In the latest edition of How to Brew, Palmer tells you to use the stir plate for only the first half of total propagation time (Notes on using a stir plate, p.122). The reason is that the yeast needs some time without oxygen in order to build up it's glycogen and trehalose reserves, which it needs during the lag phase. This also fits with what the guys from Imperial Yeast say in this Beersmith podcast: http://beersmith.com/blog/2017/12/10/imperial-yeast-and-starters-with-owen-lingley-jess-caudill-beersmith-podcast-161/ ,namely that it is detrimental to the yeast if you go on supplying oxygen after the fermentable sugars are consumed.

So I do not doubt that this is good advice. It means that if you leave your starter spinning on your stir plate till all the sugar is consumed, you''ll get yeast with depleted energy reserves. That's to be avoided, og course.

But I'm not happy about the way the advice is given. It's rather hard to know beforehand what the total propagation time will be. I asked Palmer, and he answered that maybe max 12 hours would be a better way to formulate it, but actually that doesn't help much either.

This involves, I think, knowledge about what actually goes on in a starter on a stir plate, and that's not easy to come by. I have some ideas, but nothing I could call sureness. So I'd be very happy if anyone has a link to some good information about that. But what I really, from a practical point of view, need, is a figure: At what SG value should I discontinue the spinning?

 

That may be close enough - but I really want the science.

I really don't like to risk infection by taking samples, but I'll do it until I've learned to recognize the correct level visually. Hopefully I can do that.

 

You didn't understand my question. I'm not concerned about cell count, but about the quality of the yeast I get, and I want to base what I do on as solid knowledge as I can get.

And my trouble with Palmer's advice is that he doesn't give me the numbers I want.

 

Oh, I know that one well. But again, it's not cell count/growth rates I'm asking about.

I'm new on this forum, but I'm not a novice brewer.

 

If you didn't continously supply the yeast with some oxygen through stirring, intermittent shaking would be more efiicient. And I think Braukaiser shoved that vigorous stirring on a plate without capping is the most efficient way. You get less yeast growth if you use an airlock.

Shearing of the yeast is a myth. As you say, you would have to spin the magnet much faster than we do in order to damage the yeast.

I'm not sure the oxygen will help you much if the energy reserves of the yeast are depleted, which is what is happening if you spin for too long. (But I've earned my living teaching litterature, history and philosophy, not real science, sadly. So my knowledge about these matters aren't too deep. I've tried to understand a little, though.)

I'm sorry, guys; it's nice to know what you're doing, but what I wanted was some solid science based information about when to take the starter off the stir plate. I haven't been able to find any, and I'm at a loss to understand why all those people out there haven't cried out for this in unison. I can't be the only one who have felt the need for something a little more solid to base my actions on?

 

Why do you say that? It's not a ghost, it's information I assume someone must be able to supply.

"Best practice" isn't necessarily necessary. Most of the time you won't notice any difference if you chose to neglect just one such point. But I'm fairly convinced that you get a better beer if you care about all those details. It's the sum of them that makes a difference.

And I've tasted plenty of pretty bad homebrew that the the brewer claimed was good stuff, so I'm not much inclined to listen to advice based on what people have found out works for them. I'm not very much inclined to trust my own experience, either. (But at least I know what my own beer tastes like.)

 

Thanks, but that deals with ordinary fermentations. What goes on in a starter where you continously supply oxygen isn't quite the same. So yeah, you're right; it doesn't answer my question.

 

I did some editing of my first answer, you may consider that part of this post, too.

I may elaborate a little on what I'm after.

Yeast cells are individuals, that's right. They don't all do the same at the same time, and different yeast strains are also different. So I'm not actually chasing a specific number - though I guess I actually asked for one - but rather a zone. But one would have to use SG values to describe it, I think.

The Crabtree effect (mostly) keeps the yeast from respiring even if it has access to oxygen. But when the level of fermentable sugar gets too low, that effect stops working. The yeast may then, after the sugar is consumed, go on respiring ethanol to get energy, and it won't start the hibernation process. Or rather, this is what I assume happen, and what I assume must be the reason why you should stop supplying oxygen.

What I'm after, then, is the point where the Crabtree effect is negated. This of course assuming that what I say here is right - and I'm not at all certain. It's just damned hard to find information about this that is accessible - and can be understood by someone who has no basic chemistry/biochemistry knowledge.

So the ghost I'm chasing is a scientist with a deep understanding of these processes, and the ablity to explain what's going on in there to a layman. (In my experience the test of a persons knowledge is his or her ability to make a layman understand.)