[tschmidlin]

If you really want to be nice to your yeast, rehydrate with a product called GoFerm.  Its a nutrient solution that provides a nice boost during rehydration.  I use it for my wine yeasts, never tried it for brewing.  Then again I mostly use liquid yeasts and starters for those.

I rehydrate my dry yeasts with goferm for mead too, and I have used it for beer although not in a way I could compare it to yeast rehydrated without it.  It's cheap enough that I will keep using it for meads, and I mostly repitch beers from brewery slurry so I don't need it.

[euge]

Will have to try some Goferm! I rehydrate dry yeast per the instructions with regular tap water which is somewhat hard- around 280ppm and get excellent results. I microwave it first to sterilize it and drive off the chlorine. Previously, have done it extensively with both distilled and R/O and there is a significant difference in lag time if that is anything to go by. So it is my local tap-water for me. Albeit somewhat of an anecdotal statement but practice reveals things...

LOL flbrewer you have come to the right place. You'll get probably an overload of info until you get a few batches under your belt!

[duboman]

Osmotic pressure only exists across a semi-permeable membrane, so RO water does not have more osmotic pressure than any other water.  It is water.  Osmotic pressure is better understood as the tendency of water to move across the membrane in order to balance both sides.  When there is something that can NOT move through on only one side of the membrane and not on the other it creates physical water pressure on the membrane from things like water that CAN move across the membrane - water will do so until the pressure on both sides of the membrane is in equilibrium.  In reverse osmosis, pressure is added to one side to force the water across the semi-permeable membrane, leaving the other stuff behind.  I'm sure you can google much better explanations.

Yeast in RO water experience higher osmotic pressure than non-RO water, but not higher than distilled water.  Adding extract to the water before adding yeast will relieve the osmotic pressure.

And while there is some osmotic pressure created by using distilled water, it is the best media for long term storage of yeast.

Started this post before all of these other replies.  Bah.

Thanks for this concise explanation! I was scratching my head a bit since I know plenty of brewers that use R/O, build it up to the desired profile and brew some amazing beer!