Hop science: alpha acids, beta acids and hop oils, explained

Inside the cone

A hop cone is the flower of the female hop plant: papery green bracts stacked like shingles. At the base of each one sit tiny golden glands of lupulin. Rub a cone between your palms and that's the sticky, pungent stuff you smell.

Lupulin holds two families of chemicals that matter to beer:

Resins — the alpha and beta acids. They make beer bitter, and they help keep it fresh.
Essential oils — a few drops per handful, hundreds of compounds. They make beer smell like citrus, pine, flowers or tropical fruit.

Everything below is one of those two. Each molecule is drawn the way chemists draw them: every corner and line-end is a carbon atom, hydrogens are left off, and the red Os are oxygen. Tap Spin it in 3D to see the real shape.

Alpha acids: the bite

Alpha acids are why beer is bitter — sort of. Fresh off the plant they're barely soluble and not very bitter at all. The magic happens in the boil: heat twists each alpha acid's six-sided ring into a five-sided one, making an iso-alpha acid. That new shape dissolves in beer and tastes properly bitter. Brewers call it isomerization, and it's the reason bittering hops go in early and boil for an hour.

Humulone C₂₁H₃₀O₅

Isohumulone (after the boil) C₂₁H₃₀O₅

Cohumulone C₂₀H₂₈O₅

Same atoms, different shape: humulone and isohumulone have the identical formula. The boil just rearranges them. Even so, only a fraction of the alpha acids a brewer adds ends up in the beer as iso-alpha acids — commonly around a quarter to a third.

Three siblings. Humulone, cohumulone and adhumulone differ only in one small side chain. Hop sheets list cohumulone as a % of the alpha because brewers have long thought more of it means a harsher, rougher bitterness. The research on that is mixed, but low-cohumulone hops are still sold as "smooth".
IBU (International Bitterness Units) is roughly milligrams of iso-alpha acids per litre of beer.
Skunked beer is an alpha-acid story too: light breaks iso-alpha acids apart into a sulfur compound (3-methyl-2-butene-1-thiol, below) that smells like a skunk. It's why beer comes in brown bottles and cans.
Iso-alpha acids also help beer foam hold up, and they fight off bacteria — the original reason hops went into beer at all.

The skunk molecule (MBT) C₅H₁₀S

Hardest-biting hops on file: Polaris 20.27% · TerraSurge 19.2% · Eureka! 18.45% · Moutere 18.4% · Vic Secret 17.88%

Gentlest: Contessa 2.5% · Wai-iti 2.69% · Strisselspalt 3.08% · Hersbrucker 3.29% · Saphir 3.47%

Lowest cohumulone: Glacier 12% of alpha · Saphir 14.5% of alpha · Vanguard 15% of alpha · Smaragd 15.5% of alpha · Opal 16% of alpha

Beta acids: the slow burn

Beta acids (lupulone and its siblings) look a lot like alpha acids with one more side chain, but they don't isomerize in the boil and barely dissolve, so fresh they add little bitterness. As hops age and oxidize, beta acids break down into bitter compounds — which is partly why old hops still bitter a beer even as their alpha acids fade. They're also strongly antibacterial.

Lupulone C₂₆H₃₈O₄

Hop sheets show the alpha-to-beta ratio because it hints at how a hop's bitterness holds up in storage.

The oils: the smell

Hop oil is only about 0.5–3 mL in every 100 g of hops, but it's where nearly all the aroma comes from. There are hundreds of compounds in it; a handful do most of the talking. They're volatile — they evaporate easily — which is why a long boil strips them out and why aroma hops go in late, in the whirlpool, or into the fermenter as a dry hop.

Myrcene C₁₀H₁₆

Myrcene

Green and resinous — the "fresh-cut hop" smell (it's also in bay leaves and ripe mango). Usually the biggest share of the oil, especially in American aroma hops, and the most volatile — boil it and it's gone. It's a big part of why fresh hop and heavily dry-hopped beers smell so green and juicy.

