Why you should always rehydrate dry yeast

Note from the editors (2026): This article is preserved for context but its advice is out of date for modern dry yeasts. See our newer fermentation basics article for current guidance.

Dry yeast is dormant, dehydrated cells. When you sprinkle it directly onto cold wort, the osmotic shock kills a substantial fraction of the cells before they can wake up — sometimes as much as half of them. The result is a slower start, a longer lag, and a higher risk of off-flavours.

To rehydrate properly:

1. Boil 250 ml of water and cool it to 25-30 °C.
2. Sprinkle the entire sachet on top of the water — don’t stir.
3. Wait 15 minutes for the cells to absorb water.
4. Gently stir, then pitch into your fermenter within the next 15 minutes.

This step takes 30 minutes and gives the yeast every advantage from minute one.