Hops

Bitterness is the best-known job. The alpha acids from hops turn into iso-alpha acids during the boil, which taste bitter and balance the sweetness of the malt.

But hops also give aroma. Dried hop cones contain 0.5–3% volatile oil, with compounds such as linalool (floral) and geraniol (rosy). Because that oil largely evaporates during the boil, brewers add aroma hops late or cold, when dry hopping. Hops also contain tiny amounts of thiols: sulfur compounds that you can smell at nanograms per litre. According to a 2023 review article, 4MMP smells of blackcurrant and passion fruit, and 3MH of rhubarb and grapefruit. In hops, many of these thiols are bound to other molecules; yeast can release them during fermentation. Yeast also converts geraniol into the citrusy citronellol.

And hops do more than add flavour. Iso-alpha acids inhibit gram-positive bacteria, such as lactic acid bacteria. According to review articles, hop acids also help keep the foam stable. Under the influence of light, those same iso-alpha acids do supply a building block for the infamous skunky smell.

It is true that dry hopping produces no new iso-alpha acids: that takes boiling. The alpha acids that end up in the beer through dry hopping dissolve poorly and, according to research, give no noticeable bitterness even at 28 mg/L. In addition, iso-alpha acids from the beer attach themselves to the hops. In a trial with Cascade hops (386 g/hL, three days at 16 °C), they dropped from 51 to 32 mg/L.

Even so, the beer can taste more bitter. Stored hops contain humulinones: oxidation products of alpha acids that cannot yet be detected in fresh hops. With dry hopping they end up in the beer, and they are about 66% as bitter as iso-alpha acids. In 121 beers examined they averaged 17 mg/L, and according to one study they can account for up to 28% of the bitterness in dry-hopped beer. Polyphenols from hops can also give a lingering bitterness and a dry, astringent feel.

The acidity changes too. With Cascade, the pH rose by about 0.1 per 386 g/hL of hops, and a higher pH strengthens the perception of bitterness: according to researchers, an extra 0.1 pH unit is comparable to 2–3 extra IBU.

How it nets out differs from beer to beer. The IBU figure is not a good measure here either: dry hopping can bring compounds into the beer that count in the IBU measurement but do not taste bitter.

For a long time, hops were seen as an inert ingredient on the cold side of the process. Research shows that this is not true. Hops contain enzymes, including alpha-amylase, beta-amylase and amyloglucosidase, which break down dextrins. Dextrins are sugar chains that yeast normally does not ferment. After the breakdown, the yeast can carry on with them, and fermentation starts up again. Brewers call this hop creep; according to a review article, the phenomenon was already described in 1893.

The effect can be considerable. In a 2018 trial, a fully fermented lager was given 10 grams of Cascade hops per litre and some yeast, at 20 °C. In 40 days the beer fermented on to such an extent that, once fully fermented out, it amounted to 1.3% (v/v) extra alcohol and 4.75% (v/v) extra carbon dioxide; according to the 2023 review, one of the higher values measured. In another trial, with two days of dry hopping, enough sugars were released for about 0.4% extra alcohol, if they were to ferment completely.

For the brewer that is a nuisance. The beer becomes drier and can taste empty, and in unfiltered and unpasteurised beer the pressure in the bottle or can can rise too high, with a risk of gushing bottles. Diacetyl can also form, so the beer has to mature for longer. Drying the hops at a higher temperature and using a lower temperature during fermentation or secondary fermentation can limit it, but according to the 2023 review there is as yet no way to prevent hop creep completely.

There is a kernel of truth in it. Around 1150, Hildegard of Bingen already wrote that hops, through their bitterness, keep out some of the spoilage in drinks, so that they stay good for longer. According to beer historian Martyn Cornell, unhopped ale could sometimes spoil after as little as a week or two. Modern research confirms that iso-alpha acids from hops inhibit gram-positive bacteria.

But hops do not protect against everything. Some strains of lactic acid bacteria, such as Lactobacillus brevis, are resistant to hops, and gram-negative bacteria such as Pectinatus and Megasphaera can spoil beer too.

