Confusion, could well be Alzheimers’ but we have a skirmish...

[Drover's dog]

Here’s the story: I think I brew reasonable beers, I try to follow the rules, I listen and learn…
Then that bloody skirmish when one who you follow says one thing and the other you follow says something quite different. So I now have this new bit of kit (a magnetic stir plate) always had my eye on one. It’s really to blame for the skirmish.

Yeast starters: Do this, do that, finally pour the starter in to a flask (are you ready for the skirmish) sanitise, sanitise, cover with foil and stir for however long necessary. The stir plate, with it’s magic vortex is said to induce oxygen into the starter and stir well which I am convinced it does. What about the sanitised foil cap the protects the ingress of bacteria etc. Simple thought tells us we can’t have both, we either induce oxygen and with it those bad things or we leave a lid on - what’s it to be.

 

[wide eyed and legless]

I just put some silver foil on, and you are right it does put oxygen into the mix, there is one microbiologist who recommends that is all you need, no oxygenating the wort just the yeast.
Though I do do both and it kicks off early, 1 hour on the stir plate agitate the wort on a sack truck for a couple of minutes and your good to go.

 

[MHB]

Google up Pasteur Bottle, there are some that still have samples that Louis him self set up around the turn of the last century (the 18-19 one), although open to the atmosphere bugs still don't get in.
Alfoil is (hopefully) preforming the same function, letting air (oxygen) in and keeping bugs out.

As for adding O2, well yeast needs O2 among other things to reproduce and keep its viability and vitality intact.
There isn't really any way at a home brew scale we can be expected to produce a big enough population to not need to have the yeast reproducing in the wort. There are also well know benefits to having the yeast reproducing in the wort, it consumes sterols, fatty acids (lipids) and protein that are really not wanted in the finished beer.
So if you are getting a reasonable gas exchange in your stirred starter, aeration probably isn't needed, but aerating the wort is a big step in the right direction.
Personally I would put aerating the wort properly ahead of making a starter.
Mark

 

[Drover's dog]

Sounds like I’m on the right track. I do aerate the wort too - skirmish over.
Thanks.

 

[goatchop41]

What about the sanitised foil cap the protects the ingress of bacteria etc. Simple thought tells us we can’t have both, we either induce oxygen and with it those bad things or we leave a lid on - what’s it to be.

A very, very simplistic way to put it is that those nasty bacteria and wild yeasts struggle to climb or float up past the foil cap, then down in to the starter. Same reason why you can use cling wrap over the fermenter instead of the lid and an airlock

 

[Danscraftbeer]

I just thought there is enough oxygen in the headspace of the flask rather than it drawing in air from the outside.
I also dose/purge that headspace or even airstone the starter with bottled O2 but thats not neccesary it just speeds things up. Also having the thought that when the starter kicks off it is expelling co2 preventing nasties getting in past the foil cap against the outflow of co2. Same as any ferment scenario.

 

[Drover's dog]

I just thought there is enough oxygen in the headspace of the flask rather than it drawing in air from the outside.
I also dose/purge that headspace or even airstone the starter with bottled O2 but thats not neccesary it just speeds things up. Also having the thought that when the starter kicks off it is expelling co2 preventing nasties getting in past the foil cap against the outflow of co2. Same as any ferment scenario.

Sure makes sense, can just picture those nasties with grapples climbing the sides of the flask, then to be blown off by a blast of Co2.

 

[goatchop41]

Sure makes sense, can just picture those nasties with grapples climbing the sides of the flask, then to be blown off by a blast of Co2.

The thing is, the ones that you're interested in keeping out can't climb!

 

[MHB]

I just thought there is enough oxygen in the headspace of the flask rather than it drawing in air from the outside.
I also dose/purge that headspace or even airstone the starter with bottled O2 but thats not neccesary it just speeds things up. Also having the thought that when the starter kicks off it is expelling co2 preventing nasties getting in past the foil cap against the outflow of co2. Same as any ferment scenario.

I'm big fan of thinking, rather than just guessing so lets say you have a 3L flask with 1L of wort in it leaving 2L of "Air".
Air is about 20% O2, - 2L*20% = 0.4L of O2
A Mole of O2 occupies about 24L, - Moles of O2 = 0.4/24 = 0.0167Moles
A Mole of O2 weighs 32g, - 32*0.0167 = 0.53g of O2
We want 10ppm (10mg/L) of O2, if the wort absorbed all the O2 in the 2L of head space and you have 530mg available.
So yes it looks like there is enough O2 available for a couple of doublings of the yeast population.

There are a few other factors involved, however you look at it, you wont get a population any bigger than 80-100million cells/mL.
100*10^6 X 1000mL = 1*10^11 (100 Billion), that's an absolute maximum under ideal conditions, really 60-80% of that is much more likely.
Mark