Citra's 60–70% myrcene sits at or above the 90th percentile — that is the fresh, citrus-tropical, volatile fraction that dry hopping preserves. Magnum's 30–45% humulene brackets the p75 mark of 25.1% and above, which reads as noble, woody, spicy: useful for a lager bittering charge whose oil contribution should be quiet and dignified rather than loud.
Farnesene is the sharpest divider of all. The corpus median is just 0.5%, and p75 is still 0.5% — meaning more than half of all varieties report essentially none. Saaz at 14–20% is therefore an extreme outlier, in the top few percent of the whole database, and that green-apple, linden, faintly waxy note is a large part of what "Czech pilsner aroma" actually means. The chart below compares oil composition across the three profile types.
| — | Saaz (CZ) | Citra | Magnum (US) |
|---|---|---|---|
| Total oil | 0.4–1 ml | 1.5–3 ml | 1.6–3 ml |
| Myrcene | 25–40% | 60–70% | 30–45% |
| Caryophyllene | 6–9% | 5–8% | 8–12% |
| Humulene | 15–30% | 7–13% | 30–45% |
| Farnesene | 14–20% | 0–1% | 0–1% |
Aroma descriptor frequency — what the database smells like
Hopedia's tag counts explain why "aroma hop" no longer means noble. The most common descriptor across 329 varieties is citrus (142 entries, 43% of the database), followed by floral (114), spicy (93), herbal (61) and pine (61). "Noble" appears on only 27 varieties — about 8%. Tropical shows up 50 times, grapefruit 50, pineapple 34, passionfruit 32. The centre of gravity of modern hop breeding is thoroughly American-citrus, and it is no accident that 148 of 329 varieties (45%) originate in the USA, versus 33 from Germany, 30 from the UK, 23 from New Zealand and 19 from Poland.
How to use the distinction at the kettle
1. Single-hop bittering, clean
Use a high-alpha, low-cohumulone hop at 60 minutes and do not expect flavour from it. Magnum (US) or Warrior at 17 g in 20 L (~35 IBU) contributes bitterness and almost nothing else. This is the correct move for pilsner, helles, and any hazy IPA where you want the late hops uncontested.
2. First wort hopping with an aroma variety
Add 2–3 g/L of a low-alpha aroma hop to the kettle before the boil starts. Utilisation runs ~10% higher than a 60-minute addition, and the bitterness is measurably rounder. With Cascade at 6% alpha, 50 g in 20 L first-wort gives roughly 25 IBU plus a genuine resinous-grapefruit backbone.
3. Hop bursting / whirlpool
Skip the 60-minute addition entirely and put 6–10 g/L into the last 10 minutes and the whirlpool at 75–80 °C. Whirlpool isomerisation still yields real bitterness — plan on 8–12% utilisation for a 20-minute stand — so a 10 g/L Citra whirlpool can land 25–30 IBU on its own. Here, alpha acid and oil are delivered by the same hop, and the aroma/bittering distinction collapses completely.
4. Dry hopping
Alpha acids do not isomerise at fermentation temperature, so dry hop selection is purely an oil decision. Choose by myrcene and total oil, dose 3–8 g/L for 2–4 days at 16–20 °C. A 15% alpha hop used only as a dry hop adds essentially zero IBU, though it can raise perceived bitterness through polyphenols and hop creep.
The takeaway
Treat "aroma" and "bittering" as shorthand for two independent axes, and read the numbers instead of the label:
- Alpha ≥ 12.5% (p75) — efficient kettle hop; below 6% (p25) expect large, trub-forming charges.
- Cohumulone ≤ 24.5% (p25) — safe for heavy bittering; ≥ 35% (p75) better kept under 30 IBU.
- Total oil ≥ 2.0 ml (p75) — worth dry hopping; ≤ 1.1 ml (p25) needs generous late dosing.
- Myrcene ≥ 52% (p75) — bright and fruity, fades fast; humulene ≥ 25% — noble, spicy, boil-stable.
- Farnesene ≥ 4% — already top quartile; ≥ 10% means a distinctive, old-world floral signature.
With those five thresholds you can classify any variety in the database yourself, and you will occasionally disagree with the vendor's label. That is a feature.