I have just started a transition to making yeast starters and also using a homemade stir plate and am curious how to estimate the viability of the yeast when using this method. Normally, there are definite, visible signs when using an airlock, but how does one judge the level of attenuation when the yeast and wort are constantly moving? Is it primarily by color, viscosity, foam or some combination of all of them?
I've prepared a 1000ml 1030-1040 starter with Wyeast 1057 in a room ~ 72F. After 12 hours, I see a 1/4" foam ring at the top of the wort. Is this what I should be seeing and is there a more systematic way to gauge when it's ready to pitch. I have read a lot of conflicting advice about the length required on the stir plate and would like to be able to understand what guidelines others typically follow.