What is the proper way to supplement a yeast starter with oxygen via an aeration stone?

I have two yeast starters going on my two homemade stir plates. The stir bar or the stir plate for one of them is malfunctioning and I can not increase the speed enough to get a vortex going. It is simply keeping the yeast in suspension.

On the first yeast starter I ever did, I would put an aeration stone in the flask every 5 hours, in addition to the oxygen obtained through the vortex of the stir plate. I believe I read somewhere online that too much oxygen could be a bad thing, and I haven't done it since.

What is the proper way to supplement a yeast starter with oxygen via an aeration stone?

Topic stir-plate aeration oxygenation yeast-starters yeast homebrew

Category Mac

1
answered at

Too much oxygen in a starter is almost impossible, so long as you decant the liquid and don't pitch it with the yeast. If you plan to pitch the liquid (e.g. to kraeusen a beer or restart a stuck ferment) then don't stir at all since you'll be pitching oxidized wort.

A vortex isn't necessary for oxygen uptake - just having the surface in continual motion you'll get oxygen uptake by ensuring fresh wort is always at the surface. While a vortex in theory gives more oxygen uptake, the wort will quickly become saturated to the 8-9ppm, and then O2 will enter only as quickly as the yeast consume it.

On the negative side - shear forces can impair or destroy yeast cells. Commercial centrifuges are designed accounting for shear stress. I don't know if stirring at max speed would be too stressful, but since a vortex isn't necessary, it's probably best to play safe and stirring at the minimum level that gets the job done, i.e. a gentle stir.

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