Post Number: 570
|Posted on Wednesday, January 05, 2005 - 05:00 pm: ||
My last starter was made by putting a pint of slurry into a little less than one gallon of starter wort (I use a stir plate.). The last time I did this, the fermentation took off so quickly, that I wondered if any O2 really had a chance to go into solution prior to the outflow of the CO2. Once fermentation starts, I believe this outflow prevents any influx of O2.
So, I came up with what I think's a good idea. I let the stir-plate do its thing for about an hour, THEN I pitched the slurry. I have to think this is a more reliable way to ensure good aeration of a starter, and am passing the idea along for GP or scrutiny.
Post Number: 1080
|Posted on Wednesday, January 05, 2005 - 05:05 pm: ||
Shake the piss out of your starter for 8 minutes and you will achieve 100% O2 saturation as per information provided in another thread.
Post Number: 1682
|Posted on Wednesday, January 05, 2005 - 05:13 pm: ||
Yes, shaking the bejeesus out of a small starter for 8 minutes should achieve near saturation levels of dissolved O2. I suppose the same could be said if there were a safe way to place a 6 gallon bucket of wort on a paint shaker without creating the world's biggest mess.
Belly Buster Bob
Post Number: 1966
|Posted on Wednesday, January 05, 2005 - 06:07 pm: ||
If the starter took off like mad why are you concerned about further aeration prior to pitching???
Post Number: 32
|Posted on Thursday, January 06, 2005 - 01:11 am: ||
I'm sure we'll hear from Fredrik on this one, but until we do, here's my rudimentary understanding:
The yeast need O2 in order to synthesize sterols and lipids for both cell wall construction for budding and for membrane permeability. A pint of slurry in a gallon starter, assuming a high percentage of the cells are viable, is grossly overpitching. So, you have a large, crowded population of yeast cells that are bumping into each other in there. They look around at each other, see that they don't need to divide further, shrug, and say, "Let's get to work."
You get quick starts because they're not dividing much. There's no need for reproduction, as they achieved higher than optimum density the moment you pitched them; therefore, there is no need IN THIS CASE to aerate the starter assuming that the slurry was properly cared for and a high percentage of the cells were viable.
Don't misunderstand me, I am a staunch believer in proper aeration of both starters and wort. But you need to understand why and when it is that you need to aerate, and when it becomes less important. Overpitching makes it less important.
Post Number: 469
|Posted on Thursday, January 06, 2005 - 01:22 am: ||
"...a pint of slurry" WOW!!!!!!!!!!
I would be lucky if I had a cup of slurry in 5 gallons!!! Let alone a pint!
To me a starter is to "WAKE UP" the yeast from it's dormancy of travel. Not to make it into some wort chomping single celled yeastzilla!!!
The yeast know what they are doing!!! Let them do it!
As one of my technical instructors said years ago, "Son, your pickin' fly-poop out of pepper with boxing gloves!"
Post Number: 157
|Posted on Thursday, January 06, 2005 - 02:21 am: ||
I use a stir plate but I've found that cranking it up right after aeration and pitching is not so good; the stir bar whips the O2 out of solution. I aerate the starter, wait a while, then begin stiring. I've gotten visibly more yeast this way. Just my experience.
Post Number: 571
|Posted on Thursday, January 06, 2005 - 03:19 am: ||
I was figuring that the yeast were whupped out and needed the O2 fix. It was a 12.5 gallon batch.
Look at the starter outcome though. The interesting thing is that the slurry came from a fermentation that was carried out in the boil kettle, along with four ounces of hop pellets. The hop residue just seems to have disappeared, though I did try to collect the pint of slurry from the perimeter of my 15.5 Sanke.
Post Number: 1918
|Posted on Thursday, January 06, 2005 - 06:38 am: ||
Though I don't have a stirrer yet but if your headspace and airflow around the surface is good I think Ken's idea of stirring some prior to pitching may be a very good alternative to shaking that is easier!
When one prepares yeast I think one should keep in mind and consider the different target you are trying to achieve.
1. Enough viable cells (with a pint slurry (if healthy) this is far and well satisfied, it's probably even too much
2. UFA and sterol levels of the yeast.
Assueming you intended to prepare for final pitching for a big batch, then aerating the short starter step by pre stirring or shaking might be a good idea, then when there is no more O2 in the hedspace I'd let it finish with no stirring. Once finished I'd decant and aerate the yeast once again.
A short starter step doesn't make the UFA/sterols drop as much which is why I think short steps during the aeration is good. I think of the Purpose of the wort here is to prevent total depletion of glycogen (O2 depletes glycogen initally), rather than reproduction. But keeping the "step" small, you minimize the relative production. At the last aeration step, you do not wish to reproduce the yeast, you just want to saturate the O2 demand of yest. Therefore the pitching rate in the aeration step should be extremely high, so in that respect your pint makes good sense. (Though a pint if >90% viable sounds overall like quite alot.)