What are Stir Plates supposed to do?

[BenB]

I have a stir plate. I built it. I love it. It looks great (in a kind of Lava lamp way) in the corner of the kitchen spinning round. But what is it supposed to do? I've heard two suggestions as to the supposed benefits neither or which I believe!!!

Reason 1) It increases the air / fluid interface to encourage oxygen absorption by the wort- the head space in the flask will quickly be filled with CO2 so that's a non-starter and if we wanted surface area we wouldn't bother with a piddly little whirlpool we'd use a very large shallow container for starters

Reason 2) It "knocks" the CO2 out of suspension. This might work. However again the headspace is full of CO2. If you wanted a situation guaranteed not to encourage release of CO2 it would be a carbonated fluid with a significant partial pressure of CO2 on top of it. If it's just to do with releasing CO2 there are better ways.

If #1 and #2 were correct why wouldn't we just bubble HEPA'd air through the starter? One of the Brewferm bubbler jobbies would cost about the same as a DIY stir plate!

Or is in fact to stop flocculation and ensure each yeast is exposed to fresh(ish) wort?

Just wondering! Still going to use it but.....

[Mr Squiffy]

I think one of the reasons is to keep the yeast suspended, this aids multiplication.

[jaroporter]

i read that in labs it is used to stop clumping/coagulation. not sure what/if purpose it serves for homebrew yeast though.

[AnthonyUK]

It keeps the yeast in an aerobic state to increase cell count.

[vacant]

Have a read of Shaken not stirred

I have a stir plate. It helps get a bit of air into the wort initially. After that, active yeast will be distributed throughout the wort by convection from heat of the conversion and released CO2 (that's why wort in a FV manages to ferment without being stirred constantly). When the yeast is done - no more heat, no more CO2 and it will settle/clump out and the wort gradually clears.

[BenB]

It keeps the yeast in an aerobic state to increase cell count.

I guess that's what I'm trying to understand. How does it keep it aerobic? To be aerobic you need air and quite quickly the CO2 building up will displace the air (CO2 being heavier than air) so a stir plate won't be stirring air/oxygen into the wort as there won't be any in the headspace.

[Fastline]

There was a discussion on here and the yeast whisperer seemed to be against the use of stir plates for building up starters, I think the term was shear pressure harms the yeast, not to sure on that exactly.

But I think the general favoured method was occasional shaking the living daylights out of the flask as a preferred method.

[AnthonyUK]

I understand that there is more than enough oxygen in the headspace and dissolved in solution for our requirements which is growing enough cells. Don't forget that there is only a finite amount of food so there is no point in growing so many cells that they die from lack of food. It is a balancing act that knowledgeable people have demonstrated to work consistently.
I can recommend the Yeast book by Chris White and Jamil for all you could ever want to know.

There was a discussion on here and the yeast whisperer seemed to be against the use of stir plates for building up starters, I think the term was shear pressure harms the yeast, not to sure on that exactly.

But I think the general favoured method was occasional shaking the living daylights out of the flask as a preferred method.

Admittedly the published results of using a stir plate only tend to focus on cell counts rather than yeast health but in my limited use of starters I have had no reason to question this method.

[john luc]

It's a yeast sex den where they are encouraged to go at it and make alot more yeasties,then when you have enough of them you send them to the wort to work

[Jocky]

I understand that there is more than enough oxygen in the headspace and dissolved in solution for our requirements which is growing enough cells. Don't forget that there is only a finite amount of food so there is no point in growing so many cells that they die from lack of food. It is a balancing act that knowledgeable people have demonstrated to work consistently.
I can recommend the Yeast book by Chris White and Jamil for all you could ever want to know.

There was a discussion on here and the yeast whisperer seemed to be against the use of stir plates for building up starters, I think the term was shear pressure harms the yeast, not to sure on that exactly.

But I think the general favoured method was occasional shaking the living daylights out of the flask as a preferred method.

Admittedly the published results of using a stir plate only tend to focus on cell counts rather than yeast health but in my limited use of starters I have had no reason to question this method.

Does the yeast book ever actually discuss stir plates? From memory I don't think it does. There's a picture of one, but that's about it.

[Fastline]

I found the thread, yeast whisper seems to know his onions, worth a read make your own mind up
viewtopic.php?f=12&t=70926

[rpt]

I think it's a myth that the headspace will fill with CO2 and displace all the O2. Even though CO2 is heavier than air there will be mixing of the two.

And it isn't a sex den since yeast reproduce by budding.

[AnthonyUK]

Does the yeast book ever actually discuss stir plates? From memory I don't think it does. There's a picture of one, but that's about it.

It has data from various starter propagation methods including stir plates which produced the most cells by a large margin e.g.

If you have a stir plate, that is perhaps the most effective method.
A stir plate provides good gas exchange, keeps the yeast in suspension and drives off carbon dioxide, all of which increase yeast growth (around two to three times as much yeast as a nonstirred
starter) and improve yeast health.

I found the thread, yeast whisper seems to know his onions, worth a read make your own mind up
viewtopic.php?f=12&t=70926

I only read the first sentence and had to stop.

Additionally, spinning a starter fast enough to aerate the medium during the lag phase causes the yeast cells to experience shear stress, which is why stirred starters often smell and taste foul.

I'm no expert but I really don't think this is the case. If you have too high a temperature during the initial stages of fermentation i.e. when the oxygen is being used up and yeast are multiplying then this is when most of the unwanted compounds are produced which would be the same in a starter. Factor this in with the 25°c temps recommended for maximum growth and you'd expect the starter to smell and taste bad.

[PhilB]

Hi rpt

I think it's a myth that the headspace will fill with CO2 and displace all the O2. Even though CO2 is heavier than air there will be mixing of the two.

... it's so nice that someone (finally) has identified and addressed this myth that CO2 eliminates (all) O2

And it isn't a sex den since yeast reproduce by budding.

... but that, sir, is like stabbing Father Xmas with the Tooth Fairy's wand ... some myths are best left un-busted

Cheers, PhilB

[Fastline]

I think the shaken not stirred thread was a good read, slightly spoiled by the world is flat brigade who instantly use a small amount of knowledge to dismiss another’s idea, past that it was a source of some very good information and a method that anyone can use to create a good starter.

A few members here tried the methods and performed their own experiments, they found there fermentation went off very well using the method described. In my opinion it gave the authors ideas much more credibility.

The thread basically says you don’t need a stir plate a plastic screw top bottle will do just as good a job and if you believe the guys here that have done it, it would seem that the shaken in a bottle method produces better results.

I say ditch the stir plate and treat yourself to four bottles of beer, will probably take up same space and you will have more fun drinking them, if you wanted you could even try harvesting the yeast from those bottles with your new found piece of kit, the plastic screw top bottle.