Pitching rates vs cell growth?

[Kev888]

I was reading 'yeast' again last night, in order to understand more about starter sizes and stepping up. What I do at the mo is 'about' right but its just based on the general rules of thumb for step sizes rather than a real understanding. I sort of knew that its really down to cell counts and pitching rates though, and that cell growth doesn’t have a linear relationship with pitching rate (which is partly why I’ve been avoiding the issue). The online calculators can help, but having moved to bigger batches the step sizes can be large, so I thought its time I understood it better to work out whats best.

So, there’s a graph in ‘yeast’ of cell growth vs starter size for a typical white labs vial. It shows that (without stir plates etc) in starters up to about 2L the cell growth increases quite steeply with progressively bigger starters, after that it begins a law of diminishing returns until (at around 19L) extra wort doesn't result in significantly more growth. Okay, fair enough, and it explains at least one reason why we step up instead of just making an enormo-starter to begin with. But I have a few questions that I’m hoping people can confirm/answer for me:

• 1. Would I be right in interpreting this to mean that a pitching rate of 100billion cells (assuming a new vial) in 2L would give you ‘roughly’ the optimum starter size for ‘wort capacity vs cell-growth’, and also probably the minimum worth using if you are trying to step up, as below that size you're restricting growth (presumably through lack of resourses for the yeast to use)?
  
  2. Can I safely extrapolate from that so that (for example) if instead of 100 billion viable cells in a new vial I had 1/10th of that in an old vial; the same pitching rate would suggest an optimum minimum of 2L/10 = 200ml first step size?
  
  3. If you maintained the same pitching rate, would you still get a similar percentage cell growth rate at smaller scales? Its just that I notice the graph doesn’t really go that low, and the Mr Malty calculator seems reluctant to consider small starters too, so possibly things change?
  
  4. In this case is 'a billion cells' the traditional million million or (as things seem to be more commonly these days) one thousand million?

Apologies for all those questions – as usual, reading anything chemistry or biology related exposes large gaps in my knowledge..

Cheers
kev

[gregorach]

Nobody, but nobody, uses billion to mean a million million any more - and indeed I don't think they ever have in my lifetime. 1 billion = 10^9, always. Which is handy, because it means that one million cells per ml is one billion cells per litre.

100 billion cells in 2L seems like a pretty small starter to me... On a stir plate, my propagations usually max out at around 200 million cells / ml, (200 billion cells in 1L) - so 100 billion cells in 2L (50 million cells / ml) is only a factor of 4 below my maximum cell count at the end of propagation. But you'll be lucky to get half that in an unstirred starter, so you're unlikely to get more than 1 doubling, which hardly seems worth bothering with. I certainly wouldn't bother with anything smaller.

It does seem pretty linear as you go down the scale - these days I start out with 10ml, then step up to 100ml, then 1L, and I seem to get pretty consistent growth rates all the way. Assuming I hit 200 million cells / ml at each step (not really a safe assumption, but it'll do for now) that means that my pitching rate on each step up is 20 million cells / ml, which seems to work well enough (on the stir plate at least).

I guess the question is: just how bothered are you about optimising your use of starter medium? I can see that if you were doing it on a commercial scale it could well be a significant concern, but on a home scale I'm not sure that wringing the absolute maximum out of it is as important. I'd say its more important to reduce the number of steps you need, as each step presents the potential for contamination. Plus it's less work that way...

As a sanity check, a Wyeast Propagator contains around 30 billion cells and is designed to be pitched into a 2L unstirred starter, giving a pitching rate of 15 million cells / ml. If I had your example 10% viability White Labs vial (i.e. 10 billion cells), I'd probably start off around 500ml (20 million cells / ml) in an ideal world. But that would mean I'd need 2 steps to get to a sufficient cell count for a 20L brew length... (Assuming we're talking ale fermentation here) So in reality, I'd be very tempted just to go for a single larger step and accept that I'm not maximising my yield factor.

[Kev888]

Thanks Dunc - thats all very useful!! Though now i feel irrationally old - a billion always meant a million million to me!