More important for the enthusiast: what hops contribute in aroma and flavour is precisely what is fragile. According to a 2023 review article, the aroma stability of dry-hopped beer is low. In a storage trial, two groups of hop bitter compounds (humulinones and alpha acids) dropped sharply in the first three months at 20 °C, and hardly at all at 3 °C. According to the authors, you are therefore best off storing dry-hopped beer cool, at around 4 °C. A study of eleven hoppy ales brewed in Germany does add nuance: some hop aroma compounds, such as linalool and geraniol, remained reasonably stable during storage, but in some ales the hop aroma depended on less stable esters. And light can break down iso-alpha acids, which contributes to a skunky smell.

In short: hops help against some of the spoilage, but a strongly hopped beer is not a beer for keeping.

IBU (International Bitterness Units) is, according to CraftBeer.com, a measure of the bitter compounds in beer, determined in the lab as milligrams of iso-alpha acids (converted bitter compounds from hops) per litre. The Brewers Association writes in its style guidelines that this figure measures one specific hop compound and does not consistently match how bitter someone perceives the beer. It gives three reasons for that: sensitivity to bitterness differs per person (partly through genetic make-up), other ingredients also contribute to bitterness, and hop aroma and flavour can strongly change the perceived bitterness.

The guidelines themselves give examples. For Export-Style Stout the BA writes that the bitterness can turn out high in the lab, but that the sweetness of the malt dampens the perception. Conversely the BJCP says of kölsch that drier versions can seem hoppier or more bitter than the IBU suggests. And in hazy IPA the bitterness is, according to the BJCP, often masked by the fuller body and the soft finish.

For orientation: CraftBeer.com gives 5–10 IBU for light lagers and 50–70 for robust, bitter IPAs. So use IBU as a rough indication, not as a flavour promise.

The 'skunky' or 'lightstruck' smell is a well-known beer defect with a chemical cause. According to research by, among others, Ghent University (Huvaere et al., 2006), it comes mainly from the compound 3-methylbut-2-ene-1-thiol (MBT), which stinks at just a few nanograms per litre. MBT forms because light breaks down the bitter compounds from hops (iso-alpha acids). Riboflavin (vitamin B2) in the beer absorbs the light and starts the reaction; a sulphur fragment, from amino acids among other things, completes the compound. Blue light (350–500 nm) proved the most effective.

That is why the packaging matters. According to the Brewers Association (BA), brown glass protects somewhat, green glass less and clear glass hardly at all. A Molson patent from 1981 confirms with measurements that clear glass lets through the most harmful light and brown glass the least. But even brown bottles do not belong in bright light according to the BA, and according to a guide from CraftBeer.com (also from the Brewers Association) the skunk smell can then form within a few seconds. The BA calls cans 'mini kegs' that protect beer from light.

Heat is bad for beer, but it causes other defects. According to the Brewers Association, lagers stored at 25 °C tended towards caramel-like notes, and at 30–37 °C towards a cardboard taste (ageing through oxidation). A skunk smell is not part of that.

According to the Brewers Association, the word 'juicy' is often used to describe the aroma and flavour of these beers. It says how the beer smells and tastes, not what is in it.

According to the BJCP, a hazy IPA gets its 'juicy' character because the brewer adds a lot of hops late in the process, above all by dry hopping, with hop varieties that smell of tropical fruit. The Brewers Association says the same: 'juicy' is about flavour and aroma that come from 'late, often very large, additions of hops'. According to the BA, a juicy character is not even required; citrus, pine, herbs or flowers are allowed too.

The yeast joins in as well. The BA mentions medium to medium-high fruity esters, aroma compounds from the yeast, that complement the hop profile.

If there is noticeable fruit in it, or lactose or vanilla, then according to the BJCP it is no longer an ordinary hazy IPA and the beer belongs in a special category, such as fruit beer. The BA counts a hazy IPA with fruit among the Experimental IPAs.

In master's research at Oregon State University (Barnette, 2018), dry-hopped beer was stored cool (3 °C) and warm (30 °C), with different amounts of dissolved oxygen. The storage temperature had the biggest effect on the aroma. With heat and more oxygen, tropical, citrus and hoppy aromas disappeared, and malty, dried-fruit and cardboard aromas took their place. Oxygen had a smaller, but demonstrable effect after just two weeks.

Other research, from the VLB in Berlin (2022) on eleven commercial hoppy ales, shows that it is more nuanced. Some hop aroma compounds stayed reasonably stable, other aroma compounds (esters) less so, and compounds that go with ageing increased.

The Brewers Association writes of the double IPA that the beer must show 'the fresh character of hops', without oxidised or aged flavours.

So store IPA cool and drink it fresh.