Yeast for Coopers Stout

[Reg]

The reserves are created by virtue of the propgation techniques.

Rehydration start the yeasts metabolism so it can control what passes accross the cell walls and keeps out toxic levels of sugars, hops and chemicals.

Nutrient reserves are used up within 30 mins and a starter will take at least 24 hrs to reach maximum cell density.

The yeast will multiply from 60-100 billion cells per litre in wort and there are already 140 billion in a 7g packet. There wont be any cell growth in a 1L starter, it''ll just use up the nutrient reserves and turn new yeast into old yeast.

If you damage the yeast by sprinkling directly onto the wort it has a good chance of recovery, if the yeast is reyhdrated the yeast isn't attemperated carefully it can mutate and the cells will not reproduce healthy new cells, in the main batch, only more useless mutated cells.

Okay, I think Iget what you're saying...

(I had to read this paper by MB Raines-Casselman, Ph.D.: http://www.maltosefalcons.com/tech/MB_R ... turing.php to fix it in my head though).

So to paraphrase... The dried yeast culture has been stopped in a state where it has the necessary glycogen onboard to multiply in any case without using the wort to build up stored glycogen for replication and has around 120 billion yeast cells per gramme. If the ideal is to pitch with the optimum amount of yeast to metabolise the wort to alcohol without the need to build up glycogen for replication, then for a 1040 wort your need around 5 million cells/ml and that density already covers that (near enough). Although the direct pitching shock may reduce that viability, the result should not waste too many viable microbes.

On the other hand, with a yeast starter, unless you are prepared to go the the fuss of using a magnetic stirrer to ensure optimum replication through optimum aereation, you are likely to be pitching yeast that needs to divide several times before reaching the alcohol producing stage of metabolism and because this yeast has used up its dividing energy it's going to need to rebuild its glycogen store from your carefully prepared wort before getting down to the serious business of making alcohol. So the only benefit in pitching a starter is if that starter has already made ideal fermentation volume. otherwise you are faced with a wrose lag phase and a slightly less potent beer.

In which case, just one more dumb question. If you pitch your stated 140 billion yeast cells into a starter, how come you end up with only 60-100 billion from the starter ferment? Do some die? The volume of yeast appears to get visibly bigger. I'd kind of always assumed you get what you started with plus some replicated yeast to pitch into your main batch.

[Reg]

If you pitch your stated 140 billion yeast cells into a starter, how come you end up with only 60-100 billion from the starter ferment? Do some die?

You won't, the remainder will just sit around, probably at the bottom of the FV with their thumbs up their collective arses. The point is you wont get any new growth and that's the point of making a starter.

Hrrrrrrmmmm...

Okay, that was one of the things I was considering. (Before I go on, I'm not trying to be pointlessly argumentative or didactic, it's just my Dad always made starters and I've always made starters and if it was good enough for Dad etc... etc...)

As far as I can see it one of three things could happen:-

1) The yeast lies dormant: unlikely, my starters show considerable activity, including gaining a yeast head etc.
2) The yeast, being at well over optimal fermentation concentration, will simply make alcohol and acclimatise to the wort. (The latter not being a bad thing and you that checked that your yeast is viable).
3) The yeast will take advantage of the aereation of the starter to replicate. (Also not a bad thing).

If we eliminate (1), because there is definitely something going on and accept that there is no budding going on as you suggest (that kills off (3)), then where is the stored glycogen going? It's kept in the mytochondriae to fuel the yeast when it has nothing else to metabolise and / or let the little blighters bud to reproduce. Is it used up just coping with the stress of exposure to a new environment, perhaps?

(Truth is, I still feel that the yeast volume increases in my starters, but as I've never washed it off and weighed it against an "un-started" rehydrated sample, there no way I can prove that!)

Anyway, I'd be interested to hear your thoughts.

Reg

[KevP]

I'm coming into this a little tired, drunk and a bit dim on yeast matters.

Would not, a properly rehydrated and active yeast sample continue to multiply and multiply until either the sugar source was spent or the temperature prevented its utilisation? I.E the yeast will feed and grow indefinately in ideal conditions (the sample)

[KevP]

Thats a lot of reading, will do tomorrow when I can keep my eyes open

[Reg]

Chris

I didn't mean to lose you, (I lose myself in trying to figure these things out sometimes)!

I take your point about there being a peak concentration by volume for yeast cells, although I'm not sure form the papers I've read what that is. It appears to be wholly dependent on the cells ability to bud which seems to need oxygen unless they get very stressed and go for a sexual reproduction cycle.

My point was, more basically, if these peak concentration yeast cells are sitting around with their thumbs up their proverbial backsides, why do starters show activity?

Cheers

Reg

[Reg]

Yeah! I get that, but if it's fermenting, (which is fine), and not dividing, where is the stored glycogen going?

[Reg]

You better ask the men in white coats that one but my guess is that yeast starts by metabolising the easiest sources of food whether it's in the wort or within the cells of the other yeast.

That question comes under the heading 'if the men with pointy heads at Danstar say to pitch the yeast withing 30 mins of rehydrating then its as well to do as they say'.