Most myrcene on file: Ernest 83% · Mistral 82% · Zappa 81% · Styrian Dragon 81% · Nectaron 79%

Humulene C₁₅H₂₄

Humulene

Woody, herbal, earthy — the classic "noble hop" note of German and English hops. Named after the hop itself (Humulus lupulus). As hops age or cook, humulene oxidizes into compounds brewers describe as spicy and "hoppy".

Most humulene on file: Junga 53% · Progress 51% · Whitbread Golding Variety 51% · East Kent Golding 49% · Smaragd 48%

Caryophyllene C₁₅H₂₄

Caryophyllene

Spicy, peppery, woody. The very same molecule gives black pepper and cloves part of their bite. Humulene's close cousin: both are built from the same 15-carbon starting block.

Most caryophyllene on file: Sladek 21% · Adeena 20% · HBC 638 19% · Kazbek 18% · Bramling Cross 18%

Farnesene C₁₅H₂₄

Farnesene

Floral, green, a hint of green apple. Many American hops have almost none; a few European aroma hops — Saaz and its relatives especially — are known for it.

Most farnesene on file: Saaz Late 21% · Motueka 18% · Wai-iti 18% · Sterling 18% · Saaz 17%

Linalool C₁₀H₁₈O

Linalool

Floral, citrusy, lavender-like — it's one of the main scents of lavender and coriander. A small share of the oil, but it dissolves into beer better than most, so it's one brewers watch.

Most linalool on file: Dr. Rudi 7% · Loral 2% · Bobek 2% · Vital 2% · Smaragd 1%

Geraniol C₁₀H₁₈O

Geraniol

Rose and geranium. The fun part: yeast can convert geraniol into citronellol, which smells more like citrus. That's one example of biotransformation — yeast rewriting hop aroma during fermentation, and part of why dry-hopping during active fermentation tastes different.

Most geraniol on file: Riwaka 3% · Superdelic 2% · Talus 2% · Dolcita 2% · Sabro 2%

"Most on file" is the share of each hop's oil, from the breeders' and merchants' spec sheets HopLove cites. The coloured strip on every hop card uses these same colours.

Thiols: the tiny ones that punch hardest

Some of the most powerful hop aromas come from sulfur compounds called thiols, present in parts per trillion yet easy to smell. The famous one is 4MMP: blackcurrant and passionfruit when there's a little, and — honestly — cat pee when there's a lot. It's also in Sauvignon Blanc wine. Hops like Citra and Simcoe are known for it, and much of it sits locked up in the hop until yeast frees it during fermentation.

4MMP C₆H₁₂OS

In the brewhouse: when the hops go in

Boil, 60 minutesAlpha acids isomerize into bitterness. The oils boil away. This is the bittering addition.
Late boil, 5–15 minutesSome bitterness, some flavour; a little of the oil survives.
Whirlpool, hot but not boilingLots of oil stays in, a little bitterness. The big-flavour move in hazy IPAs.
Dry hop, in the fermenterCold, no boiling: nearly all the oil makes it into the beer, and yeast can transform some of it. Almost no bitterness. This is the "juicy" aroma.
Fresh hopHops straight off the bine, never dried, used within hours. Mostly water, so brewers use several times as much. Green, grassy, bright — and only once a year. See this season's →

Reading the numbers on a hop page

Alpha acid, e.g. 11–14%: Percent of the dried cone's weight. More alpha, more potential bitterness. Under 5% is gentle; 13%+ is a bruiser.
Beta acid: Also % of cone weight. Mostly about storage and aging, not the boil.
Cohumulone, e.g. 22–26%: Not % of the cone — % of the alpha acids. Lower is traditionally called smoother.
Total oil, e.g. 1.5–3 mL/100 g: How much aroma a hop packs. Above 2 is a lot.
Myrcene 60%, humulene 15%…: Each oil as a share of the total oil — the coloured strip.
Why ranges?: Every harvest, farm and lab gives slightly different numbers, and HopLove keeps each source's figures instead of picking one. The range is where they land.