Take the 2-minute tour ×
Homebrewing Stack Exchange is a question and answer site for dedicated home brewers and serious enthusiasts. It's 100% free, no registration required.

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?

share|improve this question
    
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 '13 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? –  Matthew Moisen Sep 12 '13 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 '13 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). –  Matthew Moisen Sep 16 '13 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 '13 at 18:13
show 1 more comment

1 Answer

up vote 2 down vote accepted

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.

share|improve this answer
    
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? –  Matthew Moisen Sep 27 '13 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 '13 at 23:44
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.