Rehydrate yeast in sterile water

[Northern Brewer]

Chris answered that one back here...

viewtopic.php?f=12&t=20346&p=232152&hil ... ty#p232152

My MO for rehydrating is pretty painless. At some point whilst I'm brewing the kettle goes on and when I make myself a cup of tea I also fill up my trusty jam jar with boiling water and fit the lid. This cools over the next couple of hours. Some 30 minutes before I'm ready to pitch I pour away 3/4 of the water, add a packet of SO4, re-apply the lid and give it a good shake. This goes into the fermenter half-an-hour later and definitely gets fermentation going several hours quicker when compared to simply sprinkling the yeast directly onto the wort.

[BarryNL]

I still don't understand why hydrating yeast in water is good but making a starter with it is bad.

It's to do with the osmotic pressure across the yeast cell walls. With plain water it's easy for the dry yeast to draw water into the cell and rehydrate. With wort it's more difficult - rehydrating in plain water gives the yeast a chance to rehydrate more easily.