2 packets of yeast?

[cutnrun]

Ok I have done 2 really nice beers for my first 2 AG's and I was looking to do a stout/porter next. I have noticed that some of the recipes use 2 packs of yeast. Why?

If a pack of yeast is added to the wort won’t it reproduce and use all of the available sugar until the alcohol level gets too high and it inhibits the yeast?

By adding 2 packs, doesn’t it just speed up the rate that the sugar is used and the inhibiting alcohol level is reached?

The SG will fall to the same point with 1 or 2 packs of yeast just take longer with only 1

 

[Aleman]

My Imperial stout recipe used 3 (or was it 4??) packets of yeast for 25L . . . but then it was 1.098, and for big beers you need to increase the amount of yeast you pitch . . . after all you don't want it to fail to ferment to dryness do you?

Another reason apart for high gravity beers where you want to pitch a lot of yeast is when you pitch at cold temperatures. if you pitch W34/70 into a 'lager' wort at 22C you can get away with pitching 11g . . . if you chill that same wort down to 9C you need to pitch 75g of the same yeast.

You are correct though for 'standard' beers at 'standard' fermentation temperatures there is rarely any requirement to pitch more than one 11g sachet of yeast . . . . of course those pathetic little 6g sachets of dried yeast you get with most beer kits are completely inadequate and although you can get away with under pitching it is not best practice.

A good calculator to determine teh amount of yeast to pitch can be found at Mr Malty pitching Rate Calculator. . . Although I still think he is under estimating the pitching rate for high gravity beers.