Originally Posted by daveyohill
I understand that dry yeast viability is improved with rehydration prior to pitching (according to Jamil anyway).
This is true and here's why...
When yeast ferment certain sugars (such as maltose), they must first secrete an enzyme that breaks this sugar down so they can metabolize it. They only have the capacity to secrete these types of enzymes when they are healthy and hydrated. Under these circumstances, the yeast's cell membrane won't allow maltose in unchecked.
When a dehydrated yeast cell first becomes hydrated, it allows too many things in the hydration solution into the cell unchecked. When it allows things in that it would normally break down first (e.g., maltose), it can die. This is why it's best to rehydrate in water first. It gives the most viability and cell count.
That being said, it's not black and white and plenty of yeast survive when rehydrated directly in wort. In the case of ales, it is often desired to have a flavor profile that comes from yeast characteristics (esters, etc.). Many of these compounds are produced during the growth phase. So, having a higher cell count, such as that which comes from rehydrating dried yeast in water, is not always considered necessary depending on the brewer and the intended beer.
In the case of S-05, you may want a relatively clean flavor profile, so it may be worth rehydrating... especially if the gravity is over ~1.040.