What does White Labs say about pure pitch viability? I scanned the site briefly and didn't find anything other than a 4-month recommended shelf life.

 

PNW: What's the source of your numbers? I was poking around on White Labs' site last night and couldn't find them. Anyway, I punched them into Excel and got the following chart:



Assuming 100% viability at packaging, you can model the yeast's viability using the equation shown. Note "X" is months! So you'd have to use "Days" divided by the average month length to use this equation to model the decrease in viability. And note the R-squared is very high so this would definitely be close enough for homebrew.

I did find that White Labs lists the shelf life of their yeast at 4 months so they want to keep the viability at around 85% or greater. Again, in the "close enough for homebrew" range, the error of assuming 100% viability (I've been known to miss this step in the calculator), your error in pitch rate is 15%. I'd bet we'd be hard-pressed to detect the difference in yeast made with yeast at 100% viability and yeast made with 85%.

 

They reproduce so fast the difference there is negligible. If you have a copy of the latest issue of Zymurgy, Drew Beechum and Denny Conn address exactly this issue. I think where a lot of people early on get wrapped around the axle is that a lot of what is a good engineering solution for a very large brewery, pitch rate included, just isn't that important to us at homebrew scale. There's a big difference between a 24 hectoliter brew and our 0.24 hectoliter batches! The second link is where I found the data for the chart, the first isn't working for some reason. My take: If I'm Coors and interested in making exactly the same brew every time, getting pitch rate right is probably worth the effort, besides, I have lots of yeast, a state-of-the-art lab and some guy paid to count yeast cells in my slurry. For me at my scale, a few billion cells just doesn't make that much of a difference, an additional hour of yeast growth will more than make up for the difference. I use the yeast calculator, trust it, and make good beer. That's what matters to me.

 

I heartily agree. Besides, cell count is far less important than yeast health, though of course you shouldn't ignore it. (I do not buy the Shaken Not Stirred method, which I think takes this too far, but that's another matter.)

I use a stir plate, and I don't really need that calculator because I know that 1 liter (1.06 quarts) of 1.040 wort will give me 160 billion new cells. (Which I have learned from the calculator.) Then I just add an estimated cell count for whatever yeast I'm propagating. An error there won't make much of a difference to the end result.

(Of course I need to know how many cells I ought to have. The calculator is nice to have there. Or I use the table in How to Brew, p. 116, which gives me the same answers.)

 

The biggest problem with homebrew and science writ large is that there's no money in researching homebrew. No one is going to fund research into production of 20 liters/5 gallons at a time! There are some hobbyists out there applying science to homebrewing (yes, Brulosophy, I'm talking to you) and they come up with some interesting finds from time to time but as an engineer, I'm interested in reproducibility, not a unique result. So most of our "knowledge" is gained through scaling down industrial processes, some of which are useful at our scale, some almost useful, some downright wrong. So in this case, it's the body of experience that is more useful than the body of knowledge, hell, I can make good beer and never make a starter! Just use dry yeast and scale up by sachets. Yeast grow exponentially so if you miss the initial pitch, you've caught up a couple of generations later and at the time you're catching up, the number of yeast cells is small.... I'll end the rant here.

 

They grow exponentially, yes, but they'll stop reproducing when the store of sterols/lipids they started out with - and which is not replenished - runs out, or rather reaches a critical level, even if there is fermentable sugar left. Then they start the hibernating process. So it is wrong to presuppose that they will just go on reproducing till they've compensated for the low number pitched. This is my issue with the Shaken, Not Stirred method. No matter how healthy your yeast is, you can't ignore the need for a certain population size.

A quotation from dr. Clayton Cone of Danstar: "At each doubling it will split the lipids with out making more lipids (no O2). The first split leaves 2.5% for each daughter cell. The second split leaves 1.25% for each daughter cell. The next split leaves 0.63%. This is the low level that stops yeast multiplication. Unless you add O2 the reproduction will stop." (The source for this not available any more. I've got it from a Norwegian blog.)

The rest of what you say here, is pure Truth.

 

Good reason to aerate/oxygenate the really big beers a second time.... I agree with you that a serious under (or over) pitch will detract from the beer. I know they won't reproduce indefinitely but it's a case where close is good enough.

 

Not that I'm a yeast Guru, but aren't those numbers based on, "proper handling and storage"? IMO, that would pretty much make them iffy unless you're ordering directly from the lab and receiving the yeast at a reasonably cool temperature.

 

Definitely! Based on my own experiences - which of course don't prove anything - I do not put much trust in that 6 month guarantee. I've had a couple at 6 months or just a few days past which were quite simply dead. No sign of life even after several days on the stirrer. And I buy mine directly at the store, and I trust the routines of that store. To be fair I've had 6 months olds that were OK, too.

Imperial do not guarantee a shelf life longer than 3 months, and I trust them a lot. Seems you can rely on their yeast to perform right up to that date. (I've had yeast from them well past those 3 months, and it was still excellent.)