What is hop biotransformation?

Biotransformation is the conversion of hop compounds by active yeast during fermentation. Yeast enzymes cleave bound glycosides and reshape terpenes, producing new fruity thiols and esters that you cannot get from dry hopping a finished, still beer. It happens only while the yeast is actively fermenting.

When is the biotransformation window?

It opens once active fermentation is established, typically around day 2 to 3 at high krausen, and closes as the yeast flocculates and attenuation completes, usually by day 4 to 5. The exact timing depends on temperature and yeast speed, which this tool models.

Does temperature change the timing?

Yes. Warmer fermentation (20 to 22 Celsius) speeds the yeast, so the window opens and closes sooner. Cooler fermentation (16 to 18 Celsius) slows everything and pushes the window later. The calculator shifts the dates accordingly.

Will a biotransformation dry hop raise gravity readings?

Dry hops can cause a small apparent gravity bump and a brief renewed activity called hop creep, where enzymes in the hops free more fermentable sugar. Leave the beer a few extra days after a biotransformation charge before packaging to let any hop creep finish.

Should all my dry hops go in during fermentation?

No. A common modern approach splits the charge: part during active fermentation for biotransformation, and part cold after fermentation completes for fresh, volatile aroma. The cold charge preserves delicate oils that active fermentation would scrub out via CO2.

Dry Hop Biotransformation Window Calculator

For hazy, juicy IPAs the timing of your dry hops matters as much as the variety. Adding hops while the yeast is still actively fermenting triggers biotransformation, where yeast enzymes convert hop compounds into new fruity aromas. This tool calculates the exact window for your batch.

How it works

Biotransformation only happens during active fermentation. The window opens once the yeast reaches high krausen and closes as it flocculates and finishes attenuating. Two factors control the timing:

Temperature — warmer ferments are faster, so the window arrives earlier.
Yeast speed — vigorous hazy strains reach the active phase sooner than clean, slow strains.

The calculator models a baseline window of roughly day 2 to day 5 after pitching at 20 °C with a medium-speed yeast, then shifts it:

shift = temperature offset + yeast speed offset

Cooler temperatures and slower yeast push the window later; warmer and faster pull it earlier. The recommended dry-hop day sits near the middle of the window, at high krausen, where conversion is most active.

Why mid-window

Adding hops too early (before active fermentation) risks stripping aroma through vigorous CO2 scrubbing and offers little enzyme activity. Adding too late, after the yeast has dropped clear, means no biotransformation at all — it becomes an ordinary cold dry hop. The middle of the window balances enzyme activity against aroma loss.

Tips

Watch for hop creep: biotransformation charges can free extra fermentable sugar, so leave a few days before packaging and confirm a stable gravity.
Many brewers split the charge — part during fermentation for biotransformation, part cold afterward for fresh volatile aroma.
Keep fermentation temperature steady; swings shift the window unpredictably.

All dates are computed locally in your browser.