Pitching Yeast - Temperature

If I rehydrate my yeast at 100F (I assume this is optimum temperature for yeast growth). Why should I not pitch it at close to the same temperature (accounting for cooling and such)? 60-75F seems low.

Re-hydrating wakes up the yeast and 100° makes it wake up happy. Letting it cool to wort temperatures prevents it from being shocked by the temperature change when pitching. Shocking your yeast will slow it down because it has to recover.

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cwballard said:

If I rehydrate my yeast at 100F (I assume this is optimum temperature for yeast growth)

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You're not GROWING yeast when you rehydrate it. The higher temperature is only to minimize the number of yeast cells that die instantly upon being rehydrated, it does NOT mean that you should be fermenting your beer at that temperature.

Another reason is you need to aerate the wort for the yeast to be health. Aerating at a higher temperature increases the chances of oxidation. Although 100 degrees might be fine for dried yeast it's not god for the aeration of wort. I've pitched at 100 degrees with no problems when I first started but if you keep doing it you could eventually get burned, especially if your making a beer that needs to age longer.

cwballard said:

If I rehydrate my yeast at 100F (I assume this is optimum temperature for yeast growth). Why should I not pitch it at close to the same temperature (accounting for cooling and such)? 60-75F seems low.

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100*F seems a little on the high side. If you are a little bit above this you actually start killing the yeast.

You don't want to ferment at high temperatures as yeast will produce larger amounts of by-products the higher the temperature is. Is the temperature to high these by-producs may increase to a level at which they are condidered off-flavors for a given style of beer.

Kai

I agree with Kaiser, 100 is a little warm IMO. 85-90F is the warmest I rehydrate at.

I rehydrate my yeast with warm tap water, right around 80 and a tbl of DME I have never done it at that high of a temp.

RICLARK said:

I rehydrate my yeast with warm tap water, right around 80 and a tbl of DME I have never done it at that high of a temp.

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The tbl of DME might get you into trouble. It can easily be infected and you won't kill the nasties unless you boil it first. There is also no need to feed them during the hydration process. They'll get plenty to eat minutes later.

Kai

RICLARK said:

I rehydrate my yeast with warm tap water, right around 80 and a tbl of DME I have never done it at that high of a temp.

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that's more than rehydrating, that is called proofing...when you provide a small amount of fermentable sugar.

shouldn't be needed. if you feel you must do it, use honey. honey has a natural ability to fight off bacteria, and store bought honey is heat treated enough that you shouldn't need to worry about any wild yeast spores in the honey taking hold in your wort.

30degC or 86degF is the temperature we use in the lab for yeast growth, FWIW.

I tend to pitch at the fermentation temperature I want, to ensure a clean flavour with no fusels or dominant esters.