I am going to make my first yeast starter with my new stir plate. How long do I leave the starter on the stir plate?
Size of the starter makes some difference. I let mine go 36 hours to make sure it is fully fermented. Thirty-six hours is more than sufficient for a two liter starter at room temperature. I cold crash before pitching so as not to have the starter wort in the beer.
You’ll leave it on the stir plate until it’s done fermenting (i.e. growing). The amount of time depends on the yeast type, the size of the starter and what temperature you’re fermenting it at.
I usually use a flashlight and look to see if there are any bubbles of C02 being released. When that has stopped you know it’s pretty much done and you’re just beating up your yeast (or keeping them in suspension).
This usually occurs between 24 and 36 hours, (sometimes a little less) depending on the factors listed above.
I did a two liter test last week with Denny’s Favorite 50 and a 1.040 starter on a stirplate at 68 degrees. Tested at 12 hours and every two after that. It was at terminal gravity by the 18th hour, so I’m not going to let them run over 24 anymore. As part of the experiment I reached out to a microbiologist who works for 1 of the “big two” liquid yeast companies. I explained my intent was to refrigerate the starter to pour off most of the beer it made and pitch the slurry. Specifically I asked if the yeast needed to rest after hitting FG prior to refrigerating. The recommendation was to refrigerate as soon as possible after hitting terminal gravity. Keeping the yeast warm and especially using a stir plate post FG is not good for the yeast.