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Hops up close - alpha acids, oils and noble versus American

The bitterness of beer and its fruity, floral or resinous aroma come from one plant: hops. But it is not one simple seasoning. In the tiny yellow glands inside a hop cone hide two completely different groups of compounds: the alpha acids responsible for bitterness and the essential oils responsible for aroma. It is they, in different proportions and of different character, that decide whether a beer bitters subtly and smells of flowers, or hits you with grapefruit and resin. Understanding hop chemistry is the key to understanding why a European pilsner and an American IPA, from the same raw material, taste so extremely different. Here is a guide to hops up close: what alpha acids are, what oils are, and what really sets noble hops apart from American ones.

What a hop cone hides

The hops used in beer are the female cones of the plant, and all their value hides in the lupulin, the yellow, sticky powder in the glands at the base of the cone petals. It is in the lupulin that the compounds sit which give beer its bitterness and aroma. The rest of the cone is mostly green plant matter of little flavour importance. The lupulin holds two key groups: the hop resins, including the alpha acids responsible for bitterness, and the essential oils responsible for the smell. It is a fundamental division, because these two groups behave completely differently during brewing. Understanding that two different worlds, bitterness and aroma, sit side by side in the cone is the starting point for understanding the whole role of hops in beer.

Alpha acids and bitterness

Alpha acids are the hop resins responsible for the bitterness of beer, and the most important of them is humulone. There is a catch, though: raw alpha acids are poorly soluble and barely bitter at all. Only the boiling of the wort transforms them into a form called iso-alpha acids, well soluble and clearly bitter. This is why bittering hops are added at the start of the boil, usually for around sixty minutes, to give time for this transformation. The alpha acid content of a hop is given as a percentage, and it tells you how strong in bitterness a given hop is. The higher the percentage, the less hop is needed to reach a given bitterness. Alpha acids are therefore the chemical source of that characteristic, palate-cleansing bitterness, without which beer would be bland.

Essential oils and aroma

The second group is the essential oils, responsible for the hop smell and flavour in the finished beer. They give the notes of citrus, resin, flowers, tropical fruit, herbs or spice. The most important of them are myrcene, humulene and caryophyllene. The oils have a basic drawback from the brewer point of view, though: they are very volatile and evaporate in a flash at high temperature. That means a hop added at the start of a long boil will lose almost all its aroma, leaving only bitterness. This is why aroma hops are added late, at the end of the boil or after it, and in modern beers also cold, after fermentation. This division of roles, bitterness early, aroma late, follows directly from the different stability of these two groups of compounds.

Bittering, aroma and dual-purpose hops

From this division comes a practical classification of hops. Bittering hops are rich in alpha acids, and are added early for the bitterness alone. Aroma hops have lower alpha acids but plenty of oils, and are added late for the smell. There are also dual-purpose hops, which have plenty of both, so they suit both bitterness and aroma. The brewer chooses hops and the moment of their addition like a cook chooses spices: one for the base bitterness, another for the finishing aroma. This is why beer recipes list not only which hop, but also when it was added. Understanding this classification is the first step to reading a beer through the lens of its hops.

Noble hops

Noble hops are the classic European varieties of delicate, refined character: floral, herbal, spicy and earthy. They include, among others, the Czech Saaz and the German Hallertau, Tettnang and Spalt. It is they that build the subtle, elegant aroma of classic European lagers and pilsners. They usually have a low alpha acid content, so they serve not for strong bitterness but for refined aroma. Characteristic of them is a higher concentration of a certain oil component, farnesene, of which there is little in American varieties. This partly explains their different, more delicate profile. Noble hops are the essence of the European, restrained school of beer, where the hop is meant to complement the malt, not drown it with loud fruit.

American and New World hops

At the other pole stand the American and New World hops, known for their bold, loud flavours: citrus, pine, tropical fruit and resin. It is they that stand behind the character of the American IPA. Varieties like Cascade, Citra, Centennial or Simcoe give an intense, fruity aroma, far from the subtlety of the noble hops. Interestingly, the content of myrcene, the main oil component, is similar in them to the European ones, but in general hops from North America tend to be more aromatic, with the noble hops as the exception. Many New World varieties also have high alpha acids, so they suit both bitterness and aroma. It is they that revolutionised craft beer, moving the hop from a background role to the front of the stage.

A table: noble versus New World

Let us gather the most important differences in one place:

The table shows why such different beers come from the same raw material: it all depends on the hop variety and its profile.

When to add the hop

Since bitterness and aroma behave differently, the moment of adding the hop is crucial. A hop added at the start of the boil, for around an hour, gives mainly bitterness, because the alpha acids isomerise and the volatile oils escape. A hop added at the end of the boil or to the whirlpool keeps more aroma. Dry hopping, that is adding the hop after fermentation, gives the maximum of fresh, fruity smell with no extra bitterness, and it is what stands behind the intense nose of modern IPAs. The brewer therefore steers not only which hop, but also when to add it, to draw out bitterness, aroma or both. We cover hops in the context of IPA more in hops and the grapefruit flavour. It is timing that decides what from the hop reaches the beer.

How to sense it in the beer

Knowledge of hops immediately changes the way you drink beer. If a pilsner smells subtly floral and herbal, and the bitterness is clean and dry, noble hops added classically probably stand behind it. If a beer hits you with grapefruit, mango and pine, and the aroma is downright juicy, that is the sign of New World hops and probably dry hopping. It is worth comparing a classic European lager with an American IPA side by side, to feel the gulf created by the hop variety alone and the moment of its addition. Recognising whether an aroma is delicate and floral or loud and fruity is one of the most pleasant tasting exercises. Over time you will start to guess the hop used from the smell of the glass alone.

The essentials in brief

Let us gather it up. The value of hops hides in the lupulin, where alpha acids and essential oils sit side by side. Alpha acids, isomerised during the boil, give bitterness and are given as a percentage. The oils, including myrcene and humulene, give aroma, but are volatile and escape at high temperature, so aroma hops are added late or cold. Noble hops from Europe are delicate, floral and herbal, of low alpha acids, and build subtle lagers. New World hops are loud, citrusy and resinous, often of high alpha acids, and stand behind the IPA. Now you know where beer gets its bitterness and aroma, and why the same raw material gives such different beers.

Note every beer in GustoNote - the style, the bitterness and the hop notes you sense. Over time you will start to recognise noble and New World hops by aroma, and understand more deeply how the brewer builds the flavour of a beer. We cover the whole process more in how beer is made.