Hydrate Dry Yeast

What is the benefit of hydrating dry yeast?

Doesn't it just make it easier for the yeast to "wake up"? Adding dry yeast to water gives the yeast just one thing to deal with.....hydrating. If you add the dry yeast to wort, you are hydrating them and asking them to start working all at once. I've always thought of it like walking outside. If you walk out your door and the weather is simply cold, you have one thing to deal with. But if you walk outside and it's cold, rainy and windy, you have a harder time adjusting to the conditions and you tend to ask yourself why you ever moved away from California. Or maybe that's just me.

I'm sure someone else can answer this much better than I can.

When yeast is dehydrated, the cell walls cannot function as a gate to keep materials which are detrimental to the yeast from entering the cells.  When you throw dehydrated yeast into a highly sugar rich environment (such as wort) the cell walls cannot regulate the migration of the sugars into the interior of the yeast cells and the high concentration may kill or damage the yeast.  By rehydrating the yeast in water, the cell walls swell and start functioning before they are subject to the sugar rich wort and this enhances the survival chances of the yeast.

Thank you for your replies.  I do hydrate dry yeast before pitching.  I had home brewers tell me its not necessary.  I can answer back now with good reasons.

Cheers,
John

I cut my lag time down to almost nothing and my fermentation times in half when I started rehydrating my dry yeast in my second runnings (first run of a double batch sparge, usually about 1.035 or so) and then spin it on a magnetic spin plate for about an hour and a half.  After reading the replies to this thread though, I'm thinking about trying to rehydrate the yeast for about 15 minutes in a bit of my brew water that has been boiled and cooled, then adding it to the second runnings (also boiled and cooled) and spinning it on the plate.  This way the first wave of hydration is only with water and then add some food to the mix. 

I had home brewers tell me its not necessary. 

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While there are advantages to rehydrating the yeast, it really isn't necessary. If you follow the directions on the packet the beer will come out fine. If you rehydrate it then you should have less lag time and a higher cell count. Either way you'll get good beer.