Biotransformation

What is actually happening

A large part of a hop’s aroma potential is not free oil at all. It is bound up as glycosides — aroma compounds attached to a sugar — which are odourless in that state and pass straight through a beer that never meets an enzyme capable of cutting them loose.

Yeast makes such enzymes. Dry hopping while fermentation is still active puts the hop and the enzyme in the same vessel at the same time, and the released compounds are frequently more tropical and more intense than anything in the hop as measured.

Yeast also transforms hop compounds it did not release: geraniol converts towards citronellol, which is one reason a beer dry-hopped during fermentation can smell of a citrus the hop analysis does not show.

The trade is control. The same activity is still scrubbing volatiles out through the airlock, so timing decides whether the technique adds more than it removes, and different yeast strains do markedly different amounts of it.

Typical conditions

Ranges, not targets. These describe what is usual across the styles and breweries that use this step, and a value outside the range is a decision rather than an error.

Styles where this step is decisive

• New England IPA6–7.5% ABV

  Hazy, soft and intensely aromatic: tropical and stone fruit hop character with deliberately low bitterness and a full, juicy body.

• American IPA5.5–7.5% ABV

  The defining modern craft beer: intense citrus, tropical and resinous hop aroma over a lean malt base, finishing firmly bitter.

• Double IPA7.5–10% ABV

  IPA scaled up: more strength, more hops and more bitterness, while staying dry enough to remain drinkable.

Related steps and concepts

• Dry hopping — The same addition, timed differently.
• Primary fermentation — What must still be active for it to happen.
• Hop creep — The unwanted consequence of the same enzyme activity.