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?

  • I'm adding this as a comment, since it doesn't answer your question about method. Keep in mind that the O2 you get from a stirplate is a secondary benefit. The main reason to use a stir plate is to keep the yeast in contact with the wort.
    – Denny Conn
    Sep 12, 2013 at 22:24
  • @DennyConn, I was under the impression that, speaking generally, yeast reproduces but does not ferment in aerobic conditions (Pasteur effect), and ceases to reproduce but starts to ferment in anaerobic conditions. Wouldn't a supply of oxygen throughout the entire yeast-starter process be necessary to creating as much yeast as possible? Sep 12, 2013 at 22:49
  • Helpful, yes...necessary, no. There will still be plenty of O2 in there....it's not anaerobic. Before I got a stirplate, I made hundreds of starters by just using intermittent shaking. While it took a day or two longer for the starter to ferment out, I ended up with the same quantity and health of yeast.
    – Denny Conn
    Sep 13, 2013 at 15:15
  • @DennyConn Correct it could not be anaerobic. I was thinking more along the lines of "a stirplate has more oxygen, therefore more reproduction". I cannot find the link, but I read an experiment that someone on the Internet did, where they compared the final yeast production from four different yeast starters: one with continuous aeration from a stone, a stir plate going full speed, a stir plate going half speed, and a starter with intermittent shaking. Have you read this one? In the order I listed, the more yeast was produced (true it was only that one person's experiment). Sep 16, 2013 at 17:38
  • As I said, the O2 introduced from a stirplate is helpful, but it is in no way necessary. IIRC, in the study you cite all methods were given the same amount of time. A starter that isn't stirred will take longer to reproduce, but with intermittient shaking will still produce an acceptable amount of healthy yeast for pitching.
    – Denny Conn
    Sep 16, 2013 at 18:13

1 Answer 1


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.

  • Thanks MDMA. When I asked him why I should buy an aeration stone, My LHBS guy told me that simply sloshing the carboy gets it oxygenated to a single digit ppm (I think he said 6-9ppm), while oxygenating it with an aeration stone will get it to a double digit ppm (I think he said 15-19ppm). Can you confirm the veracity of this? If its true, why wouldn't I want to aerate my wort? Sep 27, 2013 at 22:44
  • It's true, I have a stone and a O2 tank, and a DO meter to check. But I don't use the O2 in a starter since stirring does the same job. If you want to get the O2 levels up in the starter initially, there's no harm in giving it a burst of oxygen - it will help the yeast. Too much is not a problem - the starter will be oxidized from all the stirring anyway. But for beer, the O2 levels have to be regulated - too much can cause early staling of the beer if there is more than the yeast need. I've not experienced this myself, but have been warned about it.
    – mdma
    Sep 27, 2013 at 23:44

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