What is dry hopping?

With dry hopping, hops go into the beer after the main fermentation has finished, at low temperature, so that the volatile hop oils do not evaporate.

Hops added during the boiling of the wort mainly give bitterness: the alpha acids are then converted into the form that tastes bitter. The aroma compounds in the same hops largely disappear in that heat.

By adding the hop only cold, the aroma is retained. The overview article in Molecules from 2023 therefore describes dry hopping as an aroma technique and not as a way to make bitterness.

After 24 hours of dry hopping, the spent hops still contain 33 to 51 percent of their original essential oil, according to the review article in Molecules.

What has disappeared from it is partly in the beer and partly lost. Of the alpha acids, 52 to 77 percent stays behind in the hops in that same time, so a heavily dry-hopped beer can still turn out somewhat more bitter than the brewer intended.

The usual dosage in American beers lies between 500 and 800 grams of hops per hectolitre. The review article calls that range the optimal dosage, and more hops does not simply give more aroma.

Cold hopping is most common in IPA, pale ale and NEIPA, but brewers also apply it to pils, blond beer and alcohol-free beer.

The technique says something about how the beer is made, not about the style. A pils that is cold-hopped remains a pils; the hop aroma is just stronger.

On the beer shelf at the Brink in Bathmen there are 54 beers listed in stock as IPA, from 29 brands from 9 countries, out of a total of 1,066 beers. In that group, dry hopping is the most common practice.

Brewers put dry hopped or cold hopped on the label as an announcement of a hop aroma, because it is not a legal designation.

Regulation (EU) No 1169/2011 prescribes in Article 9 which particulars are mandatory: among others the name of the product, the net quantity, the date of minimum durability and, above 1.2 percent alcohol, the actual alcoholic strength by volume. It says nothing about brewing technique. A list of ingredients is not even mandatory for drinks above 1.2 percent alcohol.

The statement is therefore voluntary. What is always on there is the best-before date, and for cold-hopped beers it matters because hop aroma fades over time.

Hop aroma fades from bottling onwards, so a cold-hopped beer tastes of hops most strongly when fresh.

Keeping it cool and dark slows that loss. Heat and light, on the other hand, speed it up; light also causes a separate fault, in which hop compounds are converted into a sulphur compound.

With an IPA or NEIPA, therefore, check the best-before date on the bottle and do not put it on the windowsill. A bock or a quadrupel tolerates long storage better than a beer that has to rely on its hop aroma.