The Fermentis website seems to have contradictory instructions for rehydration. If you go to the Tips & Tricks section, it shows:
Rehydrate the dry yeast into yeast cream by sprinkling it in ten times its own weight of water or wort. Gently stir and leave for 30 minutes. Finally, pitch the resultant cream into the fermentation vessel.
The rehydration step is done in a vessel outside the fermenter. The objective is to allow the yeast to recover all its functionalities before pitching.
Ale yeasts optimum rehydration temperature: 25 29C (77 84F)
Lager yeasts optimum rehydration temperature: 21 - 25 (69 - 77F)
After rehydration bacterial contamination can develop in the slurry. Follow our recommendations of maximum time between rehydration in sterile water and pitching depending on the storage temperature of the rehydrated yeast.
Stored at 4C Pitch within 18 hours
Stored at 20C Pitch within 6 hours
Stored at 25C Pitch within 4 hours
Water or Wort?
Fermentis yeast can be rehydrated with sterile water or sterile wort.
After the wort has been boiled for at least 15 minutes, collect the volume required for rehydration and leave to cool to the required temperature. Rehydrate the yeast for 30 minutes. Pitch immediately into the tank after checking the temperature of wort, in order to avoid foam.
Dont forget your rehydration essentials.
Respect recommended rehydration temperatures to assure the yeast membrane fluidity
Water or wort, whatever you choose make it sterile
Do not use chlorinated water it will kill the yeast
Do not use demineralized water
But going to the US-05 page (S-04 is the same), you see:
Sprinkle the yeast in minimum 10 times its weight of sterile water or wort at 27°C± 3°C (80°F ± 6°F). Leave to rest 15 to 30 minutes.
Gently stir for 30 minutes, and pitch the resultant cream into the fermentation vessel.
Alternatively, pitch the yeast directly in the fermentation vessel providing the temperature of the wort is above 20°C (68°F). Progressively sprinkle the dry yeast into the wort ensuring the yeast covers all the surface of wort available in order to avoid clumps. Leave for 30 minutes, then mix the wort using aeration or by wort addition.
Ive emailed Fermentis twice but havent gotten an answer. Ive looked through old threads but cant find anything about the contradictory instructions.
I think probably any combination of these techniques would be fine. Does anybody have a more definitive answer?