How are yeast killed?

Made a beer earlier last week. Aerated the hell out of it. Waited 72 hours and no fermentation (hydrometer). I've never had this problem before. Repitched by sprinkling some S-04 on top and w/i 20 minutes, I could see bubbles coming up. Now have an active fermentation. Not the yeast I wanted but I will have beer anyway. Who knows, maybe it will be better?

I followed the same procedure I always use (basically). Heat up a bit of water to boiling. Take off heat and let cool to about 80-85. Let sit for about 30 minutes. Pitch yeast into water (was Nottingham). I usually pitch when the wort is around 75-80. I always have active fermentation within 4 hours.

The only difference this time is that the wort was too hot when I took the IC out. It was 95. I didn't want to kill the yeast by shocking them. I waited until it was around 80 (3-4 hours later) and then pitched. Then nothing.

Did I over-hydrate them? If not, what might have happened? They were new yeast from a recent order. I've already used other yeast from that shipment and they were fine.

Ideas?

It's possible that the long delay between re-hydration and pitching starved the yeast. Dry yeast don't have much reserve energy, that's why the they manufacturer recommends pitching within 15 minutes of re-hydrating.

A second possibility is a small defect in the packaging allowed moisture into the packet. Dry yeast is only viable for 2 weeks once a packet is opened.

The third is the packet got over-heated at some point.

3-4 hours in plain water is going to seriously hurt the viability of the yeast. That was your mistake.
Ofcourse a punctured or over heated package could also contribute but I suspect the long fasting.

Now that long delay probably didn't kill all the yeast and they were probably getting close to active fermentation by the time you pitched the S-04, which is why you say such immediate results.

Keep the temperatures cool and rouse the yeast periodically and the results should be very similar to your intended yeast.

And next time you can equalize the temps slowly by adding small amounts of the warm wort to the rehydrated yeast. Do this several times a few minutes apart to bring the yeast temps in line with the wort.

Of course cooling the wort to the correct temp is even better.

Craig

Thanks for the insight. I've been out of town so hopefully everything is going smoothly. In the future, I'll make sure not to starve the yeasties.

david_42 said:

It's possible that the long delay between re-hydration and pitching starved the yeast. Dry yeast don't have much reserve energy, that's why the they manufacturer recommends pitching within 15 minutes of re-hydrating.

A second possibility is a small defect in the packaging allowed moisture into the packet. Dry yeast is only viable for 2 weeks once a packet is opened.

The third is the packet got over-heated at some point.

Click to expand...

Wow good to know, didn't think you needed pitch so quickly after re-hydration! I used dry yeast for the first time a couple weeks ago after being re-hydrated for about 4 hours...although I did put some sugar in there to them going as well.