Think I'll err on the side of caution and use much bigger volumes than my suggested minimum then - probably double in fact, or even bigger and miss out the odd step as you mention. At least in the earlier steps it probably wouldn't make much difference to me as a home brewer, but a short while ago I moved to less frequent but bigger (85L) batches and more recently into liquid yeast and now slants, so my starters are involving more steps and big enough quantities that efficiency seems more attractive (although I agree its nothing like a commercial scale!).

Cheers
kev

[Wolfy]

The thing about MrMalty, and the graphs in the book, is that they use a fresh pack of yeast pitched into a small home-brew size starter, as the basis for the experiment. However, since Wyeast packs in the UK are the smaller propagator packs and that by the time the home-brewer gets their hands on either Wyeast or WhiteLabs yeast it's often 'old' and much less viable, I'm not sure that is a valid starting point for comparisons. If you had a fresh pack of yeast (with close to 100billion cells) I'm sure all the data is valid, however if you are starting with 10% (of a 30 billion propagator pack) or less viable cells, I'm not sure that the data remains valid.

In addition, pitching 100billion cells into a home-brew starter is a huge over-pitch, even for a starter, as explained by Chris White in the various TBN podcasts, where he suggests that using 'small starters' really does nothing for yeast growth (and can actually harm the yeast) because the yeast chew through the food reserves without any need to reproduce. So yeast packs into those small volume starters is more about cell health than cell growth, however, they are still the basis for the cell growth experiments in the book and on MrMalty. Which is going to be valid, if (and only if) you are starting with the same conditions, which as home brewers outside of the USA, I'm not sure we are.

Wyeast have a new "Pitch rate and growth rate calculator" on their website: http://www.wyeastlab.com/hb_pitchrate.cfm
This allows for adjustments for a propagator pack and you can also use fractions of a pack, so you might find it more useful than MrMalty or the graphs in the book (test it out, but I'm not convinced that the data is actually valid for small fractions of packs or if it is just a linear extrapolation of data using full/fresh packs).

Like Dunc, most of my 'yeast calculations' are based on building yeast up in steps from starters, however I think this is still valid if you're working with older propagator packs anyway.
I work on the assumption that (with a stir-plate) the yeast will reach a maximum cell density 100million cells per ml which then equates to 100billion cells in 1L. (This is the number mentioned by Chris White at ANHC last year, but on Page 130 of the book it suggests a number between 100 and 200 billion per ml).
Then if you use the the 'usual' 10x steps for starters, each step will have a pitching rate of 10milion cells per ml (or 20million per ml if you use the higher figure as Dunc does) however I often use step sizes of 5x so the pitching rate comes back to 20million per mil anyway.

Having said all that, in the end - and based on the assumptions mentioned above - despite all the theory assumptions and maths - it should all come down to 3 easy steps:
*Use the industry standard pitching rate to work out how many yeast cells you need (this is exactly what MrMalty does, info on the maths here: http://www.mrmalty.com/pitching.php )
*Assume a maximum cell density to determine your starter volume size (I use 100million cells per ml, hence 100billion per L).
*Assuming a starter pitching rate (I use 10milion cells per ml)
*Step that starter size 'down' by a factor of between 5 and 10, progressively until you get to the number of yeast cells you (assume) you are starting with.
(Assuming a maximum density of 100million cells per ml, a starter pitching rate of 10mlllion cells per ml works so well with 10x step sizes and makes the maths easy).

Example: Brewing 85L of Ale at 1.045, starting with 10billion cells (1/10th viability from the old vial)
*We need 713billion cells" (number comes from MrMalty)
*Required starter size is 7.2L **
*To pitch 72billion cells into our 7.2L we need 720ml starter which is a 10x step down
*We'd need to pitch 7.2billion cells into our 720ml starter, but since we assume we're starting with 10billion we've already got enough yeast.
Result:
10billion cells from old vial, into 720ml, into 7.2L gives us the 720billion cells required in 2 easy steps.

