Evidently my stir plate is putting out more heat than I remember. I started a 3 qt starter last night in the garage, all stuffed into a cooler; yesterday morning dawned at 25, in the 50s in my garage,so I was worried about getting too cold. Anyway, this afternoon was warmer and even with the cooler lid open a inch or so the starter is at 92, with the stir plate warm to the touch. Obviously this would create problem in a beer, but what about for the yeast I will be harvesting?
Yeast won’t die until the temp gets to 110F, I think 92 would be OK but its getting kind of close to a temp that would stress the yeast. They do grow faster when its warm though. Something in the low 80’s would be ideal.
I’ve never had a starter get quite that hot but once found one at 86. The starter wort tasted pretty crazy but I decanted it more than usual and it made a fine beer. My stir plate also makes a fair amount of heat so I have to be careful of that as well - although I’m not doing starters in a cooler.