*I actually use 1/2 the standard pitching rate for a British Ale (which is actually what is recommended in the Yeast book .... somewhere) so I'd use 475billion cells instead of the 713billion MrMalty says for an ale, which would be about a 500ml starter into the 5L starter.
** If you want to assume a maximum cell density of 20million cells per ml as Dunc suggested the starter is half what I suggested, but I'm sure you can follow the maths from there

[Kev888]

Good grief, many thanks Wolfy, thats really superb - and very clear too.

I don't feel quite so daft for just having used the rules of thumb before now either, as theres clearly a lot going on and (for the homebrewer with no way to count viable cells) still a lot of estimation anyway. But i think between you and Dunc theres enough info here that i now have a good feel for all this, and am happy that I can plan a sensible ballance between effort and starter size, and have confidence in the end pitching rate being suitable. Thanks very much once again!

Cheers
Kev

[Kev888]

Just to say thanks once more. I set up a spreadsheet with the various pitching rates and maximum cell densities to calculate number of steps and their volumes; amazingly I seem to be getting the same numbers as in Wolfy's example so clearly between you both you have actually managed to hammer a new bit of understanding into my head!!

many thanks!
Kev

[greenxpaddy]

Wolfy

Is your 10x rule of thumb based upon using a stir plate?

Cheers

paddy

PS Between steps do you refrigerate to send the cells dormant and split the spent wort out? Or do you simply tip into fresh wort? Seems like refrigerating is more likely to give the opportunity of stressing cells

[coatesg]

The thing about MrMalty, and the graphs in the book, is that they use a fresh pack of yeast pitched into a small home-brew size starter, as the basis for the experiment. However, since Wyeast packs in the UK are the smaller propagator packs

Not those supplied by Brew UK - they supply the activators. But I agree on the freshness - they are generally reasonably fresh, but I'd never pitch without a starter - usually work it out from the mr malty site which assumes the use of vials/activators I believe, but my rule of thumb is one pack in a 1.4L starter on a stir plate (unless it's a big beer). I'll then crash, decant and pitch only the yeast.

[Wolfy]

Is your 10x rule of thumb based upon using a stir plate?
...
PS Between steps do you refrigerate to send the cells dormant and split the spent wort out? Or do you simply tip into fresh wort? Seems like refrigerating is more likely to give the opportunity of stressing cells

Yes, I use a stir-plate for all my starters.

When stepping starters, I simply tip the smaller one into the larger one after about 24h on the stir-plate, no need to let them finish fermenting, chill, or decant the spent starter.
Before pitching the starter into my beer, I usually give it an extra day (or more) to fully ferment, let the yeast settle (can put it in the fridge if you want, but in winter here I don't bother if the yeast will be used within a day or so) decant the spent starter beer and pitch only the yeast.

[greenxpaddy]

Cool. I've ordered all my stir plate bits and will be assembling two this week. Think I'll split the two off one power lead to get 6V each. Think that means I have to wire them "in series"?

[greenxpaddy]

Question for Wolfy. Did you cut off your fan blades or will it be ok leaving them on? Thought it might stress it if I left them on, then I thought I might unbalance it if I tried to cut them off and didn't do each blade perfectly the same

[weiht]

I just got my stirplate build and will be using it from now onwards. As a start, i am planning of pitching a vial of whitelabs into 1L of starter wort for a 5-6 gallon batch. I was told that its equivalent of 2L starter without stirplate which i used to do for my 5-6 gallons.

Is the 1L starter with stirplate sufficient? Does pitching into a 2L be the same as pitching into a 1L then adding it later to another additional 1L wort?

[greenxpaddy]

Weiht, you would be seriously underpitching with a non stirred 2L starter without a stirplate. I would stick with 2L but you'll need 20 billion cells to start with based upon Wolfy 's multiples. How viable is you vial?

[weiht]

My lhbs gets fresh yeast, and most of the time i use 3-4 weeks old yeast. Using the calculator, 2L without stirplate is about the average starter size as I do shake it from time to time.

[gregorach]

You get approximately twice the cell growth from a stirred starter vs unstirred. I use 1L as the final step in my propagations and that gives me plenty of yeast. I also find that variations in the intial cells counts tend to even out as you step up, so unless the vial is really old, going straight to 1L should be absolutely fine. If it's a relatively fresh vial, I wouldn't bother with a starter at all.